Global Change Consulting Consortium, Inc.
Forests – Water, carbon, climate, fire, exotic species, management: What are some of the major issues, and what are the concepts that inform the issues?
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The discussion here is not intended to be comprehensive, as this would require a book or set of books. It highlights a number of current issues. The GCCC is, or course, willing to address topics beyond those covered here. We have composed a brief list of the kinds of projects that we foresee as productive.
I. Introduction: forests in a broad context
4 billion (4,000 million) hectares worldwide, or about 1/3 of the land
area. In the
Forests are of diverse types in their species composition. They develop in diverse climates and over diverse soils and topography. Consequently, issues are diverse and geographically localized. We shall cover only some of the larger or more controversial issues herein.
Forests have roles in climate (particularly via energy balance of the land surface), the hydrologic cycle and the attendant availability of water for human use, and the carbon cycle with its links to climate. They provide ecosystem services beyond their functions in watersheds and as sources of timber, including providing habitat for fauna and flora, flood control, and cultural values. Forests have roles in atmospheric cycling beyond the major exchanges of oxygen, carbon dioxide, and water, by intercepting pollutants such as ozone and particulate but, conversely, by releasing terpenes that react photochemically, much as anthropogenic emissions that generate smog. (Note that forests, and all biota, do not exert the major short- or long-term control over atmospheric oxygen; burning all vegetation on earth would not change O2 content of the air by much; rather, it is geochemical control via iron and sulfur cycles, abetted by hundreds of millions of years of geologic burial of carbon made by vegetation or microbes and made resistant to decay, in particular.)
Forests are notably subject to deforestation, which is not the topic here, that being functioning forests. Forests are also subject to: fire, both wildfire and of human origin (land clearance, controlled burns, arson); insect attacks, which may be of enormous scale; diseases, whose spread is often linked back to insects, as in Dutch elm disease, but also to simple abiotic transport by wind and traffic; invasion by exotic species, often in the understory but sometimes displacing the forest overstory; climatic changes, which have been dramatic over the millenia and which are now proceeding at a great rate; and a range of management practices (harvesting, replanting, thinning, fire suppression, controlled burns, spraying for insect control) and their collateral actions (e.g., soil erosion).
II. Some of the major issues concerning forests, as currently formulated
Issues vary widely by region. We may introduce the topic of issues – that is, of management issues, primarily – by using a few examples. A number of them will be discussed in more detail later.
Many forests with a regime of frequent surface fires have been managed with fire suppression, leading to a buildup of intermediate-height trees as ladder fuels; some natural events of massive establishment after rainy seasons have also contributed. A new regime of large fires has ensued. Firefighting of fires on all land-cover classes has generated large budget deficits for these states. What management responses are appropriate, timely, and cost-effective – prescribed burns? Thinning? In what spatial pattern to reduce fire spread? Should the urban-wildland interface that takes much of the firefighting resources be afforded less protection?
On another front, drought and high temperatures occurred in unprecedented combination across western
Another issue is yield of surface water from forested watersheds. Has stand condition from fire suppression or soil condition from logging compromised water yields in some forest types? Is climate change altering water yields? What are the controls, in both weather or climate (meteorology) and plant physiology, over water use? How do they vary with spatial scale and time, both as cycles and as trends?
For several hundred million years, forests have played major roles in the carbon and water cycles and in climate. An imposing issue is climate change itself – how forests affect climate, and the converse. Does afforestation or reforestation sequester a large and stable amount of carbon as CO2 from the atmosphere, reducing its impact as a greenhouse gas? Is such forest expansion sustainable? How will the extra transpiration of water with more leaf area affect water yields? Does reforestation at high latitudes actually increase global warming? A recent climate modelling study concludes that dark forests absorbing sunlight so much more strongly that unforested land that this warming cancels or exceeds the cooling from forest uptake of CO2 as a greenhouse gas. Will climate change – and the rise in CO2 itself - induce major changes in the biogeographic distribution of tree species? Are such changes predictable? Are there strategies of social adaptation to such changes?
Returning to the phenomenon of drought and water stress:
Droughts are meteorological phenomena, episodic decreases in precipitation from
some local norm. Water stress is a
physiological phenomenon that is specific to given plant species or genotypes
and their location on the landscape (soil depth, slope, fertility…). Both are inevitable. Their extremes are, nonetheless, dramatic and
may be very disturbing to human endeavors, such as in the case of the massive
tree loss in
Introduced species have wrought changes, from local to continental and small to catastrophic, in forests. Introductions have included competing plants (e.g., Myrica faya trees in Hawaii, which reduces soil stability on slopes; cheatgrasses prone to fires that kill tree seedlings, among other things), insect pests (Asian longhorned beetle; gypsy moths in North American forests), vertebrate pests (Australian possums that kill vast numbers of trees in New Zealand’s coniferous forests), and diseases (chestnut blight and Dutch elm disease that reshaped the eastern US; Phytophthora diseases that are changing the landscape in Australia). Accidental introductions are virtually unpreventable with so many routes of entry that are prohibitive to monitor for small seeds, spores, etc. Do we have in place the rapid recognition of invasives and immediate control measures for new exotics?
To answer these questions requires a full understanding of the concepts in forest physiology, hydrology, climatology, and other fields that underpin all the phenomena. Indeed, such an understanding is needed in order to ask the right questions – questions that are meaningful, questions that are capable of validation or falsification, questions whose answers inform effective action. We must know if we are aiming at the correct targets, and the correct time scales.
A. What environmental and physiological factors, including historic legacies, structure forests? That is, What sets their density, as stem density, basal area index, or leaf area index? What sets their species composition? Individual trees experience germination, establishment, maturation, and mortality. These life-cycle events may occur on the scale of one or a few trees (canopy gap phenomena, when one mature tree dies and leaves a space for regeneration), many trees (e.g., triggered by floods, landslides), or whole stands (such as stand-replacing fires or insect attacks).
The topics of individual dynamics and stand dynamics are vast in scope, having generated many journal articles, monographs, reports, semi-popular accounts, and popular accounts. Here we cover some highlights and novel or less-discussed concepts relevant to stand density, water relations, and fire dynamics.
i. Density - a measure, and its regulation. Stand density has several measures. Common measures are basal area index (stem area per ground area), number density of trees (perhaps by age class), and leaf-area index (LAI, or leaf area per ground area). A mass establishment event typically follows clearance of an area by fire, harvesting, landslide, or other catastrophic disturbance. High resource availability favors a dense initial stand, particularly a season of high rainfall. Individuals grow and begin interfering with each others’ resource acquistion, especially light. The more resource-stressed individuals die in a process denoted as self-thinning. Some remarkable regularities occur in the process, such as the “-3/2 law”, that mean plant mass rises as number density, n, falls, as n-3/2. As an example, as growth (crowding) decreases the number density by a factor of 100, average mass changes by a factor of (1/100)-3/2 = 1000. The total biomass density increases by the factor (number density)*(average biomass) = (1/100)*1000 = 10. Detailed physiological reasons for mortality of individual plants, relating performance to local microenvironment and to genotypic variation among the individuals, are just beginning to be explored.
ii. Leaf area – a measure, and its regulation. The physiology behind development of leaf-area index and its (near) stabilization is better explored. On the individual plant, development of individual leaves has stages of cell division and cell expansion. These stages overlap in part. They respond (at least in herbaceous species such as maize, those best studied to date) via a reduction in one or both processes, to stresses of low water status, low light level, or low humidity. All these stresses indicate that it is unfavorable at the moment for support of the existing leaf area, thus, even less for added leaf area. At the level of the whole stand, competing leaf development among all individuals hits limits imposed by availability of water and light, with a modulation of the limit by temperature that affects rates of water use. A model of LAI acclimating or equilibrating to the environment has been developed and partially tested. It has been extended to include the effect of increasing atmospheric CO2, which reduces water use rates, among other changes.
Equilibration of LAI (and consequently of basal area) is a response that limits water stress and perhaps nutrient stress. One may then ask if high density should induce a greater degree of average water stress, with subsequent increases in dangers of fire or insect attack. If equilibration occurs to a set stress level, the answer would be “no.” If the tolerated average stress level is higher for smaller plants, the answer would be “yes.” The reality is unknown to date; there are no large-scale and long-term measurements of forest water stress to date, for reasons discussed below. Extant studies on smaller temporal and spatial scales have indicated some correlation of density with stress, either by observations (1, 2) or using experimental thinning. It is known that high densities that arise from wet years after a disturbance can persist as long as centuries in some forest types. In brief, self-thinning can be a prolonged process.
iii. Species composition is another major aspect of forest structure. A few basic aspects of climate have been held to determine biome type and some aspects of species composition, such as dominant functional type or even species. These pieces of climate are temperature as average or range and precipitation as annual total and possibly in its seasonal distribution. Moreover, individual (tree) species are ascribed to have natural geographic ranges determined by these same factors. These are certainly gross oversimplifications, so that somewhat deeper models have been developed. For one, ranges of species are determined as well by competition, as well as by ranges of pests, diseases, pollinators, etc.; potential ranges (fundamental niches) get restricted to actual or realized ranges. Also, the extremes of climate may be quite a bit more important than the means. Finally we note that soils matter. Their texture determines water infiltation rates, their depth and texture determine “plant available water” and stress regimes, and their parent material, among other things, determines some critical nutrient contents. Soils do develop conjointly with climate, parent material, and vegetation. The elaborate linkages do not readily allow resolving climate and parent material as the abiotic drivers and soil or vegetation as the result. In all this background of forces that form species composition, mass species replacement do occur and will occur. Megadroughts are one mechanism, and their prediction, even on a statistical basis (not exactly when they will occur, but with what mean return time at a given intensity), is challenging.
We have also projected that the human-caused rise of atmospheric CO2 content will directly change species ranges and the species composition at any geographic location. As CO2 increases, it induces changes in water-use rates, water-use efficiency, nitrogen content, and the efficiency of using nitrogen in photosynthesis and growth. The changes differ widely among species. Despite the fundamental importance of each such performance measure to plant success, species vary in the suite of adaptive responses encoded in their genes. It has been a very long time, 20 million years, by some measures, that CO2 has been as high as now. Adaptive genetic variation has surely been lost extensively by genetic drift or by selection for other traits that work against adaptation to the return of high CO2. We believe that some predictions of adaptive and maladaptive responses may be feasible for different plants, not generally as individual species but by functional types defined by, say, stature, lifespan, degree of tolerance of water and nurient stresses, temperature tolerances. These “working regions” for each functional group correlate with climate, which has correlated in the past with CO2.
iv. Climate change,
particularly as an overall warming, is apparent,
and its causes are being resolved with greater certainty. Changes in species composition are widely
expected, from a variety of direct and indirect effects. Extremes of precipitation, both high and low,
are increasing (1, 2) over
large regions of the globe; these condition survival of species. Longer warm seasons are changing water use
by plants, in some cases causing early depletions of soil water. Activities of pollinating
insects have been shifted, with some mismatches to plant flowering. This is in part a result of environmental cues
such as photoperiod,
which plants and animals use to set developmental schedules, becoming more
poorly correlated to seasonal growth conditions. Insect pests have become more
abundant in some ecosystems, including rather notably pests of conifers
v. Fire suppression. Another agent that structures stand structure as density and species composition is fire suppression, eliminating a natural agent of self-thinning. If such causation is correctly identified, how is it that self-thinning is delayed for decades or longer? Essentially, it is posited that surface fires are normally the dominant agent of self-thinning but that these are prevented. The simple presence of more fuel of intermediate height (ladder fuel) enhances the probability of fire progression from surface fire to crown fires, which are very infrequent in the fire regime of arid-zone forests.
A number of questions should be answered before we can embark on new, comprehensive management activities (mechanical thinning, prescribed burns) in a useful and cost-effective manner.
B. What environmental and plant factors, including historic legacies, set the operating points of forests in water and carbon cycles and in climate?
We will assume a rather basic knowledge of plant physiology, to keep the discussion compact. Elements of this knowledge include that water is primarily taken up by plants at their roots and primarily lost by transpiration at their leaves through myriad small pores called stomata; that CO2 is taken up through the same stomata, passing in as water vapor passes out; that photosynthesis as the source of energy capture for growth occurs primarily in the leaves and responds in general positively to increased availability of light, water, CO2, and mineral nutrients, while response to temperature shows an increase until an optimum is passed. Many nuances and some counterintuitive phenomena occur but only a few can be detailed here.
i. Leaf transpiration. Water loss at leaves by transpiration is under the control of: 1) the leaf aerial environment (sunlight, humidity, air temperature, windspeed, air pressure, CO2 content of air); 2) water status of the leaves and the roots; and 3) the physiology of the plant – the many small pores in the leaf (stomata), which also pace their operation to the photosynthetic capacity of the leaf. The action of stomata has evolved to nearly optimize various aspects related to water-use efficiency. Simply, if water stress develops or if humidity is low, stomata reduce their conductance. This cuts water loss almost in proportion, but it reduces photosynthesis much less; the water-use efficiency as the ratio of photosynthesis to water loss improves. Some additional details are given on another page here.
Meteorological conditions have greater control over transpiration rates than does physiology (stomatal control). Solar energy delivery is the most important condition. Unless a plant canopy is quite stressed for water or nutrients, transpiration rate typically changes in parallel with the flux of solar energy to the leaves. For a specified leaf area, one can visualize all the different contributions to control by computing changes in E under the normal range of variations in all factors – energy flux density in sunlight, air temperature, humidity, windspeed, CO2 mixing ratio, air pressure, and stomatal conductance. In this exercise, one must use consistent, coupled models of energy balance of a leaf (or of all leaves in a canopy), stomatal control, and photosynthesis. We have developed a Fortran program for this.
ii. Leaf area development. Stomatal
conductance (gs) is nonetheless the primary short-term physiological
control over transpiration. In the long
term, leaf-area development exerts the greatest control: a full vegetative
canopy readily evapotranspires at twice or more the rate of bare, wet ground,
which has a low aerodynamic or boundary-layer conductance for water vapor. Strong control of leaf area development is
adaptive, in the biological sense of contributing to fitness as leaving
descendants. The plant must survive to
do so, and must partition its resources to seeds or other propagules
efficiently, not growing leaves and other non-reproductive parts beyond
necessity. Individual leaves develop by
division of small sets of original cells (meristems) and their volume
expansion. In some species, these
processes have been discerned in great detail.
Division and expansion proceed for limited times as measured by thermal time, which slows down at low temperatures and
passes rapidly at high temperatures.
Both processes are permanently debited (and
leaves are thus smaller) at low light availability, low humidity, or during
tissue water stress. Such responses are
adaptive. For example, water stress indicates
that current leaf area is minimally supportable; extra leaf area is not, so
that its development should be reduced.
In extreme cases, plants lose leaves.
Many species are adapted to do so without
damage to their later growth potential.
Examples include trees in tropical dry forests as in southern
iii. Responses of stomata. To compare vegetation types with comparable leaf area development, we return to considering stomatal control. Stomata respond to light, humidity, temperature, CO2 concentration in air, windspeed, air pressure, and water stress. The metabolic and biophysical steps that mediate the responses are still being eludicated. Nonetheless, simple regularities have emerged: gs reponds in direction proportion to humidity at the leaf surface and in inverse proportion to CO2 mixing ratio at the leaf surface. All other responses – to light, temperature – fold into a direct response to photosynthetic rate that is driven by these environmental factors. The observed responses act to maintain good water-use efficiency (ratio of photosynthesis performed to water lost) or more general optima, and they are apparent at the level of the whole stand as well. Stomata also respond to signals of water stress, with a notabl decline in conductance. In herbaceous plants, this signal appears to be largely a hormone, abscisic acid, made in the roots in response to stress. In trees, a larger signal may derive from the water status of the leaf itself.
i. Roots. Plants maintain high function, or, at least, viability, by the ability of root uptake of water to match transpirational water loss. For most plants replenishment of root-accessible water relies on precipitation. The episodic nature of precipitation inputs leads to episodic shortfalls in water uptake, abetted by limitations on root development and function. Such shortfalls require control of transpirational water loss by stomata, and, more so, curtailment of leaf area development that was noted in Sec. above. Root development and function is an extensive topic that cannot be detailed here. One point to note is that stress can permanently cut root function (hydraulic conductivity) by a process of air entry into plants’ water-conducting vessels. This possibility makes stomatal control yet more important.
ii. Control varies with scale.
The balance of factors that control water use by plants shifts with scale,
going from leaf to plant to stand to landscape. We noted earlier that stomatal control is
the most important physiological control.
Control can be described by a few physiological parameters: one is a
“slope” function, the factor of proportionality between stomatal conductance
and an index comprised of
photosynthetic capacity, humidity, and CO2 mixing ratio at the leaf
surface. Another is the intercept of the
relation between gs and the index just noted. A yet more important
parameter is maximal photosynthetic capacity
Vc,max. We have
analyzed field data on trees growing in a dense stand on the banks of the
Using all the water.
Of course, leaf area index is a major scaling factor for transpiration,
also. Limitations on water and nutrient
availability constrain leaf area development and, thus, transpiration
also. Deserts are examples of water use
scaled to water supply; leaf area indices over a whole landscape in a desert
are less than unity, and may be nearly zero in the driest deserts. We may state that water use is“closely”
scaled to water supply. Plants have
evolved to use as much water as possible on the time scale of a whole season;
there is no adaptive value in leaving water unused, particularly for
competitors. Careful measurements confirm this.
There is an evolutionary challenge in metering water use over the short
term during any season, to avoid severe water stress that threatens function
and, ultimately, life of the plant. On
the converse side to episodes of water-supply deficits, water may be delivered
at rates exceeding both plant use and storage in soil. Runoff results, forming one major route of generating
surface waters, the rivers and lakes.
(The other route is infiltration of water into soil with subsequent
movement below the surface to springs.)
On a global average, about 1/3 of precipitation
on land becomes runoff. There are
notable variations among the continents, including the perhaps surprising
iv. Large scales. At larger spatial scales, stomatal control is diluted by the imposition of additional resistances to moving water vapor from the leaf interior to ambient air. Two of these are the “aerodynamic” conductances (or resistances) of boundary layers of restricted air movement that form around individual leaves and over whole plant canopies. In an extreme case, stomatal control becomes negligible and only meteorological control remains. At the scale of entire watersheds, stomatal control is not obvious at all. It has been found in Russian and US studies that yearly evapotranspiration (plant transpiration plus evaporation of water from bare soil) scales to annual precipitation, P, in a form that is a bit subtle and a bit complicated. The form involves net radiation, Rn, a measure not only of solar energy input but also gains and losses of thermal radiation, and the form involves the power 1/a of (1+[P/Rn]a). Where did stomatal control, and leaf area control, disappear? Our interpretation is that, on a landscape average, gs and LAI both acclimate to match this function of precipitation. Why it is in the complicated form is not yet to be discerned. Also not clear is why nutrient status does not enter; two biomes of the same total P can have very different nutrient availability yet appear to be covered by the same relation that does not involve nutrient status, or does so in an even more obscure fashion than for gs and LAI. These scalings must be important for forests and for understanding the diverse operating conditions of diverse forest types.
B. Water stress and its stable and unstable forms of occurrence
i. Stress is inevitable. Water stress is formative of forests, strongly contributing to determining their establishment, persistence, species composition, fire regime, and the balance of water use and surface water yield. Episodes of water stress are inevitable in virtually all ecosystems, even in the tropics, given: 1) the stochastic nature of precipitation or other water supplies, and 2) the competitive disadvantage to a plant from reducing water use to a stress-avoiding minimum; water is saved for competitors as well. Consequently, plants have various behaviors, which we may call evolved strategies, to deal with water stress. These may be categorized as: 1) stress avoidance, by developing access to privileged water supplies, typically by developing deep root systems. These are metabolically costly, particularly in construction, so that there are some elaborate tradeoffs, and they are far from common; 2) stress escape, by timing the plant’s life cycle to wet seasons. Many ephemeral plants do this; 3) stress resistance, by controlling water uptake and water loss stringently to limit the depth and duration of stress. Some draconian actions may be involved, including shedding of leaves in a dry season; 4) stress tolerance, which is the ability to undergo significant drops in water status (water potential) while retaining future function and assuring a high probability of survival. Some desert shrubs have remarkable tolerance of low cellular water potential that includes protection against additional damage such as photoinhibition of idling photosynthetic systems.
ii. Extreme events. Water stress may be ubiquitous but it varies seasonally, and by ecosystem type, and by species within an ecosystem, and so on. It also can develop into an extreme event. This is a concept that needs extreme care in its defintion in order to be useful; among other things, extremity is a continuum, not a category. Moreover, an extreme event is a specific sequence (of temperature, precipitation, etc.) rather than a point event; the same high temperature may be of no consequence in summer but lethal in early spring, because acclimation is critical and it depends upon time sequences in the environment. No evolved physiology and developmental program of a genotype assures survival of extremes nor preservation of reproductive potential, which is the ultimate measure of fitness. There are stand-replacing droughts even in natural systems when one considers long time scales, such as millenia. In lesser extremes, many individuals suffer damage or mortality because they established in poorer microsites (soils, topography, competitor density) or because their genotype did not confer sufficient stress responses. More than abiotic water relations are important in mortality events; insects attacking drought-stressed trees have mediated the greatest one-year loss of forests in history and a massive reinjection of CO2 into the atmosphere from decay of dead material.
iii. Stress is multi-dimensional. Thus, we must phrase questions about water stress, or any stress, with care to develop the full context. In particular, there will be a regime of water stress that is natural and sustainable by the species in the ecosystem. In general, we in the scientific community have very little knowledge of these regimes. We propose that the description of stress must be expanded to at least three dimensions (see also 1, 2). Stress episodes have depths (quantifiable as, say, the amount of the drop in water potential), durations, and recurrence frequencies or, more generally, recurrence probability distributions. In complementary fashion, there are regimes of plant stress responses. A given “toolbox” of stress responses encoded genetically and epigenetically in an individual plant enables it to cope with a variety of stress regimes, with varying consequences to growth, fitness, and survival probability in each regime. We expect that sustainable stands cluster in some region of the 3-dimensional “stress space,” a concept we may borrow from mechanics. If we lack a knowledge of stress regimes, associated coping strategies, and fitness consequences of the combinations of stress regime and strategy, we cannot interpret the exhibition of stress symptoms as being normal or not, or adaptive or not, or as having specific consequences for hazards of fire or insect attack or the like. We can gain this knowledge, nonetheless. It requires, first, observation of natural regimes, their causation (as, by weather), and plant responses to them.
iv. Stress is distinct from drought. Water stress may be associated with drought, but equating them is inviting confusion by attributing causation without critical thinking. Drought is a meteorological phenomenon, an almost-always temporary shift to a regime of lesser precipitation. It may contribute to water stress in a plant or stand or landscape, but so also might the chance location of an individual or stand on an unfavorable microsite, or so might human management practices.
v. Equilibrating water use. We offer two concepts that should be useful in discerning natural stress regimes. One is equilibrium leaf area index, noted earlier. Another, which we believe is new in this document, is equilbrium evapotranspiration, or, to avoid confusion with the a term of the same name in micrometeorology, acclimated evapotranspiration (AcET). We may extend the concept of equilibrium LAI to add the concept of equilibrium stomatal conductance. The mean value of gs over a longer period may be shifted by stress responses. Probably more commonly, a shift in gs and thus in ET between times or between locations results from acclimation of photosynthetic capacity of leaves, to which gs has been observed to scale fairly strictly. Photosynthetic capacity requires nutrient investment in leaves, so that nutrient availability certainly helps to mold capacity and AcET. Low photosynthetic capacities in the nitrogen-poor boreal forest appear to exemplify this limitation. The role of water in setting photosynthetic capacity is less clear to date. In one example, we have found (publ. in prep.) that leaves of the desert shrub, creosotebush (Larrea tridentata) exhibit modest photosynthetic rates per leaf area while having extremely high amounts of nitrogen per leaf area. They have abundant N in soil but limited water.
Can the acclimated ET of a stand be exceeded, escaping physiological and developmental controls within individual plants, such that water stress becomes greater in one or more of its dimensions of depth, duration, or frequency, and among many individuals in a whole stand? One condition in which such excess stress has been invoked as occurring and with attribution of a cause is a high density of even-aged plants. High density may occur naturally after a disturbance such as a stand-replacing fire is followed by abundant precipitation, allowing establishment of many more individual plants than is normal. Indeed, stand density does show legacies of high-establishment years for as long a centuries. Can stand density lead to greater rates of transpiration per ground area, thus, to depleting soil water more rapidly between precipitation events? Because transpiration is more controlled by energy delivery (sunlight interception) than by physiological (stomatal) control, modestly higher ET might occur. A full program of modelling and field measurement would be needed to answer these questions. Correspondingly, the concepts of equilibrium LAI and acclimated ET do need to be generalized to account for their recovery from disequilibrium. Perhaps most simply, one could define the characteristic time of recovery, or relaxation time.
En route to answering the questions just posed and later questions about fire and insect attacks, we must be able to quantify water stress responses of large numbers of individuals or of whole stands.
Direct measures of stress vary in the identity of the process studied. Some measures are of water potential itself, others are of photosynthetic pigment changes, others sense transpiration, leaf temperature, or CO2 uptake from bulk air. Changes in plant development are also used, ranging from delays in development to loss of leaves. Methods vary as well in applicability to large-scale detection by virtue of the effort and time required, or the degree to which direct access to plants is required, particularly to elevated parts of tall canopies.
i. Pros and cons. Large-scale measurements are often taken, and with good reason, to be the purview of remote sensing (RS) from satellites or aircraft. Satellite RS offers global coverage, as well as repeat coverage at known intervals and access to the aerial parts of the canopy. Tradeoffs include: inability to resolve single trees or other plants, such that the resultant averaging can hide some phenomena; long intervals (16 days) between repeat coverage for the high-resolution imagery such as ASTER; fixed overpass time that disallows resolution of daily time courses of plant responses; obscuration of signals by clouds or by atmospheric aerosols whose effects cannot be completely corrected. Aircraft RS offers imagery that is much more detailed spatially. The most advanced systems offer numerous wavelength bands that indicate very diverse physiological conditions. Repeat coverage is challenging; spatial coverage is limited, and the operational expense is high.
ii. Electromagnetic signals. In remote sensing, the only signals available are optical in the widest sense, that is, from electromagnetic radiation, covering not only the visible spectrum but also infrared, thermal infrared, and microwaves. Most RS is passive, in the sense that the satellite or aircraft does not emit a probe beam. Instead, RS relies on reflectance of sunlight or spontaneous emission of thermal radiation by plants and soil.
Fortunately, many changes in physiological status include changes in reflectance. These occur in discrete ranges of wavelength or wavebands. A great body of research allows interpretation of these signals with varying degrees of confidence that they each represent unique plant responses. A number of these signals or spectral indices are outlined on a separate page here. These indices do have explanatory power for carbon gain in both forests and woodlands, for one. Some have significant limitations, particularly if they are measured in narrow wavebands that require hyperspectral sensors. Satellites and many portable sensors have broader bands that dilute the signal. There are some surrogates (1, 2) using combinations of a number of these broader bands. Indices may have to be calibrated for individual tree species, adding a demand for additional ground-based information. In some species, the relation to stress is weak and less useful. Finally, indices may depend on illumination geometry that variably samples leaves that are heterogeneous by location in the canopy and by time (refs. 1, 2).
iii. Using multiple signals. There are other RS measures that use satellite-measured combinations of many reflectances and thermal infrared emissions in order to estimate evapotranspiration itself. Changes in ET indicate changes in stress as well as the direct participation of vegetation in the water cycle and climate system. Most of the methods use the radiation measurements to indicate energy exchanges between the vegetated surface and the air, the soil, or the sky (by radiation). The energy lost to the air by evapotranspiration is determined as the remainder in the total energy budget. This remainder can be the small difference between larger quantities that have associated error limits, so that the error in estimating ET is larger yet. One critical part of the energy budget is the transfer of so-called sensible heat (that is, transfer that involves changes in temperature, not changes in phase of water between liquid and vapor). This is proportional to the temperature difference between vegetation and air. The amount of error in determining this difference can be large, as noted above. Several methods reduce the problem by calibrating this difference with two locations in a scene, one that is hot (maximum heat transfer, essentially zero ET) and one that is cold (maximum ET). The method has several weak assumptions that limit its utility in comparing different locations (it is useful comparing different times at one site) and over forests that have very small differences between vegetation and air, or over sparse vegetation where (hot) soil dominates the signal. Other models have been proposed and tested. The technique is maturing, so that it is becoming practical to identify water stress by remote sensing. We thus have hopes for using the acclimated ET index in the effort to determine the natural stress regime of different vegetation types.
iv. Active remote sensing is feasible also, with more limited times and areas of deployment. One promising technique with direct relevance to vegetation water stress is chlorophyll fluorescence. A laser beam is directed at leaves. Chorophyll molecules, with the participation of auxiliary pigments, absorb the beam, as they also absorb sunlight. A small but readily measured fraction of the energy is re-emitted from chlorophyll as fluorescence. The amount of fluorescence and its timing can be interpreted in great detail. For vegetation surveys, rapid scanning is needed, limiting the type of information that can be derived, but it is highly informative about stress. Surveys with such instrumentation (1, 2, 3) can cover areas of intermediate spatial scale, up to a number of square km. For accurate representation of the status of large geographic areas, researchers must carefully choose where samples are taken, as by using hierarchical sampling (1, 2, 3, 4). Other active sensing methods, primarily for structure, include radar and its analog using light beams, LIDAR. Pulses of microwaves or of light are reflected; the time for the return signal indicates the distance travelled, hence, the variations in height of the surface. The height variations are often related to the patterns of tree crowns or other vegetation patterns, hence, to vegetation amount and type. Yet another active system uses microwave pulses whose reflections are detected with information about the wave phase, not only the amplitude. The geometric structure of the surface doing the reflecting can be inferred, though with much complexity.
v. Sensing mass. One final type of remote sensing that we will mention is not based on light or other electromagnetic radiation, but on gravity. In the GRACE experiments (an acronym for Gravity Recovery And Climate Experiment), two satellites follow each other in orbit by 220 km. Local changes in mass in the earth below, such as water accumulating in soils, speed up the lead satellite as it approaches and retard it after passage, and similarly for the following satellite. The changes in satellite separation are measured to micrometes and allow detection of changes in soil water content as small as mm depth. The resolution by spatial extent is crude, about 400 km, but this is useful in large-scale studies of weather and climate, including monsoonal rains such as in the US Southwest.
E. Long-term, integrated measures of stresses and resource use
i. Growth. Reduction in overall growth is certainly an indication of stress. This may be practical to measure in very limited circumstances, requiring, among other things, that individuals have been identified from their origin. More practical measures exist. Tree rings can be related to specific years and seasons. Their width indicates how favorable each year was, in that weather but also that tree’s microsite, which may have had significantly different water, nutrient, or light availability from the stand as a whole.
ii. Isotopic composition of tissue. Even more informative than gross ring width is its composition of carbon and oxygen isotopes. In brief, carbon on earth exists primarily as 2 stable isotopes, carbon-12 (12C) and carbon-13 (13C) that differ only in the number of neutrons in the nucleus. The average fraction of 13C is 1.1%. The isotopes behave almost identically in chemical reactions; the heavier isotope is, among other things, slightly slower to react, as explained by quantum mechanics (my writeup?). Consequently, photosynthetic reaction of CO2 with water to give sugars and then final biomass shows a depletion of the 13C/12C ratio in plant tissues. This ratio can be measured to exquisite precision by modern instrumentation (mass spectrometers). The discrimination in photosynthesis occurs in two major steps – diffusion of CO2 through the stomatal pores (a lesser discrimination), and the enzymatic reactions (much greater discrimination). If stomatal conductance is reduced by tissue water stress or by low atmospheric humidity, the lesser discrimination during diffusion predominates. The 13C/12C ratio is higher than in unstressed conditions. The change in discrimination is a reliable indicator of reduction in stomatal conductance over the period that the biomass accumulated. Leaf tissue can be fractionated to resolve components made recently (days). Tree rings can be analyzed as a whole. This measurement resolves water stress (as a combination of low humidity and tissue water stress) from other factors such as low temperature or high cloudiness.
Isotopes of oxygen, hydrogen, nitrogen, and sulfur can be measured in tissues as well as in stem water and water flux into the air. Much other information can be extracted, for example, about which sources of water or nutrients that the plant has used.
i. Eddy covariance. Transpired water goes into the atmosphere, and the fluxes can be measured at a given point from towers where sensors record the vapor concentration and the vertical wind velocity very frequently, about 20 times per second. In this technique of eddy covariance, the integral of vapor concentration multiplied by vertical windspeed yields the cumulative transport of water vapor, that is cumulative ET. When sensors are also deployed to measure CO2 in the air, one can get net CO2 exchanges or fluxes between the landscape and the air. Because parcels of air arrive at the sensors from various points upwind at different times, eddy covariance measures ET averaged over a “footprint” distributed over the land. The footprint shifts position with atmospheric condition (stability) and with wind direction. Thus, the technique does not resolve individual trees. However, it gives continuous records over time. More than 400 towers are in place around the globe in a group called Fluxnet. Cooperative data analysis has enabled the resolution of large-scale changes in ET, such as happened during the European heat wave of 2003. The Fluxnet towers also measure CO2 fluxes, informing studies of the carbon cycle and its role in climate. Some specialized towers resolve different isotopes of carbon. The isotopic ratio reflects differences in flux from different vegetation groups (so-called C3 vs. C4 plants, which differ in the enzymatic pathways that begin photosynthesis). The ratio also reflects vegetation stress that changes stomatal conductance.
ii. Limitations. The accuracy and areas of applicability of eddy covariance has its limitations, as do all technologies. Foremost, and still puzzling, is the general inability of eddy covariance systems to account for the balance of energy on the landscape. The net input of energy comes as radiation, both shortwave (sunlight) and thermal infrared, debited for energy flowing into soil as heat. This should equal the energy that goes into heating the air (sensible heat) and into evapotranspiration, with some corrections for heat staying transiently within the stand to heat the stand itself (wood, primarily). The degree of energy closure, as the outputs divided by the inputs, is disturbingly low on average, about 75% among all the Ameriflux sites. About ten possible causes have been investigated, with no conclusions possible to date. The common resolution is to scale up both sensible heat flux and ET by the same factor, but it is not known how appropriate this is. Problems are excerbated by placement of the systems on complex terrain (hills). Additionally, systems suffer data gaps when individual sensors fail.
iii. LIDAR. An even more elaborate system for measuring the flux of water vapor in air is dual-beam LIDAR that is aimed over the canopy, traversing the air. One beam is absorbed by water vapor, the other is not. Incremental differences in beam strength are detected by the scattering of the beams back to a detector. The spatial profiles of water vapor content, with the aid of data on winds and atmospheric structure, can be processed to indicate water vapor fluxes in the air – ET in great detail. The system is very expensive and rather immobile, and its senses small areas (100’s of meters).
i. Inadequate precision in single methods. There has been remarkable progress in the technology and data analysis methods for measuring many aspect of plant stress and plant participation in water and carbon cycles. Many of these methods apply well to forests, our topic here. While we can’t review all of this here, we can offer some observations about the adequacy of various methods, singly and in combination. First, it is necessary to disabuse ourselves of the idea that remote sensing will suffice to quantify the water cycle across the globe. Consider the challenge of estimating surface water yield as runoff (plus some deep percolation with later emergence of water at springs). On a global average, about 30% of precipitation over land runs off into surface waters. To predict runoff with a relative precision of 50% (far below the precision of ground-based stream gauges, where they exist) requires an absolute accuracy of 15% in estimating precipitation minus ET. (We also refer readers to discussions of the distinction between accuracy and precision.) The current precision of ET estimation from satellite remote sensing is often greatly overstated, by restricting consideration to full vegetative cover only, and to repeat measures on one site, and the like. A precision of 20% may be realistic, particularly in view of the calibration methods such as eddy covariance having yet lower precision. At this point, we already have exceeded the desired error bound. Now add the imprecision in estimating precipitation. Even ground-based raingauges suffer here, not nearly as much from their own operation as from inadequate spatial sampling. In convective storms that account for perhaps half of all precipitation but variably by location – the rest being stratiform precipition – the distribution of amount is highly varied across the landscape. An impractically large array of raingauges may be needed for high precision. Radar detection of precipitation, such as by the NEXRAD system, offers coverage of large areas on moderately fine scales (1 km), but calibration has proved extremely difficult. Most areas of the globe lack the radar systems, so that even rougher surrogates must be used, notably cloud-top temperatures.
ii. What is the solution? Most certainly, it is coupling of
remote-sensing and ground-based methods.
Remote sensing may offer relative measures of ET and precipitation that
are wholly inadequate to predict runoff by themselves, but the results may be
entered into hydrological models, often very empirical, that are calibrated to,
say, stream gauges. Will these joint
efforts be pursued adequately and in timely fashion? One of us attended a NASA workshop in 1991 at
Water balance and water stress merited a lengthy development above because of their importance for forest structure per se, plus fire dynamics, water yield, and more. Forests, in turn, have direct effects on water and carbon cycles that are important for determining climate.
A. Three major effects are: forest absorption of sunlight, or albedo; sequestration of carbon that reduces the atmosphere’s content of CO2 as a greenhouse gas; and evapotranspiration that helps load the atmosphere with water vapor as a potent greenhouse gas and as precipitable water. Early model studies implicated the absorption of sunlight by dark boreal forests as contributing significantly to air temperature on the scale of very large regions. Recent studies indicate that adding forest area in these latitudes would have a net warming effect. Admittedly, tree growth removes CO2 from the air and reduces the warming that CO2 causes by trapping thermal radiation. However, trees that replace highly reflective snowcover for long seasons directly warm that air as a larger effect. Trees are not always helpful against global warming. [As a side note, vegetation and specifically forests have done much over the last hundreds of millions of years to sequester carbon, massively reducing its content in air – one might say making their own lives harder by reducing CO2 as a photosynthetic substrate, as well as cooling the climate. The decay-resistance of wood allowed burial of much carbon as coal. Also, plant roots exuded CO2 into soil air spaces, acidifying soil water and accelerating rock weathering into soil that leads ultimately to carbon being sequestered as carbonate deposits in sediments. Time scales matter a great deal.]
B. Carbon sinks. In most locations, nonetheless, growth of stable, dense biomass is a major contribution to removing CO2 from the air. Regrowth of North American forests that were heavily harvested up to the 1950’s has created a major carbon sink, exceeding 10% of all the CO2 injection from fossil fuel use and deforestation globally. Whole nations such as Australia have embarked on tree-planting programs. There is, of course, a limit to how much growth can be sustained. Forests reach saturating biomass densities. Unless logs are buried – which has been suggested, net CO2 sequestration then ceases. A prominent global-change researcher has remarked in private that all the efforts at afforestation and reforestation might gain us the equivalent of only 2 years of human-cause CO2 addition to the air. Even on a global scale, then, forests are insufficient in themselves to mitigate climate change.
C. Carbon sources. Forests also burn with some regularity, participating in the 4% of all biomass that burns annually. Burning consequently represents a major flow in the carbon cycle. This is made clearer by the observation that photosynthesis annually fixes a mass of carbon about 10% as large as the standing biomass, so that burning represents 40% of the return flux of CO2 to the air. This is much greater than before the expansion of human populations. Regrettably, it is likely to continue or to increase.
D. Water recycling. While
CO2 circulates globally, water vapor has more regional circulation,
even if large-scale. Transpiration by
all vegetation is responsible for about 2/3
of total water injection to the air from the land; the other 1/3 is from
direct evaporation of water from wet soil and water bodies or sublimation of
ice. Forests dominate the biomass of
vegetation on earth and co-dominate total transpiring area, along with
woodlands and grasslands, so they clearly are critical in the land-to-air part
of the water cycle. In the other part of
the water cycle, total ET over land supplies about one-fifth of water for
precipitation globally (less than the 29% fraction of the earth’s surface as
land; land has less ET and less precipitation than ocean, on average). In some regions, the return of water from ET
to precipitation occurs in relatively short distances, on the order of 1000
km. The recycling
ratio over a given downwind distance is high in areas of rainforest,
E. Surface water.
Also critically for the water cycle, forests condition runoff
to rivers by their action to intercept snow and rain. They also retard overland water flows,
allowing greater infiltration of precipitation into soils and reducing soil
erosion that affects water quality and ecosystem stability itself. These ecosystem services make it an object of
great concern that forest losses are huge, particularly in poor tropical and
subtopical nations, such as
A. Very brief overview of fire ecologically and in human affairs
Burning is not only a major element of the carbon
cycle. It is also an agent for changing
habitats for many species, eliminating some habitats, creating others, and
We turn our attention now to forest fire among the topics above. The topic is very large, so we focus on several issues. We may ask how forest structure and function are related to fire regime, fire hazard to humans and their activities, and insect attack that kills trees and may increase fire probability. Conversely, how does fire contribute to structuring forests? We may ask several questions with yet finer focus. What is the role of water stress in the probability of ignition and in the pattern of fire propagation? What effect does stand density and related management have on water stress and on fire dynamics directly? What are positive and negative effects of changed fire regimes on the ecosystem services offered by forests? How will climate change alter forest structure, stress regimes, and fire regimes?
A number of monographs and reports cover the basic physical
and biological determinants of fire. A recent report addresses
the issue of overdense dry western forests in
Natural ignition of forest fires is commonly by lightning
strikes. Humans cause additional fires
by accident or as arson. In western
Both kinds of fires have recurred naturally for thousands to
millions of years, variously by location.
They generate diverse
habitats in the recovery or successional stages. They generate concurrently cycles of other ecosystem
services such as surface water yield (more runoff on burned areas, e.g.). Tree species native to a region show diverse
adaptations to withstanding fire or regenerating after fire. Fire-resistant
bark protects some species as mature trees.
Some species have fruits and cones that release
seeds only after fire to start regeneration; others regenerate from vegetative parts
underground. Yet others, notably the
Fire conditions in any given forest vary both naturally and artificially, and on a range of time scales. Episodes of precipitation vary the fuel moisture content, which is a good predictor of ignition probability. Surrogates for fuel moisture are commonly used in monitoring systems, either using ground weather data or satellite remote sensing. In the longer term, stand density varies. High densities can arise from wet years following fire and can persist for many decades. Fire suppression by human management can lead to a buildup of surface fuels and ladder fuels. The consequent change to a regime of crown fires and large fires is a concern over approximately 40 to 60% of the dry forests of the western US.
It is important to estimate both fuel amount and moisture content on large areas. Ground-based surveys are sufficiently labor-intensive as to require extensive interpolation of transect measurements. Satellite remote sensing has poor spatial resolution and a modest ability to detect water stress in canopy tissue, which has only variable correlation with the moisture of surface fuel. Hyperspectral remote sensing from aircraft or pointable satellites can reveal more detail but is expensive and has a low repeat frequency. Fuel density can be estimated with better precision using LIDAR that reveals surface roughness. It has the same problems of expense and repeat frequency for large areas. Fuel moisture is often estimated from ground data on weather with water-balance models (long download). In summary, fire condition remains challenging to estimate.
Ground-based observations and aircraft overflights have long been used to monitor fires in progress. More recently, satellite detection of hot pixels has been used in a highly automated system that maps fires in near-real time. All three techniques are employed in monitoring post-fire conditions of vegetation and soil. LIDAR has been used as well to monitor soil conditions, particularly erosion in post-fire rains.
This is a very large topic, both conceptually and
economically. Over and above the need
for monitoring as noted, forest managers and emergency agencies prepare
firebreaks, firefighting and evacuation routes, and the like. Modification of forest conditions is the largest
potential expense. In dry western
forests of the
Higher temperatures are correlated, if imperfectly, with
greater evaporation from wet surfaces (of dead fuels) and lower fuel
moisture. The temperatures act directly
on instantaneous rates of evaporation and indirectly by extending
the season of active evapotranspiration.
Perhaps more important is alteration of the precipitation regime. The hydrologic cycle is become more active globally, as expected, but precipitation has
trends upward in some regions and downward in others. Intensity of precipitation has similarly bidirectional
trends (1, 2). Vegetation growth itself is
enhanced by elevated CO2, acting to increase fuel loads. Vegetation water status
is affected variously by species and location, ranging from minimally changed
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