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2 Drought, the Soil-Plant-Atmosphere Continuum, and Canopy Processes

2 Drought, the Soil-Plant-Atmosphere Continuum, and Canopy Processes

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Forest Biogeochemistry and Drought




Q. rubra







J. virginiana






gs or A (relative units)

PLC (%)



Q. rubra





J. virginiana



Ystem (MPa)




PLC (%)

Θ (m3 m-3)










Yleaf (MPa)



10-3 10-2 10-1 100 101

Ysoil (MPa)







Q. rubra

J. virginiana







Yroot (MPa)

Fig. 29.1 Drought impacts on water potentials (C) of (a) soil, (b) root, (c) stem, and (d) foliage;

percent loss of hydraulic conductivity (PLC) of root (b) and stem (c) xylem; and stomatal (gs) and

photosynthetic (A) responses of foliage (d). Drought progression is indicated by movement along

curves from green to brown symbols. Two curves in each plot illustrate extremes of soil hydraulic

properties and tree cavitation vulnerability (Y) represents soil volumetric water content

elements of the SPAC. As water moves through distributed frictional resistances,

discrete vascular constrictions, and against gravity along its transpiration pathway,

the initial hydraulic cost set by soil water availability progressively increases:

water potential becomes more negative, and vascular and physiological function,

and canopy gas exchange, are generally reduced. Closely associated with soil water

potential is hydraulic conductivity; both are highly sensitive to soil water content.

Unsaturated conductivity drops rapidly with lower water content in many soils

(van Genuchten 1980), increasing resistance to nutrient flow to roots. Drought thus

decreases nutrient mobility at the same time that it decreases soil nutrient lability

(as discussed below). Decreased uptake of nutrients in turn reduces canopy photosynthesis (Evans 1989), feeding back to reduced SOC supply.

Moving from soil toward the plant, declining bulk soil water content can lead to

threshold-type hydraulic discontinuities at the soil–root interface. During drought,

soil and roots shrink and can physically separate, increasing hydraulic resistance at


S.A. Billings and N. Phillips

the soil–root interface (Weatherley 1979; Blizzard and Boyer 1980); this process

depends on soil texture and porosity (Bristow et al. 1984). Even water that does retain

continuity across soil–root interfaces is variably conducted into plants under influence of drought by root aquaporins. Aquaporins, integral membrane water channels,

are produced, activated, and/or relocated under water stress, facilitating water uptake

(Luu and Maurel 2005). Chemicals involved in gating aquaporins include calcium,

protons (Luu and Maurel 2005), and nitrogen (Gloser et al. 2007; Gorska et al. 2008),

but little is known about relationships between aquaporin function, root membrane

hydraulic function in general, and soil availability of biogeochemicals.

Once water passes from soil through root membranes, its pathway through root,

bole, and branch xylem is vulnerable to hydraulic failure. Drought imposes increasingly negative water potentials in xylem of roots to leaves, placing vascular tissues

closer to a failure threshold associated with tensile rupturing of liquid water, a

phenomena called cavitation. In turn, fewer pathways for water flow in xylem may

make remaining xylem even more vulnerable to cavitation (Tyree and Sperry

1988). This illustrates state-rate interdependency. The relationship between degree

of cavitation and water tension is called a “cavitation vulnerability curve” (Tyree

and Sperry 1989), and is influenced by anatomical and physicochemical properties

of xylem conduits. The wide variety of vascular anatomies found among forest trees

therefore leads to a diversity of cavitation vulnerabilities. Tree species may “tune”

xylem properties and cavitation vulnerability to soil porosity (Hacke et al. 2000).

Evidence has accumulated that refilling of embolized (cavitated) vascular material

occurs regularly (Zwieniecki et al. 2000; Bucci et al. 2004). This phenomenon

occurs when water columns are under tension. A full understanding of how refilling

under tension can occur has not yet been developed (Holbrook and Zwieniecki

1999). Little is known about how drought may impact refilling under tension.

Moving from the woody vascular system to the leaf, water transforms from the

liquid to gas phase, and here both physics and biochemistry limit exchange of both

vapor and CO2. These two distinct kinds of leaf gas exchange limitations are known as

stomatal, or diffusion limitations; and nonstomatal, or biochemical limitations. Because

water loss from stomata is a necessary consequence of C uptake in C3 plants, leaves

require a constant supply of water from vascular systems when photosynthesizing. Leaf

hydraulic resistance is a function of the leaf vascular network (Sack and Holbrook

2006), and can be a dominating control on water supply to the stomatal complex. In

addition to water supply rate, leaf water status has biochemical impacts on photosynthesis (Tezara et al. 1999). Operation of the Calvin Cycle or associated processes can be

diminished by low (negative) leaf water potential. Finally, leaf expansion is limited by

cell turgor, and so the ability for tall trees (or trees in dry soils) to produce photosynthesizing leaf area at their tops can be limited by cell water status (Woodruff et al. 2004).

Importantly, the stable isotopic signature of forest biomass and of CO2 within

forest canopies can help constrain estimates of canopy responses to moisture

availability (Bowling et al. 2008). Because drought generally induces more rapid

declines in stomatal conductance than photosynthesis, canopy discrimination (D)

tends to decline with drought (Bowling et al. 2002; Fessenden and Ehleringer

2003). This phenomenon has a negative effect on d13C of CO2 (Randerson 2005),


Forest Biogeochemistry and Drought


providing investigators with an important tool for evaluating the extent of forest

canopy responses to moisture shortages.

The tight coupling of water and C flows within trees has long been studied, but

theories have emerged over the past two decades to explain how such coupling may

influence the physical stature of forests (Yoder et al. 1994; Ryan and Yoder 1997;

Ryan et al. 2006), their rainfall use efficiency (Huxman et al. 2004), and susceptibility to mortality (McDowell et al. 2008; Adams et al. 2009). These theories, focused

on tree processes and ecosystem productivity, have obvious implications for above‐

and belowground biogeochemistry, but await incorporation into comprehensive

forest biogeochemical models.

General patterns have emerged in synthesis studies of canopy processes during

drought, and these studies continue to inform theory. Synthesis studies derive from

forest sites in the US Long-Term Ecological Research (LTER) Network and FluxNet.

Two emerging generalities in forest C response to drought are that canopy photosynthesis is more sensitive to drought than ecosystem respiration (Schwalm et al. 2010);

and that, in their driest years, aboveground net primary productivity (ANPP) of

forests falls along a line of rain use efficiency (RUE) behavior common to other

biomes including deserts (Huxman et al. 2004). These findings represent crucial

footholds toward developing general understandings of forest canopy response to

drought. However, they also demonstrate that, despite centuries of study, there

remain basic unknowns. For example, the remarkable RUE convergence in Huxman

et al. (2004), evaluated using ANPP, remains untested using NEE.

Cross-site syntheses permit examination of the limits of generalities. For example,

in a FluxNet synthesis that included 128 forest sites, ecosystem C uptake was

generally, but not always, reduced by drought (Schwalm et al. 2010). Drought

accompanies other environmental changes that impact ecosystem C exchange. For

example, tropical drought is associated with greater light availability (e.g., Graham

et al. 2003; Hutyra et al. 2007). When forests can access deep stores of soil moisture,

higher light supports greater C uptake (Hutyra et al. 2007; Saleska et al. 2007; Bonal

et al. 2008). Access to deep soil moisture indicates that generalities about forest

drought responses may depend on definitions of drought. An additional interactive

influence of drought arises from its common association with higher temperature.

Where productivity is temperature limited, the temperature increases associated with

drought can enhance C uptake through increased crown photosynthesis or increased

growing season (Schwalm et al. 2010). These different drought-associated factors

produce regional anomalies (e.g., tropical vs. boreal) in forest C exchange, and

demonstrate the need for caution against overgeneralization.


Litterfall and Its Decomposition

Because mean annual precipitation is positively correlated with forest productivity

(Lieth 1975; Schlesinger 1997), litterfall production is typically higher in forests

where water is less limiting. However, litterfall production in forests is not necessarily linked with annual or growing season precipitation (Knutson 1997), and can


S.A. Billings and N. Phillips

even exhibit negative correlations with precipitation, reflecting high rates of

senescence during severe drought (Beard 2005). The importance of litterfall as a

source of nutrient recycling in forests (Singh and Gupta 1977; Aber et al. 1991) is

realized, of course, when organic inputs to the soil profile are subject to decomposition – a process strongly linked with water availability. Thus, drought has a large

influence on the heterotrophically mediated fate of senesced biomass.

Mean annual precipitation can be an important driver of decomposition rates,

given observed links between annual mass loss and actual evapotranspiration

(Meentemeyer 1978; Schlesinger 1997). Such observations indicate that drought

can impose restrictions on the return of nutrients to available pools from litterfall

(Fig. 29.2). This effect may be particularly evident in tropical forests, where precipitation appears to be a more influential driver of decomposition rates than in

Fig. 29.2 Schematic of hypothetical influences of drought on relative magnitudes of key biogeochemical processes in forest soils, relative to well-watered conditions. Many fluxes depicted have not

been measured in situ, such as mass fluxes of exo-enzymes to substrates or substrate flow into

microbial communities, or across ecosystems with and without drought. Depicted are gaseous fluxes

(CO2, CH4, N2O) via arrows of varying width, representing hypothetical relative magnitudes that

contrast well-watered (green) with drought (brown) conditions. Drought conditions generally reduce

soil respiration and methanogenesis, and enhance methanotrophy. Production of N2O declines with

drought because anaerobic conditions promote denitrification and associated fluxes of N2O as a

byproduct. Drought can also induce a net N2O sink in some forests (not depicted here), though the

mechanism for this phenomenon remains unclear (see text for details). Relatively short-term drought

can enhance litterfall accumulation, represented here as O horizon thickness, though longer-term

drought can result in reductions in net primary productivity (NPP) and associated declines in litterfall

production. Dotted arrows (brown) depict enhanced temporal and spatial variability of the activity of

some microorganisms belowground with drought, relative to solid arrows (green) in well-watered

conditions, which represent generally larger and more consistent fluxes. Diffusivity of organic

substrates to microorganisms is reduced with drought, as is microbial production of many extracellular

enzymes. Extracellular enzyme activity (EEA) thus can decline with decreases in microbial functional

rates and enzyme diffusivity to substrate reaction sites, with associated declines in organic matter

decay and associated NH4+ production, nitrification, and NO3À leaching. In addition, drought conditions can prompt microbial uptake of solutes to mitigate dessication (not pictured here)


Forest Biogeochemistry and Drought


temperate and boreal systems (Berg et al. 1993; Gholz et al. 2000; Trofymow et al.

2002; Powers et al. 2009a, b). The decline in decomposition rates with increasingly

limited moisture availability is likely a result of water limitation of decomposer

communities (discussed below), but could also result from declines in nutrient

concentrations, especially phosphorus, in litterfall with drought (Wood et al. 2005).

Although the influence of drought on decomposition rates and, in some forests,

litterfall nutrient concentrations is at least partially characterized, the extent to which

these mechanisms negatively influence forest productivity remains unclear. To date,

we are unaware of any study that attempts to separate the direct effects of moisture

shortages from the indirect effects of drought-imposed nutrient limitation on forest

productivity. However, nutrients derived from organic matter decomposition are a

significant fraction of those taken up by forest vegetation (Aber et al. 1991), and

litterfall accumulation associated with declines in decomposition (Johnson et al.

2002) is linked with nutrient immobilization in litter and reduced cation leaching

(Johnson et al. 1998). These phenomena indicate that drought may indirectly influence a forest’s ability to assimilate C as the rate of nutrient return to plant-available

pools declines. These effects, if present, likely are realized only during drought

periods sufficient in duration to impose severe and long-lasting restrictions on litterfall decomposition. Shorter-term drought events, in contrast, can result in transient

increases in plant-available pools of nutrients due to inhibited vegetation uptake in

some ecosystems (Sardans et al. 2008). The complicated interactions between

decomposition dynamics, patterns of plant nutrient uptake, and length and severity

of drought inhibit investigators from elucidating the extent to which reduced decomposition rates impose limitations on forest C assimilation. Studies that try to characterize these effects, and the severity of drought required to induce them, remain rare.


Decomposition Dynamics Within Forest

Mineral Soil Profiles

Drought can also be an important determinant of biogeochemical transformations

within the mineral soil profile. Perhaps the most critical characteristic of soil

profiles experiencing drought with respect to biogeochemical functioning is the

decline of diffusion rates of organic and inorganic compounds in soil solution to

reaction sites (Fig. 29.2). As a result, substrate availability for many microbially

mediated transformations can limit reaction rates. This concept is typically conveyed using MichaelisMenten kinetics:


vmax S


Km ỵ S


where v is the reaction rate constant, vmax is the maximum reaction rate, S is substrate

concentration, and Km is the half-saturation constant. As S declines with moisture

availability, v is increasingly influenced by Km, and can be significantly reduced


S.A. Billings and N. Phillips

relative to vmax. Michaelis–Menten kinetics thus are an important component for

formulating predictions of biogeochemical transformations within soils as they

experience moisture fluctuation (Davidson and Janssens 2006; Dalal et al. 2008).

Microbially mediated processes within soil profiles also slow due to microbial

physiology and reductions in enzyme diffusivity (Fig. 29.2, Stark and Firestone

1995; Ford et al. 2007; Borken and Matzner 2009). Soil microorganisms either

dehydrate and die during periods of low moisture availability, or survive in a state

of reduced activity (Killham 1994). Gram-positive bacteria are generally viewed as

more drought-resistant than Gram-negative bacteria, due to their thicker cell wall

(Schimel et al. 2007), and fungi are thought to be more drought resistant than most

bacterial cells (Harris 1981). Surviving microorganisms appear to import solutes for

protection from dessication (Harris 1981, discussed in more detail below). Even

those organisms well adapted for drought conditions, however, typically experience

reduced functioning during times of moisture limitation. The extracellular enzymes

produced by still-functioning microorganisms also can experience limited diffusivity. Thus, the transport limitation of substrates, combined with slowed diffusivity of

enzymes responsible for their breakdown, result in slower rates of microbially

mediated biogeochemical transformations within soils experiencing drought.

The declines in soil microbial activity associated with drought have important

biogeochemical consequences. As decomposition slows, microbial respiration

declines, consistent with observed declines in soil respiration in multiple forest

types with drought (Fig. 29.2, Billings et al. 1998; Sotta et al. 2007), though these

measures are confounded with likely declines in autotrophic respiration as well.

Laboratory experiments that isolate microbial activity from that of plant roots,

however, indicate that microbial respiration is typically negatively affected

by water shortages, and that the magnitude of decline depends on nutrient availability, microbial community composition, the time period between wetting events,

and the magnitude of the antecedent wetting event (Fierer and Schimel 2003;

Schimel et al. 2007; Borken and Matzner 2009). These interacting effects make it

difficult to quantifiably predict microbial respiration with drought using empirical

data, and reinforce the importance of employing first principles, such as the

Michaelis–Menten model discussed above, to model reaction rates. Importantly,

because the stable isotopic composition of C substrates varies across soil horizons

(Ehleringer et al. 2000; Billings 2006) and can reflect antecedent forest water

availability (Balesdent et al. 1993; Bowling et al. 2008), the d13C of microbially

respired CO2 can be an important tool for assessing the influence of drought on

substrate accessibility and associated soil microbial activity.


Methane Fluxes and Forest Drought

Of all terrestrial ecosystems, upland forests generally exhibit the highest rates of net

soil CH4 uptake; within forests, net soil CH4 consumption is greatest in temperate

systems (Dalal et al. 2008). These fluxes of CH4 are influenced by water availability


Forest Biogeochemistry and Drought


to a much greater extent than temperature (Castro et al. 1995; Billings et al. 2000).

Methanogens transform simple C substrates into CH4 in anaerobic conditions (Kim

and Gadd 2008). Thus, when soil moisture is high and a relatively large number of

O2-depleted soil microsites exist, methanogenesis can represent a significant fraction of CH4 fluxes within a profile (McLain and Ahmann 2008). In contrast, wellaerated soil profiles support relatively more methanotrophy, the oxidation of CH4

into CO2. In such instances, net soil CH4 consumption is observed at the soil surface

(Fig. 29.2). Soil texture and bulk density can be important drivers of CH4 diffusion

into soil (Dorr et al. 1993; Boeckx et al. 1997; Del Grosso et al. 2000), but the

influence of moisture availability on air-filled pore space is an additional determinant of CH4 diffusion into the soil profile and its subsequent oxidation (Dalal et al.

2008). Thus, as moisture declines CH4 can diffuse more readily into the profile,

following the CH4 concentration gradient maintained by soil methanotrophs (Castro et al. 1995; Whalen and Reeburgh 1996; Billings et al. 2000; Gulledge and

Schimel 2000; Le Mer and Roger 2001; Borken et al. 2006).

Studies examining the influence of moisture on forest CH4 fluxes confirm the

potential importance of drought as a determinant of CH4 biogeochemistry. Davidson

et al. (2008) report that rainfall exclusion plots in a tropical forest consistently served

as a net CH4 sink, in contrast with control plots. In a spruce forest in Germany,

drought reduced methanogenesis and enhanced methane oxidation (Lamers et al.

2009). Experimentally induced drought in a temperate forest increased soil CH4

uptake rates, though the effect was small relative to the large change in soil moisture

(Borken et al. 2006). Net soil CH4 uptake was also significantly enhanced with

experimental drought in an upland boreal forest in Alaska (Billings et al. 2000).

Importantly, however, soil CH4 uptake declined with drought in a floodplain forest

composed of similar vegetation (Billings et al. 2000). These results suggest that

factors other than CH4 diffusivity, such as variation in the composition of CH4related microbial communities, may be important influences on soil CH4 fluxes

(Schimel and Gulledge 1998).

Although it remains challenging to quantify predictions of soil surface CH4 fluxes

across multiple moisture scenarios and forest types, recent modeling work suggests

that rates of forest soil CH4 consumption will likely increase with increased drought

occurrence and intensity. Curry (2009) predicts that cool temperate and subtropical

forest ecosystems will experience the largest absolute increases in soil CH4 consumption relative to other terrestrial ecosystems by the year 2100, and that boreal forests

will be one of the three ecosystems experiencing the largest relative increases in CH4

consumption. This work, however, neglects methanogenesis, which needs to be

incorporated into any future predictions of net forest soil CH4 fluxes.


Nitrogen Cycling and Forest Drought

Although the influences of drought on soil N cycling are not well understood, it is

generally accepted that soil moisture shortages impose constraints on N availability


S.A. Billings and N. Phillips

via several mechanisms. As discussed above, drought can restrict decomposition

rates of litterfall and mineral SOM, reducing the flow of N into the “leaky pipe”

(Firestone and Davidson 1989) of soil N cycling with enhanced N immobilization

in the O horizon (Johnson et al. 2002, 2008). In addition, drought can impose

constraints on N availability due to microbial N immobilization (Fig. 29.2). To

survive drought conditions, microorganisms must accumulate osmolytes to reduce

their internal water potential (Harris 1981). Although not universally true (Tiemann

2011), osmolytes can be N-rich; for example, bacteria typically employ amino acids

for this purpose. Schimel et al. (2007) calculates that the total amount of N in

microbial osmolytes can represent between 10 and 40% of annual net N mineralization. Drought, then, can impose N requirements on soil microorganisms that have

ecosystem-level implications for N availability.

Drought can also result in reductions in transformation rates of inorganic N

(Fig. 29.2). For example, net N mineralization was reduced with drying in soils

supporting a Norway spruce forest in Germany (Hentschel et al. 2007). Experimental reductions in throughfall in a temperate deciduous forest also resulted in lower N

fluxes through the soil profile (Johnson et al. 2002, 2008). Such effects may result

from reductions in microbial population size and activity, and from the declining

availability of substrates for enzymatic reaction sites discussed above (Kieft et al.

1987; Franzluebbers et al. 1994; Borken and Matzner 2009). The transformation of

ammonium to nitrate by nitrifying bacteria also slows with drought (Tietema et al.

1992). The mechanism governing this effect is unknown, but one study examining

the negative effect of drought on nitrifiers indicates that osmotic potentials in the

soil below À0.6 MPa induces severe nitrifier dehydration (Stark and Firestone

1995). In contrast, nitrification was inhibited primarily by substrate availability

when osmotic potentials were above this threshold (Stark and Firestone 1995).

Although drought can promote relative accumulations of inorganic N in soil

through reductions in plant uptake of N (Sardans et al. 2008) and nitrate leaching

from the soil system (Johnson et al. 2002, 2008; Hong et al. 2005), the negative

effects of soil drought on N cycling rates can restrict forest N dynamics.

The evolution of nitrogenous gases from forest soils is also influenced by

drought. Because of the acidic nature of forest soils, we do not consider NH3

volatilization, which tends to occur in soils with pH > 7 (Schlesinger and Peterjohn

1991; Billings et al. 2002). However, forest soils can be significant sources of nitric

oxide (NO) and nitrous oxide (N2O), byproducts of nitrification and denitrification

(Fig. 29.2). Nitrogen budgets of multiple ecosystems suggest that significant quantities of N escape via these pathways, but capturing fluxes of these gases sufficient

to close N budgets has proven challenging (Schlesinger and Peterjohn 1991;

Billings et al. 2002; Groffman et al. 2006), in part because these gases are both

produced and consumed by microbial activity within the soil.

Recent work highlights our inability to predict the response of NO and N2O

fluxes from forest soils to drought. For example, soil moisture shortages tend to

promote NO emissions relative to N2O (Firestone and Davidson 1989; Davidson

et al. 2000), but some tropical forests require repeated drought years for this effect

to be evident, for unknown reasons (Davidson et al. 2008). Moisture availability can


Forest Biogeochemistry and Drought


promote N2O production via enhancement of denitrification and, to a lesser extent,

nitrification, but when soil moisture is sufficiently high, this effect is countered by

high rates of N2O consumption (transformation of N2O into N2) (Chapuis-Lardy

et al. 2007). Paradoxically, Goldberg and Gebauer (2009) report a significant,

sustained N2O sink in a forest soil with experimentally imposed drought, conditions

under which we would predict negligible transformation of N2O into N2.

The mechanism for this unexpected response is not known. Patterns of d15N of

N2O throughout the soil profile prompted suggestions of drought resulting in

declining in N2O production, while N2O consumption was maintained (Goldberg

and Gebauer 2009). An alternative explanation for the observed phenomena of

drought-influenced soils serving as a net N2O sink is enhanced rates of N2O

consumption with the maintenance of N2O production (Billings 2008); this scenario

is consistent with the 15N2O profile observed in the German study.

Regardless of the mechanism(s) governing the net N2O sink in these dry forest

soils, Goldberg and Gebauer’s work (2009) highlights our lack of understanding of

how forest soils can consume N2O when anaerobic microsites must be relatively

rare. Because so few studies report net N2O consumption at the soil surface, and

because those that do tend to dismiss such results merely as evidence of the

challenges associated with measuring N2O fluxes (Chapuis-Lardy et al. 2007), the

relationship between moisture availability and forest soil N2O fluxes remains

enigmatic. Although atmospheric N2O is relatively long lived and well mixed,

confounding our ability to use temporal and spatial differences in its isotopic

signature to constrain N2O budgets (Rahn 2005), employing isotopic constraints

on N2O fluxes within the soil profile will be important for future efforts to elucidate

drivers of N2O production and consumption.



The nature of drought itself – its frequency, severity, and duration – may change as

part of climate change (IPCC 2007). Drought can span an almost unlimited spectrum of conditions from the episodic to the chronic, as becomes evident within a

few minutes of attempting to design an experiment to explore the influence of

drought on forest processes. However, manipulative studies are needed to explore

forest responses to previously unobserved regions of the time and frequency

domains of drought that may occur in the future. In addition to such studies,

cross-ecosystem explorations of forest responses to drought are needed, to permit

comparison between forest types and climatic regimes. Such comparisons will

augment our understanding of both the diversity and similarities of relationships

between forest biogeochemical fluxes and drought. The LTER and FluxNet networks will continue to provide excellent synthesis platforms for such studies. The

emerging National Ecological Observatory Network (NEON; Keller et al. 2008),

with its unprecedented scope of data monitoring, management, and sharing


S.A. Billings and N. Phillips

capabilities will also serve as a rich source of data for understanding forest

responses to drought.

In addition to insights gained from manipulative studies and ecological networks,

examination of extreme drought events holds great promise for contributing to

general knowledge of forest responses to drought. An emerging body of literature

has been built around such extreme events (e.g., Huxman et al. 2004; Ciais et al.

2005; Reichstein et al. 2007; Granier et al. 2007; Saleska et al. 2007; Armone et al.

2008; Marengo et al. 2008; Goldberg and Gebauer 2009). The exceptional events

examined in these studies provide crucial tests of emergent general patterns of forest

response to drought, and continued examination of extreme events will serve an

important role in the developing predictive models of forest responses to drought

under climate change scenarios.

The studies described in this chapter also reveal the broad need for a better

understanding of how moisture dynamics influence soil microbial activity. Pursuing

this wide research area will provide the knowledge needed for developing conceptual and, eventually, more quantitative models of many belowground processes,

including decomposition rates, nutrient transformations, and soil production and

consumption of multiple gases that influence Earth’s climate. The nonlinear and

interactive responses to drought of these microbially mediated fluxes (e.g., Schimel

et al. 2007; Borken and Matzner 2009; Goldberg and Gebauer 2009) represent an

immense and relatively unexplored research frontier. For example, biogeochemical

responses to drought severity – as measured by intensity and length – are frequently

not linear, likely due to interactions between water availability, microbial functioning,

and vegetation physiology, and the multiple timescales at which these factors operate.

Complex patterns of responses of these processes within and among varying forest

types challenge our efforts to develop predictive models of these fluxes with drought

and remain a critical research area for the future.


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2 Drought, the Soil-Plant-Atmosphere Continuum, and Canopy Processes

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