The Role of Biodiversity on the Evaporation of Forests

Abstract

On visiting an arboretum or walking through a mixed-species forest, an impression one receives is that there is much diversity in the form and function of trees. The leaves of angiosperm trees can be thin or fleshy, shiny, dull, or hairy.They can be large or small. Their shape can be simple or compound, linear or lobed, cordate, deltate, ovate, or elliptical, among many examples of variation.On a stem, the leaves can be grouped in clumps, arranged in whorls or extend from individual isolated petioles. As for the regulation and trans- port of water,angiosperms may possess stomata on one or both sides of a leaf and have either ring porous (e.g., Quercus or Ulmus spp.) or diffuse porous (e.g., Betula or Acer spp.) xylem. Gymnosperms, by contrast, have either needles (e.g., Pinus spp.) or scales (Junipers spp.). Their phytoelements can be arranged in shoots, as with spruce (Picea spp.), be comprised of groups of needles on fascicles, as with pine (Pinus spp.). From these simple observa- tions, one may surmise that biodiversity could affect rates of transpiration of trees and their annual water budget.But does it? The answer to this simple question is complicated due to interactions and competition among species for light energy,water and nutrients (e.g.,Allen et al. 2002), evolution (Beerling et al. 2001), and to the space and time scale at which it is asked (Waide et al. 1999).Genetic diversity, combined with biogeo- chemical forcings,produce plant species that differ in physiological and mor- phological features (Mooney 2001).A specific set of plant features contributes to a ranking of transpiration among tree species because such elements affect the energy balance of leaves and plants. For example, biodiversity, achieved through competition and evolution, is responsible for morphological differ- ences in leaf size, thickness, shape, and reflective properties. These features are important because they affect the aerodynamic resistance and radiative balance of leaves. Species-dependent differences in physiological factors affect transpiration by altering the demand for and supply of water.For example, differences in surface resistance arise through species differences in pho- tosynthetic capacity, lifespan (Reich et al. 1997),and maximum stomatal con- ductance (Schulze et al. 1994; Pataki et al. 2000). Species-dependent differ- ences in the supply of water are mediated by differences rooting depth (Lewis and Burgy 1964; Ehleringer and Dawson 1992; Jackson et al. 1996), timing of physiological activity (Xu and Baldocchi 2003), and drought tolerance or avoidance (Stephenson 1998). The second question to ask is: do all species in a mixed-species landscape make an independent and proportional contribution to canopy evaporation? The answer to this question can be debated, as there is evidence for and against.On the pro side, an analysis by Currie and Paquin (1987) shows that species richness of trees in North America increases with annual evaporation. A second line of evidence comes from grassland field studies, indicating that aboveground biomass and net primary productivity increase with species richness (Tilman et al. 1997;Hector et al. 1999;Waide et al. 1999;Roy 2001).By inference one could conclude that increasing biodiversity increases evapora- tion rates and amounts because evaporation of forests scales with net primary productivity (Law et al. 2002). A contrary view can be derived on the basis of biometeorology and ecolog- ical theory. As groups of trees come together and form a closed canopy, the coupling of individual plants with the atmosphere changes (Jarvis and McNaughton 1986). Theoretical and experimental studies show that rates of evaporation, normalized by available energy and temperature, increase with leaf area up to a threshold (a leaf area index of about 3; Jarvis and McNaughton 1986; Saugier and Katerji 1991; Kelliher et al. 1995; Baldocchi and Meyers 1998; Eamus et al. 2001). On ecological grounds, the number and combination of functional factors are limited due to convergent evolution. So the total number of species across a landscape may not be as important as the different number of functional types (Hooper and Vitousek 1997; Tilman et al. 1997; Mooney 2001; Roy 2001). An intermediate view can also be drawn, based on a recent analysis of net primary productivity by Waring et al. (2002). They found that greatest species diversity, along a transect of forests in Ore- gon, occurs at sites of intermediate productivity. As a micrometeorologist, I am presupposed to favor the view that the impact of biodiversity on evaporation is realized by how it alters the struc- tural and functional properties of a plant stand, such as its aerodynamic roughness, the amount of transpiring plant material, its physiological capac- ity to transpire,and its ability to intercept solar radiation.However, I leave the answer to this question to be drawn at the end of this essay.To arrive at a final conclusion, I explore the question of how biodiversity may or may not affect water use of plants by examining theory and experimental data across the scales of leaf, tree, and canopy.