Towards a mechanistic understanding of spatial patterns of forest transpiration, and its implications for scaling
Loranty, Michael M.
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Estimates of forest stand transpiration are typically achieved by scaling a series of representative point measurements to the stand level. This method fails to account for heterogeneity in transpiration at stand boundaries as a result of gradients in key environmental drivers such as soil moisture. To address this problem I made a series of spatially intensive transpiration estimates using sap flux measurements at two sites in northern Wisconsin. An aspen site captured a transition between an early successional forested upland dominated by quaking aspen ( Populus tremuloides Michx), and a forested wetland dominated by alder ( Alnus incana (DuRoi) and cedar ( Thuja occidentalis L.). A maple site captured a transition between a secondary successional sugar maple ( Acer saccharum Marsh.) stand and a red pine plantation ( Pinus resinosa Ait.). At the aspen site sapwood area ( A S ) was used to scale J S to whole tree transpiration ( E C ). Because spatial patterns imply underlying processes, geostatistical analyses were employed to quantify patterns of spatial autocorrelation across the site. A simple Jarvis type model parameterized using a Monte Carlo sampling approach was used to simulate E C ( E C-SIM ). E C-SIM was compared with observed E C ( E C-OBS ) and found to reproduce both the temporal trends and spatial variance of canopy transpiration. E C-SIM was then used to examine spatial autocorrelation as a function of environmental drivers. I found no spatial autocorrelation in J S across the gradient from forested wetland to forested upland. E C was spatially autocorrelated and this was attributed to spatial variation in A S which suggests species spatial patterns are important for understanding spatial estimates of transpiration. However, the range of autocorrelation in E C-SIM decreased linearly with increasing vapor pressure deficit, implying that consideration of spatial variation in the sensitivity of canopy stomatal conductance to D is also key to accurately scaling up transpiration in space. To understand the decreasing range of spatial autocorrelation in E C-SIM with D I examined differences in stomatal conductance ( G S ) between individual trees using a transpiration model. Values of reference stomatal conductance ( G Sref ) for individual trees from the aspen and maple site were compared to a series of environmental variables. For both sites no relationship was observed between G Sref and variables related to hydraulic regulation of G S including soil moisture, sapwood area ( A S ), and tree height ( H T ). There was a non-linear inverse relationship between G Sref and a canopy competition index ( C I ) designed to characterize the competitive light environment of an individual tree. This suggests that G S light limited and that differences in stomatal control of transpiration between plants may be related to photosynthetic capacity. Sap flux data was used to parameterize a series of transpiration models. Models that account for canopy heterogeneity using a clumping parameter (Omega) and 3-dimensional canopy representation (3-D) were compared to a simple Big-Leaf model (big-leaf) that does not account for heterogeneity among individuals. Values of G Sref for individual trees remained consistent between models for aspen and maple. For aspen values of transpiration per unit ground area ( E CG ) simulated using the Omega and 3-D models showed better agreement with observations that those simulated with the big-leaf model. At the maple site differences in model agreement were less pronounced, the Omega model slightly improved agreement, while the 3-D model resulted in a slight decrease in agreement between simulated and observed E CG . Clumping parameters for the Omega model did not exhibit a relationship with any physically meaningful variables and so do not appear to be useful for purposes of characterizing heterogeneity between individuals. In shade intolerant aspen trees with more neighboring competitors experienced a greater increase in model agreement as a result of accounting for canopy heterogeneity. For shade tolerant maple the 3-D model likely over compensated for competitive shading. The results suggest that substantial variability in stand transpiration exists. However it is explained largely in part by tree size, and so current point-to-area scaling methods will work as long as sample plots are appropriately selected. Additionally the results offer evidence of a link between plant hydraulics and photosynthesis, and a tie between ecosystem function and structure that could allow remote sensing to be used to characterize ecosystem processes.