Spatial and temporal variability of tree transpiration and its drivers along a soil drainage gradient in the boreal black spruce forest .

摘要

Natural ecosystems vary both in time and space creating large- and small-scale heterogeneity in processes and their driving mechanisms. Assumptions of homogeneity at one spatial or temporal scale my lead to inaccurate quantification of biological processes and their drivers. This may result in over- or underestimation of chemical fluxes when scaling from plot- to regional-level measurement. An even bigger challenge is determining how chemical fluxes will respond to global changes such as warmer temperatures, drier growing seasons, increased wildfire frequency, and longer growing seasons. In order to accurately predict ecosystem response to global change, studies of natural systems must focus on processes occurring along environmental gradients to capture the heterogeneity of the system. This research meets this goal by quantifying temporal and spatial heterogeneity of tree transpiration along a soil drainage gradient in the boreal black spruce forest. Using different measurement and analysis protocols such as heat dissipation sensors, stable isotope analyses, spatial statistics, and determining stomatal sensitivity to vapor pressure deficit (D ), I quantified (1) differences among stand age, species, and drainage condition at half-hourly to yearly time scales and the environmental drivers of this variability, (2) stomatal sensitivity to D in reference to hydraulic tradeoffs and it's associated spatial and temporal drivers at each stand, and (3) the extent of spatial autocorrelation of tree transpiration and its drivers. Lastly, I estimated the spatial and temporal variability of evapotranspiration and its components along a soil moisture and stand age gradient by comparing energy balance, sap flow, and chamber-based measurements. Interestingly, temporal drivers of tree transpiration changed dramatically depending on the spatial location of interest and included soil moisture, peat depth, water table depth, and vapor pressure deficit, but drivers of transpiration in space could be simplified into an integrated driver that encompasses all soil drainage affects; tree size.