Controls On Water Use Efficiency For Different Forest Ecosystems Across North America: From The Leaf To Landscape

Abstract:

Water use efficiency (WUE), defined as the ratio of carbon assimilation (A) to water loss via transpiration, is the key physiological parameter that explicitly links water and carbon cycling in forest ecosystems. Most studies examining the influence of climatic factors on forest-WUE have focused on site-specific and water-limited ecosystems. Much less is known about the dynamics of WUE across different forest ecosystems, along gradients of climate and soil nutrients. This study explores the variation in WUE at twelve different forested Ameriflux sites across North America spanning a wide range of forest types and climate conditions. We were interested in answering the following questions: 1) Are patterns of WUE at the tree level similar to those for whole ecosystems? 2) Is there a consistent relationship between foliar nitrogen (N) and WUE at the two different scales? 3) To what extent does species diversity explain forest ecosystem functioning and resilience to drought? Finally, 4) Can changes in GPP, ET and WUE across the studied climate gradient be estimated using remotely derived water indices, such as NDWI? We address these questions through a combination of techniques applied at the leaf to landscape level, including foliar δ¹³C, δ¹⁸O and N, eddy covariance and remote sensing data. At each site, we used foliar δ¹³C to infer intrinsic WUE (_iWUE=A/g_s) for the two dominant tree species, while foliar N and δ¹⁸O provided insights about the species-specific physiological mechanisms underlying variation in _iWUE. Furthermore, we used flux data to derive ET and WUE_e (i.e., ecosystem WUE= GPP/ET). Variations in _iWUE and WUE_e in relation to foliar N, climate parameters and water stress index (PSDI) are considered across sites. Moreover, differences in GPP, ET and WUE_e are explored in relation to forest ecosystem type, species richness and Shannon’s diversity index. Finally we examine the degree to which changes in WUE are related to NDVI and NDWI.