Why size matters: the interactive influences of tree diameter distribution and sap flow parameters on upscaled transpiration

Abstract

In stands with a broad range of diameters, a small number of very large trees can disproportionately influence stand basal area and transpiration (Et). Sap flow-based Et estimates may be particularly sensitive to large trees due to nonlinear relationships between tree-level water use (Q) and tree diameter at breast height (DBH). Because Q is typically predicted on the basis of DBH and sap flow rates measured in a subset of trees and then summed to obtain Et, we assessed the relative importance of DBH and sap flow variables (sap velocity, Vs, and sapwood depth, Rs) in determining the magnitude of Et and its dependence on large trees in a tropical montane forest ecosystem. Specifically, we developed a data-driven simulation framework to vary the relationship between DBH and Vs and stand DBH distribution and then calculate Q, Et and the proportion of Et contributed by the largest tree in each stand. Our results demonstrate that variation in how Rs is determined in the largest trees can alter estimates up to 26% of Et while variation in how Vs is determined can vary results by up to 132%. Taken together, these results highlight a great need to expand our understanding of water transport in large trees as this hinders our ability to predict water fluxes accurately from stand to catchment scales.

Sap Flow Dynamics and Responses to Grazing and Seasonal Changes in Soil Moisture for Acacia Ancistroclada and Comberatum Molle in a Kenyan Savanna

“…The linear relationship of SFD, with DBH, wood area and the crown projected area reported in our study (table 1) was also documented by other researchers (Kallarackal et al, 2012;Berry et al, 2017). The relationship between crown projected area and sap ow is explained by the pipe model theory (Shinozaki et al, 1964).…”

Edaphic factors and initial conditions influence successional trajectories of early regenerating tropical dry forests

“…Basal area can be an indicator of competition intensity early in succession because low initial basal area can reflect more space available for recruitment and growth. Conversely, high initial basal area may accelerate sapling mortality and slow sapling biomass accretion because patches with more basal area transpire more water (Berry et al, ) probably leaving little soil moisture left for saplings. Moreover, our results resemble what van Breugel et al () found in a successional forest in Mexico, where mortality via self‐thinning was higher than recruitment along a gradient from high initial basal area to low initial basal area.…”

Differential water-use strategies in co-occurring pioneers and late-successional tree species in secondary tropical montane forests of Eastern Himalaya

“…These ecophysiological traits, such as wood anatomy, rooting depth, stomatal density, sapwood density, have evolved through competition and adaptation by co-occurring species belonging to different functional groups, and are expressed as inter-specific variability (between individuals of different species) (Phillips et al 2010, Hernandez-Santana et al 2015. The intra-specific (within individuals of the same species) variations manifests as radial and azimuthal variability in sap flow, and are influenced by growth stages, size, and individual responses to the microclimate (Fiora and Cescatti 2006, Berdanier et al 2016, Berry et al 2018). Such species-based understanding is important while scaling transpiration to individual and stand-levels, which requires using appropriate scalars, including biometric scalars like sapwood area, leaf area index (LAI), basal area, and tree height (Hatton et al 1995, Barbour et al 2005, Mackay et al 2010.…”