Effects of Wood Hydraulic Properties on Water Use and Productivity of Tropical Rainforest Trees

Martyna M Kotowska,Alexander Röll,Bernhard Schuldt,Dietrich Hertel,Christoph Leuschner,Aiyen Tjoa,Pierre-André Waite,Dirk Hölscher,Gerald Moser,Roman M Link

doi:10.3389/ffgc.2020.598759

Abstract

The efficiency of the water transport system in trees sets physical limits to their productivity and water use. Although the coordination of carbon assimilation and hydraulic functions has long been documented, the mutual inter-relationships between wood anatomy, water use and productivity have not yet been jointly addressed in comprehensive field studies. Based on observational data from 99 Indonesian rainforest tree species from 37 families across 22 plots, we analyzed how wood anatomy and sap flux density relate to tree size and wood density, and tested their combined influence on aboveground biomass increment (ABI) and daily water use (DWU). Results from pairwise correlations were compared to the outcome of a structural equation model (SEM). Across species, we found a strong positive correlation between ABI and DWU. Wood hydraulic anatomy was more closely related to these indicators of plant performance than wood density. According to the SEM, the common effect of average tree size and sap flux density on the average stem increment and water use of a species was sufficient to fully explain the observed correlation between these variables. Notably, after controlling for average size, only a relatively small indirect effect of wood properties on stem increment and water use remained that was mediated by sap flux density, which was significantly higher for species with lighter and hydraulically more efficient wood. We conclude that wood hydraulic traits are mechanistically linked to water use and productivity via their influence on sap flow, but large parts of these commonly observed positive relationships can be attributed to confounding size effects.

Highlights

Water availability is probably the single most important determinant of tree size and productivity on a global scale (Moles et al, 2009; Šímová et al, 2019)
This study investigates stem sap flux, wood anatomy and aboveground biomass increment of 99 tropical tree species from 37 families using measurements from 265 tree individuals (Supplementary Table S1), combining data from two long-term studies in plot networks on the Indonesian islands Sumatra and Sulawesi comprising a total of 22 plots (Supplementary Figure S1)
Wood density (WD), on the other hand, was only weakly associated with most other variables in the dataset (Figure 2). This is reflected in the results of the standardized major axis (SMA) regressions (Figure 3), which revealed a significant positive scaling relationship between hydraulically-weighted vessel diameter and biomass increment (ABI; r = 0.38, P < 0.001), daily water use (DWU; r = 0.56, P < 0.001), sap flux density (Js; r = 0.38, P < 0.001), and tree height (H; r = 0.39, P < 0.001), while their relationships to wood density (WD) were insignificant with the exception of sap flux density

Summary

Introduction

Water availability is probably the single most important determinant of tree size and productivity on a global scale (Moles et al, 2009; Šímová et al, 2019). Evolving an efficient vascular system designed for long-distance water transport allows trees to place their carbon assimilating foliage high above the ground (Sperry, 2003), thereby gaining a competitive advantage over shorter. To enable a sufficiently high water supply to the canopy, the stem wood of tall and productive tropical trees is commonly composed of wide xylem vessels that facilitate the necessary high flow rates (Poorter et al, 2010; Schuldt et al, 2013; Kotowska et al, 2015; Hietz et al, 2016). The coordination of productivity and hydraulic functioning has long been recognized, the underlying causal links often remain unresolved and the precise role of xylem vascular anatomy for the efficiency of the water transport system, tree water use and stem growth rate is hard to quantify (Hoeber et al, 2014). The approach to use easy-to-measure plant functional traits for characterizing the hydraulic architecture of tropical trees and for understanding the coexistence and performance of tree species in communities is increasingly popular (Poorter et al, 2018; McDowell et al, 2019)

Methods

Results

Discussion

Conclusion