Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro)

Bradley O Christoffersen,Timothy R Baker,Nikolaos M Fyllas,Patrick Meir,Steven Jansen,Chonggang Xu,Sanna Sevanto,Nate G Mcdowell,David R Galbraith,Sophie Fauset,Maurizio Mencuccini,Lucy Rowland,Manuel Gloor,Brendan Choat,Oliver J Binks,Bart Kruijt,Rosie A Fisher

doi:10.5194/gmd-9-4227-2016

Abstract

Abstract. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.

Highlights

Tropical forests harbor great biodiversity (Myers et al, 2000; ter Steege et al, 2013) and play an important role in regulating regional and global climate (Gash and Nobre, 1997; Silva Dias et al, 2002)
We develop a continuous porous media approach intended for application at specific sites in the tropics to explore dynamics of water fluxes from hourly to seasonal timescales and at spatial scales ranging from individual trees to the stand-level scale
While we found no significant relationship of the residual fraction (RWCr,l) with leaf mass per area (LMA) or wood density (WD), we did find a significant relationship of RWCr,l with εleaf (r2 = 0.32; p = 0.002; Fig. 2e)

Summary

Introduction

Tropical forests harbor great biodiversity (Myers et al, 2000; ter Steege et al, 2013) and play an important role in regulating regional and global climate (Gash and Nobre, 1997; Silva Dias et al, 2002). Climate change is inducing changes to the hydrological regime of tropical forests (Feng et al, 2013; Fu et al, 2013; Gloor et al, 2013), with some consensus for a projected increase in drought frequency over the coming century via an intensification of precipitation seasonality (Joetzjer et al, 2013; Boisier et al, 2015), an increase in El Niño events (Cai et al, 2014), and chronically rising atmospheric moisture demand (McDowell and Allen, 2015), even as the directional change of total precipitation remains highly uncertain (Solomon, 2007) Because of their intrinsic value and strong coupling to the regional and global climate system, it is of paramount importance to have a predictive capability of tropical forest response to changes in water availability (Kumagai and Porporato, 2012; Oliveira et al, 2014; Meir et al, 2015). Coupled with site-specific parameterization, the plant hydraulics approach enables high-fidelity simulation of tropical forest response to moisture (Fisher et al, 2006, 2007)

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