Patterns in hydraulic architecture from roots to branches in six tropical tree species from cacao agroforestry and their relation to wood density and stem growth.

Martyna M Kotowska,Bernhard Schuldt,Henry Barus,Dietrich Hertel,Yasmin Abou Rajab

doi:10.3389/fpls.2015.00191

Martyna M Kotowska, Bernhard Schuldt + Show 3 more

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https://doi.org/10.3389/fpls.2015.00191

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Abstract

For decades it has been assumed that the largest vessels are generally found in roots and that vessel size and corresponding sapwood area-specific hydraulic conductivity are acropetally decreasing toward the distal twigs. However, recent studies from the perhumid tropics revealed a hump-shaped vessel size distribution. Worldwide tropical perhumid forests are extensively replaced by agroforestry systems often using introduced species of various biogeographical and climatic origins. Nonetheless, it is unknown so far what kind of hydraulic architectural patterns are developed in those agroforestry tree species and which impact this exerts regarding important tree functional traits, such as stem growth, hydraulic efficiency and wood density (WD). We investigated wood anatomical and hydraulic properties of the root, stem and branch wood in Theobroma cacao and five common shade tree species in agroforestry systems on Sulawesi (Indonesia); three of these were strictly perhumid tree species, and the other three tree species are tolerating seasonal drought. The overall goal of our study was to relate these properties to stem growth and other tree functional traits such as foliar nitrogen content and sapwood to leaf area ratio. Our results confirmed a hump-shaped vessel size distribution in nearly all species. Drought-adapted species showed divergent patterns of hydraulic conductivity, vessel density, and relative vessel lumen area between root, stem and branch wood compared to wet forest species. Confirming findings from natural old-growth forests in the same region, WD showed no relationship to specific conductivity. Overall, aboveground growth performance was better predicted by specific hydraulic conductivity than by foliar traits and WD. Our study results suggest that future research on conceptual trade-offs of tree hydraulic architecture should consider biogeographical patterns underlining the importance of anatomical adaptation mechanisms to environment.

Highlights

The water transport pattern in trees is mainly determined by the plant hydraulic architecture, i.e., the spatial distribution of various xylem properties from roots to branches of a tree individual (McCulloh et al, 2010)
The hydraulic efficiency of different compartments along the root-to-leaf flow path can be described by the sapwood area-specific hydraulic conductivity (KS), which is directly related to the hydraulic resistance of a given position (Tyree and Ewers, 1991; McElrone et al, 2004)
We examined the inter-relationship between sapwood area-specific hydraulic conductivity of the root, stem, and branch xylem tissue with wood anatomical traits along the water flow path across six common cacao agroforestry tree species with different biogeographical origins from either seasonally dry or perhumid tropical environments growing in cacao agroforests in Central Sulawesi (Indonesia)

Summary

Introduction

The water transport pattern in trees is mainly determined by the plant hydraulic architecture, i.e., the spatial distribution of various xylem properties from roots to branches of a tree individual (McCulloh et al, 2010). As one of the basic organizing principles of tree hydraulic architecture it has been postulated that the mean vessel diameter in the xylem tissue generally decreases acropetally from roots to branches (‘vessel tapering’: Baas, 1982; Tyree and Zimmermann, 2002; Anfodillo et al, 2013). This principle has stimulated several conceptual models on plant hydraulic architecture during the past 15 years. Recent studies in tropical forests in South America (Machado et al, 2007; Fortunel et al, 2013) and Indonesia (Schuldt et al, 2013) have produced contradictory results regarding the paradigm of continuous vessel tapering. Schuldt et al (2013) supposed that mechanisms reducing cavitation risk may not have been evolved in these moist or perhumid environments where drought stress is normally not apparent

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