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

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.

Implementing Plant Hydraulics in the Community Land Model, Version 5

“…PHS may overestimate HR, given the simplified root system architecture (Bouda & Saiers, ) and the lack of an explicit representation of fine‐root cavitation (Kotowska et al, ). In our simulations, HR increases annual total RWU by up to 52% relative to transpiration alone (2003, TFE).…”

Over‐accumulation of abscisic acid in transgenic tomato plants increases the risk of hydraulic failure

“…Recent works have shown reductions in leaf hydraulic conductivity of detached leaves fed with exogenous ABA in both Arabidopsis (Pantin et al, ) and grapevine varieties (Coupel‐Ledru et al, ), possibly through the regulation of aquaporin activity in the bundle sheath around leaf veins (Shatil‐Cohen, Attia, & Moshelion, ). An alternative is that the decrease in K s observed in the transgenic sp12 line may be due to the associated lower number of large diameter xylem vessels compared with the wild type (Figure S3), given that hydraulic properties of the xylem network strongly depends on wood anatomical features such as conduit diameter (Tyree & Ewers, ) and that a small increase in vessel diameter results in a large increase in conductivity (Hagen‐Poiseuille law; Kotowska, Hertel, Rajab, Barus, & Schuldt, ; Nobel, ). The decrease in K s in the sp12 line was balanced by an increase in root hydraulic conductivity ( Lp r ).…”

How tree roots respond to drought

“…As the most distal organs belowground, fine roots can be sacrificed in response to drought, similar to leaf fall, as observed in various temperate and boreal forests (e.g., Gaul et al, 2008 ; Chenlemuge et al, 2013 ; Hertel et al, 2013 ). Kotowska et al (2015) postulated a mechanism for fine root die off analogous to that of leaf abscission by defining a ‘hydraulic fuse,’ a concept evolved from Zimmermann’s segmentation hypothesis ( Tyree and Zimmermann, 2002 ). At the root level, this ‘hydraulic segmentation’ might additionally protect the belowground system from a reverse water flow from the roots back to the dryer soil ( Kotowska et al, 2015 ).…”

“… Kotowska et al (2015) postulated a mechanism for fine root die off analogous to that of leaf abscission by defining a ‘hydraulic fuse,’ a concept evolved from Zimmermann’s segmentation hypothesis ( Tyree and Zimmermann, 2002 ). At the root level, this ‘hydraulic segmentation’ might additionally protect the belowground system from a reverse water flow from the roots back to the dryer soil ( Kotowska et al, 2015 ).…”