Schönauer Marian, Hietz Peter, Schuldt Bernhard, Rewald Boris
Department of Forest and Soil Sciences, Institute of Forest Ecology, University of Natural Resources and Life Sciences, Vienna, Austria.
Department of Forest Work Science and Engineering, Department of Forest Sciences and Forest Ecology, Georg-August-Universität Göttingen, Göttingen, Germany.
Front Plant Sci. 2023 Jun 12;14:1127292. doi: 10.3389/fpls.2023.1127292. eCollection 2023.
Vessel traits are key in understanding trees' hydraulic efficiency, and related characteristics like growth performance and drought tolerance. While most plant hydraulic studies have focused on aboveground organs, our understanding of root hydraulic functioning and trait coordination across organs remains limited. Furthermore, studies from seasonally dry (sub-)tropical ecosystems and mountain forests are virtually lacking and uncertainties remain regarding potentially different hydraulic strategies of plants differing in leaf habit. Here, we compared wood anatomical traits and specific hydraulic conductivities between coarse roots and small branches of five drought-deciduous and eight evergreen angiosperm tree species in a seasonally dry subtropical Afromontane forest in Ethiopia. We hypothesized that largest vessels and highest hydraulic conductivities are found in roots, with greater vessel tapering between roots and equally-sized branches in evergreen angiosperms due to their drought-tolerating strategy. We further hypothesized that the hydraulic efficiencies of root and branches cannot be predicted from wood density, but that wood densities across organs are generally related. Root-to-branch ratios of conduit diameters varied between 0.8 and 2.8, indicating considerable differences in tapering from coarse roots to small branches. While deciduous trees showed larger branch xylem vessels compared to evergreen angiosperms, root-to-branch ratios were highly variable within both leaf habit types, and evergreen species did not show a more pronounced degree of tapering. Empirically determined hydraulic conductivity and corresponding root-to-branch ratios were similar between both leaf habit types. Wood density of angiosperm roots was negatively related to hydraulic efficiency and vessel dimensions; weaker relationships were found in branches. Wood density of small branches was neither related to stem nor coarse root wood densities. We conclude that in seasonally dry subtropical forests, similar-sized coarse roots hold larger xylem vessels than small branches, but the degree of tapering from roots to branches is highly variable. Our results indicate that leaf habit does not necessarily influence the relationship between coarse root and branch hydraulic traits. However, larger conduits in branches and a low carbon investment in less dense wood may be a prerequisite for high growth rates of drought-deciduous trees during their shortened growing season. The correlation of stem and root wood densities with root hydraulic traits but not branch wood points toward large trade-offs in branch xylem towards mechanical properties.
导管特征是理解树木水力效率以及生长性能和耐旱性等相关特性的关键。虽然大多数植物水力研究都集中在地上器官，但我们对根系水力功能以及各器官间性状协调性的理解仍然有限。此外，来自季节性干燥（亚）热带生态系统和山林的研究几乎空白，并且关于叶习性不同的植物可能存在的不同水力策略仍存在不确定性。在此，我们比较了埃塞俄比亚季节性干燥亚热带阿夫罗山地森林中5种干旱落叶和8种常绿被子植物树种的粗根与小枝之间的木材解剖特征和比水力导率。我们假设最大的导管和最高的水力导率存在于根中，由于常绿被子植物的耐旱策略，其根与等径枝条之间的导管逐渐变细程度更大。我们进一步假设根和枝条的水力效率无法从木材密度预测，但各器官的木材密度通常是相关的。导管直径的根枝比在0.8至2.8之间变化，表明从粗根到小枝的逐渐变细程度存在显著差异。虽然落叶树与常绿被子植物相比显示出更大的枝木质部导管，但在两种叶习性类型中根枝比都高度可变，并且常绿树种并未表现出更明显的逐渐变细程度。两种叶习性类型间经验测定的水力导率和相应的根枝比相似。被子植物根的木材密度与水力效率和导管尺寸呈负相关；在枝条中发现的相关性较弱。小枝的木材密度与茎和粗根的木材密度均无关联。我们得出结论，在季节性干燥的亚热带森林中，相同大小的粗根比小枝拥有更大的木质部导管，但从根到枝的逐渐变细程度高度可变。我们的结果表明叶习性不一定会影响粗根和枝水力性状之间的关系。然而，枝条中更大的导管以及低密度木材中较低的碳投资可能是干旱落叶树在其缩短的生长季节实现高生长速率的先决条件。茎和根木材密度与根水力性状相关但与枝木材无关，这表明枝木质部在机械性能方面存在巨大权衡。
