Differential Growth Responses to Water Balance of Coexisting Deciduous Tree Species Are Linked to Wood Density in a Bolivian Tropical Dry Forest

A seasonal period of water deficit characterizes tropical dry forests (TDFs). There, sympatric tree species exhibit a diversity of growth rates, functional traits, and responses to drought, suggesting that each species may possess different strategies to grow under different conditions of water availability. The evaluation of the long-term growth responses to changes in the soil water balance should provide an understanding of how and when coexisting tree species respond to water deficit in TDFs. Furthermore, such differential growth responses may be linked to functional traits related to water storage and conductance. We used dendrochronology and climate data to retrospectively assess how the radial growth of seven coexisting deciduous tree species responded to the seasonal soil water balance in a Bolivian TDF. Linear mixed-effects models were used to quantify the relationships between basal area increment and seasonal water balance. We related these relationships with wood density and sapwood production to assess if they affect the growth responses to climate. The growth of all species responded positively to water balance during the wet season, but such responses differed among species as a function of their wood density. For instance, species with a strong growth response to water availability averaged a low wood density which may facilitate the storage of water in the stem. By contrast, species with very dense wood were those whose growth was less sensitive to water availability. Coexisting tree species thus show differential growth responses to changes in soil water balance during the wet season. Our findings also provide a link between wood density, a trait related to the ability of trees to store water in the stem, and wood formation in response to water availability.

热带干旱林（Tropical Dry Forests, TDFs）的典型特征为季节性水分亏缺。在此类森林中，同域分布的乔木树种具有多样的生长速率、功能性状及干旱响应模式，提示各物种或许演化出了适配不同水分可利用条件的生长策略。对土壤水分平衡变化的长期生长响应开展评估，有助于阐明热带干旱林中共存乔木物种响应水分亏缺的方式与时机。此外，这类差异化的生长响应或与水分储存、导水相关的功能性状存在关联。本研究借助树木年代学（dendrochronology）与气候数据，对玻利维亚一处热带干旱林中7种共存落叶乔木的径向生长响应季节性土壤水分平衡的情况进行了回顾性评估。研究采用线性混合效应模型，量化了断面积增长量与季节性水分平衡之间的关联，并将上述关联与木材密度、边材产量相结合，以评估这些性状是否会影响树木对气候的生长响应。所有物种的生长均在湿季对水分平衡呈现正向响应，但这类响应因物种而异，且随其木材密度的不同而变化。例如，对水分可利用性具有较强生长响应的物种，其平均木材密度较低，这类性状或可促进茎干储水。与之相对，木材密度较高的物种，其生长对水分可利用性的敏感性更低。由此可见，共存乔木物种在湿季对土壤水分平衡变化的生长响应存在差异。本研究结果还揭示了木材密度——一种与树木茎干储水能力相关的功能性状——与响应水分可利用性的木材形成过程之间的关联。