Anatomical traits explain drought response of seedlings from wet tropical forests

Water availability regulates plant community dynamics, but the drought response of seedlings remains poorly known despite their vulnerability, especially for the Asian tropics. In particular, discerning how functional traits of seedlings mediate drought response can aid generalizable predictions of tree responses to global environmental change. We assessed interspecific variation in drought response explained by above- and below-ground seedling traits. We conducted a dry-down experiment in the greenhouse using 16 tree species from the humid forests of Western Ghats in southern India, chosen to represent differences in affinity to conditions of high and low seasonal drought (seasonality affiliation). We compared survival, growth, and photosynthetic performance under drought and well-watered conditions, and assessed the extent to which species’ responses were explained by seasonality affiliation and 12 traits of root, stem, and leaf. We found that the species from seasonally dry forest reduced photosynthetic rate in drought compared to well-watered conditions, but seasonality affiliation did not explain differences in growth and survival. Performance in drought vs well-watered conditions were best explained by anatomical traits of xylem, veins and stomata. Species with larger xylem reduced their growth and photosynthesis to tolerate desiccation. In drought, species with smaller stomata correlated with lower survival even though photosynthetic activity decreased by a larger extent with larger stomata. Overall, anatomical traits of xylem and stomata, directly related to water transport and gas-exchange, played a more prominent role than commonly used traits (e.g., specific leaf area, leaf dry matter content) in explaining species response to drought, and may offer a good proxy for physiological traits related to drought tolerance of seedlings. --

水分可获得性调控植物群落动态，但尽管幼苗本身极为脆弱，其干旱响应机制仍未得到充分阐明，在亚洲热带地区这一问题尤为突出。具体而言，阐明幼苗功能性状（functional traits）如何介导干旱响应，有助于对树木响应全球环境变化做出可推广的预测。
我们评估了物种间干旱响应的差异，并探究了幼苗地上与地下功能性状对该差异的解释程度。本研究以印度南部西高止山脉（Western Ghats）湿润森林中的16种树木为研究对象，在温室中开展了逐步干旱实验（dry-down experiment），所选物种涵盖了对高、低季节性干旱条件的不同适应性。我们对比了干旱与充分供水条件下各物种的存活率、生长速率及光合性能，并评估了物种的季节干旱适配性以及根、茎、叶共12项功能性状对其干旱响应的解释程度。
研究发现，来自季节性干旱森林的物种在干旱条件下的光合速率较充分供水时有所降低，但季节干旱适配性无法解释物种间生长与存活率的差异。物种在干旱与充分供水条件下的性能差异，最佳可通过木质部（xylem）、叶脉（veins）与气孔（stomata）的解剖性状（anatomical traits）予以解释。木质部面积更大的物种会通过降低生长与光合速率来耐受脱水。在干旱条件下，尽管气孔更大的物种光合活性下降幅度更大，但气孔更小的物种存活率反而更低。
总体而言，直接参与水分运输与气体交换的木质部和气孔解剖性状，在解释物种干旱响应方面比常用功能性状（如比叶面积（specific leaf area）、叶干物质含量（leaf dry matter content））发挥了更显著的作用，或可作为幼苗干旱耐受相关生理性状的良好替代指标。