Environmental Change Institute, School of Geography and the Environment, Oxford University Centre for the Environment, South Parks Road, Oxford, UK.
School of Natural Resources, Copperbelt University, Kitwe, Zambia.
Tree Physiol. 2019 Jan 1;39(1):104-112. doi: 10.1093/treephys/tpy062.
Water availability has frequently been linked to seasonal leaf display in seasonally dry ecosystems, but there have been few ecohydrological investigations of this link. Miombo woodland is a dominant seasonally dry tropical forest ecosystem type in southern Africa; however, there are few data on the relationship between seasonal dynamics in plant-water relations and patterns of leaf display for Miombo woodland. Here we investigate this relationship among nine key Miombo woodland tree species differing in drought tolerance ability and leaf phenology. Results of this study showed that seasonal patterns of leaf phenology varied significantly with seasonal changes in stem water relations among the nine species. Leaf shedding coincided with the attainment of seasonal minimum stem water potential. Leaf flush occurred following xylem rehydration at the peak of the dry season suggesting that endogenous plant factors play a pivotal role in seasonal leaf display in this forest type. Drought-tolerant deciduous species suffered significantly higher seasonal losses in xylem hydraulic conductivity than the drought-intolerant semi-evergreen tree species (P < 0.05). There was a significant and positive correlation between species drought tolerance index and species' seasonal loss in hydraulic conductivity (P < 0.05), confirming the ecological role of long-distance xylem transport in this seasonally dry tropical forest. Our results reveal that water stress in seasonally dry tropical forests selects for water conservative traits that protect the vulnerable xylem transport system. Therefore, seasonal rhythms in xylem transport dictate patterns of leaf display in seasonally dry tropical forests.
水的可获得性常常与季节性干旱生态系统中的季节性叶片展示有关，但对这种联系的生态水文学研究很少。米邦博林地是南非主要的季节性干旱热带森林生态系统类型；然而，关于米邦博林地植物水分关系的季节性动态与叶片展示模式之间的关系的数据很少。在这里，我们研究了在耐旱能力和叶片物候不同的 9 种关键米邦博林地树种之间的这种关系。研究结果表明，9 个树种的叶片物候季节性模式与茎水分关系的季节性变化显著相关。叶片脱落与茎水势达到季节性最低值相吻合。在旱季高峰期木质部重新水合后出现叶片萌发，这表明内源性植物因素在这种森林类型的季节性叶片展示中起着关键作用。耐旱落叶树种的木质部水力传导率季节性损失明显高于不耐旱半常绿树种（P<0.05）。物种耐旱指数与物种水力传导率季节性损失之间存在显著正相关（P<0.05），这证实了长距离木质部运输在这种季节性干旱热带森林中的生态作用。我们的结果表明，季节性干旱热带森林中的水分胁迫选择了保护脆弱木质部运输系统的节水特性。因此，木质部运输的季节性节律决定了季节性干旱热带森林的叶片展示模式。
