Causes and implications of dry season control of tropical wet forest tree growth at very high water levels: direct vs. indirect limitations

哥斯达黎加最近的研究表明，热带潮湿林木的生长与一系列降雨的旱季强度密切相关，按照所有标准衡量，旱季的降雨量都足以使树木生长。理解这一发现的基础至关重要，因为热带潮湿森林在全球碳循环和水循环中发挥着重要作用，因此在区域和全球气候中也发挥着重要作用。预测未来热带潮湿森林的变暖和干燥可能会对全球气候产生强烈的反馈效应。然而，亚马逊最近的卫星研究报告称，树木在旱季拥有最大的树叶覆盖率，这表明旱季不会影响产量。这两个发现表明，我们对旱季对热带树木生长和碳固存的作用的理解存在严重差距。旱季对树木生长的控制既可能是直接的水分限制，也可能是通过其他间接机制。考虑到旱季的高降雨量、土壤湿度和湿度，通过干燥土壤和干燥空气进行直接限制似乎不太可能。潜在的间接机制包括过高的叶温降低了光合作用，对干燥空气的反应过早地损失了叶片，在直射太阳期间低的光能利用效率和冠层光合作用，以及资源分配给根的生物量或生殖而不是地上生长的变化。该项目将测试直接限制水分的假设和间接机制，以确定旱季降雨量与生长相关的基础。这项研究将使用观测和实验方法来测试直接水分限制，结合整个森林光合作用的测量和能量损失的形式，树叶和整个树木的水分利用，每月树木生长和气候测量，以及旱季的加水实验。间接限制将通过测量冠层叶面积、叶温以及根产量和新陈代谢的测量来评估。将使用对生长和生态系统碳、水和能量平衡的测量来评估减少旱季生长对区域气候的重要性。该项目将支持南佛罗里达州和哥斯达黎加的外展和教育部分。在南佛罗里达州，与当地一所1-8年级私立学校建立了伙伴关系，开展环境推广活动。项目人员将继续支持迈阿密费尔柴尔德热带植物园的费尔柴尔德挑战学校环境意识计划。该项目还将支持热带研究组织在研究地点La Selva生物站的环境教育计划。该项目将支持一名博士后研究员和几名本科生，并为两名哥斯达黎加技术人员提供培训。最后，了解旱季限制对地上生物量生产的敏感性的基础，对于预测热带潮湿森林对未来气候制度的反应至关重要。

Recent research in Costa Rica has shown that the growth of tropical moist forest trees is strongly related to the strength of the dry season across a range of rainfall that by all standard measures would be considered more than sufficient for growth. Understanding the basis for this finding is crucial because tropical moist forests play important roles in the global carbon and water cycles and therefore regional and global climate. Predicted future warming and drying of tropical moist forests could have strong feedback effects on global climate. However, recent satellite studies from the Amazon report that trees have their greatest leaf cover during the dry season, suggesting that the dry season does not affect production. These two findings suggest a serious gap in our understanding of role of the dry season on tropical tree growth and carbon sequestration. Dry-season controls on tree growth could result from both direct water limitation or via other indirect mechanisms. Direct limitation via dry soil and dry air seems unlikely given the high rainfall, soil moisture, and humidity that occur during the dry season. Potential indirect mechanisms include excessive leaf temperatures lowering photosynthesis, premature leaf loss in response to dry air, low light-use efficiency and canopy photosynthesis during periods of direct sun, and changes in allocation of resources to root biomass or reproduction instead of aboveground growth. This project will test hypotheses of direct water limitation and indirect mechanisms to determine the basis for the dry season rainfall correlation with growth. The study will use both observational and experimental approaches to test for direct water limitation, combining measurement of whole forest photosynthesis and the form of energy losses, leaf-and whole-tree water use, monthly tree growth and climate measurements, and a dry-season water-addition experiment. Indirect limitations will be evaluated by measurements of canopy leaf area, leaf temperature, and measurements of root production and metabolism. Measurements of growth and ecosystem carbon, water and energy balance will be used to evaluate the importance of dry season growth reduction for regional climate.This project will support outreach and education components in both South Florida and Costa Rica. In South Florida a partnership has been developed with a local private grade 1-8 school for environmental outreach activities. Project personnel will continue to support the Fairchild Challenge school environmental awareness program at Fairchild Tropical Botanical Garden in Miami. The project will also support the environmental education programs of the Organization for Tropical Studies at the La Selva Biological Station, the location of the study. The project will support one postdoctoral fellow, several undergraduates, and provide training for two Costa Rican technicians. Finally, understanding the basis of the sensitivity of dry season limitation on aboveground biomass production is critical to forecasting the responses of tropical wet forests to future climate regimes.