RECOVER MAP 3.1.3.6 Landscape Pattern - Ridge, Slough, and Tree Island function in relation to marsh hydrology

This project will support two existing MAP projects: 1) Tree Islands in Everglades National Park (ENP) - Big Cypress and 2) Tree Island Stage Duration and the measurement of water depth on tree islands located in WCA 3A and 3B. While these projects have similar broad objectives, some of the specific monitoring design and constituents differ. Tree island elevations and species composition were measured on over 200 islands in WCA 3, while community dynamics, hydrology, soil moisture, and transpiration and growth rates of the dominant trees are monitored on tree islands located within ENP. The ENP monitoring program was based on work originally funded by the ENP in 2005. Ross and Oberbauer (2006) have shown large seasonal differences between dry and wet season transpiration rates in an island located in the eastern prairies of ENP. However, no seasonal differences were observed in the islands located in Shark Slough where the dry season water levels remained above the marsh surface. These results have implications for the management of these systems, since it appears that extended dry-downs during the November â May dry season may cause significant declines in tree island productivity. These changes in productivity, in turn, may alter the role tree islands play in nutrient cycles in the Everglades marshes. The role of tree islands in marsh nutrient cycles has been the subject of recent journal articles (Ross et al. 2006, Wetzel et al. 2005), and in the last year RECOVER has sponsored several discussions dedicated to the development of tree island conceptual models and performance measures (e.g. GEER 2008). The WCA 3 monitoring effort has begun to add many of the physiological measurements to their monitoring program, but is currently lacking the funding necessary to fully implement the effort. This work will place all the existing MAP projects within a common framework that links tree island productivity with nutrient cycles and hydrologic conditions in the marsh. The results are expected to provide information useful for the development of cost-effective, long-term monitoring tools for the MAP.
The overall objective of the study is to test the transpiration model presented by Ross et al. (2006 and Wetzel et al. (2005), which states that high transpiration is the driving force for nutrient accumulation of tree islands (Fig 1). According to this model, slough tree islands can maintain high transpiration rates during the dry season using standing marsh water around the island or groundwater, while prairie tree islands will have low transpiration rate during dry season due to low water availability. Thus, the difference in hydroperiod between slough and prairie tree islands will result in differential nutrient accumulation rates and suggests that slough tree islands can accumulate more nutrients than prairie tree islands.

The specific objectives of this study are 1) to test whether there is a decrease in transpiration from wet to dry season in prairie tree islands but not in slough tree islands, 2) to test whether the above transpiration shift is reflected in the foliar carbon isotope ratios and 3) to test whether prairie tree islands showing the decrease from wet to dry season transpiration have lower foliar nutrient concentrations compared to slough tree islands. The objective is to use Granier probes to measure sap flux rates as a proxy for transpiration, foliar carbon and nitrogen isotope analysis as well foliar phosphorus and nitrogen concentrations to answer the above questions. The results of this project will be submitted to a peer reviewed journal. The results of this research will link transpiration with nutrient accumulation and will provide guidelines to tree island models. In addition, the foliar isotopic analysis, if consistent with the above hypotheses, will provide a measure of tree island nutrient stability and may be suitable for use as a MAP monitoring tool.

本项目将支持两项现有MAP项目：1）大沼泽地国家公园（Everglades National Park, ENP）——大柏树区域的树岛研究；2）WCA 3A与3B区域树岛的阶段持续时间及水深测量研究。尽管这些项目的总体目标相似，但部分具体监测设计与监测对象存在差异。WCA 3区域的200余个树岛已完成海拔与物种组成测量；而ENP区域树岛的监测内容则包括群落动态、水文状况、土壤湿度，以及优势树种的蒸腾作用（transpiration）与生长速率。ENP监测项目基于2005年ENP资助的初始研究。Ross与Oberbauer（2006）的研究表明，ENP东部草原区域某树岛的蒸腾速率（transpiration rate）在干湿季间存在显著季节性差异；但在Shark Slough区域的树岛中未观测到此类差异——该区域枯水期水位仍维持在沼泽地表以上。这些结果对该生态系统的管理具有重要意义：11月至次年5月枯水期的持续干旱似乎会导致树岛生产力显著下降，而生产力的变化又可能改变树岛在大沼泽湿地养分循环（nutrient cycles）中的作用。树岛在湿地养分循环中的作用是近期期刊文章的研究主题（Ross等，2006；Wetzel等，2005）；此外，RECOVER在去年资助了多场专题讨论，旨在推进树岛概念模型与绩效指标的开发（如GEER 2008）。WCA 3区域的监测工作已开始将多项生理指标纳入监测方案，但目前缺乏全面实施所需的资金。本研究将把所有现有MAP项目整合至统一框架中，该框架可关联树岛生产力、湿地养分循环与水文条件。研究结果有望为MAP开发经济高效的长期监测工具提供参考依据。
本研究的总体目标是验证Ross等（2006）与Wetzel等（2005）提出的蒸腾模型——该模型指出，高蒸腾作用（transpiration）是树岛养分积累的驱动力（图1）。根据该模型，slough树岛可在枯水期利用周边湿地积水或地下水维持高蒸腾速率（transpiration rate），而prairie树岛因水资源匮乏，枯水期蒸腾速率较低。因此，slough与prairie树岛之间的水文周期（hydroperiod）差异将导致养分积累速率不同，且slough树岛的养分积累量可能高于prairie树岛。

本研究的具体目标包括：1）验证prairie树岛的蒸腾作用（transpiration）是否从湿季到枯季下降，而slough树岛无此变化；2）验证上述蒸腾变化是否反映在叶片碳同位素比值（foliar carbon isotope ratios）中；3）验证湿季至枯季蒸腾作用下降的prairie树岛，其叶片养分浓度是否低于slough树岛。本研究将采用Granier探针测量树液通量速率（sap flux rate）（作为蒸腾作用的替代指标）、叶片碳氮同位素分析，以及叶片磷氮浓度测定来解答上述问题。本项目结果将投稿至同行评审期刊。研究结果将建立蒸腾作用与养分积累的关联，并为树岛模型提供指导。此外，若叶片同位素分析（foliar isotopic analysis）与上述假设一致，将可作为树岛养分稳定性的衡量指标，且可能适用于MAP监测工具。