The Ecology of Root Lifespan in Temperate Trees

尽管它很重要，但不同物种之间的根寿命差异以及对环境变化的反应却知之甚少。被研究的物种相对较少，而且很少有多个物种在共同的环境中进行比较。这项拟议的工作将使用最先进的方法研究12个树种的根寿命，这些树种在根直径、根组织密度和潜在生长速度方面差异很大。大约九年前，树木作为一年生苗木被移植到一个普通花园的重复地块中。根寿命的变化将与植物潜在生长速率、根结构(比根长、根直径、根组织密度)和根N浓度有关。根和叶是否具有与它们的寿命相关的一系列相似的特征？有三个假说试图解释是什么控制和限制了根的寿命：“淀粉消耗假说”(SDH)、“资源优化假说”(ROH)和一个新的假说--“代谢活性假说”(MAH)。淀粉耗竭假说认为，在根形成时储存的碳水化合物(淀粉)是有限的，根呼吸耗尽碳水化合物的速率决定了根的寿命。资源优化假说假设优化了根的寿命，以在根或根簇的寿命内以最低的成本(通常以碳为单位)提供最大的水分和养分效益。代谢活性假说表明，根的寿命主要由代谢率决定；呼吸活性较高的根寿命比呼吸活性较低的根短。提出了三个实验来区分哪一种假说最能解释根寿命的模式。一项实验涉及创造不会枯竭的肥沃土地。这些贴片应该会增加氮素的获取效率，也会增加代谢率。如果补丁中增加了根寿命，则支持ROH。如果根的寿命减少，那么SDH或MAH就会得到支持，这取决于根相对于其淀粉储备死亡的速度。在另一项实验中，不同树种根的呼吸作用和非结构性碳水化合物(包括淀粉)将作为根龄的函数进行检测。最后一项实验通过用13C脉冲标记碳水化合物来检验当前光合作用对一级根根寿命的重要性。这项研究将产生几个更广泛的影响。对于那些试图模拟生态系统碳循环的人来说，更好地了解根的寿命将是有价值的。这项研究中考察的许多树木都是东部阔叶林中的森林优势种。更好地了解它们的根部寿命将有助于森林管理者以及气候变化的研究人员。这项研究将为每年培养两名研究生和几名本科生提供有力的支持。此外，艾森斯塔特实验室有吸引代表性不足的少数族裔进行暑期研究的记录，部分原因是宾夕法尼亚州立大学慷慨的支持和定义明确的项目提供高质量的暑期研究体验，包括为参加这个为期8周的暑期研究项目的学生支付项目成本的75%。

Despite its importance, variation in root lifespan among species and in response to changes in the environment is poorly understood. Relatively few species have been examined and rarely have multiple species been compared in a common environment. The work proposed will examine the root lifespan of 12 tree species that vary widely in root diameter, root tissue density and potential growth rate using state-of-the-art approaches. Trees were transplanted as 1-yr-old seedlings in replicated plots in a common garden approximately nine years ago. Variation in root lifespan will be related to plant potential growth rate, root structure (specific root length, diameter, tissue density) and root N concentration. Do roots and leaves share parallel suites of traits commonly associated with their lifespan? Three hypotheses are proposed that attempt to explain what controls and constrains root lifespan: the "Starch depletion hypothesis" (SDH), the "Resource optimization hypothesis" (ROH) and a new hypothesis proposed here, the "Metabolic activity hypothesis" (MAH). The Starch depletion hypothesis assumes that a finite amount of stored carbohydrates (starch) is deposited at root formation and that the rate the carbohydrates are depleted by root respiration determines the lifespan of the root. The Resource optimization hypothesis assumes that root lifespan is optimized to provide the greatest benefit in terms of water and nutrients for the least cost (usually measured in carbon) over the lifespan of the root or cluster of roots. The Metabolic activity hypothesis suggests that root lifespan is mainly governed by metabolic rate; roots with higher respiratory activity live shorter lives than those with lower respiratory activity. Three experiments are proposed to distinguish which hypothesis best explains patterns of root lifespan. One experiment involves creating fertile patches which do not become depleted. These patches should increase the efficiency of nitrogen acquisition and also increase metabolic rate. If root lifespan is increased in the patch, then ROH is supported. If root lifespan is decreased then either SDH or MAH is supported, depending on how quickly the roots die in relation to their starch reserves. Respiration and nonstructural carbohydrates (including starch) of the roots of the different species will be examined as a function of root age in another experiment. A final experiment examines the importance of current photosynthate on root lifespan of 1st-order roots by pulse-labeling carbohydrates with 13C. This study will have several broader impacts. A better understanding of root lifespan will be valuable to those attempting to model ecosystem carbon cycles. Many of the trees examined in this study are forest dominants in eastern hardwood forests. Better understanding of their root lifespan will be useful to forest managers as well as investigators of climate change. This study will provide strong support for the training of two graduate students and several undergraduates each year. In addition, the Eissenstat lab has a track record of attracting under-representative minorities to conduct summer research, partly because of the generous support and well-defined programs at Penn State provide a quality summer research experience, including payment of 75% of program costs for the students enrolled in this 8-week summer research program.