The Ecology of Root Lifespan in Temperate Trees

尽管它很重要，但物种间根系寿命的变化以及对环境变化的响应却知之甚少。相对较少的物种已经被检查，很少有多个物种在一个共同的环境进行比较。提出的工作将使用最先进的方法检查12种树种的根寿命，这些树种在根直径、根组织密度和潜在生长率方面差异很大。大约9年前，树木作为1岁的幼苗移植到一个普通花园的复制地块上。根系寿命的变化与植株潜在生长率、根系结构（比根长、直径、组织密度）和根系氮浓度有关。根和叶是否具有与它们的寿命相关的平行特征？本文提出了三个假说，试图解释控制和限制根系寿命的因素：“淀粉消耗假说”（SDH）、“资源优化假说”（ROH）和一个新的假说，即“代谢活性假说”（MAH）。淀粉消耗假说认为，在根的形成过程中储存了有限数量的碳水化合物（淀粉），而根呼吸消耗碳水化合物的速度决定了根的寿命。资源优化假说认为，在根或根簇的整个生命周期中，以最小的成本（通常以碳计量）提供最大的水和营养效益。代谢活性假说认为根的寿命主要受代谢率的影响；呼吸活跃度高的根比呼吸活跃度低的根寿命短。提出了三个实验来区分哪种假设最能解释根系寿命的模式。其中一项实验涉及创造不会枯竭的肥沃地块。这些补片可以提高氮的获取效率，也可以提高代谢率。如果根生命周期在补丁中增加，则支持ROH。如果根的寿命减少，那么SDH或MAH都是支持的，这取决于根死亡的速度与其淀粉储备的关系。不同树种根系的呼吸作用和非结构性碳水化合物（包括淀粉）将在另一个实验中作为根龄的函数进行检验。最后一个实验通过碳水化合物13C脉冲标记来检验当前光合作用对一级根寿命的重要性。这项研究将产生几个更广泛的影响。更好地了解根的寿命对那些试图模拟生态系统碳循环的人来说是有价值的。本研究检查的许多树木是东部阔叶林的森林优势。更好地了解它们的根系寿命将对森林管理者和气候变化研究人员有用。本研究将为每年培养两名研究生和几名本科生提供强有力的支持。此外，Eissenstat实验室在吸引代表性不足的少数民族进行夏季研究方面有着良好的记录，部分原因是宾夕法尼亚州立大学的慷慨支持和明确的项目提供了高质量的夏季研究体验，包括为参加这个为期8周的夏季研究项目的学生支付75%的项目费用。

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.