The Ecology of Root Lifespan in Temperate Trees

尽管其重要性，物种之间的根寿命的变化和对环境变化的反应知之甚少。 相对较少的物种已被检查，很少有多个物种在一个共同的环境进行比较。 拟议的工作将使用最先进的方法研究12个树种的根寿命，这些树种的根直径、根组织密度和潜在生长率差异很大。 大约9年前，将树木作为1岁的幼苗移植到一个普通花园的复制地块中。根系寿命的变化与植物潜在生长速率、根系结构（比根长、直径、组织密度）和根系氮浓度有关。 根和叶是否具有与其寿命相关的相似特征？ 提出了三个假说，试图解释是什么控制和约束根寿命：“淀粉耗竭假说”（SDH），“资源优化假说”（ROH）和一个新的假说提出这里，“代谢活性假说”（MAH）。 淀粉消耗假说假设在根形成时储存的碳水化合物（淀粉）是有限的，并且根呼吸消耗碳水化合物的速率决定了根的寿命。 资源优化假设假设根的寿命被优化，以在根或根簇的寿命期间以最少的成本（通常以碳来衡量）提供水和营养方面的最大效益。 代谢活性假说认为，根的寿命主要取决于代谢率;呼吸活性较高的根比呼吸活性较低的根寿命短。 提出了三个实验来区分最好的解释模式的根寿命的假设。一个实验涉及创造不会枯竭的肥沃斑块。 这些补丁应该增加氮的获取效率，也增加代谢率。 如果补丁中的根生命周期增加，则支持ROH。 如果根寿命减少，则支持SDH或MAH，这取决于根死亡的速度与其淀粉储备的关系。 在另一个实验中，将检查不同物种根的呼吸和非结构性碳水化合物（包括淀粉）作为根龄的函数。 最后一个实验研究的重要性，目前的光合作用对根寿命的第一阶根脉冲标记碳水化合物与13 C。 这项研究将产生几个更广泛的影响。 更好地了解根的寿命将是有价值的那些试图模拟生态系统碳循环。 在这项研究中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有助于森林管理者和气候变化调查人员。本研究将为每年培养两名研究生和多名本科生提供有力的支持。 此外，该实验室有吸引代表性不足的少数民族进行夏季研究的记录，部分原因是宾夕法尼亚州立大学的慷慨支持和明确的计划提供了高质量的夏季研究体验，包括支付75%的计划费用为学生参加这个为期8周的夏季研究计划。

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.