Collaborative Research: Biophysical and Ecological Constraints on Maximum Tree Height:Insights From the Three Tallest Tree Species.

最大树高的生物物理和生态限制：来自三种最高树种的见解北亚利桑那大学和洪堡州立大学巨大的有机体对人类有着固有的吸引力，而极端尺寸的进化和生物物理决定因素长期以来一直引起生物学家的兴趣。在树木中，过去至少有三种（红木、道格拉斯冷杉和山灰树）的高度超过了110米（360英尺）。如今，只有红杉才能长到这样的高度，大量的伐木大大减少了这三种树种的高大个体的数量。巨树的稀缺性，以及持续砍伐和预测的气候变化所带来的威胁的不确定性，强调了对地球上最高的树木进行这项新研究的必要性，这项研究旨在回答有关陆地植物高度限制的基本问题。在土壤湿润的地方，树木长得很高，对光照的竞争也会促进树木的高度增长。目前的研究支持这样一种观点，即当树木长得更高时，限制向树顶输送水的限制会减缓并最终停止树木的高度增长。重力可能通过降低较高树木传导系统内的水压来限制高度的增长，这反过来又增加了叶片的水分压力，降低了光合作用的速率。然而，高大树木光合作用较低的令人信服的证据是缺乏的，因为大多数研究都局限于不到该物种最高记录高度一半的树木。在这项研究中，研究人员将从最大高度的50%到100%获取树木的树冠，以比较三种最高的树种在不同高度的个体之间的水分压力、光合作用和高度增长。这三种树种生长在加利福尼亚北部和澳大利亚的温带雨林中。他们还将利用树木年轮生长记录来了解这些长寿（400至2000年）的物种如何对过去的气候变化做出反应，以及它们如何受到未来气候变化的影响。这项研究的见解将对其他科学学科产生影响。生态系统生态学中一个长期存在的问题是，随着林分生长和老化，净初级生产力明显下降的原因可能包括光合作用的水力限制，这是本研究的主要焦点。此外，高大树木的森林是生物多样性的宝库，这项研究将加强基础设施，为持续的长期监测和生态研究在州和联邦保护区寻求保护剩余的巨树及其依赖的生物多样性。州和联邦保护区红杉树梢死亡的问题是公园管理者的首要任务，这项研究可能会揭示这一现象的原因，以及气候变化是否会加剧这一现象。这项研究在许多方面为人力资源做出了贡献：它加强了对高中生物课的持续推广，这些课程将获得关于高大树木环境条件的实时数据，用于教学目的；支持3名研究生的论文研究；它还为本科生参与实验室研究活动提供支持，包括植物生理学研究和叶片稳定同位素组成的最新分析，叶片稳定同位素组成是植物水分胁迫的有力指标。这两个合作机构都招收了大量的美国原住民学生，并为他们参与研究提供了项目。这些学生和来自其他代表性不足群体的学生将被寻求参与这项研究。

BIOPHYSICAL AND ECOLOGICAL CONSTRAINTS ON MAXIMUM TREE HEIGHT:INSIGHTS FROM THE THREE TALLEST TREE SPECIESGeorge W. Koch1 and Stephen C. Sillett21Northern Arizona University and 2Humboldt State University Organisms of great size hold an inherent fascination for humans, and the evolutionary and biophysical determinants of extreme size have long intrigued biologists. Among trees, at least three species (redwood, Douglas-fir, and mountain ash) have exceeded 110 meters (" 360 ft) height in the past. Today, redwood alone reaches such heights, heavy logging having greatly reduced the number of tall individuals of all three species. The rarity of giant trees, and uncertainty regarding the threats of continued logging and predicted climate change, underscore the need for this new study of Earth's tallest trees, which seeks to answer fundamental questions regarding the height limits of terrestrial plants. Trees grow tall where soils are moist and competition for light places a premium on height growth. Current research supports the view that constraints on water delivery to the treetop slow and eventually halt height growth as trees grow taller. Gravity may constrain height growth by reducing water pressure within the conducting system of taller trees, which in turn increases water stress to leaves and reduces rates of photosynthesis. Convincing evidence of lower photosynthesis in tall trees is lacking, however, because most studies have been limited to trees less than half of the maximum-recorded height of the species. In this study, the researchers will access the crowns of trees from 50% to 100% of maximum height in order to compare water stress, photosynthesis, and height growth among individuals of different heights for the three tallest trees species, which grow in temperate rainforests in northern California and Australia. They will also use tree ring growth records to understand how these long-lived (400 to 2000 yr) species have responded to past climate variation and how they may be impacted by future climate change. Insights from this study will have implications for other scientific disciplines. A long-standing question in ecosystem ecology concerns the cause of the apparent decline in net primary productivity as forest stands grow and age, which may include hydraulic constraints on photosynthesis, a primary focus of this study. Furthermore, forests of tall trees are storehouses of biodiversity, and the study will strengthen the infrastructure for continued long-term monitoring and ecological research in the state and federal reserves that seek to protect the remaining giant trees and their dependent biodiversity. The problem of tree top death of redwoods in state and federal reserves is a high priority of park managers, and this study may shed light on the causes of this phenomenon and whether it may be exacerbated by climate change. The study contributes to human resources in a number of ways: it strengthens ongoing outreach to high school biology classes that will access real-time data on environmental conditions in tall trees for teaching purposes; it supports the thesis research of three graduate students; and it provides support for involvement of undergraduate students in laboratory-based research activities, including plant physiology studies and state-of-the-art analyses of stable isotope composition of leaves, a powerful index of water stress in plants. Both partner institutions have large enrollments of Native American students and programs for their involvement with research. These students and those from other underrepresented groups will be sought for involvement in this study.