BIOPHYSICAL AND ECOLOGICAL CONSTRAINTS ON MAXIMUM TREE HEIGHT: INSIGHTS FROM THE THREE TALLEST TREE SPECIES
George W. Koch1 and Stephen C. Sillett2 1Northern 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.
