HR: 14:40h
AN: GC33B-05    [Abstracts]
TI: Recent Relationships of Tree Establishment and Climate in Alpine Treelines of the Rocky Mountains
AU: * Germino, M J
EM: germmatt@isu.edu
AF: Idaho State University, 921 S 8th St, Stop 8007, Pocatello, ID 83209, United States
AU: Graumlich, L J
EM: lisag@ag.arizona.edu
AF: University of Arizona, PO Box 3308, Tuscon, AZ 85722, United States
AU: Maher, E J
EM: azile77@yahoo.com
AF: Idaho State University, 921 S 8th St, Stop 8007, Pocatello, ID 83209, United States
AB: Changes in the forest structure of alpine-forest or treeline boundaries may be a significant climate response of mountainous regions in the near future. A particularly important point of climate sensitivity for treelines is the initial survival and establishment of tree seedlings - a demographic bottleneck that may be particularly suited to early detection of treeline responses to climate change. However, concise information on climate sensitivity of seedling establishment has come primarily from direct observations of seedlings over short time periods encompassing a few years. Dendrochronological approaches have revealed tree establishment patterns at more extensive time scales of decades to millenia, but at coarser temporal resolutions. Climate variations that most directly affect initial tree seedling establishment occur at annual or smaller time scales, and climate for seedlings is modulated by landscape factors such as neighboring plant cover. Our objective was to assess climate sensitivity of tree establishment at treeline at these finer temporal and spatial scales, with consideration of treeline features that alter the climate for seedlings. Our approach combined direct observations of seedling emergence and survival with dendrochronology of older seedlings and saplings that were still small and young enough (less than 25 years and 20 cm height) to allow detecting the year of establishment and associated factors. Surveys for subject seedlings and saplings were performed for 2 years across the gradient from forest into treeline alpine in the Beartooth, Teton, and Medicine Bow mountains of Wyoming USA. No seedlings or saplings were detected above the highest elevation adult trees or krummholz, but there were up to 0.3 seedlings per square meter in subalpine meadows close to forest (within the timberline zone) where changes in tree abundance appear possible in future decades. Correlations of establishment and summer temperature ranged from weak in whitebark pine (Pinus albicaulis) and Engelmann Spruce (Picea engelmannii) to significantly positive for subalpine fir (Abies lasiocarpa). Seedling establishment was consistently associated with microsite features such as resident trees and herbs that alter sunlight and temperature for small seedlings, and the effect was strongest for subalpine fir and least evident for whitebark pine. For all species and treelines, establishment in microsites with the least amount of overhead tree cover (furthest from forest in the alpine, in exposed locations) occurred in years with warmer summer temperatures. These patterns of establishment are consistent with previous and current experimental studies of terrestrial and solar radiation and temperature effects on tree seedlings at treeline. Our findings indicate that local treeline response to climate variability may vary as a function of current landscape patterns of tree and herb cover, and tree species assemblages that are unique to different treelines. Local shifts in tree species composition that are ongoing may thus pose a significant issue in forecasting future treeline change.
DE: 0476 Plant ecology (1851)
DE: 1632 Land cover change
DE: 1851 Plant ecology (0476)
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting