Masters Thesis Abstract by Annette Nana Zolbrod (1996)

Vegetation in ecosystems located at high elevations may be particularly sensitive to changes in environmental conditions resulting from CO2-induced global warming. The gap model ZELIG was used to examine the effects of increased temperature (2oC) and altered precipitation on subalpine meadows, forests, and montane forests along elevation gradients in the Olympic Mountains, Washington, USA. Changes in tree species distribution, composition, and abundance as well as stand biomass were examined on north and south aspects in the dry NE and wet SW regions of the Olympics. Current vegetation composition and structure was sampled to provide an empirical database for model parameterization and calibration. Warmer, warmer/20% wetter, and warmer/20% drier climate changes were simulated for 1000 years from initial stand conditions. Trees establish in all current meadow sites in all warmer climate scenarios. Dominant tree species shift upwards 300-600 m in elevation in the SW region regardless of aspect and precipitation change, with subalpine meadows and Tsuga mertensiana forests being replaced by Abies amabilis forests at the higher elevations, and T. heterophylla forests replacing A. amabilis forests at the lower elevations. Stable stand composition was reached in 600-800 years. In the NE region, high elevation, drought-tolerant species become dominant approximately 200 m lower than present, with A. Lasiocarpa dominating the north aspect and Pinus contorta the south. Species composition and biomass change occur more rapidly after the onset of climate change (within 150-200 years) in the NE than the SW. Biomass levels increase in the SW and generally decrease in the NE, depending on aspect and precipitation regime. The different precipitation regimes have a greater effect on species composition and abundance in the dry NE than in the SW. Precipitation also affects the rate and elevation of transition between species dominance as well as rankings of subdominant species. Transitions between species dominance occur more rapidly and at slightly higher elevations in the drier climate change scenario than for the wetter scenario. Changes in vegetation patterns occur by responses in individual species, and vary by region and aspect. These results suggest species and site-specific response at meso- and micro-scale resolutions should be included in models that predict the impacts of climate change on vegetation in mountainous regions.