Graduate Thesis Or Dissertation
 

Competition release and climate change effects on a moisture-demanding tree species in a dry system

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  • Climate change and the increase in meteorological drought have generated global concern over the persistence of ecosystems already in decline from decreased moisture. Evidence suggests dryland ecosystems have been more impacted by drought because of their tightly coupled growth-water relationships and high sensitivity to environmental shift. Removal of competing vegetation to free available moisture can allow remaining trees to both persist and increase in vigor during periods of drought. However, in the dry forests of the western North America, the magnitude and duration of the growth response of angiosperms to such releases are not well understood. I selected quaking aspen (Populus tremuloides) stands within the dry forests of the Inland Pacific Northwest (PNW), experimentally treated them with conifer removal from within and around the aspen, and assessed them together with historically treated aspen stands. To test aspen vulnerability to future climate change, I used an adapted version of the forest growth model Physiological Principles for Predicting Growth (3-PGmix) with specific parameters for aspen in my region.My objectives were to examine the response of aspen suckers and overstory stems to removal of competing vegetation and the vulnerability of aspen stems to the stressors of future climate change. I found aspen sucker growth and density both increased in the three years following conifer removal. Plant water potential (PWP) was the strongest predictor of the observed increase. Aspen stem radial growth, as measured by basal area increment (BAI), was also greater after conifer removal. The increase over pretreatment growth in average annual BAI continued from post-treatment year 1 to year 11. PWP was the strongest predictor of the increased BAI. The modeling under 3-PGmix found most 40-year old stems are at risk of mortality while 70- and 100-year old stems in medium and high soil moisture sites are not at risk under future climate change effects. My results highlight moisture as the driver of aspen sucker growth, sucker density, and radial growth in the Inland PNW where aspen occurs in small, localized drainage catchments, and differ from Rocky Mountain studies where precipitation and site index are the key drivers. Further, I suggest that release of competition in dry conifer forests increases soil moisture availability allowing successful regeneration and growth of moisture-demanding plants. The rapid aspen mortality that occurred under drought conditions in the Rocky Mountain Region has not yet been documented in the Inland PNW. My study highlights that some climate-induced aspen mortality could be expected by 2025 with older stems and those growing in higher moisture sites not being at risk of mortality while younger stems could be vulnerable to die-off under even modest future climate conditions.
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