Understanding forest ecosystem change in Mongolia: The role of climate and fire in stand dynamics

Projections suggest that a period of rapid climatic transition is underway in northern latitude forests, supported by evidence from both meteorological and ecological studies. Initial research from boreal Russia hypothesized that warming would increase ecosystem productivity as thermal limitations were lifted and growing season length extended. However, recent research indicates a more incoherent picture of ecosystem response to warming, and non-linear responses have been observed. Change has also been documented in disturbance patterns---including fire, which is becoming more frequent and more severe in certain areas. Much of the existing research has focused on the northern edge of the boreal, but what happens at the southern edge will affect the lives and livelihoods of a great many more people through changes in forest resources and ecosystem services. My research is set in the Siberian larch (Larix sibirica) forests of north-central Mongolia, along the southern ecotone of the Eurasian boreal. Recent climate data shows that during the past century Mongolia's rate of warming has exceeded that of Asia and much of the Northern Hemisphere. Fire appears to drive forest recruitment within the forest-steppe, but little is understood about how fire regimes vary spatially and temporally.;This study analyzes 378 fire scar samples, 950 tree demography samples and 1800 tree cores recording forest productivity, collected from 22 sites located along the ecotone. Along with ancillary data on climate I asked three primary questions in order to better understand how global change will impact this, and other, transition ecosystems. Foremost, I sought to determine the role climate and fire play in the timing and intensity of tree recruitment in this region. Secondly, I reconstructed the current and historical fire regimes in this region to determine the role climate has played in driving fire over the past 500 years. Lastly, I examined growth records from over 1200 trees to identify whether moisture or temperature was limiting forest growth and whether forest decline indicative of ecotone retraction was evident. My reconstructions indicate that climate is a strong driver of regionally synchronous fire in this region, and climate and fire interact in contingent processes to drive recruitment. Climate also plays an important role in forest productivity, with moisture availability being the most limiting factor across much of the landscape. Based upon these findings, I conclude that if a drying phase were to accompany the warming already occurring in this region, sub-regional to regional scale fires will occur more frequently, and forest productivity will suffer. Although the region appears to have recently recovered from a severe drought, a warmer and drier long-term climate will likely lead to more rapid shifting and/or fragmentation of the Eurasian southern boreal forest ecotone.