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Accueil du site → Doctorat → États-Unis → 2019 → Fire-driven alternative states and spatial variability in forest resilience at a dry forest ecotone

Pennsylvania State University (2019)

Fire-driven alternative states and spatial variability in forest resilience at a dry forest ecotone

Harris, Lucas

Titre : Fire-driven alternative states and spatial variability in forest resilience at a dry forest ecotone

Auteur : Harris, Lucas

Université de soutenance  : Pennsylvania State University

Grade : Doctor of Philosophy (PhD) Geography 2019

Semi-arid forests worldwide are being altered by climate change and changes in the frequency and severity of disturbances. Future vegetation change may be especially rapid at forest/non-forest ecotones where conditions are already marginal for tree establishment and survival. However, recent and future vegetation change at landscape scales within these ecotones depends to a great extent on variability in water balance over complex terrain as well as patterns of existing vegetation types and structures, which may amplify or buffer climate change effects. This dissertation investigates influences on past tree cover change, post-fire tree regeneration and fire severity and extent at a forest-steppe ecotone in the rain-shadowed eastern Sierra Nevada, California, USA. Analysis of aerial photographs from 1953/1955 and 1999/2002 at four sites revealed 1.5 to 5-fold increases in forest cover over that period due in part to fire exclusion and livestock grazing. Increases in tree cover occurred in more mesic topoclimatic settings and in areas that were initially close to forest but with sparse tree cover, suggesting an influence of both water balance and tree density and spatial configuration on tree cover change over time. Field surveys of tree regeneration 9–15 years after wildfire at the same four sites revealed a strong influence of water availability, though not drought stress, on tree regeneration. However, the nearby presence of live trees was also highly important to tree regeneration, likely due to a combination of seed source limitations and buffering effects of trees on microclimate. An analysis of area burned, fire severity and fire-fire interactions conducted across the length of the eastern Sierra Nevada revealed an increase in area burned after 1998 related to rising summer temperatures. Overall, this work suggests that tree cover in these semi-arid ecotonal forests was strongly affected both by fire exclusion and by recent wildfire, but that over landscape scales forest dynamics hinged on variability in water balance and interactions between vegetation, local temperature and moisture and fire. By linking past tree cover change to subsequent wildfires and then post-fire tree regeneration, this dissertation investigates the geography of fire-initiated alternative vegetation states over landscapes, with potential implications for explaining and predicting vegetation change in other semi-arid forest/non-forest ecotones and rain-shadow environments worldwide.


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