Responses of boreal vegetation to recent climate change
The high northern latitudes have warmed faster than anywhere else in the globe during the past few decades. Boreal ecosystems are responding to this rapid climatic change in complex ways and some times contrary to expectations, with large implications for the global climate system. This thesis investigates how boreal vegetation has responded to recent climate change, particularly to the lengthening of the growing season and changes in drought severity with warming. The links between the timing of the growing season and the seasonal cycle of atmospheric CO2 are evaluated in detail to infer large-scale ecosystem responses to changing seasonality and extended period of plant growth. The influence of warming on summer drought severity is estimated at a regional scale for the first time using improved data. The results show that ecosystem responses to warming and lengthening of the growing season in autumn are opposite to those in spring. Earlier springs are associated with earlier onset of photosynthetic uptake of atmospheric CO2 by northern vegetation, whereas a delayed autumn, rather than being associated with prolonged photosynthetic uptake, is associated with earlier ecosystem carbon release to the atmosphere. Moreover, the photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn. Rapid warming since the late 1980s has increased evapotranspiration demand and consequently summer and autumn drought severity, offsetting the effect of increasing cold-season precipitation. This is consistent with ongoing amplification of the hydrological cycle and with model projections of summer drying at northern latitudes in response to anthropogenic warming. However, changes in snow dynamics (accumulation and melting) appear to be more important than increased evaporative demand in controlling changes in summer soil moisture availability and vegetation photosynthesis across extensive regions of the boreal zone, where vegetation growth is often assumed to be dominantly temperature-limited. Snow-mediated moisture controls of vegetation growth are particularly significant in northwestern North America. In this region, a non-linear growth response of white spruce growth to recent warming at high elevations was observed. Taken together, these results indicate that net observed responses of northern ecosystems to warming involve significant seasonal contrasts, can be non-linear and are mediated by moisture availability in about a third of the boreal zone.
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