Responses of boreal vegetation to recent climate change

Barichivich, J (2014) Responses of boreal vegetation to recent climate change. Doctoral thesis, University of East Anglia.

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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.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Users 2259 not found.
Date Deposited: 09 Jul 2014 11:52
Last Modified: 09 Jul 2014 11:52

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