Tree Physiology Advance Access published online on December 6, 2008
Tree Physiology, doi:10.1093/treephys/tpn004
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Changes in net ecosystem productivity of boreal black spruce stands in response to changes in temperature at diurnal and seasonal time scales
1 Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2E3, Canada
2 Corresponding author (robert.grant{at}afhe.ualberta.ca)
3 Faculté de Foresterie et de Géomatique, Pavillon Abitibi-Price, Université Laval, Québec, QC G1K 7P4, Canada
4 Climate Research Branch, Meteorological Service of Canada Saskatoon, SK S7N 3H5, Canada
5 Department of Soil Science, University of British Columbia, Vancouver BC V6T 1Z4, Canada
6 Department of Geography, Worcester State College, Worcester, MA 01602, USA
7 Natural Resources Canada Canadian Forest Service, Laurentian Forestry Center, Québec, QC G1V 4C7, Canada
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Abstract |
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Net ecosystem productivity (NEP) of boreal coniferous forests is believed to rise with climate warming, thereby offsetting some of the rise in atmospheric CO2 concentration (Ca) by which warming is caused. However, the response of conifer NEP to warming may vary seasonally, with rises in spring and declines in summer. To gain more insight into this response, we compared changes in CO2 exchange measured by eddy covariance and simulated by the ecosystem process model ecosys under rising mean annual air temperatures (Ta) during 2004–2006 at black spruce stands in Saskatchewan, Manitoba and Quebec. Hourly net CO2 uptake was found to rise with warming at Ta < 15 °C and to decline with warming at Ta > 20 °C. As mean annual Ta rose from 2004 to 2006, increases in net CO2 uptake with warming at lower Ta were greater than declines with warming at higher Ta so that annual gross primary productivity and hence NEP increased. Increases in net CO2 uptake measured at lower Ta were explained in the model by earlier recovery of photosynthetic capacity in spring, and by increases in carboxylation activity, using parameters for the Arrhenius temperature functions of key carboxylation processes derived from independent experiments. Declines in net CO2 uptake measured at higher Ta were explained in the model by sharp declines in mid-afternoon canopy stomatal conductance (gc) under higher vapor pressure deficits (D). These declines were modeled from a hydraulic constraint to water uptake imposed by low axial conductivity of conifer roots and boles that forced declines in canopy water potential (
c), and hence in gc under higher D when equilibrating water uptake with transpiration. In a model sensitivity study, the contrasting responses of net CO2 uptake to specified rises in Ta caused annual NEP of black spruce in the model to rise with increases in Ta of up to 6 °C, but to decline with further increases at mid-continental sites with lower precipitation. However, these contrasting responses to warming also indicate that rises in NEP with climate warming would depend on the seasonality (spring versus summer) as well as the magnitude of rises in Ta.
Keywords: autotrophic respiration, climate change, ecosys, gross primary productivity, heterotrophic respiration, modeling, net primary productivity
Supplementary data for this article are available at Tree Physiology Online.