© 2004 Heron Publishing—Victoria, Canada
Modeling topographic effects on net ecosystem productivity of boreal black spruce forests
Robert F. Grant
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2E3, Canada (robert.grant@ualberta.ca) / Received September 26, 2002; accepted June 7, 2003; published online December 1, 2003
Summary
Current regional estimates of net primary productivity (NPP) of boreal black spruce overlook the large variation in NPP caused
by small-scale topographic effects on soil water, temperature and nutrient availability. Topographic effects on black spruce
NPP could likely be modeled by simulating the lateral and vertical movement of water, and its effects on soil nutrient transformation
and uptake, through three-dimensional watersheds defined by aspects and slopes of their topographic positions. To examine
this likelihood, the ecosystem model ‘ecosys’ was run for 120 years on a transect that included upper- and lower-slope positions
and a basin in which a basal water table was set 0.5 m below the soil surface. For the run, we used soil properties and weather
conditions recorded at the 115-year-old BOREAS Southern Old Black Spruce site. Short-term model performance was tested by
comparing diurnal and annual carbon (C) transfers simulated under 1994 weather conditions during the 115th year of the model
run with those measured at this site during 1994 by eddy covariance, surface chambers and allometry. After 115 years, annual
spruce NPP simulated at the upper-slope positions was twice that at the basin (350 versus 170 g C m–2), whereas accumulated wood C was almost three times as large (6.8 versus 2.4 kg C m–2). In the model, increases in NPP and wood growth in upper-slope positions were caused by lower soil water contents, higher
soil temperatures, and more rapid O2 uptake that accelerated heterotrophic respiration and hence nutrient mineralization and uptake. Modeled differences in wood
growth with topographic position were quantitatively consistent with measurements of boreal black spruce at several research
sites differing in water table depth. Modeled differences also agreed with differences in wood growth rates derived from allometric
measurements at boreal black spruce sites differing in productivity indices as a result of differences in subsurface hydrology.
The magnitude of these differences clearly indicates the importance of accounting for subsurface hydrology in regional estimates
of boreal forest productivity.
Keywords:
boreal forests, ecosys, ecosystem modeling, NEP, NPP, topography.
