© 1996 Heron Publishing—Victoria, Canada
Declining forest productivity in aging forest stands: a modeling analysis of alternative hypotheses
Danuse Murty (1), Ross E. McMurtrie (1) and Michael G. Ryan (2)
1. School of Biological Science, University of New South Wales, Sydney, NSW 2052, Australia / 2. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, 240 West Prospect Road, Fort Collins, CO 80526-2098,
USA / Received March 2, 1995
Summary
Several explanations have been advanced to account for the decline in forest net primary productivity (NPP) with age in closed-canopy
stands including the hypotheses that: (1) sapwood maintenance respiration rate increases, reducing the availability of carbon
to support new growth; (2) stomatal conductance and hence photosynthetic efficiency decline; and (3) soil nutrient availability
declines. To evaluate these hypotheses we applied the ecosystem model G’DAY to a 40- and a 245-year-old stand of lodgepole
pine (Pinus contorta Dougl. ex Loud.), growing on infertile soils. Net primary productivity estimated from biomass data was 0.47 and 0.25 kg C
m–2 year–1 and foliar nitrogen/carbon ratio (N/C) was 0.0175 and 0.017 for the 40- and 245-year-old stands, respectively.
Productivities of the young and old stands were derived from a graphical analysis of the G’DAY model. The graphical analysis
also indicated that the observed age-related decline in NPP can be explained in terms of interacting processes associated
with Hypotheses 2 and 3. However, the relative importance of these two hypotheses differed depending on key model assumptions,
in particular those relating to variation in soil N/C ratio. Thus, if we assumed that soil N/C ratio can vary significantly
during stand development, then Hypotheses 2 and 3 jointly explain the decline in NPP, whereas if we assumed that soil N/C
ratios are constant, then Hypothesis 3 alone explains the decline in NPP. The analysis revealed that only a small fraction
of the decline of NPP can be explained in terms of increasing sapwood respiration.
Keywords:
forest decline, net primary productivity, nitrogen availability, photosynthetic efficiency, Pinus contorta, respiration, stand age, woody litter.
