Modeling productivity and transpiration of Pinus radiata: climatic effects.

Climatic effects on annual net carbon gain, stem biomass and annual transpiration were simulated for Pinus radiata D. Don at Canberra and Mt. Gambier. Simulations were conducted with an existing process-based forest growth model (BIOMASS, Model 1) and with a modified version of the BIOMASS model (Model 2) in which response functions for carbon assimilation and leaf conductance were replaced with those derived from field gas exchange data collected at Mt. Gambier. Simulated carbon gain was compared with a published report stating that mean annual stem volume increment (MAI) at Mt. Gambier was 1.8 times greater than at Canberra and that the difference could be the result solely of differences in climate. Regional differences in climate resulted in a 20% greater simulated annual transpiration at Canberra than at Mt. Gambier but only small differences in simulated productivity, indicating that climatic differences did not account for the reported differences in productivity. With Model 1, simulated annual net carbon gain and annual increase in stem biomass were greater at Canberra than at Mt. Gambier, whereas Model 2 indicated a similar annual net carbon gain and annual stem biomass increase in both regions.