Net primary production, production allocation, and foliage efficiency in second growth douglas-fir stands with differing site quality
Creators: name: Kurz, Werner Alexander
Publisher: name: University of British Columbia
Year: 2010

description: Several of the current generation of computer models which simulate biomass production in forest ecosystems require a quantitative understanding of the effects of site quality on foliage efficiency (the amount of biomass produced per unit of foliage) and on carbon partitioning between above- and belowground stand components. This study investigated changes in foliage efficiency and carbon allocation in 12 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands for which the site indices ranged from 19.5 to 41.3 m at 50 years. These stands are located on Vancouver Island, British Columbia, Canada. Regression models for aboveground biomass components were developed from 39 destructively sampled Douglas-fir trees. Foliage biomass was predicted with a model which uses diameter at breast height (dbh) and a competition index as independent variables. This model predicts that stand foliage biomass stabilizes after canopy closure. Diameter and tree mortality data of the 12 Douglas-fir stands were available for 15 to 16 years, and were used to calculate aboveground and coarse root biomass and annual production estimates. In 1985, aboveground biomass in the 12 stands, with ages from 32 to 70 years, ranged from 135 to 573 Mg ha⁻¹. Coarse root biomass was estimated to be equal to 20 - 23% of aboveground biomass. In the period between 1985 and 1987, annual aboveground production (ANPP) in these 12 stands ranged from 4.7 to 16.0 Mg ha⁻¹ year⁻¹. Coarse root production was estimated to be equal to 13 -16% of aboveground production. Fine (0-2 mm) and small (2-5 mm) root biomass and production estimates were derived by analyzing soil cores collected in five of the stands on 5 to 6 sampling dates over a 12 month period. All five stands showed similar seasonal dynamics in live fine root biomass, with the highest values occurring in May and the lowest values in October. In May 1985, biomass of living fine and small roots ranged from 1.82 to 7.91 Mg ha⁻¹ and from 0.59 to 4.10 Mg ha⁻¹, respectively. Three different methods of computing production and mortality were assessed. Different estimates were obtained for annual production and annual mortality in both fine and small roots, because fine root mortality exceeded production during the year which had a very dry summer. Estimates derived using one of the computational methods (decision-matrix) ranged from 1.12 to 5.14 Mg ha⁻¹ year⁻¹ for fine root production and from 2.15 to 4.89 Mg ha⁻¹ year⁻¹ for fine root mortality. Small root production and mortality estimates based on this computational method ranged from 0.51 to 2.22 and from 0.88 to 2.13 Mg ha⁻¹ year⁻¹, respectively. With increasing site index, a decreasing proportion of total production was allocated to belowground stand components. The site with the lowest site index allocated about 31 to 51% of total net production to belowground components while the site with the highest site index allocated about 23 to 30% belowground. About 56% of the variation in 72 estimates (12 stands and 6 measurement periods) of foliage efficiency based on aboveground production was accounted for by a regression model with foliage biomass and site index as independent variables. This model suggests that there is an optimum foliage biomass at which total aboveground production is maximized and that this optimum foliage biomass increases with increasing site index. The results of this study emphasize the importance of understanding variation in canopy function and shifts in carbon allocation from above to belowground stand components. Forest ecosystem production simulation models should include an explicit representation of changes in foliage efficiency and carbon allocation patterns to be able to accurately predict the responses of forest ecosystems to changes in environmental conditions and to silvicultural treatments.