Ontario Ministry of Natural Resources and Forestry Sault Ste. Marie, ON, Canada
Background/Question/Methods
Appropriately quantifying effects of species interactions on ecosystem function is essential to understanding the impacts of biodiversity loss and development of resource management practices and policies. In biodiversity-ecosystem functioning research, the effects of biodiversity on ecosystem functions are generally quantified through the null expectation by comparing species growth in mixtures relative to their monocultures. The determined positive biodiversity effects can result from both complementary and competitive interactions, resulting in confusion and misunderstanding on the effects of species interactions on ecosystem functions. To determine biodiversity effects that reflect species interactions, we develop a new measure called ‘competitive expectation’, i.e., more competitive species are expected to grow more and less competitive species are expected to grow less in mixtures relative to their monocultures. Because existing experiments lack partial density monocultures, we utilize a growth and yield simulation model to generate data required by the competitive expectation, including full and partial density monocultures and different compositions of mixtures, for trembling aspen and white spruce mixedwood forests in Alberta, Canada.
Results/Conclusions
Competition effect increased as the proportion of white spruce in a stand increased due to greater growth response of the fewer more competitive trembling aspen to intraspecific density reduction, while complementarity effect peaked at equal mixtures when the two species are intimately mixed. Both competition effect and complementarity effect increased with age due to cumulative effects of species interactions. The complementarity effect resulted primarily from competitive reduction, as a weak facilitative interaction (i.e., species observed yield in mixture > partial density monoculture yield) occurred only on spruce in nearly pure spruce mature mixture, possibly due to improved nutrient availability by aspen. Averaged across all ages and compositions, the net biodiversity effect estimated via the null expectation was 14.3 m3 ha-1. On a relative basis, 67% (9.6 m3 ha-1) of the net biodiversity effect was attributable to complementarity effects (including 2% species interference) and 33% (4.7 m3 ha-1) to competition effects. The competitive expectation enables ecologists to determine which communities are truly benefitting from diversity in the form of positive species interactions and which are just resulting from among-species differences in growth and competitiveness, an approach that is not possible with current methods.
