University of Montana, Maureen and Mike Mansfield Library
Abstract
Western larch (Larix occidentalis) is an important tree species exclusive to the inland northwest region of North America. It is very intolerant of shade but managed across a range of communities with both shade-tolerant and -intolerant species. Recent works have shown that tree and stand level competition in mixed-species communities can be modified depending on how the characteristics of species in a mixture interact. Such changes can alter the growth relationship of the participant species. Western larch growth has been well-characterized in empirical growth models, where growth is typically estimated from a complex of size, competition, density, and site productivity variables. Although these models have been characterized using data which span a wide-range of conditions, the effects of interspecific relationships on western larch growth dynamics have yet to be studied.
We used a network of long-term forest growth and yield research plot data to investigate (i) how mixtures modify the growth of individual western larch after accounting for other factors that influence growth and (ii) how to characterize species-mixing effects on western larch within the context of individual tree growth modelling. First, we used a generalized additive mixed model (GAMM) to estimate western larch annual basal area increment from size, competition, density, and site productivity variables, without addressing species-mixing. This model was then used in two separate analyses. In the first analysis, we identified different types of species mixtures and compared how the established western larch growth relationships in each mixture differed from those in pure larch stand conditions. In the second analysis we identified three species-informed metrics and added each to the established model. This resulted in three models which were compared to assess how each species-mixing metric affected the established growth relationship.
These analyses resulted in a GAMM that represented size, competition, density, and site variables with functional relationships similar to those identified in previous works. With this model, we showed that the way in which larch growth is characterized can differ when western larch is growing in mixtures compared to when growing in a pure stand with other western larch. When comparing species-mixing metrics in the third analysis, we showed that western larch growth may be higher in mixtures with species of higher shade-tolerance as well as in mixtures with other species in general
