Assessing Differences in Competitive Effects Among Tree Species in Central British Columbia, Canada

Peter L. Marshall,Stella Britwum Acquah

doi:10.3390/f11020167

Peter L. Marshall, Stella Britwum Acquah

Open Access

https://doi.org/10.3390/f11020167

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Abstract

Research Highlights: We investigated the competitive interactions among three tree species (interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), interior spruce (Picea glauca [Moench] Voss × Picea engelmannii Engelm.), and lodgepole pine (Pinus contorta Dougl. Ex Loud. var. latifolia Englem.)) in multi-aged stands in central British Columbia, Canada. Background and Objectives: Understanding competitive interactions among tree species in mixed-species stands is fundamental to supporting silvicultural decision-making in such stands. Using the periodic annual basal area increment for single trees as our dependent variable, we investigated whether neighboring trees competed with subject trees independently of species identity. We also examined the differences in single-tree basal area growth among the three conifer species over time under different levels of competition. Materials and Methods: We developed several spatially explicit, single-tree basal area growth models for interior Douglas-fir, interior spruce, and lodgepole pine using data from 16 plots in two blocks of a long-term study (five measurements over a 21-year period) on the response to pre-commercial thinning. We compared these equations to assess whether intraspecific or interspecific competition predominated. We also examined the differences in basal area growth among the three conifer species over time under different levels of competition. Results: We found asymmetrical relationships between the conifer trees and their neighbors for all species, indicating that the main driver limiting growth in these stands is aboveground competition for light. There was evidence of higher intraspecific competition for small (<10.0 cm DBH) interior Douglas-fir in one block. However, there was no general pattern among larger subject trees with respect to the identity of neighborhood competitive effects and the equivalence of neighbors. We observed a higher level of basal area growth over time for interior Douglas-fir than for lodgepole pine and interior spruce, irrespective of the competition intensity and, not surprisingly, the growth rate declined with increasing competition levels for the three species. Conclusions: Our results provide an understanding of how interior Douglas-fir stands will develop over time and information on species interactions that could help forest managers explore different silvicultural options and their effects on individual tree growth in these complex stands.

Highlights

Mixed-species forest stands are increasingly gaining attention as studies have highlighted their importance in providing higher levels of ecosystem services and functions [1]
We observed a higher level of basal area growth over time for interior Douglas-fir than for lodgepole pine and interior spruce, irrespective of the competition intensity and, not surprisingly, the growth rate declined with increasing competition levels for the three species
We examined 13 widely used competition indices with respect to their ability to contribute to the prediction of tree basal area growth using a similar dataset to that employed in this study (Table 2)

Summary

Introduction

Mixed-species forest stands are increasingly gaining attention as studies have highlighted their importance in providing higher levels of ecosystem services and functions [1]. Mixed-species stands may be more productive and have a higher resistance to biotic and abiotic disturbances [2,3]. Differing interactions among trees (competition (negative), facilitation (neutral or positive), and competitive reduction) are widely described in the literature [2]. Oliver and Larson [4] describe competition as an interaction between trees that results in one individual obtaining a greater share of growth resources (growing space) than another, leading to that individual gaining dominance over the other. The three processes are difficult to differentiate and are collectively referred to as complementarity [5], i.e., the effect of a species mixture on growth. Under high abiotic stress, interactions shift towards facilitation (as described by the stress-gradient hypothesis) [7]

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