Stand Structure and Composition Affect the Drought Sensitivity of Oregon White Oak (Quercus garryana Douglas ex Hook.) and Douglas-Fir (Pseudotsuga menziesii (Mirb.) Franco)

Ze’Ev Gedalof,Jennifer Franks

doi:10.3390/f10050381

Abstract

Due to a suite of environmental changes, Oregon white oak (Quercus garryana Douglas ex Hook; called Gary oak in Canada) associated ecosystems at many North American sites are being encroached upon by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) and other conifer species. Alteration of stand structure and composition is causing substantial changes in the dynamics of these ecosystems, creating an environment in which Oregon white oak is not thriving. In this study we used dendrochronology to investigate the competitive dynamics between Oregon white oak and Douglas-fir in a mixed forest stand on Southern Vancouver Island. Significant species-specific differences in radial growth sensitivity to drought were found between Oregon white oak and Douglas-fir. Oregon white oak trees growing at high densities, or competing with Douglas-fir for moisture were found to be more sensitive to drought and more sensitive to growing conditions during the prior year. The response of Douglas-fir to drought was less variable, possibly due to the relatively low conifer densities at our study site, as well as the species’ ability to root graft, its higher shade tolerance than Oregon white oak, and its rapid growth rates that allow it to achieve a more dominant canopy position. The non-stationary response to climate exhibited by Oregon white oak provides insights into the mechanisms by which Oregon white oak savannas are being converted to coniferous woodland, but also suggest that tree-ring reconstructions of climate need to explicitly address changes in stand dynamics that could influence the growth–climate relationship

Highlights

Anthropogenic activity and environmental changes have the potential to alter intra- and inter-specific competitive interactions in many forest ecosystems [1,2]
Stand Density of thisChronologies study suggest that while density does not affect the growth–climate associations of Douglas-fir at this site (Figure 3) Oregon white oak growing at high densities are more limited by Results of this study suggest that while density does not affect the growth–climate associations drought than Oregon oak growing at low densities (Figureat4).high
This study has provided insights both into the dynamics of Oregon white oak ecosystems and into the effects of competition on limiting factors

Summary

Introduction

Anthropogenic activity and environmental changes have the potential to alter intra- and inter-specific competitive interactions in many forest ecosystems [1,2]. Traits such as shade tolerance, drought resistance, growth rate and form, affect its competitive status within a forest environment [1]. Environmental site conditions such as slope, aspect, soil depth, radiation influx, and soil moisture influence climatic sensitivities and patterns of forest dynamics [3,4]. Changing competitive interactions over time may influence stand trajectories, structure, and composition, potentially affecting disturbance dynamics [5]. While the effects of changing stand structure and composition on disturbance regimes, soil properties and erosion are reasonably well understood, the role of competition in both mediating

Methods

Results

Conclusion