David Montwe

There is increased interest in mixed conifer-broadleaf forests as a way to enhance forest resilience to climate-induced droughts due to complementary functional traits of species. Based on a 23-year-old experiment in coastal British Columbia, Canada, this study assesses the effects of different red alder (Alnus rubra) and paper birch (Betula papyrifera) densities on the drought tolerance of three conifers: coastal Douglas-fir (Pseudotsuga menziesii var. menziesii), western redcedar (Thuja plicata), and western hemlock (Tsuga heterophylla). The broadleaves were planted at three densities either as single species or in a 50:50 mixture in between alternating conifers planted at a 3×3m spacing. Tree-ring data from 202 trees served to evaluate tolerance to a severe drought in 2015. Three indices of relative growth change were calculated: resistance, recovery, and resilience. Resistance is the ratio between pre-disturbance basal area increment (BAI) to during-disturbance BAI, recovery assesses during-disturbance to post-disturbance BAI, and resilience compares pre-disturbance to post-disturbance BAI. Among the conifers, Douglas-fir showed the lowest resistance, recovery, and resilience across treatments. In the high-density alder treatment, western redcedar exhibited increased resistance, and western hemlock displayed significantly greater recovery compared to the control. Additionally, western redcedar exhibited a tradeoff between its resistance and individual tree volume, displaying the highest resistance but the lowest productivity. There were net competitive effects in the birch treatments. Here, western hemlock had significantly lower drought tolerance. There were no specific patterns in the 50:50 alder and birch admixtures. In terms of productivity, there were no differences among the treatments. However, within species, hemlock generally had the highest individual tree volume, followed by Douglas-fir and western redcedar. Overall, the results suggested net facilitation for red alder admixture at 1150 stems/hectare for shade-tolerant western redcedar and western hemlock, enhancing drought resilience. Admixture of birch generally caused net competition for the conifers. This highlights the importance of functional traits like the shade tolerance of conifers along with the density of broadleaves. Delaying broadleaf introduction might be beneficial for managing mixed stands of shade-intolerant Douglas-fir and red alder.

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