Mixing tree species and density management to reduce drought susceptibility in coastal plantation forests of British Columbia

The coastal forests of British Columbia have been experiencing longer and more intense droughts in recent years. To evaluate the response of species composition and planting density (500, 1000, and 1500 stems ha −1 ) to drought, a study was conducted in a plantation consisting of 1:0, 1:1, 1:3 and 0:1 Douglas-fir ( Pseudotsuga menziesii var. menziesii (Mirb.) Franco): western redcedar ( Thuja plicata Donn ex D. Don in Lamb.) mixtures, located along the east side of Vancouver Island, Canada. Measurements were taken to evaluate soil moisture, drought tolerance, and water use efficiency of these stands. Soil moisture was significantly lower in the highest compared to the lowest density stands ( p = 0.016). Drought indices calculated from tree cores showed that drought resistance, resilience and recovery increased with decreasing stand basal area. Water use efficiency (WUE), inferred from wood δ¹ ³C, was significantly affected by density × mixture × species interaction ( p = 0.020) but not seasonal variation ( p = 0.155). In the 1:3 mixture, western redcedar at the lowest density exhibited higher WUE than at 1000 and 1500 stems ha⁻¹ ( p = 0.038 and 0.005, respectively) but pure western redcedar at moderate density (1000 stems ha⁻¹) appeared to have the highest overall WUE, exceeding several other treatment combinations by 1.32–1.92 ‰ ( p < 0.001–0.047). The results indicate that reducing stand basal area, which can be achieved by mixing species with different growth rates and controlling stand density, can help reduce the drought susceptibility of these forests. • Stand basal area, but not tree height, influenced drought tolerance. • Drought tolerance improved with lower stand basal area. • Water use efficiency varied by species, mixture, and planting density interaction. • Silvicultural design can reduce drought susceptibility in coastal forests.