Mature and old-growth Douglas-fir forests in the Pacific northwest (PNW) have failed to demonstrate changes in tandem with shifting climate as indicated by lagged range shifts, stable basal area, and little to no species turnover in individual forest stands. Recruitment represents a key process underlying current and future forest structure, and may contribute to explain the apparent climate change resistance of PNW forests. To better understand the mechanisms underlying the lagged response to climate change in PNW forests, we evaluated spatial and temporal patterns of seedling recruitment in relation to climate across a broad elevation gradient. Seedlings were counted, identified, and aged in five long-term monitoring plots in mature and old-growth Douglas-fir forests extending from the coast to the crest of the Cascade Range in western Oregon. Bayesian mixed-effects models for individual species were used to quantify the relationships between climate and seedling densities across the range of conditions and years represented in the data, and a Bayesian sensitivity analysis was applied to quantify the relative contribution of each climate covariate to observed recruitment patterns.
Results/Conclusions
Our preliminary results show that recruitment rates are low across the full range of climatic conditions for all species except western hemlock, a shade-tolerant species. For this species, recruitment did not vary systematically with elevation. Recruitment rates were highest in wetter years and locations and recruitment was most sensitive to summer precipitation. Recruitment rates were lowest in a recent dry year that was characterized by conditions consistent with climate change projections. These results indicate that recruitment is likely to decline across much of the Douglas-fir zone under increasing dry conditions associated with climate change. Recruitment limitation associated with most species appears to limit population and community turnover and is likely a key mechanism underlying lagged climate change responses.