The direct regeneration hypothesis in northern forests

Question Can the direct regeneration hypothesis (DRH) be used to predict post-disturbance regeneration after fire, wind disturbance, and clearcutting in northern forests? Do life-history traits such as regeneration strategy and shade tolerance influence post-disturbance regeneration success of tree species? Location Northern forests in North America. Methods A meta-analysis was conducted by collecting published data on pre- and post-disturbance stand compositional characteristics in the northern forests. For each tree species, compositional difference (CD) was calculated as the difference between basal area proportions of the post- and pre-disturbance stands, but for post-disturbance stands < 25 years of age, post-disturbance proportions were calculated based on relative stem density. Results Species response to disturbances was best explained by regeneration strategy, while disturbance type had no effect on CD. The proportion of broadleaf trees with either strong or weak vegetative reproduction ability increased after all disturbances. Serotinous species had CD values not significantly different from zero after fire, while CD for semi-serotinous species was negative. The post-disturbance proportions of non-serotinous conifers decreased after all forms of disturbance. Conclusions All disturbances promote broadleaf trees, regardless of regeneration strategy (suckering, sprouting, or seeding). The DRH is supported for conifers with serotinous cones after fire. Fire causes local extinction of non-serotinous conifers, while wind and clearcutting only decrease the proportion of non-serotinous conifers because of partial survival of seed sources and advanced regeneration. This study suggests that increasing stand-replacing disturbances associated with global climate change will promote broadleaf trees in northern forests.