Response of habitat quality to mixed severity disturbance regime in Norway spruce forests

Natural disturbances change forest habitat quality for many species. As the extent and intensity of natural disturbances may increase under climate change, it is unclear how this increase can affect habitat quality on different spatial scales. To support management tools and policies aiming to prevent habitat loss, we studied how habitat quality develops in the long run depending on the disturbance severity using a space-for-time substitution approach. We explored the effects of time since disturbance (0-250 years) and disturbance severity (20%-100% canopy removal) on structure-based habitat quality indicators in European primary Norway spruce Picea abies forests using 1000 m(2) circular plots in hierarchical design (a total of 407 plots in 35 stands). Disturbance history was reconstructed from tree cores. Habitat quality indicators were modelled as a function of the severity of the most severe disturbance and the time since this disturbance. We hypothesised that high within-stand habitat heterogeneity is formed by different successional stages after disturbances of various intensities. The results showed a U-shaped response of habitat quality to post-disturbance habitat succession on the plot scale. The decline deepened with disturbance severity. The U-shape response occurred in: large tree occurrence, amount of standing and lying deadwood, diversity of understory and understory openness. The spatial diversity in disturbance parameters increased spatial diversity of habitat quality on a stand level as expected. This high within-stand habitat heterogeneity also decreased with increasing age of the most recent disturbance. This suggests that the absence of young successional stages results in the absence of some important elements for biodiversity, for example sun-exposed snags. Synthesis and applications. Our results demonstrate that currently intensifying natural disturbance regime can consequently result in a lower habitat heterogeneity. In managed spruce forests after natural disturbances, we recommend at least the partial retention of biological legacies to preserve habitat heterogeneity and to avoid uniform and dense plantations resulting in a greater homogenisation. To emulate the natural disturbances pattern, spruce forests should be managed with a wide range of harvested patches of the size limited by a local natural disturbance regime creating spatial heterogeneity.

Automated summary

Natural disturbances can change forest habitat quality, but it is uncertain how this change will be affected by increasing extent and intensity of disturbances under climate change. To understand this, we studied habitat quality in European primary Norway spruce forests using a space-for-time substitution approach. We found that post-disturbance habitat succession has a U-shaped response on plot-scale habitat quality, with greater decline in quality as disturbance severity increases.