Restoration of northern temperate rainforest: An application of principles from studies of old-growth forests

Background/Question/Methods

It is a well known ecological principle that habitat heterogeneity and resource availability are key drivers of biological diversity. It is also generally recognized that most managed ecosystems, or ecosystems that are effected by human disturbances tend to have less structural heterogeneity than those that are maintained by natural disturbances.  The Tongass National Forest designed an experimental canopy gap treatment for second growth stands over 20 years ago to test these ideas.  I report on findings from a two year study on remeasuring these canopy gap treatments on Prince of Wales Island in Southeastern Alaska. Key question we were asking is: does restoration of spatial heterogeneity through canopy gaps lead to a greater sustained diversity and deer habitat quality as compared to untreated or thinned forests?

Results/Conclusions

Seventy-six gaps were selected for sampling representing a broad geographic, and ecological range of stand conditions throughout the region.   Our data shows statistically significant increases in species diversity, understory cover, forb biomass, and shrub annual growth for gap plots as compared to either thinned or untreated controls.  Canopy gap treatments create habitats that are on average 4 times the deer carrying capacity of  our thinned second growth stands in the summer or over 8 times the carrying capacity of thinned sites in the winter. A key finding here is that after 20 years nearly all the understory vegetation response from thinning treatments is lost, while canopy gaps still maintain high levels of diversity.  Our analysis of these plots suggests that canopy gaps may represent one of the most effective techniques for long-term improvement of biodiversity in second growth forests of the region. It is well known that complex spatial structure is one of the defining characteristics of old-growth temperate rainforests. Creation of canopy gaps as illustrated here offers a promising means to help restore this critical ecosystem structure, and several of its associated functions.