Does landscape context affect how forests change after ice storm damage?

Lloren and colleagues recently had their paper ‘The influence of landscape context on short‐ and long‐term forest change following a severe ice storm‘ published in Journal of Ecology. Read more about their work in the post below, written by co-author Jenny McCune.


Investigating the interplay of natural and anthropogenic disturbance

Around the world, deforestation has reduced forests to smaller patches surrounded by agricultural fields and housing. The composition of the plant communities in these patches changes as a result – for example, there is often an influx of disturbance-associated species from abundant seed sources in the non-forest matrix. These forests also continue to experience natural disturbances like wildfire, hurricanes, and ice storms. Understanding the interactions between anthropogenic and natural disturbances is crucial for predicting future changes in forest composition, with increasing deforestation and changing climate (see e.g., Catterall et al. 2008, Laurance & Cochrane 2001, Smart et al. 2014).

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At the edge of a forest patch completely surrounded by cropland (Photo: J.L. McCune).

The chopping up of large forests into smaller chunks could affect forest response to natural disturbances in two ways. First, it can change the frequency or degree of damage caused by natural disturbances. For example, small isolated forest fragments may experience forest fires less often, if fire can’t spread across non-forested areas. However, these fragmented forests may experience greater damage from hurricanes because most of the remaining forest is close to edges, which are more susceptible to wind damage. Second, it could affect which species move in after the canopy is damaged. In landscapes with little forest cover, and in stands close to the forest-matrix edge, seeds and seedlings may be dominated by disturbance-adapted, shade-intolerant species. As these species may be more likely than interior forest species to colonize canopy gaps. This second effect is more difficult to test, because we need data on forest composition over the years and decades following a natural disturbance.

Our study is based on a set of permanent forest plots near Ottawa, Canada, surveyed immediately following and three years after a severe ice storm in 1998. We resurveyed the plots again in 2017, so we could measure changes in woody stem density, species richness, and community composition 3 years and 19 years after the storm. We measured the total amount of forest within 1km of each plot, and the distance from each plot to the forest-matrix edge. Then, we tested for interactions between the amount of canopy loss from the storm and these two measures of landscape context.

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Ice storms are frequent disturbances of forests in northeastern North America. Thick layers of ice formed on branches cause breakage and canopy loss (photo credit: Deb)

We found significant interactions between landscape context and the amount of canopy loss. In the short-term, shifts in community composition were greater with more canopy damage. This effect was magnified in plots with less forest on the landscape and (unexpectedly) in plots farther from the forest-matrix edge. Over the long-term, highly damaged plots were likely to return towards their original species richness and composition only if they were far away from the forest edge. For plots close to the forest-matrix boundary, the storm may have caused a permanent shift in species richness and community composition.

It is clear that if we want to predict how forests will respond to storms in the future, we have to keep in mind that the landscape context in which these communities are embedded will affect their response. Human modification of forested landscapes has not only a direct, but also an indirect effect on the composition of the remaining forest communities, by influencing how they respond to natural disturbances.

Jenny McCune, University of Lethbridge, Canada

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