Stuck between the Mandibles of an Insect and of a Rodent: Where Does the Fate of Ash-Dominated Riparian Temperate Forests Lie?

Abstract

The beaver (Castor canadensis Khul) is a key species that is known to shape the composition of riparian forests. Ash trees (Fraxinus spp.) can be abundant in these forests. However, invasion by the emerald ash borer (Agrilus planipennis Fairmaire) in North America threatens their survival. The disappearance of ash will have a large impact on the riparian forest composition in itself. It is not known what the consequences would be for the remaining forest if ash plays an important role in the beaver diet. Inventory plots across an ash gradient were measured in Plaisance National Park, Quebec, Canada, to collect data and to establish if (1) trees and saplings of this genus were selected or avoided by beavers, (2) if other genera had a lower or a greater probability of being consumed compared to ash, and (3) if ash density could affect the probability of consumption of other genera. Of all genera present in the park, ash trees were selected in the highest number of plots. Only two genera, Carpinus and Populus, had a higher probability of being consumed than ash. These genera are not abundant in the park, and neither in riparian forests of the temperate biome, and thus are not good candidates to replace ash as a staple for beavers. The most abundant genus in riparian temperate forests, along with ash, is Acer. In this study, Acer trees were not selected, and as for Acer saplings, were less likely to be consumed than ash. Mixed results were obtained about genera that could become more likely to be consumed as ash density decreases. It would seem that the disappearance of ash would not cause a switch to a single or a few genera in the future, which may be due to the high diversity of genera present in temperate riparian forests. However, ash may not disappear completely due to its capacity to sprout following the death of the aboveground portion of ash trees. This scenario is discussed in light of the susceptibility of intermediate-sized ash stems to be colonized by the emerald ash borer and of the greater likelihood of beavers to feed on these same-sized stems.