Will aspen (Populus tremula) disappear from protected old-growth forest areas: long-term patterns of aspen dynamics

Abstract

Conservation of forest biodiversity often relies on protected areas. However, protected areas cover only a marginal proportion of land. Furthermore, their characteristics may change, for example, due to natural succession. European aspen (Populus tremula) is a keystone species for boreal forest biodiversity but there is an obvious risk of disappearance of aspen from nature reserves as a result of stand succession. More specifically, the long-term dynamics of aspen populations in conservation areas is not known empirically. For example, it is unclear if there is recruitment to balance the mature tree mortality and maintain long-term persistence of aspen in these areas.

In order to evaluate aspen dynamics in protected forests, we conducted areawide, full-coverage surveys in 15 nature reserves in North Karelia, Finland to see if a lack of recruitment is seen in the long-term age-distribution of aspen. These surveys were conducted twice over a span of 18 years, one being in 1999(1) and the other in 2017.

We found declines of on average 75 % for dead aspens and 41 % for living aspens per area, with similar declines per ha, and in volume per area and per ha between 1999 and 2017. Human-caused notching had increased the amount of dead wood in 1999 way beyond a natural level, which was also reflected in the deadwood to living wood ratio. The recent estimate from 2017 probably reflects naturally occurring mortality patterns of aspen as the deadwood to living wood ratio declined. Tentative analyses also indicate distribution shifts in diameter at breast height and decay classes (separated in fallen, standing whole trees and standing broken trees) during this 18 year period. Moreover, it appears that there is a continuous lack of recruitment despite a high amount of regeneration in each area. The current recruitment occurs primarily on open areas and edges, often adjacent to managed forests or roads bordering the conservation areas.

Our study showcases the highly dynamic nature of forest conservation areas, due to naturally occurring stand succession, and its possible consequences for biodiversity. Aspen maintains a diverse group of other species. If aspen vanishes from conservation areas like the ones studied, it may lead to the extinction of several other species. It seems evident that conservation of aspen-associated species is impossible only with the current network of conservation areas. The solution is likely to be found in managing the surrounding commercial forests for these species, although further research into this issue is required.

1. Kouki, J., Arnold, K., & Martikainen, P. (2004). Long-term persistence of aspen - a key host for many threatened species - is endangered in old-growth conservation areas in Finland. Journal for Nature Conservation, 12(1), 41-52. doi:10.1016/j.jnc.2003.08.002