Aspen demographics in relation to spatial context and ungulate browsing: Implications for conservation and forest management

Abstract

Aspen has a high conservation value and has been targeted as a priority species in sustainable forest management. However, sustained and intense browsing pressure by ungulates has raised concerns about the recruitment of aspen to mature growth stages. Here we investigate the influence of ungulate browsing on the demographic structure of European aspen populations in two managed boreal forest areas in Sweden with contrasting amounts of aspen and high densities of ungulates. The contrast in size distribution of ramets in the two areas was striking. The aspen-poor environment contained a much lower density of small-sized ramets than the aspen-rich environment. Ramet abundance was strongly associated with the abundance of ramets in the smaller size classes in both areas, reflecting demographic inertia in these populations. Ungulate use was most strongly associated with abundance of medium-sized aspen saplings growing in forest interiors. Otherwise, use of ungulates was only weakly correlated with aspen population parameters. The strong demographic momentum exhibited by these populations suggests that aspen conservation and management should pay more attention to additional factors than browsing to promote ramet establishment. Large variation in abundance of ramets in the smallest size-class shows that some aspen stands have a disproportionate effect on aspen recruitment, particularly in areas where aspen is less abundant. Identifying aspen stands with a large capacity for self regeneration may be the key to meet aspen conservation goals.