Propagule pressure is an important predictor of common buckthorn abundance, regardless of disturbance history: Evidence from Minnesota’s deciduous forests

Background/Question/Methods

Introduced plants can invade and dominate forest ecosystems, causing ecological and economic harm. Several previous studies have found propagule pressure, or the number of seeds introduced to a site, to be among the most important drivers of invasion. Others have suggested that disturbance, or a change in disturbance regime, can increase a site’s susceptibility to invasion due to an increase in available resources. Our study examined the relative influences of propagule pressure and disturbance, as well as other environmental characteristics, on forest invasibility.

We focused on common buckthorn (Rhamnus cathartica L.), an abundant invasive plant in the Upper Midwest that was introduced to North America from Europe. Among other impacts, it has been shown to change nutrient cycling and forest floor conditions. We surveyed native and exotic vegetation at 53 upland oak-dominated hardwood forest sites in Minnesota, USA, some that had been harvested for timber and others without recent major disturbance. At each site we measured abiotic and biotic site characteristics, including resident plant diversity, soil pH and texture, and light levels. We also conducted novel driving surveys to measure landscape-scale propagule availability. We used multiple regression to examine the relative contributions of the various factors to site-level buckthorn abundance.

Results/Conclusions

Propagule pressure had the strongest influence on within-site buckthorn abundance, with less buckthorn present in sites farther from forest edges and houses, and more in sites where more buckthorn was counted in the driving surveys. Litter depth, which was strongly associated with exotic earthworm invasion stage, was also negatively correlated to buckthorn abundance. The best-fit multiple regression model contained propagule pressure index and litter depth and accounted for 39% of the variability of within-site buckthorn abundance (p<0.0001). Harvested sites differed from undisturbed sites by having greater mean light levels (p<0.0001), greater mean herbaceous and woody species richness (p<0.0001, p=0.01, respectively), lower mean non-buckthorn basal area (p<0.0001), lower mean herbaceous phylogenetic diversity (p<0.0001), and greater mean woody phylogenetic diversity (p=0.05). However, buckthorn abundance did not differ between undisturbed and harvested sites, even when accounting for propagule pressure. Our results have forest management implications since uninvaded forests close to established buckthorn populations are more likely to become invaded. Close monitoring of these stands can help contain new buckthorn populations. While our results did not indicate higher susceptibility to invasion following timber harvests, managers should be aware that harvest sites in areas of high propagule pressure have a greater chance for invasion.