Invasive earthworms interact with abiotic conditions to influence the invasion of common buckthorn (Rhamnus cathartica)

Abstract

Common buckthorn (Rhamnus cathartica L.) is one of the most abundant and ecologically harmful non-native plants in forests of the Upper Midwest United States. At the same time, European earthworms are invading previously glaciated areas in this region, with largely anecdotal evidence suggesting they compound the negative effects of buckthorn and influence the invasibility of these forests. Germination and seedling establishment are important control points for colonization by any species, and manipulation of the conditions influencing these life history stages may provide insight into why invasive species are successful in some environments and not others. Using a greenhouse microcosm experiment, we examined the effects of important biotic and abiotic factors on the germination and seedling establishment of common buckthorn. We manipulated light levels, leaf litter depth and earthworm presence to investigate the independent and interactive effects of these treatments on buckthorn establishment. We found that light and leaf litter depth were significant predictors of buckthorn germination but that the presence of earthworms was the most important factor; earthworms interacted with light and leaf litter to increase the number and biomass of buckthorn across all treatments. Path analysis suggested both direct and moisture-mediated indirect mechanisms controlled these processes. The results suggest that the action of earthworms may provide a pathway through which buckthorn invades forests of the Upper Midwest United States. Hence, researchers and managers should consider co-invasion of plants and earthworms when investigating invasibility and creating preemptive or post-invasion management plans.

Single and interactive effects of deer and earthworms on non-native plants

“…Complex and unpredictable interactions among multiple factors indicate that impacts and subsequent management of invasive plant species cannot be adequately studied and managed without consideration of the myriad other influences on forest ecosystems, including other co-occurring non-native species (Rauschert and Shea, 2012;Kuebbing et al, 2013b;Roth et al, 2014), that may directly or indirectly confound experimental results or obscure interpretations. Taken together, our results emphasize the critical role of white-tailed deer in forest ecosystems and highlight the need to concurrently study multiple factors in order to understand mechanisms of plant and earthworm invasion success.…”

Increased light availability due to forestry mowing of invasive European buckthorn promotes its regeneration

“…Our results indicate a nonlinear response of buckthorn to mulch that could result in forestry mowing unintentionally enhancing buckthorn germination rates in sites with intermediate buckthorn density and consequently intermediate mulch depth. The unimodal germination results may reflect a tradeoff between mulch enhancing soil moisture retention and thus germination, and physically obstructing emergence, and detection, following germination (Brockway et al ; Roth et al ). This tradeoff and consequent unimodal response have not been reported in the context of forestry mower mulch, but literature on leaf litter layers suggests that similar mechanisms could be responsible for similar trends with respect to leaf litter depth (Kostel‐Hughes et al ; Roth et al ).…”

“…This was done to ensure that variation among plots in plant cover did not introduce variation in availability of soil resources or light. There was no litter layer present at the experimental site, likely due to the history of buckthorn invasion and persistent, severe invasion by earthworms (Heneghan et al ; Suárez et al ; Roth et al ). Once the plots were reduced to bare soil, a 0.5 m × 0.5 m sampling area was divided into four subplots using plastic garden dividers (Fig.…”

“…These impacts are part of a positive feedback loop involving earthworms, wherein both buckthorn and earthworms facilitate the other's success through niche‐construction mechanisms (i.e. invasional meltdown; Simberloff & Von Holle ; Simberloff ; Roth et al ). Buckthorn has the greatest success in forest gaps or edges, with a moist but not saturated soil (Kurylo et al ).…”

Human-mediated introduction of geoengineering earthworms in the Fennoscandian arctic

“…Note that the organic surface horizons (Oi and Oe) have been completely mixed into the mineral subsoil, resulting in a thick (*20 cm) Ahorizon in heavily invaded arctic forests main reasons for our concern. First, the observed geoengineering earthworm species are known to be drivers of dramatic environmental changes in other formerly glaciated environments, including negative effects on native plants and preferential selection for non-native plants and graminoids (Roth et al 2015;Craven et al 2016), increased greenhouse gas emissions (Lubbers et al 2013) and depletion of key nutrients in surface soils . We also note that the earthworm-driven modifications to soil morphologies appears to largely resemble observations from North American forests (Fig.…”