Role of biotic interactions in regulating conifer invasion of grasslands

Woody-plant invasions can be regulated by positive and negative interactions with the recipient community, but the sensitivity of these interactions to ontogeny or abiotic conditions is not well understood. We experimentally examined germination, seedling survival and growth of the conifer, Pseudotsuga menziesii, in response to above- and below-ground interactions with resident vegetation in low-elevation prairies of western Washington (USA). Shading, below-ground competition, and soil origin (prairie vs. forest) had strong and, at times, interacting effects on Pseudotsuga establishment. These effects varied among life stages and years. Germination was strongly enhanced by shading and inhibited by below-ground competition during a drought year, but not during a wet year. Germinant and seedling survival were strongly enhanced by shading, inhibited by below-ground competition, and unaffected by forest soil or mycorrhizal inoculation. Seedling growth was strongly negatively affected by below-ground competition, weakly affected by soil origin, and unaffected by shading. No individuals survived a single growing season in the absence of shade and presence of competing vegetation. In contrast, the presence of shade and removal of below-ground competition increased mean survival to ∼85%. Our results demonstrate that biotic factors can regulate woody-plant invasions and that the strength and direction of these effects can differ with ontogeny and abiotic conditions. Although woody-plant invasions are typically attributed to extrinsic factors (e.g., climate, fire, or grazing), our results highlight the importance of biotic interactions as critical intrinsic controls.

► We tested how above- and below-ground interactions regulate conifer establishment in grasslands. ► Directions and strengths of interactions varied with ontogeny and annual climate. ► Biotic interactions represent important intrinsic controls on conifer invasion of grasslands.