Effects of winter snowpack, fire and forest structure on invasive plant establishment

Background/Question/Methods

In montane forests of western North America, climate and forest management, in particular winter snowpack, fire, and forest harvest practices, strongly interact to influence vegetation structure. At higher elevations in these mountain ranges, where invasive plants are relatively less abundant, changes in these dynamics may interact to provide recruitment opportunities for invasive plants spreading from lower elevations. With the expectation of decreased winter snowpack duration due to climate change, longer growing seasons may promote population growth of invasive plants. Furthermore, with predicted increases in both fire occurrence and fuel reduction treatments via forest thinning, disturbance at higher elevations may also favor plant invasions. We transplanted two invasive shrubs, Scotch broom and Spanish broom, above their invasive range in a mixed-conifer forest in the Sierra Nevada of California. We investigated how fire and forest thinning interact with changes in winter snowpack to affect establishment success of these invasive shrubs. We planted seeds of each species in a nested block design across three forest structures: untreated forest, thinned forest, and clearcuts, and subjected a subset of blocks in thinned forest to prescribed fire. During winter 2012, we either increased or decreased snowpack levels at a subset of plots within each block.

Results/Conclusions

We documented effects of snowpack level, fire and forest structure on two establishment stages: winter survival of seedlings, and spring germination. Effects varied by species. Scotch broom, the shrub with the higher invasive elevation range, was unaffected by winter snowpack level at either establishment stage, regardless of forest structure or fire. For Spanish broom, however, which has a lower elevation range than Scotch broom, both spring germination and winter survival were higher under the reduced snowpack treatment. Both species had the highest germination rates in fuel treatments, and adding prescribed fire to thinned forest further increased germination rates. However, we document an interaction between fire and snowpack for Spanish broom, such that fire stimulated increased germination only in treatments receiving reduced snowpack. We conclude that, particularly for low-elevation invasive plants that may be more vulnerable to extended frost, reductions in snowpack have the potential to facilitate greater establishment success, particularly in forests managed using fuel treatments and active fire. These results suggest that future changes in climate may constrain management options in montane forests where invasive plants are a primary management concern.