Role of phenology in the colonization success of Rhamnus cathartica in forested ecosystems

Background/Question/Methods

In general, invasive plant species tend to be shade intolerant rarely invading the forest interior.  Rhamnus cathartica (buckthorn) is one exception that readily invades sites with low light levels.   How this is accomplished is unknown, but may be related to its leafing phenology.  Buckthorn breaks bud earlier in the spring and holds leaves later in the fall compared to co-occuring native understory species and the forest canopy.  This phenology may allow buckthorn to take advantage of high light levels prior to canopy closure in spring and after leaf drop in fall.  We hypothesize that this unique phenology is one mechanism that facilitates invasion of the forest interior by buckthorn. To test our hypothesis, we experimentally shaded buckthorn seedlings, reducing high light levels in the spring and fall to simulate intact canopy conditions.  Forty individuals at two sites in central MN, USA were randomly assigned to four treatments: spring shading, fall shading, both spring and fall shading and no shading. We measured spring and fall leafing phenology, light availability and seedling survival and growth.     

Results/Conclusions

Shade structures decreased light to between five and one percent of available light during the spring and fall. Shade structures also led to increases in herbivory by Lepidoptera Tortricidae caterpillars. In the spring shading treatment, 27% to 60% of the leaves were damaged, compared to 6% to 9% for the unshaded spring groups (p<0.001). We ran an ANCOVA with % damage and shade treatment as predictor variables to account for effects of both shading and insect damage on growth.  We found that regardless of insect damage, plants in the spring shading treatment had lower stem diameter growth compared to individuals who received no shading (p=0.006). We saw very little mortality in the first year likely because seedlings were buffered by root stores. We expect increased mortality in subsequent years as seedlings use up carbohydrate reserves, and are unable to replenish them in spring. These results support our hypothesis that high light levels associated with early leafing in spring increase buckthorn growth and thus may provide one mechanism for its success in low light environments of closed canopy forests.