A pie lover's paradise: Forest canopy openings and nitrogen fertilization cause a differential increase in wild blackberry (Rubus spp.) cover

Background/Question/Methods

The cover of blackberry (Rubus spp.) in the herbaceous layer of an Appalachian forest increased dramatically during 23 years of nitrogen (N) fertilization. The initial study compared a fertilized and unfertilized watershed in the Fernow Experimental Forest near Parsons, WV. However, the stand ages differed by 70 years, complicating the interpretation of the results. To better assess the effect of N inputs on blackberry, we compared two even-aged watersheds and included canopy openness as a covariate for this shade-intolerant species. We hypothesized that the effect of canopy openness on blackberry depends on N-fertilization. To test this idea we measured the cover of blackberry in a fertilized and an unfertilized watershed with stand ages of ca. 43 years. The fertilized watershed was last cut in 1970 and has received 35 kg N/ha*yr as ammonium sulfate ((NH₄)₂SO₄) since 1989. The unfertilized watershed was cut in stages from 1963-1966, maintained barren with herbicide until 1969, and then recovered naturally. Since the watershed comparison is not replicated, measurements of blackberry cover were also made in a nearby long-term soil productivity experiment (LTSP). The LTSP experiment is a randomized block design with 4 replicates of both fertilized (35 kg N/ha*yr as (NH₄)₂SO₄) and unfertilized plots in a forest re-growing after harvest in 1996.  Blackberry cover was estimated in 90 plots in each watershed, and in 6 plots within each of the four treatment replicates in the LTSP experiment. Canopy openness was measured using a spherical densitometer.

Results/Conclusions

Two-way analysis of covariance revealed that the effect of canopy openness on the relative cover of blackberry in the watersheds depended on N fertilization (p = 0.002).  Specifically, blackberry cover in the fertilized watershed was 82.5% higher at the highest level of canopy openness when compared to the unfertilized watershed but it was not different at the lowest levels of canopy openness. Results from the LTSP experiment show a similar trend (p = 0.076).  Collectively, our results indicate that increased N availability dramatically increases the dominance of blackberry only where there is sufficient light. They also explain why blackberry can be abundant early in secondary succession – when there is increased light and a disturbance initiated N pulse – only to decline in importance after canopy closure.  Thus, in regions of high N deposition, management practices or other changes that increase canopy openness will create conditions in which blackberry can dominate the herb layer and potentially reduce overall diversity.