Ecosystem effects of removing non-native trees from early successional subtropical dry forests

Background/Question/Methods

In Puerto Rico, introduced tree species frequently dominate early succession after abandonment of agriculture and they can persist for many years after establishment. Many of these non-native pioneers are leguminous and local managers and the public consider most weeds. Eradication of introduced species is often a stated as a management goal in order to protect native species and restore habitat. However, invasive species may be filling manmade niches that native species are unable to colonize and thereby perform important ecological functions. Thus, eradication of introduced species may achieve some goals at the expense of others. We evaluated the effects of removing non-native Albizzia procera trees on biodiversity and nutrient dynamics in replicated plots in the Laguna Cartagena Fish & Wildlife Reserve. We mechanically removed Albizzia by bulldozing, which is common practice in Puerto Rico and compared these plots to intact stands of Albizzia and native tree plantations. We measured understory biomass recovery, species richness, nutrient fluxes, and nitrogen fixation over the course of one year.

Results/Conclusions

Biomass in the bulldozed plots recovered slowly and no seedlings were seen, while the control plots showed a decrease in understory biomass over time.  Species richness was higher in the bulldozed plots compared to adjacent control areas, but the additional species were not native. After 6 months, fluxes of total plant available N were higher in the control plots, but the bulldozed treatments showed similar seasonal trends, eventually dropping to near zero as vegetation returned. Both increased 2 months after the start of the experiment and decreased over time to levels below what was found in the nearby native treatment. N flux in native-tree plots showed little variability over time. The total N flux was determined by the available nitrate in the system, and not ammonium.  Fluxes of available P increased in all treatments over time, but were highest in the native stands. Even though the control plots had nitrogen fixing tree species, nitrogen fixation was not detected in any treatment. At least in the short term, there appeared to be no trade off between diversity and ecosystem function as a result of bulldozing, because vegetation removal resulted in higher species richness and reduced N flux.