Cedrela odorata L. (Meliaceae: Magnoliidae), which was introduced for timber more than 50 years ago and now covers ~800 hectares on Santa Cruz Island in Galápagos, is replacing the highly threatened and endemic tree Scalesia pedunculata (Asteraceae: Magnoliidae). In spite of the evident replacement of the endemic-dominated forest by Cedrela, no studies had assess 1.if the forest dominated by this invasive tree is following novel trajectories when compared to historic ecosystems, 2.why Cedrela is so successful at replacing native trees and 3. whether or not Cedrela forest can harbor native plant species or be restored to more natural conditions. To answer these questions we established a network of plots located within both forests where we compared in a first stage, community aspects; and on a second phase, conducted experiments to assess Cedrela colonization mechanisms and the feasibility to restore (removing Cedrela adults) this invaded site.
Results/Conclusions .
First, we found Cedrela dominated forest had 37 percent higher non-native plant diversity, and highly dissimilar plant composition when compared to Scalesia forest. Also, the invaded forest had unique abiotic characteristics including a more open and taller tree canopy, and more leaf litter than native forest. Cedrela presence and its associated forest structural changes were the primary factors that best explained higher non-native plant diversity in this apparent novel ecosystem. We hypothesize that Cedrela recruitment is facilitated by diverse set of traits that include shade tolerance, low risk of herbivore attack, and allelopathy that reduces competition. Second, we show that growth (height and dry weight) of seedlings of the native tree Scalesia were significantly reduced in the presence of leaf and root extracts of Cedrela. Specifically, Scalesia seedlings irrigated with Cedrela extracts grew on average ~33% less and weighed 14% less than plants under control conditions (rainwater irrigation). And finally, our Cedrela removals demonstrate that extraction benefited some native species such as Scalesia, but also promoted increased growth and establishment of other invasive plants. In conclusion, our results demonstrate the multiple thresholds Galápagos endangered plant species must overcome to regain novel ecosystems and how some of these biological barriers could be reverted with adequate restoration practices and active management.