Comparative study of ecophysiological traits of introduced and native species in tropical novel forests

Background/Question/Methods Introduced species have been object of worry among scientists and the public in general, because they can become invasive, and potentially cause undesirable consequences to native biodiversity and the ecosystems. Most introduced species that have become dominant are able to outcompete with native species. However, the best fitness is strictly related with suitable environmental conditions, created after disturbances. There are discrepant reports about traits of introduced species in the introduced range, and about how different they are from those of native species growing side by side. I chose to study novel forests of Castilla elastica (Sessé), a naturalized tree in Puerto Rico (considering novel forests as regenerated ecosystems with species assemblages never seen before in the region). Because of its high dominance (37 percent of Importance Value), I hypothesized that C. elastica had higher competitive capacity than native species growing around. To test my hypothesis I used gas exchange measurements (light response curves a year round using an LCpro+) to compare the photosynthetic capacity (A_max=maximum photosynthetic rate at saturated conditions), the intrinsic water use-efficiency (WUE_i=transpiration/stomatal conductance), the photosynthetic nitrogen use-efficiency (PNUE=A_max/nitrogen concentration) among species in the canopy. The specific leaf area (SLA=area/ weight) was also compared.

Results/Conclusions I found that the photosynthetic capacity was similar among species (A_max = 10.7±0.5 vs. 11.5 to 12.1 μmol m^-2 s^-1, for C. elastica and native species, respectively). Native species showed a tendency of higher WUE_i (36 to 50 vs. 33±2 μmol mol^-1), but C. elastica had higher PNUE (67±4 vs. 44 to 53 μmol mol^-1 [F=3.4, p<0.05]) and higher SLA (139±13 vs. 82±1.5 g m^-2 [t=-4.3, p<0.0001]). Due to its similar A_max to that of native species, C. elastica was not considered a classical invasive species. Possibly, the environmental conditions after disturbances have provided optimal growth for C. elastica, which resulted in its dominance. Nonetheless, the higher SLA and PNUE suggest that C. elastica’s dominance could be explained by the higher area to capture light, higher CO₂ diffusion through leaves (thinner leaves exhibit higher mesophyll conductance), and higher nitrogen use-efficiency during photosynthesis. Generally, according to the literature, the higher SLA, PNUE and WUE are traits of those introduced species that have become invasive. Further, broad studies in tropical areas should be done comparing tree community of novel forests in order to produce a short list of the traits that can explain the best performance and dominance of introduced species.