Carbon fluxes in novel tropical secondary forests

Background/Question/Methods Ecologists debate the conservation value of ecological functioning of tropical secondary forests including those of novel tropical forests, which are forests that develop naturally in deforested and abandoned lands and whose species composition include introduced species. Are carbon flux rates of novel forests different from those of native secondary forests? This question is important because the area of novel forests is increasing, is likely to increase even more due to climate and global change, and there is uncertainty about their capacity to store and cycle carbon. I studied replicate 50-yr-old secondary forest stands dominated by the introduced tree species Castilla elastica, at Puerto Rico. The study was conducted from 2008 to 2009. My objective was to characterize carbon fluxes associated with photosynthesis (4 replicates measured quarterly in canopy leaves with a LCpro+), annual tree growth (320 trees), annual litterfall (20 baskets collected biweekly), and litter decomposition (20 monthly samples) to see how rates deviated from those of native secondary forests and other comparable novel forests dominated by Spathodea campanulata.

Results/Conclusions I found that the carbon flux through canopy photosynthesis was larger than soil carbon flux by 570 μmolC/m².hr, which accumulated in tree biomass, and that the litter compartment was balanced (annual turnover of 2.2, mean litter standing stock was 2.2 ± 0.1 MgC/ha). Tree growth accounted for 0.2 ± 0.1 MgC/ha.yr and litterfall flux for 4.8 MgC/ha.yr. The carbon flux rates in this novel secondary forest are higher for litterfall (3 ± 0.4 MgC/ha.yr) and for litter turnover (0.5 to 1.8) to those of native secondary forests, but slower than those of novel forests dominated by S. campanulata (6 MgC/ha.yr and 2.8, respectively). The tree growth rate was lower than the tree growth of native secondary forests (1.4 ± 0.05 MgC/ha.yr). The study suggests that one cannot generalize that all introduced species result in carbon flux rates that are higher than rates of secondary forests dominated by native species. Instead, carbon fluxes in novel and native secondary forests exhibit a wide range of variability depending on ecological factors such as site conditions and/or age of forests. Since dramatic and unexpected functional attributes are not found in the C. elastica forest, I could not conclude that the functioning of this novel forest was impaired simply because of the high dominance of an introduced tree species.