Tree demography and stand characteristics in a recovering tropical secondary rainforest

Background/Question/Methods

Recent estimates indicate that approximately 60% of all tropical forests are degraded or in secondary succession, and the importance of secondary forests to achieving conservation goals that cannot be met only by protecting remnants of primary forest is likely to continue increasing.  However, the factors influencing recovery rates of secondary forests and the ecosystem services they provide, including biodiversity maintenance and carbon sequestration, are not well understood.  We established 30 m by 30 m monitoring plots in lowland secondary rain forest near Dominical, Costa Rica on land previously logged and grazed but abandoned approximately 15 years ago, with one plot set up in 2006 and two in 2007.  All trees with diameter at breast height (dbh) greater than 2 cm were measured and tagged and their locations mapped.  In 2007-8, all previously tagged trees were remeasured and new recruits identified, measured and tagged.  We used these data to estimate tree mortality, growth and recruitment rates, to characterize stand densities and biomass, and to compare them with published data from similar-aged forests.  

Results/Conclusions

Densities of larger trees (>5 cm dbh) ranged from 1100-1756 per hectare.  Using published allometric relationships, we estimated a total aboveground biomass for all trees (>2 cm dbh) from 117-176 Mg/ha, towards the higher end of values in the literature for tropical forests of similar age (10-242 Mg/ha).  Mortality rates were low from 2006-7 (2.2%, or 29.5 individuals/ha).  We observed a between-year growth in estimated net biomass per hectare of 6% (8.3 Mg/ha).  Most of this change came from recruitment of new saplings reaching 2 cm in dbh; densities increased by 52% (net addition of 925 individuals/ha).  Established individuals were just as likely to shrink (36% of trees), primarily by death of a stem, as grow (33%).  Size distributions were skewed towards smaller individuals, although median total tree dbh varied substantially among the three plots (10.6 cm, 6.0 cm, and 5.7 cm).  In sum, these are forests dominated by small, pioneer trees that currently experience low mortality.  They appear to be steadily accumulating woody biomass, but primarily through recruitment.  Since many existing studies of tropical secondary forests use comparisons of different-aged stands to infer past successional processes rather than marking and monitoring fates of individual trees, long-term monitoring projects such as this one can contribute key information about rates of biomass accumulation and carbon fixation in recovering forests and the processes that control them.