Effects of a strong drought on Amazonian forest fragments and edges
ABSTRACT.Little is known about how climatic variability affects fragmented forests and their abrupt edges.We contrasted effects of the 1997 El Nin Ë?o drought between fragmented and continuous forests in central Amazonia, using long-term data on tree mortality.For 23 permanent 1-ha plots,annualized mortality rates of trees 10 cm diameter at breast height (dbh)were compared among a 'baseline' interval of 5 -17 y before the drought,a 12 -16-month interval during the drought, and a 12 -13-month interval after the drought,using repeated-measures ANOVA. We also examined the size distributions of dead trees for each interval. During the drought,average annual tree mortality rose signi .cantly in both forest edges (from 2.44%to 2.93%)and nteriors (from 1.13%to 1.91%),and the magnitude of this increase did not differ signi .cantly between edges and nteriors.After the drought,tree mortality declined inall plots,but most dramatically onedges. Mortality rates were more variable over time onedges thaninteriors, and there was no evidence of time lags in mortality.In forest interiors,the size distributions of trees that died did not differ signi .cantly among the three intervals. On edges, however,relatively fewer small (10 -15 cm dbh)and more medium-sized (20 -30 cm dbh)trees died in the post-drought interval,compared to other intervals. Moreover,forest edges lost a signi .cantly higher proportion of large ( 60 cm dbh) trees thandid forest interiors.These results suggest that droughts have relatively complex effects onfragmented Amazonian forests. Drought effects inour forest fragments probably were reduced by prior .oristic and structural changes near edges and by adjoining regrowth forest that partially buffered edge vegetation from desiccating conditions.