Temporal and spatial variability in seedling dynamics: a cross-site comparison in four lowland tropical forests

Spatial and temporal variation in seedling dynamics was assessed using records of community-wide seedling demography collected with identical monitoring methods at four tropical lowland forests in Panama, Malaysia, Ecuador and French Guiana for periods of between 3 and 10 y. At each site, the fates of between 8617 and 391 777 seedlings were followed through annual censuses of the 370–1008 1-m2 seedling plots. Within-site spatial and inter-annual variation in density, recruitment, growth and mortality was compared with among-site variability using Bayesian hierarchical modelling to determine the generality of each site's patterns and potential for meaningful comparisons among sites. The Malaysian forest, which experiences community-wide masting, was the most variable in both seedling density and recruitment. However, density varied year-to-year at all sites (CVamong years at site = 8–43%), driven largely by high variability in recruitment rates (CV = 40–117%). At all sites, recruitment was more variable than mortality (CV = 5–64%) or growth (CV = 12–51%). Increases in mortality rates lagged 1 y behind large recruitment events. Within-site spatial variation and inter-annual differences were greater than differences among site averages in all rates, emphasizing the value of long-term comparative studies when generalizing how spatial and temporal variation drive patterns of recruitment in tropical forests.

Altitudinal treelines of the southern Andes near 40ºS

In the southern Andes near 40ºS, altitudinal treelines are dominated by Nothofagus pumilio, a broadleaf deciduous angiosperm in the beech family (Fagaceae). Treeline elevations, ranging from 1100 to 1500 m a.s.l., are influenced by regional climate and volcanism. At the local scale, disturbance influences treeline elevation, ecotone length, and vegetation productivity. Decadal and interannual variation in climate related to El Niño-Southern Oscillation (ENSO) significantly affected radial growth of krummholz trees and seedling demography; however, climate-treeline relations were complex. Radial growth of krummholz trees and seedling demography responded differently to climate variation. These relations differed between climate regions and were unstable through time. We conclude that inter-annual variations in climate, such as those associated with ENSO, will be critical for successful reproduction and growth of Nothofagus pumilio at treeline in the Andes under the influence of global warming. Key words:Argentina, Chile, climate change, disturbance, forest dynamics, global warming, northern Patagonia, Nothofagus pumilio, South America, timberline