BayCEER Workshop 2026
08. October 2026
Seasons in the Forest Understory: Tracking Seedling Performance Across a Tropical Rainfall Gradient
2 Yale School of Environment
1.2 in Morning Session
08.10.2026, 11:30-11:45, H 36
Tropical forests are home to approximately 70% of the planet's terrestrial biodiversity. Most experience a distinct dry and wet season, and this seasonality is a fundamental driver of tropical forest dynamics. The seedling stage is an important bottleneck in tropical forest composition. This study examines the role of seasonality for seedling performance, how it varies across space and time, and whether patterns are altered during strong El Niño years, associated with drought.
The study was conducted across a pronounced rainfall gradient at the Isthmus of Panama. In eight 1-ha forest sites, we monitored growth and survival of woody seedlings. Over 5 years, including two El Niño years, we collected data before and after each season.
Both seedling mortality and growth differed strongly between seasons and sites: Mortality was consistently higher in the dry than in the wet season in all sites across the rainfall gradient. In both seasons, mortality increased towards drier sites, and seasonal differences changed little across the rainfall gradient. Growth patterns differed: In the dry season, seedling growth decreased towards wetter sites, but in the wet season it increased. Seasonal differences in growth consequently switched along the rainfall gradient, with higher dry-season growth on the dry side of the gradient, and higher wet-season growth on the wet side. During the strong 2016 El Niño event, dry-season mortality and wet-season growth peaked, significantly amplifying seasonal differences. These results highlight water limitation's key role in seedling mortality, while growth patterns indicate light and natural enemies are more important.
The study provides important insights into factors shaping seedling dynamics in tropical forests, and the effects of extreme climate events. To improve understanding of species' responses to climate and better predict tropical forest shifts, we will link plant functional traits to species responses.
Keywords: seedlings, seasons, mortality, growth, El Niño, ENSO, rainfall gradient, dry season severity
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