Can seedling functional traits explain liana and tree seedling dynamics in an everwet tropical forest?

Background/Question/Methods

For several decades researchers have reported large increases in the number of woody vines, called lianas, present in tropical forests worldwide. The majority of the work studying liana and tree dynamics in tropical forests has focused on mature plants in seasonally dry forests, where lianas enjoy a competitive advantage over trees during drought due to longer root systems and more water-use efficient leaves. In everwet forests where water is rarely limiting, lianas and trees may have a different competitive balance and there is less evidence for increasing liana abundance. Signals of increased abundances should be detectable at early life stages either through increased seed supply and seedling recruitment or through differential performance such as survival or growth. We examined evidence for increasing liana abundance using a long-term dataset on regeneration dynamics from the Yasuní Forest Dynamics Plot, Ecuador, an everwet forest in the western Amazon. We combined annual demography from over 200 species to track performance since 2003. In a subset of these we also examined the variation in seedling functional trait between trees and lianas and how these contribute to performance differences across environmental and temporal gradients.

Results/Conclusions

We tracked 27,342 seedlings from 15 cohorts recruiting between 2003 and 2018. The number of newly recruited tree and liana seedlings increased over time, but the number of trees and lianas surviving until their fifth year remained unchanged, indicating strong demographic bottlenecks at the earliest life stages. We found differences in liana and tree seedlings in both demographic performance and functional traits when examining species composition across environmental gradients. In drier ridge habitats, successful tree and liana seedlings both invested in denser tissues (low specific leaf area and specific root length) and survived at higher rates than seedlings in other habitats. Otherwise, lianas tended to have cheaper tissues (high specific leaf area and specific root length) and survive at lower rates than trees. These findings indicate that lianas are not becoming more abundant in Yasuní, suggesting the conditions necessary for liana proliferation may be limited to seasonally dry forests. Ultimately, climate change will lead to prolonged drought in some regions of the Amazon and have unknown effects in the western Amazon, making it difficult to predict how these forests’ plant communities will change in the future.