A race for light: Understanding sapling architecture for Amazon forest regeneration

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Sarah N. Williams, Chaffey College, Rancho Cucamonga, CA, Nuria Apaza and Roxana P. Arauco-Aliaga, Biological Station Cocha Cashu, San Diego Zoo Global, Peru, Cuzco, Peru, John Terborgh, Biology Department, Duke University, Durham, NC, Marcel Caritá Vaz, Department of Ecology & Evolutionary Biology, University of California, Los Angeles, CA

Background/Question/Methods
The regeneration niche is a key concept to forest restoration. In order to replicate natural regeneration, though, we must first understand the role this niche has on the forest structure. One hypothesis is that, due to allocation tradeoffs between growing tall and expanding the crown, only saplings that invest in height gain (optimists) will be able to become tall adult trees. Saplings that invest into crown development (pessimists), however, become understory trees. Moreover, it is hypothesized that while pessimists can better persist in the shade, optimists can capitalize more when illumination increases. Because forests vary in how dynamic they are, and thus have different light regimes, we compared two contrasting Amazon rainforests in order to test the above mentioned hypotheses. We thus quantified sapling architecture (height, crown projected area and crown thickness) for ~100 species at Cocha Cashu (Peru), which is a more dynamic forest with higher tree turnover rate and thus with a better lit understory compared to Manaus (Central Amazon, Brazil), where we sampled over 400 species. We predicted saplings would tend to be more optimistic in Cocha Cashu compared to Manaus and that their growth strategies would be correlated to their adult sizes (Hmax).
Results/Conclusions
Our results contradicted our predictions, thus rejecting the growth allocation hypothesis. Saplings in Manaus were more optimistic in growth than those in Peru. The saplings of Manaus invest more in height as opposed to crown development, and the Peru saplings invest in crown development versus height. We have some hypotheses to explain the differences between the two forests. First, there are relatively more understory species in Peru compared to Manaus. Since there are more understory species it is possible that our samples were not saplings, but actually adult short trees, and for this reason were displaying relatively large crowns. Additionally, the forest understory in Manaus is dominated by the large fronds of stemless palms, which are often found covering the saplings, who must then invest in height to escape these ultra-shade conditions. Finally, we also found that adult size is decoupled from species regeneration niches in both Cocha Cashu and Manaus, further refuting the original allocation hypothesis. We concluded that the hyper-diversity of the Amazon forest does not allow for simplifications and that we still need a better understanding of the complex coexistence of species in these forests in order to restore them some day.