Understanding the role of tree species dominance on forest restoration outcomes in the tropics

Andrew Whitworth, Miguel Muñoz Mazón, Maria José Mata Quirós, Hilary Brumberg, Eleanor Flatt, Ruthmery Pillco Huarcaya, Johan Ortiz, Marvin Lopez, Andres Santana and Adrian Forsyth, Osa Conservation, Osa Peninsula, Costa Rica, Andrew Whitworth, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, United Kingdom, Miguel Muñoz Mazón, MINA, Norwegian University of Life Sciences, Aas, Norway, Ana M. Durán-Quesada, Atmospheric, Oceanic and Planetary Physics Department & Climate System Observation Laboratory, School of Physics, Universidad de Costa Rica, San Jose, Costa Rica, Ana M. Durán-Quesada, Center for Geophysical Research, Universidad de Costa Rica, San Jose, Costa Rica, Ana M. Durán-Quesada, Environmental Pollution Research Center, Universidad de Costa Rica, San Jose, Costa Rica, Ruthmery Pillco Huarcaya, Conservación Amazónica ACCA, Peru

Background/Question/Methods
Active restoration strategies can speed up forest succession and are critical to achieve climate change mitigation and biodiversity conservation goals.The planting of fast-growing pioneer species is a promising strategy to facilitate forest regeneration in the tropics. However, to develop resilient and sustainable restoration strategies in the long term, it is also key to maximize species diversity in restoration efforts. In this context, understanding how trees respond to changes in the dominance and diversity of neighbouring tree species is crucial to guide decision makers and conservation practitioners towards cost-effective restoration methods. Reforestation experiments, where tree species interactions can be manipulated, offer unique opportunities to evaluate the outcomes of these different restoration strategies. In 2017, we planted ~20,000 trees of 60 native species in 40 0.5-ha plots. To understand the role of pioneer tree species dominance on tree growth and survival, we established four treatments where the abundance of the native broad-leaved pioneer tree Ochroma pyramidale (balsa) was manipulated (High 75%, Medium 33%, Low 0% and natural regeneration). Growth and survival of each species was measured in 2017-2021.
Results/Conclusions
After four years of experiment, we compiled preliminary results on tree growth and survival. We did not detect any significant difference in the average tree growth and survival between the restored treatments. However, we observed trends that may predict future outcomes of the experiment. For example, tree survival tends to be greater in the mid-abundance balsa vs the high and low abundance treatments, suggesting a potential effect of balsa abundance on neighbouring trees in the long term. Contrary to expectations, growth tends to be greater in the absence of balsa, most likely due to increased light availability. Together these findings suggest that other variables than tree interactions, such as local soil conditions, may be more important when explaining growth and mortality patterns at these early stages of forest restoration. The long term effect of tree species interactions on restoration outcomes needs to be further investigated.