Assistant Professor Oklahoma State University Stillwater, OK, United States
Abstract: Tropical forests are the most biodiverse ecosystems in the world and are critical to global ecosystem functions. Human-mediated disturbance has increased forest fragmentation, yet the effects of fragmentation on the mechanisms maintaining high tropical tree diversity are widely unknown. Fragmentation by agricultural expansion has the potential to introduce new biotic interactions into tropical forests via spillover, particularly transmission of pathogenic fungi. Fungal pathogens commonly cause conspecific density-dependent seedling mortality during recruitment which could maintain a high plant diversity in tropical forests. As global temperatures continue to rise, coffee farmers in Costa Rica are forced to move their plantations to higher elevations, resulting in forest fragmentation and an increased risk of fungal pathogen spillover. This study aimed to understand the effects of fungal pathogens on the tree seedling community in the wet tropical premontane forests in Costa Rica. We hypothesized that spillover of fungal pathogens could lead to an overall decrease in seedling diversity and negative density dependence near the forest edge. Two transects were set up with ten 1x1m plots at every ten meters from 5-95 m into the forest interior in four different forests (n=80). Plots at each ten-meter mark were randomly assigned to be sprayed with 50 mL of water or Amistar fungicide weekly. Seedling height, leaf damage, and stem diameter were recorded bi-weekly and mortality was recorded weekly for 8 weeks from late May to mid-July 2022.
Preliminary results show variable responses of tree seedling communities to the forest edge and to the fungicide treatment across forests. There was a general increase in seedling survival, seedling growth (height), and diversity as distance from the forest edge increases. In contrast, percent leaf damage due to fungal pathogen significantly decreased with distance from the forest edge and plots treated with fungicide had significantly less pathogen presence compared to untreated plots. Survival was significantly influenced by conspecific and heterospecific density, treatment, and the interaction between treatment and the densities for the forest interior of one forest and edge of another. These data provide insights into how forest fragmentation and proximity to agriculture is affecting the forest community structure at the tree seedling level in tropical forests near coffee plantations. It also provides useful feedback on how certain coffee management types may be influencing these patterns which can be relayed to farmers and foresters in this region.
