Barriers to tropical rainforest restoration: Tree seedling regeneration in a degraded tropical pasture

Background/Question/Methods

The current scale of deforestation and continuing decline of tropical forests underscores an important need to restore degraded tropical landscapes. Research here focuses on highly degraded habitats, such as pastures, where forest regeneration may be impeded or no longer possible. The study site, a former cattle-ranching pasture, borders the tropical rainforest of Los Tuxtlas. To determine tree recruitment success within pasture habitat, seedling experimental exclosures were built to mitigate herbaceous vegetation and abundant pasture rodent fauna. Exclosures evaluating seed removal by ants and rodents were also built in an adjacent pasture. Experimental seedling exclosures tested 1) the degree that pasture herbaceous vegetation inhibits or facilitates recruitment of tree species of varying successional types, 2) the intensity of seedling removal by pasture rodent fauna, and 3) the prevalence of life-history traits that correlate with seedling establishment success. Seed experimental exclosures tested 1) the rate of seed removal by ant, rodents or both on seeds of various seed masses and 2) the effect that forest proximity had on seed removal rates.

Results/Conclusions

After one year, half of 16 sowed species failed to establish in any treatment conditions within the seedling experimental exclosures. The presence of grass significantly diminished the recruitment potential of recruiting seedlings (p = 0.036). Access to rodents in experiment treatments also significantly reduced species seedling establishment (p = 0.003). Sowed in equal quantities, large seeded, persistent tree species established in higher numbers across all treatments (p = 0.044). In seed experimental exclosures, small seeds were removed at faster rates compared to large seeds by ants and rodents. (p << 0.005). Large seeds removal by rodents was species specific and not determined by seed mass.  Early results from this study suggest that rodents and ants can be a major barrier to early tree regeneration in degraded tropical habitats. This being said, for most seedling species higher establishment was observed in treatments absent of mitigated barriers with the exception of ant which could not be removed. Despite this, tree establishment potential via sowing in degraded habitat appears to be a manageable enriching effort. Future work which addresses between and within species phenotypic variation will advance the understanding of regenerating plant responses in degraded tropical habitat.