Barriers to Tropical Rainforest Reforestation: Tree Seedling Regeneration in Degraded Pasture

The current scale of deforestation and decline of tropical forest biodiversity stresses an important need to restore degraded tropical landscapes. This research focuses on highly degraded pasture habitat where tropical forest recovery may be impeded or no longer possible. Inhibition of tree recruitment in pasture was investigated using four different approaches: 1) comprehensive meta-analysis of available peer-reviewed data on major regeneration barriers (Chapter 2), 2) experimental evaluation of dominant biotic rodent-grass pasture components (Chapter 3), 3) experimental evaluation of prevailing abiotic drought-solar radiation pasture conditions (Chapter 4) and 4) calculation of consecutive recruitment stage probabilities to assess overall seedling survival probability assessment (Chapter 5). The four different approaches illuminate processes controlling tree recruitment in pasture habitat. The meta-analysis revealed that while the overall effect of pasture habitat suppresses tree propagule abundance, some life-stages (e.g. seed removal) can be positively impacted and are associated with different life-history strategies (e.g. pioneer, persistent) (Chapter 2). The exclosure experiment revealed, as expected, that aggressive pasture grasses reduce tree seedling establishment. In addition, an inhibitory effect driven by pasture rodents on seed-seedling abundance was confirmed using the exclosure design (Chapter 3). An outdoor growing experiment was similarly utilized to test if morphological traits regulated seedling survival under water and heat stress characteristic of pasture habitat. Growing house results did not reveal reduced survivorship under sun-dry conditions. Morphological traits that increased seedling survivorship under water and heat stress included number of leaves and root length (Chapter 4). Finally, estimation of recruitment stage transition probabilities found a marked regeneration bottleneck between seed removal to germination transitions (Chapter 5).