Non-native Tree Species Can Help Restore Forests in Tropical Landscapes
Catterall, C. P. 2016. Roles of non‐native species in large‐scale regeneration of moist tropical forests on anthropogenic grassland. Biotropica 48:809-824.
Understanding how and why forests regenerate in tropical landscapes is a bit like detective work. In some places, deforested pastures lie fallow for decades with only shrubs and ferns; in others, natural regeneration can be rapid, lush, and biodiverse. The difference lies in many factors – how the land was used and for how long; the amount of forest in the surrounding landscape; rainfall; soils and bedrock; and species initially present on the site. Understanding these processes is critical to predict the ultimate outcomes – the species and function of the forests that recover.
Some of these factors are challenging for restoration practitioners to control, but we know that the outcomes of regeneration can be vastly different depending on the types of species initially at the site. In particular, non-native species are often viewed as negative factors, many resources are often spent to remove them. But this choice isn’t always based on evidence, and is not necessarily effective or even possible. This paper illustrates that non-native species can either enhance or inhibit forest recovery depending on the type of species and the context. The functional role of the species is more important in how they affect forest recovery than is the origin of the species. An example from work in Australian rainforest demonstrates this clearly – although non-native pasture grass inhibited forest recovery, scattered native and non-native pasture trees stimulated succession. Because non-native pioneer trees were both short-lived and helped to recruit native species, the proportion of natives to non-native species increased rapidly over time. Furthermore, because of their ability to survive in harsh conditions, certain non-native species, such as camphor laurel (Cinnamomum camphora), are especially well suited to stimulate native regrowth early on.
Studies from around the world support these results. Some non-native species–such as herbs, grasses, vines and ferns–suppress tree growth. But tree species can go either way. Some non-native trees inhibit growth of native trees. For example, trees adapted to low-light conditions may be able to persist under the canopy of a forest and crowd out native species. In contrast, short-lived pioneer species can enhance the recovery of native species early in forest regeneration. Potentially useful non-native species for restoration have a short lifespan; high fecundity; seeds capable of wide dispersal across cleared landscapes; seeds predated by forest fauna (but not pasture fauna); light-demanding seedlings with high growth rates and high competitive abilities; and adult trees that are both attractive to seed‐dispersing fauna and not strongly competitive with seedlings or saplings of desired later‐successional native species.
In the case of tropical forests, non-native species can play a key role in stimulating succession. Although current conservation goals are often targeted at removing non‐native species, high cover or abundance of a non‐native species does not necessarily mean that its removal will facilitate forest restoration. A more strategic management plan could involve tolerating non-natives in the beginning, removing them only once succession is underway (when short-lived species may already be declining on their own). Achieving large‐scale tropical forest restoration will require increased recognition of their multiple roles, and costs and benefits of allocating resources to their removal.
The PARTNERS connection
Dr. Catterall participated in the inaugural PARTNERS meeting in 2014 and then again in 2015. Her perspective and wealth of experience on forest restoration in Australia provided a vital contribution to a PARTNERS special issue on natural regeneration for forest and landscape restoration in Biotropica.