It gets better with age

Secondary and degraded forests are one of the world’s most rapidly expanding ecosystems and now form a large proportion of global tropical forests. If secondary forest ecosystems could recover the biological diversity inherent in mature forests, their increasing spatial extent provides great potential for inexpensive large-scale restoration of biodiversity in human-impacted landscapes.

The recent paper by Goosem et al investigated tree, shrub and liana species diversity in a chronological sequence of secondary forest sites ranging in age between 3 and 60 years since formation of closed tree canopy in a region that was cleared up to 110 years ago. This aged chronological sequence of secondary rainforest sites provides information about growth and trajectories of secondary forests over a longer time frame than is generally possible elsewhere in the tropics.

The aims of this research were to:

They found no correlation between secondary rainforest age and the number of individual trees. Species richness estimates were generally low in secondary forests when compared with mature forests.

Older secondary forest sites were more similar in species composition to mature forests than younger sites. However, secondary forest age categories were more similar to each other than to mature forests.

They found that age of the secondary rainforest was an important predictor of increasing tree diversity and increasing similarity of the secondary plant community composition to that of a mature forest. To a lesser extent, they found that distance to mature forest and soil fertility also influenced the assemblages of species.

Though they observed a trend of gradual increase in similarity to mature forest in both diversity and species composition as the secondary forests aged, even their oldest sites remained clearly very dissimilar from mature forest. They found that neither species diversity nor species composition had reached mature forest levels even 30–60 years after the regrowth forest had achieved a closed canopy and up to 70 years following pasture abandonment.

Their analysis detected an influence of isolation on species richness and composition and lesser influences of soil fertility and texture on species composition. This finding emphasises the importance of resource and propagule availability in a landscape that has been cleared and subject to grazing for long periods.

Another finding was that dispersal and recruitment of larger-fruited species was adversely affected, resulting in the observation that species recruited at longer distances from primary forest fragments were almost entirely small-fruited and wind-dispersed species. They suggest that, in general, where the extent of mature forest remaining in the landscape is relatively small, dispersal from mature forest seed sources is limited. Only about 10% of mature forest cover remained in the landscape near their secondary forests, which they suggest explains the large number of mature forest species that were not detected even in the oldest secondary forest sites. In a larger-scale spatial analysis of all the secondary forest in their study area, size and isolation of mature forest remnants greatly influenced the extent and distribution of secondary forests (see related article).

Interestingly, the research found that if remnant trees were associated with a patch of secondary regrowth, their presence canaccelerate succession through a range of biotic and abiotic processes. Some of these processes include providing perching and foraging habitat for vertebrate dispersers, improving soil conditions for seedling establishment, shading out competitive grasses and reducing the microclimatic extremes associated with open habitats. Planting of small groups of trees has been postulated as an effective, less intensive and less expensive method to restore forests than large-scale restoration. The results reported in this paper provide support for this approach, implying that remnant trees may accelerate succession even after long periods of land use.

Given the extensive deforestation of the region, these secondary forests are important for biodiversity conservation even though they are currently not a replacement for mature forests. Secondary rainforests contain important plant diversity, particularly as the age of the regrowth increases. The findings of this study emphasise the importance of the protection of mature forest tracts and fragments for increasing landscape resilience by facilitating recovery of plant diversity in secondary forests. Given the very high cost of rainforest restoration through tree planting programs, passive restoration by encouraging and facilitating the development of secondary rainforest is extremely important to heavily cleared tropical landscapes in restoring ecological function, services and resilience.