New study shows that restoration areas previously degraded by logging may face long-term regeneration failures due to high seedling mortality rates.
Restoring logged rainforest areas is an important effort in bringing global biodiversity back. But how effective are these active interventions for forest recovery compared to naturally regenerating areas or intact forests? New research finds that restoration sites after logging experience higher seedling mortality and lower species richness – raising the question about long-term regeneration success.
In the rainforests of Northern Borneo a team of scientists monitored the growth of over 5000 seedlings for a year and a half to compare the recovery of areas used for logging to intact forests 30 years after logging activities ceased. In their new research, now published in Global Change Biology, the team differentiated three different types of forest areas: intact forests, naturally regenerating forests, and actively restored forests (i.e. by planting trees).
While initial results show a similarly high number of seedlings sprouting in intact and restored forests compared to naturally recovering ones, the benefits of active restoration did not last long. By the end of the experiment, actively restored forests experienced the highest seedling mortality, cutting their population down to the size found in naturally regenerating forests. In addition, species richness (a measure of biodiversity), began to significantly drop in actively restored forests, too, only six months into the experiment.
Dr. Robin Hayward from the University of Stirling, said: “We were surprised to see restoration sites having lower seedling survival. After such a productive fruiting event in the restored forest, it’s disappointing that so few were able to survive – and to think what this might mean for the long-term recovery of different tree species.”
What makes seedling survival so difficult in restored logged areas? Lead author Dr. David Bartholomew, formerly at the University of Exeter and now at Botanic Gardens Conservation International, explains: “Our findings suggest that seedlings are experiencing stress in logged forests. Logging induces changes to canopy structure, microclimate, and soil nutrient cycles, and current restoration treatments are insufficient to eliminate this stress. In particular, highly specialised species seem to struggle to survive, leaving communities with reduced species diversity compared to intact forest.”
Dr. Daisy Dent of ETH Zurich and the Smithsonian Tropical Research Institute brings another key factor up: “Animals that eat seeds – like bearded pigs – may be drawn into restored forest patches to eat the more abundant seeds and seedlings, rather than moving into adjacent poor-quality logged forests. In natural forests, however, animals potentially move more freely and so do not exhaust seed supplies in the same way.”
Ecosystem changes due to logging, seed availability, and seed predation: the study shows how complex rainforests are. Dr. Lindsay F. Banin of the UK Centre for Ecology and Hydrology sums up: “Together, these results reveal that there may be bottlenecks in recovery of particular elements of the plant community. We are now progressing this research to better understand the various stages of the regeneration process – fruiting, germination, establishment and causes of mortality – to help understand which mechanisms are driving the patterns we have observed and how we can better assist forest regeneration and support the long-term sustainability of degraded forests.”
Selective logging of forests is prevalent throughout the tropics, and their long-term recovery is crucial to maintaining carbon stocks and high biodiversity. What do these findings mean for current restoration practices as outlined in the UN Kunming-Montreal Global Biodiversity Framework and the UN Decade on Ecosystem Restoration? While active restoration does enhance initial seed production and benefits forest biomass accumulation (total amount of growth), carefully designing, monitoring, and adaptively managing restoration projects is of utmost importance – otherwise the long-term recovery of biodiversity and carbon storage is likely to face even more challenges.
The paper titled ‘Bornean tropical forests recovering from logging at risk of regeneration failure’, published in Global Change Biology can be accessed here: https://doi.org/10.1111/gcb.17209.