||It is difficult to overemphasize the importance of tropical forests for the ecological services they provide to humans across the planet and their pivotal role in conserving terrestrial biodiversity. They are estimated to hold half of the total stock of carbon in the world’s vegetation, and produce 34% of the total gross primary productivity of terrestrial systems across the planet. This represents the largest contribution to Earths' productivity from a single biome and is more than four times the contribution from boreal and temperate forests combined. Tropical forests also support staggering levels of biodiversity, and are notable hotspots for almost all terrestrial plant and animal groups, with many more species being described each year.
Yet humid tropical forests are also highly threatened by human activities. Deforestation rates in tropical forests remain high across the world, with estimates varying between 8 and 10 million hectares yr-1 from 2000-2010. Of equal concern is the fact that much of the remaining forests have already been degraded through the combined and often synergistic consequences of patch isolation and edge effects, the over-exploitation of timber and non-timber resources, fire and climate change. Degradation of these human-modified tropical forests (HMTFs) can be defined as a continuum of decline in the provision of ecosystem services resulting from increasing levels of unsustainable human-related impacts. Some 850 million ha of tropical forest were estimated to be degraded at the turn of the millennium although the true extent of degraded forests is likely to be much greater given the difficulties of reliably measuring the extent and intensity of different impacts.
Addressing the knowledge gap
There is growing consensus that the combined impact of degradation processes such as logging and fire on forest biodiversity and carbon stocks is of comparable magnitude to deforestation. In areas where no intact forests remain, these heavily modified ecosystems are the last refuges for many endemic species. Yet we still have a limited understanding of how these forests are functioning, their ability to provide critical ecosystem services, and the prospects for long-term biodiversity persistence. Progress in six inter-related areas of scientific research is needed to address these knowledge gaps and help inform appropriate management of human modified forest ecosystems.
1) Impacts of human modification on ecosystem functioning. The most fundamental knowledge gap relates to the cycling of organic matter and nutrients and relationships between biophysical processes, soil, biodiversity and climate. For example, what are the rates of carbon and nutrient cycling in human-modified forests compared to undisturbed areas, and how do they vary in relation to the intensity of human-related disturbance? What are the long-term implications of changes to ecosystem functioning for the resilience of these systems? Most research on tropical forest dynamics has hitherto focussed on apparently undisturbed forests (e.g. the CTFS, RAINFOR and AfriTRON networks). Although these studies provide an invaluable baseline, they are inadequate for predicting the future structure, composition and dynamics of HMTFs.
2) Linking ecosystem functioning and the biological traits of tropical trees. It is well established that plant and animal communities in HMTFs are heavily modified by edge and isolation effects, wildfires and the removal of commercially important trees during logging operations. Such impacts are likely to have far-reaching implications for nutrient cycling, ecosystem stability and resilience. Trait-based approaches are frequently used to assess the link between biodiversity and ecosystem functioning, and have allowed researchers to improve predictions about the ecological consequences of anthropogenic impacts on natural systems by providing a plausible mechanistic link between biodiversity loss and ecosystem functioning. Yet we have a very poor understanding of plant traits in tropical forests, how these change after human-modification, and how they link to forest dynamics and ecosystem resilience. It is plausible that moderate amounts of disturbance increase nutrient availability, partially through the flush of new decomposition, but also through a shift in community traits, as intense light competition favours community traits such as rapid growth, high nutrient demand, lack of investment in chemical defences against herbivory, and short lifespans, leading to rapid cycling of nutrients between plants, litter, heterotrophs and soils. On the other hand, very heavily disturbed forests may see sufficient loss of key plant functions as to shift the community to a resource-conservative, low nutrient supply state. Both data and theory are currently lacking to build a robust understanding of how trait-based descriptions of forest plant communities can help us understand the resilience of these degraded and regenerating systems.
3) Understanding the functional consequences of changes in animal communities. Preliminary data suggest that the loss of animals from HMTFs radically changes the composition and recruitment of trees, with implications for carbon uptake and nutrient cycling. Although animals have an important role in ecosystem functioning, these functions are much less understood than plant-based functions as they often span trophic levels and experiments are complicated by mobility. Before adopting intensive experimental efforts to link animals to functions, we need to develop a fundamental understanding of how functionally-relevant animal traits change after modification of tropical forests.
4) Understanding the generality of ecosystem responses. There is increasing evidence that tropical forests vary in their responses to environmental change, such as very high variation in rates of tree mortality in response to edge effects and fires in the Amazon and that impacts can vary depending on both interactions with natural biophysical gradients and changes in species composition. Research from multiple sites is needed to evaluate the extent to which inferences on ecosystem functioning derived from one region can be used to improve understanding of human impacts on forests elsewhere.
5) Understanding the complex consequences of human-modification for biodiversity across whole landscapes. The bulk of research on human-impacts on tropical forest systems to date, including for both the Amazon and Atlantic forest has concentrated either on the entire biome (e.g. 26) which often depends upon very coarse-scale data and obscures critically important inter- and intra-regional processes and interactions, or on detailed work on a few intensively studied research sites with limited replication, thereby only capturing a tiny fraction of the variability in environmental and land-use gradients that drive ecological change. We will only understand the value of modified tropical forest landscapes for biodiversity and ecosystem functioning, and deliver research outputs that more closely match the scale of management, by sampling across landscapes. This is particularly true in areas where no intact forests remain, as in these cases HMTFs make a key contribution to ecosystem processes and are the last refuges for many endemic species.
6) Closing the science-policy gap. Until now, environmental policy in the tropics has mostly prioritised the creation of reserves in the least disturbed areas of forest. However, in forested biomes under heavy human pressure, most of the remaining habitat is distributed either in small or degraded fragments that do not currently meet the criteria to be set apart for conservation and are given limited legal protection. Although these small, isolated or degraded remnants have often already lost many of the extinction-prone species, they nonetheless could provide key ecosystem functions and services at a local scale, or may function as important stepping stones for endangered species as a part of a larger network of habitat fragments at a larger scale. We currently lack the knowledge to inform environmental policies about the functional importance of degraded forests or what are the roles that forest remnants play at a larger scale, thus new, evidence-based policy approaches are needed to help avoid the loss or to prevent further forest degradation, and to help incentivise the regeneration of forests and enhance of carbon stocks (e.g. the UNFCCC process of Reducing Emissions from Deforestation and Degradation; REDD+).