Tropical forests are hotspots of terrestrial biodiversity. The loss, fragmentation and degradation of these forests are important drivers of global biodiversity loss.

Although remote from the UK, tropical forests are a high priority for the UK Government’s international biodiversity work in the context of the 2020 targets for biodiversity agreed at the Nagoya Summit in October 2010 under the Convention on Biological Diversity.

The loss of tropical forests has important implications for the global climate system, as well as a range of other ecosystem services. Deforestation is second only to the combustion of fossil fuels for energy generation as a source of greenhouse gas emissions.

Converting forest into alternative land-uses (particularly agriculture) has major implications for biogeochemical cycles. The net loss of carbon to the atmosphere has been well documented, but it is also becoming clear that forest conversion alters the nitrogen and water cycles and the emission of volatile organic compounds (VOCs).

Changes in emissions of biogenic VOCs impact on the generation of surface pollutants such as ozone, and may change the oxidation rate of methane in the tropics. Uncertainty in how the tropical biosphere will respond to global change is one of the major constraints on predicting the climate of the end of this century and therefore in assessing threshold values of greenhouse gas emissions that may avoid dangerous climate change.

Tropical forest biodiversity and biogeochemical cycles are commonly studied in isolation. The biodiversity community has tended to focus on changes in population and community dynamics rather than the functional role biodiversity plays in ecosystem processes. Biogeochemical cycles have tended to be studied from the perspective of physical and chemical processes, with relatively limited attention to the biological components of the system. There is an increasing need to bring these areas together, both to gain a synthetic understanding of tropical forest ecosystems, and to provide evidence for policy decisions.

Integrated science that addresses both biodiversity and biogeochemical cycles is in its infancy and limited mainly to describing large-scale spatial patterns in ecosystem services and biodiversity. Significant knowledge gaps exist, particularly with respect to the role biodiversity plays in regulating biogeochemical cycles in tropical forests (including biosphere-atmosphere interactions), and the links between these processes and species that are of conservation concern, which is a key component of REDD+.

Developing the science requires integrated observations and modelling linked to gradients in forest modification (loss, fragmentation, degradation) and derived land-uses (eg agriculture). This integration is challenging. Tropical biodiversity is poorly described compared with temperate regions.

Above-below ground interactions will play a key role in regulating biogeochemical cycles. While tropical plants above ground are routinely censured other key biodiversity groups such as soil organisms and consumers are not. This means that there is a need for basic assessments (distribution, abundance, community composition) of these important groups as well as links with measurements of their functional roles. These assessments are labour intensive and technically demanding.

There are major challenges in bringing state of the art observational science associated with biosphere-atmosphere into the field. Taken together, these challenges argue against a dispersed, multi-site approach to developing the integrated science and in favour of detailed observations and measurements initially from a single site. This in itself creates a challenge concerning how data and models from a single site can be generalised to provide insights into contrasting locations across the tropics.

The HMTF programme aims to address these knowledge gaps.