This chapter provides an introduction to the biodiversity of forest ecosystems and highlights the currently acting drivers of forest biodiversity loss. Recent findings on relationships between biodiversity patterns and ecosystem functions are summarized, including the functional consequences of biodiversity loss for the stable provision of forest ecosystem services. Finally, implications for the protection and management of forest ecosystems as important means for biodiversity conservation and climate change mitigation are addressed.

Forest ecosystems host a huge proportion of the Earth’s terrestrial biodiversity and play a crucial role in global biogeochemical cycles. However, dramatic losses of forest area currently constitute an important driver of global biodiversity loss, with unprecedented consequences for the functioning of forest ecosystems and the services they provide. This applies to tropical rain forests in particular, which are estimated to support about two-thirds of the global biodiversity, despite covering less than 15% of the world’s land surface. For the years 1990–2005, the net loss of natural tropical forest area was estimated to 135 million hectares. As a consequence of losses of forest area, more than 5000 tree species from 180 countries are currently threatened with extinction. Declining forest area and associated biodiversity loss in turn will feedback on important functions of forest ecosystems. Declining forest area (in the decade 2003–2012) generated a mean biophysical warming on land corresponding to about 18% of the global biogeochemical signal due to CO2 emission from land-use change. Primary producer diversity, for example of tree species, enhances forest productivity due to resource partitioning, facilitation, natural enemy partitioning or selection effects. As a consequence, maintaining tree diversity is an important prerequisite for both the long-term preservation of ecosystem functioning and the provision of ecosystem services such as timber production or climate change mitigation. It is assumed that a 10% decline of tree species richness will result in a 2–3% reduction of forest productivity at the global scale. The monetary value of tree species richness in maintaining commercial forest productivity is estimated to amount to $166 to $490 billion per year, highlighting the functional importance of forest biodiversity and the need for safeguarding forest biodiversity for human well-being.

Besides the establishment of extensive protected forest areas (wilderness areas) across forest biomes, forest management is considered an important tool for the preservation of biodiversity and ecosystem functioning, as key attributes for forest species conservation and ecological processes critically depend on management intensity. Therefore, sustainable forest management strategies (i.e. ecosystem-based approaches) across forest biomes are required that (1) avoid deforestation and land-use changes, (2) approach key attributes of ‘natural forest communities’ (e.g. biome-specific tree species composition and diversity), (3) allow for and maximize the natural dynamics typical of the respective forest ecosystems, and (4) prioritize the minimization of silvicultural interventions over the maximization of forest timber exploitation, thus optimizing biodiversity protection and forest ecosystem functioning (including ecosystem resistance and resilience against global change). Moreover, we highlight the importance of ecological continuity for safeguarding forest biodiversity and its functional role in mediating the response of forest ecosystems to multiple environmental changes.