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This synthesis chapter of the book Old-Growth Forests: Function, Fate and Value reviews the challenges of old-growth research, summarises the major findings, and defines future research needs. Several processes that are influenced by the characteristic structural features of old-growth forests were identified. (1) A complex, tall structure enhances radiation interception, resilience of productivity to drought stress, leakiness for nutrients, and the provision of habitat for plants and animals. (2) Up to 600 years of age, old-growth forests remain carbon sinks and exhibit similar sink strength as younger forests. Old-growth forests accumulate carbon both in the biomass and woody detritus, and to a lesser extent in the soil; however, data for soil are scare and ambiguous. (3) In most old-growth forests studied to date, nutrient cycling, productivity, biomass and woody detritus accumulation are still influenced by successional species turnover and associated changes in functional tree traits. (4) A successional decline in net primary productivity is not a ‘universal feature’ of natural forests and we identified several processes that work against such a decline. Changes in productivity are more closely related to changes in canopy structure than to age per se. (5) In the past, old-growth forests may have accounted for between 20 and 90% of the natural forest landscape. This is in contrast with their current extent in the temperate region, which is below 0.5%. Current rates of old-growth forest destruction are alarmingly high in the tropics and some boreal regions. (6) The assertion of carbon neutrality in old-growth forests has led to their exclusion from the Kyoto-Protocol. This book challenges this paradigm and provides a refined picture of the functioning of old-growth forests that shall serve as an additional basis for future political decisions in the field of climate change mitigation and conservation of biodiversity.
