Tree Mortality: Revisited Under Changed Climatic and Silvicultural Conditions

When trees within forests increase their dimension, the limitation of space (and thus resources) can cause competition-based mortality. Thus, growth and mortality interact, and acceleration of growth triggered by climatic change or forest management may be accompanied by increased mortality. We here concentrate on evaluating the underlying causes of and the expected changes in this natural non-catastrophic mortality, which should not be mixed up with catastrophic mortality, generally induced by extreme conditions such as fire, storms or insects. The main objectives of this review are to (1) quantify the natural non-catastrophic mortality in monospecific stands, (2) analyze how mortality is modified by growth acceleration due to climate change, and (3) estimate the impact of widely promoted tree species mixing on tree mortality. The presented analysis is built on previously published findings but also on our own research on the mortality of Norway spruce (Picea abies [L.] Karst), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.) and sessile oak (Quercus petraea (MATT.) LIEBL.) using a unique set of mono- and mixed-species experimental plots across Europe. During stand development competition-based mortality is responsible for about a third of the total forest production. Global warming or/and increased availability of CO2 and nitrogen have generally accelerated the competition-based mortality and turnover in forests. Compared to monospecific stands, mixed forests show significantly higher growth rates, but also accelerated mortality and volume losses. Thus, the increase in gross primary production and growth is generally stronger than net biomass growth, but relative net growth tends to be lower in mixed than in monospecific stands. We discuss the relevance of the results for measuring, understanding, modelling and managing forest stands. Reduced management intensity, increased growth, and a higher degree of tree species mixing, also result in higher competition-based mortality. This high turnover rate, which depends on various environmental conditions, strongly suggests to pay higher attention to mortality and to include permanently unmanaged stands in long-term experiments. We also emphasize the need to consider competition-based approaches to describe tree mortality with models. In a second step, disturbances or catastrophic-driven mortality needs then to be included. The role of mortality is important to consider in discussions about withdrawing from forest management in order to increase carbon sequestration since it is indicated that more than a third of the total stand production is supposed to end up for decomposition rather than as long-term carbon storage.