Quantifying tree and volume mortality in Italian forests

Tree mortality plays a key role in forest development and function, reducing net annual increments (NAI) even in the absence of major disturbances, although its extent in semi-natural forests is often overlooked. While losses are most commonly quantified as a fraction of stand density, in many cases this could prove misleading due to the non-random nature of competition and mortality, and a quantification of volume mortality rates would provide additional insight. Of great interest is also the quantification of losses relative to gross annual increments (GAI), both from an ecological perspective and in order to better assess net increments and the sustainability margins of forest management. All three perspectives were compared in the present study, based on available data from permanent sample plots across Italy covering a wide range of species, ages and management types. The frequency distribution of all three measures of mortality was highly skewed, with a high number of stands showing little losses; this was apparently not the result of an insufficient monitoring area or time span. Mortality resulted in an average loss of 2.25 m3 ha−1 yr−1, which corresponds to 20.7% of GAI. Mortality rates by number and by volume amounted on average to 1.35% and 0.51%, respectively, demonstrating the need to distinguish between the two perspectives. Two rather distinct clusters were apparent in the relationship between the two mortality rates, with a 1:1 relationship in thinned or disturbed stands but much lower volume mortality rates when self-thinning prevailed, resulting in the loss of many small, suppressed trees. When considering volume mortality rates, however, the relative contribution from dominant or co-dominant trees to total stand mortality was much more important, as a result of their larger volume. If extrapolated to national level, these results could help refine current estimates of net annual increments in Italian forests, and of the margin of sustainability of current harvest levels, which would amount to 45% of NAI.