Growth and structural changes in Swiss uneven-aged forests over 100 years, and comparisons between 15 uneven-aged forest types of Europe, North America and Australia

Abstract

Abstract Long-term changes in growth, physiology and stand structure, as well as differences due to species composition, were examined in six types of Swiss plenter forests: (1) Picea abies dominated, (2) mixtures of P. abies and Abies alba, (3) mixtures of P. abies, A. alba and Fagus sylvatica, (4) mixtures of Larix decidua, Pinus cembra/sylvatica and P. abies, (5) F. sylvatica dominated and (6) species diverse stands. Long-term changes were examined using 19 plots measured for an average of 94 years. These data were then combined with the literature plenter forest data to compare the growth and structure of 15 species compositions from Europe, North America and Australia. Over the past 100 years, common trends included increases in growth, maximum tree sizes, species diversity, and increasingly convex diameter distributions with fewer medium sized trees and more very large trees. Differences in growth between species compositions were generally consistent with their light-use efficiency, transpiration and water-use efficiency, which were calculated using a process-based model. Stand volume of each forest type was negatively correlated with the shade tolerance (but not crown architectures) of the species it contained, but no such correlations existed for stand basal area. This reflects the high structural variability of these forests (through time and between sites) as well as the high tree neighbourhood variability within plots, which appear to buffer the effects of stand density on regeneration and recruitment. This study shows that despite significant temporal and spatial variability in structure, the negative exponentially shaped diameter distributions were maintained in the long term by applying simple silvicultural principles rather than requiring a very specific stand structure, i.e. a certain stand density, or that all diameter classes strictly lie on the curve of a negative exponentially shaped diameter distribution. The values of stand structural characteristics reported in this study could be used as a guide for the conversion of relatively simply structured forests into plenter forests.