Effects of tree-tree competition on growth in post-disturbance, drought-prone montane forests

The regeneration phase of forests is a crucial and vulnerable life stage in tree and forest development, which is likely to be intensified by climate change leading to increased drought events. In addition, the functioning of protective forests in mountain areas also needs to be continuously maintained or quickly restored after disturbances. It is widely thought that competition between trees negatively affects tree growth also in early live stages. Therefore, this study examines the effects of tree-tree competition on tree growth at a reforested post-disturbance site in the Northern Calcareous Alps. Despite high precipitation, the severe site conditions namely, shallow soils, steep slopes and southeastern aspect, result in drought-prone forests and site conditions likely to increase under climate change.

Following a windthrow, an experimental afforestation trail was established in 2010 with four coniferous and three broadleaved tree species. To calculate a distance-weighted competition index (CI), the tree height of the focal tree as well as the distances and heights of its three main competitors were measured in 2023. The CI was then calculated from the sum of distance-weighted ratios of the tree’s height to that of its competitors. Due to the high survival rate, this study focuses on results of the European larch (Larix decidua) and the Norway spruce (Picea abies).

Our results show that European larch has the highest growth rate, with mean tree height of 6.4 m after 12 growing seasons. Furthermore, a strong negative correlation (Pearson r = -0.789) is observed between its height and CI, suggesting that competition has a negative effect on growth. The Wilcoxon-Mann-Whitney test confirmed that tree height was significantly lower under high competition. The opposite was observed for Norway spruce, with a median tree height of 2.65 m with low CI and a tree height of 4.7 m at high CI values.

These results highlight the complex interactions in a mixed forest, where pioneer species such as European larch thrive under extreme site conditions and maintain their leading role in early succession stages. Norway spruce, however, appears to benefit from con- and interspecific clustering at this life stage, which we interpret as advantages of favourable microclimate under severe site conditions. These results highlight the dual role of competition: while it limits growth for some species, it can also create favourable microclimatic conditions for others. These different characteristics play a key role in restoration of forests and their ecosystem services after disturbance. Furthermore, the resilience of a mixed forest structure provides essential benefits during early succession and the crucial regeneration phase despite many challenges posed by climate change.