Neighborhood effects on tree growth in a Fagus sylvatica - Abies alba forest following an ice storm

Ice storms can cause substantial damage to tree crowns and lead to growth reduction. However, in uneven-aged stands, the growth of an individual tree may also increase due to crown release caused by the damage or mortality of neighboring trees. Three years after the devastating ice storm in 2014 in mixed uneven-aged Dinaric forests (Croatia), we cored 156 European beech (Fagus sylvatica) and 85 silver fir (Abies alba) trees across 20 permanent sample plots to study the post-storm growth response as a function of tree, stand, site, spatial arrangement and local competition factors. The ice storm damaged over 84% of trees on the sampled plots. Among the cored trees, 52.7% exhibited growth reduction, which on average amounted to −3.1% relative to the pre-disturbance average. Trees with less than 40% crown damage maintained their pre-disturbance growth rates or experienced only minor growth suppression. While 60% of beech trees suffered a growth reduction at an average rate of −7.2%, the average radial increment of fir after the storm was 14.0% higher compared to the pre-storm rate. A linear mixed-effects model suggests that the growth response can largely be explained by the focal and neighboring tree species identity, tree competition pressure, focal and neighboring tree damage, crown size, slenderness index and stoniness. Growth release was positively associated with fir, inverse distance-weighted crown damage of the nearest neighbor, shorter crowns, slenderness, less stony sites and less damaged trees. The analysis suggests that at the same level of local competition load, trees with a broadleaved and damaged nearest neighbor are more likely to experience growth release than those with a coniferous or undamaged nearest neighbor. This implies that uneven-aged stands with a substantial presence and mingling of both conifers and broadleaves are expected to be more resilient to ice storms and are less likely to suffer growth reduction.