Differential modulation of crown allometry and stem growth at gap edges in five European tree species by drought conditions

Background In Central Europe, forests are increasingly affected by various disturbances, resulting in an increasing gap formation in the canopy. In order to support goal-oriented management, more knowledge is required about the acclimation of the crown and its effects on the basal area growth of trees at the edge of a gap. Methods This work compared trees' growth and crown structure at the edge of a transient gap, with a gap size of more than 80 ​m2, with trees in the stand that were at least 30 ​m away from the gap. A total of 249 European beeches (Fagus sylvatica L.), Norway spruces (Picea abies L. Karst), Scots pines (Pinus sylvestris L.), oaks (Quercus spp.; Quercus petraea (Matt.) Liebl., Quercus robur L.), and silver firs (Abies alba Mill.) were examined on long-term experimental plots in southern Germany. Various crown measures were developed and calculated using high-resolution terrestrial laser scanning (TLiDAR) to capture the three-dimensional crown structures. Growth responses to edge conditions were measured based on tree rings. Using linear mixed models, we predict the basal area increment of edge trees relative to trees in the stand under wet and dry soil moisture conditions after the gap formation. Results We identified i) species-specific acclimation of the crown of edge trees after the gap formation, ii) under wet soil moisture conditions a growth increase of 25%–45% for beech, pine, and oak edge trees and growth losses of 5%–60% for spruce and fir and iii) coniferous tree species benefited from the edge position regarding their basal area increment under dry soil moisture conditions and deciduous tree species grew regardless of the soil moisture conditions at the edge of a gap. Conclusion Gaps have a species-specific effect on the habitus and growth of edge trees and can have both positive and negative impacts on silviculture.