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 80m^(2),with trees in the stand that were at least 30m 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.

Competitive effect, but not competitive response, varies along a climatic gradient depending on tree species identity

Background: Understanding the role of species identity in interactions among individuals is crucial for assessing the productivity and stability of mixed forests over time. However, there is limited knowledge concerning the variation in competitive effect and response of different species along climatic gradients. In this study, we investigated the importance of climate, tree size, and competition on the growth of three tree species: spruce(Picea abies), fir(Abies alba), and beech(Fagus sylvatica), and examined their competitive response and effect along a climatic gradient.Methods: We selected 39 plots distributed across the European mountains with records of the position and growth of 5,759 individuals. For each target species, models relating tree growth to tree size, climate and competition were proposed. Competition was modelled using a neighbourhood competition index that considered the effects of inter-and intraspecific competition on target trees. Competitive responses and effects were related to climate.Likelihood methods and information theory were used to select the best model.Results: Our findings revealed that competition had a greater impact on target species growth than tree size or climate. Climate did influence the competitive effects of neighbouring species, but it did not affect the target species? response to competition. The strength of competitive effects varied along the gradient, contingent on the identity of the interacting species. When the target species exhibited an intermediate competitive effect relative to neighbouring species, both higher inter-than intraspecific competitive effects and competition reduction occurred along the gradient. Notably, species competitive effects were most pronounced when the target species' growth was at its peak and weakest when growing conditions were far from their maximum.Conclusions: Climate modulates the effects of competition from neighbouring trees on the target tree and not the susceptibility of the target tree to competition. The modelling approach should be useful in future research to expand our knowledge of how competition modulates forest communities across environmental gradients.