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.

Diversity of beetle species and functional traits along gradients of deadwood suggests weak environmental filtering

Background: Gradients in local environmental characteristics may favour the abundance of species with particular traits, while other species decline, or favour species with different traits at the same time, without an increase in average species abundances. Therefore, we asked: do variations in species and traits differ along gradients of deadwood variables? Do species abundance and trait occurrence change with species richness within or between functional groups? Thus, we analysed the beetle assemblages of five forest sites located in Italy, along the Apennines mountains.Methods: From 2012 to 2018 we sampled beetles and five deadwood types in 193 plots to characterise the deadwood gradient: standing dead trees, snags, dead downed trees, coarse woody debris, and stumps. We modelled beetle species relative abundances and trophic traits occurrences against the deadwood variables using joint species distribution models.Results: Out of 462 species, only 77 showed significant responses to at least one deadwood type, with a weak mean response across species. Trophic groups showed mostly negative responses to deadwood variables. Species abundance increased with species richness among sites only for phytophagous and saproxylophagous. Trait occurrence did not increase with species richness among sites, except for phytophagous and saproxylophagous.However, trait occurrence changed significantly with species richness of several trophic groups within some sites.We found that increases in species richness do not result in decreases in species abundance of a given trophic group, but rather null or positive relationships were found suggesting low interspecific competition.Conclusions: Our findings suggest that in Mediterranean mountain forests there is still room for increasing the level of naturalness, at least for what concerns deadwood management. On one side, our findings suggest that competition for deadwood substrates is still low, on the other side they indicate that increasing deadwood volume and types to improve overall beetle richness may increase also beetle abundances.

Monitoring the abundance of saproxylic red-listed species in a managed beech forest by landsat temporal metrics

Background:Rapid climate changes lead to an increase in forest disturbance,which in turn lead to growing concerns for biodiversity.While saproxylic beetles are relevant indicators for studying different aspects of biodiversity,most are smaller than 2 mm and difficult to sample.This,together with a high number of species and trophic roles,make their study remarkably challenging,time-consuming,and expensive.The Landsat mission provides data since 1984 and represents a powerful tool in this scenario.While we believe that remote sensing data cannot replace on-site sampling and analysis,in this study we aim to prove that the Landsat Time Series(TS)may support the identification of insects’hotspots and consequently guide the selection of areas where to concentrate field analysis.Methods:With this aim,we constructed a Landsat-derived NDVI TS(1984–2020)and we summarised the NDVI trend over time by calculating eight Temporal Metrics(TMs)among which four resulted particularly successful in predicting the amount of saproxylic insects:(i)the slope of the regression line obtained by linear interpolating the NDVI values over time;(ii)the Root Mean Square Error(RMSE)between the regression line and the NDVI TS;(iii)the median,and the(iv)minimum values of the NDVI TS.The study area consists of four monitoring sectors in a Mediterranean-managed beech forest located in the Apennines(Molise,Italy),where 60 window flight traps for flying beetles were installed.First,the saproxylic beetle's biodiversities of monitoring sectors were quantified in terms of species richness and alpha-diversity.Second,the capability of TMs in predicting the richness of saproxylic beetles family and trophic categories was assessed in terms of Pearson's product-moment correlation.Results:The alpha diversity and species richness analysis indicate dissimilarities across the four monitored sectors(Shannon and Simpson's index ranging between 0.67 to 2.31 and 0.69 to 0.88,respectively),with Landsat TS resulting in effective predictors for estimating saproxylic beetle richness.The strongest correlation was reached between the Monotomidae family and the RMSE temporal metric(R=0.66).The mean absolute correlation(r)between the NDVI TMs and the saproxylic community was 0.46 for Monotomidae,0.31 for Cerambycidae,and 0.25 for Curculionidae.Conclusions:Our results suggest that Landsat TS has important implications for studying saproxylic beetle distribution and,by helping the selection of monitoring areas,increasing the amount of information acquired while decreasing the effort required for field analysis.