Contrasting patterns of tree species mixture effects on wood δ13C along an environmental gradient

Abstract

Context Over the last decades, there has been an increasing interest in mixed species forests because of their expected positive impacts on ecosystem services (including productivity), ecosystem stability and risk management. However, the mechanisms at play in species interaction and their dependency on spatial and temporal variations of environmental conditions are still insufficiently understood.Methods To assess the impact of species mixture on tree response to water availability, we constructed 22-year time series of wood carbon isotope compositions from a network of sites covering a large gradient of environmental conditions throughout Europe. Each site included a mixed Fagus sylvatica L. / Pinus sylvestris L. stand and one monospecific stand of each species, with all the stands at a given site in similar environmental conditions.Results A positive species-mixture effect for both species was found on dry sites. On moderately wet sites, the results were contrasted, with pine showing a negative effect and beech a positive one. The contrasted results can be explained by the differences in how each species manages the trade-off between carbon acquisition and water loss, which are highlighted in pure plots. No species-mixture effect was found on extremely dry or extremely wet sites. There were no differences in reactions to drought between pure and mixed stands. ConclusionMixing species did not improve trees response to a drought event but influenced their average isotopic composition according to the species-specific functional traits and average site conditions. The pattern of mixing effect along the gradient of water availability was not linear but showed threshold points.

Tree species richness modulates water supply in the local tree neighbourhood: evidence from woodδ13C signatures in a large-scale forest experiment

“…Moreover, a study on tropical tree seedlings using physiological variables, such as stomatal conductance, found that drought resistance was higher in heterospecific than in conspecific neighbourhoods [11]. However, the vast majority of research on leaf physiological responses to tree diversity has focused on tree diversity at the community level [4,6,7,[12][13][14], whereas only few studies dealt with local neighbourhood conditions [11,15,16]. Importantly, neighbourhood diversity effects were addressed in these studies by neighbourhood treatments (e.g.…”

Mortality Reduces Overyielding in Mixed Scots Pine and European Beech Stands Along a Precipitation Gradient in Europe

“…Accompanying measurements on the 32 triplets at the first survey suggested that the superior gross stem volume growth on the mixed plots may result from a higher canopy packing density , complementary light and nutrient use (de Streel et al, 2020, changes in stem and crown allometry (Barbeito et al, 2017, Pretzsch 2019, improved drought resilience or resistance and temporal stability related to between species growth asynchrony (del Río et al, 2017, de Streel et al, 2022 and a decrease of wood density (Zeller et al, 2017). In the long term, the soil fertility of the mixed stands can improve compared with the monospecific stands (Błońska et al, 2018).…”

Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe

“…First, it does not account for non linearity of the relationship between resource availability and mixing effects. For instance, while their focus was on the mixing effect on drought exposure, de Streel et al (2019) highlighted tipping points along a nearly similar environmental gradient. Second, the above framework does not take into account simultaneous limiting factors.…”