The impact of insect herbivory on biogeochemical cycling in broadleaved forests varies with temperature

Herbivorous insects alter biogeochemical cycling within forests, but the magnitude of these impacts, their global variation, and drivers of this variation remain poorly understood. To address this knowledge gap and help improve biogeochemical models, we established a global network of 74 plots within 40 mature, undisturbed broadleaved forests. We analyzed freshly senesced and green leaves for carbon, nitrogen, phosphorus and silica concentrations, foliar production and herbivory, and stand-level nutrient fluxes. We show more nutrient release by insect herbivores at non-outbreak levels in tropical forests than temperate and boreal forests, that these fluxes increase strongly with mean annual temperature, and that they exceed atmospheric deposition inputs in some localities. Thus, background levels of insect herbivory are sufficiently large to both alter ecosystem element cycling and influence terrestrial carbon cycling. Further, climate can affect interactions between natural populations of plants and herbivores with important consequences for global biogeochemical cycles across broadleaved forests

Major axes of variation in tree demography across global forests

The future trajectory of global forests is closely intertwined with tree demography, and a major fundamental goal in ecology is to understand the key mechanisms governing spatio-temporal patterns in tree population dynamics. While previous research has made substantial progress in identifying the mechanisms individually, their relative importance among forests remains unclear mainly due to practical limitations. One approach to overcome these limitations is to group mechanisms according to their shared effects on the variability of tree vital rates and quantify patterns therein. We developed a conceptual and statistical framework (variance partitioning of Bayesian multilevel models) that attributes the variability in tree growth, mortality, and recruitment to variation in species, space, and time, and their interactions – categories we refer to as organising principles (OPs). We applied the framework to data from 21 forest plots covering more than 2.9 million trees of approximately 6500 species. We found that differences among species, the species OP, proved a major source of variability in tree vital rates, explaining 28–33% of demographic variance alone, and 14–17% in interaction with space, totalling 40–43%. Our results support the hypothesis that the range of vital rates is similar across global forests. However, the average variability among species declined with species richness, indicating that diverse forests featured smaller interspecific differences in vital rates. Moreover, decomposing the variance in vital rates into the proposed OPs showed the importance of unexplained variability, which includes individual variation, in tree demography. A focus on how demographic variance is organized in forests can facilitate the construction of more targeted models with clearer expectations of which covariates might drive a vital rate. This study therefore highlights the most promising avenues for future research, both in terms of understanding the relative contributions of groups of mechanisms to forest demography and diversity, and for improving projections of forest ecosystems

Mycorrhizal feedbacks influence global forest structure and diversity

One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure

Long-term forest soil acidification, nutrient leaching and vegetation development: Linking modelling and surveys of a primeval spruce forest in the Ukrainian Transcarpathian Mts.

The biogeochemical model MAGIC was applied to simulate long-term (1880–2050) soil and stratified soil solution (30 and 90 cm depth) chemistry at a spruce dominated site in the western Ukraine (Pop Ivan, 1480 m a.s.l.) to evaluate the effects of acid deposition on soil acidification in a less polluted region of Europe. Since 2008, sulphur (S) deposition of 9 kg ha−1 year−1 and nitrogen (N) deposition of 8.5 kg ha−1 year−1 have been measured at Pop Ivan. The recent deposition of S and N is about 30% and 50% of those values estimated for the early 1980s, respectively. Acidic deposition caused the depletion of base cations (Ca, Mg, Na, K) from the soil cation exchange complex, which resulted in a decrease of calcium and magnesium saturation between 1935 and 2008 in the top mineral soil (0–30 cm) and deeper mineral soil (30–80 cm) by 67% and 88%, respectively. Base cation leaching acted as the major buffer mechanism against incoming acidity, therefore the measured inorganic aluminium (Al) concentration in soil solutions is ca. 10 μmol L−1 and the subsequent molar (Ca + Mg + K)/Al ratio above 1. Recovery of the soil solution pH and Al is expected within the next 40 years, whereas the soil base saturation will only increase slowly, from 6% to 9.8% in the top soil and from 5.5% to 11% in the deeper mineral soil. Since the 1960s, modelled inorganic N leaching (as NO3) has started to increase following the trend in N deposition. Modelling and experimental evidence suggest that N availability from mineralization and deposition exceeds the rate of microbial and plant immobilization. Thus, soil N accumulation since the 1960s has been limited. A significant increase in nitrophilous species as well as a decrease of herb layer diversity was observed between 1936 and 1997