Rapid functional shifts across high latitude forests over the last 65 years

Long-term stability of productivity increases with tree diversity in Canadian forests

The temporal stability of forest productivity is a key ecosystem function and an essential service to humanity. Plot-scale tree diversity experiments with observations over 10 to 11 y indicate that tree diversity increases stability under various environmental changes. However, it remains unknown whether these small-scale experimental findings are relevant to the longer-term stability of natural forests. Using 7,500 natural forest plots across much of Canada, monitored over three to four decades on average, we provide strong evidence that higher temporal stability (defined as the mean productivity divided by its SD over time) is consistently associated with greater tree functional, phylogenetic, and taxonomic diversity across all lengths of observations. Specifically, increasing functional diversity from its minimum to maximum values increases stability, mean productivity, and the temporal SD of productivity by 14%, 36%, and 28%, respectively. Our results highlight that the promotion of functionally, phylogenetically, and/or taxonomically diverse forests could enhance the long-term productivity and stability of natural forests.

Resource availability enhances positive tree functional diversity effects on carbon and nitrogen accrual in natural forests

Forests harbor extensive biodiversity and act as a strong global carbon and nitrogen sink. Although enhancing tree diversity has been shown to mitigate climate change by sequestering more carbon and nitrogen in biomass and soils in manipulative experiments, it is still unknown how varying environmental gradients, such as gradients in resource availability, mediate the effects of tree diversity on carbon and nitrogen accrual in natural forests. Here, we use Canada's National Forest Inventory data to explore how the relationships between tree diversity and the accumulation of carbon and nitrogen in tree biomass and soils vary with resource availability and environmental stressors in natural forests. We find that the positive relationship between tree functional diversity (rather than species richness) and the accumulation of carbon in tree biomass strengthens with increasing light and soil nutrient availability. Moreover, the positive relationship between tree functional diversity and the accumulation of carbon and nitrogen in both organic and mineral soil horizons is more pronounced at sites with greater water and nutrient availabilities. Our results highlight that conserving and promoting functionally diverse forests in resource-rich environments could play a greater role than in resource-poor environments in enhancing carbon and nitrogen sequestration in Canada's forests.

Functional diversity enhances dryland forest productivity under long-term climate change

Short-term experimental studies provided evidence that plant diversity increases ecosystem resilience and resistance to drought events, suggesting diversity to serve as a nature-based solution to address climate change. However, it remains unclear whether the effects of diversity are momentary or still hold over the long term in natural forests to ensure that the sustainability of carbon sinks. By analyzing 57 years of inventory data from dryland forests in Canada, we show that productivity of dryland forests decreased at an average rate of 1.3% per decade, in concert with the temporally increasing temperature and decreasing water availability. Increasing functional trait diversity from its minimum (monocultures) to maximum value increased productivity by 13%. Our results demonstrate the potential role of tree functional trait diversity in alleviating climate change impacts on dryland forests. While recognizing that nature-based climate mitigation (e.g., planting trees) can only be partial solutions, their long-term (decadal) efficacy can be improved by enhancing functional trait diversity across the forest community.

Tree diversity increases decadal forest soil carbon and nitrogen accrual

Increasing soil carbon and nitrogen storage can help mitigate climate change and sustain soil fertility^1,2. A large number of biodiversity-manipulation experiments collectively suggest that high plant diversity increases soil carbon and nitrogen stocks^3,4. It remains debated, however, whether such conclusions hold in natural ecosystems^5-12. Here we analyse Canada's National Forest Inventory (NFI) database with the help of structural equation modelling (SEM) to explore the relationship between tree diversity and soil carbon and nitrogen accumulation in natural forests. We find that greater tree diversity is associated with higher soil carbon and nitrogen accumulation, validating inferences from biodiversity-manipulation experiments. Specifically, on a decadal scale, increasing species evenness from its minimum to maximum value increases soil carbon and nitrogen in the organic horizon by 30% and 42%, whereas increasing functional diversity enhances soil carbon and nitrogen in the mineral horizon by 32% and 50%, respectively. Our results highlight that conserving and promoting functionally diverse forests could promote soil carbon and nitrogen storage, enhancing both carbon sink capacity and soil nitrogen fertility.

Global methyl halide emissions from biomass burning during 2003-2021

Methyl halides (CH₃Cl, CH₃Br, and CH₃I) are ozone-depleting substances. Biomass burning (BB) is an important source of methyl halides. The temporal variations and global spatial distribution of BB methyl halide emissions are unclear. Thus, global methyl halide emissions from BB during 2003-2021 were estimated based on satellite data. A significant decreasing trend (p < 0.01) in global methyl halide emissions from BB was found between 2003 and 2021, with CH₃Cl emissions decreasing from 302 to 220 Gg yr^-1, CH₃Br emissions decreasing from 16.5 to 11.7 Gg yr^-1, and CH₃I emissions decreasing from 8.9 to 6.1 Gg yr^-1. From a latitudinal perspective, the northern high-latitude region (60-90° N) was the only latitude zone with significant increases in BB methyl halide emissions (p < 0.01). Based on an analysis of the drivers of BB methyl halide emissions, emissions from cropland, grassland, and shrubland fires were more correlated with the burned area, while BB emissions from forest fires were more correlated with the emissions per unit burned area. The non-BB emissions of CH₃Cl increased from 4749 Gg yr^-1 in 2003 to 4882 Gg yr^-1 in 2020, while those of CH₃Br decreased from 136 Gg yr^-1 in 2003 to 118 Gg yr^-1 in 2020 (global total CH₃I emissions are not available). The finding indicates that global CH₃Cl and CH₃Br emissions from sources besides BB increased and decreased during 2003-2020. Based on our findings, not only searching for unknown sources is important, but also re-evaluating known sources is necessary for addressing methyl halide emissions.

Climate-driven divergent long-term trends of forest beetles in Japan

Concerning declines in insect populations have been reported from Europe and the United States, yet there are gaps in our knowledge of the drivers of insect trends and their distribution across the world. We report on our analysis of a spatially extensive, 14-year study of ground-dwelling beetles in four natural forest biomes spanning Japan's entire latitudinal range (3000 km). Beetle species richness, abundance and biomass declined in evergreen coniferous forests but increased in broadleaf-coniferous mixed forests. Further, beetles in evergreen coniferous forests responded negatively to increased temperature and precipitation anomalies, which have both risen over the study's timespan. These significant changes parallel reports of climate-driven changes in forest tree species, providing further evidence that climate change is altering forest ecosystems fundamentally. Given the enormous biodiversity and ecosystem services that forests support globally, the implications for biodiversity change resulting from climate change could be profound.

Complex range shifts among forest functional types under the contemporary warming

The direction and magnitude of species distribution shifts tend to differ among species and functional types (FTs). Quantifying functional trait variation and species interactions will improve our understanding of the complex mechanisms that govern ecosystem dynamics and their responses to climate change. Here, we analyzed differences in the juvenile and adult temperature ranges of Japanese tree species at the mean, colder edge, and warmer edge of their distributions to reveal how functional traits affect interactions between different FT groups (e.g., deciduous and evergreen broad-leaved trees), using linear models and permutation tests. Overall, juveniles preferred cooler sites, but with high variation. The variation among species was partly explained by the difference in seed mass where species with lighter seeds tend to colonize colder sites. On the other hand, the distribution range of FTs showed complex behavior at the ecotones of different FTs. Specifically, in three of eight ecotones, nonparallel range shifts between FTs were detected, which includes cold shifting in deciduous broad-leaved FT where a warm shift by subalpine FT happened, and cold shifting in subtropical FT where warm shifts by either the deciduous broad-leaved or the evergreen broad-leaved FTs happened. Our results suggest that past warming has caused a general cold shift at species level, whereas different mechanisms, such as light seeds disperse farther in distribution's colder edge and heavy seeds (e.g., evergreen broad-leaved) compete better in warmer edge, create nonparallel responses of FT distribution ranges leading to the observed homogenization at several ecotones among FTs. These complex range shifts at FT level have crucial implications for climate change mitigation and adaptation.