Higher tree diversity is linked to higher tree mortality

Tree Diversity Increases Carbon Stocks and Fluxes Above-But Not Belowground in a Tropical Forest Experiment

International commitments advocate large-scale forest restoration as a nature-based solution to climate change mitigation through carbon (C) sequestration. Mounting evidence suggests that mixed compared to monospecific planted forests may sequester more C, exhibit lower susceptibility to climate extremes and offer a broader range of ecosystem services. However, experimental studies comprehensively examining the control of tree diversity on multiple C stocks and fluxes above- and belowground are lacking. To address this gap, we leverage data from the Sardinilla experiment in Panama, the oldest tropical tree diversity experiment, which features a gradient of one-, two-, three- and five-species mixtures of native tree species. Over 16 years, we measured multiple above- and belowground C stocks and fluxes, ranging from tree aboveground C, over leaf litter C production, to soil organic carbon (SOC). We show that tree diversity significantly increased aboveground C stocks and fluxes, with a 57% higher gain in aboveground tree C in five-species mixtures compared to monocultures (35.7 ± 1.8 vs. 22.8 ± 3.4 Mg C ha-1) 16 years after planting. In contrast, we observed a net reduction in SOC (on average -11.2 ± 1.1 Mg C ha-1 across diversity levels) and no significant difference in SOC3 stocks (the predominantly tree-derived, i.e., C3 plant-derived SOC fraction) between five-species mixtures and monocultures (13.0 ± 0.9 vs. 15.1 ± 1.3 Mg C ha-1). Positive tree diversity effects persisted despite repeated climate extremes and strengthened over time for aboveground tree growth. Structural equation models showed that higher tree growth in mixtures enhanced leaf litter and coarse woody debris C fluxes to the soil, resulting in a tightly linked C cycle aboveground. However, we did not observe significant links between above- and belowground C stocks and fluxes. Our study elucidates the mechanisms through which higher tree diversity bolsters the climate mitigation potential of tropical forest restoration. Restoration schemes should prioritize mixed over monospecific planted forests.

Large-sized trees regulating the structural diversity-productivity relationships through shaping different productive processes in a tropical forest

Forest structural diversity, a measurement indicating the spatial and size distribution of individual trees, is critical for forest productivity, which stems from the combination of different ecological processes, such as tree mortality, recruitment and growth. Here, we evaluated the relationship between structural diversity and productivity caused by different ecological processes, and tested the roles of different-sized trees in influencing this relationship in a Forest Global Earth Observatory (ForestGEO) rainforest site on the Barro Colorado Island between 2000 and 2015. Generally, we found a negative relationship between structural diversity and forest productivity. Specifically, tree mortality-induced productivity loss increased, while tree recruitment-induced productivity gain decreased, with structural diversity. In addition, the structural diversity-productivity relationship varied with tree size, which was negative for small trees but positive for large trees. Furthermore, we revealed the important role of large-sized trees, which significantly promoted structural diversity but decreased productivity through increasing biomass loss. By disentangling the components of productivity, our results provide insights on the mechanism of the relationship between structural diversity and productivity, and highlight the role of large trees in shaping this relationship.

Early overyielding in a mixed deciduous forest is driven by both above- and below-ground species-specific acclimatization

BACKGROUND AND AIMS

Mixed forest plantations are increasingly recognized for their role in mitigating the impacts of climate change and enhancing ecosystem resilience. Yet, there remains a significant gap in understanding the early-stage dynamics of species trait diversity and interspecies interactions, particularly in pure deciduous mixtures. This study aims to explore the timing and mechanisms by which trait diversity of deciduous species and competitive interactions influence yield, carbon allocation and space occupation in mixed forests, both above and below ground.

METHODS

A forest inventory was conducted in planted monocultures, two-species and four-species mixtures of European Acer, Tilia, Carpinus and Quercus, representing a spectrum from acquisitive to conservative tree species. Effects of competition were assessed with linear mixed-effects models at the level of biomass and space acquisition, including leaf, canopy, stem and fine root traits.

KEY RESULTS

Early above-ground growth effects were observed 6 years post-planting, with significant biomass accumulation after 8 years, strongly influenced by species composition. Mixtures, especially with acquisitive species, exhibited above-ground overyielding, 1.5-1.9 times higher than monocultures. Fine roots showed substantial overyielding in high-diversity stands. Biomass allocation was species specific and varied markedly by tree size and the level of diversity and between acquisitive Acer and the more conservative species. No root segregation was found.

CONCLUSIONS

Our findings underscore the crucial role of species trait diversity in enhancing productivity in mixed deciduous forest plantations. Allometric changes highlight the need to differentiate between (active) acclimatizations and (passive) tree size-related changes, but illustrate major consequences of competitive interactions for the functional relationship between leaves, stem and roots. This study points towards the significant contributions of both above- and below-ground components to overall productivity of planted mixed-species forests.

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.

Mixing oak and pine trees in Mediterranean forests increases aboveground hydraulic dysfunctions

Increasing tree species diversity in Mediterranean forests could reduce drought-induced hydraulic impairments through improved microclimate and reduced competition for water. However, it remains unclear if and how species diversity modulates tree hydraulic functions and how impacts may shift during the growing season. Using unmanaged Mediterranean forest stands composed of one (i.e., monospecific) or four (i.e., multispecific) tree species, we examined the seasonal dynamics of in-situ hydraulic traits (predawn and midday leaf water potential - Ψpd and Ψmd, xylem- and leaf-specific hydraulic conductivity - KS and KL, percentage loss of conductivity - PLC, specific leaf area - SLA, and Huber value - HV) in four co-existing Pinus and Quercus species over two years. We mainly observed adverse impacts of species diversity with lower Ψpd, Ψmd, KS, KL, and higher PLC in multispecific compared to monospecific stands, especially for the two pines. These impacts were observed all along the growing season but were stronger during the driest periods of the summer. Beneficial impacts of diversity were rare and only occured for oaks (Q. faginea) after prolonged and intense water stress. Our findings reveal that mixing oaks and pines could mainly enhance hydraulic impairments for all species during the dry season, suggesting a potential decline in mixed Mediterranean forests under future climate.

Relative decline in density of Northern Hemisphere tree species in warm and arid regions of their climate niches

Although climate change is expected to drive tree species toward colder and wetter regions of their distribution, broadscale empirical evidence is lacking. One possibility is that past and present human activities in forests obscure or alter the effects of climate. Here, using data from more than two million monitored trees from 73 widely distributed species, we quantify changes in tree species density within their climatic niches across Northern Hemisphere forests. We observe a reduction in mean density across species, coupled with a tendency toward increasing tree size. However, the direction and magnitude of changes in density exhibit considerable variability between species, influenced by stand development that results from previous stand-level disturbances. Remarkably, when accounting for stand development, our findings show a significant change in density toward cold and wet climatic conditions for 43% of the species, compared to only 14% of species significantly changing their density toward warm and arid conditions in both early- and late-development stands. The observed changes in climate-driven density showed no clear association with species traits related to drought tolerance, recruitment and dispersal capacity, or resource use, nor with the temperature or aridity affiliation of the species, leaving the underlying mechanism uncertain. Forest conservation policies and associated management strategies might want to consider anticipated long-term species range shifts alongside the integration of contemporary within-distribution density changes.

Effects of plant diversity on productivity strengthen over time due to trait-dependent shifts in species overyielding

Plant diversity effects on community productivity often increase over time. Whether the strengthening of diversity effects is caused by temporal shifts in species-level overyielding (i.e., higher species-level productivity in diverse communities compared with monocultures) remains unclear. Here, using data from 65 grassland and forest biodiversity experiments, we show that the temporal strength of diversity effects at the community scale is underpinned by temporal changes in the species that yield. These temporal trends of species-level overyielding are shaped by plant ecological strategies, which can be quantitatively delimited by functional traits. In grasslands, the temporal strengthening of biodiversity effects on community productivity was associated with increasing biomass overyielding of resource-conservative species increasing over time, and with overyielding of species characterized by fast resource acquisition either decreasing or increasing. In forests, temporal trends in species overyielding differ when considering above- versus belowground resource acquisition strategies. Overyielding in stem growth decreased for species with high light capture capacity but increased for those with high soil resource acquisition capacity. Our results imply that a diversity of species with different, and potentially complementary, ecological strategies is beneficial for maintaining community productivity over time in both grassland and forest ecosystems.

Positive effects of tree species diversity on productivity switch to negative after severe drought mortality in a temperate forest experiment

The synthesis of a large body of evidence from field experiments suggests more diverse plant communities are more productive as well as more resistant to the effects of climatic extremes like drought. However, this view is strongly based on data from grasslands due to the limited empirical evidence from tree diversity experiments. Here we report on the relationship between tree diversity and productivity over 10 years in a field experiment established in 2005 that was then affected by the 2018 mega-drought in central Europe. Across a number of years, tree species diversity and productivity were significantly positively related; however, the slope switched to negative in the year of the drought. Net diversity effects increased through time, with complementarity effects making greater contributions to the net diversity effect than selection effects. Complementarity effects were clearly positive in three- and five-species mixtures before the drought (2012-2016) but were found to decrease in the year of the drought. Selection effects were clearly positive in 2016 and remained positive in the drought year 2018 in two-, three-, and five-species mixtures. The survival of Norway spruce (Picea abies) plummeted in response to the drought, and a negative relationship between species diversity and spruce survival was found. Taken together, our findings suggest that tree diversity per se may not buffer communities against the impacts of extreme drought and that tree species composition and the drought tolerance of tree species (i.e., species identity) will be important determinants of community productivity as the prevalence of drought increases.

Denser forests across the USA experience more damage from insects and pathogens

Forests across much of the United States are becoming denser. Trees growing in denser stands experience more competition for essential resources, which can make them more vulnerable to disturbances. Forest density can be expressed in terms of basal area, a metric that has been used to assess vulnerability of some forests to damage by certain insects or pathogens. A raster map of total tree basal area (TBA) for the conterminous United States was compared with annual (2000-2019) survey maps of forest damage due to insects and pathogens. Across each of four regions, median TBA was significantly higher within forest areas defoliated or killed by insects or pathogens than in areas without recorded damage. Therefore, TBA may serve as a regional-scale indicator of forest health and a first filter for identifying areas that merit finer-scale analysis of forest conditions.