Tree species matter for forest microclimate regulation during the drought year 2018: disentangling environmental drivers and biotic drivers.

Tree canopies are considered to effectively buffer climate extremes and to mitigate climate change effects. Droughts, which are predicted to become more frequent in the course of climate change, might alter the microclimatic cooling potential of trees. However, our understanding of how microclimate at the tree canopy level is modulated by environmental and tree characteristics and their interactions is still limited. Here, we investigated canopy temperature regulation for five mature co-occurring tree species for two contrasting hydrological situations during the severe drought in 2018. Even though we observed a significant drought-induced decline in canopy cover and transpiration across tree species, we found evidence that differences in the water use strategies of trees affected cooling mechanisms differently. Although a large share of the variations in the cooling potential of trees was explained by direct and indirect effects of meteorological factors, we identified a gradual shift in importance from latent heat flux to components defining the magnitude of sensible heat flux on the energy budget of tree as the drought gained severity. The decrease in latent heat fluxes, approximated by sap flow rates, furthermore resulted in a reduced cooling potential and an equalization of tree species canopy temperatures.

Divergent above- and below-ground biodiversity pathways mediate disturbance impacts on temperate forest multifunctionality.

Biodiversity plays a fundamental role in provisioning and regulating forest ecosystem functions and services. Above-ground (plants) and below-ground (soil microbes) biodiversity could have asynchronous change paces to human-driven land-use impacts. Yet, we know very little how they affect the provision of multiple forest functions related to carbon accumulation, water retention capacity and nutrient cycling simultaneously (i.e. ecosystem multifunctionality; EMF). We used a dataset of 22,000 temperate forest trees from 260 plots within 11 permanent forest sites in Northeastern China, which are recovering from three post-logging disturbances. We assessed the direct and mediating effects of multiple attributes of plant biodiversity (taxonomic, phylogenetic, functional and stand structure) and soil biodiversity (bacteria and fungi) on EMF under the three disturbance levels. We found the highest EMF in highly disturbed rather than undisturbed mature forests. Plant taxonomic, phylogenetic, functional and stand structural diversity had both positive and negative effects on EMF, depending on how the EMF index was quantified, whereas soil microbial diversity exhibited a consistent positive impact. Biodiversity indices explained on average 45% (26%-58%) of the variation in EMF, whereas climate and disturbance together explained on average 7% (0.4%-15%). Our result highlighted that the tremendous effect of biodiversity on EMF, largely overpassing those of both climate and disturbance. While above- (ß = 0.02-0.19) and below-ground (ß = 0.16-0.26) biodiversity had direct positive effects on EMF, their opposite mediating effects (ß = -0.22 vs. ß = 0.35 respectively) played as divergent pathways to human disturbance impacts on EMF. Our study sheds light on the need for integrative frameworks simultaneously considering above- and below-ground attributes to grasp the global picture of biodiversity effects on ecosystem functioning and services. Suitable management interventions could maintain both plant and soil microbial biodiversity, and thus guarantee a long-term functioning and provisioning of ecosystem services in an increasing disturbance frequency world.

Context-dependency of tree species diversity, trait composition and stand structural attributes regulate temperate forest multifunctionality.

High species diversity is generally thought to be a requirement for sustaining forest multifunctionality. However, the degree to which the relationship between species-, structural-, and trait-diversity of forests and multifunctionality depend on the context (such as stand age or abiotic conditions) is not well studied. Here, we hypothesized that context-dependency of tree species diversity, functional trait composition and stand structural attributes promote temperate forest multifunctionality including above- and below-ground multiple and single functions. To do so, we used repeated forest inventory data, from temperate mixed forests of northeast China, to quantify two above-ground (i.e. coarse woody productivity and wild edible plant biomass), five below-ground (i.e. soil organic carbon, total soil nitrogen, potassium, phosphorus and sulfur) functions, tree species diversity, individual tree size variation (CVDBH) and functional trait composition of specific leaf area (CWMSLA) as well as stand age and abiotic conditions. We found that tree species diversity increased forest multifunctionality and most of the single functions. Below-ground single and multifunctionality were better explained by tree species diversity. In contrast, above-ground single and multifunctionality were better explained by CVDBH. However, CWMSLA was also an additional important driver for maintaining above- and below-ground forest multifunctionality through opposing plant functional strategies. Stand age markedly reduced forest multifunctionality, tree species diversity and CWMSLA but substantially increased CVDBH. Below-ground forest multifunctionality and tree species diversity decreased while above-ground forest multifunctionality increased on steep slopes. These results highlight that context-dependency of forest diversity attributes might regulate forest multifunctionality but may not have a consistent effect on above-ground and below-ground forest multifunctionality due to the fact that those functions were driven by varied functional strategies of different plant species. We argue that maximizing forest complexity could act as a viable strategy to maximizing forest multifunctionality, while also promoting biodiversity conservation to mitigate climate change effects.

Environmental filtering, predominance of strong competitor trees and exclusion of moderate-weak competitor trees shape species richness and biomass.

Strong competitor (i.e. big-sized) trees are globally crucial for promoting aboveground biomass. Still, we do not fully understand the simultaneous influences of different levels of competitor (i.e. strong, moderate, medium and weak) trees at stand level in shaping forest diversity and biomass along a climatic gradient. We hypothesized that few strong competitor trees shape the positive relationship between tree species richness and aboveground biomass better than moderate, medium and weak competitor trees along a climatic gradient. Using the forest inventory data (i.e. tree diameter, height and crown diameter), we quantified strong (i.e. 99th percentile; top 1%), moderate (i.e. 75th percentile; top 25%), medium (i.e. 50th percentile) and weak (i.e. 25th percentile) competitor trees as well as species richness and aboveground biomass of 248 plots (moist temperate, semi-humid, and semi-arid forests) across 12 sites in Iran. The main results from three piecewise structural equation models (i.e. tree diameter, height and crown based models) showed that, after considering the simultaneous fixed effects of climate and random effects of sites or forest types variation, strong competitor trees possessed strong positive effects on tree species richness and biomass whereas moderate, medium and weak competitor trees possessed negligible positive to negative effects. Also, different levels of competitor trees promoted each other in a top-down way but the effects of strong competitor trees on moderate, medium and weak competitor trees were relatively weak. This study suggests that the simultaneous interactions of different tree sizes at stand level across forest sites should be included in the integrative ecological modeling for better understanding the role of different levels of competitor trees in shaping positive forest diversity - functioning relationship in a changing environment.

Impacts of climatic and edaphic factors on the diversity, structure and biomass of species-poor and structurally-complex forests.

Understanding the impacts of multiple climatic and edaphic factors on forest diversity, structure and biomass is crucial to predicting how forests will react to global environmental change. Here, we addressed how do forest structural attributes (i.e. top 1% big, top 25% big medium and small trees; in terms of tree height, diameter, and crown), species richness, and aboveground biomass respond to temperature-related and water-related climatic factors as well as to edaphic factors. By assuming disturbance as a constant factor in the study forests, we hypothesize that water-related and temperature-related climatic factors play contrasting roles whereas edaphic factors play an additional role in shaping forest diversity, structure and aboveground biomass in species-poor and structurally-complex forests. We used forest inventory and environmental factors data from 248 forest plots (moist temperate, semi-humid, and semi-arid) across 12 sites in Iran. We developed multiple linear mixed-effect models for each response variable by using multiple climatic and edaphic factors as fixed effects whereas sites as a random effect. Top 1% big, top 25% big, medium, and small trees enhanced with mean annual temperature but declined with water-related climatic (i.e. mean annual precipitation, cloud cover, potential evapotranspiration, and wet day frequency) factors, whereas soil texture (i.e. sand content) and pH were of additional importance. Species richness increased with precipitation and cloud cover but decreased with temperature, potential evapotranspiration, soil fertility and sand content. Aboveground biomass increased along temperature gradient but decreased with potential evapotranspiration, clay and sand contents. Temperature seemed to be the main driver underlying the increase in forest structure (i.e. diameter-related attributes) and biomass whereas precipitation did so for species richness. We argue that the impacts of multiple climatic factors on forest structural attributes, diversity and biomass should be properly evaluated in order to better understand the responses of species-poor forests to climate change.

Topography, grazing, and soil textures control over rangelands' vegetation quantity and quality.

Topography, grazing disturbances, and soil textures are the main determining factors of natural herbaceous plant communities. Yet, while interesting efforts have been made to link topography, soil conditions, grazing disturbances, species diversity and aboveground biomass, we still lack a comprehensive understanding of how soil textural properties and grazing disturbances co-vary along topographic gradients, and how they jointly shape vegetation quantity and quality in natural rangelands. In this study, we used abiotic and biotic datasets from 735 quadrats of natural rangelands located in the southern Alborz Province of Iran. We quantified topographic variables (i.e. elevation, slope, and aspect), grazing disturbance intensities, soil textural properties (i.e. gravel, sand, silt, and clay contents) as predictor variables. Vegetation quantity (i.e. aboveground biomass, vegetation coverage, and vegetation density) and quality (i.e. species richness, Shannon's diversity, and species evenness) variables were used as response variables. We used boosted regression trees (BRT) models for assessing the relative contribution and effects of multiple predictors on each response variable. We found that vegetation quantity and quality were jointly explained by topography, grazing disturbances, and soil textural properties. Vegetation quantity increased gradually or showed a hump-backed type pattern whereas vegetation quality decreased with elevation. Intensive grazing decreased vegetation quantity of shrubs and graminoids, which in turn determined the vegetation quantity of whole-community (i.e. all species). Higher vegetation quantity of shrubs was located on sandy soils while high vegetation quality was located on silty soils, whereas forbs and graminoids showed an opposite trend. Although the drivers of rangelands' vegetation quantity and quality are not mutually exclusive, the magnitude, shape and complexity of these relationships are highly dependent on plant growth forms. This study suggests that high grazing at lower elevation should be managed properly in order to conserve graminoids and to enhance their functioning in line with forbs and shrubs species.

The positive relationships between plant coverage, species richness, and aboveground biomass are ubiquitous across plant growth forms in semi-steppe rangelands.

The positive relationships between biodiversity and aboveground biomass are important for biodiversity conservation and greater ecosystem functioning and services that humans depend on. However, the interaction effects of plant coverage and biodiversity on aboveground biomass across plant growth forms (shrubs, forbs and grasses) in natural rangelands are poorly studied. Here, we hypothesized that, while accounting for environmental factors and disturbance intensities, the positive relationships between plant coverage, biodiversity, and aboveground biomass are ubiquitous across plant growth forms in natural rangelands. We applied structural equation models (SEMs) using data from 735 quadrats across 35 study sites in semi-steppe rangelands in Iran. The combination of plant coverage and species richness rather than Shannon's diversity or species diversity (a latent variable of species richness and evenness) substantially enhance aboveground biomass across plant growth forms. In all selected SEMs, plant coverage had a strong positive direct effect on aboveground biomass (ß = 0.72 for shrubs, 0.84 for forbs and 0.80 for grasses), followed by a positive effect of species richness (ß = 0.26 for shrubs, 0.05 for forbs and 0.09 for grasses), and topographic factors. Disturbance intensity had a negative effect on plant coverage, whereas it had a variable effect on species richness across plant growth forms. Plant coverage had a strong positive total effect on aboveground biomass (ß = 0.84 for shrubs, 0.88 for forbs, and 0.85 for grasses), followed by a positive effect of species richness, and a negative effect of disturbance intensity across plant growth forms. Our results shed light on the management of rangelands that is high plant coverage can significantly improve species richness and aboveground biomass across plant growth forms. We also found that high disturbance intensity due to heavy grazing has a strong negative effect on plant coverage rather than species richness in semi-steppe rangelands. This study suggests that proper grazing systems (e.g. rotational system) based on carrying capacity and stocking rate of a rangeland may be helpful for biodiversity conservation, better grazing of livestock, improvement of plant coverage and enhancement of aboveground biomass.

Abiotic and biotic drivers of aboveground biomass in semi-steppe rangelands.

Rangelands play an important role in the biodiversity conservation and ecosystem functions. Yet, few studies have assessed the effects of biotic and abiotic factors on aboveground biomass across plant growth forms and at whole-community level in rangelands. Here, we hypothesized that aboveground biomass is driven by both biotic (plant coverage, species richness and evenness) and abiotic factors (soil textural properties and topographic factors) but biotic factors may best predict aboveground biomass, probably due to small spatial scale. To test this hypothesis, we performed multiple linear mixed model by including abiotic and biotic factors as fixed effects while sites aspects and plant community types across sites, and disturbance intensities as random effects, using data from 735 quadrats across 35 sites in semi-steppe rangelands in Iran. The optimal model for shrubs showed that aboveground biomass was positively related to plant coverage, species richness, elevation, sand, silt and clay. Aboveground biomass of forbs and grasses was positively related to plant coverage, species richness, elevation and slope. Whole-community aboveground biomass was positively related to plant coverage, species richness and elevation, but negatively to species evenness and slope. We conclude that higher aboveground biomass is related to high species richness and plant coverage, and located on high elevation and/or slope across plant growth forms while having medium-coarse-textured to fine-textured soils for adaptation of shrubs only. Few dominant species or niche overlap in whole-community may also drive high aboveground biomass, and located on high elevation with gentle slope. Therefore, we found support for both the niche complementarity and selection effects across plant growth forms and at whole-community. In addition, this study shows that plant coverage is the best proxy for aboveground biomass in the studied rangelands.