Effects of rainfall manipulation and nitrogen addition on plant biomass allocation in a semiarid sandy grassland

Effects of rainfall patterns in dry and rainy seasons on the biomass, ecostoichiometric characteristics, and NSC content of Fraxinus malacophylla seedlings

With global climate change and rising temperatures, rainfall will change. The impact of global rainfall changes on ecosystems has prompted people to delve deeper into how changes in rainfall affect plant growth; Plant biomass, nutrient element content, and non-structural carbohydrate content are very sensitive to changes in precipitation. Therefore, understanding the impact of rainfall changes on seedlings is crucial. However, it is currently unclear how the seedlings of Fraxinus malacophylla Hemsl in rocky desertification areas respond to changes in rainfall. In this study, the response of biomass, nutrient accumulation, and NSC content of Fraxinus malacophylla Hemsl seedlings to different rainfall intervals and rainfall during the dry and rainy seasons was studied. Use natural rainfall duration of 5 days (T) and extended rainfall duration of 10 days(T+) as rainfall intervals; average monthly rainfall was used as the control (W), with a corresponding 40% increase in rainfall (W+) and a 40% decrease in rainfall (W-) as rainfall treatments. The research results indicate that the biomass of roots, stems, and leaves, as well as the accumulation of C, N, and P in Fraxinus malacophylla Hemsl seedlings increase with the increase of rainfall, while the soluble sugar and starch content show a pattern of first increasing and then decreasing. The biomass and nutrient accumulation of each organ showed root>leaf>stem. Except for the beginning of the dry season, prolonging the duration of rainfall in other periods inhibits the biomass accumulation of Fraxinus malacophylla Hemsl seedlings, and promotes the accumulation of C, N, and P nutrients and an increase in soluble sugar and starch content. There was a significant positive correlation (P<0.05) between the nutrient contents of C, N, and P in various organs, as well as between soluble sugar and starch content; And N: P>16, plant growth is limited by P element. These results indicate that changes in rainfall can affect the growth and development of Fraxinus malacophylla Hemsl seedlings, increasing rainfall can promote biomass and nutrient accumulation of Fraxinus malacophylla Hemsl seedlings, and prolonging rainfall intervals and reducing rainfall have inhibitory effects on them. The exploration of the adaptation of Fraxinus malacophylla Hemsl seedlings to rainfall patterns has promoted a basic understanding of the impact of rainfall changes on the growth of Fraxinus malacophylla Hemsl. This provides a theoretical basis for understanding how Fraxinus malacophylla Hemsl can grow better under rainfall changes and for future management of Fraxinus malacophylla Hemsl artificial forests in rocky desertification areas.

Elevation Influences Belowground Biomass Proportion in Forests by Affecting Climatic Factors, Soil Nutrients and Key Leaf Traits

Forest biomass allocation is a direct manifestation of biological adaptation to environmental changes. Studying the distribution patterns of forest biomass along elevational gradients is ecologically significant for understanding the specific impacts of global change on plant resource allocation strategies. While aboveground biomass has been extensively studied, research on belowground biomass remains relatively limited. Furthermore, the patterns and driving factors of the belowground biomass proportion (BGBP) along elevational gradients are still unclear. In this study, we investigated the specific influences of climatic factors, soil nutrients, and key leaf traits on the elevational pattern of BGBP using data from 926 forests at 94 sites across China. In this study, BGBP data were calculated from the root biomass to the depth of 50 cm. Our findings indicate considerable variability in forest BGBP at a macro scale, showing a significant increasing trend along elevational gradients (p < 0.01). BGBP significantly decreases with increasing temperature and precipitation and increases with annual mean evapotranspiration (MAE) (p < 0.01). It decreases significantly with increasing soil phosphorus content and increases with soil pH (p < 0.01). Key leaf traits (leaf nitrogen (LN) and leaf phosphorus (LP)) are positively correlated with BGBP. Climatic factors (R2 = 0.46) have the strongest explanatory power for the variation in BGBP along elevations, while soil factors (R2 = 0.10) and key leaf traits (R2 = 0.08) also play significant roles. Elevation impacts BGBP directly and also indirectly through influencing such as climate conditions, soil nutrient availability, and key leaf traits, with direct effects being more pronounced than indirect effects. This study reveals the patterns and controlling factors of forests' BGBP along elevational gradients, providing vital ecological insights into the impact of global change on plant resource allocation strategies and offering scientific guidance for ecosystem management and conservation.

Effects of Grazing, Extreme Drought, Extreme Rainfall and Nitrogen Addition on Vegetation Characteristics and Productivity of Semiarid Grassland

Grassland use patterns, water and nutrients are the main determinants of ecosystem structure and function in semiarid grasslands. However, few studies have reported how the interactive effects of rainfall changes and nitrogen deposition influence the recovery of semiarid grasslands degraded by grazing. In this study, a simulated grazing, increasing and decreasing rainfall, nitrogen deposition test platform was constructed, and the regulation mechanism of vegetation characteristics and productivity were studied. We found that grazing decreased plant community height (CWMheight) and litter and increased plant density. Increasing rainfall by 60% from May to August (+60%) increased CWMheight; decreasing rainfall by 60% from May to August (-60%) and by 100% from May to June (-60 d) decreased CWMheight and coverage; -60 d, +60% and increasing rainfall by 100% from May to June (+60 d) increased plant density; -60% increased the Simpson dominance index (D index) but decreased the Shannon-Wiener diversity index (H index); -60 d decreased the aboveground biomass (ABG), and -60% increased the underground biomass (BGB) in the 10-60 cm layer. Nitrogen addition decreased species richness and the D index and increased the H index and AGB. Rainfall and soil nitrogen directly affect AGB; grazing and rainfall can also indirectly affect AGB by inducing changes in CWMheight; grazing indirectly affects BGB by affecting plant density and soil nitrogen. The results of this study showed that in the semiarid grassland of Inner Mongolia, grazing in the nongrowing season and grazing prohibition in the growing season can promote grassland recovery, continuous drought in the early growing season will have dramatic impacts on productivity, nitrogen addition has a certain impact on the species composition of vegetation, and the impact on productivity will not appear in the short term.

The Effect of Soil Water Deficiency on Water Use Strategies and Response Mechanisms of Glycyrrhiza uralensis Fisch

We aimed to investigate the water use strategies and the responses to water shortages in Glycyrrhiza uralensis, which is a dominant species in the desert steppe. Water stress gradients included control, mild, moderate, and severe. The time intervals were 15, 30, 45, and 60 d. Our study suggested that with the aggravation of water stress intensity, the total biomass of Glycyrrhiza uralensis gradually decreased and allometric growth was preferred to underground biomass accumulation. From 30 d and mild to moderate water stress, the water potential (WP) of leaves decreased considerably compared to the CK. The relative water content (EWC) decreased over time and had a narrow range of variation. Proline (PR) was continuously increased, then declined at 45−60 d under severe and more severe water stress. The δ13C values increased in all organs, showed roots > stems > leaves. The net photosynthetic rate (Pn) and transpiration rate (Tr) decreased to varying degrees. The instantaneous water use efficiency (WUEi) and limiting value of stomata (Ls) increased continuously at first and decreased under severe water stress. Meanwhile, severe water stress triggered the most significant changes in chloroplast and guard cell morphology. In summary, Glycyrrhiza uralensis could maintain water content and turgor pressure under water stress, promote root biomass accumulation, and improve water use efficiency, a water-conservation strategy indicating a mechanism both avoidable dehydration and tolerable drought.

Above- and belowground biomass allocation and its regulation by plant density in six common grassland species in China

Above- and belowground biomass allocation is an essential plant functional trait that reflects plant survival strategies and affects belowground carbon pool estimation in grasslands. However, due to the difficulty of distinguishing living and dead roots, estimation of biomass allocation from field-based studies currently show large uncertainties. In addition, the dependence of biomass allocation on plant species, functional type as well as plant density remains poorly addressed. Here, we conducted greenhouse manipulation experiments to study above- and belowground biomass allocation and its density regulation for six common grassland species with different functional types (i.e., C₃ vs C₄; annuals vs perennials) from temperate China. To explore the density regulation on the biomass allocation, we used five density levels: 25, 100, 225, 400, and 625 plant m^-2. We found that mean root to shoot ratio (R/S) values ranged from 0.04 to 0.92 across the six species, much lower than those obtained in previous field studies. We also found much lower R/S values in annuals than in perennials (C. glaucum and S. viridis vs C. squarrosa, L. chinensis, M. sativa and S. grandis) and in C₄ plants than in C₃ plants (C. squarrosa vs L. chinensis, M. sativa and S. grandis). In addition to S. grandis, plant density had significant effects on the shoot and root biomass fraction and R/S for the other five species. Plant density also affected the allometric relationships between above- and belowground biomass significantly. Our results suggest that R/S values obtained from field investigations may be severely overestimated and that R/S values vary largely across species with different functional types. Our findings provide novel insights into approximating the difficult-to-measure belowground living biomass in grasslands, and highlight that species composition and intraspecific competition will regulate belowground carbon estimation.

Disentangling the effects of climatic variability and climate extremes on the belowground biomass of C3- and C4-dominated grasslands across five ecoregions

Elucidating the variation in grassland belowground biomass (BGB) and its response to changes in climatic variables are key issues in plant ecology research. In this study, BGB data for five ecoregions (cold steppe, temperate dry steppe, savanna, humid savanna, and humid temperate) were used to examine the effects of climatic variability and extremes on the BGB of C₃- and C₄-dominated grasslands. Results showed that BGB varied significantly across the ecoregions, with the highest levels in cold steppe and the lowest in savanna. The results indicated that growing-season temperature, maximum and minimum temperatures and their interactions had significantly positive effects on the single-harvest BGB of C₃ plants in colder ecoregions (i.e., humid temperate and cold steppe) and of C₄ plants in arid ecoregions (i.e., temperate dry steppe and savanna). The single-harvest BGB of C₃ plants in arid ecoregions and C₄ plants in humid savanna ecoregion declined with increasing temperature during the growing season. Growing-season precipitation exerted significant positive effects on the single-harvest BGB of C₄ plants in arid ecoregions. Annual temperature variables negatively impacted the annual BGB of humid temperate ecoregion, because of the dominance of C₃ plants. Increasing cumulative growing-season precipitation elevated and the mean annual temperature reduced the annual BGB of both categories of plants in arid ecoregions. Compared with normal climates, extreme dry events during the growing season enhanced single-harvest BGB in colder ecoregions. The single-harvest BGB of C₄ plants in savanna tended to increase during extreme wet and decrease during moderate dry events compared to normal climates. This study suggests that the differential effects of climatic variability and extremes on BGB can be explained by differences in plant types, and ecoregions. These findings on the responses of the BGB to climatic variability and extremes constitute important scientific evidence emphasizing the need to maintain ecosystem stability across ecoregions.