Terrestrial carbon (C) dynamics heavily rely on plant carbon economics, with biomass allocation between above- and belowground organs (root:shoot ratio, R/S) being a crucial factor in estimating terrestrial C storage. Global change factors alter resource supply for plants, potentially shifting optimal allocation patterns and thus R/S. Drought and elevated CO2 generally increase root allocation, while increased precipitation and nitrogen deposition favor shoot allocation. Climate warming's impact on net primary production and potential soil nutrient deficits can further influence R/S. Antecedent climatic conditions (mean annual temperature and precipitation) are significant predictors of R/S variation, but their relationship with immediate R/S responses to warming remains unclear. Belowground ecosystem properties, such as mycorrhizal fungi (MF), also play a role. MF's diverse nutrient foraging strategies may influence plant carbon allocation. Arbuscular mycorrhizal fungi (AMF) hyphae penetrate root cells, while ectomycorrhizal fungi (EMF) hyphae do not. AMF enhance inorganic nutrient uptake, while EMF improve access to organic nutrient pools. MFT dominance varies across biomes, potentially explaining variable biomass allocation. This study hypothesizes that warming alters biomass allocation, may homogenize R/S across biomes, and that mean annual precipitation (MAP) is a key factor.

The literature extensively documents the impact of various environmental factors on plant biomass allocation. Studies have shown that drought and elevated CO2 increase root biomass relative to shoot biomass, while increased precipitation and nitrogen deposition have the opposite effect. Climate warming’s effect on net primary production and the potential for soil nutrient deficits further complicates the response of R/S. The relationship between long-term correlations of R/S with climatic conditions and immediate R/S responses to warmed environments remains unclear, necessitating a comprehensive analysis. The role of belowground properties, particularly mycorrhizal associations, is also crucial, as different types of mycorrhizal fungi exhibit distinct nutrient foraging strategies, influencing carbon allocation. The existing literature lacks a global synthesis integrating these different factors and their interactions.

This study conducted a meta-analysis of 322 warming experiments (280 field, 42 laboratory) from peer-reviewed articles (1950-2020) identified through ISI Web of Science. Data were extracted using GetData software. Inclusion criteria ensured warming treatments in terrestrial biomes, measurement of at least one relevant variable (TB, AGB, BGB, R/S, WUE, SIN, soil NH4+, soil NO3−, MB, MB C/N), clear indication of temperature, warming methods, dominant plant species, and sufficient data for statistical analysis. Environmental data (MAP, MAT, DUR, WM, BD, CLAY, SOC) were gathered from publications or WorldClim and Harmonized World Soil Database. Biome types, PFTs, taxonomic categories (evergreen vs. deciduous, broadleaf vs. coniferous, annual vs. perennial, angiosperm vs. gymnosperm), and MFTs (AMF, EMF, AM-EMF) were recorded. Response ratio (RR) was calculated as the natural log of the ratio of means in warming and control groups. Weighted response ratio (RR++) was calculated considering variance. Statistical analyses included Q-statistic for heterogeneity, stepwise linear regression, Blomberg's K for phylogenetic signal, ANOVA for the effects of biomes, MFTs, PFTs, experimental methods, WM, and DUR, and structural equation modeling (SEM) to examine the effects of PFTs, MFTs, climate, and warming treatment on RR(R/S) via AGB and BGB.

The meta-analysis, based on 94 pairs of observations, revealed that average warming of 2.5 °C increased R/S by 6.1% to 8.8% (mean 8.1%), significantly greater than 0 (p < 0.01). The intercept between warmed and ambient R/S was positive (CI > 0, p < 0.05), indicating greater root allocation under warming. The slope was less than 1 (CI < 1, p < 0.05), suggesting homogenization of R/S among biomes under warming. 164 vascular plant species (144 angiosperms, 20 gymnosperms) were included, with more perennials than annuals and comparable numbers of woody and herbaceous species. While R/S responses to warming showed a significant phylogenetic signal, MAP was the most critical factor affecting RR(R/S), showing a negative correlation (R² = 0.104). RR(R/S) reversed from positive to negative at MAP > 900 mm. MFT was the second most important factor. MAP's impact was stronger in AMF biomes than EMF biomes. Warming magnitude and duration did not significantly affect RR(R/S). Warming enhanced total biomass (+10.0%) by increasing BGB (+13.1%), even when decreasing AGB (-8.9%). Warming decreased AGB in AMF-associated plants but increased R/S. In EMF biomes, warming increased TB, AGB, BGB, and R/S. The path model showed that MFTs explained 22.9% of AGB response variation, while PFTs and MAP were major pathways explaining 25.4% of BGB response variation. AGB and BGB responses together explained 46.3% of RR(R/S) variation. RR(R/S) correlated with RR(BGB) but not RR(AGB).

The findings support the hypotheses of warming-induced vertical downward biomass allocation and horizontal homogenization of R/S. The dependence of warming effects on soil moisture is a primary reason for the upregulation of R/S, except in frozen tundra and alpine soils. Even with decreased total biomass, belowground allocation was higher, potentially due to lower turnover rates of root-associated carbon. Biomes with lower initial R/S showed greater belowground allocation under warming, implying resource demand. The upregulated R/S could alter carbon transfer between atmosphere and soil. In areas with MAP < 900 mm, warming increases belowground allocation, possibly to exploit deeper soil layers. In areas with MAP > 900 mm, adequate moisture could enable positive warming effects on nutrient availability, leading to decreased belowground allocation, especially for AMF-associated plants. Mycorrhizal association further regulated biomass allocation, with different mechanisms for AMF and EMF-associated plants. EMF plants show increased AGB, BGB, and R/S, likely due to SOM degradation and nutrient absorption. AMF plants might slow shoot growth due to water deficiency and carbon transfer to AMF. Mixed AMF/EMF biomes show stable R/S and productivity. Variations in effect sizes might be due to warming magnitude and duration.

Climate warming enhances belowground biomass allocation, particularly in drier habitats. A 2.5°C warming, similar to IPCC projections, significantly impacts biomass allocation. Habitat dryness and mycorrhizal type are crucial for predicting biomass allocation responses to warming. Future research should investigate the long-term implications of these shifts on carbon sequestration and ecosystem functioning across diverse biomes.

The meta-analysis relies on published data, potentially leading to publication bias. The variability in experimental designs, warming methods, and measurement techniques across studies might influence the results. Furthermore, the study focused primarily on the effects of warming on biomass allocation, and future research could integrate other important factors, such as nutrient availability, soil properties, and plant functional traits, to gain a more comprehensive understanding of ecosystem responses.