Increasing calcium scarcity along Afrotropical forest succession

Soil pH-dependent nitrogen stimulation of plant biomass: magnesium and calcium as key constraints

Anthropogenic nitrogen (N) deposition can alleviate N limitation and stimulate plant growth in many terrestrial ecosystems. While theoretical models often emphasize phosphorus limitations as a constraint on this positive N effect, the impact of N-induced magnesium (Mg) and calcium (Ca) deficits due to soil acidification has been largely overlooked. Here, we synthesized data from 243 experiments across diverse terrestrial ecosystems to investigate the role of Mg and Ca in plant biomass responses to N addition. We found that the effect of N addition on aboveground biomass (AGB) shifted from neutral in low pH (≤ 4.5) to positive in medium (4.5-7.5) and high pH (> 7.5) soils. By contrast, belowground biomass (BGB) responses to N addition were independent of soil pH, leading to asymmetric increases in AGB and BGB. These variations in biomass accumulation across pH levels were primarily explained by changes in foliar Mg and Ca concentrations, which were negatively affected by N addition in low-pH soils but remained stable in medium and high-pH soils. Our findings underscore the critical role of Mg and Ca in modulating plant responses to N fertilization, providing new insights for improving Earth system models and better predicting climate-biosphere feedback.

Unraveling the drivers of optimal stomatal behavior in global C3 plants: A carbon isotope perspective

Understanding the drivers of stomatal behavior is critical for modeling terrestrial carbon cycle and water balance. The unified stomatal optimization (USO) model provides a mechanistic linkage between stomatal conductance (gs) and photosynthesis (A), with its slope parameter (g1) inversely related to intrinsic water use efficiency (iWUE), providing a key proxy to characterize the differences in iWUE and stomatal behavior. While many studies have identified multiple environmental factors influencing g1, the potential role of evolutionary history in shaping g1 remains incompletely understood. Leaf organic matter 13C discriminations (Δ13C) can be applied to estimate g1 over timescales from days to whole growing season. However, most applications assume that mesophyll conductance (gm)-a critical parameter in the Δ13C model-is infinite, due to limited information. Here, we incorporated new insight of gm to allow for more realistic parameterization of this variable, and subsequently to enable improved estimation of g1 based on a global bulk leaf Δ13C dataset comprising 2215 observations of 1521 species that span major biomes. Our analysis revealed a significant phylogenetic signal in g1 values, which differed among phylogenetic groups. Through a Bayesian phylogenetic linear mixed model, we found that species and phylogeny together explained 36.63 % of g1 variance, a contribution comparable to that of the environmental factors (44.59 %). Our findings uncovered for the first time that environmental factors, species-level and phylogenetic effects jointly shape g1 variability, thereby contributing to a more comprehensive understanding of optimal stomatal behavior in the context of global environmental change.

Wood nutrients: Underexplored traits with functional and biogeochemical consequences

Resource storage is a critical component of plant life history. While the storage of nonstructural carbohydrates in wood has been studied extensively, the multiple functions of mineral nutrient storage have received much less attention. Here, we highlight the size of wood nutrient pools, a primary determinant of whole-plant nutrient use efficiency, and a substantial fraction of ecosystem nutrient budgets, particularly tropical forests. Wood nutrient concentrations also show exceptional interspecific variation, even among co-occurring plant species, yet how they align with other plant functional traits and fit into existing trait economic spectra is unclear. We review the chemical forms and location of nutrient pools in bark and sapwood, and the evidence that nutrient remobilization from sapwood is associated with mast reproduction, seasonal leaf flush, and the capacity to resprout following damage. We also emphasize the role wood nutrients are likely to play in determining decomposition rates. Given the magnitude of wood nutrient stocks, and the importance of tissue stoichiometry to forest productivity, a key unresolved question is whether investment in wood nutrients is a relatively fixed trait, or conversely whether under global change plants will adjust nutrient allocation to wood depending on carbon gain and nutrient supply.

Nitrogen and potassium limit fine root growth in a humid Afrotropical forest

Nutrient limitations play a key regulatory role in plant growth, thereby affecting ecosystem productivity and carbon uptake. Experimental observations identifying the most limiting nutrients are lacking, particularly in Afrotropical forests. We conducted an ecosystem-scale, full factorial nitrogen (N)-phosphorus (P)-potassium (K) addition experiment consisting 32 40 × 40 m plots (eight treatments × four replicates) in Uganda to investigate which (if any) nutrient limits fine root growth. After two years of observations, added N rapidly decreased fine root biomass by up to 36% in the first and second years of the experiment. Added K decreased fine root biomass by 27% and fine root production by 30% in the second year. These rapid reductions in fine root growth highlight a scaled-back carbon investment in the costly maintenance of large fine root network as N and K limitations become alleviated. No fine root growth response to P addition was observed. Fine root turnover rate was not significantly affected by nutrient additions but tended to be higher in N added than non-N added treatments. These results suggest that N and K availability may restrict the ecosystem's capacity for CO2 assimilation, with implications for ecosystem productivity and resilience to climate change.

Changes in nitrogen and phosphorus availability driven by secondary succession in temperate forests shape soil fungal communities and function

The soil fungal community plays an important role in forest ecosystems and is crucially influenced by forest secondary succession. However, the driving factors of fungal community and function during temperate forest succession and their potential impact on succession processes remain poorly understood. In this study, we investigated the dynamics of the soil fungal community in three temperate forest secondary successional stages (shrublands, coniferous forests, and deciduous broad-leaved forests) using high-throughput DNA sequencing coupled with functional prediction via the FUNGuild database. We found that fungal community richness, α-diversity, and evenness decreased significantly during the succession process. Soil available phosphorus and nitrate nitrogen decreased significantly after initial succession occurred, and redundancy analysis showed that both were significant predictors of soil fungal community structure. Among functional groups, fungal saprotrophs and pathotrophs represented by plant pathogens were significantly enriched in the early-successional stage, while fungal symbiotrophs represented by ectomycorrhiza were significantly increased in the late-successional stage. The abundance of both saprotroph and pathotroph fungal guilds was positively correlated with soil nitrate nitrogen and available phosphorus content. Ectomycorrhizal fungi were negatively correlated with nitrate nitrogen and available phosphorus content and positively correlated with ammonium nitrogen content. These results indicate that the dynamics of fungal community and function reflected the changes in nitrogen and phosphorus availability caused by the secondary succession in temperate forests. The fungal plant pathogen accumulated in the early-successional stage and ectomycorrhizal fungi accumulated in the late-successional stage may have a potential role in promoting forest succession. These findings contribute to a better understanding of the response of soil fungal communities to secondary forest succession and highlight the importance of fungal communities during the successional process.

Complementary belowground strategies underlie species coexistence in an early successional forest

Unravelling belowground strategies is critical for understanding species coexistence and successional dynamics; yet, our knowledge of nutrient acquisition strategies of forest species at different successional stages remains limited. We measured morphological (diameter, specific root length, and root tissue density), architectural (branching ratio), physiological (ammonium, nitrate, and glycine uptake rates) root traits, and mycorrhizal colonisation rates of eight coexisting woody species in an early successional plantation forest in subtropical China. By incorporating physiological uptake efficiency, we revealed a bi-dimensional root economics space comprising of an 'amount-efficiency' dimension represented by morphological and physiological traits, and a 'self-symbiosis' dimension dominated by architectural and mycorrhizal traits. The early pioneer species relied on root-fungal symbiosis, developing densely branched roots with high mycorrhizal colonisation rates for foraging mobile soil nitrate. The late pioneer species invested in roots themselves and allocated effort towards improving uptake efficiency of less-mobile ammonium. Within the root economics space, the covariation of axes with soil phosphorus availability also distinguished the strategy preference of the two successional groups. These results demonstrate the importance of incorporating physiological uptake efficiency into root economics space, and reveal a trade-off between expanding soil physical space exploration and improving physiological uptake efficiency for successional species coexistence in forests.