Abstract Soil nitrogen (N) and phosphorus (P) availability generally decrease with forest stand development, yet how tree species acclimate to this decline remains largely unclear. The functional coordination hypothesis proposes that trees concurrently improve leaf nutrient recycling and root nutrient acquisition to cope with nutrient deficiency. Conversely, the resource limitation hypothesis posits that trees prioritize either strategy, as simultaneous employment of both strategies is resource costly. To test these conflicting hypotheses, we leveraged a chronosequence of Pinus sylvestris var. mongolica stands with six age classes ranging from 16‐ to 62‐year‐old. We measured a comprehensive set of 70 plant and soil variables including needle nutrient retranslocation and resorption, and root architectural, morphological, physiological and mycorrhizal traits to assess the response of needle nutrient recycling and root nutrient acquisition to changes in stand nutrient status. Stand development reduced soil N and P availability and caused a shift from N to P limitation as evidenced by increasing needle N:P ratios and decreasing N:P‐acquisition enzyme ratios. There was no evidence of N or P retranslocation from old to young needles, and nutrient resorption efficiency was independent of soil nutrient availability, indicating that adjustments of needle nutrient recycling contribute little to mitigating nutrient deficiency. Root length density decreased exponentially with stand development, whereas cord‐forming ectomycorrhizal fungi (EMF), root phosphatase activity and rhizosphere effects on plant‐available soil P logarithmically increased, implying a shift from root nutrient scavenging to combined EMF nutrient scavenging and root P mining. Synthesis . Our findings suggest that tree species do not necessarily rely on both improved recycling and acquisition to acclimate to soil nutrient scarcity with stand development. However, trees can simultaneously depend on physiological adaptations of absorptive fine roots themselves, as well as on a constrained selection of EMF. This reveals that complex sets of adaptations, both root‐based ‘do‐it‐yourself’ and mycorrhizal‐based ‘outsourcing’, can be effectively used by trees to cope with changes in soil nutrient availability.
Abstract Predicting plant responses to climate change, such as shifting of mountain treelines, requires a mechanistic understanding of how abiotic and biotic factors interact to regulate tree regeneration. Although climate warming is anticipated to alleviate thermal constraints on tree growth beyond the current treeline limits, the interaction between biotic facilitation and climate in regulating tree regeneration remains poorly understood. In this study, we aimed to unravel the interactive role of microclimate, functional traits, soil properties and shrub facilitation on the recruitment of Himalayan birch tree, Betula utilis . For this, we combined controlled seed germination and seedling production with in‐situ transplant experiment at two sites along the treeline elevational gradient: below treeline (BTL), at treeline (TLN) and above treeline (ATL), supplemented with 3‐year field monitoring and multi‐driver modelling. We ran response‐surface and facilitation‐surface models to simulate the combined effects of soil warming and co‐varying environmental shifts on seedling survival and facilitation stability. The tree recruitment revealed a unimodal pattern, with both seed germination (23–31%) and seedling survival peaking at TLN. Seedling survival was predominantly dependent on biotic facilitation, with a 45% mean reduction in survival probability in open microsites compared to those beneath shrub canopies in TLN and ATL. Soil temperature emerged as the dominant abiotic driver, explaining 60–80% of the variation in modelled seedling survival. Our results reveal a novel warming‐facilitation paradox: while warming of +1.66°C could create thermal conditions in ATL analogous to the current optimum recruitment conditions of TLN, this same warming was predicted to cause a catastrophic decline in facilitation stability, with beneficial effects of shrubs rapidly reducing beyond a threshold of +1.5–1.7°C. Synthesis . Our findings demonstrate that the abiotic potential for treeline shift is nullified by the biotic collapse of critical facilitative recruitment niches. The current treeline represents a microclimatically modulated and biotically buffered optimum with no future analogue under climate warming. Our study provides a mechanistic explanation for widely reported treeline vegetation lag to climate and also posits that predictive models failing to incorporate the thermal sensitivity of biotic facilitation and its interaction with soil and resource‐acquisition strategies will overestimate climate warming‐induced treeline shifts.
Abstract Maintaining temporal community stability during ecological restoration is essential for sustaining ecosystem services, yet how stabilizing pathways shift directionally across restoration stages remains poorly understood. Here, we investigated the temporal dynamics of three stabilizing effects (dominance, asynchrony and averaging effects) during alpine grassland restoration on the Qinghai–Tibetan Plateau over a 13‐year sequence. Temporal stability was partitioned into dominance, asynchrony and averaging effects and tracked across nine consecutive 5‐year rolling windows, integrating longitudinal monitoring (Years 1–5) with a chronosequence approach (Years 6–13). Community evenness, species richness and stability trajectories were modelled using linear mixed‐effects models, and mediation analysis examined the pathway linking evenness to the dominance effect via Taylor's scaling exponent b . Community evenness increased overall despite declining species richness ( R 2 = 0.834, p < 0.001). The composition of stabilizing pathways shifted progressively along the restoration sequence, from asynchrony‐dominated in early stages to averaging effect‐dominated in the later, chronosequence‐based windows. Notably, the averaging effect reversed from destabilizing ( ω > 1) when diversity was low to strongly stabilizing as the community developed. Evenness regulated the dominance effect primarily through Taylor's scaling exponent b , with the indirect pathway accounting for most of the total mediation effect (Sobel test: Z = −4.54, p < 0.001). Synthesis . Stabilizing pathways shift sequentially during restoration, with statistical averaging superseding asynchrony‐based stabilization in the later stages. Rather than net stability alone, it is the changing composition of stabilizing mechanisms that best characterizes how communities are stabilized as restoration proceeds. These findings identify a management opportunity in mid‐to‐late restoration, during which promoting species evenness may strengthen the mechanisms underpinning long‐term community stability.
Abstract Silicon (Si) is increasingly linked to shifts in plant carbon (C), nitrogen (N) and phosphorus (P) stoichiometry. However, reported responses vary widely across records. A quantitative synthesis is therefore needed to understand the overall direction and magnitude of these responses. We conducted a meta‐analysis of 168 records to quantify responses of plant C, N and P stoichiometry and nutrient use efficiency (NUE) to Si addition. Overall, Si addition reduced C concentration by 2.3% and C:P by 5% and increased P concentration by 8% and NUE by 14%. Plant N concentrations and C:N ratios showed no significant response. Reductions in C and C:P and increases in NUE were stronger in Poaceae, whereas the P increase was larger in non‐Poaceae. The Si‐C relationship was nonlinear and differed between families, and the P increase occurred only in substrate‐grown plants, not in hydroponics. This suggests no single mechanism accounted for the stoichiometric responses across plant families. Synthesis. Si could regulate plant C and P stoichiometry with potential ecological implications. By mobilising P and improving NUE, Si may help mitigate nutrient constraints on biomass production and shape plant performance. Furthermore, reduced plant C:P ratios translate into P‐rich and high‐quality litter, which may in turn alter litter decomposition and soil nutrient cycling.
Abstract Drought legacies can shape how grasslands respond to increasingly frequent climate extremes, yet their influence on future resistance and recovery remains unclear, particularly in mesic grasslands, where productivity is strongly linked to precipitation availability. Using a long‐term field experiment, we tested how zero, one or two prior experimentally imposed extreme droughts influenced grassland resistance to a subsequent 4‐year extreme drought (2018–2021) and recovery after drought in 2022. We quantified above‐ground net primary productivity and species composition, as well as plant functional traits and arbuscular mycorrhizal (AM) fungal colonization of the dominant grasses at the site, to evaluate above‐ and below‐ground legacy effects. Differences in productivity resistance among drought‐legacy treatments were small during the first three drought years and became evident only in the final, most severe drought year, when resistance was lowest in plots exposed to two prior droughts. However, above‐ground productivity recovered to similar levels across all treatments in 2022, demonstrating substantial short‐term recovery despite contrasting drought histories. The dominant grass, Andropogon gerardii , exhibited higher specific leaf area and greater leaf production during the recovery year, indicating increased investment in rapid resource capture and regrowth. Additionally, AM fungal colonization of A. gerardii declined with repeated drought exposure and remained lowest in plots exposed to two prior droughts even after above‐ground productivity recovered. In contrast to these traits and below‐ground changes, species composition, including richness, evenness, turnover and absolute cover, remained stable across all treatments and years. Synthesis: Repeated drought reduced resistance under the most severe drought conditions, yet this mesic grassland recovered rapidly once drought ended. At the same time, AM fungal colonization of the dominant grass remained reduced, revealing a mismatch between above‐ground recovery and below‐ground legacies and showing that different ecosystem components can recover at different rates.
