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Biodiversity across multiple trophic levels is required to maintain multiple ecosystem functions. Yet it remains unclear how multitrophic diversity and species interactions regulate ecosystem multifunctionality. Here, combining data from 9 different trophic groups (including trees, shrubs, herbs, leaf mites, small mammals, bacteria, pathogenic fungi, saprophytic fungi, and symbiotic fungi) and 13 ecosystem functions related to supporting, provisioning, and regulating services, we used a multitrophic perspective to evaluate the effects of elevation, diversity, and network complexity on scale-dependent subalpine forest multifunctionality. Our results demonstrated that elevation and soil pH significantly modified species composition and richness across multitrophic groups and influenced multiple functions simultaneously. We present evidence that species richness across multiple trophic groups had stronger effects on multifunctionality than species richness at any single trophic level. Moreover, biotic associations, indicating the complexity of trophic networks, were positively associated with multifunctionality. The relative effects of diversity on multifunctionality increased at the scale of the larger community compared to a scale accounting for neighboring interactions. Our results highlight the paramount importance of scale- and context-dependent multitrophic diversity and interactions for a better understanding of mountain ecosystem multifunctionality in a changing world.

The 2020 fire season punctuated a decades-long trend of increased fire activity across the western United States, nearly doubling the total area burned in the central Rocky Mountains since 1984. Understanding the causes and implications of such extreme fire seasons, particularly in subalpine forests that have historically burned infrequently, requires a long-term perspective not afforded by observational records. We place 21st century fire activity in subalpine forests in the context of climate and fire history spanning the past 2,000 y using a unique network of 20 paleofire records. Largely because of extensive burning in 2020, the 21st century fire rotation period is now 117 y, reflecting nearly double the average rate of burning over the past 2,000 y. More strikingly, contemporary rates of burning are now 22% higher than the maximum rate reconstructed over the past two millennia, during the early Medieval Climate Anomaly (MCA) (770 to 870 Common Era), when Northern Hemisphere temperatures were ∼0.3 °C above the 20th century average. The 2020 fire season thus exemplifies how extreme events are demarcating newly emerging fire regimes as climate warms. With 21st century temperatures now surpassing those during the MCA, fire activity in Rocky Mountain subalpine forests is exceeding the range of variability that shaped these ecosystems for millennia.

Coniferous forest nitrogen (N) budgets indicate unknown sources of N. A consistent association between limber pine (Pinus flexilis) and potential N₂‐fixing acetic acid bacteria (AAB) indicates that native foliar endophytes may supply subalpine forests with N. To assess whether the P. flexilis–AAB association is consistent across years, we re‐sampled P. flexilis twigs at Niwot Ridge, CO and characterized needle endophyte communities via 16S rRNA Illumina sequencing. To investigate whether endophytes have access to foliar N₂, we incubated twigs with ¹³N₂‐enriched air and imaged radioisotope distribution in needles, the first experiment of its kind using ¹³N. We used the acetylene reduction assay to test for nitrogenase activity within P. flexilis twigs four times from June to September. We found evidence for N₂ fixation in P. flexilis foliage. N₂ diffused readily into needles and nitrogenase activity was positive across sampling dates. We estimate that this association could provide 6.8–13.6 μg N m⁻² d⁻¹ to P. flexilis stands. AAB dominated the P. flexilis needle endophyte community. We propose that foliar endophytes represent a low‐cost, evolutionarily stable N₂‐fixing strategy for long‐lived conifers. This novel source of biological N₂ fixation has fundamental implications for understanding forest N budgets.

Climatic warming alters the onset, duration and cessation of the vegetative season. While previous studies have shown a tight link between thermal conditions and leaf phenology, less is known about the impacts of phenological changes on tree growth. Here, we assessed the relationships between the start of the thermal growing season and tree growth across the extratropical Northern Hemisphere using 3,451 tree-ring chronologies and daily climatic data for 1948–2014. An earlier start of the thermal growing season promoted growth in regions with high ratios of precipitation to temperature but limited growth in cold–dry regions. Path analyses indicated that an earlier start of the thermal growing season enhanced growth primarily by alleviating thermal limitations on wood formation in boreal forests and by lengthening the period of growth in temperate and Mediterranean forests. Semi-arid and dry subalpine forests, however, did not benefit from an earlier onset of growth and a longer growing season, presumably due to associated water loss and/or more frequent early spring frosts. These emergent patterns of how climatic impacts on wood phenology affect tree growth at regional to hemispheric scales hint at how future phenological changes may affect the carbon sequestration capacity of extratropical forest ecosystems. The authors use tree-ring width data across the extratropical Northern Hemisphere to show that earlier growing season onsets lead to enhanced tree radial growth in cold humid but not in dry areas.

1. Species distribution shifts in response to climate change require that recruitment increase beyond current range boundaries. For trees with long life spans, the importance of climate-sensitive seedling establishment to the pace of range shifts has not been demonstrated quantitatively. 2. Using spatially explicit, stochastic population models combined with data from long-term forest surveys, we explored whether the climate-sensitivity of recruitment observed in climate manipulation experiments was sufficient to alter populations and elevation ranges of two widely distributed, high-elevation North American conifers. 3. Empirically observed, warming-driven declines in recruitment led to rapid modelled population declines at the low-elevation, 'warm edge' of subalpine forest and slow emergence of populations beyond the high-elevation, 'cool edge'. Because population declines in the forest occurred much faster than population emergence in the alpine, we observed range contraction for both species. For Engelmann spruce, this contraction was permanent over the modelled time horizon, even in the presence of increased moisture. For limber pine, lower sensitivity to warming may facilitate persistence at low elevations — especially in the presence of increased moisture — and rapid establishment above tree line, and, ultimately, expansion into the alpine. 4. Synthesis. Assuming 21st century warming and no additional moisture, population dynamics in high-elevation forests led to transient range contractions for limber pine and potentially permanent range contractions for Engelmann spruce. Thus, limitations to seedling recruitment with warming can constrain the pace of subalpine tree range shifts.

High severity fire may promote or reduce plant understory diversity in forests. However, few empirical studies have tested long-standing theoretical predictions that productivity may help to explain observed variation in post-fire plant diversity. Support for the influence of productivity on disturbance-diversity relationships is found predominantly in experimental grasslands, while tests over large areas with natural disturbance and productivity gradients are few and have yielded inconsistent results. Here, we measured the response of post-fire understory plant diversity to natural gradients of fire severity and productivity in a large-scale observational study in California’s subalpine forests. We found that plant species richness increased with increasing fire severity and that this trend was stronger at high productivity. We used plant traits to investigate whether release from competition might contribute to increasing diversity and found that short-lived and far-dispersing species benefited more from high severity fire than their long-lived and near-dispersing counterparts. For far-dispersing species only, the benefit from high severity fire was stronger in high productivity plots where unburned species richness was lowest. Our results support theoretical connections between fire severity, productivity and plant communities that are key to predicting the consequences of increasing fire severity and frequency on diversity in the coming decades.

AimsNitrogen (N) deposition increased forest carbon (C) sink significantly, hence exploring the microscopic mechanisms is critical to predicting future global ecosystem C cycle, especially the effects of enhanced N deposition on soil microbial carbon use efficiency (CUE), which still unclear.MethodsWe evaluated the responses of soil microbial CUE to long-term (5 years) N addition in an evergreen broad-leaved forest and a mature coniferous forest by using a 13C isotope tracing method.ResultsThe results showed that the soil microbial CUE ranged from 0.38 to 0.51, which was smaller than the results obtained from the previous studies based the same method and forest type. In evergreen broad-leaved forest, the microbial CUE had no significant changes in the low N-addition treatment, but it was increased by 9.23% and 12.69% in medium and high N-addition treatments compared to the control. In coniferous forest, soil microbial CUE was increased by 14.64%, 21.89% and 24.34% in low, medium and high N-addition treatments, respectively. Moreover, the soil C:P and N:P are negatively relate to soil microbial CUE.ConclusionsOur findings indicate that the enhancing N deposition can increase soil microbial CUE and ultimately promote C sequestration, especially in coniferous forest. The imbalance of soil stoichiometry is the main impact factor of CUE under N addition. However, we speculate that the key to increase forest soil microbial CUE is to promote the decomposition rate of litter and thus increase the available C content.

Aims Subalpine forest ecosystems are sensitive to climate change, and extracellular enzyme activities and microbial metabolic limitation in soils are influenced by multiple factors including climate. The study aims to reveal the extracellular enzyme characteristics and microbial metabolic limitation in soils and their key drivers along an elevation gradient in subalpine forest ecosystems. Methods Microbial metabolic limitations in bulk and rhizosphere soils under Quercus aquifolioides forest along an elevation gradient (3100-4180 m a.s.l) of Pamuling Mountain on the eastern Qinghai-Tibetan Plateau were investigated by enzyme stoichiometry theory and path modelling analysis. Results Elevation had significant effects on the carbon (C)-, nitrogen (N)-, and phosphorus (P)-acquiring enzyme activities, but the difference between bulk and rhizosphere soil was non-significant. Microbial metabolism was mainly limited by C and P, and showed a tendency to gradual shift from C and P limitation to C and N limitation at high elevation. The C limitation of microbial metabolism showed an increasing trend with elevation, implying greater C limitation at higher elevations where temperatures were lower. Therefore, we infer that the increase in temperature may help to alleviate C limitation of microbial metabolism in subalpine forest. Soil available nutrients affected N and P limitations of microbial metabolism, and soil pH, nutrient ratios and available nutrients mainly affected C limitation. Conclusions There were significant influences of elevation on soil enzyme activities and C limitation of microbial metabolism. Soil pH, nutrient stoichiometric characteristics and available nutrients were the key factors affecting soil microbial metabolism in subalpine Q. aquifolioides forest ecosystems.

In forests adapted to infrequent (> 100-year) stand-replacing fires, novel short-interval (< 30-year) fires burn young forests before they recover from previous burns. Postfire tree regeneration is reduced, plant communities shift, soils are hotter and drier, but effects on biogeochemical cycling are unresolved. We asked how postfire nitrogen (N) stocks, N availability and N fixation varied in lodgepole pine (Pinus contorta var. latifolia) forests burned at long and short intervals in Grand Teton National Park (Wyoming, USA). In 2021 and 2022, we sampled 0.25-ha plots that burned as long-interval (> 130-year) stand-replacing fire in 2000 (n = 3) or 2016 (n = 3) and nearby plots of short-interval (16-year) fire that burned as stand-replacing fire in both years (n = 6 ‘reburns’). Five years postfire, aboveground N stocks were 31% lower in short- versus long-interval fire (77 vs. 109 kg N ha−1, respectively) and 76% lower than 21-year-old stands that did not reburn (323 kg N ha−1). However, soil total N averaged 1,072 kg N ha−1 and dominated ecosystem N stocks, which averaged 1,235 kg N ha−1 and did not vary among burn categories. Annual resin-sorbed nitrate was highest in reburns and positively correlated with understory species richness and biomass. Lupinus argenteus was sparse, and asymbiotic N fixation rates were modest in all plots (< 0.1 kg N ha−1 y−1). Although ecosystem N stocks were unaffected, high-severity short-interval fire reduced and repartitioned aboveground N stocks and increased N availability. These shifts in N pools and fluxes suggest reburns can markedly alter N cycling in subalpine forests.

Gaseous ammonia (NH3) is an important precursor for secondary aerosol formation and contributes to reactive nitrogen deposition. NH3 dry deposition is poorly quantified due to the complex bidirectional nature of NH3 atmosphere-surface exchange and lack of high time-resolution in situ NH3 concentration and meteorological measurements. To better quantify NH3 dry deposition, measurements of NH3 were made above a subalpine forest canopy in Rocky Mountain National Park (RMNP) and used with in situ micrometeorology to simulate bidirectional fluxes. NH3 dry deposition was largest during the summer, with 47 % of annual net NH3 dry deposition occurring in June, July, and August. Because in situ, high time-resolution concentration and meteorological data are often unavailable, the impacts on estimated deposition from utilizing more commonly available biweekly NH3 measurements and ERA5 meteorology were evaluated. Fluxes simulated with biweekly NH3 concentrations, commonly available from NH3 monitoring networks, underestimated NH3 dry deposition by 45 %. These fluxes were strongly correlated with 30 min fluxes integrated to a biweekly basis (R2 = 0.88), indicating that a correction factor could be applied to mitigate the observed bias. Application of an average NH3 diel concentration pattern to the biweekly NH3 concentration data removed the observed low bias. Annual NH3 dry deposition from fluxes simulated with reanalysis meteorological inputs exceeded simulations using in situ meteorology measurements by a factor of 2.