Catherine Cleveland

Natural disturbances induce variable recovery pathways, and long‐term studies are key to understanding how that variability affects ecosystem function over time and across space. In subalpine forests, high‐severity wildfires produce high variability in postfire tree density, but how this variability relates to nitrogen (N) cycling remains unclear. We resampled lodgepole pine ( Pinus contorta var. latifolia ) stands 35 years after stand‐replacing fire and asked (1) how N concentrations, pools, and mineralization changed with increasing lodgepole pine biomass from 15 to 35 years postfire; (2) how N concentrations, pools, and rates of change varied across the gradient of early postfire stand density; and (3) whether among‐stand structure and function were converging over time. Vegetation, forest floor litter, downed coarse wood, surface mineral soil (0–15 cm), and resin‐sorbed N were measured in fourteen 0.25‐ha plots in Yellowstone National Park (Wyoming, USA). From 25 to 35 years postfire, average total aboveground N pools increased from 252 to 405 kg N ha ⁻¹ . Over the same 10‐year period, soil N pools averaged 986 kg N ha ⁻¹ and did not change, resin‐sorbed soil N remained low, and the only common symbiotic N‐fixing plant in the plots (silvery lupine, Lupinus argenteus ) remained sparse. Aboveground lodgepole pine biomass increased at similar rates from 15 to 25 and 25 to 35 years postfire (2.5 and 2.2 Mg ha ⁻¹ year ⁻¹ , respectively) along with N pools (4.6 and 3.8 kg N ha ⁻¹ year ¹ ) and forest floor litter N (1.2 and 2.2 kg N ha ⁻¹ year ¹ ). Among stands 35 years postfire, total aboveground N varied threefold (212–638 kg N ha ⁻¹ ) and was positively correlated with lodgepole pine aboveground net primary productivity and biomass but unrelated to stem density. Soil N pools ranged from 570 to 1668 kg N ha ⁻¹ and were negatively correlated with lodgepole pine stem density but unrelated to tree productivity or biomass. Among‐stand variation in stem density remained high but declined over time while N pools became less variable even as pools increased and variability in tree productivity increased from 25 to 35 years postfire. Rapid tree growth and biomass accumulation strongly regulate the N cycle in 35‐year‐old lodgepole pine stands, but sources of recovered N remain unresolved.

Enemy release may promote plant invasions and trigger rapid evolution in traits related to growth and defence in non‐native populations. However, to what extent plants escape enemies, such as root‐associated pathogens, is little known as comparisons between native and non‐native ranges remain scarce. Also, whether plant–pathogen interactions vary between native and non‐native populations under common garden conditions, and if such differences coincide with plant performance, is poorly understood. We identified putative fungal pathogens (hereafter pathogens) in roots and rhizosphere soil of the invader Conyza canadensis . We sampled 17 populations in the native range and 17 in the non‐native range spanning wide and comparable environmental gradients across both ranges. We also sampled soil from adjacent plant communities where C. canadensis was absent. Potential evolutionary shifts in how C. canadensis shapes pathogen communities were assessed by growing native and non‐native populations in native soil in the glasshouse (round 1). We then grew seedlings from the same populations in their conditioned soil from round 1 and compared growth to plants grown in sterile soil (round 2). Pathogen communities were dominated by generalist taxa and differed between ranges for both C. canadensis and adjacent communities. However, pathogen richness associated with C. canadensis was greater in the non‐native range and proportionally more pathogens occurred in the rhizosphere than in roots relative to C. canadensis in the native range. The ratio of pathogen abundance in rhizosphere versus roots correlated positively with C. canadensis shoot biomass in the non‐native, but not native, range. In the glasshouse, non‐native C. canadensis promoted more and different pathogens in the rhizosphere than native C. canadensis in round 1, yet was less suppressed by soil biota in round 2. Synthesis . Differences in pathogen communities across ranges may have caused evolutionary changes in plant–pathogen associations in C. canadensis . Populations from the non‐native range accumulated pathogens in their rhizosphere while appearing better defended, which coincided with greater performance in both the field and glasshouse relative to native populations. Our findings are relevant to shifting defences, fungal multifunctionality, and the overlooked role of generalist pathogen accumulation in exotic plant invasions.

Severe, stand‐replacing wildfire substantially depletes nitrogen (N) stocks in subalpine conifer forests, potentially exacerbating N limitation of net primary productivity in many forested regions where fire frequency is increasing. In lodgepole pine (Pinus contorta var. latifolia) forests in the Greater Yellowstone Ecosystem (GYE), long‐term data show surface soil and biomass N stocks are replenished during the first few decades following wildfire, but the source(s) of that N are unclear. We measured acetylene reduction rates in multiple cryptic niches (i.e., lichen, moss, pine litter, dead wood, and mineral soil) in 34‐year‐old lodgepole pine stands in the GYE to explore the rates, temporal patterns, and climate controls on cryptic N fixation. Acetylene reduction rates were highest in late May (0.376 nmol C₂H₄ g⁻¹ h⁻¹) when moisture availability was high compared with early August and mid‐October when moisture was relatively low (0.112 and 0.002 nmol C₂H₄ g⁻¹ h⁻¹, respectively). We observed modest rates of nitrogenase activity in a few niches following a mid‐summer rain event, suggesting that moisture is an important factor regulating field‐based N fixation rates. In a laboratory experiment, moss responded more strongly to temperature and moisture variation than all other niches. Acetylene reduction rates in dead wood increased with temperature but not moisture content. No other niches showed clear responses to either moisture or temperature manipulation. Together, the field and laboratory results suggest that frequent asynchrony between favorable temperature and moisture conditions may limit N fixation rates in the field. Overall, total annual cryptic N fixation inputs (mean: 0.26; range: 0.07–2.9 kg N ha⁻¹ year⁻¹) represented −1 year⁻¹), pointing to a still unknown source of ecosystem N following fire.

Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high‐severity wildfires in forests with historically frequent, low‐severity fire regimes are increasingly common, but long‐term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20‐year responses to thinning and prescribed burning treatments commonly used in dry, low‐elevation forests of the western United States from a long‐term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short‐term (Dendroctonus ponderosae) outbreak that impacted the site 5–10 years post‐treatment and describe 20‐year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB‐induced changes to forest structure and fuels increased the fire hazard 20 years post‐treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.

Increased plant growth under elevated carbon dioxide (CO₂) slows the pace of climate warming and underlies projections of terrestrial carbon (C) and climate dynamics. However, this important ecosystem service may be diminished by concurrent changes to vegetation carbon‐to‐nitrogen (C:N) ratios. Despite clear observational evidence of increasing foliar C:N under elevated CO₂, our understanding of potential ecological consequences of foliar stoichiometric flexibility is incomplete. Here, we illustrate that when we incorporated CO₂‐driven increases in foliar stoichiometry into the Community Land Model the projected land C sink decreased two‐fold by the end of the century compared to simulations with fixed foliar chemistry. Further, CO₂‐driven increases in foliar C:N profoundly altered Earth's hydrologic cycle, reducing evapotranspiration and increasing runoff, and reduced belowground N cycling rates. These findings underscore the urgency of further research to examine both the direct and indirect effects of changing foliar stoichiometry on soil N cycling and plant productivity.

Plant element stoichiometry and stoichiometric flexibility strongly regulate ecosystem responses to global change. Here, we tested three potential mechanistic drivers (climate, soil nutrients, and plant taxonomy) of both using paired foliar and soil nutrient data from terrestrial forested National Ecological Observatory Network sites across the USA. We found that broad patterns of foliar nitrogen (N) and foliar phosphorus (P) are explained by different mechanisms. Plant taxonomy was an important control over all foliar nutrient stoichiometries and concentrations, especially foliar N, which was dominantly related to taxonomy and did not vary across climate or soil gradients. Despite a lack of site‐level correlations between N and environment variables, foliar N exhibited intraspecific flexibility, with numerous species‐specific correlations between foliar N and various environmental factors, demonstrating the variable spatial and temporal scales on which foliar chemistry and stoichiometric flexibility can manifest. In addition to plant taxonomy, foliar P and N:P ratios were also linked to soil nutrient status (extractable P) and climate, especially actual evapotranspiration rates. Our findings highlight the myriad factors that influence foliar chemistry and show that broad patterns cannot be explained by a single consistent mechanism. Furthermore, differing controls over foliar N versus P suggests that each may be sensitive to global change drivers on distinct spatial and temporal scales, potentially resulting in altered ecosystem N:P ratios that have implications for processes ranging from productivity to carbon sequestration.

Biological nitrogen (N₂) fixation represents the largest source of new nitrogen to ecosystems. Many N-fixers are well documented, but new sources of N via fixation – or N-fixing niches – are still emerging. Here, the authors reveal a surprising new site of N fixation: free-living bacteria living in biofilms on the shells of fiddler crabs in mangrove ecosystems. This newly revealed "cryptic" source of N fixation could represent an important source of N fertility to mangroves globally, and this new discovery underscores the likely importance of cryptic N-fixers in most, if not all, ecosystems.

This very well-presented synthesis offers a comprehensive, rigorous, and balanced review of the evidence for a carbon dioxide (CO₂) fertilization effect on the terrestrial biosphere. Like the Intergovernmental Panel on Climate Change (IPCC), the authors present evidence for the effects of CO₂ on various ecosystem pools/processes with varying levels of confidence assigned to each, showing where some of the most significant opportunities remain in carbon cycle science inquiry. This approach was honest and refreshing. This comprehensive and up-to-date review of the potential large-scale effects of increasing atmospheric CO₂ should be a valuable resource for both researchers and teachers.