Reintroducing bison results in long-running and resilient increases in grassland diversity.

The widespread extirpation of megafauna may have destabilized ecosystems and altered biodiversity globally. Most megafauna extinctions occurred before the modern record, leaving it unclear how their loss impacts current biodiversity. We report the long-term effects of reintroducing plains bison (Bison bison) in a tallgrass prairie versus two land uses that commonly occur in many North American grasslands: 1) no grazing and 2) intensive growing-season grazing by domesticated cattle (Bos taurus). Compared to ungrazed areas, reintroducing bison increased native plant species richness by 103% at local scales (10 m2) and 86% at the catchment scale. Gains in richness continued for 29 y and were resilient to the most extreme drought in four decades. These gains are now among the largest recorded increases in species richness due to grazing in grasslands globally. Grazing by domestic cattle also increased native plant species richness, but by less than half as much as bison. This study indicates that some ecosystems maintain a latent potential for increased native plant species richness following the reintroduction of native herbivores, which was unmatched by domesticated grazers. Native-grazer gains in richness were resilient to an extreme drought, a pressure likely to become more common under future global environmental change.

Contrasting shrub and grass hydraulic responses to experimental drought.

Whole-plant hydraulics provide important information about responses to water limitation and can be used to understand how plant communities may change in a drier climate when measured on multiple species. Here, we measured above- and belowground hydraulic traits in Cornus drummondii, an encroaching shrub within North American tallgrass prairies, and Andropogon gerardii, a dominant C4 grass, to assess the potential hydraulic responses to future drought as this region undergoes woody expansion. Shelters that reduced precipitation by 50% and 0% were built over shrubs and grasses growing in sites that are burned at 1-year and 4-year frequencies. We then measured aboveground (Kshoot), belowground (Kroot), and whole-plant maximum hydraulic conductance (Kplant) in C. drummondii and Kroot in A. gerardii. We also measured vulnerability to embolism (P50) in C. drummondii stems. Overall, we show that: (1) A. gerardii had substantially greater Kroot than C. drummondii; (2) belowground hydraulic functioning was linked with aboveground processes; (3) above- and belowground C. drummondii hydraulics were not negatively impacted by the rainfall reductions imposed here. These results suggest that a multi-year drought will not ameliorate rates of woody expansion and highlight key differences in aboveground and belowground hydraulics for dominant species within the same ecosystem.

Linking resource- and disturbance-based models to explain tree-grass coexistence in savannas.

Savannas cover a significant fraction of the Earth's land surface. In these ecosystems, C3 trees and C4 grasses coexist persistently, but the mechanisms explaining coexistence remain subject to debate. Different quantitative models have been proposed to explain coexistence, but these models make widely contrasting assumptions about which mechanisms are responsible for savanna persistence. Here, we show that no single existing model fully captures all key elements required to explain tree-grass coexistence across savanna rainfall gradients, but many models make important contributions. We show that recent empirical work allows us to combine many existing elements with new ideas to arrive at a synthesis that combines elements of two dominant frameworks: Walter's two-layer model and demographic bottlenecks. We propose that functional rooting separation is necessary for coexistence and is the crux of the coexistence problem. It is both well-supported empirically and necessary for tree persistence, given the comprehensive grass superiority for soil moisture acquisition. We argue that eventual tree dominance through shading is precluded by ecohydrological constraints in dry savannas and by fire and herbivores in wet savannas. Strong asymmetric grass-tree competition for soil moisture limits tree growth, exposing trees to persistent demographic bottlenecks.

African savanna grasses outperform trees across the full spectrum of soil moisture availability.

Models of tree-grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade-off between water use under wet conditions and drought tolerance. We measured whole-plant hourly transpiration in a growth chamber and quantified drought tolerance using leaf osmotic potential (Ψosm ). We also quantified whole-plant water-use efficiency (WUE) and relative growth rate (RGR) under well-watered conditions. Grasses transpired twice as much as trees on a leaf-mass basis across all soil water potentials. Grasses also had a lower Ψosm than trees, indicating higher drought tolerance in the former. Higher grass transpiration and WUE combined to largely explain the threefold RGR advantage in grasses. Our results suggest that grasses outperform trees under a wide range of conditions, and that there is no evidence for a trade-off in water-use patterns in wet vs dry soils. This work will help inform mechanistic models of water use in savanna ecosystems, providing much-needed whole-plant parameter estimates for African species.

Evolutionary lineage explains trait variation among 75 coexisting grass species.

Evolutionary history plays a key role driving patterns of trait variation across plant species. For scaling and modeling purposes, grass species are typically organized into C3 vs C4 plant functional types (PFTs). Plant functional type groupings may obscure important functional differences among species. Rather, grouping grasses by evolutionary lineage may better represent grass functional diversity. We measured 11 structural and physiological traits in situ from 75 grass species within the North American tallgrass prairie. We tested whether traits differed significantly among photosynthetic pathways or lineages (tribe) in annual and perennial grass species. Critically, we found evidence that grass traits varied among lineages, including independent origins of C4 photosynthesis. Using a rigorous model selection approach, tribe was included in the top models for five of nine traits for perennial species. Tribes were separable in a multivariate and phylogenetically controlled analysis of traits, owing to coordination of important structural and ecophysiological characteristics. Our findings suggest grouping grass species by photosynthetic pathway overlooks variation in several functional traits, particularly for C4 species. These results indicate that further assessment of lineage-based differences at other sites and across other grass species distributions may improve representation of C4 species in trait comparison analyses and modeling investigations.

Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal.

Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure from local (land use/land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here we report on a decade-long watershed-scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before-after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, associated with a decline in streamflow, loss of grassy plant species, and other ecosystem-scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for 3 years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when reseeded with grassland species. Rapid expansion of shrubs (Cornus drummondii, Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every 2 years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands, resulting in inexorable movement toward a new ecosystem state. Human pressures, such as climate change, atmospheric CO2 increases, and elevated atmospheric nitrogen deposition, could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well-studied sites.

Kernel weight contribution to yield genetic gain of maize: a global review and US case studies.

Over the past century of maize (Zea mays L.) breeding, grain yield progress has been the result of improvements in several other intrinsic physiological and morphological traits. In this study, we describe (i) the contribution of kernel weight (KW) to yield genetic gain across multiple agronomic settings and breeding programs, and (ii) the physiological bases for improvements in KW for US hybrids. A global-scale literature review concludes that rates of KW improvement in US hybrids were similar to those of other commercial breeding programs but extended over a longer period of time. There is room for a continued increase of kernel size in maize for most of the genetic materials analysed, but the trade-off between kernel number and KW poses a challenge for future yield progress. Through phenotypic characterization of Pioneer Hi-Bred ERA hybrids in the USA, we determine that improvements in KW have been predominantly related to an extended kernel-filling duration. Likewise, crop improvement has conferred on modern hybrids greater KW plasticity, expressed as a better ability to respond to changes in assimilate availability. Our analysis of past trends and current state of development helps to identify candidate targets for future improvements in maize.

N and P constrain C in ecosystems under climate change: Role of nutrient redistribution, accumulation, and stoichiometry.

We use the Multiple Element Limitation (MEL) model to examine responses of 12 ecosystems to elevated carbon dioxide (CO2 ), warming, and 20% decreases or increases in precipitation. Ecosystems respond synergistically to elevated CO2 , warming, and decreased precipitation combined because higher water-use efficiency with elevated CO2 and higher fertility with warming compensate for responses to drought. Response to elevated CO2 , warming, and increased precipitation combined is additive. We analyze changes in ecosystem carbon (C) based on four nitrogen (N) and four phosphorus (P) attribution factors: (1) changes in total ecosystem N and P, (2) changes in N and P distribution between vegetation and soil, (3) changes in vegetation C:N and C:P ratios, and (4) changes in soil C:N and C:P ratios. In the combined CO2 and climate change simulations, all ecosystems gain C. The contributions of these four attribution factors to changes in ecosystem C storage varies among ecosystems because of differences in the initial distributions of N and P between vegetation and soil and the openness of the ecosystem N and P cycles. The net transfer of N and P from soil to vegetation dominates the C response of forests. For tundra and grasslands, the C gain is also associated with increased soil C:N and C:P. In ecosystems with symbiotic N fixation, C gains resulted from N accumulation. Because of differences in N versus P cycle openness and the distribution of organic matter between vegetation and soil, changes in the N and P attribution factors do not always parallel one another. Differences among ecosystems in C-nutrient interactions and the amount of woody biomass interact to shape ecosystem C sequestration under simulated global change. We suggest that future studies quantify the openness of the N and P cycles and changes in the distribution of C, N, and P among ecosystem components, which currently limit understanding of nutrient effects on C sequestration and responses to elevated CO2 and climate change.

Climate variability supersedes grazing to determine the anatomy and physiology of a dominant grassland species.

Grassland ecosystems are historically shaped by climate, fire, and grazing which are essential ecological drivers. These grassland drivers influence morphology and productivity of grasses via physiological processes, resulting in unique water and carbon-use strategies among species and populations. Leaf-level physiological responses in plants are constrained by the underlying anatomy, previously shown to reflect patterns of carbon assimilation and water-use in leaf tissues. However, the magnitude to which anatomy and physiology are impacted by grassland drivers remains unstudied. To address this knowledge gap, we sampled from three locations along a latitudinal gradient in the mesic grassland region of the central Great Plains, USA during the 2018 (drier) and 2019 (wetter) growing seasons. We measured annual biomass and forage quality at the plot level, while collecting physiological and anatomical traits at the leaf-level in cattle grazed and ungrazed locations at each site. Effects of ambient drought conditions superseded local grazing treatments and reduced carbon assimilation and total productivity in A. gerardii. Leaf-level anatomical traits, particularly those associated with water-use, varied within and across locations and between years. Specifically, xylem area increased when water was more available (2019), while xylem resistance to cavitation was observed to increase in the drier growing season (2018). Our results highlight the importance of multi-year studies in natural systems and how trait plasticity can serve as vital tool and offer insight to understanding future grassland responses from climate change as climate played a stronger role than grazing in shaping leaf physiology and anatomy.

Fire frequency, state change and hysteresis in tallgrass prairie.

Hysteresis is a fundamental characteristic of alternative stable state theory, yet evidence of hysteresis is rare. In mesic grasslands, fire frequency regulates transition from grass- to shrub-dominated system states. It is uncertain, however, if increasing fire frequency can reverse shrub expansion, or if grass-shrub dynamics exhibit hysteresis. We implemented annual burning in two infrequently burned grasslands and ceased burning in two grasslands burned annually. With annual fires, grassland composition converged on that of long-term annually burned vegetation due to rapid recovery of grass cover, although shrubs persisted. When annual burning ceased, shrub cover increased, but community composition did not converge with a long-term infrequently burned reference site because of stochastic and lagged dispersal by shrubs, reflecting hysteresis. Our results demonstrated that annual burning can slow, but not reverse, shrub encroachment. In addition, reversing fire frequencies resulted in hysteresis because vegetation trajectories from grassland to shrubland differed from those of shrubland to grassland.

Microanatomical traits track climate gradients for a dominant C4 grass species across the Great Plains, USA.

BACKGROUND AND AIMS: Andropogon gerardii is a highly productive C4 grass species with a large geographic range throughout the North American Great Plains, a biome characterized by a variable temperate climate. Plant traits are often invoked to explain growth rates and competitive abilities within broad climate gradients. For example, plant competition models typically predict that species with large geographic ranges benefit from variation in traits underlying high growth potential. Here, we examined the relationship between climate variability and leaf-level traits in A. gerardii, emphasizing how leaf-level microanatomical traits serve as a mechanism that may underlie variation in commonly measured traits, such as specific leaf area (SLA). METHODS: Andropogon gerardii leaves were collected in August 2017 from Cedar Creek Ecosystem Science Reserve (MN), Konza Prairie Biological Station (KS), Platte River Prairie (NE) and Rocky Mountain Research Station (SD). Leaves from ten individuals from each site were trimmed, stained and prepared for fluorescent confocal microscopy to analyse internal leaf anatomy. Leaf microanatomical data were compared with historical and growing season climate data extracted from PRISM spatial climate models. KEY RESULTS: Microanatomical traits displayed large variation within and across sites. According to AICc (Akaike's information criterion adjusted for small sample sizes) selection scores, the interaction of mean precipitation and temperature for the 2017 growing season was the best predictor of variability for the anatomical and morphological traits measured here. Mesophyll area and bundle sheath thickness were directly correlated with mean temperature (annual and growing season). Tissues related to water-use strategies, such as bulliform cell and xylem area, were significantly correlated with one another. CONCLUSIONS: The results indicate that (1) microanatomical trait variation exists within this broadly distributed grass species, (2) microanatomical trait variability appears likely to impact leaf-level carbon and water use strategies, and (3) microanatomical trait values vary across climate gradients, and may underlie variation in traits measured at larger ecological scales.

Transcriptional responses to water stress and recovery in a drought-tolerant fescue wild grass (Festuca ovina; Poaceae).

Water stress associated with drought-like conditions is a major factor limiting plant growth and impacts productivity of natural plant communities and agricultural crops. Molecular responses of plants to water stress have been studied most extensively in model species and crops, few of which have evolved natural drought tolerance. In the current study, we examined physiological and transcriptomic responses at multiple timepoints during increasing water stress and following initial recovery from stress in a drought-tolerant C3 species, Festuca ovina. Results demonstrated non-linear transcriptomic changes during increasing stress, but largely linear declines in physiological measurements during this same period. Transcription factors represented approximately 12.7% of all differentially expressed genes. In total, 117 F. ovina homologs of previously identified and molecularly characterized drought-responsive plant genes were identified. This information will be valuable for further investigations of the molecular mechanisms involved in drought tolerance in C3 plants.

Fire and browsing interact to alter intra-clonal stem dynamics of an encroaching shrub in tallgrass prairie.

The expansion of woody species into grasslands has altered community structure and ecosystem function of grasslands worldwide. In tallgrass prairie of the Central Great Plains, USA, decreased fire frequency and intensity have increased the cover and abundance of woody species. In particular, clonal shrub cover has increased at accelerated rates due to vegetative reproduction and resprouting after disturbance. We measured the intra-clonal stem demography and relative growth rates (estimated change in woody biomass) of the shrub Cornus drummondii in response to fire frequency (4 vs 20 year burn intervals) and simulated browsing during the 2018 and 2019 growing seasons at Konza Prairie Biological Station (Manhattan, Kansas). Overall, infrequent fire (4 year burn interval) increased intra-clonal stem relative growth rates and shrub relative growth rates. Intra-clonal stem relative growth rates were reduced in unbrowsed clones in 2018 due to drought and simulated browsing reduced intra-clonal stem relative growth rates in 2019. Additionally, simulated browsing nearly eliminated flower production within clones but did not affect intra-clonal stem mortality or recruitment within a growing season. Fire in conjunction with simulated browsing reduced estimated relative growth rates for entire shrub clones. Browsed shrubs that experienced prescribed fire in 2017 had reduced intra-clonal stem densities compared to unbrowsed shrubs and stem densities of browsed shrubs did not recover in 2018 or 2019. These results illustrate that infrequent fire alone promotes the expansion of clonal shrubs in tallgrass prairie and multiple interacting disturbances (e.g., fire and browsing) are required to control the spread of clonal shrubs into grasslands.

Lineage-based functional types: characterising functional diversity to enhance the representation of ecological behaviour in Land Surface Models.

Process-based vegetation models attempt to represent the wide range of trait variation in biomes by grouping ecologically similar species into plant functional types (PFTs). This approach has been successful in representing many aspects of plant physiology and biophysics but struggles to capture biogeographic history and ecological dynamics that determine biome boundaries and plant distributions. Grass-dominated ecosystems are broadly distributed across all vegetated continents and harbour large functional diversity, yet most Land Surface Models (LSMs) summarise grasses into two generic PFTs based primarily on differences between temperate C3 grasses and (sub)tropical C4 grasses. Incorporation of species-level trait variation is an active area of research to enhance the ecological realism of PFTs, which form the basis for vegetation processes and dynamics in LSMs. Using reported measurements, we developed grass functional trait values (physiological, structural, biochemical, anatomical, phenological, and disturbance-related) of dominant lineages to improve LSM representations. Our method is fundamentally different from previous efforts, as it uses phylogenetic relatedness to create lineage-based functional types (LFTs), situated between species-level trait data and PFT-level abstractions, thus providing a realistic representation of functional diversity and opening the door to the development of new vegetation models.

Rooting depth varies differentially in trees and grasses as a function of mean annual rainfall in an African savanna.

A significant fraction of the terrestrial biosphere comprises biomes containing tree-grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree-grass vertical root separation occurs in these ecosystems, how this overlap varies across large-scale environmental gradients, and what these rooting differences imply for water resource availability and tree-grass competition and coexistence. To assess the extent of tree-grass rooting overlap and how tree and grass rooting patterns vary across resource gradients, we examined landscape-level patterns of tree and grass functional rooting depth along a mean annual precipitation (MAP) gradient extending from ~ 450 to ~ 750 mm year-1 in Kruger National Park, South Africa. We used stable isotopes from soil and stem water to make inferences about relative differences in rooting depth between these two functional groups. We found clear differences in rooting depth between grasses and trees across the MAP gradient, with grasses generally exhibiting shallower rooting profiles than trees. We also found that trees tended to become more shallow-rooted as a function of MAP, to the point that trees and grasses largely overlapped in terms of rooting depth at the wettest sites. Our results reconcile previously conflicting evidence for rooting overlap in this system, and have important implications for understanding tree-grass dynamics under altered precipitation scenarios.

An assessment of diurnal water uptake in a mesic prairie: evidence for hydraulic lift?

Hydraulic lift, the passive movement of water through plant roots from wet to dry soil, is an important ecohydrological process in a wide range of water-limited ecosystems. This phenomenon may also alter plant functioning, growth, and survival in mesic grasslands, where soil moisture is spatially and temporally variable. Here, we monitored diurnal changes in the isotopic signature of soil and plant xylem water to assess (1) whether hydraulic lift occurs in woody and herbaceous tallgrass prairie species (Rhus glabra, Amorpha canescens, Vernonia baldwinii, and Andropogon gerardii), (2) if nocturnal transpiration or grazing by large ungulates limits hydraulic lift, and (3) if a dominant grass, A. gerardii, utilizes water lifted by other tallgrass prairie species. Broadly, the results shown here suggest that hydraulic lift does not appear to be widespread or common in this system, but isolated instances suggest that this process does occur within tallgrass prairie. The isolated instance of hydraulic lift did not vary by grazing treatment, nor did they result in facilitation for neighboring grasses. We suggest that the topographic complexity of this tallgrass prairie and the high rates of nocturnal transpiration observed in this study likely limit the frequency and occurrence of hydraulic lift. These results suggest that hydraulic lift can be a patchy process, particularly in heterogeneous landscapes.

A safety vs efficiency trade-off identified in the hydraulic pathway of grass leaves is decoupled from photosynthesis, stomatal conductance and precipitation.

A common theme in plant physiological research is the trade-off between stress tolerance and growth; an example of this trade-off at the tissue level is the safety vs efficiency hypothesis, which suggests that plants with the greatest resistance to hydraulic failure should have low maximum hydraulic conductance. Here, we quantified the leaf-level drought tolerance of nine C4 grasses as the leaf water potential at which plants lost 50% (P50 × RR ) of maximum leaf hydraulic conductance (Ksat ), and compared this trait with other leaf-level and whole-plant functions. We found a clear trade-off between Ksat and P50 × RR when Ksat was normalized by leaf area and mass (P = 0.05 and 0.01, respectively). However, no trade-off existed between P50 × RR and gas-exchange rates; rather, there was a positive relationship between P50 × RR and photosynthesis (P = 0.08). P50 × RR was not correlated with species distributions based on precipitation (P = 0.70), but was correlated with temperature during the wettest quarter of the year (P < 0.01). These results suggest a trade-off between safety and efficiency in the hydraulic system of grass leaves, which can be decoupled from other leaf-level functions. The unique physiology of C4 plants and adaptations to pulse-driven systems may provide mechanisms that could decouple hydraulic conductance from other plant functions.

Tight coupling of leaf area index to canopy nitrogen and phosphorus across heterogeneous tallgrass prairie communities.

Nitrogen (N) and phosphorus (P) are limiting nutrients for many plant communities worldwide. Foliar N and P along with leaf area are among the most important controls on photosynthesis and hence productivity. However, foliar N and P are typically assessed as species level traits, whereas productivity is often measured at the community scale. Here, we compared the community-level traits of leaf area index (LAI) to total foliar nitrogen (TFN) and total foliar phosphorus (TFP) across nearly three orders of magnitude LAI in grazed and ungrazed tallgrass prairie in north-eastern Kansas, USA. LAI was strongly correlated with both TFN and TFP across communities, and also within plant functional types (grass, forb, woody, and sedge) and grazing treatments (bison or cattle, and ungrazed). Across almost the entire range of LAI values and contrasting communities, TFN:TFP ratios indicated co-limitation by N and P in almost all communities; this may further indicate a community scale trend of an optimal N and P allocation per unit leaf area for growth. Previously, results from the arctic showed similar tight relationships between LAI:TFN, suggesting N is supplied to canopies to maximize photosynthesis per unit leaf area. This tight coupling between LAI, N, and P in tallgrass prairie suggests a process of optimal allocation of N and P, wherein LAI remains similarly constrained by N and P despite differences in species composition, grazing, and canopy density.

Imposing antecedent global change conditions rapidly alters plant community composition in a mixed-grass prairie.

Global change drivers are altering climatic and edaphic conditions of ecosystems across the globe, and we expect novel plant communities to become more common as a result. In the Colorado Front Range, compositional changes have occurred in the mixed-grass prairie plant community in conjunction with shifts in winter precipitation and atmospheric nitrogen (N) deposition. To test whether these environmental changes have been responsible for the observed plant community change, we conducted an in situ manipulative experiment in a mixed-grass meadow near Boulder, CO. We simulated historical conditions by reducing N availability (+500 g C m(-2) year(-1)) and winter precipitation (with rainout shelters) for 2 years (2013-2014) and compared vegetation response to these treatments with that of ambient conditions. The site experienced an extreme precipitation event in autumn 2013 which allowed comparison of an exceptionally wet year with an average year. We measured pre-treatment species composition in 2012, and treatment responses in the spring and summer of 2013 and 2014. As predicted, simulating historical low N-winter dry conditions resulted in a plant community dominated by historically abundant species. Cool-season introduced species were significantly reduced in low N-winter dry plots, particularly the annual plants Bromus tectorum and Alyssum parviflorum. These same species responded strongly to the extreme precipitation event with large increases, while native grasses and forbs showed little change in productivity or composition under varying climatic or edaphic conditions. This work provides clear evidence linking on-going global change drivers to altered plant community composition in an otherwise relatively undisturbed grassland ecosystem.

Evidence of recovery of Juniperus virginiana trees from sulfur pollution after the Clean Air Act.

Using dendroisotopic techniques, we show the recovery of Juniperus virginiana L. (eastern red cedar) trees in the Central Appalachian Mountains from decades of acidic pollution. Acid deposition over much of the 20th century reduced stomatal conductance of leaves, thereby increasing intrinsic water-use efficiency of the Juniperus trees. These data indicate that the stomata of Juniperus may be more sensitive to acid deposition than to increasing atmospheric CO2. A breakpoint in the 100-y δ(13)C tree ring chronology occurred around 1980, as the legacy of sulfur dioxide emissions declined following the enactment of the Clean Air Act in 1970, indicating a gradual increase in stomatal conductance (despite rising levels of atmospheric CO2) and a concurrent increase in photosynthesis related to decreasing acid deposition and increasing atmospheric CO2. Tree ring δ(34)S shows a synchronous change in the sources of sulfur used at the whole-tree level that indicates a reduced anthropogenic influence. The increase in growth and the δ(13)C and δ(34)S trends in the tree ring chronology of these Juniperus trees provide evidence for a distinct physiological response to changes in atmospheric SO2 emissions since ∼1980 and signify the positive impacts of landmark environmental legislation to facilitate recovery of forest ecosystems from acid deposition.