Elevation alters ecosystem properties across temperate treelines globally.

Temperature is a primary driver of the distribution of biodiversity as well as of ecosystem boundaries. Declining temperature with increasing elevation in montane systems has long been recognized as a major factor shaping plant community biodiversity, metabolic processes, and ecosystem dynamics. Elevational gradients, as thermoclines, also enable prediction of long-term ecological responses to climate warming. One of the most striking manifestations of increasing elevation is the abrupt transitions from forest to treeless alpine tundra. However, whether there are globally consistent above- and belowground responses to these transitions remains an open question. To disentangle the direct and indirect effects of temperature on ecosystem properties, here we evaluate replicate treeline ecotones in seven temperate regions of the world. We find that declining temperatures with increasing elevation did not affect tree leaf nutrient concentrations, but did reduce ground-layer community-weighted plant nitrogen, leading to the strong stoichiometric convergence of ground-layer plant community nitrogen to phosphorus ratios across all regions. Further, elevation-driven changes in plant nutrients were associated with changes in soil organic matter content and quality (carbon to nitrogen ratios) and microbial properties. Combined, our identification of direct and indirect temperature controls over plant communities and soil properties in seven contrasting regions suggests that future warming may disrupt the functional properties of montane ecosystems, particularly where plant community reorganization outpaces treeline advance.

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains.

Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere.

The productivity-stability trade-off in global food systems.

Historically, humans have managed food systems to maximize productivity. This pursuit has drastically modified terrestrial and aquatic ecosystems globally by reducing species diversity and body size while creating very productive, yet homogenized, environments. Such changes alter the structure and function of ecosystems in ways that ultimately erode their stability. This productivity-stability trade-off has largely been ignored in discussions around global food security. Here, we synthesize empirical and theoretical literature to demonstrate the existence of the productivity-stability trade-off and argue the need for its explicit incorporation in the sustainable management of food systems. We first explore the history of human management of food systems, its impacts on average body size within and across species and food web stability. We then demonstrate how reductions in body size are symptomatic of a broader biotic homogenization and rewiring of food webs. We show how this biotic homogenization decompartmentalizes interactions among energy channels and increases energy flux within the food web in ways that threaten their stability. We end by synthesizing large-scale ecological studies to demonstrate the prevalence of the productivity-stability trade-off. We conclude that management strategies promoting landscape heterogeneity and maintenance of key food web structures are critical to sustainable food production.

Exotic Grasses Reduce Infiltration and Moisture Availability in a Temperate Oak Savanna.

Biological invasions represent one of the most urgent conservation challenges. Oregon white oak (Quercus garryana) savannas, a complex of grassland and transitional forest, are especially sensitive to these invasions. These ecosystems have been severely degraded and fragmented over the past century and are being encroached by conifers, and oak seedlings are failing to emerge from the understory at many locations. Understanding competitive interactions between Oregon white oak and associated native and exotic vegetation would provide insight into forest-grassland dynamics and the role of exotic grasses in the decline of native species, the processes that maintain temperate savanna ecosystems, and the role of soil water uptake by individual savanna species in contributing to overall species assemblages. In this study, we quantified the soil moisture budget for invaded and uninvaded oak-associated ecosystems. From February to October 2007 we used a split paired plot experiment in Duncan, British Columbia, Canada to measure soil moisture on treatment sites where exotic grasses were removed with herbicide and control plots where they were not, using three depths (5, 20, and 35 or 50 cm) in the soil profile. Our results show that the plots that contained exotic vegetation had a faster rate of soil drying following precipitation events at the 5 cm depth than plots with the predominantly native species. We attribute this difference to the capacity of exotic vegetation to exploit soil moisture more rapidly than native vegetation at times of the year when native vegetation cannot. These results provide insight into one mechanism by which exotic grasses affect associated native plants and could help guide restoration efforts.

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE.

Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770-780 and 990-1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.

Author Correction: Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE.

The original version of this Article contained an error in the Data Availability section, which incorrectly read 'All data will be freely available via https://www.ams.ethz.ch/research.html .' The correct version states ' http://www.ams.ethz.ch/research/published-data.html ' in place of ' https://www.ams.ethz.ch/research.html '. This has been corrected in both the PDF and HTML versions of the Article.

Predicting potential global distributions of two Miscanthus grasses: implications for horticulture, biofuel production, and biological invasions.

In many regions, large proportions of the naturalized and invasive non-native floras were originally introduced deliberately by humans. Pest risk assessments are now used in many jurisdictions to regulate the importation of species and usually include an estimation of the potential distribution in the import area. Two species of Asian grass (Miscanthus sacchariflorus and M. sinensis) that were originally introduced to North America as ornamental plants have since escaped cultivation. These species and their hybrid offspring are now receiving attention for large-scale production as biofuel crops in North America and elsewhere. We evaluated their potential global climate suitability for cultivation and potential invasion using the niche model CLIMEX and evaluated the models' sensitivity to the parameter values. We then compared the sensitivity of projections of future climatically suitable area under two climate models and two emissions scenarios. The models indicate that the species have been introduced to most of the potential global climatically suitable areas in the northern but not the southern hemisphere. The more narrowly distributed species (M. sacchariflorus) is more sensitive to changes in model parameters, which could have implications for modelling species of conservation concern. Climate projections indicate likely contractions in potential range in the south, but expansions in the north, particularly in introduced areas where biomass production trials are under way. Climate sensitivity analysis shows that projections differ more between the selected climate change models than between the selected emissions scenarios. Local-scale assessments are required to overlay suitable habitat with climate projections to estimate areas of cultivation potential and invasion risk.