The greening of the Northern Great Plains and its biogeochemical precursors

Impact of aridity rise and arid lands expansion on carbon-storing capacity, biodiversity loss, and ecosystem services

Drylands, comprising semi-arid, arid, and hyperarid regions, cover approximately 41% of the Earth's land surface and have expanded considerably in recent decades. Even under more optimistic scenarios, such as limiting global temperature rise to 1.5°C by 2100, semi-arid lands may increase by up to 38%. This study provides an overview of the state-of-the-art regarding changing aridity in arid regions, with a specific focus on its effects on the accumulation and availability of carbon (C), nitrogen (N), and phosphorus (P) in plant-soil systems. Additionally, we summarized the impacts of rising aridity on biodiversity, service provisioning, and feedback effects on climate change across scales. The expansion of arid ecosystems is linked to a decline in C and nutrient stocks, plant community biomass and diversity, thereby diminishing the capacity for recovery and maintaining adequate water-use efficiency by plants and microbes. Prolonged drought led to a -3.3% reduction in soil organic carbon (SOC) content (based on 148 drought-manipulation studies), a -8.7% decrease in plant litter input, a -13.0% decline in absolute litter decomposition, and a -5.7% decrease in litter decomposition rate. Moreover, a substantial positive feedback loop with global warming exists, primarily due to increased albedo. The loss of critical ecosystem services, including food production capacity and water resources, poses a severe challenge to the inhabitants of these regions. Increased aridity reduces SOC, nutrient, and water content. Aridity expansion and intensification exacerbate socio-economic disparities between economically rich and least developed countries, with significant opportunities for improvement through substantial investments in infrastructure and technology. By 2100, half the world's landmass may become dryland, characterized by severe conditions marked by limited C, N, and P resources, water scarcity, and substantial loss of native species biodiversity. These conditions pose formidable challenges for maintaining essential services, impacting human well-being and raising complex global and regional socio-political challenges.

Timing and magnitude of drought impacts on carbon uptake across a grassland biome

Although drought is known to negatively impact grassland functioning, the timing and magnitude of these impacts within a growing season remain unresolved. Previous small-scale assessments indicate grasslands may only respond to drought during narrow periods within a year; however, large-scale assessments are now needed to uncover the general patterns and determinants of this timing. We combined remote sensing datasets of gross primary productivity and weather to assess the timing and magnitude of grassland responses to drought at 5 km² temporal resolution across two expansive ecoregions of the western US Great Plains biome: the C₄ -dominated shortgrass steppe and the C₃ -dominated northern mixed prairies. Across over 700,000 pixel-year combinations covering more than 600,000 km² , we studied how the driest years between 2003-2020 altered the daily and bi-weekly dynamics of grassland carbon (C) uptake. Reductions to C uptake intensified into the early summer during drought and peaked in mid- and late June in both ecoregions. Stimulation of spring C uptake during drought was small and insufficient to compensate for losses during summer. Thus, total grassland C uptake was consistently reduced by drought across both ecoregions; however, reductions were twice as large across the more southern and warmer shortgrass steppe. Across the biome, increased summer vapor pressure deficit (VPD) was strongly linked to peak reductions in vegetation greenness during drought. Rising VPD will likely exacerbate reductions in C uptake during drought across the western US Great Plains, with these reductions greatest during the warmest months and in the warmest locations. High spatiotemporal resolution analyses of grassland response to drought over large areas provide both generalizable insights and new opportunities for basic and applied ecosystem science in these water-limited ecoregions amid climate change.

Changes in forest nitrogen cycling across deposition gradient revealed by δ15N in tree rings

Tree rings provide valuable insight into past environmental changes. This study aimed to evaluate perturbations in tree ring width (TRW) and δ¹⁵N alongside soil acidity and nutrient availability gradients caused by the contrasting legacy of air pollution (nitrogen [N] and sulphur [S] deposition) and tree species (European beech, Silver fir and Norway spruce). We found consistent declines of tree ring δ¹⁵N, which were temporarily unrelated to the changes in the TRW. The rate of δ¹⁵N change in tree rings was related to the contemporary foliar carbon (C) to phosphorus (P) ratio. This observation suggested that the long-term accumulation of ¹⁵N depleted N in tree rings, likely mediated by retained N from deposition, was restricted primarily to stands with currently higher P availability. The shifts observed in tree-ring δ¹⁵N and TRW suggest that acidic air pollution rather than changes in stand productivity determined alteration of N and C cycles. Stable N isotopes in tree rings provided helpful information on the trajectory of the N cycle over the last century with direct consequences for a better understanding of future interactions among N, P and C cycles in terrestrial ecosystems.

Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes-including elevated atmospheric carbon dioxide (CO₂) levels and rising temperatures-are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.

Contrasting plant responses to multivariate environmental variations among species with divergent elevation shifts

The general predictions of climate impacts on species shifts (e.g., upward shift) cannot directly inform local species conservation, because local-scale studies find divergent patterns instead of a general one. For example, our previous study found three shift patterns with elevation (strong down-, moderate down-, and up-slope shifts) in temperate mountain forests. The divergent shifts are hypothesized to arise from both multivariate environmental variations with elevation and corresponding species-specific responses. To test this hypothesis, we sampled soils and leaves to measure elevation variations in soil conditions and determined plant responses using discriminations against heavier isotopes, carbon (¹³ C) and nitrogen (¹⁵ N). Functional traits of the species studied were also extracted from a public trait dataset. We found that: (1) With low soil water contents at low elevations, only the leaves of up-shifters had lower ¹³ C discriminations at low vs. high elevations; (2) With low soil P contents at high elevations, only the leaves of moderate down-shifters had higher ¹⁵ N discriminations at high vs. low elevations; (3) The leaves of strong down-shifters did not show significant elevation patterns of the discriminations; (4) The contrasting responses among the three types of shifters agree with their functional dissimilarity, suggested by their separate locations in a multitrait space. Taken together, the divergent shifts are associated with the elevation variations in environmental conditions and contrasting plant responses. The contrasting responses could result from the functional dissimilarity among species. Therefore, a detailed understanding of both local environmental variations and species-specific responses can facilitate accurate predictions of species shifts to inform local species conservation.

Is the grass always greener? Land surface phenology reveals differences in peak and season-long vegetation productivity responses to climate and management

Vegetation phenology-the seasonal timing and duration of vegetative phases-is controlled by spatiotemporally variable contributions of climatic and environmental factors plus additional potential influence from human management. We used land surface phenology derived from the Advanced Very High Resolution Radiometer and climate data to examine variability in vegetation productivity and phenological dates from 1989 to 2014 in the U.S. Northwestern Plains, a region with notable spatial heterogeneity in climate, vegetation, and land use. We first analyzed interannual trends in six phenological measures as a baseline. We then demonstrated how including annual-resolution predictors can provide more nuanced insights into measures of phenology between plant communities and across the ecoregion. Across the study area, higher annual precipitation increased both peak and season-long productivity. In contrast, higher mean annual temperatures tended to increase peak productivity but for the majority of the study area decreased season-long productivity. Annual precipitation and temperature had strong explanatory power for productivity-related phenology measures but predicted date-based measures poorly. We found that relationships between climate and phenology varied across the region and among plant communities and that factors such as recovery from disturbance and anthropogenic management also contributed in certain regions. In sum, phenological measures did not respond ubiquitously nor covary in their responses. Nonclimatic dynamics can decouple phenology from climate; therefore, analyses including only interannual trends should not assume climate alone drives patterns. For example, models of areas exhibiting greening or browning should account for climate, anthropogenic influence, and natural disturbances. Investigating multiple aspects of phenology to describe growing-season dynamics provides a richer understanding of spatiotemporal patterns that can be used for predicting ecosystem responses to future climates and land-use change. Such understanding allows for clearer interpretation of results for conservation, wildlife, and land management.

Seasonal patterns of bison diet across climate gradients in North America

North American plains bison (Bison bison) have been reintroduced across their former range, yet we know too little about their current diet to understand what drove their past migrations as well as observed continental-scale variation in weight gain and reproduction. In order to better understand the seasonal diets of bison at the continental scale, bison fecal material was collected monthly from April to September in 2019 across 45 sites throughout the conterminous United States. Fecal material was analyzed for dietary quality using near infrared spectroscopy and dietary composition with DNA metabarcoding. As observed in previous research, dietary quality peaked in June and was on average greatest for sites with cold, wet climates. Yet, in April, dietary quality was highest in warmer regions, likely reflecting earlier phenology of plants in southern than northern regions. Independent of climate and season, bison that consumed more warm-season grasses had lower dietary protein concentrations. Interpreting the relative abundance of sequences from different plant species as the relative intake of protein from those species, only 38% of bison protein intake came from grasses. An equal amount of dietary protein came from legumes (38%) and 22% from non-leguminous forbs. Seasonal shifts in bison diet were also clear, in part, following the phenology of functional groups. For example, cool-season grass protein intake was highest in May, while legume protein intake was highest in August. Comparing data taken in June and September 2018 in a previous study with corresponding data in 2019, on average, June [CP] was 20% higher in 2019 than 2018, while September [CP] did not differ between years. Dietary functional group composition was generally similar in amounts and relationships with climate between years, yet in September 2019, legumes contributed 20% more protein and warm-season grasses 14% less than in September 2018. In all, this research demonstrates that bison consistently rely on eudicots for protein with the functional group composition of their diet in some ways consistent across space and time, but also spatially and temporally variable. The early-season inversion of plant quality gradients would have been a strong driver of migratory behavior for large numbers of bison optimizing protein intake. As most bison currently experience protein deficiency, optimizing protein intake under current non-migratory conditions will require increasing the relative abundance of high-protein species such as N₂-fixing species.

Rangeland Fractional Components Across the Western United States from 1985 to 2018

Monitoring temporal dynamics of rangelands to detect and understand change in vegetation cover and composition provides a wealth of information to improve management and sustainability. Remote sensing allows the evaluation of both abrupt and gradual rangeland change at unprecedented spatial and temporal extents. Here, we describe the production of the National Land Cover Database (NLCD) Back in Time (BIT) dataset which quantified the percent cover of rangeland components (bare ground, herbaceous, annual herbaceous, litter, shrub, and sagebrush (Artemisia spp. Nutt.) across the western United States using Landsat imagery from 1985 to 2018. We evaluate the relationships of component trends with climate drivers at an ecoregion scale, describe the nature of landscape change, and demonstrate several case studies related to changes in grazing management, prescribed burns, and vegetation treatments. Our results showed the net cover of shrub, sagebrush, and litter significantly (p < 0.01) decreased, bare ground and herbaceous cover had no significant change, and annual herbaceous cover significantly (p < 0.05) increased. Change was ubiquitous, with a mean of 92% of pixels with some change and 38% of pixels with significant change (p < 0.10). However, most change was gradual, well over half of pixels have a range of less than 10%, and most change occurred outside of known disturbances. The BIT data facilitate a comprehensive assessment of rangeland condition, evaluation of past management actions, understanding of system variability, and opportunities for future planning.

Looking back in time to reconstruct nitrogen availability trajectories

This is a commentary on Brookshire et al. 26, 5404-5413 For the Northern Great Plains, Brookshire, Stoy, Currey, and Finney (Global Change Biology, 2020) analyze satellite-based reconstructions of greenness and foliar nutrition and isotopic composition from herbarium samples. Their results of greater productivity coupled with reduced N availability are part of an inflection in our understanding of the global N cycle as much of the terrestrial biosphere appears to be experiencing reduced N availability.