Perennial Grassland Research Articles

Experimental drought indirectly enhances the individual performance and the abundance of an invasive annual weed

During environmental change, invasive species may be favored by increased resource input or reduced resource use of the resident community. Plasticity in certain plant traits of invasive species may be one possible mechanism behind their ability to quickly exploit unused resources. We tested whether rainfall manipulations (severe drought, moderate drought, watering) alter the growth and reproductive success of the invasive annual Conyza canadensis, and if it translates into a change in the abundance of the species in a semiarid perennial grassland in Central Hungary. Overall, C. canadensis exhibited greater individual performance and higher abundance in drought plots than in control and watered plots. At individual level, plants showed the strongest response to moderate drought: they grew 2.5-times taller than in control and watered plots, and produced twice and 2.5-times more seeds than in watered and control plots, respectively. Reproductive phenology was advanced in response to rain exclusions. Although severe drought caused 40% mortality, the cumulative performance of C. canadensis, expressed as plot-level aboveground biomass, was consistently greater in severe drought plots than in control and watered plots throughout the 3 years of the study. The higher performance of C. canadensis in drought plots is most likely due to the decreased abundance and, thus, decreased competitive effect of previously dominant perennial grasses caused by the rain exclusions. We conclude that drier summers that suppress perennial grasses will favor this alien annual forb, and phenotypic plasticity in growth and reproduction may contribute to its invasion success.

Interactive effects of elevated CO2 , warming, reduced rainfall, and nitrogen on leaf gas exchange in five perennial grassland species.

Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi-factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open-air experiment in Minnesota, USA, including elevated CO2 (eCO2 ), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context-dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO2 depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C4 grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.

Long-term nitrate removal in three riparian buffers: 21 years of data from the Bear Creek watershed in central Iowa, USA

Riparian buffers are a conservation practice that increases vegetation diversity on the agricultural landscape while providing environmental benefits. This study specifically focused on the ability of riparian buffers to remove nitrate from shallow groundwater. There are many studies that assessed nitrate removal within buffers, but not many have a long-term, continuous data set that can analyze for variation in nitrate removal rates over time. Here we report on 21 years of nitrate well data, from 1996 through 2017, for three buffers in the Bear Creek watershed in central Iowa. These buffers are named using abbreviations to help keep landowners anonymous (e.g. RN, RS, and ST). Studied buffers RS and ST showed greater nitrate reduction (or removal) after 10 and 6 years of its establishment, respectively. Buffer RN did not experience a significant nitrate removal increase with time, but instead had higher nitrate removal rates when compared to buffers RS and ST of 10.3 g NO3−-N m−1 day−1 from the start of this study. From this data, we suggest that past land management played a major role in the responses observed. RN had previously been established in cool-season grasses for grazing before being converted to a buffer, while RS and ST had been managed in a corn and soybean rotation. RN was thought to have higher denitrification immediately with increased labile soil carbon input and enhanced soil aggregation due to the grassland perennials, while buffer vegetation establishment increased soil carbon inputs and soil aggregation over time for RS and ST. These nitrate removal trends would not have been observed without access to long-term, continuous data. This study highlighted the importance of long-term data sets and the need to assess conservation practices over time to determine their longevity and efficiency with time.

Principal Components and Classification Analysis of Manganese Content in Spring of Perennial Grassland Forage from Microecosystems of Banat Hills

The purpose of this research was to monitor the manganese (Mn) content of forage from perennial grassland microecosystems situated in the west part of Romania. Mineral and organic fertilization, since 2003, influenced the microecosystems conditions. It was used fermented sheep manure as organic fertilizer. Calamagrostis epigejos, Festuca rupicola and Trifolium repens represented mainly the Poaceae and Fabaceae families of grassland floristic matrix. It was determined the spring Mn content in the middle and at the end of May by ContrAA300 High-Resolution Continuum Source spectrometer of Analytik Jena. It was selected a number of 17 variables to compute the PC&CA model. The first three components described around 75% of total variance. The correlation coefficients for spring Mn contents of forage and mineral fertilization data were high, in 0.6 – 0.9 range. The correlation coefficients between fermented sheep manure data and spring Mn contents of forage were negatively.

Repeated fire altered succession and increased fire behavior in basin big sagebrush–native perennial grasslands

AbstractThe structure and composition of sagebrush‐dominated ecosystems have been altered by changes in fire regimes, land use, invasive species, and climate change. This often decreases resilience to disturbance and degrades critical habitat for species of conservation concern. Basin big sagebrush (Artemisia tridentata ssp. tridentata) ecosystems, in particular, are greatly reduced in distribution as land has been converted to agriculture and other land uses. The fire regime, relative proportions of shrub and grassland patches, and the effects of repeated burns in this ecosystem are poorly understood. We quantified postfire patterns of vegetation accumulation and modeled potential fire behavior on sites that were burned and first measured in the late 1980s at John Day Fossil Beds National Monument, Oregon, USA. The area partially reburned 11 yr after the initial fire, allowing a comparison of one vs. two fires. Repeated burns shifted composition from shrub‐dominated to prolonged native herbaceous dominance. Fifteen years following one fire, the native‐dominated herbaceous component was 44% and live shrubs were 39% of total aboveground biomass. Aboveground biomass of twice‐burned sites (2xB; burned 26 and 15 yr prior) was 71% herbaceous and 12% shrub. Twenty‐six years after fire, total aboveground biomass was 113–209% of preburn levels, suggesting a fire‐return interval of 15–25 yr. Frequency and density of Pseudoroegneria spicata and Festuca idahoensis were not modified by fire history, but Poa secunda was reduced by repeated fire, occurring in 84% of plots burned 26 yr prior, 72% of plots burned 15 yr prior, and 49% in 2xB plots. Nonnative annual Bromus tectorum occurred at a frequency of 74%, but at low density with no differences due to fire history. Altered vegetation structure modified fire behavior, with modeled rates of fire spread in 2xB sites double that of once‐burned sites. This suggests that these systems likely were historically composed of a mosaic of shrub and grassland. However, contemporary increases in fire frequency will likely create positive feedbacks of more intense fire behavior and prolonged periods of early‐successional vegetation in basin big sagebrush communities.

EFFECTS OF DROUGHT AND FIRE ON NATIVE STIPA PULCHRA (POACEAE) RECOVERY IN SOUTHERN CALIFORNIA GRASSLANDS

Mediterranean-type climate California experiences both frequent wildfires and periodic drought. Climate change models predict increased severity of drought, and wildfire size and frequency, leading to concern about the ability of some plant species to recover to pre-fire conditions when wildfires and drought coincide. During exceptional drought, drought-adapted native vegetation, such as perennial native grassland, can experience substantial die-off. Here we use three post-fire case studies in southern California to assess the combined effects of fire and drought on the resilience of a native perennial bunchgrass, Stipa pulchra Hitchc. (Poaceae). We hypothesized that resilience to fire, assessed as post-fire survival, resprouting and fecundity (i.e., number of flowering plants and inflorescences), would decrease if the fire occurred during a drought. Unburned (control) sites had about double the survival and resprouting, and about three times the fecundity compared to moderate-severity burn sites. Resprouting and fecundity in the low-severity burn sites was about twice that of moderate-severity burn sites, and survival was five times higher. Responses to two springtime fires that occurred during drought were similar (average mortality 37%), in contrast to a fall fire with no drought (6%). We cannot separate the relative contribution of drought effects from those of the time of year (spring vs. fall), burn severity, and other site factors. However, moderate-severity burns, in combination with drought, decreased survival, resprouting, and fecundity of S. pulchra.

Drought survival is positively associated with high turgor loss points in temperate perennial grassland species

Abstract Turgor loss point (πtlp) has been suggested to be a key trait for drought resistance in woody species. In herbaceous grassland species, the role of πtlp for species drought survival has not yet been tested, although grasslands are projected to experience more frequent and intense droughts with climate change. To gain insights into the role of πtlp for drought resistance of temperate perennial grassland species, we assessed πtlp of 41 species common in Germany (20 forbs, 21 grasses). We directly related them to the species’ comparative whole‐plant drought survival and midday leaf water potentials under drought (ΨMD) assessed in a common garden drought experiment, and to species moisture association. Species drought survival increased with increasing πtlp across all species as well as within forbs or grasses separately. ΨMD was positively related to πtlp and drought survival. Our results imply that high πtlp promotes drought survival of common perennial European temperate mesic grassland species by enabling them to maintain high leaf water potentials under drought, that is, a desiccation avoidance strategy. However, πtlp was not related to species moisture association. The positive relationship between πtlp and drought survival in herbaceous grassland species was opposite to the negative relationship previously established in woody plants, implying that mechanisms of drought resistance differ between woody and herbaceous species. Our results highlight the necessity of directly testing the relationship of functional traits to whole‐plant drought survival in different plant life forms, before using trait assessments for predicting plant responses to drought. A free plain language summary can be found within the Supporting Information of this article.

Nutrient addition increases grassland sensitivity to droughts.

Grasslands worldwide are expected to experience an increase in extreme events such as drought, along with simultaneous increases in mineral nutrient inputs as a result of human industrial activities. These changes are likely to interact because elevated nutrient inputs may alter plant diversity and increase the sensitivity to droughts. Dividing a system's sensitivity to drought into resistance to change during the drought and rate of recovery after the drought generates insights into different dimensions of the system's resilience in the face of drought. Here, we examine the effects of experimental nutrient fertilization and the resulting diversity loss on the resistance to and recovery from severe regional droughts. We do this at 13 North American sites spanning gradients of aridity, five annual grasslands in California, and eight perennial grasslands in the Great Plains. We measured rate of resistance as the change in annual aboveground biomass (ANPP) per unit change in growing season precipitation as conditions declined from normal to drought. We measured recovery as the change in ANPP during the postdrought period and the return to normal precipitation. Resistance and recovery did not vary across the 400-mm range of mean growing season precipitation spanned by our sites in the Great Plains. However, chronic nutrient fertilization in the Great Plains reduced drought resistance and increased drought recovery. In the California annual grasslands, arid sites had a greater recovery postdrought than mesic sites, and nutrient addition had no consistent effects on resistance or recovery. Across all study sites, we found that predrought species richness in natural grasslands was not consistently associated with rates of resistance to or recovery from the drought, in contrast to earlier findings from experimentally assembled grassland communities. Taken together, these results suggest that human-induced eutrophication may destabilize grassland primary production, but the effects of this may vary across regions and flora, especially between perennial and annual-dominated grasslands.

Identification of suites of traits that explains drought resistance and phenological patterns of plants in a semi-arid grassland community

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (Ksat), resistance to hydraulic failure (P50), leaf gas exchange, plant and soil water stable isotope ratios (δ18O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P50 and δ18O of xylem water explained 67% of differences in phenology, with lower P50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Assessing soil resilience across an agricultural land retirement chronosequence

Reestablishment of perennial vegetation can improve soil conditions on intensively cultivated lands, but the timeframe required to recover uncultivated soil quality is not well understood. This study aimed to measure the recovery of soil physical (bulk density, aggregate stability, and water infiltration rate), biological (total microbial biomass and soil respiration), and chemical (surface soil organic carbon [SOC]) parameters across a 30-year land retirement chronosequence. Soil parameters on lands retired under long- and short-term Conservation Reserve Program (CRP) enrollment periods were compared to undisturbed and actively cropped lands, and chronofunctions were utilized to estimate the time required to achieve uncultivated soil quality. Results showed that short-term enrollment was not sufficient to restore any parameters to uncultivated quality, although bulk density, water infiltration, soil respiration, and total organic carbon (C) levels generally trended toward the undisturbed condition. No significant differences between the uncultivated and 30-year CRP enrollment for aggregate stability and total microbial biomass suggests uncultivated soil quality was restored within 30 years of land retirement. Based on chronofunction analysis, an estimated period of 53 years would be necessary to return bulk density and soil respiration to undisturbed quality on retired land. Water infiltration and SOC had the longest predicted recovery time of 75 and 128 years, respectively. These results suggest soil properties recover at varying rates once retired into perennial grassland systems, and long-term land retirement is necessary to restore soil properties to precultivation condition and maintain high quality soils capable of providing key ecosystem services.

Vegetation mapping using hierarchical object‐based image analysis applied to aerial imagery and lidar data

AbstractAims: The primary objective of this study is to map the distribution and quantify the cover of vegetation alliances over the entirety of San Clemente Island (SCI). To this end, we develop and evaluate the mapping method of hierarchical object‐based classification with a rule‐based expert system.Location: San Clemente Island, California, USA.Methods: We developed and tested an approach based on hierarchical object‐based classification with a rule‐based expert system to effectively map vegetation communities on SCI following the Manual of California Vegetation classification system. In this mapping approach, the shrub species defining each vegetation community and non‐shrub growth forms were first mapped using aerial imagery and lidar data, then used as input in an automated mapping rule set that incorporates the percent cover rules of a field‐based mapping rule set.Results: The final vegetation map portrays the distribution of 19 vegetation communities across SCI, with the largest areas comprised of California Annual and Perennial Grassland (35%) and three types of coastal sage scrub and maritime succulent scrub, comprising a combined 53% of the area. Map accuracy was assessed to be 79% based on fuzzy methods and 61% with a traditional accuracy assessment. The accuracy of tree identification was assessed to be 81%, but species‐level tree accuracy was 45%.Conclusions: Semi‐automated approaches to vegetation community mapping can produce repeatable maps over large spatial extents that facilitate ecological management efforts. However, some low‐statured shrub community types were difficult to differentiate due to patchy canopies of co‐occurring species including abundant non‐native grasses characteristic of complex disturbance histories. Species‐level tree mapping accuracy was low due to the difficulty of identifying species within poorly illuminated canyons, resulting from sub‐optimal image acquisition timing.

Response of three mesic South African perennial grassland forbs to defoliation and competition

The importance of maintaining biodiversity in natural grasslands under livestock production systems has been recognised. Forbs contribute more to species richness than grasses in South African grasslands. However, little is known about the impact of grazing and grass competition on the population dynamics of forbs. The aim of this study was to determine the response of three mesic grassland perennial forb species to simulated intensive defoliation and interspecific competition from a neighbouring dominant grass, Themeda triandra, in a field experiment. Two of the selected forb species are sensitive to defoliation (Afroaster hispida and Gerbera ambigua) and one is potentially resistant to defoliation (Hypoxis hemerocallidea). Defoliation resulted in smaller plants of all three species following regrowth. Only A. hispida showed an interaction response to defoliation and competition. Thus, a competitive release was observed for A. hispida in that the height of defoliated plants was reduced by 45% compared with undefoliated plants under full competition, but there was no difference under partial or no competition. Gerbera ambigua was unaffected by competition. Hypoxis hemerocallidea was sensitive to competition irrespective of being defoliated or undefoliated. Findings of this study highlighted an individual response such that a general response cannot be predicted.

Climate Benefits of Increasing Plant Diversity in Perennial Bioenergy Crops

Bioenergy from perennial grasses mitigates climate change via displacing fossil fuels and storing atmospheric CO2 belowground as soil carbon. Here, we conduct a critical review to examine whether increasing plant diversity in bioenergy grassland systems can further increase their climate change mitigation potential. We find that compared with highly productive monocultures, diverse mixtures tend to produce as great or greater yields. In particular, there is strong evidence that legume addition improves yield, in some cases equivalent to mineral nitrogen fertilization at 33–150 kg per ha. Plant diversity can also promote soil carbon storage in the long term, reduce soil N2O emissions by 30%–40%, and suppress weed invasion, hence reducing herbicide use. These potential benefits of plant diversity translate to 50%–65% greater life-cycle greenhouse gas savings for biofuels from more diverse grassland biomass grown on degraded soils. In addition, there is growing evidence that plant diversity can accelerate land restoration.

Divergent carbon cycle response of forest and grass-dominated northern temperate ecosystems to record winter warming.

Northern temperate ecosystems are experiencing warmer and more variable winters, trends that are expected to continue into the foreseeable future. Despite this, most studies have focused on climate change impacts during the growing season, particularly when comparing responses across different vegetation cover types. Here we examined how a perennial grassland and adjacent mixed forest ecosystem in New Hampshire, United States, responded to a period of highly variable winters from 2014 through 2017 that included the warmest winter on record to date. In the grassland, record-breaking temperatures in the winter of 2015/2016 led to a February onset of plant growth and the ecosystem became a sustained carbon sink well before winter ended, taking up roughly 90g/m2 more carbon during the winter to spring transition than in other recorded years. The forest was an unusually large carbon source during the same period. While forest photosynthesis was restricted by leaf-out phenology, warm winter temperatures caused large pulses of ecosystem respiration that released nearly 230gC/m2 from February through April, more than double the carbon losses during that period in cooler years. These findings suggest that, as winters continue to warm, increases in ecosystem respiration outside the growing season could outpace increases in carbon uptake during a longer growing season, particularly in forests that depend on leaf-out timing to initiate carbon uptake. In ecosystems with a perennial leaf habit, warming winter temperatures are more likely to increase ecosystem carbon uptake through extension of the active growing season. Our results highlight the importance of understanding relationships among antecedent winter conditions and carbon exchange across land-cover types to understand how landscape carbon exchange will change under projected climate warming.

Time in pasture rotation alters soil microbial community composition and function and increases carbon sequestration potential in a temperate agroecosystem

Soil carbon (C) sequestration plays an important role in mitigating global climate change, and certain land utilization strategies can exert a pronounced effect on carbon storage. Land use practices, such as planting previously cropped lands into perennial grasslands, can increase soil C sequestration; however, the temporal response of soil C pools to such changes in land use are likely complex and not well quantified. In the current study, a space-for-time approach was used to assess the response of soil C sequestration and microbial community composition during a five-year grazed pasture rotation following three years of vegetable production on a central Kentucky farm. After 5 years in pasture, soil organic C and N in the top 15 cm increased 20.6% and 20.1%, respectively, from year 1 levels, and particulate organic matter C (POM C) increased 53.5%. A carbon mineralization (CM) assay indicated that the potential release of CO2 also increased with time in pasture rotation. When compared to permanent pasture (not previously used for vegetable production), soil microbial community composition differed in rotation years 1–3 but became similar in years 4 and 5. Multi-response permutation procedure (MRPP) analysis showed that CM and POM were key factors affecting microbial community composition. Soil microbial community composition also varied with time of year (season), but to a lesser degree than with pasture duration. Overall, incorporation of perennial pasture into cropping systems can have profound effects on microbial community composition and function, increasing soil organic C, and consequently enhancing the potential for C sequestration; however, whether these increases in C storage persist throughout the full cropping sequence (i.e., once the pasture has been returned to vegetables) and/or how these changes influence subsequent vegetable production remains to be evaluated.

Evidence for rapid evolution in a grassland biodiversity experiment.

In long-term grassland experiments, positive biodiversity effects on plant productivity commonly increase with time. Subsequent glasshouse experiments showed that these strengthened positive biodiversity effects persist not only in the local environment but also when plants are transferred into a common environment. Thus, we hypothesized that community diversity had acted as a selective agent, resulting in the emergence of plant monoculture and mixture types with differing genetic composition. To test our hypothesis, we grew offspring from plants that were grown for eleven years in monoculture or mixture environments in a biodiversity experiment (Jena Experiment) under controlled glasshouse conditions in monocultures or two-species mixtures. We used epiGBS, a genotyping-by-sequencing approach combined with bisulphite conversion, to provide integrative genetic and epigenetic (i.e., DNA methylation) data. We observed significant divergence in genetic and DNA methylation data according to selection history in three out of five perennial grassland species, namely Galium mollugo, Prunella vulgaris and Veronica chamaedrys, with DNA methylation differences mostly reflecting the genetic differences. In addition, current diversity levels in the glasshouse had weak effects on epigenetic variation. However, given the limited genome coverage of the reference-free bisulphite method epiGBS, it remains unclear how much of the differences in DNA methylation was independent of underlying genetic differences. Our results thus suggest that selection of genetic variants, and possibly epigenetic variants, caused the rapid emergence of monoculture and mixture types within plant species in the Jena Experiment.

Preferential water flow: Influence of alfalfa (Medicago sativa L.) decayed root channels on soil water infiltration

Soil water infiltration is an important part of the land surface hydrological cycle, and plays an important role in the hydrological response of the soil, such as soil erosion. High infiltration rates favor an increase in the soil water storage capacity that allows maintaining vegetation restoration in arid and semi-arid regions. Alfalfa (Medicago sativa L.) is a quality perennial legume grassland, which is widely planted in semi-arid areas. In this study, the effects of the root channels formed by the decay of alfalfa on preferential flow were evaluated as a driving force to improve soil water infiltration and soil water supply. A double-ring infiltrometer (30-cm inner diameter and 60-cm outer diameter) was used to measure the infiltration process with the falling head method. Methylene blue was used to visualize the pathways followed by the infiltrated water. The results showed that the initial infiltration rate in the alfalfa grassland increased by 27.7%, compared with the control bare land, and the total cumulative infiltration was 1.13 times higher. The steady infiltration rate of the grassland increased by 31.8% compared to the bare land. The root channel diameters were measured with a Vernier caliper, and the average root channel area (RCA) was calculated through the average root channel diameter (ARCD). The values of ARCD and RCA were significantly and positively correlated with the infiltration rates, being the coefficients of determination 0.815 and 0.789, respectively. Our results indicated that root channels formed by the decayed roots of alfalfa played an important role in increasing soil water infiltration and soil water supply under semi-arid conditions. Our research improves the understanding of the hydrological cycle processes at the plant-soil interface in semi-arid areas.

Summer drought decreases Leymus chinensis productivity through constraining the bud, tiller and shoot production

AbstractExtreme drought events can directly decrease productivity in perennial grasslands. However, for rhizomatous perennial grasses it remains unknown how drought events influence the belowground bud bank which determines future productivity. Ninety‐day‐long drought events imposed on Leymus chinensis, a rhizomatous perennial grass, caused a 41% decrease in the aboveground biomass and a 28% decrease in belowground biomass. Aboveground biomass decreased due to decrease in both the parent and the daughter shoot biomass. The decreases in daughter shoot biomass were due to reductions in both the shoot number and each individual shoot weight. Most importantly, drought decreased the bud bank density by 56%. In addition, drought induced a bud allocation change that decreased by 41% the proportion of buds that developed into shoots and a 41% increase in the buds that developed into rhizomes. Above results were supported by our field experiment with watering treatments. Thus, a 90‐day‐long summer drought event decreases not only current productivity but also future productivity, because the drought reduces the absolute bud number. However, plasticity in plant development does partly compensate for this reduction in bud number by increasing bud development into rhizomes, which increases the relative allocation of buds into future shoots, at the cost of a decrease in current shoots.

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment, independent of nitrogen supply

Uncertainty about long-term leaf-level responses to atmospheric CO2 rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO2 (eCO2 ) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long-lived perennials. Here, we report the effects of eCO2 on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free-Air CO2 Enrichment experiment, BioCON. Monocultures of species belonging to C3 grasses, C4 grasses, forbs, and legumes were exposed to two levels of CO2 and nitrogen supply in factorial combinations over 21years. eCO2 increased photosynthesis by 12.9% on average in C3 species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO2 was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO2 was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C4 grasses the least as expected, but did not further diverge over time. Leaf-level water-use efficiency increased by 50% under eCO2 primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO2 in nitrogen-limited systems, and that significant and consistent declines in stomatal conductance and increases in water-use efficiency under eCO2 may allow plants to better withstand drought.

The Importance of Shifting Disturbance Regimes in Monarch Butterfly Decline and Recovery

The Eastern migratory monarch butterfly has declined in recent decades, partly because widespread adoption of herbicide-resistant corn and soybean has nearly eliminated common milkweed from crop fields in the US Midwest. We argue that in addition to milkweed loss, monarch declines were likely exacerbated by shifting disturbance regimes within their summer breeding range. Monarchs prefer to lay eggs on younger, vegetative milkweed stems. They also benefit from enemy-free space, as most eggs and early-instar larvae succumb to predators. Historically, ecological disturbances during the growing season could have provided these conditions. During most of the 19th and 20th centuries, milkweed was abundant in crop fields where manual weeding and mechanical cultivation set milkweed back, but rather than killing it would often stimulate regrowth later in the summer. Before European settlement, large mammals and fires (natural and anthropogenic) perturbed grasslands during the summer and could have had similar effects. However, presently most remaining milkweed stems in the Midwest are in perennial grasslands like roadsides, old-fields, parks, and conservation reserves, which often lack growing season disturbance. As a result, monarchs may be left with limited options for oviposition as the summer progresses and could have lower survival in grasslands where predation pressure is high. Our recent work has shown that targeted disturbance during the growing season produces milkweed stems that are attractive to ovipositing monarchs and harbor fewer arthropod predators. Targeted disturbance in perennial grasslands could improve habitat heterogeneity and phenologic diversity of milkweeds, and should be explored as a monarch conservation strategy.