Melilotus officinalis (Yellow Sweetclover) Causes Large Changes in Community and Ecosystem Processes in Both the Presence and Absence of a Cover Crop

Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C3 grass cover crop (Pascopyrum smithii), and a commonly seeded non-native C3 grass (Bromus inermis) on the establishment of target native C4 prairie grass species. All treatments contained the same seed density of target C4 species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen (15N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C3 grass species on non-native forbs or C4 target species, but seeded P. smithii did increase species diversity. Non-native plants therefore impeded native C4 grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient availability. The effects of M. officinalis were not reduced by the presence of a cover crop

An Empirical Comparison of Beta Diversity Indices in Establishing Prairies

Whittaker (1960, 1972) first proposed the idea that species diversity has spatial components, with alpha diversity estimating diversity within individual stands (or communities) and beta diversity estimating the number of community types in an area (or in Whittaker’s terminology, ‘‘differentiation of communities along gradients’’). These two values combined make up gamma diversity. Beta diversity is important because it provides the conceptual link between local and regional diversity, more directly measures how soil types, disturbance, and dispersal affect diversity, and is helpful in understanding why species loss is sometimes smaller than predicted by theory (Wilsey et al. 2005). Many interesting and longstanding questions are applied across scales, such as how much diversity is found within islands vs. across islands? Is the number of habitat types (i.e., beta) within islands key to explaining diversity at larger scales or is it the greater population sizes found on large islands? Furthermore, a consideration of both alpha and beta is necessary for understanding how diversity arises and is maintained in diverse systems. For example, in the northern Great Plains, we have found that remnant prairies can contain over 120 plant species within a small area (Wilsey et al. 2005); this occurs because of high diversity at the neighborhood scale where 20–25 species are found per square meter (Martin et al. 2005), and from species accumulation across neighborhoods (i.e., beta)

Native cover crops and timing of planting: Effects on 15N uptake, weed invasion and prairie establishment

Cover crops have been used for several purposes in prairie restorations. This project looked at whether the assumed benefits are supported by research results

Grassland Plant Composition Alters Vehicular Disturbance Effects in Kansas, USA

Many ‘‘natural’’ areas are exposed to military or recreational off-road vehicles. The interactive effects of different types of vehicular disturbance on vegetation have rarely been examined, and it has been proposed that some vegetation types are less susceptible to vehicular disturbance than others. At Fort Riley, Kansas, we experimentally tested how different plant community types changed after disturbance from an M1A1 Abrams tank driven at different speeds and turning angles during different seasons. The greatest vegetation change was observed because of driving in the spring in wet soils and the interaction of turning while driving fast (vegetation change was measured with Bray-Curtis dissimilarity). We found that less vegetation change occurred in communities with high amounts of native prairie vegetation than in communities with high amounts of introduced C3 grasses, which is the first experimental evidence we are aware of that suggests plant communities dominated by introduced C3 grasses changed more because of vehicular disturbance than communities dominated by native prairie grasses. We also found that vegetation changed linearly with vehicular disturbance intensity, suggesting that at least initially there was no catastrophic shift in vegetation beyond a certain disturbance intensity threshold. Overall, the intensity of vehicular disturbance appeared to play the greatest role in vegetation change, but the plant community type also played a strong role and this should be considered in land use planning. The reasons for greater vegetation change in introduced C3 grass dominated areas deserve further study

Species composition but not diversity explains recovery from the 2011 drought in Texas grasslands

Extreme droughts can have profound direct consequences for grassland ecosystems, but it is poorly known how ecosystems recover from drought and what ecological factors are associated with recovery. Recovery occurs when ecosystem functioning returns to values observed prior to a perturbation. Here, we tested for ecosystem recovery after an extreme drought in 2011 in previously established native and exotic experimental communities in Central Texas. Planted mixtures of all native and all exotic species were crossed with a summer irrigation treatment, with eight community compositions (random draws) per treatment. Prior to the drought, native plots had higher diversity than exotic plots, which sets up the prediction that the high-diversity native plots will recover more quickly than exotics. The extreme drought decreased rain-use efficiency ([RUE], annual biomass production per unit of rainfall) by 82%. Rain-use efficiency remained well below pre-drought levels during the growing season after the drought. However, on average, RUE recovered to pre-drought levels by the second growing season following drought. Exotic communities showed higher RUE than native communities, and irrigation significantly reduced RUE in both exotic and native communities across years. Interestingly, not all of the mixtures recovered from the drought, and recovery was associated with species composition, but not diversity. Rain-use efficiency recovery from drought was greatest in native communities in which the proportion of C3 forb biomass increased during and following drought and in exotic communities with a low proportion of short grass biomass. Extreme droughts can exert differential impacts on plant functional groups, leading to a drought legacy effect that reduces recovery with possible long-term repercussions

Dominant Grass Effects on Diversity and Functioning of Restored Grassland

Native grasslands provide a multitude of benefits to society including forage production, wildlife habitat, and nutrient and CO2 uptake and storage. There has been continuing interest within the conservation community in restoring grasslands to maximize these multiple benefits. In addition to achieving the most common objectives of reducing soil erosion and increasing organic carbon and nutrient availabilities, restored grasslands also produce important wildlife habitat, and they have the potential to uptake and store greenhouse gases like CO2 . Grassland plantings have been found to increase game and non-game bird abundance and diversity and to improve deer habitat

Spectrally derived values of community leaf dry matter content link shifts in grassland composition with change in biomass production

Leaf traits link environmental effects on plant species abundances to changes in ecosystem processes but are a challenge to measure regularly and over large areas. We used measurements of canopy reflectance from grassland communities to derive a regression model for one leaf trait, leaf dry matter content (LDMC). Partial least squares regression (PLSR) analysis was used to model community‐weighted (species abundance‐weighted) values of LDMC as a function of canopy reflectance in visible and near‐infrared (NIR) wavebands. The PLSR model then was applied to airborne measurements of canopy reflectance to determine how community LDMC interacts with inter‐annual variation in precipitation to influence the normalized difference vegetation index (NDVI), a surrogate of aboveground biomass production, of restored grassland during spring over 4 years. LDMC was well‐described by a PLSR model that included reflectance measurements located primarily in red edge and NIR portions of the spectrum. Community LDMC decreased as annual forb species became more abundant and was negatively correlated with maximum values of NDVI. Decreased precipitation reduced NDVI (biomass production) both by increasing community LDMC (LDMC response) and reducing the slope of the NDVI‐LDMC relationship (LDMC effect on NDVI). We find that grassland LDMC is well‐described by a regression model using canopy reflectance in red edge and NIR wavebands. Our results demonstrate the utility of spectral estimates of LDMC for discerning shifts in grassland composition and predicting consequences for production‐related ecosystem functions

Biotic Regulation of CO2 Uptake–Climate Responses: Links to Vegetation Properties

Identifying the plant traits and patterns of trait distribution in communities that are responsible for biotic regulation of CO2 uptake–climate responses remains a priority for modeling terrestrial C dynamics. We used remotely sensed estimates of gross primary productivity (GPP) from plots planted to different combinations of perennial grassland species in order to determine links between traits and GPP–climate relationships. Climatic variables explained about 50% of the variance in temporal trends in GPP despite large variation in CO2 uptake among seasons, years, and plots of differing composition. GPP was highly correlated with contemporary changes in net radiation (Rn) and precipitation deficit (potential evapotranspiration minus precipitation) but was negatively correlated with precipitation summed over 210 days prior to flux measurements. Plots differed in GPP–Rn and GPP–water (deficit, precipitation) relationships. Accounting for differences in GPP–climate relationships explained an additional 11% of variance in GPP. Plot differences in GPP–Rn and GPP–precipitation slopes were linked to differences in community-level light-use efficiency (GEE*). Plot differences in GPP–deficit slopes were linked to differences in a species abundance-weighted index of specific leaf area (SLA). GEE* and weighted SLA represent vegetation properties that may regulate how CO2 uptake responds to climatic variation in grasslands

Importance of Species Replication in Understanding Plant Invasions into North American Grasslands

The global homogenization of the Earth’s biota is expected to increase due to the increase in movement of people and goods between regions, and many introduced species are having a negative economic impact. The increase of introduced species can be thought of as a major global change, because ecosystems throughout the world are now impacted by exotics [1, 2]. Grasslands, which cover roughly 25% of the globe, contain perhaps the most disrupted and homogenized communities in the world. Native grasslands have been lost because of land conversion, and native species have been replaced or displaced with introduced grasses and legumes. Many species were intentionally introduced during the early 20th century to prevent erosion or to improve grazing, and many have undoubtedly done so. However, as management objectives for grasslands have expanded to include wildlife habitat, biodiversity, and C sequestration, it has become critical to understand how introduced species are affecting these new objectives as well. For example, Christian and Wilson [3] found that areas in Saskatchewan, Canada, dominated by the introduced forage grass Agropyron cristatum are sequestering less C into their soils compared to developing native prairie stands with similar land use histories