Leanne M. Martin

Biodiversity, photosynthetic mode, and ecosystem services differ between native and novel ecosystems

Abstract Human activities have caused non-native plant species with novel ecological interactions to persist on landscapes, and it remains controversial whether these species alter multiple aspects of communities and ecosystems. We tested whether native and exotic grasslands differ in species diversity, ecosystem services, and an important aspect of functional diversity (C3:C4 proportions) by sampling 42 sites along a latitudinal gradient and conducting a controlled experiment. Exotic-dominated grasslands had drastically lower plant diversity and slightly higher tissue N concentrations and forage quality compared to native-dominated sites. Exotic sites were strongly dominated by C4 species at southern and C3 species at northern latitudes with a sharp transition at 36–38°, whereas native sites contained C3:C4 mixtures. Large differences in C3:C4 proportions and temporal niche partitioning were found between native and exotic mixtures in the experiment, implying that differences in C3:C4 proportions along the latitudinal gradient are caused partially by species themselves. Our results indicate that the replacement of native- by exotic-dominated grasslands has created a management tradeoff (high diversity versus high levels of certain ecosystem services) and that models of global change impacts and C3/C4 distribution should consider effects of exotic species.

Differences in beta diversity between exotic and native grasslands vary with scale along a latitudinal gradient

Biodiversity can be partitioned into alpha, beta, and gamma components, and beta diversity is not as clearly understood. Biotic homogenization predicts that exotic species should lower beta diversity at global and continental scales, but it is still unclear how exotic species impact beta diversity at smaller scales. Exotic species could theoretically increase or decrease beta diversity relative to natives depending on many factors, including abiotic conditions, community assembly history, management, dispersal rates of species, and connectivity among patches. We sampled plant species abundances in 42 novel, exotic- and native-dominated (remnant) grasslands across a latitudinal gradient in the tallgrass prairie region, and tested whether exotic and native grasslands differed in beta diversity at three scales: across sites within the entire biome, across sites within regions, and across locations within sites. Exotic-dominated grasslands differed from native-dominated grasslands in beta diversity at all scales, but the direction of the difference changed from positive to negative as scales went from large to small. Contrary to expectations, exotic-dominated grasslands had higher beta diversity than native-dominated grasslands at the largest scale considered. This occurred because the identity of dominant exotic species varied across the latitudinal gradient, with many exotic grassland pairs exhibiting zero similarity, whereas native-dominated grasslands differed more gradually with distance. Beta diversity among sites within a region was variable, with exotic-dominated grasslands having 29% higher beta diversity than native grasslands in the south and 33% lower beta diversity in the north. Within sites, beta diversity was 26% lower in exotic-dominated than native grasslands. Our results provide evidence that different regional identities and abundances of exotics, and lack of connectivity in fragmented landscapes can alter beta diversity in unexpected ways across spatial scales.

Phenology differences between native and novel exotic‐dominated grasslands rival the effects of climate change

1. Novel ecosystems can differ from the native systems they replaced. We used phenology measures to compare ecosystem functioning between novel exotic-dominated and native-dominated grasslands in the central US. 2. Phenology, or timing of biological events, is affected by climate and land use changes. We assessed how phenology shifts are being altered by exotic species dominance by comparing remotely sensed Normalized Difference Vegetation Index (NDVI) within growing seasons at exotic- and native-dominated sites along a latitudinal gradient. Exotic species were dominated by the C3 species functional group in the north and the C4 species functional group in the south. 3. Date of senescence was an average of 36 days later in exotic than native-dominated grasslands, and this effect was consistent across latitudes. 4. Exotic-dominated grasslands greened up an average of 10.7 days earlier than native-dominated grasslands, but this effect was highly dependent on latitude and the plant functional group that dominated at that latitude. Greenup differed between native and exotic sites the most in central and northern regions that had dominant C3 grasses. 5. We estimated the effects of an increase in global temperatures on green-up and senescence with a space-for-time substitution, and by comparing growing degree day differences between historical average temperatures and +2.5° C. Green-up was significantly earlier and senescence was significantly later with a 2.5 ° C increase in temperature. The native-exotic difference was significantly greater than the difference due to increased temperature for senescence, but not for green-up. 6. Synthesis and applications. Native to exotic plant conversions in central US grasslands have led to highly altered phenology, especially in terms of senescence, and this effect should be considered along with global warming in models moving forward. This conversion will have to be considered in developing estimates of how global change will affect phenology in locations where exotics are present, especially in cases where their abundance is increasing concurrent with climate change. Global change models and policy should consider exotic species invasion as an additional widespread factor behind changes in phenology. This article is protected by copyright. All rights reserved.

Phenology and temporal niche overlap differ between novel, exotic- and native-dominated grasslands for plants, but not for pollinators

AbstractThe temporal dynamics of ecosystem functioning and services are largely regulated by seasonal patterns, or phenology, of Earth’s biota. Recent evidence suggests that species in exotic-dominated systems may exhibit altered phenology relative to species in native-dominated systems. However, whether phenology of plants and pollinator communities differ between existing novel, exotic-dominated ecosystems and native-dominated systems remains poorly understood. We tested whether exotic- and native-dominated grasslands differed across a growing season for plant production, species and functional diversity, and bee pollinator abundances and diversity in the Northern Great Plains tallgrass prairie region, Iowa, USA. We found that niche overlap and some aspects of phenology in exotic-dominated systems were different from native systems for plants, but not for pollinators. Exotic dominated grasslands peaked earlier in biomass production, and exhibited consistently lower levels of species and functional diversity, including higher levels of proportion of C3 biomass, than native grasslands. These results were related to higher levels of temporal niche overlap in exotic grasslands, where species were similar across the growing season in exotic, but not in native, grasslands. Surprisingly, bee pollinator communities did not differ between exotic and native grasslands despite higher forb:grass ratios in native sites, implying that factors other than exotic-native status may be important in structuring pollinator communities at this scale in highly fragmented landscapes. Our results imply that phenological differences between communities in novel and native grasslands can have important consequences to diversity and ecosystem functioning, and that exotic or native status of plant species should be considered in future studies of phenology.

Harvesting and Recolonization of Wild Populations of Oshá (Ligusticum porteri) in Southern Colorado

ABSTRACT: Land managers face the challenge of conserving medicinal plants that may be threatened by harvest pressure, often with limited biological information available to inform management decisions. Oshá (Ligusticum porteri) is an important medicinal plant whose roots are harvested as an herbal remedy for flu, sore throat, and other illnesses. However, little is known about population structure, root production, or the capacity of oshá to recover from harvest in different environmental contexts. We compared oshá population structure and root production within a gradient of canopy cover from meadow to forested habitat. We experimentally harvested roots of mature oshá plants and recorded oshá recolonization of pits created by root harvest. Prior to harvest, the number and percent cover of reproductive plants and the number of flowering stems per plot were higher in the meadow than in forested habitat. Canopy cover had a significant negative relationship to these variables, suggesting that oshá populations benefit from increased light availability. Average root weight per plant in meadow plots was three times higher than in forested plots. One year after harvest, the majority of all harvest pits across the canopy cover gradient were recolonized by oshá. Our results suggest that oshá population structure and root production are significantly influenced by canopy cover, but that plants have a high capacity for post-harvest recolonization under variable light conditions. These results demonstrate the need to account for environmental factors that influence population structure when addressing concerns about the overharvest of wild medicinal plants.