Weather and Fuel as Modulators of Grassland Fire Behavior in the Northern Great Plains.

Fuel and weather interact to affect wildland fire behavior, but little is known about associations between these variables in the northern Great Plains of North America. Few studies consider rate of spread or statistically test the influence of fuel and weather. We measured overall fuel load and moisture ahead of prescribed fires in North Dakota, USA, and used a thermocouple array to measure two-dimensional rate of spread, soil surface temperature, and aboveground flame temperature, to compare with fire weather data. Flame temperatures averaged 225 °C during spring burns and 250 °C during fall burns, and were generally higher with greater fuel loads and lower overall fuelbed moisture. Surface temperatures averaged ≈100 °C, although 50% of observations were ≤60 °C. Fires spread at an average of 2.5 m min-1, increasing with wind speed. As such, prescribed fire in northern Great Plains working rangeland appear to spread slowly and effect low soil surface temperatures, often limited by high fuelbed moisture. Fire behavior measurements respond differently to variability in fuel and weather. Belowground heating is likely minimal. We suggest ecologists ought to consider which fire behavior measurements best relate to fire effects, and managers consider weather and ignition pattern mitigations when fuels constrain desired fire behavior to ensure effective burns.

Effects of nutrient addition on endophyte-associated grass invasion in a long-term, old-field community experiment.

Strictly vertically transmitted (hereditary) Epichloë spp. fungal endophytes are symbionts with cool-season pooid host grasses. Such endophytes may increase host invasiveness in the non-native, introduced ranges. However, because costs and benefits for the host can vary with the growing conditions, the endophyte may become locally or temporally extinct when costs outweigh benefits. Our long-term field experiment involved the introduction of seven Schedonorus pratensis (meadow fescue) cultivars hosting Epichloë uncinata endophyte, which represent host-grass populations differing in genetic backgrounds and Epichloë infection frequencies, to an unmanaged old field. In the first 6 years, the host grasses persisted but did not become invasive in the plant community, regardless of their endophyte infection frequency. Subsequently, we hypothesized that increasing nutrient availability would decrease endophyte costs and thus increase the host's success and abundance. We fertilized half of the plots for four additional years and re-examined S. pratensis invasiveness. We predicted that increased nutrient availability would increase S. pratensis abundance and E. uncinata frequency and concentration, as well as decrease plant community diversity, relative to unfertilized plots. Fertilization increased endophyte concentrations in three low-endophyte host populations. However, E. uncinata did not enable S. pratensis populations to achieve high abundance or to reduce plant community diversity in the old field, with or without fertilization. Thus, nutrient availabililty and host invasiveness appear to be decoupled in this study system.

The effect of endophyte presence on Schedonorus arundinaceus (tall fescue) establishment varies with grassland community structure.

The endophyte Neotyphodium coenophialum in Schedonorus arundinaceus (tall fescue) may alter host interactions with specific resident species or shift the host's niche. These effects can be quantified by assessing tall fescue responses to, and effects on, the variation among resident species (selection) and resident species interactions (complementarity). To determine how N. coenophialum affects tall fescue, grassland microcosms containing 16 transplants of two, four, or eight resident species were seeded with endophyte-infected (E+) or endophyte-free (E-) Kentucky-31 (KY-31) tall fescue. All resident species were also grown in monoculture. Aboveground biomass was harvested 9 weeks after tall fescue was added (18 weeks' total growth). At harvest, more E+ than E- individuals were present and they represented a larger portion of the aboveground biomass across richness treatments, despite similar germination in concurrent trials. Tall fescue individuals were larger in microcosms dominated by more productive resident species (greater selection). In contrast, fewer E-, but not E+, individuals were present in microcosms with more facilitative interactions among the resident species (greater complementarity). E- and E+ tall fescue also affected resident species differently. High-richness E+ microcosms were more diverse and less dominated by productive species (lower selection) than E- microcosms. Thus, E+ KY-31 may more readily establish in, and affect, species-rich, functionally diverse communities as a result of niche shifts during establishment and negative effects on specific resident species. Although results need to be further tested under field conditions, it appears that endophyte presence may only facilitate KY-31 invasion into a limited suite of community types.

A Mixed Model for Assessing the Effect of Numerous Plant Species Interactions on Grassland Biodiversity and Ecosystem Function Relationships.

In grassland ecosystems, it is well known that increasing plant species diversity can improve ecosystem functions (i.e., ecosystem responses), for example, by increasing productivity and reducing weed invasion. Diversity-Interactions models use species proportions and their interactions as predictors in a regression framework to assess biodiversity and ecosystem function relationships. However, it can be difficult to model numerous interactions if there are many species, and interactions may be temporally variable or dependent on spatial planting patterns. We developed a new Diversity-Interactions mixed model for jointly assessing many species interactions and within-plot species planting pattern over multiple years. We model pairwise interactions using a small number of fixed parameters that incorporate spatial effects and supplement this by including all pairwise interaction variables as random effects, each constrained to have the same variance within each year. The random effects are indexed by pairs of species within plots rather than a plot-level factor as is typical in mixed models, and capture remaining variation due to pairwise species interactions parsimoniously. We apply our novel methodology to three years of weed invasion data from a 16-species grassland experiment that manipulated plant species diversity and spatial planting pattern and test its statistical properties in a simulation study.Supplementary materials accompanying this paper appear online. Supplementary materials for this article are available at 10.1007/s13253-022-00505-2.

Helianthus maximiliani and species fine-scale spatial pattern affect diversity interactions in reconstructed tallgrass prairies.

Biodiversity and Ecosystem Function analyses aim to explain how individual species and their interactions affect ecosystem function. With this study, we asked in what ways do species interact, are these interactions affected by species planting pattern, and are initial (planted) proportions or previous year (realized) proportions a better reference point for characterizing grassland diversity effects?We addressed these questions with experimental communities compiled from a pool of 16 tallgrass prairie species. We planted all species in monocultures and mixtures that varied in their species richness, evenness, and spatial pattern. We recorded species-specific biomass production over three growing seasons and fitted Diversity-Interactions (DI) models to annual plot biomass yields.In the establishment season, all species interacted equally to form the diversity effect. In years 2 and 3, each species contributed a unique additive coefficient to its interaction with every other species to form the diversity effect. These interactions were affected by Helianthus maximiliani and the species planting pattern. Models based on species planted proportions better-fit annual plot yield than models based on species previous contributions to plot biomass.Outcomes suggest that efforts to plant tallgrass prairies to maximize diversity effects should focus on the specific species present and in what arrangement they are planted. Furthermore, for particularly diverse grasslands, the effort of collecting annual species biomass data may not be necessary when quantifying diversity effects with DI models.

Across species-pool aggregation alters grassland productivity and diversity.

Plant performance is determined by the balance of intra- and interspecific neighbors within an individual's zone of influence. If individuals interact over smaller scales than the scales at which communities are measured, then altering neighborhood interactions may fundamentally affect community responses. These interactions can be altered by changing the number (species richness), abundances (species evenness), and positions (species pattern) of the resident plant species, and we aimed to test whether aggregating species at planting would alter effects of species richness and evenness on biomass production at a common scale of observation in grasslands. We varied plant species richness (2, 4, or 8 species and monocultures), evenness (0.64, 0.8, or 1.0), and pattern (planted randomly or aggregated in groups of four individuals) within 1 × 1 m plots established with transplants from a pool of 16 tallgrass prairie species and assessed plot-scale biomass production and diversity over the first three growing seasons. As expected, more species-rich plots produced more biomass by the end of the third growing season, an effect associated with a shift from selection to complementarity effects over time. Aggregating conspecifics at a 0.25-m scale marginally reduced biomass production across all treatments and increased diversity in the most even plots, but did not alter biodiversity effects or richness-productivity relationships. Results support the hypothesis that fine-scale species aggregation affects diversity by promoting species coexistence in this system. However, results indicate that inherent changes in species neighborhood relationships along grassland diversity gradients may only minimally affect community (meter) - scale responses among similarly designed biodiversity-ecosystem function studies. Given that species varied in their responses to local aggregation, it may be possible to use such species-specific results to spatially design larger-scale grassland communities to achieve desired diversity and productivity responses.

Effects of the Epichloë fungal endophyte symbiosis with Schedonorus pratensis on host grass invasiveness.

Initial studies of grass-endophyte mutualisms using Schedonorus arundinaceus cultivar Kentucky-31 infected with the vertically transmitted endophyte Epichloë coenophiala found strong, positive endophyte effects on host-grass invasion success. However, more recent work using different cultivars of S. arundinaceus has cast doubt on the ubiquity of this effect, at least as it pertains to S. arundinaceus-E. coenophiala. We investigated the generality of previous work on vertically transmitted Epichloë-associated grass invasiveness by studying a pair of very closely related species: S. pratensis and E. uncinata. Seven cultivars of S. pratensis and two cultivars of S. arundinaceus that were developed with high- or low-endophyte infection rate were broadcast seeded into 2 × 2-m plots in a tilled, old-field grassland community in a completely randomized block design. Schedonorus abundance, endophyte infection rate, and co-occurring vegetation were sampled 3, 4, 5, and 6 years after establishment, and the aboveground invertebrate community was sampled in S. pratensis plots 3 and 4 years after establishment. Endophyte infection did not enable the host grass to achieve high abundance in the plant community. Contrary to expectations, high-endophyte S. pratensis increased plant richness relative to low-endophyte cultivars. However, as expected, high-endophyte S. pratensis marginally decreased invertebrate taxon richness. Endophyte effects on vegetation and invertebrate community composition were inconsistent among cultivars and were weaker than temporal effects. The effect of the grass-Epichloë symbiosis on diversity is not generalizable, but rather specific to species, cultivar, infection, and potentially site. Examining grass-endophyte systems using multiple cultivars and species replicated among sites will be important to determine the range of conditions in which endophyte associations benefit host grass performance and have subsequent effects on co-occurring biotic communities.