Fire, rodent herbivory, and plant competition: implications for invasion and altered fire regimes in the Mojave Desert.

Biological invasions are responsive to changing wildfire regimes related to human activities that are altering biological communities. Our objective was to investigate how fire, rodent activity, and competition among plant species modify plant community structure, invasion patterns, and vulnerability to altered fire regimes. We imposed experimental fires, and reduced rodent density using fencing in a full factorial design and quantified competitive interactions among plant species in the northeast Mojave Desert that has experienced dramatic increases in plant invasion and fire in recent years. Vegetation surveys were conducted in the experimental plots to determine plant density, cover, and biomass of herbaceous plants over a 5-year period. Rodent exclusion increased the density, cover, and biomass of Bromus rubens, an invasive annual grass, and density of forb species. In contrast, rodent exclusion decreased the density, cover, and biomass of Schismus spp. another dominant annual invader. Fire increased Schismus spp. and forb species density, cover, and biomass but decreased B. rubens density. Negative spatial correlation between B. rubens and Schismus spp., and forbs indicated interspecific competition among the dominant plant species. Fire reduced rodent community diversity (Shannon's) 2.5-fold, which was correlated with increases in B. rubens cover and biomass, and native forb diversity. Fire, high rodent diversity, and competition from the other plant species may decrease fire potential in our study system by reducing the density and biomass of B. rubens, which because of its taller growth form tends to ignite and carry fire better than Schismus spp. and forbs.

CO2 enrichment and soil type additively regulate grassland productivity.

Atmospheric CO2 enrichment usually increases the aboveground net primary productivity (ANPP) of grassland vegetation, but the magnitude of the ANPP-CO2 response differs among ecosystems. Soil properties affect ANPP via multiple mechanisms and vary over topographic to geographic gradients, but have received little attention as potential modifiers of the ANPP-CO2 response. We assessed the effects of three soil types, sandy loam, silty clay and clay, on the ANPP response of perennial C3 /C4 grassland communities to a subambient to elevated CO2 gradient over 10 yr in Texas, USA. We predicted an interactive, rather than additive, effect of CO2 and soil type on ANPP. Contrary to prediction, CO2 and soil additively influenced grassland ANPP. Increasing CO2 by 250 µl l-1 increased ANPP by 170 g m-2 across soil types. Increased clay content from 10% to 50% among soils reduced ANPP by 50 g m-2 . CO2 enrichment increased ANPP via a predominant direct effect, accompanied by a smaller indirect effect mediated by a successional shift to increased dominance of the C4 tallgrass Sorghastrum nutans. Our results indicate a large, positive influence of CO2 enrichment on grassland productivity that resulted from the direct physiological benefits of CO2 augmented by species succession, and was expressed similarly across soils of differing physical properties.

A longitudinal study of attitudes toward evolution among undergraduates who are members of the Church of Jesus Christ of Latter-day Saints.

Polling data reveal a decades-long residual rejection of evolution in the United States, based on perceived religious conflict. Similarly, a strong creationist movement has been documented internationally, including in the Muslim world. Members of the Church of Jesus Christ of Latter-day Saints (LDS, Mormon), a generally conservative denomination, have historically harbored strong anti-evolution sentiments. We report here a significant shift toward acceptance, compared to attitudes 30 years earlier, by students at Brigham Young University, which is owned and operated by the LDS church. This change appears to have multiple explanations. Students currently entering the university have been exposed to a much-improved introduction to evolution during high school. More importantly, there has been a significant decrease in negative messaging from Church authorities and in its religious education system. There is also evidence that current students have been positively influenced toward evolution by their parents, a large percentage of whom were BYU students, who earlier were given a strong science education deemed compatible with the maintenance of religious belief. A pre-post comparison demonstrates that a majority of current students become knowledgeable and accepting following a course experience focused on evolutionary principles delivered in a faith-friendly atmosphere. Elements of that classroom pedagogy, intended to promote reconciliation, are presented. Our experience may serve as a case-study for prompting changes in acceptance of evolution in other conservative religious groups.

Niche opportunities for invasive annual plants in dryland ecosystems are controlled by disturbance, trophic interactions, and rainfall.

Resource availability and biotic interactions control opportunities for the establishment and expansion of invasive species. Studies on biotic resistance to plant invasions have typically focused on competition and occasionally on herbivory, while resource-oriented studies have focused on water or nutrient pulses. Through synthesizing these approaches, we identify conditions that create invasion opportunities. In a nested fully factorial experiment, we examined how chronic alterations in water availability and rodent density influenced the density of invasive species in both the Mojave Desert and the Great Basin Desert after fire. We used structural equation modeling to examine the direct and mediated effects controlling the density of invasives in both deserts. In the first 2 years after our controlled burn in the Great Basin, we observed that fire had a direct effect on increasing the invasive forb Halogeton glomeratus as well as a mediated effect through reducing rodent densities and herbivory. 4 years after the burn, the invasive annual grass Bromus tectorum was suppressing Halogeton glomeratus in mammal exclusion plots. There was a clear transition from years where invasives were controlled by disturbance and trophic interactions to years were resource availability and competition controlled invasive density. Similarly, in the Mojave Desert we observed a strong early influence of trophic processes on invasives, with Schismus arabicus benefitted by rodents and Bromus rubens negatively influenced by rodents. In the Mojave Desert, post-fire conditions became less important in controlling the abundance of invasives over time, while Bromus rubens was consistently benefitted by increases in fall rainfall.

Plant community change mediates the response of foliar δ(15)N to CO 2 enrichment in mesic grasslands.

Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf δ(15)N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, (15)N-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m(-2)) and foliar δ(15)N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf δ(15)N only for prairie plants on a silty clay. δ(15)N increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar δ(15)N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, δ(15)N-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher δ(15)N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland δ(15)N by shifting species relative abundances.

Gopher mounds decrease nutrient cycling rates and increase adjacent vegetation in volcanic primary succession.

Fossorial mammals may affect nutrient dynamics and vegetation in recently initiated primary successional ecosystems differently than in more developed systems because of strong C and N limitation to primary productivity and microbial communities. We investigated northern pocket gopher (Thomomys talpoides) effects on soil nutrient dynamics, soil physical properties, and plant communities on surfaces created by Mount St. Helens' 1980 eruption. For comparison to later successional systems, we summarized published studies on gopher effects on soil C and N and plant communities. In 2010, 18 years after gopher colonization, we found that gophers were active in ~2.5% of the study area and formed ~328 mounds ha(-1). Mounds exhibited decreased species density compared to undisturbed areas, while plant abundance on mound margins increased 77%. Plant burial increased total soil carbon (TC) by 13% and nitrogen (TN) by 11%, compared to undisturbed soils. Mound crusts decreased water infiltration, likely explaining the lack of detectable increases in rates of NO3-N, NH4-N or PO4-P leaching out of the rooting zone or in CO2 flux rates. We concluded that plant burial and reduced infiltration on gopher mounds may accelerate soil carbon accumulation, facilitate vegetation development at mound edges through resource concentration and competitive release, and increase small-scale heterogeneity of soils and communities across substantial sections of the primary successional landscape. Our review indicated that increases in TC, TN and plant density at mound margins contrasted with later successional systems, likely due to differences in physical effects and microbial resources between primary successional and older systems.

N-P co-limitation of primary production and response of arthropods to N and P in early primary succession on Mount St. Helens volcano.

BACKGROUND: The effect of low nutrient availability on plant-consumer interactions during early succession is poorly understood. The low productivity and complexity of primary successional communities are expected to limit diversity and abundance of arthropods, but few studies have examined arthropod responses to enhanced nutrient supply in this context. We investigated the effects of nitrogen (N) and phosphorus (P) addition on plant productivity and arthropod abundance on 24-yr-old soils at Mount St. Helens volcano. METHODOLOGY/PRINCIPAL FINDINGS: We measured the relative abundance of eight arthropod orders and five families in plots that received N, P, or no nutrients for 3-5 years. We also measured plant % cover, leaf %N, and plant diversity. Vegetation responded rapidly to N addition but showed a lagged response to P that, combined with evidence of increased N fixation, suggested P-limitation to N availability. After 3 yrs of fertilization, orthopterans (primarily Anabrus simplex (Tettigoniidae) and Melanoplus spp (Acrididae)) showed a striking attraction to P addition plots, while no other taxa responded to fertilization. After 5 yrs of fertilization, orthopteran density in the same plots increased 80%-130% with P addition and 40% with N. Using structural equation modeling, we show that in year 3 orthopteran abundance was associated with a P-mediated increase in plant cover (or correlated increases in resource quality), whereas in year 5 orthopteran density was not related to cover, diversity or plant %N, but rather to unmeasured effects of P, such as its influence on other aspects of resource quality. CONCLUSIONS/SIGNIFICANCE: The marked surprising response to P by orthopterans, combined with a previous observation of P-limitation in lepidopteran herbivores at these sites, suggests that P-mediated effects of food quantity or quality are critical to insect herbivores in this N-P co-limited primary successional system. Our results also support a previous suggestion that the availability of N in these soils is P-limited.

Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2.

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.

Linking community and ecosystem development on Mount St. Helens.

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N2-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.

Nonlinear grassland responses to past and future atmospheric CO(2).

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO(2) concentrations (C(a)) as C(a) increases to more than 500 micromol mol(-1) this century from interglacial levels of less than 200 micromol mol(-1) (refs 1 6). However, such carbon storage depends on feedbacks between plant responses to C(a) and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past C(a) than to those forecast for the coming century. Along a continuous gradient of 200 to 550 micromol mol(-1) (refs 9, 10), increased C(a) promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient C(a), but was unchanged at elevated C(a) where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in C(a) there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future C(a) and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.