Effects of plant species and soil history on root morphology,arbuscular mycorrhizal colonization of roots,and biomass in four tallgrass prairie species

Although dominant C₄ grasses in tallgrass prairie are highly mycotrophic, for many non-dominant species neither extent of mycorrhizal colonization nor root morphology effects on plant–soil feedback interactions are known. In a laboratory study conducted from November 2013 to February 2014 at Governors State University (University Park, IL), we grew plants of a dominant C₄ grass (Andropogon gerardii) and three non-dominant forbs (Symphyotrichum laeve var. laeve, Symphyotrichum novae-angliae and Parthenium integrifolium) individually in soil collected in the field beneath a conspecific, collected beneath another study species, or in sterilized soil. The study addressed the following questions: (1) Is extent of mycorrhizal colonization of roots related to root structure? (2) How does soil history interact with plant root traits to influence plant–soil feedback? (3) How might plant–soil feedback patterns influence competitive interactions among study species? We found that proportion arbuscular mycorrhizal fungi (AMF) colonization decreased with increasing specific root length. Soil history had a stronger influence than plant species on total biomass of plants, with all species having highest total biomass when grown in Andropogon soil. Consequently, net, or heterospecific, feedback did not differ among pairwise species combinations, and was not different from 0. While these results suggest that no study species should have a competitive advantage in the field, Andropogon might still have an advantage through mechanisms such as competition for light. Future work in the field and including less mycotrophic species is needed to better understand AMF effects on competitive interactions.

     

Fire‐mediated niche‐separation between two sympatric small mammal species

Fire is a key ecological process influencing the population dynamics of small mammals. Whilst shifting competitive advantage amongst small mammal species following a single fire event is well‐documented, there has been little investigation of the potential influence of fire frequency on small mammal interspecific interactions. In this study, we investigated the effect of fire frequency on the abundance of two small dasyurid mammals, Antechinus stuartii and A. flavipes, which occur sympatrically in some parts of their range. The two antechinus species are known to have different habitat preferences, so it is possible that fire regimes may promote their coexistence in areas of sympatry by altering vegetation structure. To investigate this possibility, we estimated the abundance of both species using replicate sites which differed in the number of times burnt (1–4) during the last four decades, but with identical time‐since‐fire. Proportionally, we captured greater numbers of A. stuartii in less frequently burnt sites and greater numbers of A. flavipes in more‐frequently burnt sites. Hence, fire may mediate niche‐separation between these two species. To clarify further this pattern of response to fire frequency, we investigated which structural habitat variables differed between fire frequencies, and compared antechinus abundances with structural vegetation characteristics. We found a trend for lower ground cover density under higher fire frequencies. This offers one potential explanation of the patterns of abundance that we observed. Our study provided insights into the complexities of small mammal responses to fire, and strongly suggests that fire could mediate competitive interactions between species.     

Measures of ecological association

Summary The relationships of photosynthetic characteristics to the competitive interactions of a C₃ plant, Chenopodium album, and a C₄ plant, Amaranthis retroflexus, were investigated in different temperature and water supply regimes. Both species had similar photosynthetic rates at 25°C, but at higher temperatures, Amaranthus had substantially greater rates than Chenopodium. Conversely, at lower temperatures, Chenopodium had an advantage. The competitive abilities in mixtures exhibited a close parallel to the photosynthetic performances with Amaranthus having an advantage at high temperatures and Chenopodium an advantage at low temperatures. These competitive outcomes were determined primarily by differences in relative growth rates prior to canopy closure. In the respective, temperature regimes, the species having the highest photosynthetic rate, which resulted an more rapid growth, overtopped and shaded the other species at the time of canopy closure. These results demonstrate that differences in photosynthetic temperature response between C₄ and C₃ plants can be an important determinant in competitive interactions, but at least in this case, the influence is primarily through, events prior to the actual initiation of competition.In contrast to temperature, growth of the plants under limited water supply had no influence on the competitive interactions. Thus, the presence of the C₄ pathway alone was not sufficient to yield a competitive advantage over the C₃ species under water limited conditions.     

DISPLACEMENT OF ANDROPOGON SCOPARIUS ON THE NEW JERSEY PIEDMONT BY THE SUCCESSIONAL SHRUB MYRICA PENSYLVANICA

Andropogon scoparius, a perennial grass found in old fields on the New Jersey Piedmont, can be invaded and displaced by a nitrogen-fixing shrub, Myrica pensylvanica. The progression of Andropogon displacement was followed over a season, and possible contributing mechanisms (shading, allelopathy, physical effects of Myrica litter) were tested through field and greenhouse experiments. In the field, Andropogon seedling growth was inhibited beneath Myrica clumps, and mature plant living crown area was reduced. In the greenhouse, Andropogon seedling growth was reduced significantly in pots with mature Myrica. Both seedling and mature growth of Andropogon were reduced under shaded conditions. No inhibitory physical effects of Myrica litter on Andropogon seed germination or mature plant growth were found; however, there may ba allelopathic effects of Myrica litter leachate on Andropogon seedlings growing under shade or within grass litter, and on mature Andropogon in shaded conditions. Andropogon displacement appears to be the result of a complex interaction of Myrica shade, allelochemic, and competitive effects, rather than being attributable to any single mechanism.     

Exotic plant species in a C4-dominated grassland: invasibility, disturbance, and community structure

We used data from a 15-year experiment in a C₄-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C₃ plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C₄ grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C₄-dominated grassland can also be directly related to community structure independent of disturbance. Received: 9 February 1999 / Accepted: 12 May 1999     

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Prescribed spring burning often contributes to a predominance of C₄ grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C₄ grasses, C₃ graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C₄ grasses were significantly more abundant after fire than after raking. Burning reduced some C₃ forbs and grasses and favored the dominance of C₄ grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C₄ grasses to the same extent.     

Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie

In tallgrass prairie, plant species interactions regulated by their associated mycorrhizal fungi may be important forces that influence species coexistence and community structure; however, the mechanisms and magnitude of these interactions remain unknown. The objective of this study was to determine how interspecific competition, mycorrhizal symbiosis, and their interactions influence plant community structure. We conducted a factorial experiment, which incorporated manipulations of abundance of dominant competitors, Andropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal symbiosis using the fungicide benomyl under two fire regimes (annual and 4-year burn intervals). Removal of the two dominant C₄ grass species altered the community structure, increased plant species richness, diversity, and evenness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community structure, although plant species richness and diversity increased. Responses of individual plant species were associated with their degree of mycorrhizal responsiveness: highly mycorrhizal responsive species decreased in abundance and less mycorrhizal responsive species increased in abundance. The combination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, indicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that mycorrhizal fungi affect plant communities indirectly by influencing the pattern and strength of plant competitive interactions. Burning strongly influenced the outcome of these interactions, which suggests that plant species diversity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis. Received: 7 April 1999 / Accepted: 30 July 1999     

Plant demographic responses to mycorrhizal symbiosis in tallgrass prairie

The effects of mycorrhizal symbiosis on seedling emergence, flowering and densities of several grasses and forbs were assessed in native tallgrass prairie and in sown garden populations at the Konza Prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed with the fungicide benomyl. Flowering and stem densities of the cool-season grass, Dichanthelium oligosanthes, sedges (Carex spp.), and the forb Aster ericoides were higher in non-mycorrhizal (benomyl-treated) than in mycorrhizal plots and the magnitude of these differences was significantly affected by burning. Mycorrhizae significantly enhanced flowering of the warmseason grasses Andropogon gerardii and Sorghastrum nutans in burned prairie, but not in unburned sites. These patterns suggest that mycorrhizal effects on the dynamics of cool-season graminoid and forb populations are likely to be mediated indirectly through effects of the symbiosis on the competitive dominance of their neighbors. Seedling emergence rates of the cool-season C₃ grasses Elymus canadensis and Koeleria cristata were significantly reduced in the benomyl-treated plots, whereas benomyl treatment had no significant effect on seedling emergence of the warm-season C₄ grasses A. gerardii and Panicum virgatum. The forbs showed variable responses. Seedling emergence of Liatris aspera was greater under mycorrhizal conditions, but that of Dalea purpurea was unaffected by mycorrhizal treatment. These results show that effects of mycorrhizal symbiosis on the population dynamics of co-occurring prairie plants vary significantly both among species and among different life history stages within species. The results also indicate that mycorrhizas and fire interact to influence competitive interactions and demographic patterns of tallgrass prairie plant populations.     

Immediate and longer-term effects of fire on seed predation by ants in sclerophyllous vegetation in south-eastern Australia

Following a prescribed spring fire at a woodland site with heathy understorey in south-eastern Australia, the following features changed markedly: the composition of the ant fauna taken in pitfall traps; the abundance of seed-eating species; seedfall from woody-fruited Eucalyptus obliqua and Casuarina pusilla; and rates of seed predation by ants. Broadly adapted species of Monomorium and Rhytidoponera, uncommonly recorded before the fire, became the most abundant ants caught. Seed removal from baits dropped by almost half during the week after fire, despite an increased catch of seed-eating Rhytidoponera tasmaniensis. This coincided with a massive fire-induced release of seeds stored inside woody fruits, suggesting that predator satiation occurred. The fall of Casuarina seeds and Eucalyptus chaff returned to pre-fire (low) levels 1 week after the fire, but a substantial proportion of Eucalyptus seeds fell after several weeks. By this time rates of seed removal were increasing rapidly as the abundance of seed-eating R. tasmaniensis continued to increase. Weekly removal from baits reached 100% 7 weeks after the fire, which was much higher than any level recorded before it. Results from a nearby heathland site, burned by an earlier fire, indicated that unusually high rates of seed removal, paralleling increases in abundance of R. tasmaniensis, can persist for at least 2 years following fire. These results have potentially important implications for post-fire recruitment.     

Restoration of C4 grasses with seasonal fires in a C3/C4 grassland invaded by Prosopis glandulosa,a fire‐resistant shrub

Questions: Can prescribed fire restore C₄ perennial grasses in grassland ecosystems that have become dominated by fire‐resistant C₃ shrubs (Prosopis glandulosa) and C₃ grasses? Do fires in different seasons alter the direction of change in grass composition? Location: Texas, USA. Methods: We quantified short‐ and long‐term (12 yr post‐fire) herbaceous functional group cover and diversity responses to replicated seasonal fire treatments: (1) repeated‐winter fires (three in 5 yr), (2) repeated‐summer fires (two in 3 yr), and (3) alternate‐season fires (two winter and one summer in 4 yr), compared with a no‐fire control. Results: Summer fires were more intense than winter fires, but all fire treatments temporarily decreased Prosopis and C₃ annual grass cover. The alternate‐season fire treatment caused a long‐term increase in C₄ mid‐grass cover and functional group diversity. The repeated‐summer fire treatment increased C₄ short‐grass cover but also caused a long‐term increase in bare ground. The repeated winter fire treatment had no long‐term effects on perennial grass cover. Mesquite post‐fire regrowth had increasingly negative impacts on herbaceous cover in all fire treatments. Conclusions: Summer fire was necessary to shift herbaceous composition toward C₄ mid‐grasses. However, the repeated‐summer fire treatment may have been too extreme and caused post‐fire herbaceous composition to “over‐shift” toward less productive C₄ short‐grasses rather than C₄ mid‐grasses. This study provides some of the first long‐term data showing a possible benefit of mixing seasonal fires (i.e., the alternate‐season fire treatment) in a prescribed burning management plan to restore C₄ mid‐grass cover and enhance overall herbaceous diversity.     

EFFECTS OF COMPETITION AND DISTURBANCE ON THE RESPROUTING PERFORMANCE OF THE MEDITERRANEAN SHRUB ERICA MULTIFLORA L. (ERICACEAE)

Two field experiments were designed to evaluate the importance of competition, fire, repeated disturbance, and their interactions on the vegetative and reproductive performance of the Mediterranean shrub Erica multiflora over a 2.5-yr period. In a burn experiment, fire was applied to the ground-level stumps of previously clipped 13-yr-old plants with a propane torch and competition was diminished by removal of neighboring plants. Fire resulted in a reduction of sprout vigor and biomass of flowers; mature neighbors also reduced E. multiflora sprout vigor and flowering. The interaction between fire and competition was nonsignificant. In a stand burned by a wildfire we studied the effects of regenerating neighbors on target plants by removing all neighbors or only Quercus coccifera, the most dominant species in the burned stand. In this stand we also simulated herbivory by repeatedly clipping the sprouts of E. multiflora. Regenerating neighbors did not affect target plant sprout vigor after the wildfire, but did cause a decrease in the biomass of flowers per plant. Survival decreased after repeated clipping but was not affected by neighborhood treatment. The results suggest that the importance of competition on resprouting vigor was temporally variable. Variables related to plant size rather than species determined competitive superiority: resprouting neighbors did not affect resprouting performance of target plants, but mature neighbors did. In nature, fire may directly reduce vegetative and reproductive biomass by the heating effect. But it may have an indirect positive effect on biomass, by reducing competition among plants. Frequent disturbances that removed aboveground biomass of E. multiflora had a detrimental effect on target plant survival independent of neighborhood effect.     

Interspecific competition among macroparasites in a density-dependent host population

 We analyze the dynamics of a community of macroparasite species that share the same host. Our work extends an earlier framework for a host species that would grow exponentially in the absence of parasitism, to one where an uninfected host population is regulated by factors other than parasites. The model consists of one differential equation for each parasite species and a single density-dependent nonlinear equation for the host. We assume that each parasite species has a negative binomial distribution within the host and there is zero covariance between the species (exploitation competition). New threshold conditions on model parameters for the coexistence and competitive exclusion of parasite species are derived via invadibility and stability analysis of corresponding equilibria. The main finding is that the community of parasite species coexisting at the stable equilibrium is obtained by ranking the species according t! o th e minimum host density H ^* above which a parasite species can grow when rare: the lower H ^* , the higher the competitive ability. We also show that ranking according to the basic reproduction number Q ₀ does not in general coincide with ranking according to H ^* . The second result is that the type of interaction between host and parasites is crucial in determining the competitive success of a parasite species, because frequency-dependent transmission of free-living stages enhances the invading ability of a parasite species while density-dependent transmission makes a parasite very sensitive to other competing species. Finally, we show that density dependence in the host population entails a simplification of the portrait of possible outcomes with respect to previous studies, because all the cases resulting in the exponential growth of host and parasite populations are eliminated.. Received: 24 June 1996 / Revised version: 28 April 1998     

Competition at the population level along a standing crop gradient: a field experiment in successional grassland

I measured competitive responses of experimentally-established populations of the perennial grass, Andropogon gerardi, across a complex gradient of standing crop and species composition in the successional grasslands of southwest Michigan. The goal was to assess whether long-term (three year) population-level responses of Andropogon to competition matched the inferences made from a previous phytometer study that examined transplant responses to competition across this same gradient over a single growing season.Replicate experimental populations of Andropogon were established at seven grassland sites by sowing seed into 0.5×0.5 m plots that had been denuded of all vegetation. During the first year of the study, all Andropogon populations were maintained as monocultures by hand weeding. At the end of the first growing season, half of the monocultures were selected for continued weeding and half were left open to invasion by competitors for three years. Invasion of the unweeded populations by neighboring plants varied strongly among sites and was positively correlated with standing crop. Increased susceptibility to invasion and competition resulted in the extinction of the unweeded Andropogon populations at the two most productive sites, supporting the hypothesis that Andropogon is restricted by competition to low productivity sites in these grasslands. The finding that the intensity of competition was positively correlated with standing crop is consistent with the previous transplant study, suggesting that short-term experimental assays of competition on the growth of individual transplants may have predictive value for longer-term outcomes of competition at the population level.     

Lizard responses to wildfire in arid interior Australia: Long‐term experimental data and commonalities with other studies

Life in terrestrial Australian ecosystems has evolved over the past 10 million years to thrive in habitats kept in a dynamic state through fire succession cycles. Previous studies support the notion that wildfires promote species diversity in plant and animal communities by creating a heterogeneous mix of habitats, each habitat more suitable for particular subsets of species. We document population and community responses to fire in a species‐rich lizard assemblage in the Great Victoria Desert of Western Australia. Lizards were censused by pit trapping at a long‐unburned flat spinifex site in the Great Victoria Desert in Austral springs of 1992 and 1995. A controlled burn was undertaken in mid‐October of 1995, and lizards were censused thereafter in late 1995 and early 1996, and then again in the Austral springs of 1998, 2003 and 2008. Forty‐six species of lizards (2872 individuals) were collected and their stomach contents analysed over the course of a 16‐year fire succession cycle at this single study site. Most strikingly, relative abundances of two species of agamids varied inversely, responding oppositely to habitat clearing effects of fire. The military dragon Ctenophorus isolepis reached higher abundances when vegetation was dense, and decreased in abundance in open vegetation following fire. The netted dragon Ctenophorus nuchalis was rare when vegetation coverage was high but increased rapidly after fire. Abundances of five species of Ctenotus skinks, C. ariadnae, C. calurus, C. hanloni, C. pantherinus and C. piankai, tracked those of C. isolepis. Abundance of a termite‐specialized nocturnal gecko, Rhynchoedura ornata, increased in abundance following fire. Lizard diets changed during the course of the fire succession cycle, returning to near pre‐burn conditions after 16 years. In addition to short‐term fire succession cycles that contribute to structuring local communities, changes in long‐term rainfall also impact desert food webs and regional biotas.     

Biomass production of C3- and C4-plant species in pure and mixed culture with different water supply

Summary Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C₃ plant) and Amaranthus retroflexus L (C₄) on the one hand and of the grasses Avena sativa L (C₃) and Panicum miliaceum L. (C₄) on the other hand were maintained in a standard soil with different ground water tables. After 12 weeks the length, dry weight and nitrogen-content of the aboveground and belowground parts of the plants, and in addition the carbon-and ash-content and the ¹³C value of the aboveground parts were determined. It turned out that the length and the dry weight of the shoots of the C₃ species showed on increasing tendency with increased water supply, while the values of the C₄ species were drastically diminished at the highest water level only. The roots showed in most cases an increased length and dry weight at drier conditions, more pronounced in the C₄ than in the C₃ species. The nitrogen content of the shoots was mostly higher in the shoots of the C₃ plants and in the roots of the C₄ plants; it changed in a non-regular manner with variations in water supply. Since the carbon content did not change markedly, the C/N ratio was variable. There was a slight tendency for a higher carbon content and mostly also for a higher C/N-ratio in the shoots of C₄ plants. The ¹³C values of both C₃ as C₄ plants were in general not at all influenced by the water supply; they were fixed genetically. The ash content of the analyzed species did not show a clear relationship to the type of photosynthetic CO₂-fixation or to the water regime.The influence of light intensity was studied with mixed cultures of all four plant species, again with different water supply. There was a strong effect of light intensity on the competitive behaviour of the C₃ and C₄ plants under modified water conditions. The wild C₃ plant Atriplex hortensis was most successful under conditions of relatively low light intensity and high water availability, while the cultivated artificial species Avena sativa showed much less differences between full-light grown and shadow plants. The C₄ plant Amaranthus retroflexus is most successful under competitive conditions at high water stress in full light. The C₄ grass Panicum miliaceum showed maximum shoot growth in light, but was successful under competitive conditions especially also with good water supply. The light intensity had no effect on the ¹³C values. — There was no indication that the soil-type as such has a distinct influence on the success of C₃ or C₄ plants in mixed cultures.Dedicated to Prof. Dr. M. Evenari, Jerusalem, and to Dr. K.F. Springer, Heidelberg     

INTERSPECIFIC COMPETITIVE INTERACTIONS AND THE MAINTENANCE OF GENOTYPIC VARIATION WITHIN TWO PERENNIAL GRASSES

Theoretical models predict that natural selection acting through competitive interactions should lead to increased divergence in resource use and to more equal competitive abilities among different genotypes. The consequent overcompensatory interactions and similar competitive abilities will favor maintenance of genotypic diversity. In field experiments we found that naturally co-occurring genotypes of two perennial grasses, Danthonia spicata and Anthoxanthum odoratum, differed in interspecific competitive performance. The competitive performance of a given genotype often depended on the genotypic identity of the competing species, especially in Danthonia spicata. Both overcompensatory and undercompensatory interactions were found, but neither was prevalent for both species. These data indicate that interactions between species are complex and have an important genetic component. The results from this experiment are compatible with a competitive explanation for the maintenance of genetic variation, but the persistence of weakly competitive genotypes cannot be explained.     

Why is Abundance of Herbaceous Legumes Low in African Savanna? A Test with Two Model Species

Although fire is frequent in African savanna ecosystems and may cause considerable loss of nitrogen (N), N₂-fixing herbaceous legumes—which could be expected to benefit from low N conditions—are usually not abundant. To investigate possible reasons for this scarcity, we conducted a pot experiment using two common plants of humid African savannas as model species, the legume Cassia mimosoides and the C₄ grass Hyperthelia dissoluta. These species were grown at different levels of water, N and phosphorus (P), both in monoculture and in competition with each other. In the monocultures, yields were significantly increased by the combined addition of N and P in pots receiving high water supply. In pots with interspecific competition, the legume grew poorly unless P was added. Foliar δ¹⁵N values of legume plants grown in mixtures were considerably lower than those in monocultures, suggesting that rates of symbiotic N-fixation were higher in the presence of the grass. Grass δ¹⁵N values, however, were also lower in mixtures, while N concentrations were higher, indicating a rapid transfer of N from the legume to the grass. We conclude that the main reason for the low abundance of C. mimosoides is not low P availability as such, but a greater ability of H. dissoluta to compete for soil N and P, and a much higher N-use efficiency. If other C₄ grasses have a similar competitive advantage, it could explain why herbaceous legumes are generally sparse in African savannas. We encourage others to test these findings using species from other types of savanna vegetation.     

Post‐fire resprouting responses of native and exotic grasses from Cumberland Plain Woodland (Sydney,Australia) under elevated carbon dioxide

This study investigated the effect of elevated CO₂ on the post‐fire resprouting response of a grassland system of perennial grass species of Cumberland Plain Woodland. Plants were grown in mixtures in natural soil in mesocosms, each containing three exotic grasses (Nassella neesiana, Chloris gayana, Eragrostis curvula) and three native grasses (Themeda australis, Microlaena stipoides, Chloris ventricosa) under elevated (700 ppm) and ambient (385 ppm) CO₂ conditions. Resprouting response after fire at the community‐ and species‐level was assessed. There was no difference in community‐level biomass between CO₂ treatments; however, exotic species made up a larger proportion of the community biomass under all treatments. There were species‐level responses to elevated CO₂ but no significant interactions found between CO₂ and burning or plant status. Two exotic grasses (N. neesiana and E. curvula, a C₃ and a C₄ species respectively), and one native grass (M. stipoides, a C₃ species) significantly increased in biomass, and a native C₄ grass (C. ventricosa) significantly decreased in biomass under elevated CO₂. These results suggest that although overall productivity of this community may not change with increases in CO₂ and fire frequency, the community composition may alter due to differential species responses.     

Upward expansion of fire‐adapted grasses along a warming tropical elevation gradient

High mountain regions in the tropics have thus far been impacted relatively little by anthropogenic activity or plant invasions, however, they are unlikely to be immune to impacts of global change, including climate change and other anthropogenic disturbances. Changes in fire regimes are known to accelerate the spread of invasive C₄ grasses and interactions between changes in fire and climate can alter species distributions. The aim of this study was to compare grass distributions along an elevational gradient in Hawai‘i between 1966–1967 and 2008 to determine whether C₄ and C₃ grass distributions are shifting upward in response to alterations in fire and climate patterns. Field plots at Hawai‘i Volcanoes National Park were surveyed for grass species and cover at ?150 m elevation intervals and compared to previous surveys done in 1966–1967. We found that the transition elevation, marking a shift in dominance between C₄ and C₃ grasses based on relative cover, shifted upward over 40 yr (95% confidence interval = 1476 m ± 130 m in 2008 versus 1200 m ± 106 m in 1966–1967). On the other hand, maximum elevations of all C₄ or C₃ grasses as a group were not significantly greater than 1966–1967 elevations; however, a subset of C₄ (and fewer C₃) grasses moved to substantially higher elevations, and these were the species adapted to fire. 100% of fire‐adapted grasses moved up in elevation compared to 29% of non‐fire adapted species, and the change in elevation of those species (=+454 m) was significantly greater than the change in elevation of non‐fire adapted species (p = 0.003). Our study documents an upward expansion of fire‐adapted grasses at high elevations in the tropics as an important threat that seems to be compounded by warming trends.