Germinable soil seed banks in a tropical savanna: seasonal dynamics and effects of fire.

Abstract The germinable soil seed bank of a tropical eucalypt savanna of north‐eastern Australia was found to be dominated by grasses and forbs, with seed bank density ranging from 58 to 792 seeds per square metre, from a total of 53 species. Late dry season fires and the fire‐related cues, heat shock and smoke, broke the seed dormancy of a range of tropical savanna species. Heat shock promoted the germination of the species groups natives, exotics, subshrubs, ephemeral and twining perennial forbs, and the common species Indigofera hirsuta, Pycnospora lutescens and Triumfetta rhomboidea. Exposure to smoke at ambient temperature promoted germination from the soil seed bank of the species groups combined natives, upright perennial forbs and grasses, as well as the common grasses Digitaria breviglumis and Heteropogon triticeus. The germinable soil seed bank varied seasonally, increasing from the mid wet season (February) and early dry season (May) to a maximum in the late dry season (October). The effect of recent fire history on soil seed bank dynamics was limited to the immediate release of some seed from dormancy; a reduction in seed densities of subshrubs and monocots, other than grasses, in recently burnt savanna; and enhanced seed density of the ephemeral I. hirsuta in the year following fire. The seed banks of most savanna species were replenished in the year following burning.     

Evidence for direct effects of fire‐cues on germination of some perennial forbs common in grassy ecosystems

In grassy ecosystems of south‐eastern Australia, fire maintains richness of native forbs. It is commonly thought that fire promotes regeneration indirectly by reducing competition for light and providing gaps for recruitment, rather than directly stimulating germination. However, physiological dormancy and morphophysiological dormancy are common, and few studies have explored responses to fire‐cues among dormant or hard‐to‐germinate forbs. Recent studies from other fire‐prone ecosystems suggest that in some cases, fire‐cues may not alleviate physiological or morphophysiological dormancy, but instead promote germination in combination with treatments which alleviate dormancy. We experimentally tested the prevailing hypothesis that perennial forbs common in south‐eastern Australian grassy ecosystems do not germinate in direct response to fire. Responses to fire‐cues both inherently and in combination with treatments which alleviate dormancy were investigated for seven species. Two fire‐cues (smoke and heat) plus a treatment of both heat + smoke were applied to fresh seed at three temperatures (35/25°C, 30/20°C and 25/15°C). Following this, the effect of fire‐cues on seed that had undergone warm stratification, cold stratification and dry‐after‐ripening was investigated. Three species — Arthropodium strictum, Cheiranthera cyanea and Dianella revoluta — responded to fire‐cues inherently, although germination in C. cyanea was low. High germination of D. revoluta was found when fire‐cues were combined with warm stratification. Fire‐cues had no effect on germination of Brunonia australis, Burchardia umbellata and Eryngium ovinum. Germination of Stypandra glauca was zero following all treatment combinations. Our finding that fire‐cues promote germination of three of the seven study species did not provide sufficient evidence to reject the current hypothesis that germination of perennial forbs is not typically promoted by fire‐cues. However, this study highlights the important direct role fire‐cues can play in promoting germination of some perennial forbs both inherently and in combination with treatments used to alleviate physiological dormancy.     

Effect of fire regime on plant abundance in a tropical eucalypt savanna of north‐eastern Australia

Abstract Changes in plant abundance within a eucalypt savanna of north‐eastern Australia were studied using a manipulative fire experiment. Three fire regimes were compared between 1997 and 2001: (i) control, savanna burnt in the mid‐dry season (July) 1997 only; (ii) early burnt, savanna burnt in the mid‐dry season 1997 and early dry season (May) 1999; and (iii) late burnt, savanna burnt in the mid‐dry season 1997 and late dry season (October) 1999. Five annual surveys of permanent plots detected stability in the abundance of most species, irrespective of fire regime. However, a significant increase in the abundance of several subshrubs, ephemeral and twining perennial forbs, and grasses occurred in the first year after fire, particularly after late dry season fires. The abundance of these species declined toward prefire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance with fire intervals of 4 years. The density of trees (>2 m tall) significantly increased in the absence of fire for 4 years, because of the growth of saplings; and the basal area of the dominant tree Corymbia clarksoniana significantly increased over the 5‐year study, irrespective of fire regime. Conservation management of these savannas will need to balance the role of regular fires in maintaining the diversity of herbaceous species with the requirement of fire intervals of at least 4‐years for allowing the growth of saplings >2 m in height. Whereas late dry season fires may cause some tree mortality, the use of occasional late fires may help maintain sustainable populations of many grasses and forbs.     

Climate, phylogeny and the ecological distribution of C4 grasses

'C4 photosynthesis' refers to a suite of traits that increase photosynthesis in high light and high temperature environments. Most C4 plants are grasses, which dominate tropical and subtropical grasslands and savannas but are conspicuously absent from cold growing season climates. Physiological attributes of C4 photosynthesis have been invoked to explain C4 grass biogeography; however, the pathway evolved exclusively in grass lineages of tropical origin, suggesting that the prevalence of C4 grasses in warm climates could be due to other traits inherited from their non-C4 ancestors. Here we investigate the relative influences of phylogeny and photosynthetic pathway in determining the ecological distributions of C4 grasses in Hawaii. We find that the restriction of C4 grasses to warmer areas is due largely to their evolutionary history as members of a warm-climate grass clade, but that the pathway does appear to confer a competitive advantage to grasses in more arid environments.     

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.     

The food web of a severely contaminated site following reclamation with warm season grasses

We used the stable isotope ¹³C to distinguish between food web components that depended on warm season grasses with the C₄ photosynthetic pathway and those that depended on plants with the C₃ pathway. The study site was contaminated by heavy metals from a zinc smelter that operated near Palmerton, Pennsylvania, U.S.A. C₃ plants only contributed 1.16% of aboveground primary productivity, whereas recently seeded (5–7 year old) warm season C₄ grasses contributed the remaining 98.84%. Analyses of tissue samples revealed that the carbon content of invertebrates and vertebrates did not reflect the composition of the vegetation. Of 135 samples, 48 (36%) had greater than 75% of their carbon from C₄‐derived sources, while 32 (24%) of the samples had less than 25%. However, carbon from C₄ grasses passed through to higher trophic levels, as shown by the abundance of predators with a high proportion of C₄‐derived carbon. We document three channels of carbon flux through the food web, one based on warm season grasses, now supporting a functioning ecosystem at all key trophic levels, one based on C₃ plants, and a third based on detritus. Theoretical and empirical studies have shown that relative configurations of such channels are important to ecosystem stability. Our results suggest that functional groupings of plants based on photosynthetic pathway or other plant traits likely form the basis for food web compartments. By using diverse functional groups of plants for reclamation or restoration, practitioners may be able to aid the development of channels and thereby promote desired ecosystem states.     

Fire Season and Dominance in an Illinois Tallgrass Prairie Restoration

North American prairie remnants and restorations are normally managed with dormant‐season prescribed fires. Growing‐season fire is of interest because it suppresses dominant late‐flowering grasses and forbs, thereby making available light and other resources used by subdominant grasses and forbs that comprise most prairie diversity. Here we report a twofold increase in mean frequency and richness of subdominant species after late‐summer fire. Stimulation of subdominants was indiscriminate; richness of prairie and volunteer species increased in species that flowered in early, mid‐, or late season. Early spring fire, the management tool used on this site until this experiment, had no effect on subdominant richness or frequency. Neither burn treatment affected reproductive tillering of the tallgrasses Sorghastrum nutans or Panicum virgatum. Flowering of Andropogon gerardii increased 4‐fold after early‐spring fires and 11‐fold after late‐summer fires. These preliminary results suggest that frequency and species richness of subdominants can be improved by late growing‐season fire without compromising vigor of warm‐season tallgrasses.     

内蒙古浑善达克沙地97种植物的光合生理特征(英文)

 报告了内蒙古浑善达克沙地不同生境下97种不同科、属植物的光合速率、蒸腾速率和水分利用效率特征。结果表明：猪毛菜（Salsola collina）、沙米（Agriophyllum pungens）、黑沙蒿（Artemisia ordosica）、狗尾草（Setaria viridis）、柠条（Caragana microphylla）等具有C4光合碳同化途径或具固氮能力的植物种具有较高的光合能力,其净光合速率大于30 μmol CO2·m-2·s-1, 而大部分具C3途径和无固氮能力的植物种的净光合速率较低,为1.29～10 μmol CO2·m-2·s-1;71%的植物种蒸腾速率集中在2～10 mmol H2O·m-2·s-1。所选出的高光效植物种在当地植被恢复与重建过程中应有很高利用价值。C4植物种占所测植物种的1/5左右,主要分布于固定沙丘上,且随着生境土壤湿度的增大其与C3植物种的光合速率差异逐渐缩小。在3种生境条件下,灌木和草本植物的光合速率和蒸腾速率顺序为：低湿地> 滩地 > 固定沙丘(p<0.01),而乔木为固定沙丘>滩地 (p<0.01) 。不同功能型植物的气体交换特征随生境的不同而异, 在固定沙丘上, 草本的蒸腾速率最高,乔木的水分利用率最高,三者光合速率相差不大。     

Trait differences between grass species along a climatic gradient in South and North America

Question: Are trait differences between grasses along a gradient related to climatic variables and/or photosynthetic pathway? Location: Temperate grassland areas of South and North America. Methods: In a common garden experiment, we cultivated C₃ and C₄ grasses from grasslands under different climatic conditions, and we measured a set of 12 plant traits related to size and resource capture and utilization. We described (1) interspecific plant trait differences along a climatic gradient defined by the precipitation and temperature at the location where each species is dominant and (2) the association between those plant trait differences and the photosynthetic pathway of the species. Results: Trait differences between grasses were related to the precipitation at the area where each species is dominant, and to the photosynthetic pathway of the species. Leaf length, leaf width, plant height, leaf area per tiller, specific leaf area, leaf δ¹³C ratio, and nitrogen resorption efficiency increased while leaf dry matter content and nitrogen concentration in senesced leaves decreased as precipitation increased. A proportion of these changes along the gradient was related to the photosynthetic pathway because dominant grass species in cold areas with low precipitation are mainly C₃ and those from warm and wet areas are C₄. Conclusions: A previous worldwide analysis showed that traits of graminoid species measured in situ changed slightly along climatic gradients (< 10% variance explained). In contrast, under a common environment we observed that (1) grass traits changed strongly along a climatic gradient (30‐85% variance explained) and, (2) a proportion of those changes were related to the association between photosynthetic pathway of the species and precipitation.     

Short‐term effects of herbicides and a prescribed fire on restoration of a shrub‐encroached pine savanna

Shrub encroachment occurring worldwide in savannas and grasslands has commonly been hypothesized to result from anthropogenically altered environments. Two disturbance‐based approaches to restoration have involved: (1) application of selective herbicides to reduce density/cover of shrubs; (2) reinstatement of natural fire regimes to generate environmental conditions favoring herbaceous species. We studied short‐term responses of native shrubs, vines, and grasses in a Louisiana pine savanna to herbicides followed by a prescribed fire and fire alone. Plots established in the summer, 2013, were hand‐sprayed in the fall with Imazapyr and Triclopyr, Triclopyr alone, or no herbicide, then prescribed burned the following spring. Numbers of species of shrubs and vines at scales of 1 and 100 m², numbers of stems and regrowth of stems produced by six common species of shrubs, and the number of flowering culms of perennial C₄ grasses were assessed postfire in 2014. Compared with fire alone, herbicides followed by fire resulted in (1) small reductions in species richness of shrubs and no effects on vines, (2) fewer stems comprising shrub genets, but similar postfire regrowth of resprouting shrub stems, and (3) fewer flowering culms of C₄ grasses. Underground storage organs of savanna shrubs and vines survived both aboveground disturbances. Thus, single applications of herbicides followed by fires reduced, but did not reverse shrub encroachment and negatively affected grasses. Because effects of herbicides overrode those of prescribed fires, these disturbances did not act synergistically, suggesting that reinstating natural fire regimes should be a priority in restoration of savannas and grasslands.     

Temperature thresholds of physically dormant seeds and plant functional response to fire: variation among species and relative impact of climate change

Variation in dormancy thresholds among species is rarely studied but may provide a basis to better understand the mechanisms controlling population persistence. Incorporating dormancy‐breaking temperature thresholds into existing trait frameworks could improve predictions regarding seed bank persistence, and subsequently species resilience in response to fire, climate change and anthropogenic management. A key ecological strategy for many species from fire‐prone ecosystems is the possession of a long‐lived seed bank, ensuring recovery after fire. Physical dormancy is dominant in these ecosystems and maintaining this dormancy is directly linked to seed bank persistence. We identified a suite of seed‐related factors relevant to maintaining populations in fire‐prone regions for 14 co‐occurring physically dormant species. We measured variation in initial levels of dormancy and then applied experimental heating treatments, based on current seasonal temperatures and those occurring during fires, to seeds of all study species. Additionally, higher seasonal temperature treatments were applied to assess response of seeds to temperatures projected under future climate scenarios. Levels of germination response and mortality were determined to assess how tightly germination response was bound to either fire or seasonal cues. Six species were found to have dormancy cues bound to temperatures that only occur during fires (80°C and above) and were grouped as having obligate pyrogenic dormancy release. The remaining species, classified as having facultative pyrogenic dormancy, had lower temperature dormancy thresholds and committed at least 30% of seeds to germinate after summer‐temperature treatments. Evidence from this study supports including dormancy‐breaking temperature thresholds as an attribute for identifying functional types. High temperature thresholds for breaking dormancy, found in our obligate pyrogenic group, appear to be a fire‐adapted trait, while we predict that species in the facultative group are most at risk to increased seed bank decay resulting from elevated soil temperatures under projected climate change.     

Fire-driven alien invasion in a fire-adapted ecosystem

Disturbance plays a key role in many alien plant invasions. However, often the main driver of invasion is not disturbance per se but alterations in the disturbance regime. In some fire-adapted shrublands, the community is highly resilient to infrequent, high-intensity fires, but changes in the fire regime that result in shorter fire intervals may make these communities more susceptible to alien plant invasions. This study examines several wildfire events that resulted in short fire intervals in California chaparral shrublands. In one study, we compared postfire recovery patterns in sites with different prefire stand ages (3 and 24 years), and in another study we compared sites that had burned once in four years with sites that had burned twice in this period. The population size of the dominant native shrub Adenostoma fasciculatum was drastically reduced following fire in the 3-year sites relative to the 24-year sites. The 3-year sites had much greater alien plant cover and significantly lower plant diversity than the 24-year sites. In a separate study, repeat fires four years apart on the same sites showed that annual species increased significantly after the second fire, and alien annuals far outnumbered native annuals. Aliens included both annual grasses and annual forbs and were negatively correlated with woody plant cover. Native woody species regenerated well after the first fire but declined after the second fire, and one obligate seeding shrub was extirpated from two sites by the repeat fires. It is concluded that some fire-adapted shrublands are vulnerable to changes in fire regime, and this can lead to a loss of native diversity and put the community on a trajectory towards type conversion from a woody to an herbaceous system. Such changes result in alterations in the proportion of natives to non-natives, changes in functional types from deeply rooted shrubs to shallow rooted grasses and forbs, increased fire frequency due to the increase in fine fuels, and changes in carbon storage.     

Effects of aqueous smoke solutions and heat on seed germination of herbaceous species of the Sudanian savanna-woodland in Burkina Faso

Smoke generated by burning of plant materials has widely been recognized as a germination cue for some species from both fire prone and fire-free ecosystems. It is an important factor for the understanding of vegetation dynamics and could have potential use for ecological management and rehabilitation of disturbed areas. Individual species, however, seem to have a specific requirement for the type and dosage of smoke treatments. In the present study, six different concentrations of smoke solution were tested on 13 herbaceous species by soaking the seeds for 24 h. The germination of a forb species, Borreria scabra, was significantly stimulated (p<0.05) by the smoke treatment while that of the annual grass species, Euclasta condylotricha, was significantly inhibited (p<0.05) by 100% smoke solution treatment. Contrary to our expectation that another fire-related cue, heat shock, would break the physical dormancy of the species tested, it was not promotive. For non-dormant seeds of B. scabra and Borreria radiata, high temperatures were lethal while low temperature induced physiological dormancy that was overcome in the former species within 30 days of the germination trial. For some species, responses to smoke did not corroborate with the field-observed response to fire, making ecological interpretation challenging. For responsive species, the smoke treatment could be a simple approach for promoting their re-establishment in areas where it is needed. More investigations are needed to assess the spread of response to smoke.     

Colonization of native Andean grasses by arbuscular mycorrhizal fungi in Puna: a matter of altitude, host photosynthetic pathway and host life cycles

The relationships of altitude, host life cycle (annual or perennial) and photosynthetic pathway (C(3) or C(4) ) with arbuscular mycorrhiza (AM) root colonization were analysed in 35 species of Andean grasses. The study area is located in north-western Argentina along altitudinal sites within the Puna biogeographical region. Twenty-one sites from 3320 to 4314 m were sampled. Thirty-five grasses were collected, and the AM root colonization was quantified. We used multivariate analyses to test emerging patterns in these species by considering the plant traits and variables of AM colonization. Pearson's correlations were carried out to evaluate the specific relationships between some variables. Most grasses were associated with AM, but the colonization percentages were low in both C(3) and C(4) grasses. Nevertheless, the AM root colonization clearly decreased as the altitude increased. This distinctive pattern among different species was also observed between some of the populations of the same species sampled throughout the sites. An inverse relationship between altitude and AM colonization was found in this Southern Hemisphere Andean system. The effect of altitude on AM colonization seems to be more related to the grasses' photosynthetic pathway than to life cycles. This study represents the first report for this biogeographical region.     

C4 plants in the vegetation of Mongolia: their natural occurrence and geographical distribution in relation to climate

The natural geographical occurrence, carbon assimilation, and structural and biochemical diversity of species with C₄ photosynthesis in the vegetation of Mongolia was studied. The Mongolian flora was screened for C₄ plants by using ¹³C/¹²C isotope fractionation, determining the early products of ¹⁴CO₂ fixation, microscopy of leaf mesophyll cell anatomy, and from reported literature data. Eighty C₄ species were found among eight families: Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Poaceae, Polygonaceae, Portulacaceae and Zygophyllaceae. Most of the C4 species were in three families: Chenopodiceae (41 species), Poaceae (25 species) and Polygonaceae, genus Calligonum (6 species). Some new C₄ species in Chenopodiaceae, Poaceae and Polygonaceae were detected. C₄ Chenopodiaceae species make up 45% of the total chenopods and are very important ecologically in saline areas and in cold arid deserts. C₄ grasses make up about 10% of the total Poaceae species and these species naturally concentrate in steppe zones. Naturalized grasses with Kranz anatomy,of genera such as Setaria, Echinochloa, Eragrostis, Panicum and Chloris, were found in almost all the botanical-geographical regions of Mongolia, where they commonly occur in annually disturbed areas and desert oases. We analyzed the relationships between the occurrence of C₄ plants in 16 natural botanical-geographical regions of Mongolia and their major climatic influences. The proportion of C₄ species increases with decreasing geographical latitude and along the north-to-south temperature gradient; however grasses and chenopods differ in their responses to climate. The abundance of Chenopodiaceae species was closely correlated with aridity, but the distribution of the C₄ grasses was more dependent on temperature. Also, we found a unique distribution of different C₄ Chenopodiaceae structural and biochemical subtypes along the aridity gradient. NADP-malic enzyme (NADP-ME) tree-like species with a salsoloid type of Kranz anatomy, such as Haloxylon ammodendron and Iljinia regelii, plus shrubby Salsola and Anabasis species, were the plants most resistant to ecological stress and conditions in highly arid Gobian deserts with less than 100 mm of annual precipitation. Most of the annual C₄ chenopod species were halophytes, succulent, and occurred in saline and arid environments in steppe and desert regions. The relative abundance of C₃ succulent chenopod species also increased along the aridity gradient. Native C₄ grasses were mainly annual and perennial species from the Cynodonteae tribe with NAD-ME and PEP-carboxykinase (PEP-CK) photosynthetic types. They occurred across much of Mongolia, but were most common in steppe zones where they are often dominant in grazing ecosystems. Received: 17 March 1999 / Accepted: 1 November 1999     

Distribution of seed dormancy classes across a fire-prone continent: effects of rainfall seasonality and temperature

Background and AimsDifferent seed dormancy classes control the timing of germination via different cues. The ecological dissimilarities between classes therefore suggest that they are likely to be subject to different selective pressures, and that species within each class will have diverse functional responses. We aimed to investigate this by assessing how variation in the distribution of dormancy classes is correlated with regional environmental factors, in particular rainfall seasonality and temperature. Additionally, we compare the relative proportions of species with physiological (PD) or physical (PY) dormancy to assess whether dormancy class influences their ability to persist under different rainfall seasonality regimes.MethodsDormancy class was assigned for 3990 species from 281 genera occurring across two climate regions, with either winter or aseasonal rainfall, across temperate fire-prone Australia. All regions have similar vegetation and fire regimes. Using a Bayesian framework, we compared the distribution of dormancy classes across temperature and rainfall climate gradients, for threatened and common species.Key ResultsA high dormant:non-dormant species ratio highlighted the critical role of dormancy across our study regions. Critically, species showing PD were more likely to be threatened in aseasonal rainfall climate regions.ConclusionsOur results support the assumption that dormancy is favoured in environments with stochastic disturbance     

Germination responses of native and invasive Cerrado grasses to simulated fire temperatures

Background: Fire is an important ecological factor in the Cerrado (Brazilian savanna). However, comparative studies on the effect of high temperatures experienced during fires on seed germination of native and invasive grass species are few.

Aims: To assess germination responses to simulated fire temperatures by seeds of invasive and native Cerrado grasses.

Methods: Heat-shock treatments (50 °C, 70 °C, 90 °C, 110 °C, 130 °C or 150 °C) were applied to seeds of 10 species of native and invasive grasses. For each temperature, the seeds were heated in a dry-air flow for 2 or 5 min. This combination of temperatures and exposure times simulated the soil conditions during typical Cerrado fires.

Results: Temperature treatment was significantly related to germination, and the effect varied according to species. Heat shock did not increase germination in either the native or the invasive species. Exposure time was important for only two species, and four species showed a significant increase in mean germination time.

Conclusions: Species showed different tolerances to high temperatures. It was not possible to differentiate the native and invasive grasses only by their tolerance to high temperatures, suggesting that fire alone may not be an efficient management tool to control the invasive species studied here.     

Observations of responses to re‐introducing fire in a Basalt Plains grassland after the removal of grazing: Implications for restoration

Natural grasslands in southern Australia commonly exist in altered states. One widespread altered state is grassland pasture dominated by cool‐season (C3) native grasses maintained by ongoing grazing. This study explores the consequences of removing grazing and introducing fire as a conservation management tool for such a site. We examined the abundance of two native and three exotic species, across a mosaic of fire regimes that occurred over a three‐year period: unburnt, summer wild‐fire (>2 years previous), autumn management fire (<1 year previously) and burnt in both fires. Given that one aim of conservation management is to increase native species at the expense of exotics, the impacts of the fires were largely positive. Native grasses were at higher cover levels in the fire‐managed vegetation than in the unburnt vegetation. Of the three exotic species, one was consistently at lower density in the burnt plots compared to the unburnt plots, while the others were lower only in those plots burnt in summer. The results show that the response of a species varies significantly between different fire events, and that the effects of one fire can persist through subsequent fires. Importantly, some of the effects were large, with changes in the density of plants of over 100‐fold. Fire is potentially a cost‐effective tool to assist the ecological restoration of retired grassland pastures at large scales.     

Ecological selection pressures for C4 photosynthesis in the grasses

Grasses using the C₄ photosynthetic pathway dominate grasslands and savannahs of warm regions, and account for half of the species in this ecologically and economically important plant family. The C₄ pathway increases the potential for high rates of photosynthesis, particularly at high irradiance, and raises water-use efficiency compared with the C₃ type. It is therefore classically viewed as an adaptation to open, arid conditions. Here, we test this adaptive hypothesis using the comparative method, analysing habitat data for 117 genera of grasses, representing 15 C₄ lineages. The evidence from our three complementary analyses is consistent with the hypothesis that evolutionary selection for C₄ photosynthesis requires open environments, but we find an equal likelihood of C₄ evolutionary origins in mesic, arid and saline habitats. However, once the pathway has arisen, evolutionary transitions into arid habitats occur at higher rates in C₄ than C₃ clades. Extant C₄ genera therefore occupy a wider range of drier habitats than their C₃ counterparts because the C₄ pathway represents a pre-adaptation to arid conditions. Our analyses warn against evolutionary inferences based solely upon the high occurrence of extant C₄ species in dry habitats, and provide a novel interpretation of this classic ecological association.