Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Plant Community Recovery Following Restoration in Semiarid Grasslands

The Conservation Reserve Program (CRP) is an extensive land use in the United States, which restores cultivated land to perennial vegetation through seeding. Low precipitation and high potential evapotranspiration are major limitations to the establishment and growth of seeded species in semiarid regions. We tested the rate of development of plant functional types across a chronosequence of restored fields using a model of plant succession. We also determined how the seeding of non‐native (introduced) relative to native perennial grasses influenced plant community recovery. In contrast to the native shortgrass steppe (SGS), recently seeded CRP fields had high cover of annuals, forbs, C₃, and introduced species. The seed mix determined which perennial grasses dominated the plant community within 18 years, but slow establishment prolonged early seral stages, allowed for the spread of colonizing perennial grasses, and limited recovery to less than half the canopy cover of undisturbed shortrass steppe. Species density declined in restored fields as seeded perennial grass cover increased and was lower in CRP fields seeded with introduced compared to native perennial grasses. Plant community composition transitioned to C₄ and native species, even if fields were not seeded with these species, and was modified by shifts in the amount and seasonality of precipitation. Thus, in semiarid CRP fields, we found that the potential for recovery depended on time since CRP enrollment, seed mix, and climatic variability. Full recovery, based on similarity to vegetation cover and composition of undisturbed SGS, requires greater than 20 years.     

Spatial patterns of desert annuals in relation to shrub effects on soil moisture

Questions: What are the effects of a shrub (Haloxylon ammodendron) on spatial patterns of soil moisture in different seasons? How does productivity of understorey annuals respond to these effects? Are such effects always positive for annuals under shrubs? Location: South Gurbantunggut Desert, northwest China. Methods: Using geostatistics, we explored seasonal patterns of topsoil moisture in a 12 × 9‐m plot over the growing season. To determine spatial patterns of understorey annuals in response to H. ammodendron presence, biomass of annuals was recorded in four 0.2 × 5.0‐m transects from the centre of a shrub to the space between shrubs (interspace). We also investigated vertical distribution of root biomass for annuals and soil moisture dynamics across soil profiles in shrub‐canopied areas and interspaces. Results: Topsoil moisture changed from autocorrelation in the wet spring to random structure in the dry season, while soil moisture below 20 cm was higher in shrub‐canopied areas. Across all microhabitats, soil moisture in upper soil layers was higher than in deeper soil layers during the spring wet season, but lower during summer drought. Topsoil was close to air‐dry during the dry season and developed a ‘dry sand layer’ that reduced evaporative loss of soil water from deeper layers recharged by snowmelt in spring. Aboveground biomass of understorey annuals was lowest adjacent to shrub stems and peaked at the shrub margin, forming a ‘ring’ of high herbaceous productivity surrounding individual shrubs. To acclimate to drier conditions, annuals in interspaces invested more root biomass in deeper soil with a root/shoot ratio (R/S) twice that in canopied areas. Conclusions: Positive and negative effects of shrubs on understorey plants in arid ecosystems are commonly related to nature of the environmental stress and tested species. Our results suggest there is also microhabitat‐dependence in the Gurbantunggut Desert. Soil water under H. ammodendron is seasonally enriched in topsoil and deeper layers. Understorey annuals respond to the effect of shrubs on soil water availability with lower R/S and less root biomass in deeper soil layers and develop a ‘ring’ of high productivity at the shrub patch margin where positive and negative effects of shrubs are balanced.     

Variation in the establishment of a non-native annual grass influences competitive interactions with Mojave Desert perennials

Competition between native and non-native species can change the composition and structure of plant communities, but in deserts, the highly variable timing of resource availability also influences non-native plant establishment, thus modulating their impacts on native species. In a field experiment, we varied densities of the non-native annual grass Bromus madritensis ssp. rubens around individuals of three native Mojave Desert perennials—Larrea tridentata, Achnatherum hymenoides, and Pleuraphis rigida—in either winter or spring. For comparison, additional plots were prepared for the same perennial species and seasons, but with a mixture of native annual species as neighbors. Growth of perennials declined when Bromus was established in winter because Bromus stands had 2–3 months of growth and high water use before perennial growth began. However, water potentials for the perennials were not significantly reduced, suggesting that direct competition for water may not be the major mechanism driving reduced perennial growth. The impact of Bromus on Larrea was lower than for the two perennial grasses, likely because Larrea maintains low growth rates throughout the year, even after Bromus has completed its life cycle. This result contrasts with the perennial grasses, whose phenology completely overlaps with (Achnatherum) or closely follows (Pleuraphis) that of Bromus. In comparison, Bromus plants established in spring were smaller than those established in winter and thus did not effectively reduce growth of the perennials. Growth of perennials with mixed annuals as neighbors also did not differ from those with Bromus neighbors of equivalent biomass, but stands of these native annuals did not achieve the high biomass of Bromus stands that were necessary to reduce perennial growth. Seed dormancy and narrow requirements for seedling survivorship of native annuals produce densities and biomass lower than those achieved by Bromus; thus, impacts of native Mojave Desert annuals on perennials are expected to be lower than those of Bromus.     

Mowing Reduces Exotic Annual Grasses but Increases Exotic Forbs in a Semiarid Grassland

Cheatgrass (Bromus tectorum) and other exotic winter‐active plants can be persistent invaders in native grasslands, growing earlier in the spring than native plants and pre‐empting soil resources. Effective management strategies are needed to reduce their abundance while encouraging the reestablishment of desirable native plants. In this 4‐year study, we investigated whether mowing and seeding with native perennial grasses could limit growth of exotic winter‐actives, and benefit growth of native plants in an invaded grassland in Colorado, United States. We established a split‐plot experiment in October 2008 with 3 mowing treatments: control, spring‐mowed, and spring/summer‐mowed (late spring, mid‐summer, and late summer), and 3 within‐plot seeding treatments: control, added B. tectorum seeds, and added native grass seeds. Cover of plant species and aboveground biomass were measured for 3 years. In March and June of 2010, 2011, and March of 2012, B. tectorum and other winter‐annual grasses were half as abundant in both mowing treatments as in control plots; however, cover of non‐native winter‐active forbs increased 2‐fold in spring‐mowed plots and almost 3‐fold in spring/summer‐mowed plots relative to controls. These patterns remained consistent 1 year after termination of treatments. Native cool‐season grasses were most abundant in spring‐mowed plots, and least abundant in control plots. There was higher cover of native warm‐season grasses in spring/summer‐mowed plots than in control plots in July 2011 and 2012. The timing of management can have strong effects on plant community dynamics in grasslands, and this experiment indicates that adaptive management can target the temporal niche of undesirable invasive species.     

Fertilizer impacts on the understorey of a regenerating Mountain Ash (Eucalyptus regnans F. Muell.) forest,Victoria

Abstract We examined the effects of adding fertilizers on nutrient concentrations in foliage, and on species composition and density of the understorey in a 10 year old Mountain Ash (Eucalyptus regnans F. Muell.) forest. Nutrient concentrations in foliage of three woody understorey species (Acacia dealbata, Pomaderris aspera and Olearia argophylla) showed no significant response to the addition of phosphorus and nitrogen either alone, in combination or combined with other nutrients. The phosphorus concentration in foliage and stems of two herbaceous species (Australina pusilla and Urtica incisa) was significantly increased by applying phosphorus. The addition of fertilizers had no significant effect on species composition and density of the understorey.     

Floodplain rehabilitation in North Cameroon: Impact on vegetation dynamics

Abstract. Since the construction in 1979 of a dam in the Logone floodplain in the Sahelo‐Sudanian zone of Cameroon, annual inundations have decreased, reducing perennial vegetation as important grazing source for nomadic herds and wildlife during the dry season. Presently, possibilities exist to release excess water for floodplain rehabilitation. In 1994 a pilot release was executed, reflooding 200 km², to verify predicted advantages. Vegetation has been studied from 1984 onwards along a transect covering flooded, recently reflooded and desiccated parts of the floodplain. Since 1993, the floristic composition has also been monitored in a grid in the centre of the impact zone. Cover of perennial grasses, most notably Echinochloa pyr amidalis and Oryza longistaminata increased from 41 to 61% in the reflooded zone. Vetiveria nigritana, a tussock grass that used to be dominant, disappeared slowly after the dam construction and has not shown a comeback. The cover of annual species, most notably Sorghum arundinaceum, a dominant annual grass only since the mid‐1980s, decreased in the reflooded zone from 58% to 34%. If the present conversion rate of annual into perennial grassland is extrapolated, recovery towards a 100% perennial state may be reached after the 2003 flooding season. Apart from favourable climatic conditions, recovery might be dependent on the restoration of soil fertility, limiting an approach focusing on flooding depth only.     

Seasonal pattern of antioxidant enzyme system in the roots of perennial forage grasses grown in alpine habitat, related to freezing tolerance

The relationship between active oxygen species (AOS) and membrane damage, and between antioxidant enzyme activity and chilling tolerance has been documented, but the mechanisms responsible for perennial forage grass to survive winter with temperatures at ?30°C in temperate alpine regions is not well understood. In this study, the seasonal pattern of enzymatic antioxidant systems superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11) and lipid peroxidation in roots and leaves of alpine perennial grasses grown in their natural environment were investigated to understand the role of the enzymatic antioxidant system in freezing tolerance of perennial grasses. Four grasses, Poa sphyondylodes Trine., Bromus inermis Leyss., Bromus sinensis Keng. and Elymus nutans Griseb., were established in alpine conditions in 1993. The grasses were sampled at approximately semi‐monthly intervals in the autumn of 1995 and spring of 1996. The results showed that leaves were dead in the autumn and membrane damage seems to play a key role in the decline of this organ. Antioxidant enzyme activities of the roots strongly changed with declining temperature in the autumn and winter or increasing temperature in the spring. With the decrease in temperature in the autumn the antioxidant enzyme activities increased rapidly, reaching maximum values in early November and then slowly declining during the following winter period, although they were still higher than in September In the spring, antioxidant enzymes activities increased again in the roots with the rise of temperature from mid April to early May when the shoots began re‐growth. In contrast, thiobarbituric acid‐reactive substances content in the roots increased markedly in the autumn, reaching maximum values in early October and remaining constant with little fluctuation during the following winter. In the autumn when the roots experienced winter acclimation, the formation of freezing tolerance in the roots was correlated with the activities of the antioxidant enzyme, indicating that antioxidant activity systems in the roots played an important role in limiting the production of free radicals to protect membrane integrity. Freezing tolerance in alpine grasses correlated with an increased capacity to scavenge or detoxify activated AOS by the antioxidant enzymatic system. AOS accumulated with decreasing temperature in early cold acclimation may be an inducer in activating the antioxidant enzyme defence system for the formation of freezing tolerance in roots.     

Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals

Understanding processes that underlie ecological resistance to weed invasion is critical for sustainable restoration of invaded plant communities. Experimental studies have demonstrated that invasive nitrophilic annuals can be controlled by addition of carbon to reduce soil nitrate concentrations, sometimes leading to enhanced establishment of native plants. However, effects of carbon supplements on soil nitrate are temporary, and the longer-term value of carbon supplementation as a restoration tool is dependent on the resistance of the re-established ecosystem to repeat invasion. We investigated whether re-established swards of the tussock grass Themeda australis (R.Br.) Stapf (a natural understorey dominant in mesic grassy woodlands of SE Australia) could suppress soil nitrate concentrations, and through this or other means, could impart ongoing resistance to exotic invasion in restored woodlands. In a remnant invaded by exotic annuals, we applied three plot treatments (carbon supplements, annual spring burns and untreated control) and two seed treatments (± Themeda seed) in a replicated, factorial design. Within 3 years, successful establishment of Themeda swards on burnt and carbon-supplemented plots was associated with a reduction in soil nitrate to levels comparable with non-invaded, Themeda-dominated reference sites in the region (<3 mg/kg), and significantly reduced exotic cover compared with unseeded plots. By contrast, on plots not seeded with Themeda, soil nitrate increased after cessation of carbon addition and exotic cover returned to levels comparable with untreated control plots, despite a high cover of other native perennial grasses. Few persistent effects of carbon supplements or spring burning on soil nutrients were evident 9–19 months after cessation of these treatments. Results suggest that Themeda is a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals.     

Effect of ploughing on plant species abundance and diversity in the northwestern coastal desert of Egypt

This study focuses on the effect of ploughing on plant abundance,vegetation cover, species richness, and taxonomic diversity during the growingseasons (winter and spring) of 1992 and 2000 in the habitat of inland plateau(natural habitat), 21 km south of Mersa-Matrouh (Egypt).Ninety-five species belonging to 27 families were recorded. High percentages oflife-forms and a large number of species were recorded in ploughed andunploughed stripes in the winter and spring of 2000. Higher averages of importancevalues (IVs) and absolute frequencies were recorded for most perennial andannual species in the unploughed stripes compared to the ploughed ones. This may beattributed to crop failure and consequently unfavourable soil conditions. On theother hand, some shrubby species (e.g. Noaea mucronata andHaloxylon scoparium) and perennial herbs (e.g.Gynandriris sisyrinchium) attained higher IVs in theploughed stripes compared to unploughed ones. This may be attributed to thecultivation of Prosopis juliflora trees in the elevatedpart of the ploughed stripes, which have an ecological role in protecting andenriching the soil with organic matter, thus favouring the growth of theseshrubs and perennial herbs. Higher species richness and diversity wereassociated with low concentration of dominance and low taxonomic diversity inthe spring of 2000 in ploughed and unploughed stripes compared to the winter of1992, for both perennials and annuals. The lowest taxonomic diversities wereexhibited in the spring of 2000 for ploughed and unploughed stripes where thevegetation had the largest number of congeneric species and confamilial genera.Higher species richness and diversity characterized the vegetation of theunploughed stripes, especially in winter and spring 2000, as compared to those ofploughed ones. The present study also reveals low species richness and diversityof therophytes in winter for both ploughed and unploughed stripes.     

Isotopic assessment of the hydrologic importance of fog deposition on tall snow tussock grass on southern New Zealand uplands

The stable isotopic composition of rain, fog and groundwater was determined for three sites on the Otago upland snow tussocklands, southeastern South Island, New Zealand, for the snow‐free period of November to June in 1996–1998. The rain and fog were collected concurrently, but the rain was consistently isotopically more depleted than the fog. The compositions of the rain form a line of the equation δD = 8.31δ¹⁸O + 13.6, which is similar to the line of δD = 8.11δ¹⁸O + 13.6 formed by the fog. The groundwater has compositions usually between those of the fog and rain, shows no isotopic evidence of evaporation, and is assumed to be a mixture of the two in sub‐equal proportions. This pattern has been observed previously only in forested regions where the tree crowns act as collection mechanisms for fog water. On the Otago uplands, the only fog collection mechanism is interception gains through fog deposition on the relatively short (0.8–1.2 m) fine wispy foliage of the native tall snow tussock grasses (Chionochloa spp.). These results support earlier but debated claims that interception of fog by the foliage of the dominant tall snow tussock grasses makes a substantial contribution to the water yield from these uplands.     

A three year (1993–1995) calendar of pollen and Alternaria mould in the atmosphere of south western Sydney

The relevance of allergy skin prick testing in the diagnosis and treatment of seasonal allergic rhinitis and pollen asthma can usefully be interpreted in relation to the timing and duration of seasonal symptoms and the presence of pollen and mould spores in the air. This calendar has been constructed from three years continuous observations of pollen and Alternaria mould spore counts between January 1993 and December 1995, using a Burkard 7‐day volumetric spore trap. Of the total airborne pollen, tree pollen comprises 65%, weeds and herbs 11% and grasses 18%. Unidentified pollen, “other”; group, accounts for 6% of the total airborne pollen. The most numerous of the tree pollen is that of the introduced trees cypress (Cupressus spp.) and privet (Ligustrum spp.). Grass pollen is seen in small numbers throughout the winter but shows a rapid increase in spring to peak in mid to late November. Weeds pollinate from early spring through to summer. Alternaria mould, which is a risk factor for childhood asthma, occurs mainly in late spring and summer but is present in small numbers intermittently throughout winter.     

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.     

California perennial grasses are physiologically distinct from both Mediterranean annual and perennial grasses

In the Central Valley of California, native perennial grass species have been largely replaced by Eurasian annual species, while in many parts of the Mediterranean Basin native perennial grasses continue to dominate, even on disturbed or degraded sites. We assessed whether differences in summer rainfall patterns have lead to the development of different plant-water strategies between grasses from these two regions. We compared six measures of plant-water physiology for three guilds of grasses: California perennial grasses, Mediterranean perennial grasses, and Mediterranean annual grasses. Discriminant analysis distinguished between the three guilds; Mediterranean perennial grasses were characterized by a more conservative water-relations physiology than Mediterranean annual grasses, whereas California perennial grasses were in some ways intermediate between the two Mediterranean grass guilds. For individual traits, California perennial grasses were either intermediate or more like Mediterranean annuals than Mediterranean perennials. Our results suggest California perennials are more drought tolerant than Mediterranean annuals but less drought tolerant than Mediterranean perennials, despite the fact that California??s Central Valley has a more intense summer drought than the Mediterranean Basin. These patterns may help explain why Mediterranean annuals, but not Mediterranean perennials, have been more successful invaders of interior California grasslands.     

Long-Term Effects of Reclamation Treatments on Plant Succession in Iceland

The long‐term effects (20–45 years) of reclamation treatments on plant succession are examined at two localities in Iceland that were fertilized and seeded from 1954 to 1979 with perennial grasses or annual grasses, or left untreated. The areas that underwent reclamation treatments had significantly higher total plant cover (7–100%) than the untreated control plots (<5%), and floristic composition was usually significantly different between treated and untreated plots. Dwarf‐shrubs (Calluna vulgaris and Empetrum nigrum), bryophytes, biological soil crust, grasses, and shrubs characterized the vegetation in the treated plots, but low‐growing herbs that have negligible effects on the environment, such as Cardaminopsis petraea and Minuartia rubella, and grasses characterized the control plots. The seeded grass species had declined (<10%, the perennials) or disappeared (the annuals) but acted as nurse species that facilitated the colonization of native plants. It seems that by seeding, some factors that limit plant colonization were overcome. Soil nutrients, vegetation cover, litter, and biological soil crust were greater in the treated areas than the control plots. This may have enhanced colonization through an increase in soil stability and fertility, increased availability of safe microsites, increased moisture, and the capture of wind‐blown seeds. This study demonstrates the importance of looking at the long‐term effects of reclamation treatments to understand their impact on vegetation succession.     

Restoring Kangaroo Grass (Themeda triandra) to grassland and woodland understoreys: a review of establishment requirements and restoration exercises in south-east Australia

Summary Temperate grassy ecosystems are amongst Australia's most endangered ecosystems. Most remnants are small, fragmented and highly degraded. Practical methods for restoring native understorey species are urgently required. Dominant native grasses such as Kangaroo Grass (Themeda triandra Forssk.) and Tussock‐grasses (Poa species) have been eliminated from many remnants by heavy grazing in the past. The reintroduction of these grasses is a critical step for understorey restoration. This paper (i) reviews the literature on Themeda seed biology and seedling establishment; (ii) summarizes the lessons learnt from three major attempts to establish Themeda stands in south‐east Australia; and (iii) identifies the research needed to enhance Themeda restoration. Considerable information is available on Themeda seed and establishment biology, and restoration exercises have shown that Themeda stands can be readily established by surface‐spreading awned seeds in seed‐bearing hay. However, many practical challenges remain, including the need to identify optimal sowing periods, create better seedbed conditions, develop practical mulching techniques, and improve weed control. The use of seed‐bearing hay has constrained restoration to relatively small areas in the past. Future trials may benefit by using more concentrated seed products such as seed‐bearing florets and pure seeds which permit larger areas to be restored at one time.     

The role of seed limitation and resource availability in the recruitment of native perennial grasses and exotics in a South Australian grassland

Abstract We investigated what factors lead to invasion of exotics or re‐colonization of native perennial grasses in the South Australian mid‐north grasslands. We manipulated 160 experimental quadrats by clipping, irrigation and seed addition and assessed recruitment by exotics in an area dominated by perennial grasses and perennial grass recruitment in an area dominated by exotics. Treatment effects differed with season for exotics: their biomass increased with irrigation in autumn and seed addition in winter. However, in both periods other factors, probably soil properties, also had a strong effect. We detected no perennial grass seedlings in the quadrats over 1 year, possibly due to unsuitable environmental conditions or persistent high competition levels. Under controlled conditions the presence of the invasive annual Avena barbata had a strong negative effect on the recruitment of the native perennial Austrodanthonia caespitosa at any moisture and nutrient availability. Avena also germinated faster and more frequently than Austrodanthonia, especially at low soil moisture. During an imposed drought Austrodanthonia seedlings survived longer in the absence of Avena. The results suggest that annual exotics are highly responsive to resources and can quickly invade areas, while the re‐colonization of invaded areas by native grasses requires a complex (and less likely) rainfall regime.     

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.     

Establishment of native grasses and their impact on exotic annuals in degraded box gum woodlands

Restoration goals often involve the addition of new species to resident, degraded communities but in box gum woodlands such restoration is often constrained by competition from persistent exotic annuals that control critical ecological processes. Nutrient reduction (via carbon addition) and seed bank depletion are two approaches to reduce competition from exotic annuals but to be effective these treatments must allow establishment of species such as native grasses. This experiment was conducted in two degraded Austrostipa understoreys in the box gum woodlands of south‐east Australia. It compares the effects of carbon addition (sugar), seed depletion (spring burning or spring grazing) and combinations of carbon addition and seed depletion treatments on the establishment of C3 and C4 native grasses, and measured the effects of their establishment on soil nitrate concentration and exotic annuals. Treatments that reduced exotic annual abundance did not increase initial germination of the C4 native grasses, Bothriochloa or Themeda. However, sugar increased seedling survival of Themeda and Bothriochloa and grazing increased seedling survival of Bothriochloa, presumably by reducing effects of exotic annuals. Poa and Rytidosperma (C3 native grasses) failed to establish. Although we were unable to detect any reduction in soil nitrate concentration, swards with successful recruitment of C4 grasses suppressed exotic annuals more than the Austrostipa‐only swards at one site (the other was affected by wildfire). Further, Austrostipa‐Themeda swards were more effective than Austrostipa‐Bothriochloa for suppressing exotics, pointing to a role for both functional and species identity in the degree of resistance conferred.     

The worst drought in 50 years in a South African savannah: Limited impact on vegetation

South Africa experienced a severe multiyear drought from 2014 to 2016. Here, we explore the response of a South African savannah ecosystem to this drought focusing on tree and grass dynamics. We used open long‐term monitoring plots established in 2000 and distributed across broad rainfall gradients in the Hluhluwe‐iMfolozi Park in KwaZulu‐Natal. Analysis showed negligible tree mortality due to drought in 2016 (0.03%) and 2017 (0.49%). However, there was an apparent increase in the tree population especially among the small size classes (0.1–0.3 m) in 2016 and 2017 relative to the predrought 2012 census. Drought effects may be confounded with changes in browser populations. The impala population declined in a mesic savannah (Hluhluwe), as measured with dung counts; however, this decline started before the drought. Impala decline in a semi‐arid savannah (iMfolozi) was more coincident with drought. Grass biomass and cover decreased in 2016 compared to predrought 2012 but showed rapid recovery once rains began. In iMfolozi grass species, composition improved from a grazing perspective as a result of the recovery of the decreaser species, an increase in palatable species and a marked decline of unpalatable species. The minor changes in woody plants and grasses from before the drought until rain resumed and the rapid recovery of grasses suggest that this South African savannah ecosystem was resilient to severe drought.