Contrasting demographics of tropical savanna and temperate forest eucalypts provide insight into how savannas and forests function. A case study using Corymbia clarksoniana from north‐eastern Australia

Eucalypts (Eucalyptus spp. and Corymbia spp.) dominate many communities across Australia, including frequently burnt tropical savannas and temperate forests, which receive less frequent but more intense fires. Understanding the demographic characteristics that allow related trees to persist in tropical savannas and temperate forest ecosystems can provide insight into how savannas and forests function, including grass–tree coexistence. This study reviews differences in critical stages in the life cycle of savanna and temperate forest eucalypts, especially in relation to fire. It adds to the limited data on tropical eucalypts, by evaluating the effect of fire regimes on the population biology of Corymbia clarksoniana, a tree that dominates some tropical savannas of north‐eastern Australia. Corymbia clarksoniana displays similar demographic characteristics to other tropical savanna species, except that seedling emergence is enhanced when seed falls onto recently burnt ground during a high rainfall period. In contrast to many temperate forest eucalypts, tropical savanna eucalypts lack canopy‐stored seed banks; time annual seed fall to coincide with the onset of predictable wet season rain; have very rare seedling emergence events, including a lack of mass germination after each fire; possess an abundant sapling bank; and every tropical eucalypt species has the ability to maintain canopy structure by epicormically resprouting after all but the most intense fires. The combination of poor seedling recruitment strategies, coupled with characteristics allowing long‐term persistence of established plants, indicate tropical savanna eucalypts function through the persistence niche rather than the regeneration niche. The high rainfall‐promoted seedling emergence of C. clarksoniana and the reduction of seedling survival and sapling growth by fire, support the predictions that grass–tree coexistence in savannas is governed by rainfall limiting tree seedling recruitment and regular fires limiting the growth of juvenile trees to the canopy.     

Residual herbicide treatments reduce Andropogon gayanus (Gamba Grass) recruitment for mine site restoration in northern Australia

Weed invasion is a major threat to Australian tropical savannas, and controlling weeds is essential for successful re‐establishment of native species on disturbed sites. Gamba Grass (Andropogon gayanus) is an African grass which has invaded large areas of tropical savanna across northern Australia. Current management strategies in northern Australia focus on fire and glyphosate to effectively control mature plants; however, re‐establishment of infestations from the soil seed bank remains a major challenge to eradication efforts. This study focused on the effects of soil seed bank treatments on Gamba Grass recruitment on a mine site in northern Australia. Adult Gamba Grass plants within test plots were killed with glyphosate to exclude resource competition. Chemical, physical and biological treatments were then applied, and the treatment effects on subsequent Gamba Grass seedling emergence and survival quantified. Seedling emergence was significantly reduced by three of the four residual herbicide treatments tested. The most effective herbicide treatments, dalapon and sulfometuron, reduced emergence by 90% compared to the standard glyphosate treatment alone. This equated to a reduction in Gamba Grass seedling emergence from 1 seedling/m² to 1 seedling 10 m^?2, a major improvement for Gamba Grass management. These residual herbicide treatments significantly reduced the population density of Gamba Grass for at least 5 months after emergence. The physical and biological treatments did not have a significant effect on seedling emergence. This significant reduction in Gamba Grass seedling emergence and survival can substantially improve Gamba Grass management. Reducing re‐colonisation from the soil seed bank using residual herbicides provides a valuable management tool for land managers, integrating readily with established strategies for controlling the mature plants.     

A grass–fire cycle eliminates an obligate‐seeding tree in a tropical savanna

A grass–fire cycle in Australian tropical savannas has been postulated as driving the regional decline of the obligate-seeding conifer Callitris intratropica and other fire-sensitive components of the regional flora and fauna, due to proliferation of flammable native grasses. We tested the hypothesis that a high-biomass invasive savanna grass drives a positive feedback process where intense fires destroy fire-sensitive trees, and the reduction in canopy cover facilitates further invasion by grass. We undertook an observational and experimental study using, as a model system, a plantation of C. intratropica that has been invaded by an African grass, gamba (Andropogon gayanus) in the Northern Territory, Australia. We found that high grass biomass was associated with reduced canopy cover and restriction of foliage to the upper canopy of surviving stems, and mortality of adult trees was very high (>50%) even in areas with low fuel loads (1 t·ha⁻¹). Experimental fires, with fuel loads >10 t·ha⁻¹, typical of the grass-invasion front, caused significant mortality due to complete crown scorch. Lower fuel loads cause reduced canopy cover through defoliation of the lower canopy. These results help explain how increases in grass biomass are coupled with the decline of C. intratropica throughout northern Australia by causing a switch from litter and sparse perennial grass fuels, and hence low-intensity surface fires, to heavy annual grass fuel loads that sustain fires that burn into the midstorey. This study demonstrates that changes in fuel type can alter fire regimes with substantial knock-on effects on the biota.     

BIODIVERSITY RESEARCH: Turning up the heat: the impacts of Andropogon gayanus (gamba grass) invasion on fire behaviour in northern Australian savannas

Aim This study aimed to quantify changes in fire severity resulting from the invasion of Australia’s tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass). Location Mesic savannas of the Northern Territory, Australia. Method Byram’s fire‐line intensity (I_f), fuel load and architecture, and two post‐fire indicators of fire intensity – scorch height (SH) and char height (CH) of woody vegetation – were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire’s radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between I_f and the post‐fire indices of fire intensity. Results A relationship between A. gayanus I_f and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low I_f. In contrast, A. gayanus I_f was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given I_f, there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha^?1 in native grasses compared to 11.6 t ha^?1 in A. gayanus) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus) of the standing fine fuel. Main conclusion Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000–15,000 km²) and predicted (380,000 km²) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low‐intensity fire.     

Disturbance of forest by trampling: Effects on mycorrhizal roots of seedlings and mature trees of Fagus sylvatica

The effects of disturbance by recreational activities (trampling) on changes in soil organic matter (SOM) and on mycorrhizal roots of seedlings and mature trees were studied in four stands of a beech (Fagus sylvatica L.) forest near Basel, Switzerland. At each site, comparable disturbed and undisturbed plots were selected. Disturbance reduced ground cover vegetation and leaf litter. Beech seedlings had lower biomass after disturbance. Ergosterol concentration in seedling roots, an indicator of mycorrhizal fungi, was lower in two of the four disturbed plots compared to undisturbed plots; these two disturbed sites had especially low litter levels. Based on ergosterol measurements, mycorrhizas of mature trees did not appear to be negatively affected by trampling. Total fine roots and SOM were higher in the disturbed than in the undisturbed plots at three sites. At the fourth site, fine roots and SOM in the disturbed areas were lower than in the undisturbed areas most probably due to nutrient input following picnic activities. Principal component analysis revealed a close correlation between SOM and fine roots of mature trees as well as litter and seedling biomass. Trampling due to recreational activities caused considerable damage to the vegetation layer and in particular to the beech seedlings and their mycorrhizal fine roots, whereas, roots of mature trees were apparently resilient to trampling.     

Andropogon gayanus (Gamba Grass) Invasion Increases Fire-mediated Nitrogen Losses in the Tropical Savannas of Northern Australia

Invasive alien grasses can substantially alter fuel loads and fire regimes which could have significant consequences for fire-mediated nutrient losses. The effects of the alien grass Andropogon gayanus Kunth. (Gamba grass) on fire-mediated nutrient losses was evaluated in Australia’s tropical savannas. Losses of macronutrients during fire were determined by comparing the nutrient pools contained in the fine fuel before fire and in the ash after fire. Pre-fire grass nutrient pools were significantly higher in A. gayanus plots than in native grass plots for all nutrients measured (N, P, K, S, Ca, and Mg). Nutrient losses were substantially higher in A. gayanus plots, with 113% higher losses for N, 80% for P, 56% for K, 63 for S, 355% for Ca, and 345% for Mg. However, only losses of N and Mg varied significantly between grass types. A simplified savanna ecosystem nutrient budget estimated that A. gayanus fires led to the net N loss of 20 kg ha⁻¹ y⁻¹. This is a conservative estimate because total fuel loads were relatively low (7.85 t ha⁻¹) for A. gayanus invaded plots leading to a relatively moderate intensity fire (6,408 kW m⁻¹). Higher A. gayanus fuel loads and fire intensities could potentially lead to losses of up to 61.5 kg N ha⁻¹ from the grass fuel. Over the long term, this is likely to lead to depletion of soil nutrients, particularly N, in the already low-fertility tropical savanna soils.     

Seed and microsite limitation in Rheum nobile,a rare and endemic plant from the subnival zone of Sino-Himalaya

Background: Species persistence, particularly in monocarpic species, depends on the successful recruitment of individuals. An understanding of the factors that limit the recruitment of rare monocarpic plant species is therefore vital for their conservation.

Aims: To identify the factors limiting the recruitment of Rheum nobile, a rare and highly specialised monocarpic giant herb endemic to the high eastern Himalayas.

Methods: Seed sowing (seeds added or not added) and seedling transplanting experiments were conducted in disturbed (vegetation removed) and undisturbed plots in the vicinity of established populations of R. nobile to explore the mechanisms of recruitment limitation. Four levels of photosynthetically active radiation (0, 15, 30 and 50 μmol m^?2 s^?1) and two sowing positions (beneath and above grass litter or moss layer) were manipulated in the laboratory to determine how ground cover limited seedling emergence.

Results: Seed addition increased seedling recruitment. Disturbance significantly increased seedling emergence and establishment. Seed germination significantly decreased with the reduction of light availability, but 31.7% of all seeds germinated in complete darkness. Seedling emergence was close to zero when seeds were sown on top of a layer of grass litter or moss, but rose to 34.5% when the seeds were sown beneath such layers.

Conclusions: Our results indicate that the recruitment of R. nobile is limited by a combination of seed and microsite availability. Therefore, in order to conserve this species, we suggest adding seeds to suitable sites and implementing soil disturbances in existing populations to create suitable microsites.     

Habitat Complexity and Invasive Species: The Impacts of Gamba Grass (Andropogon gayanus) on Invertebrates in an Australian Tropical Savanna

Habitat complexity is an important factor structuring local faunal assemblages. Many invasive plant species alter vegetation complexity because they are both functionally and structurally different from native flora. This study investigated the effects of an invasive grass (Andropogon gayanus Kunth.), which is invading undisturbed savannas in northern Australia. Although A. gayanus is a similar lifeform to that dominant in the native understory (i.e., a C4 grass), it is structurally different and significantly alters habitat complexity. We, therefore, hypothesized that there would be a substantial effect of the invasive grass on ant, spider, and other invertebrate assemblages. Contrary to our hypothesis, there was no effect of A. gayanus on ant species richness, abundance, or composition, nor were spiders or other invertebrates affected. Instead the change in weather conditions with season was more important in structuring the local invertebrate assemblages. Change in habitat complexity was most pronounced vertically, rather than horizontally at ground level, thus although there was a clear difference in the vertical structure between invaded and noninvaded habitats, the limited invertebrate response to different A. gayanus densities suggests some invertebrates are less sensitive to vertical changes in vegetation structure.     

The role of seed depth,litter, and fire in the seedling establishment of amphicarpic peanutgrass (Amphicarpum purshii)

Summary Amphicarpum purshii is an annual grass which mostly grows in disturbed areas of the New Jersey Pine Barrens, USA. It is amphicarpic, producing spikelets (and seeds) both above and below the soil surface. Previous research has shown that subterranean seed production ensures reproduction in the event of a major disturbance such as fire and results in rapid post-burn colonization of these sandy habitats. The effects of fire, litter, and seed depth were further examined by planting subterranean seeds at four depths in 16 litter-covered flats buried at ground level and comparing plants arising from burned flats with those in undisturbed litter-covered flats. At 0 and 1 cm depth, rates of seedling emergence were lowest in burned flats. Surface-sown seeds produced seedlings more likely to desiccate. Sowing depth had a greater influence on most measured characters than burning treatments. The mean depth of subterranean seed placement by Amphicarpum is 3.5 cm and this coincides with the seed depth from which plants showed the greatest height growth, shoot biomass, and reproductive output. In a second experiment, subterranean seeds on the bare soil surface in clay pots were more likely to lose viability and less likely to germinate than seeds protected by litter or burial in soil. In addition to providing protection from fire, placement of seeds below ground in the sandy habitat of peanutgrass provides conditions more suitable for seed survival and subsequent seedling establishment.     

Exotic Grass Invasions: Applying a Conceptual Framework to the Dynamics of Degradation and Restoration in Australia’s Tropical Savannas

Plant invasions can cause severe degradation of natural areas. The ability of an ecosystem to recover autogenically from degradation following weed control is in part determined by the type and magnitude of changes to both biotic and abiotic processes caused by the invasion and how these interact with structural and functional components of the ecosystem. Recently, a number of conceptual frameworks have been proposed to describe the dynamics of degradation and regeneration in degraded ecosystems. We assessed the utility of one of these frameworks in describing the degradation and restoration potential of Australia’s tropical savannas following exotic grass invasion. First, we identified easily measured structural characteristics of putative states. We found that a continuous cover of the exotic grasses Gamba grass (Andropogon gayanus Kunth.) and Perennial mission grass (Pennisetum polystachion (L.) Schult.) under an intact tree canopy was a common state with an understorey characterized by reduced species richness and abundance and a change in the relative contribution of functional groups. Further degradation led to a state where the canopy was severely reduced and the impacts on the understorey were more severe. In both states, the seed bank was substantially less degraded than the understorey vegetation. Guided by the framework, we combined our study with other studies to construct a conceptual model for degradation in exotic grass‐invaded savannas.     

Resource competition and fire‐regulated nutrient demand in carnivorous plants of wet pine savannas

Question: What are the mechanisms by which fire reduces competition for both a short‐lived and a long‐lived species in old‐growth ground‐cover plant communities of wet pine savannas (originally Pinus palustris, replaced by P. elliottii)? Location: Outer coastal plain of southeastern Mississippi, USA. Methods: I reviewed previous competition experiments and proposed a new hypothesis to explain the relationship between fire, competition, and species co‐existence in wet longleaf pine savannas. The first study is about growth and seedling emergence responses of a short‐lived carnivorous plant, Drosera capillaris, to reduction in below‐ground competition and above‐ plus below‐ground competition. The second study deals with growth and survival responses of a long‐lived perennial carnivorous plant, Sarracenia alata, to neighbour removal and prey‐exclusion to determine if a reduction in nutrient supply increased the intensity of competition in this nutrient‐poor system. Results: Fire increased seedling emergence of the short‐lived species by reducing above‐ground competition through the destruction of above‐ground parts of plants and the combustion of associated litter. Prey exclusion did not increase competitive effects of neighbours on the long‐lived species. However, because the experiment was conducted in a year without fire, shade reduced nutrient demand, which may have obviated competition for soil nutrients between Sarracenia alata and its neighbours. Conclusion: Repeated fires likely interact with interspecific differences in nutrient uptake to simultaneously reduce both above‐ground competition and competition for nutrients in old‐growth ground cover communities in pine savannas. Restoration practitioners should consider the possibility that the composition of the plant community is just as important as fire in ensuring that frequent fires maintain species diversity.     

Grazing and neighborhood interactions limit woody encroachment in wet subtropical savannas

Woody encroachment in savannas is a worldwide concern, and there is growing consensus that anthropogenic activities play a central role in changing tree – grass interactions. We evaluated the influence of livestock grazing and neighborhood interactions on seedling emergence and survival of the native tree Acacia caven in wet savannas of northeastern Argentina. We hypothesized that grazing and grass competition act as biotic barriers limiting tree recruitment, but the relative magnitude of such barriers differs according to grass patch type. In two consecutive years (cohort 1 and 2) we sowed seeds and transplanted seedlings of Acacia in two grass patch types (prostrate/palatable and tussock/unpalatable grasses) in both, grazed and ungrazed plots. Each grass patch type was further manipulated to create three levels of grass competition (unclipped control, above-ground biomass removal and total biomass removal).Cattle grazing diminished seedling emergence of both cohorts and seedling survival of cohort 1. The effect of grass competition changed according to grass patch type. Prostrate grass cover enhanced emergence but lowered early survival, while tussock grass cover and also its total biomass removal facilitated early survival. During the second year, a severe drought drastically reduced Acacia recruitment, and it was strong enough to eliminate any grazing effects although the effect of grass competition on seedling establishment remained significant.Our results suggest that grazing and grass competition additively diminished the risk of woody establishment in this wet savanna. However, the stocking rate should be carefully balanced, thus contributing to the maintenance of a competitive grass cover to limit tree recruitment.     

Limiting factors for seedling emergence and establishment of the invasive non-native Rosa rugosa in a coastal dune system

The relative importance of seed, habitat and microhabitat limitation has rarely been investigated for invasive non-native species, although this is critical for their effective management and for predicting future range expansion. Rosa rugosa is an abundant non-native shrub in coastal habitats of NW Europe; it is common along the Baltic coast but more scarce in exposed dunes of the North Sea. To investigate whether invasion of exposed dunes is limited by seed, habitat and microhabitat limitation, seedling emergence and establishment were examined in a factorial sowing, transplant and disturbance experiment. Twenty plots were randomly placed in each of five dune habitats (white dune, Empetrum dune, grey dune, outer dune heath, inner dune heath), and studied over 2 years. Seedling emergence in control subplots was zero in all habitats, whereas 96% and 98% of the undisturbed and disturbed seeded subplots produced seedlings. Disturbance had a positive effect on emergence and subsequent survival in white dune, outer and inner dune heath. Seedling survival and growth, and growth of transplanted seedlings, were markedly lower in grey dune. These findings indicate that establishment of R. rugosa is seed-limited in coastal dune habitats, and that the species is able to establish in both active and fixed dunes once seeds have arrived. Although differential seedling emergence and growth indicate that habitats differ in their degree of invasion susceptibility by R. rugosa, the positive influence of small-scale disturbance suggests microhabitat limitation in some dune habitats as well. Dune management should aim to reduce seed production and dispersal of R. rugosa in near-natural sites, and anthropogenic changes of habitat dynamics should be prevented.     

Contrasting fire-related resilience of ecologically dominant ants in tropical savannas of northern Australia

This paper examines the role of fire in mediating the relative abundance of two of the world's major ecologically dominant ant genera, Iridomyrmex and Oecophylla, where they coexist across the tropical savanna landscapes of northern Australia. These taxa have contrasting biogeographical histories, which are predicted to lead to contrasting responses to fire. Iridomyrmex is an autochthonous Australian genus that has radiated primarily in the arid zone; as such, its abundance is predicted to be promoted by frequent fire because this maintains an open habitat. In contrast, Oecophylla is a genus of leaf‐nesting ants occurring in the canopies of Old World tropical rainforest, and is a recent arrival to Australia in geological time; the abundance of these ants is predicted to decline under frequent fire. We test these predictions using results from a landscape‐scale fire experiment, where three experimental fire regimes (including no fire) were applied to replicated subcatchments over a 5‐year period. Using sweep nets, ants were sampled in the grass layer (the habitat layer of greatest overlap between Iridomyrmex and Oecophylla) in eucalypt woodland (canopy cover < 30%) and open eucalypt forest (canopy cover about 50%) habitats. A total of 27 species from 11 genera were collected during the study; eight were common enough for statistical analysis, and the abundances of four of these were significantly affected by fire treatment. As predicted, the abundance of Iridomyrmex was promoted by fire, whereas that of Oecophylla declined. These changes occurred only under late‐season (relatively high intensity) fires, and for Oecophylla occurred only in open forest (not woodland) habitat. This fire‐mediated relationship between Iridomyrmex and Oecophylla mirrors the much broader, ecosystem‐wide dynamic between eucalypt‐dominated savanna and rainforest in tropical Australia, with savannas dominated by fire‐resistant sclerophyll elements of Australian origin, and rainforest dominated by fire‐sensitive mesophyll elements of South‐East Asian origin.     

A continental-scale analysis of tree cover in African savannas

Aim We present a continental‐scale analysis that explores the processes controlling woody community structure in tropical savannas. We analyse how biotic and abiotic factors interact to promote and modify tree cover, examine alternative ecological hypotheses and quantify disturbance effects using satellite estimates of tree cover. Location African savannas. Methods Tree cover is represented as a resource‐driven potential cover related to rainfall and soil characteristics perturbed by natural and human factors such as fire, cattle grazing, human population and cultivation. Within this framework our approach combines semi‐empirical modelling and information theory to identify the best models. Results Woody community structure across African savannas is best represented by a sigmoidal response of tree cover to mean annual precipitation (MAP), with a dependency on soil texture, which is modified by the separate effects of fire, domestic livestock, human population density and cultivation intensity. This model explains c. 66% of the variance in tree cover and appears consistent across the savanna regions of Africa. Main conclusions The analysis provides a new understanding of the importance and interaction of environmental and disturbance factors that create the broad spatial patterns of tree cover observed in African savannas. Woody cover increases with rainfall, but is modified by disturbances. These ‘perturbation’ effects depend on MAP regimes: in arid savannas (MAP < 400 mm) they are generally small (< 1% decrease in cover), while in semi‐arid and mesic savannas (400–1600 mm), perturbations result in an average 2% (400 mm) to 23% (1600 mm) decrease in cover; fire frequency and human population have more influence than cattle, and cultivation appears, on average, to lead to small increases in woody cover. Wet savannas (1600–2200 mm) are controlled by perturbations that inhibit canopy closure and reduce tree cover by, on average, 24–34%. Full understanding of the processes determining savanna structure requires consideration of resource limitation and disturbance dynamics.     

Physical disturbance of an upland grassland influences the impact of elevated UV-B radiation on metabolic profiles of below-ground micro-organisms

This investigation determined the response of soil microbial communities to enhanced UV‐B radiation and disturbance in upland grassland. A factorial field experiment encompassing two levels of UV‐B supplementation (simulating ambient and a 30% increase in stratospheric ozone) and two levels of disturbance (disturbed and undisturbed) was established at Buxton Climate Change Impacts Laboratory, Derbyshire, UK, and maintained for 7 years prior to sampling. Enhanced UV‐B increased microbial utilization of carbohydrates, carboxylic acids, polymers and aromatic compounds present in Biolog® GN plates when inoculated with soils taken from disturbed plots, but did not affect carbon utilization of soil microbial communities associated with undisturbed plots (UV‐B×Disturbance interaction, P<0.05 for each substrate type). UV‐B treatment did not affect numbers of bacteria or fungi. Direct microscopic counts showed fewer bacteria in soil originating from disturbed plots than from undisturbed plots (Disturbance, P<0.001), although a greater number of culturable bacteria and fungi were isolated from disturbed than from undisturbed soils (Disturbance, P<0.001). No UV‐B‐ or disturbance‐related differences in protein, starch or urea hydrolysis were exhibited by bacterial isolates. UV‐B treatment did not affect total plant biomass within undisturbed plots or the biomass of individual groupings of grasses, forbs and mosses. Per cent root length colonized by arbuscular mycorrhizal fungi (AMF) was not affected by enhanced UV‐B radiation in the undisturbed plots. Neither AMF nor plant biomass was measured in disturbed plots. The key findings of this study show that UV‐B‐mediated alterations in carbon utilization occurred in soil microbial communities subjected to disturbance, but such changes were not observed in communities sampled from undisturbed grassland. Differences in the catabolic potential of microbial communities from disturbed grassland subjected to enhanced UV‐B are probably related to plant‐mediated changes in resource availability or quality.     

Plant functional group responses to fire frequency and tree canopy cover gradients in oak savannas and woodlands

Questions: How do fire frequency, tree canopy cover, and their interactions influence cover of grasses, forbs and understorey woody plants in oak savannas and woodlands? Location: Minnesota, USA. Methods: We measured plant functional group cover and tree canopy cover on permanent plots within a long‐term prescribed fire frequency experiment and used hierarchical linear modeling to assess plant functional group responses to fire frequency and tree canopy cover. Results: Understorey woody plant cover was highest in unburned woodlands and was negatively correlated with fire frequency. C4‐grass cover was positively correlated with fire frequency and negatively correlated with tree canopy cover. C3‐grass cover was highest at 40% tree canopy cover on unburned sites and at 60% tree canopy cover on frequently burned sites. Total forb cover was maximized at fire frequencies of 4–7 fires per decade, but was not significantly influenced by tree canopy cover. Cover of N‐fixing forbs was highest in shaded areas, particularly on frequently burned sites, while combined cover of all other forbs was negatively correlated with tree canopy cover. Conclusions: The relative influences of fire frequency and tree canopy cover on understorey plant functional group cover vary among plant functional groups, but both play a significant role in structuring savanna and woodland understorey vegetation. When restoring degraded savannas, direct manipulation of overstorey tree canopy cover should be considered to rapidly reduce shading from fire‐resistant overstorey trees. Prescribed fires can then be used to suppress understorey woody plants and promote establishment of light‐demanding grasses and forbs.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Safe Site and Seed Limitation in Cytisus Scoparius (Scotch Broom): Invasibility, Disturbance, and the Role of Cryptogams in a Glacial Outwash Prairie

In all plant populations, establishment is controlled by two factors: the supply of propagules and their access to ‘safe sites’ for growth. An infestation of invading pest plants results in a seed-production gradient, from the edge where seeds are limiting, to the center where seeds may be in excess. Do invaded sites become ‘saturated’ with seeds? How rapidly does this occur, and how does the process depend on the availability of safe sites? Are safe sites, and consequently invasion, promoted by disturbance? I quantified the response of seedling establishment to seed input and disturbance in Cytisus scoparius (Scotch broom), an exotic shrub invading glacial outwash prairie remnants in western Washington, USA. I used disturbance treatments to investigate the role of the thick cryptogamic layer in these prairies, disturbing cryptogams by scraping or by fire. The effect of fire was partitioned into two factors: burning of the background vegetation/substrate versus breaking C. scoparius seed dormancy, by adding seeds either before or after the burn. Seed treatments ranged from 20 to 1000 seeds per m². Both seed number and surface treatment showed significant effects on seedling density, along with a significant interaction between the two factors. Disturbance did not promote C. scoparius establishment; undisturbed plots produced more seedlings than burned or scraped plots. Within the burned plots, fire scarification appeared to increase germination but this effect was not significant. For germinated seedlings, mortality through the dry season (June–August) was not significantly different among surface treatments, nor did survivorship depend on density, with the result that initial differences in germination among the treatments persisted. The message that the undisturbed cryptogam layer facilitates C. scoparius establishment suggests that ‘ecosystem management’ strategies promoting healthy, undisturbed sites will not always be effective against invasive pest species.     

Limitations to seedling establishment in a mesic Hawaiian forest

While invasive species may be visible indicators of plant community degradation, they may not constitute the only, or even the primary, limitation to stand regeneration. We used seed-augmentation and grass-removal experiments under different canopy conditions to assess the relative importance of dispersal limitation, resource availability, and competition on seedling establishment in the understory shrubs Sophora chrysophilla, Dodonea viscosa, and Pipturus albidus in a montane mesic forest in Hawaii. The study location was an Acacia koa-Metrosideros polymorpha forest at 1000–1500 m elevation on the leeward side of Hawaii Island; it is a closed-canopy forest historically subject to logging and grazing by cattle and sheep and currently dominated by the exotic grass, Ehrharta stipoides, in the herb layer. Seedling establishment after 1 and 2 years was strongly dispersal limited in Sophora and Dodonea, but not in Acacia, a non-augmented species in which abundant seedlings established, nor in Pipterus, in which only one seedling established in 2 years. Grass cover reduced seedling establishment in Acacia, Sophora, and Dodonea and, for the latter two species, seedling establishment was substantially greater in the warmer, more moist forest at the lowest elevation. Light, moisture, and resin-captured N and P were more strongly affected by elevation and canopy composition than by grass cover, but in most cases seedling establishment was not positively correlated with resource availability. Limitations to the establishment of woody seedlings in this forest-grassland mixture vary among species; however, both dispersal limitation and competition from a shade-tolerant grass are important deterrents to regeneration in these forests.