Impact of fire on leaf nutrients,arthropod fauna and herbivory of native and exotic eucalypts in Kings Park,Perth, Western Australia

The vegetation of Kings Park, near the centre of Perth, Western Australia, once had an overstorey of Eucalyptus marginata (jarrah) or Eucalyptus gomphocephala (tuart), and many trees still remain in the bushland parts of the Park. Avenues and roadsides have been planted with eastern Australian species, including Eucalyptus cladocalyx (sugar gum) and Eucalyptus botryoides (southern mahogany), both of which have become invasive. The present study examined the effect of a recent burn on the level of herbivory on these native and exotic eucalypts. Leaf damage, shoot extension and number of new leaves were measured on tagged shoots of saplings of each tree species in unburnt and burnt areas over an 8‐month period. Leaf macronutrient levels were quantified and the number of arthropods on saplings was measured at the end of the recording period by chemical knockdown. Leaf macronutrients were mostly higher in all four species in the burnt area, and this was associated with generally higher numbers of canopy arthropods and greater levels of leaf damage. It is suggested that the pulse of soil nutrients after the fire resulted in more nutrient‐rich foliage, which in turn was more palatable to arthropods. The resulting high levels of herbivory possibly led to reduced shoot extension of E. gomphocephala, E. botryoides and, to a lesser extent, E. cladocalyx. This acts as a negative feedback mechanism that lessens the tendency for lush, post‐fire regrowth to outcompete other species of plants. There was no consistent difference in the levels of the various types of leaf damage or of arthropods on the native and the exotic eucalypts, suggesting that freedom from herbivory is not contributing to the invasiveness of the two exotic species.     

Mammalian herbivory in post-fire chaparral impacts herbaceous composition but not N and C cycling

火后灌丛中哺乳动物食草性影响草本植物组成而不影响氮和碳循环经典理论预测,草食动物通过选择性植物啃食和富含氮的排泄物的沉积影响草本植物组成和土壤氮循环,其影响取决于生态系统氮的有效性。据预测,草食动物在氮有效性高时可加速氮循环,而在氮有效性低时减慢其循环。然而,在自然系统中对这些理论的实验验证比较有限,并且有些结果相互矛盾。美国加利福尼亚州广泛分布的灌木林地为验证这些理论提供了一个可行的系统。它们易于发生周期性的林冠火,这会暂时移除活的灌木覆盖物,使土壤中的矿质氮沉积,并利于各种草本植物组合在3–5 年内形成主导景观。灌丛也越来越容易受到草本植物的入侵;哺乳动物的食草性可能会限制灌丛下层非本地草本植物的建立。我们设置一项为期两年的植食动物-围封实验,评估火灾后早期灌丛演替期间哺乳动物食草性对草本植物组合和土壤氮和碳循环的影响。我们预测,在火灾后高氮条件下,哺乳类食草动物不会更倾向于取食固氮草本植物,会加速氮循环并减少非本地草本植物的丰富度。研究结果表明,排除哺乳类食草动物后,尽管草本植物的现存生物量增加了54%,但固氮与非固氮草本植物的相对丰富度没有改变,也没有改变氮和碳循环的指标。在两年的实验期间内,草食动物对养分循环的影响可能并不显著,草食动物活动的物理影响可能抵消了植物凋落物和动物粪/尿输入的影响。哺乳类食草动物主要以典型的非本地草本植物为食,略微降低了它们的相对丰度;然而,在火灾后的前两年,哺乳动物的食草性并不能有效地阻止侵入性草本植物对灌丛的入侵,也不能改变植物的碳和氮循环。     

Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community

Summary The effects of spring grazing by sheep and of natural levels of insect herbivory were studied in 1985 on a limestone field abandoned from arable land for four years. A split-plot design was adopted in which paddocks, arranged in Latin squares, were either left ungrazed or heavily grazed by sheep for ten days in April. Within each paddock plots were either sprayed regularly with Malathion-60 or untreated.Natural levels of insect herbivory, compared to the reduced levels in insecticide-treated plots, had effects of similar magnitude to those from the short burst of spring grazing. Many attributes of the grazed/insecticide-treated sward were either increased or decreased by a factor of two within a season. Both types of herbivore caused changes in the direction of plant succession as well as in its rate. Effects on early successional species were large and similar when caused by either type of herbivore. Effects on later successional species were often smaller, but also showed differences in the action of the two herbivore types, as did effects on sward height, species richness and total cover. The effects of sheep and insect herbivory were not always additive or in the same direction.The results suggest that manipulations of both mammal and insect herbivores may be powerful tools for directing changes in plant community composition.     

Patterns of phytophagous insect herbivory and abundance on juvenile regrowth of Eucalyptus subgenera coexisting in southeastern Tasmania

Abstract Eucalypts from the subgenus Monocalyptus tend to be more abundant and dominate species from the subgenus Symphyomyrtus where they occur together in the mixed species stands of southeastern Australia. The differential impacts of herbivory by phytophagous insects has been postulated as a causal mechanism in the creation and maintenance of such stands. This research aimed to quantify phytophagous insect abundance and herbivory in mixed species juvenile regrowth of Eucalyptus globulus, E. viminalis (Symphyomyrtus), E. obliqua and E. pulchella (Monocalyptus} southwest of Hobart, Tasmania. Monocalyptus experienced a higher level of herbivory than Symphyomyrtus. However, mean damage levels were relatively low at less than 11% throughout. Furthermore, due to the positively skewed nature of herbivory data the mean was an inappropriate measure of central tendency; median damage levels ranged from 4.9% to 8.4%. Patterns of herbivory tended to be different for each eucalypt species: E. obliqua was particularly prone to chewing damage, E. pulchella and E. globulus suffered higher levels of distortion while E. viminalis was least affected by insect attack. Even though some trends in insect community structure seemed apparent at the level of eucalypt subgenus, closer examination revealed patterns of abundance were characteristic of each Eucalyptus species. The composition of foliar damage corresponded with the prominence of particular insect groups. Sucking insects tended to dominate the fauna except on E. obliqua where chewing insects in general, and chrysomelids in particular, were most prevalent. Both the distribution and magnitude of herbivory suggested that phytophagous insects had a negligible effect on competition between coexisting juvenile eucalypts and were unlikely to be responsible for the dominance of Monocalyptus or the maintenance of mixed species stands.     

Regional and local variation in insect herbivory,vegetation and soils of eucalypt associations in contrasted landscape positions along a climatic gradient

Abstract Relationships with climate and local resources are developed for soils, vegetation and tree foliage as well as levels of herbivory for the dominant eucalypts at sites representing a regional gradient in climate and local contrasts in landscape position. Indicators of site productivity such as soil nitrogen and phosphorus, canopy height and cover, foliar nitrogen and water, and average leaf area tended to increase as climate became more favourable. Many were also higher in locally richer parts of the landscape. In contrast, specific leaf weight, an indication of sclerophylly, decreased as climate and local resources became more favourable. Rates of herbivory tended to increase with increasing site productivity and the associated changes in soil, vegetation and foliar properties, in broad agreement with models relating herbivory to resource availability and plant vigour. We found no evidence to support models relating high herbivory to low-resource environments and plant stress. The apparent level of herbivore damage on mature leaves was highest at intermediate levels of resources; this could reflect interactions between resource availability, rates of herbivory and rates of leaf replacement. Implications of these findings are discussed with respect to ways of measuring herbivory, regional patterns in rates and levels of herbivory, and the regional distribution of rural dieback associated with high herbivory.     

Impacts of urbanization on insect herbivory and plant defences in oak trees

Systematic comparisons of species interactions in urban versus rural environments can improve our understanding of shifts in ecological processes due to urbanization. However, such studies are relatively uncommon and the mechanisms driving urbanization effects on species interactions (e.g. between plants and insect herbivores) remain elusive. Here we investigated the effects of urbanization on leaf herbivory by insect chewers and miners associated with the English oak Quercus robur by sampling trees in rural and urban areas throughout most of the latitudinal distribution of this species. In performing these comparisons, we also controlled for the size of the urban areas (18 cities) and gathered data on CO₂ emissions. In addition, we assessed whether urbanization affected leaf chemical defences (phenolic compounds) and nutritional traits (phosphorus and nitrogen), and whether such changes correlated with herbivory levels. Urbanization significantly reduced leaf chewer damage but did not affect leaf miners. In addition, we found that leaves from urban locations had lower levels of chemical defences (condensed and hydrolysable tannins) and higher levels of nutrients (nitrogen and phosphorus) compared to leaves in rural locations. The magnitude of urbanization effects on herbivory and leaf defences was not contingent upon city size. Importantly, while the effects of urbanization on chemical defences were associated with CO₂ emissions, changes in leaf chewer damage were not associated with either leaf traits or CO₂ levels. These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here. Overall, our simultaneous assessment of insect herbivory, plant traits and abiotic correlates advances our understanding of the main drivers of urbanization effects on plant–herbivore interactions.     

Seasonal changes in foliar macronutrients (N,P, K,Ca and Mg) inEucalyptus saligna Sm. andE. wandoo Blakely growing in rehabilitated bauxite mine soils of the Darling Range,Western Australia

Summary Seasonal changes in the foliar concentration of macronutrients (N, P, K, Ca and Mg) in sapling trees ofEucalyptus saligna Sm. andE. wandoo Blakely growing in rehabilitated bauxite mined areas in the Darling Range of Western Australia are described. Foliar N concentration decreased with age of the fully expanded leaf tissue. Leaf N concentrations were also high when rates of litter decomposition were expected to be high during the period of early spring. The greatest foliar N difference between trees growing in good soil conditions and those from poorer soil conditions also occurred during this period. Levels of P in leaves were highest in young developing leaves but once the leaves reached full size, no seasonal trend in P concentration was observed. Foliar K was lower during the winter and probably related to the period of maximum leaching by precipitation. High foliar K during summer, however, could be related to the role of K in lowering cellular water potential. Leaf Ca was highest during early sping. Low mobility of cellular Ca during the cool portion of the year was indicated. Foliar Mg showed a weak pattern of decreasing concentration with leaf age. The best season for sampling for these broadleafed evergreen species to provide information on plant nutrient status appears to be in spring.     

Effects of sclerophylly and host choice on gall densities and herbivory distribution in an Australian subtropical forest

We hypothesize that upper canopies contrast with the understorey vegetation in gall density and survival because of greater mortality in the latter. We expect that high sclerophylly rates in the upper canopy leaves are a main cause of such pattern, and more important than other environmental traits, for instance related to altitude. The study was conducted in Lamington National Park, Queensland, Australia. Four independent vertical cylindrical transects through the forest canopy and one equivalent, horizontal understorey transect (20 m long and 1 m in diameter) were sampled at different altitude (300, 700, 900 and 1100 m above sea level) during two seasons. Total and damaged leaves were counted, and galls were collected and opened to determine if they were alive or what may have been the cause of death. Sclerophylly was estimated as specific foliar mass. Out of 72 sampled plant species, 29 presented galls, of which the greatest densities were concentrated on seven host species. A significant increase in sclerophylly with increasing canopy stratum height was observed, but had no direct effect on gall distribution. Total and live galls were more abundant in the canopy than in the understorey for all altitudes but 300 m, where a specific infestation on saplings of the canopy tree Arytera divaricata occurred. We found a positive gall establishment and survivorship in the upper canopy, along with a decrease in chewing herbivory, which resulted in decreasing risks of gall death by herbivory. An overall high sclerophylly rate in both canopy and understorey and the total number of galled host species suggest that the plant community studied is prone to gall establishment and evolution. However, a few hosts species with extreme infestation, such as A. divaricata and Ficus watkinsiana, override the community‐wide effect of sclerophylly. Our results emphasize how scarce and patchy distributed galls are.     

Foliar and twig macronutrients (N,P, K,Ca and Mg) in selected species ofEucalyptus used in rehabilitation: sources of variation

Summary The macronutrient variation within four 6 year oldEucalyptus saligna and four 5 year oldE. wandoo growing on rehabilitated bauxite pits was determined. Significant differences in mean nutrient concentrations were generally recorded between good soil condition sites and poor soil sites, between tree individuals, branch height, and plant organ type; but mean nutrient values were not different among canopy aspects. Fully expanded leaves of the current year provided the most uniform nutrient levels among the plant organs and showed major differences between sites with good soil nutrient conditions and those with poor conditions. Differences in foliar and branch levels of N, P, K, Ca and Mg, the variation between sites, canopy heights and plant organ types, and the use of foliar nutrient levels to indicate deficiencies are discussed.     

The effects of light on foliar chemistry,growth and susceptibility of seedlings of a canopy tree to an attine ant

Summary Seedlings of Inga oerstediana Benth. (Mimosaceae) growing in three different light environments (the understory, tree-fall gaps and full sun) were tested for differences in chemistry (nutrients and tannins), wound-induced increases in tannins, growth, and susceptibility to leaf-cutter ants, Atta cephalotes (L.) (Formicidae: Attini). I hypothesized that seedlings of I. oerstediana would contain higher concentrations of tannins when growing in high light conditions and, therefore, would be less susceptible to leaf-cutter ants.Foliar concentrations of condensed tannins were much higher in plants growing in full sun compared to those growing in the understory. The concentrations of condensed tannins did not increase following damage. Despite higher concentrations of condensed tannins in sun foliage, leaf-cutter ants found these leaves more acceptable. The preference for sun leaves was consistent with higher concentrations of foliar nutrients. I suggest that the magnitude of the increase in condensed tannins was not great enough to override the benefits of increased concentrations of foliar nutrients. Finally, based on these results and those of others, I suggest that foraging by leaf-cutter ants may be an important factor determining patterns of succession in early successional habitats.     

Influence of fire severity on the regeneration,recruitment and distribution of eucalypts in the Cotter River Catchment,Australian Capital Territory

Abstract Plant responses to fire are variable between and within species and are influenced by numerous factors including fire severity. This study investigated the effects of fire severity on the regeneration and recruitment of forest eucalypts in the Cotter River Catchment, Australian Capital Territory (ACT). This study also examined the potential for the obligate seeder Eucalyptus delegatensis R.T. Baker (Myrtaceae) to expand into adjacent stands dominated by the facultative resprouter Eucalyptus fastigata H. Deane & Maiden (Myrtaceae) by seed shed and seedling establishment beyond the pre‐fire boundary. Sites were located in areas of either higher or lower fire severity, and transects were placed across the boundary of stands of E. delegatensis and E. fastigata. Species distributions, tree survival and seedling densities and heights were recorded, and the location of each boundary was determined as the region of maximum change in species composition along the transects. Eucalyptus delegatensis was the only eucalypt killed by higher severity fire. However, E. delegatensis seedling density was greater at higher severity sites than lower severity sites. Eucalyptus fastigata seedling density was low across all sites, with other eucalypts producing few, if any, seedlings. There was no evidence that E. delegatensis had increased its range into downslope stands dominated by E. fastigata. Patterns of vegetative recovery and seedling recruitment may be related to a number of factors, including differences in allocation patterns between seeders and sprouters, and the effects of overstory and understory competition. It is unclear what processes impede E. delegatensis seedling establishment beyond the stand boundary, but may involve an inability of E. delegatensis to shed seed sufficiently far downslope; unsuitable conditions for germination beyond the boundary; or, competition from a retained or resprouting overstory, despite the potential for increased dispersal distance soon after fire.     

Effects of seedling conspecific density and heterospecific frequency on insect herbivory in a tropical dry forest

1. Conspecific plant density and heterospecific frequency are key drivers of herbivore damage. However, most studies have investigated their effects separately and for single (rather than multiple) focal plant species.
2. We conducted an experiment involving three tree species, namely: Cordia dodecandra (Boraginaceae), Manilkara zapota (Zapotaceae), and Piscidia piscipula (Fabaceae). We manipulated understory densities of M. zapota and C. dodecandra (focal species) and their frequency relative to P. piscipula. Three months after planting, we surveyed insect leaf chewer and sucking damage on the former two. Because these species are attacked by different herbivores, we predicted a negative effect of heterospecific frequency on herbivory.
3. Density and frequency varied in the direction and function of their effects on herbivory depending on the plant species and attacking herbivore. As expected, Piscidia piscipula frequency had a negative linear effect on M. zapota leaf-chewer damage, whereas conspecific density did not affect chewer damage on this species. In contrast, density and frequency had non-linear effects on C. dodecandra chewer damage, namely positive (hump-shaped) and negative (U-shaped) relationships, respectively. In addition, density and frequency had positive linear effects on C. dondecandra damage by leafhoppers.
4. These findings call for more work jointly assessing plant inter-specific variation in density- and frequency-dependent variation in herbivory and its underlying drivers.

     

Altered N,P and C dynamics with absence of fire in Eucalyptus forests affected by premature decline

Absence of fire is increasingly recognized as an important driver of soil nutrient budgets in Eucalyptus forest, especially in forests affected by premature Eucalyptus decline, due to the effects of soil nutrient accumulation on nutrient balances and forest community dynamics. In this study, we present a dataset of soil and foliar nutrient analyses, and vegetation measurements from a fire chronosequence survey in native E. delegatensis forest. Measured indices include total soil and extractable soil nitrogen (N), or phosphorus (P), soil organic carbon (C), soil acid‐phosphatase (PME) activity, foliar N and foliar P, and understorey and overstorey vegetation canopy height. We show that in some cases indices are strongly linked to time since fire (2–46 years). Time since fire correlated positively with foliar N, total and extractable soil N, soil organic C, and also soil PME activity; the latter an indicator of biotic P demand. Differences in the strength of these relationships were apparent between two geology types, with stronger relationships on the potentially less‐fertile geology. The strong positive correlation with time since fire and understorey canopy height reflected increasing shrub biomass and thickening of the shrub layer. The strong positive correlation for soil or foliar N, but not P, with time since fire, indicates that P does not increase relative to N over time. P may, therefore, become limiting to growth in this plant community. Similarly, the significantly higher concentrations of soil N but not P, also found in both older and long‐unburnt forest stands (>100 years since management), may exacerbate a situation of soil nutrient limitation over several decades. A characteristic feature of long unmanaged stands is a developing tea tree (Leptospermum sp.) understorey, which may benefit from elevated soil N availability and increasing organic C accumulation with prolonged fire absence. This increased shrub biomass would outcompete Eucalyptus for resources, including soil nutrients and water.     

Correlations between leaf constituent levels and the densities of herbivorous insect guilds in an Australian forest

Abstract Significant correlations were found between herbivorous insect densities and leaf constituent levels across 18 species of understorey shrub in an Australian forest. In general, insect densities were positively correlated with nitrogen and water and negatively correlated with fibre and its constituents (cellulose and lignin), but not all insect guilds were significantly correlated with all leaf constituents. When mature leaf traits were considered, total herbivore density among plant species was most strongly correlated with levels of nitrogen and water. However, when insects were divided into functional guilds, only densities of sucking insects were significantly correlated with mature leaf nitrogen and water, whereas chewing insect densities were significantly correlated with mature leaf fibre and lignin. Low leaf nitrogen levels were recorded for all plants surveyed (among the lowest reported for woody angiosperms), and many plant species also had high levels of leaf fibre. These features are characteristic of Australian forests and have been linked to the generally nutrient‐poor soils of this continent. Levels of new leaf nitrogen (% fresh weight) were generally less than or equal to levels of mature leaf nitrogen (% fresh weight). Hence the new leaf preference of several herbivore guilds could not be explained by higher levels of leaf nitrogen. However, significant negative correlations were found between chewer densities and levels of leaf fibre, cellulose and lignin for new and mature leaves, suggesting that higher levels of fibre, cellulose and lignin in mature leaves may be contributing more strongly to the new leaf preference of chewers than leaf nitrogen levels. Despite the significant correlations between leaf constituent levels and the densities of individual guilds, multivariate analyses found no significant relationship between leaf constituent levels and the taxonomic or guild composition of herbivorous insect assemblages.     

Prescribed burning as an alternative management in grasslands of temperate Europe: The impact on seeds

Fire is not an integral part of terrestrial ecosystems in temperate Europe, nevertheless prescribed burning is proposed to be an alternative to traditional management applied to grasslands. Thus, anthropogenic fire represents a serious challenge to plant species, and there is no information on how the recruitment of species responds to fire. The aim of our study was to assess the effect of fire on seed germination of 16 herbaceous grassland and ruderal species belonging to four common families. We first assessed the fuel load in open habitats in early spring and measured soil temperatures during experimental fires. After that we performed a controlled pot experiment involving heat treatments and experimental fire applied to seeds and followed their germination. Our measurements showed that maximum temperatures are between 73 °C and 264 °C on the ground surface and fire passage is fast, with short residence times of high temperatures. In deeper soil layers (1 cm and 5 cm), temperature increase is negligible. Seed germination of half of the species was decreased by the passage of fire, and a heat shock of 100 °C for 5 min had an even stronger adverse effect. Seeds of three Fabaceae species were stimulated by heat or fire, while negative effects prevailed among species belonging to other families. Anthropogenic fire in grasslands of temperate Europe might reduce recruitment by seed, particularly in species of the Asteraceae and Poaceae, two very important families with a large representation in temperate grasslands. Our results indicate that prescribed burning should be carefully applied in order not to endanger the local persistence of grassland species whose seedling emergence is negatively affected by fire.     

黑土长期施肥及养分循环再利用的作物产量及土壤肥力质量变化Ⅰ.作物产量

在一丰P黑土(Olsen P25.8mg·kg^-1)上进行13年中长期田间试验的结果表明,氮肥平均增产率为275%,年增产粮食(大豆、玉米、小麦混合)724kg·hm^-2,1kgN增产粮食9.4kg;磷肥前期增产不明显,13年平均增产率为7%,年增产粮食241kg·hm^-2,1kgP增产粮食12.7kg.每年施用循环回田猪圈肥(以处理区收获产品的80%喂猪、垫圈经堆制而成),粮食平均年增产量在不施化肥、施N、施NP基础上,分别为268、258、255kg·hm^-2,相应的增产率为9.8%、7.6%和7.0%.试验期间循环回田猪圈肥的增产效果有逐渐增长趋势,表明存在着猪圈肥残效的叠加效应.     

The adaptive value of young leaves being tightly folded or rolled on monocotyledons in tropical lowland rain forest: an hypothesis in two parts

In tropical lowland rain forest, we find that species with the leaves tightly folded or rolled until they reach at least 50% of final length occur in 10 of the 15 monocot families with >100 species, and in 12 of the 24 monocot families with <100 species, but in only seven of the 212 dicot families (eudicots and magnoliids). Earlier researchers have described how examples of tightly folded and rolled leaves develop, but most have not considered the potentially adaptive value of this pattern of growth. We hypothesize that it is a protection against herbivorous invertebrates. For tropical and temperate dicots, the young leaves have been found to suffer much smaller losses to herbivores while folded and rolled than after they are unfolded or unrolled. Being folded or rolled until a late stage involves an ‘opportunity cost’ in the loss of photosynthesis. Among dicots, defences involving such a cost (notably late development of photosynthetic systems in pendent soft young leaves) are typical of shade-tolerant species, which have longer-lived leaves than light-demanders. In contrast, among monocots late folding and rolling are found in both shade-tolerators and light-demanders. We hypothesize that late folding and rolling bring a net advantage to monocots in general, whether shade-tolerant or light-demanding, despite the opportunity cost, because they mostly have fewer leaves per plant of a given size, and therefore an individual leaf is relatively more valuable to the plant. As a coda, we suggest that the ‘sleep movements’ of some tropical plants, and the circinate vernation of ferns and some cycads, provide protection against invertebrate herbivores through the apposition of two or more layers of leaf.     

Effects of plant-plant interactions and herbivory on the plant community structure in an arid environment of Saudi Arabia

There is currently considerable evidence support that plant community structures are driven by plant-plant interactions (e.g., competition and facilitation). In contrast, there is also evidence demonstrating that plant community structure is affected by the impact of consumer pressure (e.g., grazing). In this study, 15 and 10 Acacia gerrardii nurse plants were selected inside and outside Sudyrah natural reserve (protected) area in western Saudi Arabia, respectively. The understory vegetation abundance (e.g. cover and density) was measured among quadrats around the nurse trees in both protected and unprotected areas to examine the impact of grazing and the positive interaction on the understory species. I found that understory vegetation associated with nurse trees (A. gerrardii) has been driven by both the positive impact of nurse plant and the grazing. Although the understory vegetation was positively affected by the impact of facilitation, the composition of such vegetation has been changed due to the impact of herbivory.     

Effect of season of burning and removal of herbaceous cover on seedling emergence in a eucalypt savanna of north‐eastern Australia

Abstract Seedling emergence in a eucalypt savanna of north‐eastern Australia was documented over a 12‐month period, between May 1999 and May 2000. Seedling emergence for grasses, forbs and subshrubs was found to mainly occur in a brief pulse at the start of the wet season following fire or the removal of grass biomass. Only a minor number of tree and shrub seedlings were detected overall. Burning, or cutting away the grass layer in unburnt savanna, in both the early (i.e. May) and the late (i.e. October) dry seasons significantly increased seedling emergence over undisturbed savanna that had been unburnt for 3 years. Removing the grass layer in unburnt savanna, during either the early or the late dry season, triggered similar seedling densities to savanna burnt in the early dry season. Late dry season fires promoted the greatest seedling density. We attribute this to the higher intensity, late dry season fires releasing a greater proportion of seed from dormancy, coupled with the higher density of soil seed reserves present in the late dry season.