Fire and soil-plant nutrient relations in a pine-wiregrass savanna on the coastal plain of North Carolina

Summary Changes in soil and plant nutrient conditions were evaluated following various burn and clip treatments in a longleaf pine-wiregrass savanna in Bladen Co., N.C., USA. Ground fires were found to add substantial quantities of N, P, K, Ca, and Mg to the soil, though not necessarily in forms immediately available to plants. Less than 1% of the total nitrogen in the charred residue (ash) is present as nitrate or ammonium. Considerable quantities of all nutrients examined were lost to the atmosphere during burning. Green leaf tissue in recently burned areas was consistently higher in N, P, K, Ca, and Mg compared to unburned areas. Howerver, when compared to similar tissues from clipped plots, burned area tissues were significantly higher in N, Ca, and Mg only. Data presented here suggest that tissue age significantly affects nutrient content and must be considered in any analysis of tissue nutrient content following burning. Within 4–6 months following fire, burned-area tissue nutrient content decreases to concentrations found in the unburned area. Burning resulted in initial enrichment of available soil nutrients including PO₄, K⁺, Ca⁺⁺, and Mg⁺⁺, however, NO₃ ^-, and NH₄ ⁺ concentrations in burned soil were not significantly different from unbruned soil. Soil and plant nutrient changes in an area burned two years in succession indicate that repeated burning may diminish nutrient availability. Plant response to various nutrient enrichment treatments of the soil indicated that nitrogen is limiting growth in both burned and unburned soils and that burning may alter some factors other than nutrients which may retard plant growth in unburned areas.     

Fire drives the reproductive responses of herbaceous plants in a Neotropical swamp

Palm swamps (veredas) are unique and diverse plant communities associated with the headwaters of streams in central Brazil, and they are frequently subjected to fires. We evaluated the effect of fire and the role of different fire-related cues on inducing reproduction by palm swamp vegetation. We compared the responses of species in burned plots, in plots in which the aboveground vegetation was clipped and then removed, and in unburned and unclipped control plots. Both the number of reproductive species and the total number of flowers/fruits produced by all species monthly were significantly greater in the burned than in the clipped and control plots, and greater in the clipped than in the control plots. For 34 of the 48 individual species analyzed the number of flowers/fruits produced per m²/month was greater in the burned than in the control plots, whereas the clipping treatment significantly increased the reproductive rate of only six species. This indicates that increased light availability was not the only factor inducing plant reproduction. Most likely, plant reproduction was also stimulated by the availability of soil nutrients whose concentrations increased significantly after burning. Although our results indicate that most plant species that occur in palm swamps are fire-recruiters, care must be taken in using fire as management tool, especially as the frequency of human-induced fires in palm swamps have increased dramatically in recent years.     

火烧及微地貌对高原湿草甸优势种华扁穗草地上部灰分含量及热值的影响

为探讨火烧干扰以及微地貌差异对高原湿草甸植被的影响,以高原湿草甸优势种华扁穗草( Blysmus sinocompressus Tang et F. T. Wang)为研究对象,分别在经历火烧干扰和未烧的湿地中选择平地和地垄作为样地,研究了不同生长期以及不同年份华扁穗草地上部的灰分含量和热值(包括干质量热值和去灰分热值)变化。结果表明：在火烧平地、火烧地垄、未烧平地和未烧地垄4类样地中,不同生长期以及不同年份间华扁穗草地上部的灰分含量和热值均有一定差异。在火烧平地、火烧地垄和未烧平地3类样地中,与生长初期相比,生长旺盛期华扁穗草地上部的灰分含量均降低,其干质量热值和去灰分热值均升高;而在未烧地垄样地中,与生长初期相比,生长旺盛期华扁穗草地上部的灰分含量显著升高,其干质量热值和去灰分热值则均略降低。从样地类型看,在生长旺盛期,平地样地中华扁穗草地上部的灰分含量均低于地垄样地,其干质量热值和去灰分热值均高于地垄样地,其中,其灰分含量在火烧平地样地中最低、在未烧地垄样地中最高;其干质量热值和去灰分热值在火烧地垄样地中均最低、在未烧平地样地中均最高,但总体无显著差异。随时间推移,火烧样地特别是火烧平地样地中华扁穗草地上部的灰分含量增加,而其干质量热值和去灰分热值则有所降低但总体上与未烧样地间无显著差异。总体上看,在火烧样地中,华扁穗草地上部的干质量热值和去灰分热值低于未烧样地;而在平地样地中,其干质量热值和去灰分热值高于地垄样地。研究结果显示：火烧干扰和微地貌差异对华扁穗草地上部的灰分含量和热值均有一定的影响,据此,建议将植物地上部的灰分含量和热值作为高原湿草甸植被优势种响应环境扰动的参考指标。     

Elevated CO2 enhances resprouting of a tropical savanna tree

The savannas (cerrado) of south-central Brazil are currently subjected to frequent anthropogenic burning, causing widespread reduction in tree density. Increasing concentrations of atmospheric CO₂ could reduce the impact of such frequent burning by increasing the availability of nonstructural carbohydrate, which is necessary for resprouting. We tested the hypotheses that elevated CO₂ stimulates resprouting and accelerates replenishment of carbohydrate reserves. Using a factorial experiment, seedlings of a common Brazilian savanna tree, Keilmeyera coriacea, were grown at 350 ppm and 700 ppm CO₂ and at two nutrient levels. To simulate burning, the plants were either clipped at 15 weeks or were left unclipped. Among unclipped plants, CO₂ and nutrients both stimulated growth, with no significant interaction between nutrient and CO₂ effects. Among clipped plants, both CO₂ and nutrients stimulated resprouting. However, there was a strong interaction between CO₂ and nutrient effects, with CO₂ having a significant effect only in the presence of high nutrient availability. Under elevated CO₂, carbohydrate reserves remained at higher levels following clipping. Root total nonstructural carbohydrate remained above 36% in all treatments, so carbohydrate reserves did not limit regrowth. These results indicate that under elevated CO₂ this species may be better able to endure the high frequency of anthropogenic burning in the Brazilian savannas. Received: 19 May 1999 / Accepted: 8 November 1999     

Responses in plant, soil inorganic and microbial nutrient pools to experimental fire, ash and biomass addition in a woodland savanna

In order to investigate the effects of savanna fires on nutrient cycling a field experiment was carried out in an open woodland savanna of southwest Ethiopia. This involved manipulations of fire, fuel load and ash fertilisation in a fully factorial design, and recording of responses in plants, soil inorganic and microbial nutrient pools up to 1 year after the disturbances. As plant biomass nitrogen (N) was only 3.5% of that in topsoil the N loss in a single fire event was relatively small. The microbial N pool size in the topsoil was similar to the N pool size in the aboveground part of the plants. Soil microbial biomass carbon increased slightly 12 days after the low severity fire, but the effect was transient and was not accompanied by an increase in microbial N. Instead, the soil inorganic N concentration increased strongly 1 day after the fire, remained higher up to 3 months after the fire and probably caused the 40% higher grass biomass in burned than unburned plots, and the similar sized increase in grass nitrogen, phosphorus and potassium pools in the following rainy season. In contrast, broad-leaved herbs showed less strong increments in biomass and nutrient pool sizes. Fire interacted with fuel load, as burning of plots with double plant biomass led to reduced microbial biomass, plant nutrient pools and herb (but not grass) biomass. Low-severity-fire nutrient losses appear to be moderate and may be replenished from natural sources. However, in areas with frequent fires and high grass biomass (fuel) loads, or with late fires, nutrient losses could be much larger and non-sustainable to the persistence of the woodland savanna ecosystem.     

Herbaceous vegetation responses to experimental fire in savannas and forests depend on biome and climate

Fire–vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C₄ grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.     

C3 shrub expansion in a C4 grassland: positive post-fire responses in resources and shoot growth

Changes in land management and reductions in fire frequency have enabled woody species to increase in grasslands worldwide. Nevertheless, fire is rarely eliminated from grasslands, and for shrubs to survive, they must be able cope with fire and replace aboveground structures. Because new shoots may have more available solar radiation, greater root?:?shoot ratios, and thus more resources available belowground after fire compared to undisturbed shrub communities, we hypothesized that carbon, nutrient, and water relations may be enhanced in stems compared to those in an undisturbed grassland. However, this same post-fire resource pulse stimulates the grasses and may intensify competitive interactions between shrubs and grasses. To test these predictions, we measured seasonal patterns in net photosynthesis (A), predawn xylem pressure potentials (XPP), leaf nitrogen (N) content, and productivity of Cornus drummondii shoots from shrub patches (islands) of different sizes in mesic grasslands burned annually, burned infrequently, and protected from fire. Seasonal average A was 20% higher (P = 0.016) in burned than in unburned shrubs, regardless of island size. Shrubs in burned sites also produced shoots with higher leaf N than unburned shrubs, and N content was higher in leaves from small islands compared to large islands (P < 0.0001). Burning caused a decrease in late summer predawn XPP in small islands (-3.1 MPa), whereas burned large islands did not differ from unburned shrubs. Post-fire productivity of new shoots was significantly greater compared to shoots in unburned sites. These results indicate that a transient period of high resource availability after fire allows for increased growth and rapid recovery of grassland shrubs. Thus, although fire has a negative effect on aboveground biomass of shrubs, the post-fire increases in resource availability, which enhance growth in the dominant grasses, are also important for recovery of woody species.     

Effects of fire on sandhill herbs: nutrients, mycorrhizae, and biomass allocation

Differences in growth responses, tissue and soil inorganic nutrients, and mycorrhizal relationships of four herbaceous species were studied on burned and unburned sandhill sites in south-central Florida, USA. Three species, (Aristida stricta, Liatris tenuifolia var. laevigata, and Pityopsis graminifolia) responded positively to conditions following the burn by increased vegetative growth and flowering. The fourth species, Balduina angustifolia, is a fire-sensitive biennial and its first-year rosettes were, with an occasional exception, unable to survive or resprout following fire. Availability of all soil inorganic nutrients examined (Ca, K, Mg, and P) was low, as were total nitrogen, soil organic matter, and pH. There was a slight nutrient pulse of phosphorus into the soil following burning. For two species (Aristida and Liatris), shoot tissue concentrations of several inorganic nutrients (especially N and P) were higher on the burned site than the unburned site following burning. These differences generally dissipated over time since burning. The high concentration of tissue nutrients postburn followed by a decline on the burned site may result from rapid nutrient uptake after fire and dilution of this concentration following restoration of plant mass. Despite low levels of soil inorganic nutrients, including phosphorus, mycotrophy was absent or weakly developed among the herbaceous species examined, except for the tap-rooted Balduina angustifolia. Colonization of host plants by vesicular mycorrhizal fungi was unaffected by burning. Mycorrhizal inoculum potentials of sandhill soil were extremely low, varying seasonally from (mean +/- 1 SE) 0.3 +/- 0.2 to 3.8 +/- 0.7%.     

Fire and Litter Effects on Seedling Establishment in Western Oregon Upland Prairies

Prescribed burning is an important tool for managing and restoring prairies and other ecosystems. One effect of fire is plant litter removal, which can influence seedling establishment. Four experimental treatments (burned, clipped and raked to remove litter, burned with litter reapplied, and unmanipulated) were applied to 2 × 2.5–m plots in three western Oregon, United States, upland prairies to determine how burning affects seedling establishment. Seeds of common exotic and native prairie species were sowed into the experimental plots after treatments. Seedlings were censused the following spring. The experiment was repeated on each of the three sites, representing three common types of prairie vegetation: an Annual Exotic Grass site, a Perennial Exotic Grass site, and a Native Bunchgrass site. In both the Annual Exotic Grass and the Perennial Exotic Grass sites, burning significantly improved native, but not exotic, seedling establishment over those on unburned plots. Litter removal was a significant component of this burn effect, particularly on the Perennial Exotic Grass site. In these winter‐moist systems, the net effect of litter is to inhibit seedling establishment. Burning treatments on the Native Bunchgrass site significantly increased seedling establishment only of short‐lived exotic species. These results suggest that in prairie ecosystems similar to the Annual and Perennial Exotic Grass sites, prescribed burning followed by sowing native seeds can be an effective restoration technique. Burning alone or sowing alone would be counter‐productive, in the first case because increased establishment would come from exotic species and in the second case because establishment rates are low in unburned plots.     

Fire enhances litter decomposition and reduces vegetation cover influences on decomposition in a dry woodland

Dry woodlands frequently experience fire, and the heterogeneous spatial patterning of vegetation cover and fire behavior in these systems can lead to interspersed burned and unburned patches of different vegetation cover types. Biogeochemical processes may differ due to fire and vegetation cover influences on biotic and abiotic conditions, but these persistent influences of fire in the months or years following fire are not as well understood as the immediate impacts of fire. In particular, leaf litter decomposition, a process controlling nutrient availability and soil organic matter accumulation, is poorly understood in drylands but may be sensitive to vegetation cover and fire history. Decomposition is responsive to changes in abiotic drivers or interactions between abiotic conditions and biotic drivers, suggesting that decomposition rates may differ with vegetation cover and fire. The objective of this study was to assess the role of vegetation cover and fire on leaf litter decomposition in a semi-arid pinyon-juniper woodland in southern New Mexico, USA, where prescribed fire is used to combat increasing woody cover. A spatially heterogeneous prescribed burn led to closely co-located but discrete burned and unburned patches of all three dominant vegetation cover types (grass, shrub, tree). Decomposition rates of leaf litter from two species were measured in mesh litterbags deployed in factorial combination of the three vegetation cover types and two fire treatments (burned and unburned patches). For both litter types, decomposition was lower for unburned trees than for unburned grass or shrubs, perhaps due to greater soil–litter mixing and solar radiation away from tree canopies. Fire enhanced litter mass loss under trees, making decomposition rates similarly rapid in burned patches of all three vegetation cover types. Understanding decomposition dynamics in spatially heterogeneous vegetation cover of dry woodlands is critical for understanding biogeochemical process responses to fire in these systems.     

Long-term post-fire resprouting dynamics and reproduction of woody species in a Brazilian savanna

Resprouting is an efficient life history strategy by which woody savanna species can recover their aboveground biomass after fire. However, resprouting dynamics after fire and the time it takes to start producing flowers and fruits are still poorly understood, especially for the Brazilian savanna (Cerrado biome), where fire is an important driver of vegetation structure and ecosystem functioning. We investigated the resprouting dynamics and production of flowers and fruits of 26 woody species (20 tree and 6 shrub species for a total of 485 individuals) that were burned and the production of flowers and fruits for a subset of 12 species (139 individuals) in an unburned area in a Brazilian savanna. We classified the species’ resprouting strategies as hypogeal (at the soil level, with main stem death), epigeal (on the main stem or crown), and hypogeal + epigeal. We used generalized linear mixed-effect models to identify the post-fire recovery patterns for five years. Individuals with basal resprouts (hypogeal and hypogeal + epigeal resprouting) produced an average of 6 basal resprouts, but only 33% of resprouts survived after five years. Individuals in burned areas produced fewer flowers and fruits than individuals in unburned areas. At least a subset of individuals in all the resprouting strategies started to produce flowers and fruits in the first-year post-fire. About 68% of the species with hypogeal resprouts produced flowers and fruits in the first-year post-fire, but the intensity of flowering and fruiting was lower compared to individuals with other resprouting strategies over time. Although woody species have invested in post-fire growth and sexual reproduction in all resprouting strategies, the long time needed to recover these processes can make these species more vulnerable to frequent fires.     

Burning the Seasonal Mosaic: Preventative Burning Strategies in the Wooded Savanna of Southern Mali

Data are presented indicating a seasonal mosaic pattern of burning in the savanna of southern Mali. A seasonal mosaic is a landscape that is annually re-created by people, and which contains patches of unburned, early burned, and recently burned vegetation. A survey of over 100 farmers and in-depth interviews demonstrates that rural inhabitants of southern Mali begin an annual burning regime early in the dry season in order to fragment the landscape, with the goal of preventing later fires that can damage natural resources. The process of gradually burning off the driest vegetation creates a seasonal mosaic of habitat patches that increases the potential of the landscape for a variety of dry season land uses, including hunting, gathering of savanna products, and grazing. An analysis of a series of Landsat images shows that the practice of mosaic burning is widespread in the wooded savanna, in which burning usually begins early and large fires are rare. On the basis of recent developments in ecological theory and empirical evidence from similar burning regimes in parts of Australia, it is suggested that seasonal mosaic burning in Mali not only prevents damaging late-season fires but increases biodiversity. It is concluded that discourse on African savanna burning overemphasizes the ecologically detrimental aspects of fire, while neglecting the beneficial ones resulting in misguided policies that pose a threat to human livelihoods and savanna ecosystems.     

Foraging decisions underlying restricted space use: effects of fire and forage maturation on large herbivore nutrient uptake

Recent models suggest that herbivores optimize nutrient intake by selecting patches of low to intermediate vegetation biomass. We assessed the application of this hypothesis to plains bison (Bison bison) in an experimental grassland managed with fire by estimating daily rates of nutrient intake in relation to grass biomass and by measuring patch selection in experimental watersheds in which grass biomass was manipulated by prescribed burning. Digestible crude protein content of grass declined linearly with increasing biomass, and the mean digestible protein content relative to grass biomass was greater in burned watersheds than watersheds not burned that spring (intercept; F_1,251 = 50.57, P < 0.0001). Linking these values to published functional response parameters, ad libitum protein intake, and protein expenditure parameters, Fryxell's (Am. Nat., 1991, 138 , 478) model predicted that the daily rate of protein intake should be highest when bison feed in grasslands with 400–600 kg/ha. In burned grassland sites, where bison spend most of their time, availability of grass biomass ranged between 40 and 3650 kg/ha, bison selected foraging areas of roughly 690 kg/ha, close to the value for protein intake maximization predicted by the model. The seasonal net protein intake predicted for large grazers in this study suggest feeding in burned grassland can be more beneficial for nutrient uptake relative to unburned grassland as long as grass regrowth is possible. Foraging site selection for grass patches of low to intermediate biomass help explain patterns of uniform space use reported previously for large grazers in fire‐prone systems.     

Long-Term Consequences of Grazing and Burning on Northern Peatland Carbon Dynamics

Abstract Using a 50-year-old field experiment, we investigated the effects of the long-term land management practices of repeated burning and grazing on peatland vegetation and carbon dynamics (C). Plant community composition, C stocks in soils and vegetation, and C fluxes of CO₂, CH₄ and DOC, were measured over an 18-month period. We found that both burning and grazing reduced aboveground C stocks, and that burning reduced C stocks in the surface peat. Both burning and grazing strongly affected vegetation community composition, causing an increase in graminoids and a decrease in ericoid subshrubs and bryophytes relative to unburned and ungrazed controls; this effect was especially pronounced in burned treatments. Soil microbial properties were unaffected by grazing and showed minor responses to burning, in that the C:N ratio of the microbial biomass increased in burned relative to unburned treatments. Increases in the gross ecosystem CO₂ fluxes of respiration and photosynthesis were observed in burned and grazed treatments relative to controls. Here, the greatest effects were seen in the burning treatment, where the mean increase in gross fluxes over the experimental period was greater than 40%. Increases in gross CO₂ fluxes were greatest during the summer months, suggesting an interactive effect of land use and climate on ecosystem C cycling. Collectively, our results indicate that long-term management of peatland has marked effects on ecosystem C dynamics and CO₂ flux, which are primarily related to changes in vegetation community structure.     

Fire temperatures and postfire plant community dynamics in Ecuadorian grass páramo

Three aspects of the páramo vegetation's response to fires were investigated: the measurement of fire temperatures, general observations of changes in plant communities following fires, and monitoring the fate of individual plants after burning.Fire temperatures were strongly influenced by the physiognomy of the vegetation, dominated by tussocks of Calamagrostis spp. Temperatures were highest amongst the upper leaves of the tussock (sometimes >500°C). The middle levels of the tussock experienced temperatures in excess of 400°C, but in the dense leaf bases temperatures were often below 65°C. On the ground between tussocks, temperatures were variable, whereas 2 cm below ground temperatures failed to reach 65°C.Plant survival depended on the intensity of the fire and the plant's position within the tussock structure. Survival was often the result of high temperature avoidance (with buds shielded by other plant parts or buried beneath the soil surface).Post-fire Calamagrostis tiller mortality rates were high and tussock regrowth was slow. Some other species appear to maintain their populations by exploiting this recovery phase for seedling establishment on tussocks.Between tussocks, changes of occupancy at the level of the individual plants were greater after fire than in control vegetation. Most transitions were random. Those which departed from random often involved gaps and were related to post-fire mortality, regrowth from below-ground parts, colonisation or, in the case of a clonal mat-forming species, to spatial rearrangement of rosettes. Recovery was slower at higher altitude. Recovery was much slower in burned plots when the upper 2 cm of soil was removed (along with buried plant parts) compared with burned plots.Qualitative observations suggest that recovery may consist of a cyclical process, mediated by the serial dominance of several species that are physiognomically important.The frequency of fires determines the amount of fuel accumulated within grass tussocks and some plants may be unable to survive repeated burning. Chance survival of species in unburned patches of vegetation and random colonisation of gaps may be important determinants of subsequent community structure.     

Herbivory effects on saplings are influenced by nutrients and grass competition in a humid South African savanna

Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGR_L). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGR_D), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGR_L and sapling biomass of three mesic species. Positive effects of nutrients on RGR_L of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGR_L with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.     

Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

Effects of fire on bird diversity and abundance in an East African savanna

Fire is an important determinant of many aspects of savanna ecosystem structure and function. However, relatively little is known about the effects of fire on faunal biodiversity in savannas. We conducted a short‐term study to examine the effects of a replicated experimental burn on bird diversity and abundance in savanna habitat of central Kenya. Twenty‐two months after the burn, Shannon diversity of birds was 32% higher on plots that had been burned compared with paired control plots. We observed no significant effects of burning on total bird abundance or species richness. Several families of birds were found only on plots that had been burned; one species, the rattling cisticola (Cisticola chiniana), was found only on unburned plots. Shrub canopy area was negatively correlated with bird diversity on each plot, and highly correlated with grass height and the abundance of orthopterans. Our results suggest that the highest landscape‐level bird diversity might be obtained through a mosaic of burned and unburned patches. This is also most likely to approximate the historical state of bird diversity in this habitat, because patchy fires have been an important natural disturbance in tropical ecosystems for millennia.     

The effects of gap size and disturbance type on invasion of wet pine savanna by cogongrass, Imperata cylindrica (Poaceae)

Cogongrass is a nonindigenous species perceived to threaten native communities of the southeastern United States through modification of species composition and alteration of community processes. To examine how gap size and disturbance type influence the invasion of wet pine savannas by cogongrass, we performed three field experiments to evaluate the response of cogongrass seeds and transplanted seedlings to four different gap sizes, four types of site disturbance, and recent burning of savanna vegetation. Cogongrass germinated, survived, and grew in all gap sizes, from 0 to 100 cm in diameter. Similarly, disturbance type had no effect on germination or seedling and transplant survival. Tilling, however, significantly enhanced transplanted seedling growth, resulting in a tenfold increase in biomass over the other disturbance types. Seedling survival to 1 and 2 mo was greater in burned savanna than unburned savanna, although transplant survival and growth were not affected by burning. Results of this study suggest that cogongrass can germinate, survive, and grow in wet pine savanna communities regardless of gap size or type of disturbance, including burning. Burning of savanna vegetation may enhance establishment by improving early seedling survival, and soil disturbance can facilitate invasion of cogongrass by enhancing plant growth.     

Impact of hemiparasitic Rhinanthus angustifolius and R. minor on nitrogen availability in grasslands

Root hemiparasites like Rhinanthus angustifolius C.C. Gmel and R. minor L. have a potential to accelerate the restoration of semi-natural grasslands because they may decrease above-ground biomass of the vegetation. This, in turn, may be beneficial for species diversity. It is known that hemiparasites often accumulate high nutrient concentrations in their above-ground parts, resulting in high quality litter. Because of the short life cycle of many parasitic plants, litter is released early in the season and the main part is not removed from the grassland by hay-making. This has been shown to yield an increased nutrient availability locally. We performed an introduction experiment with R. angustifolius and R. minor in three semi-natural grasslands in Flanders (Belgium). In the second year after sowing, the above-ground nitrogen (N) content of the grasses and of the potential host vegetation (excluding the hemiparasite), was increased in the parasitized plots. The reduction of grass (and legume) above-ground biomass in parasitized plots resulted in a decrease in the total above-ground N uptake of grasses, host and total vegetation (ex- and including the parasite, respectively) of the parasitized plots compared to the control. Furthermore, with a tracer experiment (¹⁵N), we demonstrated that the N from the added tracer was relatively less available in parasitized plots, suggesting larger soil N pools in these treatments. This is probably the consequence of increased mineralization, resulting from the high-quality, parasitic litter. Further experiments should be conducted to investigate the impact of hemiparasitic Rhinanthus spp., e.g. on the availability of other nutrients such as phosphorus.