Invasion of maritime chaparral by the introduced succulentCarpobrotus edulis

Invasion by the alien succulent,Carpobrotus edulis, has become a common occurrence after fire in maritime chaparral in coastal California, USA. We studied post-burnCarpobrotus establishment in chaparral that lackedCarpobrotus plants before the fire and compared seedbank and field populations in adjacent burned and unburned stands.Carpobrotus seeds were abundant in deer scat and in the soil before burning. Burning did not enhance germination: many seeds were apparently killed by fire and seed bank cores taken after fire revealed no germinable seeds. Laboratory tests showed that temperatures over 105°C for five minutes killedCarpobrotus seeds. In a field experiment involving use of herbivore exclosures, we found that herbivory was an important source of mortality for seedlings in both burned and unburned chaparral. All seedlings, however, died outside of the burn regardless of the presence of cages. Establishment there is apparently limited by factors affecting plant physiology. In the burned area, seedlings that escaped herbivory grew very rapidly. Overall, it appears that herbivory limited seedling establishment in both burned and unburned sites but that the post-burn soil environment supportedCarpobrotus growth in excess of herbivore use, thus promoting establishment.     

Creosote growth rate and reproduction increase in postfire environments

Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7‐fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re‐establishment of native plant communities in this desert system. Additional research is needed to assess if repeat fires that are characteristic of invasive grass‐fire cycles may limit these benefits.     

Allocation of nitrogen to an inducible defense and seed production in Nicotiana attenuata

Resource-based tradeoffs in the allocation of a limiting resource are commonly invoked to explain negative correlations between growth and defense in plants, but critical examinations of these tradeoffs are lacking. To rigorously quantify tradeoffs in a common currency, we grew Nicotiana attenuata plants in individual hydroponic chambers, induced nicotine production by treating roots with methyl jasmonate (MJ) and standardized leaf puncturing, and used ¹⁵N to determine whether nitrogen-based tradeoffs among nicotine production, growth, and seed production could be detected. Plants were treated with a range of MJ quantities (5, 45 or 250 μg plant^?1) to effect a physiologically realistic range of changes in endogenous jasmonic acid levels and increases in nicotine production and accumulation; MJ treatments were applied to the roots to target JA-induced nicotine production, since nicotine biosynthesis is restricted to the roots. Leaf puncturing and 5 μg MJ treatments increased de novo nicotine synthesis and whole-plant (WP) nicotine pools by 93 and 66%, while 250 μg MJ treatments increased these values 3.1 and 2.5-fold. At these high rates of nicotine production, plants incorporated 5.7% of current nitrogen uptake and 6.0% of their WP nitrogen pools into nicotine. The ¹⁵N-labeled nicotine pools were stable or increased for the duration of vegetative growth, indicating that the N-nicotine was not metabolized and re-used for growth. Plants with elevated nicotine production grew more slowly and the differences in plant biomass gain between MJ-treated plants and controls were linearly related to the differences in nicotine accumulation. Despite the reductions in rosette-stage growth associated with nicotine production, estimates of lifetime fitness (cumulative lifetime seed production, mass/seed, seed viability) were not affected by any treatment. Only two treatments (leaf puncturing and 250 μg MJ) increased the allocations of ¹⁵N acquired at the time of induction to seed production. On average, plants used only 14.9% of their WP nitrogen pool for seed production, indicating that either the nitrogen requirements for seed production or the reproductive effort of these hydroponically-grown plants are low. To determine if seed production is strongly influenced by the amount of vegetative biomass attained before reproduction, the experiment was repeated with plants that had 44% of their leaf area (or 29% of their WP biomass) removed before MJ treatments with a removal technique that minimized the nicotine response. MJ treatments of these plants dramatically increased nicotine production and accumulation, but these plants also suffered no measurable fitness consequences from either the leaf removal or MJ treatments. We conclude that when N. attenuata plants are grown in these individual hydroponic chambers, their allocation to reproduction is sufficiently buffered to obscure the large increases in nitrogen allocations to an inducible defense. To determine whether soil-grown plants are similarly buffered, we grew two genotypes of plants in the high-nutrient soil from a 1-year-old burn in a piñyon-juniper forest (the plants' natural habitat) and in low-nutrient soil from an adjacent unburned area, and induced nicotine production in half of the plants with a 500 μg root MJ treatment. Plants grown in burned soils had an estimated lifetime fitness that was on average 2.8-fold greater than that of plants grown in unburned soils. MJ treatment reduced fitness estimates by 43% and 71% in the burned and unburned soils, respectively. We conclude that while hydroponic culture allows one to rigorously quantitate nitrogen allocation to growth, reproduction and defense, the allocation patterns of plants grown in hydroponic culture differ from those of plants grown in soil. Under hydroponic conditions, plants have low reproductive allocations and reproductive-defense tradeoffs are not detected. Reproductive-defense tradeoffs are readily discernible in soil-grown plants, but under these growing conditions, the nitrogen-basis for the tradeoff is difficult to quantify.     

不同火烈度火烧迹地内油松叶功能性状的变化

叶功能性状对林火的响应是林火生态领域的研究热点之一,研究火后油松叶功能性状变化能够揭示油松为适应火环境形成的生长策略,为促进油松火后恢复提供参考。以山西省沁源县火烧迹地内油松为研究对象,选择当年生叶片分析叶功能性状在不同火烈度(未过火、轻度火烧、中度火烧)火烧迹地间的变化规律,并研究不同火烧迹地内叶经济谱的变化特征。结果表明: 除氮磷比外,叶功能性状在不同火烈度的火烧迹地间存在显著差异,其中,叶面积的差异最为明显,是最敏感的性状。随火烧迹地内火烈度的增加,叶面积、叶厚度、叶干物质含量、叶氮含量和叶磷含量升高,比叶面积、叶有机碳含量降低。部分叶功能性状间存在显著的相关关系,但其相关性在不同火烈度的火烧迹地间存在差异。叶经济谱沿着“未过火-轻度火烧-中度火烧”的火烧迹地环境总体向“快速投资-收益型”的资源权衡策略移动,低烈度火烧迹地内油松的生长恢复会加快。     

Flower production in relation to individual plant age and leaf production among different patches of Corydalis intermedia

The life history of an organism can be viewed as the combination of allocations made to maintenance, growth, and reproduction. Allocation to these functions are constrained by trade-offs as increased investment to one function may happen at the expense of another. Moreover, because fecundity and survival probabilities are affected by both the state of an individual and by its surrounding environment, optimal allocation to reproduction and growth may vary with both individual size/age and with the habitat in which it lives. In this study we aim to describe how flower production varies with individual plant age and leaf production among different patches of the perennial herb Corydalis intermedia. We take advantage of the construction of the underground storage organ to estimate the age of individual plants which allows us tacitly to relate flower and leaf production to individual age and successional status of the patch. We sampled all individuals present in nine patches from the same forest and estimated their age, flower production and total leaf area. The age distributions showed that each patch was most often dominated by a few and consecutive age classes. In patches where individuals had the oldest mean age, very few or no juvenile age classes were found suggesting that recruitment had ceased. Based on the age distribution of the patches we propose that the dynamics may best be described as metapopulational with colonization of newly formed open forest gaps and a successionally determined extinction as the patch gradually becomes too shaded for recruitment. Both mean flower production, leaf area and age varied significantly among patches. Flower production increased with both increasing age and leaf area. We found no indication of a trade off between reproduction and vegetative growth since flower production showed a positive relation with leaf production even after removing the effect of age. Number of flowers produced by plants of the same age but growing in different patches did not vary indicating that the difference among patches mainly was due to a difference in age distribution. No individuals produced flowers before they reached an estimated age of three years. Production of flowers followed a power function with increasing age. Our data suggests that C. intermedia plants change their allocation strategy with age investing a relatively large amount of energy in flower production immediately after the immature growth phase when recruitment in their patch may be high. Production of flowers then reaches a plateau around the age of 11 years after which number of flowers produced stays constant. This revised version was published online in June 2006 with corrections to the Cover Date.     

The influence of fire on the phenological behaviour of Mediterranean plant species in Bas-Languedoc (southern France)

Fire is an ancient ecological factor influencing the Mediterranean vegetation of southern France. The study was carried out on three areas to determine the phenological behaviour of plants with regard to fire. First we studied the flowering responses of perennials in relation to the time since fire: in a Quercus coccifera garrigue most species flower during the year following burning. In comparing species by species between burned and unburned areas most species did not show major differences in the phenological stages. However, fire did increase the number of inflorescences of grasses. A phenological synthesis showed that differences at the community level existed for the flowering stages between the burned areas and the unburned control sites during the first and second years following fire. The growth of some woody species was also studied; the elongation and growth of the plants were biggest during the first or second year after fire. The lack of differences in phenological response between burned and unburned plants may be an adaptive trait to fire.     

Recruitment pattern of Rhus integrifolia populations in periods between fire in chaparral

Population structure, fruit production and dispersal, and recruitment of Rhus integrifolia, a gynodioecious sclerophy llous shrub living in coastal chaparral, were studied in two localities in southern California last burned 60 and 90 years ago. Though hermaphroditic plants produce some fruits, only the male-sterile plants bore significant numbers. Among the sample plants, one individual accounted for 50% of the crop measured. Terrestrial animals, through the loss of some seeds taken for consumption, play a decisive role in the seed dispersal, promoting the establishment of seedlings outside the parent's canopy. Birds have not been demonstrated to contribute to long distance dispersal, but they are responsible for dropping 25% of the fruits collected below the R. integrifolia canopies, 36%) of which were completely destroyed. Allelopathic effects were not observed in laboratory assays, and germination is probably not influenced by the species beneath which seeds have been deposited. This pattern can explain the non-aggregated distribution of individuals observed in the studied populations. We conclude that establishment of new individuals has been continuous at both localities and that the populations of R. integrifolia are increasing in these long unburned stands. This may be indicative of a successional trend in unburned chaparral and coastal scrub toward a sclerophy llous woodland.     

Comparative physiology of burned and unburned Rhus laurina after chaparral wildfire

Summary Laurel Sumac (Rhus laurina) is a dominant member of the coastal chaparral community of southern California that survives periodic burning by wildfires by resprouting from a lignotuber (root crown). We investigated the physiological basis for resprouting by comparing shoot elongation, leaf nitrogen content, tissue water status, leaf conductance to water vapor diffusion, and photosynthetic rates of post-fire R. laurina to those of adjacent unburned shrubs. Resprouts had higher rates of shoot elongation, leaf conductance, and photosynthesis than mature, unburned shrubs. Leaf nitrogen contents were elevated in burned shrubs even though their leaves developed interveinal chlorosis. A comparison of soil water potential to predawn water potential indicated that roots of R. laurina remain active below 2 m during the first summer drought after wildfire. Our results support the hypothesis that lignotubers not only contain dormant buds that develop into aerial shoots after wildfire but they also supply nutrient resources that enhance shoot elongation. Because R. laurina is relatively sensitive to drought, yet very successful in its rapid recovery after fire, maintaining an active root system after shoot removal may be the primary function of the massive lignotuber formed by this species.     

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.     

Barriers to shrub reestablishment following fire in the seasonal submontane zone of Hawai'i

Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas.     

Structural determinants of increased susceptibility to dehydration‐induced cavitation in post‐fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem‐specific hydraulic conductivity (K_s) and resistance to cavitation (P₅₀) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co‐occurring post‐fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K_s. Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter‐vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.     

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.     

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.     

Fire increases aboveground biomass,seed production and recruitment success of Molinia caerulea in dry heathland

During the last decades, the perennial tussock grass Molinia caerulea has shown an increased abundance in European heathlands, most likely as a result of increased nitrogen deposition and altered management schemes. Because of its deciduous nature, Molinia produces large amounts of litter each year, which may affect the intensity and frequency of accidental fires in heathlands. These fires may influence plant population dynamics and heathland community organization through their effects on plant vital attributes and competitive interactions. In this study, fire-induced changes in competitive ability and invasiveness of Molinia through changes in biomass production, seed set and seed germination under both natural and laboratory conditions were investigated. We found that fire significantly increased aboveground biomass, seed set and germination of Molinia. Seed set was twice as high in burned compared to unburned heathland. Two years after fire, seedling densities in natural conditions were on average six times higher in burned than in unburned heathland, which resulted in increased abundance of Molinia after burning. The seed germination experiment indicated that seeds harvested from plants in burned heathland showed higher germination rates than those from unburned heathland. Hence, our results clearly demonstrate increased invasive spread of Molinia after large and intense fires. Active management guidelines are required to prevent further encroachment of Molinia and to lower the probability of large fires altering the heathland community in the future.     

Contrasting effects of fire frequency on plant traits of three dominant perennial herbs from Chaco Serrano

Fire frequencies are currently increasing in many regions across the world as a result of anthropic activities, affecting ecological processes and plant population dynamics. Fire can generate important changes in soil properties, altering nutrient dynamics and thereby plant growth. Here we analyse fire frequency effects on soil quality and plant traits of three native perennial herbaceous plants (Cologania broussonetii, Desmodium uncinatum and Rhynchosia edulis; Fabaceae) with the capacity for biological N₂ fixation that resprouts and are abundant after fire in Chaco Serrano forests. Based on 22‐year fire history, we assessed plant traits in sites with low and high fire frequencies along with unburned scenarios. We found significantly lower water content, nitrates and electrical conductivity in frequently burned soils. As a result, the three species showed consistently lower leaf area and specific leaf area in both fire frequencies, implying lower growth rates in comparison to unburned sites. However, total leaf biomass was not affected by fire, leaf phosphorus concentration was higher in R. edulis in high fire frequency and leaf N concentration was twice as large in plants growing in sites of high fire frequency in C. broussonetii and R. edulis. Such an increase in N and phosphorus concentrations is likely a result of both their conservative use of resources and their biological N₂ fixation capacity. To our knowledge, this is the first record of such contrasting fire effects observed consistently in three co‐occurring species: while plant growth decreased with fire frequency, leaf nutritional traits remain unchanged or increased in frequently burned sites. Quality‐depleted and drier soils that result from increased fire frequencies may not only affect trait variation at the intraspecific level but can also drive to a homogenization of the plant community, selecting species with particular combinations of morphological and functional traits.     

Frequent fires do not affect sexual expression and reproduction in Vachellia caven

In environments with high fire frequency the impoverishment of abiotic resources may favour male sexual expression in plants as it is less costly than female expression. Also, fire can modify pollinator communities and thus affect plant reproduction. Here we evaluate the effect of frequent fires on sexual expression, pollination and reproductive success of Vachellia caven (Leguminosae), an andromonoecious tree that is highly dependent on animal pollination and is abundant in burned sites. We expect that increased fire frequency will favour maleness but it will decrease reproductive success due to abiotic resource depletion in repeated burned sites. To test this, we selected focal plants in three unburned sites and three frequently burned sites and measured their sexual expression, basal diameter, pollination and fruit set. The proportion of male inflorescences per plant was not affected by fire and it was negatively related with the diameter of the plant. The proportion of pollinated flowers was not affected by fire, and fruit set increased with maleness only in frequently burned sites. These results indicate that V. caven is adapted to regimes of high fire frequency: not only was there similar fruit set in both burned and unburned sites, but more male plants had higher fruit set in burned sites. Despite the soil impoverishment triggered by repeated fires, V. caven is able to maintain its sexual and reproductive functions, allowing it to persist and maintain viable populations in fire‐prone environments. Abstract in Spanish is available with online material.     

Habitat Use and Selection by California Spotted Owls in a Postfire Landscape

ABSTRACT Forest fire is often considered a primary threat to California spotted owls (Strix occidentalis occidentalis) because fire has the potential to rapidly alter owl habitat. We examined effects of fire on 7 radiomarked California spotted owls from 4 territories by quantifying use of habitat for nesting, roosting, and foraging according to severity of burn in and near a 610-km²fire in the southern Sierra Nevada, California, USA, 4 years after fire. Three nests were located in mixed-conifer forests, 2 in areas of moderate-severity burn, and one in an area of low-severity burn, and one nest was located in an unburned area of mixed-conifer-hardwood forest. For roosting during the breeding season, spotted owls selected low-severity burned forest and avoided moderate- and high-severity burned areas; unburned forest was used in proportion with availability. Within 1 km of the center of their foraging areas, spotted owls selected all severities of burned forest and avoided unburned forest. Beyond 1.5 km, there were no discernable differences in use patterns among burn severities. Most owls foraged in high-severity burned forest more than in all other burn categories; high-severity burned forests had greater basal area of snags and higher shrub and herbaceous cover, parameters thought to be associated with increased abundance or accessibility of prey. We recommend that burned forests within 1.5 km of nests or roosts of California spotted owls not be salvage-logged until long-term effects of fire on spotted owls and their prey are understood more fully.     

Effects of Prescribed Fires on Young Valley Oak Trees at a Research Restoration Site in the Central Valley of California

Woodland restoration sites planted with Quercus lobata (valley oak) often have serious invasions of nonnative annual grasses and thistles. Although prescribed fire can effectively control these exotics, restoration managers may be reluctant to use fire if it causes substantial mortality of recently planted saplings. We studied the effects of prescribed fires on the survival and subsequent growth of 5‐ and 6‐year‐old valley oak saplings at a research field near Davis, California. One set of blocks was burned in summer 2003 at a time that would control yellow star thistle, a second set of blocks was burned in spring 2004 at a time that would control annual grasses, and a third set was left unburned. Very few oaks died as a result of either fire (3–4%). Although a large proportion was top‐killed (66–72%), virtually all these were coppiced and most saplings over 300 cm tall escaped top‐kill. Tree height, fire temperature, and understory biomass were all predictive of the severity of sapling response to fire. Although the mean sapling height was initially reduced by the fires, the growth rates of burned saplings significantly exceeded the growth rates of unburned control trees for 2 years following the fires. By 2–3 years after the fires, the mean height of spring‐ and summer‐burned saplings was similar to that of the unburned control saplings. The presence of valley oak saplings does not appear to preclude the use of a single prescribed burn to control understory invasives, particularly if saplings are over 300 cm tall.     

Can salvage logging affect seed dispersal by birds into burned forests?

The recovery of vegetation in Mediterranean ecosystems after wildfire is mostly a result of direct regeneration, since the same species existing before the fire regenerate on-site by seeding or resprouting. However, the possibility of plant colonization by dispersal of seeds from unburned areas remains poorly studied. We addressed the role of the frugivorous, bird-dependent seed dispersal (seed rain) of fleshy-fruited plants in a burned and managed forest in the second winter after a fire, before on-site fruit production had begun. We also assessed the effect on seed rain of different microhabitats resulting from salvage logging (erosion barriers, standing snags, open areas), as well as the microhabitats of unlogged patches and an unburned control forest, taking account of the importance of perches as seed rain sites. We found considerable seed rain by birds in the burned area. Seeds, mostly from Olive trees Olea europaea and Evergreen pistaches Pistacia lentiscus, belonged to plants fruiting only in surrounding unburned areas. Seed rain was heterogeneous, and depended on microhabitat, with the highest seed density in the unburned control forest but closely followed by the wood piles of erosion barriers. In contrast, very low densities were found under perches of standing snags. Furthermore, frugivorous bird richness seemed to be higher in the erosion barriers than elsewhere. Our results highlight the importance of this specific post-fire management in bird-dependent seed rain and also may suggest a consequent heterogeneous distribution of fleshy-fruited plants in burned and managed areas. However, there needs to be more study of the establishment success of dispersed seeds before an accurate assessment can be made of the role of bird-mediated seed dispersal in post-fire regeneration.     

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.