The interaction between Cistaceae and a highly specific seed-harvester ant in a Mediterranean scrubland

We studied the interaction between the ant Goniomma kugleri and Cistaceae in a Cistus ladanifer -dominated scrubland, in southwestern Spain. We monitored seed harvesting, and studied ant preferences among Cistaceae seeds and their capture efficiencies for preferred seeds. For the stand of C. ladanifer , we estimated seed losses due to the ants. Harvesting was restricted to two seasons: mid-autumn to late winter, and late spring. Ant diet relied on Cistaceae seeds: during autumn and winter 90% of seeds returned to nests were of C. ladanifer , and the remaining fraction also comprised Cistaceae seeds. At this time, the ants harvested seeds directly from the plants. In late spring, the ant diet consisted of Tuberaria guttata s.l. seeds. Goniomma kugleri selectively collected Cistaceae seeds. For preferred species, seed removal rates at the colony level and seed capture times invested by individual workers were correlated with seed size. Because of shorter capture time and higher success frequency, capture efficiency in terms of number of seeds captured per unit time was higher for small-seeded species. Although each ant colony collected large numbers (up to 10⁵) of C. ladanifer seeds over the autumn–winter season, the impact of ant removal on the annual seed output was moderate, at around 20%. It is likely that, in C. ladanifer , the staggered seed release period, and the pulsed exposure of seed clumps in capsules through progressive locule dehiscence, effectively minimise seed losses to the ants.     

Fire and summer temperatures interact to shape seed dormancy thresholds

Background and AimsIn Mediterranean ecosystems, the heat shock of wildfire disrupts physical seed dormancy in many plant species. This triggers germination in the post-fire environment where seedling establishment is optimal due to decreased competition and increased resource availability. However, to maintain the soil seed bank until a fire occurs, the minimum heat capable of breaking seed dormancy (i.e. the lower heat threshold) must be above the maximum temperatures typically observed in the soil during the summer. We therefore hypothesized that summer temperatures have shaped heat requirements for physical dormancy release. Specifically, we predicted that seeds from populations growing under warmer summers will have higher values of the lower heat threshold.MethodsTo evaluate this prediction, we collected seeds from two Cistus species in 31 populations (20 Cistus albidus and 11 Cistus salviifolius) along a climate gradient of summer temperatures on the eastern coast of Spain. For each population, seeds were treated to 10 min heat shocks, from 30 to 120 °C in 5 °C increments (19 treatments), to simulate increasing heat doses from summer to fire-related temperatures. Seeds were then germinated in the lab.Key ResultsFor all populations, maximum germination was observed when applying temperatures associated with fire. Lower heat thresholds varied among populations, with a positive relationship between summer temperatures at seed population origin and the heat dose required to break dormancy.ConclusionsOur results suggest that fire drives maximum dormancy release for successful post-fire germination, while summer temperatures determine lower heat thresholds for ensuring inter-fire seed bank persistence. Significant among-population variation of thresholds also suggests that post-fire seeder species have some potential to modify their dormancy release requirements in response to changing climate.     

Germination of Cistus ladanifer seeds in relation to parent material

Variability in seed germination behaviour of Cistus ladanifer L. (rockrose), a Cistaceous species widely distributed in the Mediterranean Basin, was studied in a central Spanish population under controlled conditions. No correlation between seed moisture content and germination was found. Great variability in germination was found among seeds of the population studied, not only between seeds belonging to different mother plants, but also between those collected from different capsules on the same plant. In most cases, seeds preheated at 100 °C for 30 minutes showed a significant increase in germination. This germination behaviour is related to fire regimes as this plant is a typical shrubby element of the mediterranean shrublands. Percent germination did not vary significantly after several months of seed storage at room temperature. In the same way, no difference was found in final germination percentage of seeds stored under room temperature vs. seeds stored under cold conditions.     

Ecophysiology of fire-stimulated seed germination in Cistus incanus ssp. creticus (L.) Hey wood and C. salvifolius L.

Abstract. Seed germination characteristics were investigated in the most common Cistus species in Greece, namely C. incanus ssp. creticus and C. salvifolius. In addition to the soft seed subpopulation, both species produce a large fraction of hardcoated, water-impermeable seeds which can be softened and, thus, promoted to germinate by mechanical scarification and thermal pretreatment. Temperature and light control of seed germination are unimportant. In the ecological context of the Mediterranean ecosystems, the eventually advantageous, opportunistic strategy of germination is based on: (a) seed heterospermy (which allows the smaller, softcoated fraction to germinate promptly each year while the majority of the seeds, the hard ones, accumulate in the soil); (b) the seed population heterogeneity in relation to coat hardness (so that any heat conditions produced by fire induce the softening and germination of a certain seed fraction); (c) the notably low germination rate (which suppresses premature germination); (d) the wide, Mediterranean-type (relatively cool), temperature range of germination (while higher temperatures simply inhibit but do not induce any dormancy); and (c) the apparent lack of photo-sensitivity (enabling germination under every light regime). In non-fire years, the temporal distribution of field germination and seedling appearance might be partly determined by the seed dispersal strategy of the individual Cistus species. Nevertheless, the post-fire regeneration response is manifested in the form of a huge wave of germination (of practically all seeds softened by the fire heat), shortly after the onset of the rainy season.     

Influence of nutrient availability and tree wildling density on nutrient uptake by Oxalis acetosella and Acer saccharum

The different weight-number strategies of seed production displayed by individuals of a Mediterranean fire-prone plant species (Cistus ladanifer) were investigated in relation to seed germination responses to pre-germination heating. A control (no heating), a high temperature during a short exposure time (100 degrees C during 5 min) and a high temperature during a long exposure time (100 degrees C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Seed germination was related to heat treatments and seed mass. Seed heating increased the proportion of seeds germinating compared with the control treatment. Mean seed weight was positively correlated to the proportion of germinated seeds but only within heat treatments. These results suggest that in periods without fire, the relative contributions to the population dynamics are equal for all seeds, regardless of their mass, whereas heavier seeds would be the main contribution after wildfire events. Since lighter seeds can be produced in higher quantities than heavier ones within a given fruit, the number of seedlings produced per fruit depended strongly on the germination conditions. In the absence of wildfire, fruits producing lighter seeds gave rise to more seedlings; nevertheless, they were numerically exceeded by those producing heavy seeds after a wildfire. The implications of these results are discussed in relation to their consequences on the population dynamics of this species, considering also additional information on stand flammability and changes in seed mass with plant age.     

Effect of fire on hard-coated Cistaceae seed banks and its influence on techniques for quantifying seed banks

The impact of fire on hard-coated Cistaceae (Halimium ocymoides, Cistus ladanifer, and C. salvifolius) soil seed banks in a Mediterranean 'maquis' shrubland, and its effect on seed germinability were studied. The study also contrasts the effectiveness of two widely used techniques for quantifying seed banks, the seedling emergence and the physical separation methods, in relation to fire. The null hypothesis that a massive enhancement of physically-dormant Cistaceae seed germination by fire would make use of the time-consuming physical separation technique unnecessary was tested. Fire reduced Cistaceae seed banks in the 0–2 cm deep soil layer by both seed fire-consumption and lethal temperatures, revealed by the significant decreasing of the seed bank density and by the increase of apparently-intact but soft-unviable seeds, respectively. In contrast, no damage was recorded in the 2–5 cm soil layer. A dramatic seed bank depletion (> 90%) in both soil layers was recorded one year after fire in the burnt area, coinciding with a significant increase of seedling density confined to the first post-fire year. The ecological consequences of this massive post-fire seed bank input are discussed. A germinability test revealed that germination of surviving Cistaceae seeds was significantly enhanced in all cases except for the C. salvifolious seed bank in the deeper soil layer. However, final germination levels (60–75%) did not correspond to the magnitude of seed bank depletion, especially for C. salvifolious, which suggests that other environmental factors not exclusively associated with fire may also be important in softening Cistaceae seeds. Germination enhancement by fire soil-heating was not high enough to reject the physical separation technique, at least in the deeper soil layer. The simultaneous use of both seedling emergence and physical separation is recommended for reliable seed bank estimates when a physically-dormant hard-seeded component can be expected in the soil, as in many Mediterranean ecosystems, regardless of fire occurrence.     

Seed bank dynamics of two obligate seeders, <Emphasis Type="Italic">Cistus monspeliensis</Emphasis> and <Emphasis Type="Italic">Rosmarinus officinalis</Emphasis>, in relation to time since fire

Many species in Mediterranean-type ecosystems regenerate after fire by seed germination from soil seed banks. Seed bank dynamics of two of those obligate seeders, Cistus monspeliensis and Rosmarinus officinalis, were investigated in relation to stand age since fire in southwestern Portugal. Soil seed density, annual seed input, annual seed losses through germination and seed persistence were compared between species at stands differing in age since fire (5, 10 and 35 years). Soil seed density and seed input increased over the first decade after fire and were lowest at 35-year-old stands for C. monspeliensis. In R. officinalis, few seeds were produced and found in the soil at early stages, and maximum seed input and soil seed density were attained at 35-year-old stands. Soil seed density was mostly driven by seed production in both species, which is largely dependent on plant traits and population dynamics related to fire. Overall, stand age since fire had a negligible effect on seed germination, seed persistence and viability. Ten to 39% of buried seeds were not recovered after 1 year, and viability of seeds recovered was 97–100% for C. monspeliensis and only 0–3% for R. officinalis. Variation in plant traits within the seeder syndrome was evidenced by this study. R. officinalis evidenced lower seed persistence, lower proportion of viable seed produced and lower density of viable soil seed than C. monspeliensis at any stage after fire. R. officinalis is expected to depend largely on previous year seed production for population replacement after fire.     

Fire-type and forestry management effects on the early postfire vegetation dynamics of a Pinus pinaster woodland

The objective of this research was to study the effects of type of fire, prefire-, and postfire-management on the postfire vegetation dynamics of a Pinus pinaster woodland in Central Spain, burned at 15 yr of age. The effects of type of fire (crown-, or surface-fire), prefire-management (thinning out of trees and clearing of brush or no such actions) and postfire-management (removal of burned trees one year after the fire or no such action) on the postfire vegetation were studied during the first three years after the fire. Herbaceous plant abundance, species richness, and diversity, as well as abundance, growth and density of the dominant shrub species (Cistus ladanifer) were measured during the first three years after the fire. Our results show that the effects of the type of fire on the vegetation were minimal. Prefire-management effects were significant on the abundance of herbaceous species, mainly during the second and third year after fire, in particular for the Leguminosae species. Prefire managed areas were more diverse in species, and produced higher plant biomass than unmanaged areas. Postfire-management effects on the shrubs and herbs were minimal, except for the Leguminosae, which increased their cover where the trees had been removed. Plant dynamics were marked by the interaction between prefire-management and fire-type through the dynamics of the shrub cover. On most occasions, plots that resulted in lower cover of C. ladanifer had greater abundance of herbaceous plants and, in particular, of the Leguminosae. In general, our results show that irrespective of fire-type, prefire-, or postfire-management all areas tended to be very similar in their vegetation three years after the fire.     

Functional features and ontogenetic changes in reproductive allocation and partitioning strategies of plant modules

In the concept of modularity, plant modules are considered as iterative units and their changes are analyzed in terms of number or size. This paper, however, analyses changes with respect to the reproductive functional performance of modules and individual plant age. Patterns of resource allocation and partitioning in reproductive modules (fruits) are compared between two different age groups of a bushy perennial plant, Cistus ladanifer.Although modules do not differ in their allocation strategies (young plant modules produce the same seed and packing/protective structure biomass as old plant modules), their partitioning strategies change with plant age: young plant modules produce a larger number of lighter seeds than old plants. These differences have a direct consequence on the plant pre-dispersal fitness, which is not counteracted by insect predation on reproductive modules. These results are empirical evidence of a differentiation in the performance of reproductive modules with the ontogenetic development of this plant species. We think that the consideration of such kind of changes in module features is essential in the analysis of the iterative construction of plants.     

The influence of dominant shrubs,fire, and time since fire on soil seed banks in mixed chaparral

The composition and density of soil seed banks beneath co-occurring Adenostoma fasciculatum and Ceanothus greggii shrubs from three chaparral stands last burned 9, 35 and 85 years before 1986 were investigated. The overall density of seeds in the soil, as estimated by germinations under greenhouse conditions, increased with time since fire (ca. 8000 to 25000/m²). However, this increase was due entirely to the accumulation of A. fasciculatum seed in the soil (ca. 2000 to 21000/m²). In contrast, the density of C. greggii seed was different in each of the three stands, but was not correlated with time since fire: maximum densities were recorded from the 35 year old stand (ca. 2000/m²).A total of 31 taxa germinated and 17 occurred in sufficient numbers to be analyzed statistically. Germinable seed densities of three herb species were not influenced by soil source (beneath A. fasciculatum or C. greggii), time since fire, or the direct effects of a controlled fire treatment. Germinable seed densities of a further nine species were significantly influenced by the elapsed time since stands last burned. The densities of four decreased and five increased. Four of the species that increased in seed density over the three stands were annuals, suggesting that the chaparral sub-canopy habitat is not as unfavorable for annuals as is often assumed. The fire treatment decreased germinable seed densities of four annual species by 40–70%, but increased the germinable seed densities of the shrubs A. fasciculatum and C. greggii, and the annual Phacelia brachyloba. Our results indicate that seeds of A. fasciculatum will increase in the soil bank for at least 85 years after fire in chaparral where it is dominant. In contrast, seed reserves of C. greggii appear to be influenced primarily by site-specific patterns of seed production and by the intensity of post-dispersal seed predation.     

The role of seed traits as segregation factors of hybrids in wild populations of Cistus (Cistaceae)

Hybridization has been postulated as a main speciation mechanism in Cistus, a hard-seeded group of plants widely distributed in fire-prone Mediterranean shrublands. However, ecological evidence about how the segregation of hybrids from parents might occur is nonexistent. The presence of two hybrid populations in Morocco and Spain from Cistus albidus and Cistus heterophyllus led us to investigate their seed trait differences (seed size, mass and coat thickness) with the parentals and their seed germinability. A sowing experiment to compare the seedling emergence ability and seedling fitness of hybrids and their parentals was also carried out. Hybrid seeds germinated in a similar proportion to those of the parents. The hybrid seeds were significantly lighter, smaller and had thinner seed coats in both populations. However, these seed trait differences did not affect the seedling fitness of hybrids. These results were interpreted as a successful shift in the regeneration niche of the hybrids due to a greater proportion of soft-coated seeds, able to emerge from the soil seed bank in absence of fire. Smaller seeds also would favour further and deeper seed dispersal to escape from a fire. Monitoring of the seed yield of the Spanish population showed a competitive ability of hybrids.     

Seed size and seed germination in the Mediterranean fire-prone shrub <Emphasis Type="Italic">Cistus </Emphasis><Emphasis Type="Italic">ladanifer</Emphasis>

The germination response of different sized seeds from individuals of a Mediterranean fire-prone shrub (Cistus ladanifer) was investigated in relation to pre-germination heating. A control (no heating), a low temperature during a short exposure time (50°C during 5 min), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, low intensity pasture fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Mean seed weight only showed a marginally significant positive correlation with the proportion of germinated seeds whatever the pre-germination treatment. These results suggest that seed dormancy is unrelated to seed size and that under the experimental conditions used in this study, the effect of seed size on seed germination is low. Nevertheless, larger seeds could be favoured in natural conditions, especially under the high competition scenario which arise after wildfires. Control seeds showed a negative correlation between seed size and germination velocity suggesting that lighter seeds could take advantage from early germination in recruitment events in the absence of wildfires. Nevertheless, even the lower pre-germination heating treatment turns this correlation in not significant, suggesting a strong selection pressure (unrelated to seed size) for early germination after fire events. In our study, different sized seeds of C. ladanifer seem to perform better under different germination scenarios suggesting that seed size variation could be maintained by the alternation of recruitments without wildfires and recruitments after wildfire events.     

Enhanced UV-B radiation increases the reproductive effort in the Mediterranean shrub Cistus creticus under field conditions

Seedlings of the Mediterranean shrub Cistus creticus L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras, 38.3°W, 29.1°E) for 20 months. During this period, measurements of photosynthetic capacity, photochemical efficiency of PS II, chlorophylls and carotenoids were performed once per season. Supplemental UV-B radiation had no significant effect on these parameters nor on the total, above ground biomass accumulation, plant height and leaf specific mass measured at plant harvest. It was observed, however, that UV-B supplementation increased the number of seeds per fruit as well as mean individual seed mass. As a result, seed number and total seed mass per plant were considerably increased. Germination rates of produced seeds were not affected. We may conclude that C. creticus is a UV-B resistant plant whose competitive ability may be improved by enhanced UV-B radiation through an increase in its reproductive effort and a higher contribution to the seed bank.     

Dispersal and recruitment dynamics in the fleshy‐fruited Persoonia lanceolata (Proteaceae)

Question: What is the role of dispersal, persistent soil seed banks and seedling recruitment in population persistence of fleshy‐fruited obligate seeding plant species in fire‐prone habitats? Location: Southeastern Australia. Methods: We used a long‐term study of a shrubby, fleshy‐fruited Persoonia species (Proteaceae) to examine (1) seed removal from beneath the canopy of adult plants; (2) seedling recruitment after fire; (3) the magnitude and location of the residual soil seed bank; and (4) the implications for fire management of obligate seeding species. We used demographic sampling techniques combined with Generalised Linear Modelling and regression to quantify population changes over time. Results: Most of the mature fruits (90%) on the ground below the canopy of plants were removed by Wallabia bicolor (Swamp wallaby) with 88% of seeds extracted from W. bicolor scats viable and dormant. Wallabies play an important role in moving seeds away from parent plants. Their role in occasional long distance dispersal events remains unknown. We detected almost no seed predation in situ under canopies (< 1%). Seedling recruitment was cued to fire, with post‐fire seedling densities 6‐7 times pre‐fire adult densities. After fire, a residual soil seed bank was present, as many seeds (77‐100%) remained dormant and viable at a soil depth where successful future seedling emergence is possible (0‐5 cm). Seedling survival was high (> 80%) with most mortality within 2 years of emergence. Plant growth averaged 17 cm per year. The primary juvenile period of plants was 7–8 years, within the period of likely return fire intervals in the study area. We predicted that the study population increased some five‐fold after the wildfire at the site. Conclusions: Residual soil seed banks are important, especially in species with long primary juvenile periods, to buffer the populations against the impact of a second fire occurring before the seed bank is replenished.     

Effects of soil disturbance,fire and litter accumulation on the establishment of Cistus clusii seedlings

Cistus clusii is an early successional shrub, the most drought-resistant species of Cistus which colonises perturbed areas in the southern Iberian Peninsula and regenerates entirely from seeds after a fire. Some of the factors controlling germination and seedling establishment were examined in a field experiment in which we tested the effect of soil disturbance, fire, and litter accumulation on seedling emergence. In a first experiment, soil disturbance and fire were applied to plots in the field with a factorial design in April 1992. In a second experiment, we measured the effect of adding C. clusii litter. The number of seedlings, annual plants and grasses, and the coverage of mosses were recorded in 1993 and 1994.Soil disturbance increased the number of C. clusii seedlings almost seven-fold but did not change the number of grasses, annual plants nor moss cover. Fire significantly increased the number of C. clusii seedlings only in combination with soil disturbance, and it did not affect the presence of annuals, grasses nor moss cover. Litter increased the number of C. clusii seedlings in the following two years.Overall, soil disturbance as a single factor had the most significant effect on seedling emergence, though plots disturbed and treated with fire had the highest number of seedlings, implying that germination of C. clusii seeds was enhanced by processes that alter the hard seed coat and break the physical dormancy imposed by the testa. In addition, recruitment of C. clusii was dependent on rainfall, as drought significantly reduced the number of seedlings appearing in winter.     

Impact of the non-indigenous shrub species Spartium junceum (Fabaceae) on native vegetation in central Spain

Aims The introduction of potentially invasive species through ornamental cultivation or for rehabilitation purposes is a serious environmental problem. They cause damage to biodiversity through loss, increased mortality or ' in situ ' selection phenomena in natural flora. Spartium junceum is a Mediterranean shrub that is not native in most areas of the Iberian Peninsula, although it is extensively grown for the rehabilitation of roadsides. We have investigated the effect on the native vegetation of an old S. junceum (Fabaceae) plantation in a conservation area in the centre of the Iberian Peninsula: the Cuenca Alta del Manzanares nature reserve in Madrid.Methods We compared S. junceum stands with the native nanophanerophytic Cistus ladanifer community at different ecosystem properties: soil properties, temporal soil seed bank contents, standing vegetation and net primary production of annual grasslands growing in these shrublands.Important findings The results highlighted S. junceum 's ability to become established in the new environment (marginal areas of the nature reserve) and ensure its successful growth. This is more apparent in northern and eastern exposures where this formation contacts with the core of the best conserved native vegetation in the nature reserve. Soils under Spartium showed a higher nitrogen content, indicating its capacity—in common with other legumes—to fix N, and conferring an advantage over Cistus, which is N-limited. Other soil nutrients such as phosphorus, magnesium and calcium and water availability are also higher in Spartium soils than in Cistus. Phosphorus is usually a constraint for N-fixers, but in our study, it is the most significant soil variable in both shrub formations and is important to the success of Spartium. Water availability is a key factor for Mediterranean vegetation, and particularly in autumn when soils are recharged. The Spartium formation is able to retain water as its growth produces a closer canopy than Cistus, thereby preventing water evaporation and contributing to the success of this species. Perennials are more frequent in the standing vegetation than in the seed bank, whereas therophytes are similar. The standing vegetation has therophytes and chamaephytes as the predominant growth forms in Spartium sites, and hemicryptophytes and phanerophytes in Cistus. Therophytes are dominant in Spartium and Cistus seed banks, although the first formation has more species. Spartium has a higher number of hemicryptophytes and Cistus is higher in phanerophytes. Northern and eastern aspects show significant differences in richness—with a predominance of annual weed species in Spartium —and in above-ground net primary production, probably as a consequence of the nutrients present in the soils. Ruderal annual species under Spartium (Bromus tectorum, Chenopodium album) have higher germination rates in the greenhouse than in the standing vegetation, suggesting they are at potential risk if environmental conditions change.     

Time of germination and establishment success after fire of three obligate seeders in a Mediterranean shrubland of central Spain

Aim Determining how differences in time of germination can affect plant establishment in plant communities that, after a disturbance, must reestablish from seeds under climatic conditions subject to extremes, such as the Mediterranean. Although early germination may be beneficial for survival in summer, when drought is severe, this may expose the seedlings to winter extremes, thus to higher mortality. Understanding how sensitive is the establishment of different species to temporal patterns of germination will help to understand the factors that control species distribution and community stability in disturbance‐prone environments, as well as its sensitivity to changes in weather patterns as climate changes. Methods An experimental fire was made in early fall in an old Cistus–Erica shrubland in Toledo (central Spain). After fire, germination, survival and growth of the three dominant seeder species (Cistus ladanifer, Erica umbellata and Rosmarinus officinalis) were monitored during the first 3 years after fire. Seedlings were tagged to identify their time of emergence, and divided into cohorts according to their month of germination. Differences in survival of the various cohorts were evaluated by means of a Wilcoxon (Gehan) statistic. Height of surviving, tagged plants was compared among cohorts by means of a Kolmogorov–Smirnov test. Results The year following fire was one of the driest on record, while the next one was one of the wettest. Germination was more abundant during the first than during the second year. Establishment was mainly from first‐year germination, as the majority of second‐year germinated seedlings died. Temporal patterns of germination within a year and between years varied between species. Seedling mortality was highest immediately following germination, not in summer. Mortality was related to time of germination: during a given period of time, the mortality of younger seedlings was higher than that of older ones. However, survival was not highest for the first cohorts. In general, the earlier the seedlings germinated the more vigourous they became, more clearly so for Cistus than for Rosmarinus, but differences tended to disappear with time. Overall, time of germination varied between species and affected differently seedling survival and vigour of the various species. Rosmarinus and Cistus had sufficient survivors to reestablish the initial population. Erica, despite abundant germination, suffered a strong population reduction. Main conclusions Mediterranean shrub species differ in their temporal patterns of germination and survival after fire. The effect of time of germination is complex: germinating early is advantageous since old seedlings fared better than younger ones when confronted with the same rigours. However, germinating early might expose the seedlings to greater hazards and the first cohort might not survive best. The temporal window for establishment is narrow and mainly restricted to the first year after fire. Second year seedlings, irrespective of most favourable conditions, survived very little. Missing the window of establishment might virtually lead to a population collapse, despite having very high germination, as found for Erica.     

The coupling of recruitment and disturbance by fire in two resprouting Proteaceae species

Recruitment in plant populations is often tightly coupled to major disturbances such as fires. For species with persistent seed banks, fire-related cues may allow or enhance germination. The litter layer influences germination and may modify the impact of seed predators on seeds and seedlings. The litter layer is obviously affected by fire, providing one mechanism by which disturbance can determine recruitment. We tested the role of litter in the disturbance–recruitment coupling of two species with contrasting seed release timing after fire—Banksia serrata (canopy seed bank) and Telopea speciosissima (transient seed bank) by planting their seeds both early and late in the post-fire recruitment period (PRP) and manipulating litter density in orthogonal treatments. Vertebrate seed predators were excluded. Both species established more seedlings late in the PRP, although results were strongly influenced by very poor establishment at one site. Invertebrate seed predators consumed more T. speciosissima seeds in sites early (69.5%) than late in the PRP (51.2%), while consumption of B. serrata seeds was lower overall and comparable across sites (average 47.3%). Surprisingly, litter had very little effect on establishment and none on invertebrate seed predation, suggesting that other factors are more important. Recruitment was only loosely coupled to disturbance for the canopy seed bank species; for the transient seed bank species, the coupling was tighter but separated in time from the disturbance. Understanding both the strength and temporal aspects of the disturbance–recruitment coupling is necessary for appropriate management of plant functional diversity in fire-prone habitats.     

Harvester ant foraging and plant species distribution in annual grassland

Summary The harvester antVeromessor andrei Mayr is a major seed predator on annual grassland growing on serpentine soil at Jasper Ridge, N. California. Ants forage intensively during morning and evening sessions in areas surrounding nests. Activity is at its most intense in early summer, coinciding with peak seed release for most annual plant species. Ants show strong preferences for seeds of non-dominant species, notablyMicroseris douglasii (DC.) Sch.-Bip., but preferences alter over the season in response to seed availability. Seeds of the dominant annual species,Lasthenia californica DC ex Lindley are not foraged until later in the summer when seeds of other species are less abundant.Seedling densities and species compositions on ant nests differ markedly from surrounding areas with species relative abundances being similar to those found on gopher mounds. An exclosure experiment in areas adjacent to nests indicated that ants significantly reduced the densities of species with preferred seeds. Ants may therefore significantly affect plant distribution and abundance within the serpentine grassland.     

POPULATION BIOLOGY OF BONAMIA GRANDIFLORA (CONVOLVULACEAE): EFFECTS OF FIRE ON PLANT AND SEED BANK DYNAMICS

The effects of disturbance on reproduction and plant and seed bank dynamics in the perennial herb Bonamia grandiflora were studied by comparing populations in recently burned, mechanically disturbed, and undisturbed habitats in central Florida over a 3-year period. Plant densities, seed production, and the occurrence of herbivory and predispersal seed predation varied considerably between sites and between years, with recently disturbed sites supporting the densest and most dynamic populations. Death of established plants was rare in all sites. In each site, the soil seed bank was several-fold larger than single season seed rains suggesting that B. grandiflora seeds are long-lived and accumulate in the soil. There was no evidence that postdispersal predation or pathogens have any significant influence on the seed bank dynamics. Fire resulted in large increases in stem densities due to both increased clonal stem production and new genet recruitment from seed. Burning also caused significant increases in the percentage of flowers producing seed and the numbers of capsules and seeds per plant. The seed rain was ten to thirty times greater in the burned site relative to adjacent unburned site during the 3 years after burning. However, additions to the seed bank from the postfire seed rain were balanced by equivalent losses due primarily to seed mortality during fire, and to a much lesser extent due to germination and new genet establishment. As a result, the subsequent densities of seeds stored in the soil in these two sites were similar, indicating that fire results in a significant turnover in the seed bank population but no immediate change in its size. These effects on seed bank dynamics, in addition to new genet recruitment, suggest that periodic fires may play an important role in the maintenance of genetic variability as well as the size of these populations.