Response of plant-pollinator communities to fire: changes in diversity, abundance and floral reward structure

Globally, plant‐pollinator communities are subject to a diverse array of perturbations and in many temperate and semi‐arid systems fire is a dominant structuring force. We present a novel and highly integrated approach, which quantifies, in parallel, the response to fire of pollinator communities, floral communities and floral reward structure. Mt Carmel, Israel is a recognised bee‐flower biodiversity hotspot, and using a chronosequence of habitats with differing post‐fire ages, we follow the changes in plant‐pollinator community organisation from immediately following a burn until full regeneration of vegetation. Initially, fire has a catastrophic effect on these communities, however, recovery is rapid with a peak in diversity of both flowers and bees in the first 2 years post‐fire, followed by a steady decline over the next 50 years. The regeneration of floral communities is closely matched by that of their principal pollinators. At the community level we quantify, per unit area of habitat, key parameters of nectar and pollen forage known to be of importance in structuring pollinator communities. Nectar volume, nectar water content, nectar concentration and the diversity of nectar foraging niches are all greatest immediately following fire with a steady decrease as regeneration proceeds. Temporal changes in energy availability for nectar, pollen, total energy (nectar+pollen) and relative importance of pollen to nectar energy show a similar general decline with site age, however, the pattern is less clear owing to the highly patchy distribution of floral resources. Changes in floral reward structure reflect the general shift from annuals (generally low‐reward open access flowers) to perennials (mostly high‐reward and restricted access flowers) as post‐fire regeneration ensues. The impact of fire on floral communities and their associated rewards have clear implications for pollinator community structure and we discuss this and the role of other disturbance factors on these systems.     

ceh-16/engrailed patterns the embryonic epidermis of Caenorhabditis elegans

engrailed is a homeobox gene essential for developmental functions such as differentiation of cell populations and the onset of compartment boundaries in arthropods and vertebrates. We present the first functional study on engrailed in an unsegmented animal: the nematode Caenorhabditis elegans. In the developing worm embryo, ceh-16/engrailed is predominantly expressed in one bilateral row of epidermal cells (the seam cells). We show that ceh-16/engrailed primes a specification cascade through three mechanisms: (1) it suppresses fusion between seam cells and other epidermal cells by repressing eff-1/fusogen expression; (2) it triggers the differentiation of the seam cells through different factors, including the GATA factor elt-5; and (3) it segregates the seam cells into a distinct lateral cellular compartment, repressing cell migration toward dorsal and ventral compartments.     

Relationships between tree size, crown shape, gender segregation and sex allocation in Pinus halepensis, a Mediterranean pine tree

Background and Aims

Sex allocation has been studied mainly in small herbaceous plants but much less in monoecious wind-pollinated trees. The aim of this study was to explore changes in gender segregation and sex allocation by Pinus halepensis, a Mediterranean lowland pine tree, within tree crowns and between trees differing in their size or crown shape.

Methods

The production of new male and female cones and sex allocation of biomass, nitrogen and phosphorus were studied. The relationship between branch location, its reproductive status and proxies of branch vigour was also studied.

Key Results

Small trees produced only female cones, but, as trees grew, they produced both male and female cones. Female cones were produced mainly in the upper part of the crown, and male cones in its middle and lower parts. Lateral branch density was correlated with the number of male but not female cones; lateral branches were more dense in large than in small trees and even denser in hemispherical trees. Apical branches grew faster, were thicker and their phosphorus concentration was higher than in lateral shoots. Nitrogen concentration was higher in cone-bearing apical branches than in apical vegetative branches and in lateral branches with or without cones. Allocation to male relative to female function increased with tree size as predicted by sex allocation theory.

Conclusions

The adaptive values of sex allocation and gender segregation patterns in P. halepensis, in relation to its unique life history, are demonstrated and discussed. Small trees produce only female cones that have a higher probability of being pollinated than the probability of male cones pollinating; the female-first strategy enhances population spread. Hemispherical old trees are loaded with serotinous cones that supply enough seeds for post-fire germination; thus, allocation to males is more beneficial than to females.     

Composition and diversity of herbaceous patches in woody vegetation: The effects of grazing,soil seed bank,patch spatial properties and scale

Herbaceous plants contribute much to plant diversity in Mediterranean-type ecosystems though mostly occupying relatively small patches within the dense woody vegetation. While studying species diversity in the herbaceous patches, we hypothesized that grazing, soil seed bank, and spatial properties of the patch affect plant diversity and composition at different spatial scales. The study site was in an LTER site located in the Mediterranean region in north Israel. We determined herbaceous species composition in: (1) randomly sampled quadrats in herbaceous patches in grazed and un-grazed plots; (2) soil seed bank samples taken from the same patches and germinated under optimal greenhouse conditions; (3) quadrats in the same patches sown with a homogenous mixture of local soil samples. Using GIS methods, we determined small-scale spatial characteristics of the herbaceous patches. Alpha and beta diversities were calculated at the patch and plot scales using Shannon's entropy H. Grazing increased alpha diversity of local untreated seed bank samples but decreased alpha diversity of the artificial homogenous soil seed bank mixture at both patch and plot scales. Positive relation between alpha diversity and patch area was detected only under grazing. Grazing increased beta diversity in all three treatments at the patch scale. Grazing decreased the similarity in species composition between above-ground vegetation and soil seed bank. The results indicate that moderate cattle-grazing affects species diversity in the herbaceous patches within the dense maquis. This effect is scale-dependent, and interacts with the effects of soil seed bank and patch spatial-properties: without grazing soil seed bank plays a more important role than patch spatial properties, but under grazing the size and the accessibility of the patch are more important in the determination of herbaceous species composition.     

Nectar resource diversity organises flower-visitor community structure

Communities of nectar‐producing plants show high spatio‐temporal variation in the patterns of volume and concentration presentation. We illustrate a novel approach for quantifying nectar reward structures in complex communities, demonstrating that nectar resource diversity (defined as the variety of nectar volume–concentration combinations available) may be a fundamental factor organising nectarivore communities. In a series of diverse bee and entomophilous flower communities in Israel, our measure of nectar resource diversity alone explains the majority of variation in bee species richness, while other nectar variables (volume, concentration, energy value, and water content) have little predictive value per se. The new measure of nectar resource diversity is highly correlated with floral species richness and particularly with the species richness of annuals, yet it is additive in its effect on bee diversity. We conclude that relying solely upon measurements of mean nectar volume and mean nectar concentration overlooks a key characteristic of community‐level reward structure, nectar resource diversity, so that previous studies may have failed to identify an important determinant of flower‐visitor community structure.     

The response of Mediterranean herbaceous community to soil disturbance by native wild boars

Native wild boar (Sus scrofa lybicus) populations in Israel increased in the last decades, and their rooting activity presents an apparent severe disturbance that may affect herbaceous plant communities. We studied the extent of wild boar rooting and its effect on herbaceous vegetation in an east Mediterranean garrigue. We compared the effects of rooting soil disturbance (RSD) between summer, winter, and RSD-free control plots on herbaceous plants. RSD affected four percent of the area covered by herbaceous vegetation in winter and spring, and only 2 % in summer. Summer RSD increased herbaceous species diversity in the following spring by decreasing the abundance of the dominant species, but it did not affect total herbaceous biomass. Winter RSD lowered herbaceous biomass but did not affect herbaceous plant abundance or diversity. We conclude that Mediterranean herbaceous plant community appears to be resilient to RSD, causing only minor changes in species composition and small decrease of their biomass. The contrast between the relatively large extent of wild boars RSD, and its limited impact on the herbaceous plant community, can be attributed to the long mutual evolutionary history of wild boars and these vegetation communities.     

Woody species as landscape modulators: their effect on the herbaceous plants in a Mediterranean maquis

“Landscape modulators” are ecosystem engineers that have an impact on community structure by creating patches in the landscape mosaic. Our aim was to study the effect of evergreen-trees, as landscape modulators, on herbaceous plants in a Mediterranean maquis system in northern Israel. We examined the effects of canopy removal and cattle grazing on species richness, plant functional types, and rare plant species in two patch-types: (1) woody—under tree canopy (or the location of a removed canopy); (2) herbaceous—in open areas with no tree canopy. Patch-type and tree removal affected species richness and plant functional types. The extreme negative effect of the woody patch-type on species richness disappeared soon after the removal of the landscape modulator canopy. We conclude that the dominant effect of the evergreen woody landscape modulators can be regulated by canopy removal and grazing for maintaining patch-type and landscape diversities, and consequent high species richness in Mediterranean ecosystems, which is a main goal of global nature conservation policy.     

Morning floral heat as a reward to the pollinators of the Oncocyclus irises

Relationships between flowering plants and their pollinators are usually affected by the amount of reward, mainly pollen or nectar, offered to pollinators by flowers, with these amounts usually positively correlated with floral display. The large Oncocyclus iris flowers, despite being the largest flowers in the East Mediterranean flora, are nectarless and have hidden pollen. No pollinators visit the flowers during daytime, and these flowers are pollinated only by night-sheltering solitary male bees. These iris flowers are partially or fully dark-colored, suggesting that they gather heat by absorbing solar radiation. Here we test the hypothesis that the dark-colored flowers of the Oncocyclus irises offer heat reward to their male solitary bee pollinators. Floral temperature was higher by 2.5°C than ambient air after sunrise. Solitary male bees emerged earlier after sheltering in Oncocyclus flowers than from other experimental shelter types. Pollination tunnels facing east towards the rising sun hosted more male bees than other aspects. We suggest that floral heat reward can explain the evolution of dark floral colors in Oncocyclus irises, mediated by the pollinators’ behavior.     

Reproductive traits of Pinus halepensis in the light of fire – a critical review

Fire is known to be a major factor in shaping plants and vegetation worldwide. Many plant traits have been described as adaptations for surviving fire, or regenerating after it. However, many of the traits are also advantageous for overcoming other disturbances. The fact that fire in the Mediterranean Basin has been almost exclusively of anthropogenic origin, and thus is of short duration in an evolutionary time scale, cast doubt on the possibility that fire can act as a selective force in the Mediterranean Basin. Our aim here is to review the ecological advantages of Pinus halepensis traits and their possibility to be selected by fire. The non-self pruning of cones and branches, and the high resin content increase the probability of canopy fires and consequent death of P. halepensis trees. Post-fire regeneration of P. halepensis depends totally upon its canopy-stored seed bank. The seedlings grow quickly and they first reproduce at an early age. Young reproductive trees function first as females with a high percentage of serotinous cones. Thus, young P. halepensistrees allocate many resources to seed production, reducing their `immaturity risk' in a case of an early successive fire. The proportion of serotinous cones is higher in post-fire naturally regenerating stands than in unburned stands, and seeds from serotinous cones germinate better under simulated post-fire conditions. The extremely high pH of the ash-bed under the burned canopies creates the post-fire regeneration niche of P. halepensis exactly under their parent trees. All these traits are advantageous for post-fire regeneration, but could they also be selected during the time scale of anthropogenic fires in the Mediterranean Basin? Pinus halepensis is a relatively short living tree with almost no recruitment under forest canopy. The longest estimated fire-return interval and generation length are about 125 years. The earliest solid evidence for the first hominid-controlled fire in the Mediterranean basin is 780,000 years ago, and thus the estimated number of post-fire generations is 6240. We suggest that such a number of generations is sufficient for the selection and radiation of fire adaptive traits in P. halepensis.