Jacques Lepart

Diversity and Stability in Garrigue Ecosystems After Fire

Vegetation dynamics after fire was studied in six communities in Bas-Languedoc (Southern France). 47 plots were observed by means of a permanent transect for ten years. In the first part, we describe floristic richness, species fugacity and the way by which, the ‘terminal’ community (as defined by the last observation) appears. The dynamics of all these communities follows a simple and general model: floristic richness reaches its maximum during the first two years after a fire, then decreases and becomes stable. Fugacity follows a similar model, whereas the mergence of the ‘terminal’ community is rapid: one year after fire 70% of the plots have already acquired 75% of the species of the ‘terminal’ community. There is no succession (in the general sense of the word), but a progressive reappearance of the species belonging to the original community. In the second part, we study floristic similarities between our plots and corresponding associations as described in literature. It appears that after a fire the floristic diversity of the landscape remains high; while the communities rapidly reach a relative maturity. In the study area fire seems to be a rather superficial phenomenon; it does not lead to an important modification of the community dynamics, because probably the most frequent species in Bas-Languedoc developed adaptations to withstand fire.

Seed bank dynamics and coexistence of annual macrophytes in a temporary and variable habitat

The seed bank dynamics of annuals in two Mediterranean temporary marshes were studied in order to investigate the impact of the disturbance regime on coexistence. The disturbance regime is made up of drought periods in the marshes and of predation. The density of diaspores buried in the sediment was measured in 4 consecutive years together with the biomass produced during two growing seasons. The total density of diaspores in the sediment varied markedly from year to year but always remained at a high level for all species (between 73000 and 800000 per m 2 ). These perennial seed banks allowed the populations to be maintained despite two successive years without any reproduction. In experimental conditions, the seed bank was not depleted after the sediment samples had been submerged during 5 successive periods. The majority of the seedlings emerged as soon as the first period of flooding and temporary droughts stimulated the germination of Zannichellia spp. and Ranunculus baudotii. The decrease in the number of seeds buried in the soil was essentially due to germination. This was stronger when the seeds were recently produced and situated at the surface of the sediment. The seed density varied only slightly with the depth of the marsh, except in the case of Ruppia maritima. The seed bank is highly variable on a spatial microscale but no aggregation pattern could be demonstrated. In the field, the biomass of a population generally appeared not to be limited by the availability of the diaspores, notably because of the ability to propagate vegetatively which is shown by all species studied. Similarly, a high vegetative biomass does not guarantee a high reproductive output as the droughts occur in a density-independent way. The success of a species is related to the interaction between its life history traits and the environmental conditions met each year. The macrophyte communities we studied function in non-equilibrium, with patterns close to those described in lottery models of coexistence. Establishment, growth and reproduction fluctuate strongly from one year to another for all species, in a partially asynchronous way. This would enable competitive exclusion to be indefinitely delayed. The seed bank introduces a storage effect by magnifying the effect of the favourable years in comparison with the bad years.

Ornithochory and plant succession in mediterranean abandoned orchards

Data on the occurrence of species with fleshy diaspores and on breeding birds were collected in three abandoned orchards, resp. 8, 11 and 24 years after abandonment. Most of the 41 phanerophytes and vines with fleshy diaspores are also found in the borders of the orchards, the more so if the number of years since abandonment increases. Most species have small seeds and red or black coloured diaspores, most of which ripen in autumn. The frequent species show a characteristic distribution pattern in relation to the distance of the fruit trees: high densities near the trunk and uniform decrease with distance. This is explained by the behaviour of frugivorous birds. No correlation was found between distribution patterns and soil conditions. Age structure of colonizing species shows a distribution conforming an inversed J curve in the more recently abandoned orchard. The main conclusions are: 1. Pioncor trees are attractive for frugivorous birds and may act as nuclei. This supports the facilitation model. 2. The distribution of diaspores by birds helps to homogenize the species distribution on the regional level but at the site level individual differences in nucleation cause a heterogeneity. Pioncor trees are attractive for frugivorous birds and may act as nuclei. This supports the facilitation model. The distribution of diaspores by birds helps to homogenize the species distribution on the regional level but at the site level individual differences in nucleation cause a heterogeneity.

Changes in Mediterranean plant succession: old‐fields revisited

. Old-field plots used for a study of succession in Mediterranean France were revisited after 12–14 yr. Our aims were: (1) to verify if predicted patterns of species richness, turnover and composition are confirmed; (2) to compare the development in disturbed plots with that in undisturbed ones; (3) to discuss the impact of management changes. In undisturbed plots species richness and turnover decreased with successional age. Floristic composition changed in a way consistent with the predicted successional development in most plots. Therophytes decreased and phanerophytes increased; anemochorous species decreased and endozoochorous species increased, as expected. In plots disturbed since the first analysis richness decreased with successional age, but generally remained higher than in undisturbed plots. Floristic composition, species turnover and an increase in therophytes indicated changes towards younger successional stages. Thus, disturbance changed succession but not much. This is probably linked with the regeneration abilities typical of mediterranean species, e.g. resprouting. At the landscape scale, richness did not change and species turnover was low. The plots studied were situated in two distinct locations. One had not been disturbed between the two observation periods, while the other is a mosaic of undisturbed and disturbed sites. Observations fitted predictions much more closely at the undisturbed location. We conclude that permanent plot studies are powerful in identifying successional trends and can also provide additional insights into the effects of disturbance some of the mechanisms underlying the dynamics of diversity.

Small scale disturbances and the maintenance of species diversity in Mediterranean old fields

In three Mediterranean old fields, one, seven, and fifteen years after the last ploughing, small scale disturbances (0.25 m 2 ) by contact herbicide associated or not with litter removal and scratching of the top 3 cm of the soil were generated in October 1988, December 1988, and March 1989. Subsequent changes in cover, species richness and composition were observed over the two following vegetation cycles (until May 1990), and compared to the dynamics in undisturbed samples. The identity of the colonizers for each disturbance date were compared to the soil readily-germinable seed content, sampled simultaneously to the disturbances. The species richness of the samples within the fields increased the first year after disturbance. Within-field heterogeneity for taxonomic composition was unchanged, but species turnover was increased by disturbances

Shrub facilitation of Quercus humilis regeneration in succession on calcareous grasslands

. In southern France, the natural invasion by Quercus humilis of calcareous grassland takes place in a mosaic of herbaceous and scrubby patches. We hypothesized that the presence of the shrubs Buxus sempervirens and Juniperus communis alter the rate and the pathway of the succession by facilitating the regeneration of Q. humilis. To infer the process of facilitation at a large scale, the spatial distribution of Q. humilis was studied in relation to acorn sources and the type of plant cover in grazed and ungrazed sites. Abundant recruitment up to 80 m from the wood margins and from isolated oak trees in grassland shows that acorns are dispersed effectively. At the three study sites, the density of Q. humilis individuals was higher under shrubs than in grassland, suggesting that facilitation may occur. This density difference was much higher in the grazed sites than in the ungrazed site. Moreover, before grazing by livestock, the distribution of first-year seedlings is independent of vegetation cover. Thus, shrubs improve Q. humilis regeneration by protecting individuals from grazing. The high density of individuals at the northern edge of shrubs suggests that a second facilitation mechanism may exist, probably related to improved germination conditions. Facilitation by shrubs appears to be very important for Q. humilis dynamics.

Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes

The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhône delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure. In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged. In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity. The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years.

Establishment of woody plants in Mediterranean old fields: opportunity in space and time

The establishment of woody plants following agricultural abandonment in the Mediterranean region is a very widespread process which underlines the extent of the rural exodus. The installation windows in space and time were studied in the French Mediterranean region for two common woody plants, Buxus sempervirens and Fraxinus angustifolia and for a group of common woody fleshy-fruited species. These plants differ in their principal modes of dispersal which are respectively, barochory, anemochory and ornithochory. Their installation was analyzed in relation to the seed shadows, the spatial patterns and the age structures of the seedlings. The majority of the seeds were dispersed over short distances, although some animal vectors may promote a limited amount of long distance dispersal. Hence, whatever the mode of dispersal, a few seeds are often dispersed far from the maternal plant. The combination of several dispersal types in one plant species is a frequently observed feature, one being dominant at a small scale, and related to successional processes, the other being dominant at a larger scale and related to invasive processes. In the old fields the spatial pattern of seedlings closely follow the observed seed shadows. However, competition with the maternal plants may lead to, in some cases, a recruitment deficit close to the seed-bearers. Age structures show that woody plants often install very early after the abandonment of cultivation and that the installation window in time is shortened by the development of a dense herbaceous cover. It is concluded that the installation of woody plants in Mediterranean old fields cannot be reduced to a general rule. The rate and extent of installation depends mainly on the spatial distribution of the seed-bearers, therefore of the spatial patterns of the landscape.

Human Impact on Landscape Patterning: Mediterranean Examples

Humans have modified large areas of landscape throughout the world. At a local level, humans may change dominant plant species, vegetation structure, and characteristics of ecotones between vegetation patches. At a regional level, humans may alter land use, arrangement of tessera in the landscape mosaic, and characteristics of ecotones between mosaics. Analysis of past and present human activities is essential in many locations for understanding landscape patterns. In this chapter, we analyze and discuss the importance of anthropic disturbances over approximately the past 8000 years in the Mediterranean region, and more specifically the Languedoc (southern France). We address the following questions: How does natural landscape structure influence human settlement patterns? How is human action distributed in time and space? Do anthropic disturbances differ from natural disturbances in kind or only in scale? Do human actions typically lead to more homogeneous landscapes? What are the long-term consequences of human activity patterns? To what extent do social and economic factors drive human land use and, in turn, landscape patterning? All these questions are interrelated. We perform the analysis with reference to a hierarchy of time and space scales, as is often done for natural disturbances (Pickett and White 1985, Urban et al. 1987).

Simulating the dynamics of a vegetation mosaic: a spatialized functional model

Abstract Space needs to be taken into account in vegetation dynamics models which deal with heterogeneous systems. Most landscapes, at a given scale, are mosaics of relatively homogeneous patches. Some patches act as sources of energy, matter or individuals, others as sinks. The dynamics of each patch depends partly on the flows which relate it to its neighbours. Simulating the functioning of these systems lead to combine, in the same model, within-patch dynamics, ecotone dynamics, and the interactions between patches and flows. The model TLALOC uses these principles to simulate the dynamics of a two-patch system from an arid zone which clearly depend on surface water flows. In each m2 of a transect perpendicular to an ecotone, both functional (water and carbon budgets) and dynamics processes (recruitment and survival) were simulated. Despite the simplifications, it satisfactorily modelled the functioning of the studied vegetation. It highlighted the importance of considering various temporal and spatial scales and the possible consequences of including the studied transect in an even larger landscape.