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

Abstract

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.

The fate of seeds in the soil: a review of the influence of overland flow on seed removal and its consequences for the vegetation of arid and semiarid patchy ecosystems

“…The first evidence that runoff may act as a vector of seed transport was indirect and based on observations of seed dispersal strategies in runoff-prone areas (Friedman and Orshan, 1975;Friedman and Stein, 1980), comparisons of plant distribution with different dispersal mechanisms between slopes and wadis (Reichman, 1984), or descriptions of seed distribution patterns in different microhabitats (Ellner and Schmida, 1981) in desert ecosystems worldwide. In the 1990s, it was argued that seed removal by runoff led to seed loss and might explain the lack or scarcity of vegetation on semiarid and arid hillslopes (Debusche and Lepart, 1992;Francis, 1991;Chambers and Mac Mahon, 1994). Although frequently invoked, this assumption was not empirically checked until the pioneering studies of García-Fayos and his collaborators about seed transport by runoff flow.…”

The Spatiotemporal Dynamics of Forest–Heathland Communities over 60 Years in Fontainebleau, France

“…In our study sites, P. sylvestris was one of the two main pioneer woody species. Debussche & Lepart [41] and Debain [42], working on this species, showed that its principal front of colonisation was situated a few tens of meters around old pine stands, and individuals that have become established ahead of the colonisation front of old pine stands colonise the space around them. Here, the maturity of the woody populations that were present prior to 1946 permitted the colonisation of heathland relatively quickly.…”

Etude des successions après abandon des pratiques culturales en Bretagne

“…fruticosus et surtout de Pteridium aquilinum. Ces processus ont cgalemcnt etc reconnus dans des successions en region mediterraneenne (Debussche et al, 1982(Debussche et al, , 1985(Debussche et al, , 1992 Fig. 5 pour Quercus robur) transforme en quelques annees un paysage de friches ou de prairies en un paysage preforestier, voire forestier.…”

“…Mais il est evident qu'une partie non negligeable des diaspores, notamment ceUes des especes ligneuses, resulte d'apports exterieurs, notamment dans le contexte bocagcr. II s'agit de processus competitifs par interference, c'est-a-dire d'une interaction directe entre individus qui recherchent activement ce que d'autres utiliscnt dans un pool commun de ressources (Connell, 1990;Debussche et Lepart, 1992).…”