The effects of protection on steppic vegetation in the Mediterranean arid zone of southern Tunisia

Published source details Floret C. (1981) The effects of protection on steppic vegetation in the Mediterranean arid zone of southern Tunisia. Vegetatio (now Plant Ecology), 117-129.
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Summary

Large areas in North African arid regions have been subject to over-grazing by domestic livestock. In this study, five areas of different degraded steppic vegetation types in southern Tunisia which had been subjected to extensive overgrazing were fenced over seven years to exclude large herbivores. The objective was to study regeneration of the natural vegetation in order to recommend an optimum period for protection from grazing to allow vegetation to recover and establish a sustainable pastoral grazing regime. The results of these observations are summarised here.

Study areas: The study took place in the Djeffara and Basses Plaines Meridionales Orientales regions in the Mediterranean arid zone of southern Tunisia. Large herbivores were excluded from areas of five different desert steppe vegetation types (representative of the steppic vegetation found in large areas of the region) by barbed wire fencing for seven years. (No information is given on the size of the fenced areas within the original paper).

Steppe vegetation and monitoring: From 1972 to 1979, the development of the plant and litter cover was followed in each of the five steppe vegetation types generally characterized by small perennial shrub-like chamaephytes (up to 20-30 cm height) with annual plants appearing if sufficient rain fell:

i) a steppe on a gypseous crust resulting from overgrazing of the Zygophyllum album-Anarrhinum brevifolium association;

ii) a steppe on a sandy, gypseous, rocky-surfaced colluvium resulting from overgrazing of the Rhantherium suaveolens-Artemisia campestris association;

iii) an open steppe on a deep, gypseous, sandy colluvium-alluvium of a Rhantherium suaveolens association;

iv) a steppe in good condition on a deep, sandy-soiled plain with a Rhantherium suaveolens-Artemisia campestris association;

v) a post-cultivated Artemisia herba-alba-Arthrophytum scoparium association on a deep loamy and gypseous colluvium soil, traditionally cultivated with cereals.

Precipitation: Rainfall measurements were taken throughout the study period. The precipitation in the first 4 years was about equal to or above the average precipitation (175-185 mm). The year 1975-76 experienced 2-3 times above average rainfall, while the last 3 years were, in general, quite dry.

There was great variability in plant cover according to season and rainfall, mainly due to annual species (which increased dramatically when sufficient rain fell), but often also as the cover of perennials varied (by as much as 25%). An increase in the cover of annuals following seven years of protection from large herbivores was not obvious as this cover was so much affected by the amount and seasonal distribution of rainfall. During dry periods, often only the woody parts of perennials remained, with any plant litter generally redistributed by the wind.

The response to protection was mainly an overall increase in cover of the perennial species. The steppe vegetation in the deep, sandy zones responded best with more-palatable perennial species, subject to greatest overgrazing benefitting most. Plant cover increases were greatest on these sandy soils compared with those steppes on loamy or gypseous crusts. The protection of the gypseous crusted steppes (formed as a result of the loss of the surface horizon and underlying gypseous layers which hardened rapidly once exposed) resulted in no indications of recovery to a natural vegetation state resembling that which would have existed prior to habitat degradation.

Conclusions: On the basis of these results, the author concludes that it is difficult to recommend an optimal duration of protection for regeneration of land in arid zones with the objective of setting up a sustainable pastoral grazing regime. The duration is governed very much upon the amount of rainfall following protection, and upon local conditions, especially the initial state of the vegetation.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at:

http://springerlink.metapress.com/content/g512mg23h290270x/fulltext.pdf