The dynamics of mediterranean shrubs species over 12 years following perturbations

Published source details Calvo L., Tarrega R. & De Luis E. (2002) The dynamics of mediterranean shrubs species over 12 years following perturbations. Plant Ecology, 160, 25-42.
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Summary

The response of woody species to experimental burning, cutting and ploughing was studied for a period of 12 years in an upland shrub community in northwest Spain dominated by Erica australis aragonensis. The treatments represent the perturbations most frequently imposed by man on these shrub communities: they were frequently burnt to open up areas for sheep grazing or cultivation; cut to provide material for e.g. domestic fuel, fodder and fertilizers; and were also ploughed in order to cultivate cereal crops. These traditional land-uses were common practices in the Province of León up to the 1960s, since when rural depopulation and an ageing population has led to abandonment of arable land and a decrease in the number of sheep flocks. This has led to a decrease in the floristically rich shrubland communities through secondary succession.

Study area: A study area was selected in an upland heathland community (Genistelo tridentatae-Ericetum aragonensis-Cytisetosum laurifolii) at around 1050 m altitude in Castilla Y Leon. Vegetation was dominated by Erica australis with other common woody species including bear berry Arctostaphylos uva-ursi, Erica umbellata, heather Calluna vulgaris, Chamaespartium tridentatum (Syn: Genista tridentata), Halimium alyssoides, Halimium umbellatum and shrubby Pyrenean oak Quercus pyrenaica (the characteristic climax community species).

Experimental design: In early summer 1985, three 10 × 10 m plots (5 m between each) were randomly established within an area with homogeneous shrub on flat ground. In 100 units of 1 m², covering each plot; percentage covers of each woody species, and all herbaceous species combined (as they comprised only a very small percentage of total cover) were estimated visually.

In July 1985, three treatments were applied:

i) Cutting - all woody shrubs cut to ground level manually;

ii) Burned - plots burned;

iii) Ploughing - ploughed with a tractor to 50 cm depth; stumps removed.

Cover values were subsequently estimated in the same way as prior to treatment application, 1, 3, 4, 5, 9 and 12 years after the treatments. From the fourth year, visible differences in the height of the woody species began to show, and the height of the tallest plant of each species was then also recorded in each sampling unit.

Vegetation recovery after burning was much faster than cutting, whilst response to ploughing was slowest as plants had to regenerate from seed, as opposed to having the opportunity to resprout. E.australis and C.tridentatum (the latter <1% original cover and benefitting in the early years from reduced competition from E.australis) regenerated by sprouting in the burnt and cut plots, and by germination in the ploughed plot. A.uva-ursi only recovered well after burning (original cover 43%; year 12 - 36%), recovery in the ploughed plot began in the third year but was subsequently very slow; it was lost in the cut treatment where it originally had a cover of 15%. H.alyssoides, H.umbellatum, E.umbellata and C.vulgaris regenerated only by seed in all three plots.

In general E.australis, which re-established as the dominant species, influenced regeneration patterns of the other species. There was a significant increase in herb cover until the third year. Subsequently, E.australis attained dominance, returning to its original cover (c.38%) within the cut and burned plots after 4 years, and around 7 years in the ploughed plot. Herb cover declined quickly under its developing canopy, and it also negatively impacted the growth of some woody species e.g. H.umbellatum, H.alyssoides and Q.pyrenaica.

12 years after treatment most woddy species (an exception being A.uva-ursi in the cut plot) had recovered to their initial cover values.

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