Abstract
It is becoming more and more common in the Mediterranean basin to guard against forest fires by creating wooded firebreak areas by reducing the overstory density, pruning the remaining trees and slash mulching. Nevertheless, very few attempts have been made to analyse the effect of this practice upon tree growth and the productivity of the understory plant species in semi-arid areas by following a well defined experimental design over any length of time. To remedy this lack of information, during the spring of 2005 we thinned the overstory in a semi-arid Aleppo pine (Pinus halepensis Mill.) afforestation in SE Spain (planted in the winter of 1993-1994) from an initial density of about 1,500 trees per ha according to three different thinning regimes. The annual growth data of each stand (diameter at breast height, canopy cover and basal area) were measured in randomly situated plots of 20 × 20 m and the above-ground biomass of understory plant species was also harvested from quadrats of 0.5 × 0.5 m during the early summers of 2005 to 2009. Our results showed a positive and significant response from trees in thinned stands compared to the controls, in spite of periods of low-rainfall and plague stress that unfortunately occurred during the study period. There were also statistical differences in understory biomass production within the different areas of overstory thinning as a response to the new conditions. Nevertheless, this biomass was only a small fraction of the total above-ground biomass of the afforestation, which was much higher in control plots. These tree responses and structural changes to the afforestation could be of great interest because of their implications for forest management in the context of global change in an area where increases in temperature and reductions and irregularity in precipitation, together with higher fire risks, are forecast.
Highlights
Recent decades have seen the planting of millions of hectares of dense, monospecific reforestations and afforestations in the Mediterranean basin
Traditional fire-breaks are narrow strips of land totally devoid of vegetation, but lately these are being replaced to some extent by wooded areas that serve the same function though more efficiently and at the same time in a more ecologically acceptable way (Ruiz-Mirazo et al, 2007). Such firebreaks are more aesthetically acceptable to the general public than clear-cut strips. They are created by reducing the overstory tree density, pruning the remaining trees and using the branches for slash mulching
It is known that thinning reduces forest transpiration (Bréda et al, 1995) and increases the availability of water and nutrients to the soil, and sunlight (Aussenac and Granier, 1987; Aussenac, 2000; Misson et al, 2003)
Summary
Recent decades have seen the planting of millions of hectares of dense, monospecific reforestations and afforestations in the Mediterranean basin. Many studies have demonstrated that a reduction in forest overstory by means of thinning or partial cutting stimulates ground-flora development because the removal of trees allows more light to reach the understory and results in a higher availability of water and mineral nutrients for plant species (Kleintjes et al, 2004; Deal, 2007; French et al, 2008) These activities can change the species composition of ground vegetation, at least in temperate forests (Hannertz and Hånell, 1993; Bergstedt and Milberg, 2001), on occasions some authors have found that there was no such response whatsoever (Götmark et al, 2005; Wilson and Puettmann, 2007). More information is needed concerning the effectiveness of different treatments upon the enhancement of forest productivity and the conservation of biodiversity as a basis for sound investment in silviculture, especially in Mediterranean environments To this end, we have studied the short-term effects caused by overstory thinning upon ground vegetation and tree growth in an Aleppo-pine afforestation in southeast Spain. Within this context we approached the question as to whether these forest treatments could change the allocation pattern of above-ground biomass production of plant species and forest productivity
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