Biomass turnover as an ecological indicator of vegetation dynamics in agrosilvopastoral systems of SW Galicia

Resumo

Traditional land use in Galician (NW Spain) coastal hills has changed dramatically over the last few decades. Fast-growing tree plantations and secondary natural vegetation dominate the present-day landscape. The current agrosilvopastoral systems have changed in composition and physical structure of vegetation due to the abandonment of ancient resource-use practices, recurrent fires and subsequent soil erosion. A common Galician agrosilvopastoral system was studied, composed of two principal ecosystems - pine woodland and gorse shrubland. Currently, only limited grazing by 'wild' horses exists as a relict from an ancient, integrated, extensive exploitation system. Pseudarrhenatherum longifolium is the dominant perennial grass species in both ecosystems. Vegetative biomass, standing dead biomass, and percentage of dead matter in above-ground biomass of P. longifolium was lower in woodland than in shrubland. A very significant positive non-linear pattern of dead biomass increase with increasing vegetative aboveground biomass was detected in both ecosystems. This pattern means a reduction in aboveground biomass turnover rate which was significantly higher in woodland than in shrubland. The results indicate a very low or no P. longifolium leaf biomass consumption by horses in this agrosilvopastoral system, and lower grass production and decomposition rates in woodland than in shrubland. They suggest that the preservation of traditional hill exploitation systems, such as free-ranging horses, is advisable, and constitutes an efficient tool for biological diversity conservation and fire and soil erosion prevention.