Effects of land use change on plant composition and ecosystem functioning in an extensive agro-pastoral system : plant functional traits and ecosystems processes

摘要

As a consequence of the industrial and socio-demographic changes of the last decades, the characteristic land use practices of extensive agro-pastoral systems in the region of Alentejo, Southern Portugal are being gradually abandoned and the consequences are still little understood. Land use changes are important drivers of environmental degradation, modification and fragmentation of habitats with the subsequent alterations of global carbon and hydrological cycles, global and regional climate, and decline in biodiversity. This work aims to contribute to a better understanding of the effects of abandonment in ecosystems with a long history of human management, as is the case of the agro-pastoral systems of Southern Portugal. Three land use categories were selected to represent a decrease in land use intensity (‘grazing’, ‘intermediate succession’ and advanced succession’). Following abandonment, secondary succession is expected to occur with consequent changes in soil characteristics and vegetation composition and structure. Because plant species differ in their functional traits and in their effects on ecosystem processes, an effect of land use change at this level may also be expected. Plant traits relate to universal plant functions of growth (e.g. light and nutrient acquisition, water use efficiency) and persistence (e.g. recruitment, dispersal, defence against herbivores and other disturbances). They provide a widely applicable framework for interpreting community shifts along environmental gradients, including secondary succession. The main objectives were to (1) identify changes in vegetation composition and structure in response to decreasing land use intensity; (2) Identify functional groups and changes in species traits in response to decreasing land use intensity; (3) Identify the effect of decreasing land use intensity in key ecosystem processes such as decomposition and above-ground net primary productivity (ANPP). Overall, the results from the present study showed that abandonment resulted in changes in species composition and richness, in plant functional traits and functional groups and in ecosystem processes (biomass, ANPP and decomposition). Secondary succession resulting from abandonment of grazing showed stronger changes in vegetation composition and structure than in soil characteristics. Among the soil chemical properties tested, only phosphorus, carbon and organic matter were affected by land use change. Phosphorus decreased with abandonment while organic matter and carbon showed an increase. Species richness decreased sharply after land abandonment, and this was associated with a strong turnover in species composition from grazed to abandoned sites as the vegetation changed from annual herbaceous to shrub-dominated communities. One single species (Cistus ladanifer) accounted for more than 50% of relative cover in the areas abandoned for a longer time, suggesting that this species might have an important role in possible changes in ecosystems processes. The species that colonized the different land use categories differed in plant functional traits. Therophyte life form, short canopy height, high specific leaf area (SLA), low leaf dry matter content (LDMC) and small seeds with dispersal structures were dominant at grazed plots. Within abandoned plots, chamaephytes dominanted at ‘intermediate succession’ plots and decrease in favour of nanophanerophytes in ‘advanced succession’ plots. Canopy height, LDMC and seed mass increase with abandonment time while SLA decreased. Functional response groups were found by combining life form and SLA and these were found to be sufficient to describe vegetation change. Therophytes with medium SLA were the dominant functional group in grazed areas, while nanophanerophytes with medium or low SLA were associated with later phases of abandonment. At intermediate stages of succession the dominant group was chamaephytes with medium SLA but functional diversity was highest as all the groups, except hemicriptophytes with medium SLA, were represented. Our study showed an increase in total above-ground biomass with abandonment indicating a positive effect of the shift to woody vegetation on total above-ground biomass. ANPP significantly increased in plots abandoned for longer time. This increase was strongly related with the increase in the cover of Cistus ladanifer. This is a pioneer species that colonises degraded areas and forms one of the first stages of succession of woody communities. Under favourable conditions, C. ladanifer can grow fast and attain large amounts of biomass in a short time and quickly spreads over recently disturbed areas. Regarding decomposition, shrub litter was found to be higher than herbaceous litter in nutrient content, especially nitrogen, which seemed to favour higher initial decomposition rates but lower decomposition rate in the longer term. Overall, decomposition was slower in abandoned than in grazed plots and this was positively correlated with the content of cellulose and hemicellulose of initial litter. Lower rates of decomposition were also found to be related to the increase in LDMC, a trait strongly linked to physical attributes of the leaves.