Plant response to disturbance in a Mediterranean grassland: How many functional groups?

Data referring to changes in vegetation composition resulting from cattle e xclosure and ploughing in a Portuguese pasture dominated by annuals were us ed to test hypotheses regarding the biology of species favoured or eliminat ed by disturbance in semi-natural herbaceous communities. These hypotheses were tested in two ways. First we compared the distribution of six a priori groups - grasses, small rosettes, large rosettes, small species with leafy stems, large species with leafy stems, legumes - across grazed, ploughed a nd undisturbed plots. In a second set of analyses we examined changes in th e frequencies of individual biological attributes in response to grazing an d ploughing. These analyses were carried out separately for grasses and dic ot forbs. Overall, the species composition showed little response to either grazing or ploughing, though species dominance changed. This lack of respo nsiveness of species composition was attributed to the long history of inte nsive land use which has resulted in the loss of disturbance-intolerant spe cies over entire landscapes. When considering a priori groups, small rosettes were indifferent to distur bance. grazing and ploughing showed that dominated. Large rosettes, large s pecies with leafy stems and legumes were generally intolerant to both grazi ng and ploughing, though individual species may increase in response to dis turbance. Small species with leafy stems were the only group favoured by gr azing whereas ploughing favoured grasses. As to individual traits, grazing excluded large grass species with heavy se eds and promoted a flat rosette canopy structure and a small size, along wi th a moderate dormancy and protected inflorescences. In forbs, grazing favo ured small species, as expected, while it excluded tall species, and, in co ntrast to earlier results, a rosette canopy. These attributes were consiste nt with responses of the a priori groups, though it would not have been pos sible to reconstruct groups directly from the attribute list. Ploughing had no effect on any of the forb traits. As to grass traits, flat- and short-s tatured species increased and heavy-seeded species decreased. Our analysis revealed two advantages of establishing plant functional class ifications within life forms. Subgroups within forbs had contrasting types of behaviour. For the same trait patterns could differ within the grass gro up from within the forb group. Finally, this analysis emphasizes the need f or plant functional classifications aiming at the identification of syndrom es of co-occurring attributes rather than of lists of isolated traits of wh ich actual combinations are not specified.