Livestock grazing in subtropical pastures: steps in the analysis of attribute response and plant functional types

Abstract

Summary In order to investigate the use of plant functional types as an alternative to floristic descriptions of response to grazing, we analysed plant communities from a cattle grazing experiment conducted in grassy eucalypt woodland in subtropical Queensland, Australia. The two variables analysed (landscape position and stocking rate) accounted for similar proportions of variation in the vegetation ground layer, although forbs were more sensitive to slope position and perennial grasses were more sensitive to stocking rate. As grazing pressure increased, perennial grasses declined, while the relative proportion of forbs and annual grasses increased. Detailed functional group analyses were conducted for the perennial grass and forb life‐forms. Annual grasses were represented by only two species, preventing identification of functional types within this life‐form. We conducted a five‐step analysis for both perennial grasses and forbs as follows: (i) defining grazing‐related species response groups; (ii) defining species groups based on natural attribute correlations; (iii) identifying attributes that changed significantly with grazing; (iv) identifying syndromes by relating (ii) and (iii); (v) describing functional types from (iii) and (iv), and assessing them against actual species response. Eight grass and eight forb functional types were identified. Of the taxa that had an observed response to grazing, 54% of the grass taxa and 57% of the forb taxa corresponded to one of these functional types in terms of meeting both grazing response and trait criteria. The five‐stage analysis provided a comprehensive but complex approach to functional type identification. The functional types identified can be summarized as follows. Low levels of grazing were associated with more medium‐sized, moderately leafy perennial grasses with wind‐dispersed or adhesive seeds, and more erect or twining forbs with large to medium sized seeds. High levels of grazing were associated with: more annual grasses; low‐growing leafy perennial grasses with small seeds having no dispersal appendages; mat‐forming large‐seeded forbs; and low‐growing, scrambling, small‐seeded forbs.