Common leaf life span of co-dominant species in a continuously grazed temperate pasture

Abstract

Short leaf life span is thought to be associated with low construction costs and consequently short payback times. Frequent defoliation could decrease the available payback time and force individuals to decrease construction costs to maintain a positive carbon balance. Therefore, we hypothesize that dominant species of frequently grazed pastures show similar and short leaf life span. Leaf life span was measured on four co-dominant species, two grasses and two dicots, in a pasture that was intensively grazed by cattle for six years. Leaf life span was analysed in terms of its components: the phyllotherm and the number of live leaves. Phyllotherm and number of live leaves were determined in eight to 20 individuals of Lolium perenne, Poa pratensis, Taraxacum officinale and Trifolium repens, during autumn 2006 and autumn and spring 2007. Leaf life span was estimated as the product of phyllotherm and number of live leaves. After critically assessing five operational definitions of leaf death, a leaf was considered alive during the time elapsed between its appearance and the senescence of 25% of its area. In general, leaf life span did not differ between species nor was there clear evidence for seasonal effects. The leaf life span ranged between 400 and 520 growing degree days (base temperature: 4°C). The similarity of leaf life span between species was based on different mechanisms: dicots had shorter phyllotherms and more live leaves than grasses. Observed leaf life spans were short when compared with data from plants growing in less disturbed habitats, but similar to the dominant species of grazed grasslands with a similar disturbance regime. The similarity of leaf life spans resulted in a common community-level leaf life span. It is suggested that this trait could control community-scale biogeochemical features, such as the residence time of carbon in the aboveground structural biomass of this leafy ecosystem.