Wayne Polley & Brian Wilsey
Plant growth or productivity varies though time in response to fluctuations in environmental conditions, such as variation between years in precipitation. The amount by which the productivity of mixed-species plant communities varies in response to precipitation fluctuations, as to other weather fluctuations, depends in turn on means and temporal variation in biological attributes of plant communities. In other words, community attributes influence how much changes in precipitation affect plant productivity. We evaluated the influence of means and temporal variation in two community attributes, species diversity and specific leaf area (SLA; leaf area per unit of leaf dry weight, weighted by species abundance), on the productivity response of perennial grassland communities to inter-annual change in precipitation. We calculated inter-annual variability in aboveground productivity of communities by dividing the standard deviation (a statistical measure of variation) of community productivity over 5 years by mean productivity. Thus variability can be reduced by reducing the standard deviation in productivity among years, increasing mean productivity, or both. Precipitation differed by as much as a factor of 3 among years, but productivity varied less over time in communities with high than low values of SLA, because high values of SLA were associated with increased mean productivity. On the other hand, productivity varied more in time when species diversity was high than low because high diversity communities had low mean productivity. High diversity communities contained few of the species, including exotic grasses, that were most productive. Our results implicate both species diversity and weighted SLA as potential indicators of variability in grassland productivity through time.