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A resting plate meter was tested in the laboratory and on a field site to determine its effectiveness for estimating peak herbage mass on semi-arid grasslands. In laboratory tests, data from the plate meter closely predicted the herbage mass of four perennial bunch grasses (thirteen of sixteen tests with r 2 ≥ 0·90) but the closeness of the prediction varied with the pressure of the plate. Field tests took place on a southern Arizona, USA semi-arid grassland in 2005, 2006, 2007 and 2008. Twenty teams of two or three people sampled the site; five teams in each year measured eight to thirteen plate heights and clipped the vegetation beneath. Consistent with a need for yearly calibration, a significantly different ( P < 0·05) linear relationship was detected between 3 of the 4 years which was associated with differences in average herbage mass of dry matter (DM) (1525 kg ha−1 for 2005, 2093 kg ha−1 for 2006, 1338 kg ha−1 for 2007 and 1370 kg ha−1 for 2008). Plate height poorly predicted herbage mass within years ( r 2 = 0·21, 0·51, 0·49 and 0·41 respectively) with plate heights explaining half or less of the variability in field herbage mass and having a mean prediction error of 466 kg DM ha−1. The plate meter technique had limited potential for estimating peak herbage mass in semi-arid grasslands. [ABSTRACT FROM AUTHOR]

Small gaps and clumped species distributions are common in grasslands. In California annual grasslands, patches of Lolium multiflorum Lam. and Bromus hordeaceus L. are often separated by gaps. These gaps potentially limit the productivity and associated resource use of these grasslands. The effect that differences in spatial aggregation, gap distribution and species mixing on 20-cm-diameter plots has on overall forage production by these two grasses was tested. There were three levels of aggregation: whole plots planted; half planted/half empty; two opposing quarters planted/two empty. Each species was planted in each distribution, and they were combined as mixed, half L. multiflorum /half B. hordeaceus and two quarters L. multiflorum /two quarters B. hordeaceus (nine treatments). Plant aggregation had no significant effect on above-ground production of whole plots, but individual tillers near gaps were significantly larger than others. Plasticity in the growth of individual annual grasses effectively buffered against variation in average productivity resulting from variations in plant distribution. There were significant (P < 0·001) differences in forage production as a result of the species the plots contained. Plots containing only L. multiflorum produced 4053 kg of dry matter (DM) ha–1 , B. hordeaceus plots produced 2448 kg of DM ha–1 , and plots containing both species produced 4712 kg of DM ha–1 . At small scales, spatial distribution was less important than species composition in determining annual grassland productivity. [ABSTRACT FROM AUTHOR]