Germination of California Annual Range Plants in Response to a Summer Rain 1

During the period 7 to 9 July, 1974 the northern half of California experienced an unusual summer rain of 40 to 150 mm. This once‐in‐a‐century occurrence provided an opportunity to observe germination responses of resistant annual grasses, legumes, and forbs. Annual plants which have evolved in a Mediterranean climate generally are assumed to have acquired mechanisms for resistance to summer germination, which ordinarily would result in catastrophic losses of seedlings. Growth of resident and introduced annual range plants in response to the July rain was observed on grazed ranges at the University of California's Sierra Foothill Range Field Station, Browns Valley, Yuba Co. Transects were located on north‐ and south‐facing slopes consisting entirely of herbaceous vegetation. Square‐decimeter quadrats were sampled along the transects on 19 July, 30 August, and 18 October, and the seedlings found were counted and identified. Seeds of only a few annual species germinated and few plants were established in response to the summer rain. Species found were principally Bromus mollis L. (soft chess), Erodium botrys (Cav.) Bertol. (broadleaf filaree), Trifolium hirtum L. (rose clover), and T. subterraneum L. (subterranean clover). Seedling densities were low compared with emergence under normal fall rainfall. Moderation of the Mediterranean climate summer temperature regime by soil surface irregularities or by litter cover favored germination and emergence. Emergence also was better on north‐facing aspects than on south‐facing aspects. The data indicate that one characteristic of the many herbaceous annual species which have successfully invaded California since Gold Rush times is effective seed dormancy during summer. It is possible that this trait could prove useful in plant introduction screening.