Population Regulation of Eschscholzia Californica by Competition and Edaphic Conditions

Abstract

Eschscholzia californica Cham., the California poppy, occurs naturally in western North America from the Columbia River south to Baja California and from the coast of California eastward into the Great Basin. Within this extensive region it grows in open sites, frequently associated with grasses. Both the poppy and the grasses require ample light for optimal development and begin growth contemporaneously after the advent of winter rains. In dry years, when grass develops poorly, poppies apparently do well; in wet years the reverse is true. The poppy grows on well-drained soils of dunes, alluvial fans, river terraces, steep banks, and rocky places. These soils may be derived from a wide range of parent materials, including serpentine. It has also been observed to colonize burned-over areas and endure on them for a number of years (Bowerman 1944). The poppy has behaved as a benign weed under conditions created by man. Small populations arise atop gravel piles, in gravelly ballast of railroads, or on road cuts and other areas where the established vegetation has been destroyed by excavation. Occasionally poppies may be seen in grain fields, where their growth is apparently favoured by mowing the overtopping grain. Although the plants themselves are cut back, new shoots emerge from ground level and flower from mid-summer to autumn. These observations suggested the following hypotheses: (1) Genetic adaptation to specific soil type is involved in the poppy's adaptation to a broad range of soil types. Such response has been reported for other herbaceous species (Kruckeberg 1951; Jowett 1958, 1959; Bradshaw & Snaydon 1959; Bradshaw 1960). (2) Growth of associated grass is antagonistic to Eschscholzia. (3) Eschscholzia cannot tolerate soils with high water content but can become established on porous soils high above the water table. These hypotheses were tested experimentally at the Botanical Garden of the University of California at Berkeley during 1959 and 1960.