MORPHOLOGICAL AND PHYSIOLOGICAL-RESPONSES OF 3 CALIFORNIA OAK SPECIES TO SHADE

Interspecific differences in morphological and physiological responses to shade commonly correspond to ecological roles in mesic woodlands but rarely have been studied in semiarid woodlands and savannas. I compared shoot mass, root mass, root: shoot ratio, taproot elongation rate, whole-seedling leaf area, whole-seedling specific leaf area, leaf area ratio, and photosynthetic capacity among three species of California oaks in three experimental shade treatments: full sunlight (I(full)), 50% of full sunlight (I50%), and 10% of full sunlight (I10%). Although there were no intraspecific differences in the percentage of seedling biomass allocated to leaves, stems, and roots in different shade treatments, interspecific differences in biomass allocation, shade-induced changes in leaf morphology, whole-seedling photosynthetic capacities, and taproot elongation rates may explain, in part, shade tolerance of Quercus agrifolia and shade intolerance of Q. lobata seedlings reported in the literature. However, morphological and physiological responses of Q. douglasii, which recruits in shade, were similar to those of Q. lobata and do not adequately explain the high recruitment of Q. douglasii seedlings in shade reported in the literature. At I10%, root weights of Q. lobata and Q. douglasii seedlings decreased by 46% and 60%, respectively, and showed similar decreases in total seedling biomass and root: shoot ratios. Shade did not affect whole-seedling biomass, root weight, or root: shoot ratio of Q. agrifolia seedlings. Total seedling leaf area of Q. lobata and Q. douglasii did not increase in shade, whereas at I10% Q. agrifolia seedlings doubled in leaf area. Specific leaf area and leaf area ratio of Q. agrifolia seedlings also increased substantially more than the other two species. For seedlings grown at I10%, photosynthetic capacity per leaf area did not differ among species, but whole-seedling photosynthetic capacity at 150 muE m-2 s-1 Was 111.3 mumol g-1 s-1 of oxygen evolved for Q. agrifolia grown at I10% compared with 34.7 and 51.3 mumol g-1 s-1 for Q. lobata and Q. douglasii, respectively. At I10%, the taproot elongation rate of Q. agrifolia seedlings was 63% of the rate at I(full), while Q. douglasii and Q. lobata elongation rates dropped to 54% and 26%of rates at I(full), respectively. This variation in shade acclimation may partially explain interspecific differences in ecological functions, including interactions with other species, patterns of recruitment, and successional roles.