Ceanothus megacarpus chaparral: A synthesis of ecosystem processes during development and annual growth

Abstract

The evergreen sclerophyll shrub,Ceanothus megacarpus, dominates the chaparral in many areas of the Santa Ynez Mountains, Santa Barbara County, California, USA. Often occurring in pure stands,C. megacarpus chaparral accumulates large amounts of aboveground live biomass during growth after fire as a result of high rates of net primary productivity (850 g/m2/yr) compared to other chaparral areas and Mediterranean scrublands of the world. Populations ofC. megacarpus are even-aged, growing from buried seed which requires heat treatment for germination. During the early years of stand development after fire, competition for water among the shrubs results in mortalities of up to 50% of the initial population. This competition is evident by changes in the population size-structure and dispersion pattern of the living shrubs. Water potential measurements during the summer drought show that smaller individuals are more severely stressed than larger individuals. Similarly, shrubs in younger stands develop lower water potentials than shrubs in older stands. There is little mortality after 15 to 20 years of growth; competition for water is alleviated in older stands presumably because the surviving shrubs have developed more extensive root systems. Leaf area reaches a steady-state of 1.6 m2/m2 in 20-year-old stands. At this stage of development, competition for light is manifest in shrub physiognomy, particularly in the death of lower branches.