NUTRIENT CHANGES IN TWO CHAPARRAL SHRUBS ALONG A FIRE-INDUCED AGE GRADIENT

Adenostoma fasciculatum H. & A. and Ceanothus leucodermis Green in chaparral stands of the foothill zone of Sequoia National Park, California, show a rapid decline in foliage concentrations of nitrogen over the first six years following fire. This is followed by a more gradual decline over succeeding years. Phosphorus concentrations show a similar early decline, followed by an increase in older age stands. Considerable data suggest luxury consumption of nutrients in both species following fire. Following a sharp increase in aboveground nutrients per unit of Adenostoma canopy area for the first 16 years of growth, a plateau is reached. The contemporaneous occurrence of restricted net nutrient uptake and observed stand senescence suggest a possibility of a causal relationship. Natural chaparral fire frequencies promote fire-cycling of nutrients at intervals consistent with periods when nutrient availability becomes limiting. The fire-nutrient interactions are of critical importance in influencing quality and quantity of suitable browse for vertebrate herbivores.