SEASONAL PATTERNS OF NUTRIENT DEPOSITION IN A QUERCUS-DOUGLASII WOODLAND IN CENTRAL CALIFORNIA

The monthly deposition of total nitrogen, phosphorus, potassium, calcium and magnesium via canopy throughfall, and various components of the litterfall was measured for 31 months under mature Quercus douglasii and in the bulk precipitation in the surrounding open grassland. Seasonal patterns of nutrient concentration in leaf litter, throughfall, and precipitation were also measured. Total annual subcanopy deposition exceeded open precipitation deposition by approximately 45-60X for nitrogen, 5-15X for phosphorus, 30-35X for potassium, 25-35X for calcium, and 5-1OX for magnesium. Total annual subcanopy deposition was low in comparison to other oak woodland sites reported in the literature. Throughfall and leaf litter were the primary sources of nutrients and thus determined the seasonal peaks of nutrient deposition. The first autumn rains and leaf fall were associated with one peak in nutrient deposition, and throughfall during early spring leaf emergence was associated with a second peak in potassium, magnesium and phosphorus. Non-leaf plant litter (excluding acorns) provided approximately 15-35% of most nutrients, with twigs and bark depositing over 12% of the annual calcium flux in 1987-1988, and flower litter depositing over 8% of the annual nitrogen flux in 1986-1987. Acorns had high concentrations of phosphorus and nitrogen and during the mast season of 1987-1988 they contained a large proportion of the total subcanopy annual flux of these elements. With acorns excluded, total annual nutrient deposition was similar between years, but timing of nutrient deposition differed. Late summer leaf fall associated with drought, variation in precipitation, and variation in deposition of non-leaf parts were associated with seasonal differences in nutrient deposition between years.