Phenology, productivity, and nutrient accumulation in the post-fire chaparral shrub Lotus scoparius

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Abstract
Lotus scoparius is a drought-deciduous shrub which is an early and abundant colonizer of sites following fire in southern California chaparral. Productivity, seasonal nutrient concentrations, nutrient accumulations and phenology were studied in a 4-year-old burn site in Adenostoma chaparral in which L. scoparius had established 49% cover. Net aboveground primary production for L. scoparius was 105 g m-2 y-1; leaves accounted for 40% of the annual production. The true increment to biomass was only 17 g m-2 y-1; 83% of the net production entered the litter layer or standing dead components. In response to the Mediterranean climatic regime, most of the annual net production and plant activity occurred from May through June when photoperiod and temperatures were favorable and moisture was available. In July leaf abscission occurred in response to the summer drought conditions. Correlation and principal component analysis suggested consistent seasonal behavior in the foliar concentrations of N, P, Zn, and Mn. Nitrogen, P, K, and Zn were strongly reabsorbed from leaf tissues before abscission. Calcium, Mg, and Fe formed a second functional group of elements which increased in concentration throughout leaf maturation and which were not reabsorbed from senescing foliage. The seasonal pattern of nitrogen-containing organic compounds (chlorophylls and proteins) was most associated with the leaf phenology and water stress. The rapid growth of Lotus scoparius plays a role in conserving nutrients that might be lost through runoff and erosion after fire in the chaparral.