Decomposition of Ocotillo (Fouquieria splendens) Wood in the Colorado Desert of California

Abstract

The weight loss of stem sections of ocotillo (Fouquieria splendens) was determined for 11 years in the Colorado Desert of California. Weight of a stem section was a function of original weight, time in the field and presence or absence of termites. A section weighing 10 g is estimated to have a half-life of 29 years without termites and 17 years if termites find the section. Small sections of stem decayed slightly more rapidly than larger sections. INTRODUCTION Decomposition in desert ecosystems is considered to be a slow process and, because of this, there can be more dead than live aboveground biomass (Hadley and Szarek, 1981). There are few measurements of natural rates of decomposition in deserts; however, examples of such studies include Haverty and Nutting (1975), who measured loss in wood due to termites over a period of one year; Comanor and Staffeldt (1978), who documented loss of weight by creosote bush (Larrea tridentata) parts over 5 months, and Fowler and Whitford (1980), who examined L. tridentata litter in bags over a period of 1-2 months. The purpose of this note is to report results of a study of weight loss in pieces of ocotillo (Fouquieria splendens) stem over 11 years. The study was prompted by a question about the stability of ocotillo populations: dead ocotillos are common in the western Colorado Desert of California, but small living individuals are very rare. Either dead ocotillos take a very long time to decay and, hence, the rarity of small living individuals reflects stationary recruitment or the population in the Colorado Desert of California is in a state of decline, with high rates of death and decomposition and a low rate of recruitment. This work was conducted at the southern end of the Anza-Borrego State Park, California (33 N lat, 116 W long). The area is a bajada between the Coyote Mountains to the N and the Jacumba Mountains to the W. The substrate is recent alluvium made up of locally derived andesitic volcanic rock mixed with granite and sedimentary rocks from the surrounding hills. The area is finely dissected with a microtopography of small water courses. The elevation is 275 m above sea level. In addition to Fouquieria splendens, Larrea tridentata, Ambrosia dumosa, Opuntia echinocarpa, 0. bagelovii and Encelia farinosa are abundant. METHODS Stems of living ocotillo were collected in January 1971 at the study site, cut into sections ca. 8.5 cm long (range 8.0-9.5 cm), dried at 85 C for 2 weeks, and then weighed to the nearest milligram. Average weight of the dried sections was 15.1 g 4 7.8 SD (range 5.0-30.9 g, n = 22). A wire was fastened around 15 sections together with a numbered metal tag. In the field, wires were fastened to wooden stakes and sections were left on the soil surface. Two sections were collected in 1972, 1973, 1974, 1977 and 1978. Five sections were collected in 1982. All sections were randomly selected. Sand was brushed from the sections, they were dried at 85 C and then weighed to determine loss. Analysis is based on a simple model in which the final weight is related to original weight by an allometric function: