Moisture Stress Effects on Biomass Partitioning in Two Sonoran Desert Annuals

Abstract

The relative biomass accumulation rate and allocation to reproduction were quantified for Plantago insularis and Schismus barbatus, growing under four different levels of moisture stress. Analysis of growth by an allometric method indicated that seasonal growth rates were modified by water input, and for all water input treatments Schismus grew more rapidly than Plantago. As the growing season progressed and ambient temperatures rose, incremental water input had an inverse effect upon relative root growth of both species. Relative foliar growth rate of either species was not markedly altered by water input. When reproductive structures were present, their growth rates were higher than those of vegetative structures in both species. Schismus responded to water input by significantly increasing the biomass allocated to reproduction, which increased relative reproductive allocation. The computed reproductive allocation of Plantago is not correlated with water input, although the ultimate reproductive biomasses were correlated with the summation of plant moisture stress during reproduction. Thus, the bar-day formulation is useful in estimating the relative sensitivity of reproduction to drought for native annual species growing in similar habitats with varying levels of moisture stress. INTRODUCTION Several recent studies of biomass accumulation and allocation by desert annuals have been reported since the pioneer investigation by Klikoff (1966). His studies indicated the magnitude of biomass accumulation for plants of Plantago insularis was dependent upon the degree of moisture stress and was modified in some moisture conditions by plant density. Subsequently, an extensive survey of biomass accumulation and allocation was conducted with 32 species of Mojave Desert annuals (Turner, 1973, 1974). In the relatively dry year 1972, 22 of these species allocated a higher percentage of biomass to reproductive structures, as compared to the relatively wet year 1973, five species allocated a lower percentage of biomass to reproductive structures. The remaining five species allocated an equivalent percentage of biomass to reproductive structures during both dry and wet years. This investigation, along with the recent studies of Bell et al. (1979), Clark and Burk (1980) and Monson and Szarek (1981), established that biomass allocation in different moisture conditions is species-specific. However, a common deficiency of these three field studies is that no single species was investigated in similar habitats with different levels of moisture stress. Klikoff's investigation suggests that habitat-specific variations in moisture stress modify the rate of biomass accumulation, but he did not address the question of moisture stress-induced variations in allocation patterns. To evaluate the extent of habitat-specific modifications of biomass accumulation and allocation of desert annual species, we have expanded the original study of Klikoff (1966). Our purpose in this investigation was to quantify the biomass accumulation rate and allocation patterns of two abundant Sonoran Desert annuals, Plantago insularis and Schismus barbatus, treated with four different levels of moisture stress in adjacent experimental plots. One method of quantifying the biomass accumulation rate employs an allometric concept of growth analysis (Batschelet, 1979). The allometric concept assumes that the biomass accumulation rate of a plant part or organ may be different from, but dependent upon, the growth rate of the whole plant. Hence, relative growth rates are deterI Present address: Laboratory of Biomedical and Environmental Sciences, University of California, Los Angeles 90024.