An expanded allometric model for crowns of four co-existing desert shrubs

Abstract

Key messageA new expanded allometric model with two asymptotic biometrics is built, revealing desert shrub crown scaling exponent varies with respect to soil water availability.AbstractMany biological metrics behave asymptotically when facing biophysical and resources constraints. However, simple allometric model (SAM) of power equation does not include any asymptotic variables. Thus, we built a new expanded allometric model (EAM) with two asymptotic independent variables. Then to test EAM statistically valid, we investigated and measured the crown radius and height of four co-existing shrubs species, including Nitraria sibirica, Reaumuria songarica, N. tangutorum and Tamarix ramosissima, at a saline desert site outside an oasis in southeast Junggar Basin, central Eurasia. Additionally, we also measured leaf biomass and leaf water potential, and surveyed soil profiles of water content and fine root to identify how factors influenced shrub crown ontogeny processes in desert ecosystems. The results showed that both SAM and EAM were statistically valid. The invariable scale exponent of crown radius vs. height from SAM reflected the average state of that from EAM, which had the common initial and ultimate values and the respective maximum for the four co-existing species. The crown horizontal extension was prior to vertical growth because the latter was uneconomic and shrub took an avoiding-self-shading strategy. Across the four species, the crown radius at the timing of the crown scaling exponent maximum was positively related to pre-dawn leaf water potential, which increased with plant rooting depth. It demonstrated that soil water availability limited the expansion of the shrub crown area. To sum up, shrubs grow asymptotically in a horizontal crown expansion manner avoiding self-shading and constrained by soil water availability in water-limited ecosystems.