Plant phenology, shoot growth, and branching pattern in Mulinum spinosum (Apiaceae), a cushion shrub of the arid Patagonian steppe of Argentina

Abstract

Although species of arid environments share physiological and morphological strategies to reduce plant transpiration and protect leaves from excessive radiation and wind desiccation, they differ significantly in growth form, aboveground architecture, and rate of leaf turnover. We studied shoot growth pattern and leaf phenology in relation to rainfall and air temperatures during the growing season in Mulinum spinosum, a cushion shrub having a broad distribution in the Patagonian steppe, a windy, temperate region with summer water deficit. The partitioning of aerial biomass in plants with different basal areas was also studied. Shoot formation began in late summer when annual shoots (AS) produced temporary short shoots which restarted extension again in spring. AS exhibited continuous leaves senescence during the mid and late growing season. Delayed branching of AS was related to total leaves senescence on parent shoots in late summer. Fast leaf turnover reduces the transpiring area and would allow plant re-hydration from soil water reserves, thus facilitating branching initiation. Plant annual biomass was reduced to a thin external layer, which became denser in plants of higher basal area, protecting buds and leaves in this arid and windy environment.