Population rules can apply to individual plants and affect their architecture: an evaluation on the cushion plant Mulinum spinosum (Apiaceae)

Javier G. Puntieri 1 2 Mara A. Damascos 0 Yanina Llancaqueo 0 Maya Svriz 2 Background aims 0 Departamento de Bota nica, Universidad Nacional del Comahue , Quintral 1250, (8400) Bariloche, Argentina 1 Universidad Nacional de Ro Negro , Sede Andina, (8400) Bariloche, Argentina 2 Instituto de Investigacion en Biodiversidad y Medioambiente (INIBIOMA) , Quintral 1250, (8400) Bariloche, Argentina Methodology Principal results Conclusions Plants are regarded as populations of modules such as axes and growth units (GUs, i.e. seasonally produced axis segments). Due to their dense arrays of GUs, cushion plants may resemble crowded plant populations in the way the number of components (GUs in plants, individuals in populations) relates to their individual sizes. The morphological differentiation of GUs and its relationship with biomass accumulation and plant size were studied for the cushion subshrub Mulinum spinosum (Apiaceae), a widespread species in dry areas of Patagonia. In 2009, GUs were sampled from one-quarter of each of 24 adult plants. Within- and between-plant variations in GU length, diameter, number of nodes and biomass were analysed and related to whole-plant size. Each year, an M. spinosum cushion develops flowering GUs and vegetative GUs. Flowering GUs are larger, twice as numerous and contain two to four times more dry mass (excluding reproductive structures) than vegetative GUs. The hemispherical area of the cushions was positively correlated with the biomass of last-year GUs. The biomass of flowering GUs was negatively correlated with the density of GUs. Mulinum spinosum plants exhibited a notable differentiation between flowering and vegetative GUs, but their axes, i.e. the sequences of GUs, were not differentiated throughout the plants. Flowering GUs comprised a major proportion of each plants photosynthetic tissues. A decrease in the size of flowering GUs and in their number relative to the total number of GUs per plant, parallel to an increase in GU density, is predicted as M. spinosum plants age over years. The assimilative role of vegetative GUs is expected to increase in summer because of their less exposed position in the cushion. These GUs would therefore gain more from warm and dry conditions than flowering GUs. Introduction In plants with rhythmic growth, their constituting axes increase in length at their distal ends through the production of structural units known as growth units (GUs; Barthelemy and Caraglio 2007). Within each plant, and depending on the species, GUs of several forms and functions may coexist (Reffye et al. 1991; Costes 1993; Puntieri et al. 2003). For instance, the coexistence of short GUs, long GUs and flowering GUs is common in adult individuals of woody plants and parallels a hierarchical axis organization (Edelin 1991; Kozlowski and Pallardy 1996; Yagi 2000; Ishihara and Kikuzawa 2009). In other species, all coexisting GUs are alike and each plant consists of a non-hierarchical system of axes (Prevost 1978). This seems to be the case in many plant species with dome-like aerial growth, such as cushion-forming plants (Rauh 1939; see Leeuwenberg model in Halle et al. 1978). Under the interpretation of plants as metapopulations of modules (e.g. White 1979; Enquist et al. 1998; Kikuzawa 1999), interactions between GUs in plants with dense arrangements of GUs, such as cushion plants, may resemble those between plants in a crowded population. Competition among plants in a crowded population leads to a negative relationship between the number of plants per unit of area occupied by the population (plant density) and mean plant size (Kira et al. 1953); the dispersion around the mean of individual plant size is also affected by competition (Weiner 1990, 1995; Puntieri and Pysek 1993; Kikuzawa 1999; Damgaard et al. 2002). At plant level, an increase in the number of GUs per unit of surface of a cushion plant (GU density) may be expected to affect both the mean and the dispersion of GU size. Mulinum spinosum (Apiaceae) has been described as a hemispherical cushion subshrub with densely arranged peripheral GUs on a relatively loose and dry core (Rauh 1939; Fig. 1A). Each M. spinosum plant develops vertical or slanted GUs (Fig. 1B). In adult plants of this species, each GU extended in a particular growing season develops between one and three lateral and/or terminal GUs in the following growing season (Damascos et al. 2008), so that the total number of GUs at the surface of a plant tends to increase exponentially from one year to the next. Meanwhile, the yearly increase in the outer surface of a 30- to 80-cm-high M. spinosum cushion is much slower, since new GUs add a layer ,12 cm thick to the plant each year (Damascos and Ghermandi 2007). This means an increase in GU density over the years. Under these assumptions, older plants would have a higher number and density of GUs and total mass, but a lower mean GU size than younger cushions. The present study evaluates the extent to which individual M. spinosum plants may be equated to populations of competing GUs regarding size density relationships and variations in GU size. Materials and methods Study species Mulinum spinosum is one of the most characteristic species in dry and disturbed areas of Patagonia (Cabrera and Willink 1980; Anchorena and Cingolani 2002). It is frequently dominant, replacing palatable tussock grasses in regions under intense grazing pressure (Sala et al. 1989; Aguiar and Sala 1998), and invading disturbed areas in nearby temperate forests (Cabrera and Willink 1980). An adult M. spinosum cushion reaches 50 100 cm in height, and regenerates its aerial plant biomass in a few years after fire (Damascos and Ghermandi 2007). Each leaf base consists of a close sheath that embraces the stem tightly; the stem is hidden under the leaf bases until the leaves decay, which occurs several years after their growth (Soriano 1983). Leaf blades are trisect and spiny, and dry out completely in the autumn following the growing season of their extension, although they remain standing for years (Damascos and Ghermandi 2007; Damascos et al. 2008; Fig. 1). Plants have a 35- to 45-cm-long taproot with dense horizontal ramifications (Soriano 1983). The annual growth of adult M. spinosum cushions consists of the development, in spring, of long GUs derived from short GUs initiated at the end of the previous summer period (the latter are described as temporary short shoots in Damascos et al. 2008). Most of the GUs extended in spring develop a terminal inflorescence. Both flowering and vegetative (i.e. non-flowering) GUs may develop short GUs from axillary buds in late summer; the number of these branches is higher in flowering than in vegetative GUs (Damascos et al. 2008). The biomass produced by an M. spinosum plant in 1 year is circumscribed to the peripheral layer, including all GUs extended in that year. Study area This study was performed in a temperate semi-arid area of northw (...truncated)