Competition intensity as a function of resource availability in a semiarid ecosystem

Abstract

Two field experiments were conducted using three dominant perennial species of the Chihuahuan Desert: Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a flat-stemmed succulent cactus). Two hypotheses concerning competition in arid plant communities were tested. (1) Marked resource partitioning with no interspecific competition could be expected since the three species belong to different life-forms, and that plant growth in deserts is basically limited by harsh environmental conditions. (2) Alternatively, resource scarcity (particularly water) will result in strong plant competition. In a 1-year removal experiment, water status and plant growth of the three species were monitored in twelve 10 m × 10 m plots randomized in three blocks and assigned to the following treatments: (a) removal of all species, except H. mutica; (b) removal of all species, except L. tridentata; (c) removal of all species, except O. rastrera, and (d) control without any manipulation. In a watering experiment, under two neighbourhood conditions (growing isolated or in associations of plants of the three species in plots of 20 m), the water status of the three species and the growth of H.mutica and L.tridentata were studied for 32 days after an irrigation equivalent to 30 mm of rain, similar to a strong storm event at the site. In the removal experiment, where plants were free to capture water, no evidence of competition was observed. However, during the watering experiment, in which water was forced into the soil, competitive effects were observed. Associated individuals of L. tridentata had lower xylem water potentials and osmotic potentials (OPs) and produced shorter twigs and less leaves and nodes. Although less pronounced, neighbours also had a negative effect on the OP in O. rastrera. According to these results, the intensity of the interspecific competition for water seems to depend on the level of resource availability in the soil. Thus, the validity of the two hypotheses tested in this study also depends on the level of resources. Competition could be absent or very low in years of low precipitation, as in the year of this study (173 mm against a 25-year average of 264 mm). However, when soil water availability is high, e.g. following heavy rain, the negative interactions between species could be more intense.

Demographic variation in a desert shrub, Larrea tridentate, in response to a thinning treatment

“…Furthermore, they suggest that negative densitydependence was not acting within this population through competitive interactions, or at least not at a level that was detectable in our study. The apparent lack of intra-specific competition in this desert shrub population is consistent with the notions of early desert ecologists reflecting on the harshness of the environment and suggesting competition is infrequent or generally absent in desert plant communities (Shreve, 1942;Went, 1955), as well as being consistent with many of the empirical results obtained from other perturbation experiments (Fonteyn & Mahall, 1978Briones et al, 1998).…”

“…This is consistent with the general demographic characteristics of Larrea populations (Goldberg & Turner, 1986), as well as being consistent with the results from perturbation experiments where the effects of competition were detected either infrequently (Fonteyn & Mahall, 1981) or only under specific environmental conditions (Briones et al, 1998). Therefore, to definitively determine the relative importance of intra-specific competition and densitydependent population regulation for this long-lived desert shrub will probably require following a perturbation experiment over decades.…”

“…However, Larrea removals resulted in a significant effect on xylem pressure potentials of target individuals only during one of three wetting and drying cycles. A similar removal experiment by Briones et al (1998) showed a similar lack of experimental evidence for competition affecting physiological performance, as well as little effect on short-term measures of growth and reproduction. Only when supplied with water did isolated Larrea exhibit greater twig, node, and leaf growth in comparison to individuals in unmanipulated areas.…”

“…These notions of the strength of competition in desert communities are relevant to both intra-specific competition and inter-specific competition (Yeaton & Cody, 1976;Yeaton et al, 1977;Briones et al, 1996Briones et al, , 1998). Here we focus on intra-specific competition because of its relevance to understanding the relative importance of densitydependent vs. density-independent population regulation in plant populations (Antonovics & Levin, 1980;Fowler, 1988).…”

Leaf growth dynamics in four plant species of the Patagonian Monte, Argentina

“…Despite that C. avellanedae did not produce new growth during the period of study, this species has xerophitic leaf attributes (Campanella and Bertiller 2011) that contribute to overcome extending hot and dry periods (Wright et al 2004). Because temporal and spatial variation in water availability could modify competitive interactions among plants in desert ecosystems (Briones et al 1998) it is important to perform long term research. Thus, additional studies will be required to get a better understanding of growth pattern of the species studied with above average precipitation and/or with high number of large precipitation events.…”