Facilitation between higher plant species in a semiarid environment

Ecology, July, 1996 by Francisco I. Pugnaire, Peter Haase, Juan Puigdefabregas

INTRODUCTION

Recent papers have emphasized the role of positive feedbacks in the dynamics of plant communities (Hobbie 1992, Wilson and Agnew 1992), in particular the ability of plant species to improve their environment by enhancing nutrient availability or resource capture. Some papers have also reported a major role of facilitation among higher plants, particularly in stressful environments such as salt marshes (Bertness and Shumway 1993, Callaway 1994), arctic regions (Walker and Chapin 1986, Carlsson and Callaghan 1991, Chapin et al. 1994), and desert environments (McAuliffe 1988, Franco and Nobel 1989). This suggests that positive relationships between species are more important than recognized to date in the dynamics of plant communities (e.g., Bertness and Shumway 1993).

Ever since Clements' work (1916), facilitation, i.e., the modification of the environment by one species that makes it more suitable for other species, has been considered to be merely a mechanism for succession (e.g., Connell and Slatyer 1977, Van Andel et al. 1993), of far less importance than competition in structuring plant communities. For this reason, the best example of facilitation involves fungi rather than higher plants (Van Andel et al. 1993). Bertness and Callaway (1994) proposed that the buffering of environmental harshness enhances positive interactions in communities under high physical stress, while competition predominates in more mesic habitats. Interaction among species should then lie somewhere on a continuum between facilitation and competition.

Facilitation could be important in semiarid environments, in which shrubs often have a layer of annual and perennial herbs underneath their canopies (Went 1942, Muller 1953, Fowler 1986). Herbs may benefit from greater availability of water under the canopy (Richards and Caldwell 1987, Joffre and Rambal 1988, 1993, Dawson 1993) as well as from greater availability of nutrients (Callaway et al. 1991, Rostagno et al. 1991, Gutierrez et al. 1993). It is also possible that the shrub benefits from the effect of herbs on the soil, for example, protecting the soil from erosion, direct insolation, and over-heating. However, the effect of the herb layer on the shrub has not yet been reported to our knowledge.

Some regions of southeastern Spain, with [approximately equal to]250 mm of annual rainfall, resemble desert ecosystems in which biomass is concentrated around individual or grouped shrubs, to form "islands of fertility" (Garcia-Moya and McKell 1970, Barth and Klemmedson 1982, Virginia and Jarrel 1983, Garner and Steinberger 1989). In southeastern Spain, the leguminous shrub, Retama sphaerocarpa (L.) Boiss., is a dominant species over large areas of rangeland and abandoned land (Losa and Rivas-Goday 1974). Retama is a practically leafless, N-fixing shrub with photosynthetic stems, called cladodes, and a deep root system that may draw water from a depth of [less than or equal to]20-30 m (Haase et al., 1996) that successfully colonizes perturbed habitats. Typically, there is an undergrowth of annual and perennial herbs associated with Retama. One of the most conspicuous species at our field site is Marrubium vulgare L. (Lamiaceae), a mesic, woolly perennial herb [less than or equal to]80 cm tall that often accounts for most of the understory biomass but also occurs in the open independently of Retama.

In this paper we investigate the association between Retama and the understory herb Marrubium vulgare in a semiarid region. We performed a natural, rather than manipulative, experiment, testing the hypothesis that Retama improves its own environment and facilitates the growth of Marrubium and other species while at the same time benefits from the sheltering herbs underneath it.

FIELD SITE

Our field site is located in the Rambla Honda, a valley near Tabernas, Almeria, Spain (37 [degrees] 08[seconds] N, 2 [degrees] 22[seconds] W; 630 m altitude) with an ephemeral water course that only carries water after heavy rains. Rambla Honda is in the southern foothills of the Sierra de Los Filabres mountain range. The local climate is semiarid with a mean annual precipitation of 218 mm and a pronounced dry season from June to September, broken only by occasional thunderstorms, but with little rain in most years. The mean annual temperature at Tabernas (490 m altitude, 9 km south of the field site) is 17.9 [degrees] C with monthly means of 10.7 [degrees] C in January and 27.1 [degrees] C in August (R. Lazaro, personal communication). Vapor pressure deficits are high during summer months, reaching [greater than] 4 kPa (Pugnaire et al., 1996).

The mostly flat, sandy bottom of the valley is disturbed by sporadic floods, and may best be described as a desert-like environment because of the low availability of water, the very low water holding capacity of the soil, and the sparse vegetation. This area is dominated by Retama, with a density of [approximately equal to]500 plants/ha covering [approximately equal to]40% of the surface. The largest shrubs may reach [greater than] 4 m in height, but most are between 2 and 3 m tall. Gaps between shrubs are almost bare of vegetation, with only occasional patches of winter annuals and negligible cover of other shrub species.