Abstract
Harsh environmental conditions in arid ecosystems limit seedling recruitment to microhabitats under nurse structures, such as shrubs or rocks. These structures, however, do not necessarily afford the same benefits to plants because nurse rocks provide only physical nurse effects, whereas nurse plants can provide both physical and biological nurse effects. Nevertheless, if the nurse plant is a conspecific, the benefits it provides may be outweighed by higher mortality due to negative density-dependent processes; consequently, negative density-dependence is expected to limit plants from acting as nurses to their own seedlings. The degree to which an abiotic nurse may be more beneficial than a conspecific one remains largely unexplored. Here, we examine the role and elucidate the mechanisms by which conspecific plants and rocks promote plant establishment in a hyper-arid desert. For 4 years, we examined establishment patterns of Myrcianthes coquimbensis (Myrtaceae), a threatened desert shrub that recruits solely in rock cavities and under conspecific shrubs. Specifically, we characterized these microhabitats, as well as open interspaces for comparison, and conducted germination, seed removal and seedling survival experiments. Our results revealed that conspecific shrubs and nurse rocks modified environmental conditions in similar ways; soil and air temperatures were lower, and water availability was higher than in open interspaces. We found no evidence on negative density-dependent recruitment: seed removal was lowest and seedling emergence highest under conspecific plants, moreover seedling survival probabilities were similar in rock cavities and under conspecific plants. We conclude that the probability of establishment was highest under conspecific plants than in other microhabitats, contrasting what is expected under the Janzen-Connell recruitment model. We suggest that for species living in stressful environments, population regulation may be a function of positive density-dependence and intraspecific facilitation may be a process that promotes the persistence of some plant species within a community.
Highlights
It is widely recognized that in desert environments plant regeneration via seedling recruitment is rare and sporadic (Squeo et al 2007)
We found significant differences in ψs and gravimetric water content (GWC) among microhabitats and between seasons (Table 1); soils in open interspaces had more negative values of ψs and lower values of GWC than under conspecific shrubs or rock cavities (Fig. 2C and D)
Our results suggest plant parental care; adult M. coquimbensis plants ameliorate the harsh environmental conditions of this hyper-arid environment, which promotes recruitment and compensates for expected negative density-dependent processes when growing at high densities
Summary
It is widely recognized that in desert environments plant regeneration via seedling recruitment is rare and sporadic (Squeo et al 2007). The set of environmental conditions that allows seeds to germinate and become established (i.e. the recruitment or establishment niche; Young et al 2005) are found beneath the canopies of adult plants or within rock crevices; seedling recruitment in arid environments is frequently associated to these elements, which are referred to as ‘nurse plants’ and ‘nurse rocks’, respectively (Shreve 1931; Wied and Galen 1998; Flores and Jurado 2003; Munguia-Rosas and Sosa 2008; Peters et al 2008) These nurse elements provide seedlings with a more benign microclimate, as well as with improved soil water and nutrient availability (Callaway 1995). Seedlings sheltered by nurse rocks do not compete with their nurse at any stage during their ontogeny; in contrast, seedlings under nurse plants may compete with their nurse as they grow and their resource requirements change (i.e. ontogenetic niche shifts) resulting eventually in lower growth and/ or survival of either partner (Miriti 2006)
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