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.

PREMISE OF THE STUDY: Drought is the most limiting factor for plant growth and recruitment in arid environments. For widespread species, however, plant responses to drought can vary across populations because environmental conditions can vary along the range of the species. Here, we assessed whether plants of Encelia canescens from different populations along an aridity gradient in the Atacama Desert respond differently to water‐deficit conditions.METHODS: We conducted a common‐garden experiment using plants grown from seeds from three populations distributed along an aridity gradient to test for differences in relative growth rate (RGR), biomass, root to shoot ratios, and photosynthesis between watered and water‐deficit plants. Additionally, we examined the relationship between root to shoot ratios with RGR and total plant biomass along the gradient.KEY RESULTS: Water deficit affected root to shoot ratios, biomass, and RGR, but not photosynthesis. Populations varied in RGR and biomass; plants from the most arid population had higher RGRs, but lower biomass than those from the least arid population. In watered conditions, root to shoot ratios did not vary with RGR or biomass. Conversely, with the water deficit, root to shoot ratios were negatively and positively related to biomass and RGR, respectively.CONCLUSIONS: Response to water deficit differed among E. canescens populations; plants from the lowest rainfall environment adjusted root to shoot ratios, which may have allowed for equal biomass production across treatments. In contrast, plants from the wettest population did not adjust root to shoot ratios, but were reduced in biomass. These morphological and physiological changes to water availability showed that populations can use different strategies to cope with water deficit.

Many plant species bear fruits that suggest adaptation to seed dispersal by extinct megafauna. Present-day seed dispersal of these megafaunal plants is carried out by rodents, which can act as predators or dispersers; whether this interaction is primarily positive or negative can depend on the context. Here, we parameterized a stochastic model using data from the field and experimental arenas to estimate the effect of rodents on the recruitment of Myrcianthes coquimbensis -an Atacama Desert shrub with megafaunal fruits- and examine whether environmental conditions can alter the sign and strength of these rodent-plant interactions. We show that the outcome of these interactions is context-dependent: in wet conditions seed removal by rodents negatively impacts the recruitment probability of M. coquimbensis; in contrast, in dry conditions, the interaction with rodents increases recruitment success. In all cases, the strength of the effect of rodents on the recruitment success was determined mainly by their role as dispersers, which could be positive or negative. This study demonstrates that by caching seeds, rodents can be effective dispersers of a megafaunal fruit plant, but that the sign and magnitude of their effect on recruitment changes as a function of the environmental context in which the interaction occurs.

Plants that bear oversized fruits, but do not co‐occur with present‐day large‐bodied frugivores to disperse their seeds, are considered to represent seed dispersal anachronisms. It is hypothesized that these plants were originally dispersed by the megafauna that became extinct at the end of the Pleistocene. Accordingly, many plant species in South America bearing the so‐called megafaunal fruits occur in ecosystems lacking native modern megafauna. In Chile, Lardizabala biternata is an endemic vine that produces a large edible fruit, with a morphology suggesting a megafaunal seed dispersal syndrome. This species grows across the Chilean Winter Rainfall‐Valdivian Forests and has a rather wide distribution, along which there are currently no large native vertebrates that could ingest its fruits and disperse the seeds. We hypothesized that foxes may be its present‐day surrogate dispersers, because they are the only remnant mesofauna across L. biternata's range that include fruits in their diet. We collected fox scats periodically for 1 year and analyzed their contents, which revealed the presence of intact L. biternata seeds. These results suggest that foxes may be acting as present‐day dispersers of this vine; however, it is unknown whether they provide effective dispersal. Our findings stress the fragmentary state of the knowledge of ecological interactions among species in Chilean ecosystems, opening new perspectives in terms of the role of terrestrial fauna in maintaining ecosystem functions.

Seed loss to predators is a common phenomenon across plant communities and an important determinant of plant recruitment. Although seed predators commonly destroy the entire seed, partial seed consumption has been reported for many species; however, the degree to which seed mass loss affects germination dynamics and survival of new individuals has been poorly documented. We simulated seed damage in natural conditions to examine how different levels of cotyledonary reserve removal affect germination dynamics and seedling performance of Myrcianthes coquimbensis (Myrtaceae), a threatened Atacama Desert shrub. The experiment combined two levels of seed maturity with three levels of seed mass loss.