Coexistence mediated by ontogenetic shifts in plant-plant interactions: Insights from a rare cycad within angiosperm communities

Background/Question/Methods

Angiosperms have gained a competitive edge due to more modern reproductive and physiological traits compared to gymnosperms. This traditional view is challenged when both plant groups coexist in overlapping habitats. We studied how ontogenetic shifts in competition and facilitation may promote the coexistence of a rare cycad with its evolutionary more-modern angiosperm neighbors in three cycad populations in northwestern Mexico. We collected four years of data on growth and survival in the rare cycad Dioon sonorense from three life-stages growing closely associated with one of three different neighbors (conspecifics, heterospecifics –angiosperms, or abiotic elements –rocks). Around each target plant, we measured canopy openness, distance to its closest neighbors, and stem density to calculate a competition index. We used mixed-effects models to evaluate how survival and relative height growth rate (RHGR) responded to intraspecific and interspecific competition, canopy openness, identity and distance to nearest neighbor. We fully stem-mapped the largest D. sonorense population and associated species to test for positive spatial associations with potential nurse plants. We further constructed photosynthetic light curves to estimate key photosynthetic traits related to shade tolerance at different life-stages.

Results/Conclusions

In early ontogeny (seedling stage), shading increased survival whereas intraspecific competition decreased both survival and RHGR via nearest neighbor mediated density-dependence. Seedling survival was higher in association with rocks where intraspecific and interspecific competition were generally lower. Intraspecific competition became weaker while interspecific competition grew in importance at more advanced stages. Shade tolerance decreased with cycad ontogeny in synchrony with a progressive increase in canopy openness and the increasing spatial disassociation from an endemic palm species with the highest nurse-effect potential. We provide empirical support to the long-standing assumption that marginal habitats (e.g. rocky, nutrient poor) provide refugia to cycads from angiosperm competition, but also from intraspecific competition at early ontogeny when, paradoxically, facilitative interactions by angiosperms may be critical. The reinforcement of these interactions may provide a mechanism for stabilizing selection and conservation of ancient cycad traits. Cycads may be tracking the environmental factors for which they exhibit narrow physiological tolerances especially during their regeneration phase at the cost of a higher intraspecific competition. The lack of knowledge of plant-plant interactions in rare or endangered species may underestimate a valuable management strategy under global climate change scenarios.