Nurse plants vs. nurse objects: effects of woody plants and rocky cavities on the recruitment of the Pilosocereus leucocephalus columnar cactus.
OBJECTIVE
Most studies on cactus recruitment have focused on the role of woody plants as seedling facilitators. Although the spatial association of cacti with objects had been described, the mechanisms underlying this association remain unknown. The aims of this study were to identify which mechanisms facilitate the establishment of a columnar cactus under the shade and protection of objects and to compare these mechanisms with those involved in plant-plant facilitation.


METHODS
Three split-split-plot field experiments were conducted to compare the effects of two microhabitats (inside rocky cavities and beneath plant canopies) on seed removal, germination, seedling survivorship and dry weight. Flat, open spaces were used as the control. For each microhabitat, the effect of seed or seedling protection and substrate limitation were explored; aboveground microclimate and some soil properties were also characterized.


RESULTS
The permanence of superficial seeds was greater inside rocky cavities than beneath woody plant canopies or on flat, open areas. Germination was similar in cavities and beneath plant canopies, but significantly higher than on flat, open areas. Seedling survivorship was greater beneath plant canopies than inside cavities or on flat, open spaces.


CONCLUSIONS
The mechanisms of plant facilitation are different from those of object facilitation. There are seed-seedling conflicts involved in the recruitment of P. leucocephalus: nurse plants favour mainly seedling survivorship by providing a suitable microenvironment, while nurse objects mainly favour seed permanence, by protecting them from predators.
Departamento de Ecología Aplicada, Instituto de Ecología A. C., Apartado Postal 63, Xalapa, Veracruz, CP 91000, Mexico. miguel.munguia@posgrado.inecol.edu.mx
Miguel Angel
MA
Munguía-Rosas
Vinicio J
VJ
Sosa
D029421:Cactaceae / Q000254:growth & development / Q000502:physiology*; D002980:Climate; D017753:Ecosystem; D018525:Germination; D011157:Population Dynamics; D036226:Seedlings / Q000254:growth & development / Q000502:physiology; D012639:Seeds / Q000254:growth & development / Q000502:physiology; D012987:Soil
D012987:Soil