Responses of the dominant shrubs of a semiarid thorn scrub community in north-central Chile to climate variability

Background/Question/Methods

In 1989, we initiated a large-scale ecological experiment on the role of biotic interactions –including interactions among predators, small herbivores and plants- as regulatory factors of a thorn scrub community in north-central Chile. Since then we have monitored the seasonal perennial cover and phenology of the main shrub species (principally Adesmia bedwellii, Porlieria chilensis and Proustia cuneifolia), as well as their functional characteristics. Cover was monitored using permanent line transects and point-intercept techniques on replicated 0.56 ha fenced exclosures with two different treatments; exclusion of small rodent herbivores and control plots. Since 2003 we have monitored monthly phenology of three individuals of the three dominant shrub species, and tallied vegetative buds, new leaves, flower buds, flowers or fruits on four stems per shrub each facing one of the four cardinal directions. 

Results/Conclusions

Woody vegetation is homogeneous both in species composition and cover distribution. Total perennial cover averages 60% and the three above-mentioned shrubs and one suffrutescent species (Chenopodium petiolare) make up around 85 to 100% of this shrub cover. Seasonal cover is higher in spring and lower in fall in this winter-wet summer-dry semiarid region. High rainfall years are usually associated with the El Niño Southern Oscillation (ENSO) phenomenon, when perennial cover is enhanced; the converse occurs when the warm phase of ENSO, La Niña, affects the west coast of South America. Plant cover of these woody shrub species showed seasonal and inter-annual variations, but these changes were not consistent through time. Both climate -mainly rainfall characteristics of the preceding and ongoing year of measure-, and biotic –density of herbivores- factors shape the pace and production of each shrub phenological cycle. However, there was a significant effect of herbivore exclusion on the production of vegetative and reproductive organs only for the leguminous species, Adesmia bedwellii. Functional traits of these shrubs appear to explain how they respond to these environmental factors, their strategies of coping with drought and nutrient-poor soils, and their relatively constant cover values through time.