Extending Grime’s CSR model to predict plant demographic responses across resource availability gradients: evidence from the Patagonian steppes

Sexual reproduction, growth, and survival are crucial demographic
strategies for plant population viability. Here, we propose a conceptual
model predicting demographic responses of species based on their
ecological strategy and the heterogeneity of environmental conditions
within a biogeographical unit and then applied it to a case study from a
5-degree latitudinal gradient in the Patagonian steppes. We also aim to
disentangle genetic from environmental effects on demographic responses.
We performed in-situ and common garden experiments with two species from
six local populations of the Occidental Phytogeographical District of the
Patagonian steppes. Species differ in key ecological traits, and thus fit
into Grime´s model for evolutionary strategies in plants: one as
competitive species and the other as stress-tolerant species. We
calculated population growth rate (λ) and performed elasticity analyses to
compare the contribution of each demographic strategy to population
fitness between species and among local populations distributed along 600
km latitudinal gradient with differences in mean annual precipitation
(MAP). We highlight four results. First, the competitive species change
from sexual reproduction to growth as MAP increases. Second, the
stress-tolerant species relied on growth and survival along the MAP
gradient. Third, interannual variation in resource availability modulated
demographic responses for both strategies. Fourth, based on the comparison
of the in-situ and common garden experiments, we submit that demographic
responses were genetically driven. Our study shows that demographic
responses can be roughly predicted by the ecological strategy across
environmental gradients. We show that differences arise not only between
species, but also were genetically driven differences within species among
local populations. Scaling up plant-level responses to population-level
dynamics allows for a process-based understanding of current and future
biogeographical species organization. Furthermore, conservation and
restoration efforts should be guided by demographic strategies underlying
population viability.