Growth rates of saguaro (Carnegiea gigantea): a giant cactus that sometimes grows really slow, but sometimes not
Modeling plant growth for perennial plants of arid ecosystems is challenging. Plants grow very slowly because they are constrained by climate and weather extremes. Also, local factors such as neighbors and soils affect the growth rates (GR) of individuals. Finally, individual GR are related allometrically to size. All these lead to temporal and spatial variation in growth rates. In this context, we analyzed the variability in GR at the intra- and inter-population level by exploring the macro-scale differences in GR among populations, the meso-scale differences in GR caused by changes in site quality, and the micro-scale differences in GR related to the local environment and to the size class of individuals. Permanent plots were established at thirteen populations across the distributional range. Direct measurements of height were taken for at least 100 plants on each site.
We found that the relationship between plant size and age was highly dependent on site. Some populations, had significantly steeper growth curves. In some populations, GR varied with site quality, but in others this pattern was not clear. Growth rates showed significant variation among individuals, and differences between populations were also significant. Growth rates were higher in warmer or wetter sites. Spatial variation in GR will allow us to estimate age with greater certainty and to model population structure with enough precision as to predict accurately plant population dynamics, the effect of recent climate changes, and to plan future conservation and management of the species.