Shrub encroachment at northeastern Patagonia. Where? When?

Abstract

Shrub encroachment is a global phenomenon in arid and semiarid regions. Where detailed long-term studies are available, encroachment is spatially and temporally heterogeneous. Notwithstanding, numerous encroaching environments globally suffer from inadequate data for detailed process reconstruction. One of these areas is northeastern Patagonia, where Chuquiraga avellanedae (Asteraceae) cover and abundance more than doubled in the last 50 years. Therefore, we aimed to estimate the establishment dates of 1595 C. avellanedae shrubs, measured in 2019, growing on sandy and clayey soils. We modelled the annual growth in each shrub's area backward in time, considering available soil water, shrub size, and soil texture. Of the shrubs on sandy soils, only 13 out of 305 (4.2%) were present in 1968. In contrast, of the 1290 shrubs growing on clayey soil, 716 (55.5%) had already been established in 1968. Shrub establishment was more continuous on sandy soils than on clayey soils, leading to a progressive increase in the proportion of shrubs established in sandy areas. Water availability from the current year was positively related to shrub establishment in both soils, but the previous year's water was also significant in clayey soils. We confirmed that soil properties regulate shrub encroachment patterns in northeastern Patagonia.