How to be a successful shrub in northwestern Patagonia grasslands: Population dynamics of Senecio bracteolatus in a fire prone ecosystem

Abstract

In arid and semi-arid environments, demographic processes of shrubs are affected by fire and precipitation. Understanding the impact on the demographic processes of the population is crucial for ecosystem conservation and management. Our aim was to assess the population dynamics of the native shrub Senecio bracteolatus and its relation to fire frequency and precipitation in northwestern Patagonia. We constructed a stochastic matrix model and we conducted numerical simulations to evaluate scenarios varying fire frequency and precipitation. To parameterize the model, we conducted greenhouse and common garden experiments and collected previous field and precipitation data. We divided the S. bracteolatus life cycle into four classes (current year seedlings, previous year seedlings, juvenile, and adult plants) and two recruitment microsites (under tussock canopy and gaps between tussocks). The climate was included in the model as accumulated spring and summer precipitation. The former affects seedling germination, while the latter affects fire regimes. The model predicts that frequent fires (every four years) increase the population of S. bracteolatus, but if fires are infrequent (more than 16 years), the population increase with an annual rate of growth of around 8.5 %. However, for annual and biannual fire frequencies, the population experiences a decline, and with a three-year fire return interval, it stabilizes. High fire frequencies lead to population decrease, attributed to the reduction of the protective canopy of tussock grasses, essential for the recruitment of the shrub. The population growth rate of S. bracteolatus increases significantly more when spring precipitation is above historical averages, and fire frequency increases when summers are dry. Our study contributes to understanding the influence of fire and precipitation on shrub demographic processes in the context of climate change, providing useful information for fire management to improve the sustainable use of grasslands.