Estimating annual rainfall threshold for establishment of tree species in water-limited ecosystems using tree-ring data

In arid and semi-arid ecosystems, water availability is discontinuous, highly variable, and characterized by discrete pulse events separated by long periods of limited resource availability. Plant recruitment in these ecosystems is also episodic and dependent on the water available during and after these discrete rainfall events. Precipitation thresholds for plant establishment have been estimated mainly for herbaceous plants and tree seedlings, but extrapolation of short-term results based on seedlings to natural tree populations is difficult. Nevertheless, estimations of water availability thresholds for tree recruitment are essential for successful policies on forest conservation and restoration. We propose a methodology to estimate precipitation thresholds for adult tree populations using tree-ring series and precipitation data. We used this methodology with two Prosopis species from South America: Prosopis pallida and Prosopis chilensis. Results indicate a precipitation threshold of around 85 mm for the establishment of P. pallida trees, whereas the threshold for P. chilensis is likely to be much higher.