Variation in δ15N of fog-dependent Tillandsia ecosystems reflect water availability across climate gradients in the hyperarid Atacama Desert

Abstract

Abstract The Atacama Desert is considered one of the driest places on Earth, where the availability of water plays a crucial role in determining the presence of plants. The sparse vegetation is limited to the coastal mountains, where abundant fog provides the main source of water and nutrients for unique Tillandsia landbeckii ecosystems. The apparent retreat of this fog-dependent vegetation over the past decades, however, may relate to changing climatic conditions, in particular increasing aridity. In this study, we used the nitrogen isotopic composition (δ15N) of plant organic matter as a measure of water availability and atmospheric nitrogen input in present and past Tillandsia dune fields. We compiled an extensive data set on δ15N values of living plants and corresponding site factors (latitude, elevation, cloud cover and precipitation) along a coastal transect. We present radiocarbon-based ages of relict T. landbeckii layers preserved in sand dunes that evolved episodically over the past 2500 years. Site-averaged δ15N values range from +2‰ to −16‰, with variations of up to 4‰ observed within one site that can be related to changes in elevation. The spread in δ15N values is surprising and considerably larger than previously reported for T. landbeckii. In contrast, δ15N values of Huidobria fruticosa and Ophryosporus spp. leaves collected mostly below and above the fog zone vary between +4‰ and + 17‰, largely in agreement with global observations from water-limited systems. Comparison with satellite-based meteorological data and modelling results revealed significant correlations between δ15N values of T. landbeckii and total cloud cover (r = −0.90; p