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

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 (delta N-15) 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 delta N-15 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 delta N-15 values range from +2 parts per thousand to -16 parts per thousand, with variations of up to 4 parts per thousand observed within one site that can be related to changes in elevation. The spread in delta N-15 values is surprising and considerably larger than previously reported for T. landbeckii. In contrast, delta N-15 values of Huldobria fruticosa and Ophryosporus spp. leaves collected mostly below and above the fog zone vary between +4 parts per thousand and + 17 parts per thousand, largely in agreement with global observations from water-limited systems. Comparison with satellite-based meteorological data and modelling results revealed significant correlations between delta N-15 values of T. landbeckii and total cloud cover (r = - 0.90; p < .01), cloud height (r = -0.93; p < .001) and precipitation (r = -0.98; p < .001) along the investigated transect. The gradient in delta N-15 values further coincides with surface ocean nutrient concentrations in austral summer when ocean primary production is highest suggesting a potential marine source for the large spread in delta N-15 values. Reconstruction of past changes in fog water supply based on fossil T. landbeckii remains indicate a distinct dry episode that is consistent with a known period of extreme long-lasting droughts during late Medieval times.