Dry season plant water sourcing in contrasting tropical ecosystems of Costa Rica

Abstract

Tracer-aided studies to understand plant water uptake sources and dynamics in tropi- cal ecosystems are limited. Here, we report the analysis of dry season source water uptake patterns of five unique ecosystems of Costa Rica across altitudinal (<150– 3,400 m asl) and latitudinal (Caribbean and Pacific slopes) gradients: evergreen and seasonal rainforests, cloud forest, Páramo and dry forest. Soil and plant samples were collected during the dry season in 2021. Plant and soil water extractions were con- ducted using centrifugation. Stem water extracted volume and stem total water con- tent were calculated via gravimetric analysis. Water source contributions were estimated using a Bayesian mixing model. Isotope ratios in soil and stems exhibited a strong meteoric origin. Enrichment trends were only detected in stems and cactus samples within the dry forest ecosystem. Soil profiles revealed nearly uniform isoto- pic profiles; however, a depletion trend was observed in the Páramo ecosystem below 25 cm. More enriched compositions were reported in cactus samples for extracted water volumes above 20% (adj. r2 = 0.34, p < 0.01). The most prominent dry season water source in the evergreen rainforest (74.0%), seasonal rainforest (86.4%) and cloud forest (66.0%) corresponded to well-mixed soil water. In the Páramo ecosystem, recent rainfall produced by trade wind incursions resulted in the most significant water source (61.9%), whereas in the dry forest, mean annual precip- itation (38.6%) and baseflow (33.1%) were the dominant sources. The latter highlights the prevalence of distinct water uptake sources between recent cold front rainfall (near-surface soil storage) to more well-mixed soil moisture during the dry season, revealing ecohydrological processing previously unknown in this tropical region.