Resumen de Point processes statistics of stable isotopes: analysing water uptake patterns in a mixed stand of Aleppo pine and Holm oak

Carles Comas, Jorge del Castillo, Jordi Voltas Velasco, P. Ferrio

• Aim of study: Understanding inter- and intra-specific competition for water is crucial in drought-prone environments. However, little is known about the spatial interdependencies for water uptake among individuals in mixed stands. The aim of this work was to compare water uptake patterns during a drought episode in two common Mediterranean tree species, Quercus ilex L. and Pinus halepensis Mill., using the isotope composition of xylem water (δ18O, δ2H) as hydrological marker.Area of study: The study was performed in a mixed stand, sampling a total of 33 oaks and 78 pines (plot area= 888 m2). We tested the hypothesis that both species uptake water differentially along the soil profile, thus showing different levels of tree-to-tree interdependency, depending on whether neighbouring trees belong to one species or the other.Material and Methods: We used pair-correlation functions to study intra-specific point-tree configurations and the bivariate pair correlation function to analyse the inter-specific spatial configuration. Moreover, the isotopic composition of xylem water was analysed as a mark point pattern.Main results: Values for Q. ilex (δ18O= -5.3 ±0.2‰, δ2H=-54.3±0.7‰) were significantly lower than for P. halepensis (δ18O= -1.2±0.2‰, δ2H = -25.1±0.8‰), pointing to a greater contribution of deeper soil layers for water uptake by Q. ilex. Research highlights: Point-process analyses revealed spatial intra-specific dependencies among neighbouring pines, showing neither oak-oak nor oak-pine interactions. This supports niche segregation for water uptake between the two species.Key words: Cross-pair correlation function; Deuterium; Mark correlation function; Oxygen-18; Point patterns; Xylem.Abbreviations: d18O, oxygen isotope composition; d2H, hydrogen isotope composition; BA, basal area.