Discussion
In these semi-arid forest ecosystems on rocky soils with little capacity of water storage, temporal water limitation may last between 3 to 6 months. Trees in these environments do not only search and explore for alternative geological water sources (occupation of alternative spatial/geohydrological and temporally emerging/seasonal niches) but employ specific functional adaptations to cope with the highly variable water availability. Once the rain season ended, we attested a humidity gradient as the dry season progressed. Overall, electric resistivity increased with depth (Fig. 2) with the lower regolith and fresh rock being the most resistive (but see Rodríguez-Robles et al. 2017 ;Ψsoil and electric resistivity). Still, only oak was able to explore all niches along this humidity gradient, while pine was specialized in using the top soil and upper regolith (Table 1, Fig. 4). Geophysical prospection revealed a spatially complex substrate including up to four niches generating a fine-scale partitioning of humidity (within less than 1 m, Fig. S2). Within this geological context, tree species traits in response to the wetting and drying cycles in the substrate generated up to nine niche configurations of water partitioning together forming a highly dynamic nichescape that has allowed the two tree species to co-exist in this forest. Oak and pine shared water from the same niche only when water was abundant, as occurred at the end of the summer months (Table 1(A), Fig. 4a). Later in the year, oak and pine used niches differentially, with pine acquiring water only from the superficial soil and the upper regolith layer stemming either from rain or remobilization by oak from deeper rock fractures to the surface (Table 1(C), Fig. 4c).
These species-specific water uptake mechanisms of oak and pine are related to different vertical fine root distribution (Figs. 3a - i). Pine located a greater proportion of thin roots responsible for water uptake in the top 10 cm (Fig. 3a), whereas oak positioned the same thin root types at 20 to 30 cm depth (Fig. 3f). Because of anatomical adaptations of fine roots, oak was able to explore the lower regolith and rock fractures (Figs. 4b, c, e, f, g, i) during the dry depletion periods. However, oak also used water from the upper regolith and surface soil thus exploiting water from a total of four identified spatial niches (Figs. 2, 4). In the case of superficial niches in the soil stratum, where oak and pine shared water use, species accessed different proportions of water from each niche (Figs. 4 c, d, h) likely to reduce competition. In one particular case, during the recovery period in spring 2013 (Fig. 4c), oak supplied water to pine through water remobilization, hence partially facilitating the recovery of the water status of pine. In another case, water stored in oak’s trunk acted as a biotic/functional niche (trunk capacitance, Table 1(F), Figs. 4f and S3.3 b, c). This trait emerged only after winter rains led to mild dry conditions (in February 2014; Fig. 4f); this unique water storage we interprete as a biotic dimension of a niche. In March-May 2014, oak switched water use from the lower regolith (Table 1(E), Figs. 4e, S3.3b, c) to trunk water (trunk capacitance) stored from rain events in December 2013 and January 2014. Pine roots, however, continued taking up water from surface soil and the upper regolith layer as evidenced by the stable isotope signature in the twigs (Table 1(F), Figs. 4f and S3.3a, b).