Spatial and temporal niche characterization
The forest ground is characterized by exfoliated volcanic rocks (0 to 45 cm high) and rock outcrops. With geophysical imaging (GPR), we visualized differential species-specific vertical root placement in the soil surface, regolith, and rock fractures (Fig. 2). For oak, the finest detectable roots (diameter < 3 cm) occurred preferentially in soil pockets and rock fractures at depths > 25 cm, whereas the thicker roots (5 to 7 cm) were located at the soil surface (differences in vertical root distribution, P < 0.001 , Figs. 2b; 3d, f). Pine in contrast, concentrated its finest roots (< 3cm) near the soil surface (top 10 cm, P <0.001; Figs. 2a; 3a), however the thicker roots were observed in the regolith between 20 – 30 cm depth (P <0.001; Figs. 2a; 3c).
We confirmed that oak and pine trees used different water sources throughout the year by comparing monthly stable isotope ratios of xylem water with soil and rainwater; these data provide direct evidence for spatial and temporal niche occupation by the two species. When combining geophysical methods with stable isotope ratio information of water from different soil/rock substrates and xylem of trees, we discovered that trees of both species occupied between one to four spatial niches (Table 1, Fig. 4) distributed in three geologically distinct substrates (Fig. S2). In the regolith layer, in several periods two separate pools of water were apparent, one in the upper regolith at the interface with the shallow soil and the other in the lower regolith within rock pockets at its deeper distribution (Figs. 2, 4).
When considering the soil/rock strata and the wetting/drying cycles in the course of three years, as well as specific tree functional traits employed, we identified nine niche configurations together forming a complex nichescape. With nichescape we refer to the set of traits employed by plants to garner water, that is spatially and temporally distributed in the soil-rock continuum (Table 1, Fig. 4).Single-niche-sharing only occurred during the rainy season of September 2012, when oak and pine satisfied all their water needs from surface soil (Table 1(A), Figs. 4a and S3.1 i).Double niche sharing occurred in the wet season 2013, when oak and pine roots acquired water from both the surface soil and the upper regolith (Table 1(D), Figs. 4d, S3.2 f-i), however the two tree species exploited differential proportions from each niche. While pine got most of the water from the surface soil (90%), oak acquired most from the upper regolith (85%). Another case, yet ofindirect double niche sharing occurred in the dry season (depletion period 2012-2013, Table 1(C), Figs. 4c, S3.1 d, S3.2 c), when oak in both stands acquired most water (92%) from rock fractures and a small portion (8%) from the surface soil/upper regolith. In contrast, pine (in mixed stands) took up water from surface soil/upper regolith plus water originating from rock fractures, which had previously been hydraulically lifted by oak roots from the rock fracture to the surface soil, where pine could access it (xylem water δD = -38.6‰).
Oak and pine exhibit remarkable differences in wood and root anatomy. While in pine roots, a parenchyma is surrounding the tracheids, in oak roots this tissue is lacking (Figs. S6g, S4). Oak trunks show a particular structural anatomy of specialized tissue formed by fiber tracheids connecting vessel tissue (Cai, Li, Zhang, Zhang, & Tyree, 2014) (conductive structures, Fig. S6b, d), which serves for water storage (Fig. S6c). Besides, oak has a considerable number of vessels adapted to xeric conditions, i.e., it has mixture of vessels with different diameters. The placements of oak’s finest roots in rock fractures suggests specific adaptations in functional anatomy (Rodríguez-Robles et al., 2017) (Fig. 3f). These roots exhibit a triple layer of epidermal tissue and contain calcium oxalate crystals (druzes), and under extremely dry conditions, oak vessel diameter of roots decreases through the formation of tyloses (Gottwald, 1972) (Fig. S5d). Deep oak roots exhibited 83% more vessels with tyloses and 60% more druzes than surface roots (Fig. S5). Pine roots did not exhibit these kinds of anatomical adaptations.