Niche multidimensionality and landscape consequences
The distribution of oak and pine trees at the landscape level in pure and mixed stands, appears to be controlled by the same suite of highly specialized functional traits (anatomy, morphology and vertical placement of roots; stem parenchyma with water storage capacity) adapted to an apparently simple geodiversity consisting of four geologic strata (shallow soil, upper and lower regolith and fractured rock), however at the microscale level these strata are highly complex considering the high temporal dynamics of water availability, small-scale spatial water gradients and the suite of plant traits adapted to garner water. These interacting biotic and abiotic elements have generated a complex environment with multidimensional niches together forming a nichescape controlled by a highly dynamic pattern of water distribution and availability, to which two coexisting tree species have adapted to and simultaneously co-shaped these niches to fully exploit the highly limited and growth controlling resource elucidating the mixed stands of these forests (Table 1, Figs. 4). The hydrological dynamics of these ecosystems are highly unpredictable and ultimately driven by extremely variable seasonal, inter-seasonal, and interannual water availability, high temporal variability of seasonal and non-seasonal droughts, heterogeneous geospatial distribution of water stocks, and temporally variable direct and/or indirect access, uptake, storage and use of water. Next to the multidimensional configuration of the nichescape in mixed stands, our data from the pure oak and pine stands reveal an apparent niche segregation of tree communities at the landscape scale. Sites with greater vertical micro-geodiversity favor the co-existence of both tree species in mixed stands (Figs. S9i,j,k,l), whereas less geodiverse microsites are covered by pure stands. Pure pine stands occur where slightly deeper soils (~30 cm) have developed over little fragmented rock (Figs. S9a,b,c,d). The absence of oak trees on this apparently more favorable substrate suggests a strong competitive exclusion mechanism and a suitable niche for pure pine stand formation. On the other hand, pure oak stands occurred only on poorly developed surface soil (> 10 cm deep) over rhyolitic rocks (Figs. S9e,f,g,h). The absence of pine under these conditions suggests that beyond the minimum soil depth of 20 cm over fractured rocks, oak trees find a suitable niche for pure stand formation.
These results significantly modify the current paradigm of water use by plants, as water stored between rocks has not been regarded as a potentially vital source sustaining mixed forest ecosystem in a semiarid region (but see Schwinning 2010; Jackson, Moore, Hoffmann, Pockman, & Linder, 1999) Here, we demonstrate for the first time that rocky soils generate a high degree of spatiotemporal heterogeneity in water distribution against which adaptive tree physiological and anatomical responses present a strong offset. Incorporating the multidimensional nichescape as a major component of ecosystem structure and function may not only mechanistically clarify species coexistence and other species interactions (i.e., competition, community diversity), but also shed new insight on patterns of the progress of landscape phenomena such as forest mortality.