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.