Dynamics of competition over water in a mixed oak-pine Mediterranean forest: Spatio-temporal and physiological components

- We studied water-use interactions in a holm oak-Aleppo pine mixed forest.
- Stable isotopes revealed species-specific seasonal patterns of water uptake.
- Two levels of interaction emerged: spatial and physiological (dynamic).
- Intra-specific interactions were more dynamic than inter-specific interactions.

Understanding inter- and intra-specific plant interactions and competition over water is challenging because of the lack of effective approaches for accessing and monitoring root distribution and activity. In this context, stable isotopes are excellent eco-hydrological tracers that allow characterizing the dynamics of water uptake patterns in trees and shrubs. Here, we studied biotic interactions for water uptake between two typical Mediterranean tree species, Aleppo pine (Pinus halepensis) and holm oak (Quercus ilex), coexisting in a mixed forest. We measured stable isotope composition (δ18O and δ2H) of xylem water in all trees found in the studied stand during one growing season, covering an exceptionally long summer drought and subsequent recovery. We applied point-process statistics together with stand density information to evaluate tree-to-tree interactions for water use. In pines, we observed a clear uncoupling between soil and xylem water isotope composition after two months of persistent drought. Conversely, the isotope composition of xylem water in oaks tracked observed changes in the soil during the first two months of drought, but began to depart from soil values after three months. These results suggest that during drought the oaks were able to keep active for longer using alternative soil water sources, not available for the pines. Point-process statistics revealed more positive isotope compositions at distances below 4-6 m, but only between con-specific individuals (i.e. pine-pine, oak-oak). These intra-specific responses were first seen in the pines (after two months of drought) and subsequently in oaks (after three months), coinciding with the onset of soil-xylem uncoupling for each species. On the other hand, the isotope composition of individual oaks decreased with increasing neighbor pine density, but increased in response to oak density. Conversely, the pines showed more positive values with increasing oak density. Our results suggest that the use of shallow water in oaks is limited by the presence of pines, which force them to shift to deep-soil water use, whereas pines have more restricted access to deep water in the presence of oaks, leading to more positive isotope values. According to the dynamics of interaction patterns, we conclude that inter-specific differences in pine-oak mixed forests hold two components: a static, spatial component determined by root distribution, and a dynamic, physiological component related to water uptake capacity within the soil profile.

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