The ecological applications of stable isotopes have, so far, been mostly confined to physiological and ecosystem level studies. Participants in the symposium discussed novel applications of stable isotope techniques to understand community processes, such as plant–plant interactions (Ramírez et al. 2009
). Indeed, the use of stable isotopes to understand plant interactions, which allows for an examination of the mechanisms underlying plant competition and facilitation, may shed new light on the study of these key community processes. This is important due to the fact that a large number of studies in this field are based on empirical, pair-wise comparisons, where the underlying mechanisms have been largely overlooked (McGill et al. 2006
). Can we move beyond this limitation through stable isotopes?
Cristina Moreno-Gutiérrez and José I Querejeta showed that bulk leaf δ18O reflects the intensity of interspecific competition for water between the woody shrub Rhamnus lycioides and neighbouring pine trees. Analyses of δ18O and δ2H in xylem water indicated that Pinus halepensis and R. lycioides rely on soil water stored at similar depth in a pine plantation in semiarid southeast Spain. Interestingly, R. lycioides shrubs growing in the close vicinity (less than 100 cm) of adult pine trees showed lower stem water contents and more enriched leaf δ18O values (which indicates a reduction in stomatal conductance and transpiration) than those located further away from the nearest tree. In contrast, Anthyllis cytisoides shrubs using isotopically enriched water stored in surface soil layers showed less-enriched bulk leaf δ18O values (suggesting increased stomatal conductance and transpiration) when growing within the canopy edge of pine trees. This pattern of decreasing leaf δ18O enrichment in the close proximity of pine trees may indicate potential facilitative effects of tree canopy shading on A. cytisoides physiology.
This new application of stable isotopes is not exempt from new problems. For instance, it is yet to be clarified how mesophyll hydraulic conductance might affect the oxygen isotopic enrichment of leaf water and organic matter. Ferrio et al. (2009)
have shown that the rapid response of mesophyll hydraulic conductance (gm
) to changes in water availability may influence leaf water 18
O enrichment. This change in gm
may complicate the interpretation of δ18
O data as indicators of changes in stomatal conductance and transpiration.
Juan-Carlos Linares showed that δ13
C in tree rings may also record past changes in the intensity of plant competition (Linares et al. 2009
), and Jordi Voltas warned us about the difficulties in using tree rings as proxies of growth conditions. There are strong interspecific differences in physiological strategies under drought, which develop in a species-specific, intra-annual pattern of δ13
C in tree rings. Whereas seasonal δ13
C changes in tree rings of some species will largely reflect fluctuations in photosynthetic gas exchange (Klein et al. 2005
), in other instances δ13
C variations may not coincide temporally with an increase or relief of water stress, but rather may represent conditions at a time earlier than when the differentiation of xylem cells occurs (Drew et al. 2009