Plant invasion modifies isohydricity in Mediterranean tree species

Understanding of plant hydraulic strategies (i.e., the degree of
iso-/anisohydricity) is crucial to predict the response of plants to
changing environmental conditions such as climate-change induced extreme
drought. Several abiotic factors, including evaporative demand, have been
shown to seasonally modify the isohydricity of plants. However, the impact
of biotic factors such as plant-plant interactions on hydraulic strategies
has seldom been explored. Here, we investigated adaptations and changes in
hydraulic strategies of two woody species in response to seasonal abiotic
conditions, experimental drought, and plant invasion in a Mediterranean
cork oak (Quercus suber) ecosystem with a combined shrub invasion (Cistus
ladanifer) and rain exclusion experiment. From the dry to wet season, Q.
suber shifted from a partial isohydric to an anisohydric behaviour while
C. ladanifer shifted from strict anisohydric to partial isohydric. During
drought, water competition by plant invasion significantly modified the
hydraulic strategy of invaded Q. suber, which was accompanied by lower
pre-dawn leaf water potentials, sap flow density, leaf area index, and
trunk increment rates. This altered isohydricity of invaded Q. suber trees
was most likely caused by interspecific competition for water resources by
water spending C. ladanifer shrubs. Both species do have the highest
proportion of fine roots in the topsoil and thus, an additional water
consumer, such as C. ladanifer can lead to more stressful conditions for
Q. suber during times of water scarcity. Further underlying mechanisms of
the altered isohydricity of Q. suber, such as potential allelopathic
effects of C. ladanifer exudates on root growth of Q. suber, have to be
investigated in the future. In conclusion, we demonstrate that the degree
of isohydricity of two woody Mediterranean plant species is dynamically
determined by the interplay of species-specific hydraulic traits and their
abiotic and biotic environment.