Expensive and hydraulically efficient leaves maintain deep rooted cold desert shrubs active during the dry season

Background/Question/Methods

Temporal and spatial variation in water availability is one of the main drivers of ecological diversity in arid ecosystems. Leaf hydraulic conductance (K_leaf) is a key trait governing plant hydraulics, water use and gas exchange under water deficit conditions. As the transpiration stream in leaves occurs through vascular and extra-vascular tissues, a suite of morphological and anatomical traits implying substantial carbon investment can affect K_leaf. We investigated the relationships between K_leaf and drought resistance traits in shrub species of the Patagonian steppe and its functional relationship with leaf phenology. Leaf hydraulics and water relations traits and leaf mass per area (LMA) were determined in seven shrub species including deciduous and functionally deciduous species with more restricted soil water access and evergreen species tapping water from deeper soil layers.

Results/Conclusions

Evergreen species had access to more stable water sources; however their leaves exhibited traits related to drought tolerance compared to leaves from deciduous and functionally deciduous species with relatively shallow roots and lower soil water availability. Leaves from evergreen species had lower osmotic potentials, solute content, and hydraulic capacitance and higher modulus of elasticity and leaf mass per area. Nevertheless these species had higher K_leaf both on area and mass basis and stomatal conductance than species exposed to larger water deficits. The strong positive relationships observed between LMA and K_leaf on both area and mass basis suggests that leaf water use and hydraulic efficiency are not optimized in species that maximize carbon gain possibly because they are uncoupled from seasonal changes in soil water availability by exploring deeper and wetter soil layers. However the construction of leaves with long lifespan (evergreen species) with high resistance to hydraulic failure and turgor loss, and the production of roots achieving deeper soil water sources implies a large energy cost with negative consequences on carbon gain at whole-plant level as photo-assimilates are allocated to roots, to active osmotically compounds and probably to a higher density of leaf vasculature.