Hydraulic and photosynthetic plant traits variation along a rainfall gradient: a multiscale perspective

To date, both hydraulic and photosynthetic plant traits have been extensively collected and analysed on global or local scale. However, the stand-alone effect of aridity on plant traits remains uncertain as aridity was always confounded by temperature. This study utilized plant traits collected from seven one-hectare plots along a rainfall gradient in Ghana, West Africa. The seven plots share very similar mean annual temperature (25.0 to 26.4 oC) but feature a gradient of mean annual precipitation from 2050 to 1200 mm, which provided a natural laboratory to disentangle the hydraulic impact on plant traits from temperature. The study started with a review on recently proposed theory, or findings regarding the variation of traits along aridity, including ratio of leaf-internal to ambient CO2 (), carboxylation (Vcmax) and electron-transport (Jmax) capacities, leaf mass per area (LMA), Huber value (vH), turgor loss point (tlp) and maximum sapwood hydraulic conductivity (Ksm). Our analysis on plot scales agrees with previous studies on increase vH, Vcmax, Jmax, and decrease , tlp, which further reinforce the impact of drought on such plant traits, while we also discussed the pattern associated with LMA and Ksm. However, due to the large variation sourced from scales finer than species that may dilute the pattern identified on site scale, species scale trait-trait correlation was found not significant between any pair of above traits. Given that some pairs of trait-trait correlation were found significant from global datasets, our findings imply either geographic sampling bias in previous global datasets, or a strong confounding effect combining multiple climate variables.