Altitudinal variations of hydraulic traits in Faxon fir (Abies fargesii var. faxoniana): Mechanistic controls and environmental adaptability

Global climate change has been seen to result in marked impacts on forest ecosystems such as accelerated tree mortality worldwide due to incidental hydraulic failure caused by intensified and more frequent occurrence of extreme drought and heat-waves. However, it is well understood how the tree hydrological strategies would adjust to environmental variability brough about by climate changes. Here we investigated the hydraulic adjustment as a mechanism of acclimation to different climate conditions along an altitudinal gradient in Faxon fir (Abies fargesii var. faxoniana) ― a tree species that plays a key role in conservation of wildlife and maintenance of ecosystem services in subalpine forests. The hydraulic traits and selective morphological and physiological variables were measured seasonally along an altitudinal gradient from 2,800 to 3,600 ​m a.s.l. We found that the native percentage loss of conductivity (PLC) increased with altitude across the seasonal measurements. Both the native sapwood-specific hydraulic conductivity (Ks) and native leaf-specific hydraulic conductivity (Kl) significantly decreased with altitude for measurements in July and October, coinciding with the timing for peak growth and pre-dormancy, respectively. The morphological traits varied toward more conservative tree hydrological strategies with increases in altitude, exhibiting trade-offs with hydraulic traits. The total non-structural carbohydrates in both needle (NSCNeedle) and branch (NSCBranch) as well as photosynthetic capacity of current-year leaves played variable roles in maintaining the integrity of the hydraulic functioning and shaping the hydraulic adjustment under prevailing environmental conditions. Our findings indicate that Faxon fir possesses some degree of hydraulic adaptability to water limitation imposed by climate fluctuations in subalpine region through morphological and physiological modifications. [Display omitted]