Contrasting patterns of radial growth rate between Larix principis-rupprechtii and Pinus sylvestris var. mongolica along an elevational gradient are mediated by differences in xylem hydraulics

Species-specific environmental adaptability results in different species having different suitable distribution ranges in nature. Investigating the change of relative advantages in growth performances among tree species along elevational gradients and the underlying mechanism are essential for selecting elevation-specific optimal species in forest management in mountainous regions. Xylem hydraulics has been widely reported to play a key role in tree growth and survival under water-limited environments, but whether and how it regulates tree growth performances at different elevations with contrasting water availability remain unclear. Here we combined tree-ring analyses and measurements of xylem hydraulics to investigate the patterns of tree radial growth and their linkages with water-related physiology for Prince Rupprecht's larch (Larix principis-rupprechtii Mayr) and Mongolian pine (Pinus sylvestris var. mongolica Litvinov) along a 500-m elevational gradient between 1336 and 1825 m a.s.l. in a semi-arid area of northern China where both species were widely used for afforestation. Our results showed that the radial growth rate, reflected by the slope of linear regression line for cumulative basal area increments and tree age, was higher in Prince Rupprecht's larch than in Mongolian pine at higher elevations, which coincided with the advantages in xylem hydraulic efficiency, i.e. higher sapwood area-specific hydraulic conductivity and leaf area to sapwood area ratio for the former species relative to the latter one at the middle and high elevation sites. However, the reversed pattern appeared in comparing radial growth rate for the two conifers at the low elevation sites that might be due to obviously greater loss of hydraulic conductivity caused by drought-induced xylem embolism for Prince Rupprecht's larch. This is corroborated by strong correlations between the tree-ring width indices and water-related climate variables during the growing season in Prince Rupprecht's larch but not in Mongolian pine at low elevations. Our study provides a mechanismic explanation for contrasting growth performances between the two species across elevations and implies a potential risk of growth decline under the influence of climate change in Prince Rupprecht's larch, the currently most widely used species for afforestation in this region, particularly under the drought-prone environment in the low-elevation sites.