Seasonal and Daily Xylem Radius Variations in Scots Pine Are Closely Linked to Environmental Factors Affecting Transpiration

Seasonal and daily radius variations in the xylem (XRV) and inner bark (IBV) of mature Scots pine trees (Pinus sylvestris) were determined during April 2019-October 2021 at a drought-prone inner alpine site (c. 750 m asl; Tyrol, Austria) by applying point dendrometers. XRVs were also related to environmental factors to evaluate the drivers of XRV during the growing season. XRV records revealed that the xylem width (i) started to shrink around the onset of radial stem growth in April, (ii) consistently decreased by c. 50 mu m at the time when air temperature (T) and vapor pressure deficit (VPD) reached their maximum in late June through mid-July, and (iii) recovered until November/December. Although in daily cycles of radius variations XRV preceded IBV by about two hours and the daily amplitude of XRV was about 1/10 that of IBV, XRV and IBV (seasonal trends removed) were closely linked (rho = 0.755; p < 0.001), indicating tight hydraulic coupling between these tissues. Furthermore, the daily amplitude of XRV was linearly and closely related to daily maximum T (rho = 0.802; p < 0.001), mean daily solar radiation (rho = 0.809; p < 0.001), and non-linearly related to daily maximum VPD (R2 = 0.837; p < 0.001), indicating that the xylem of Pinus sylvestris reacts like a transpiration-driven passive hydraulic system.

Automated summary

Seasonal and daily variations in the xylem and inner bark of mature Scots pine trees were studied using point dendrometers. The results showed that the xylem width started to shrink during radial stem growth in April, decreased significantly in late June through mid-July, and recovered until November/December. The study also found a tight hydraulic coupling between xylem and inner bark, with the daily amplitude of xylem variations closely related to environmental factors such as maximum temperature, solar radiation, and vapor pressure deficit.