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

SIMPLE SUMMARY: Combined measurements of radial variations of the water and carbon conducting tissue (i.e., the xylem and phloem, respectively) are valuable in order to study water exchange between tissues and water shortages in tree stems. In this study, we determined diurnal and seasonal radius variations of the xylem and inner bark (i.e., the tissue including phloem, parenchyma and meristem cells) of mature Scots pine trees (Pinus sylvestris) at a drought prone site, using devices that continuously record variations in stem radius with high resolution, so-called point dendrometers. The results of our study revealed that daily and seasonal radial variations of the xylem and inner bark were closely linked, indicating intensive water exchange between these tissues. A comparison of radial fluctuations in the xylem (XRV) with environmental variables affecting transpiration revealed a close coupling of XRV to vapor pressure deficit (VPD), i.e., the drying power of the air and air temperature, pointing to a strong dependence of stem water status on changes in atmospheric conditions. Because VPD increases exponentially with the increase in temperature and thus causes the water content in the stem to decrease especially under drought conditions, tree growth and mortality will be increasingly affected by climate warming. ABSTRACT: 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 µm 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 (ρ = 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 (ρ = 0.802; p < 0.001), mean daily solar radiation (ρ = 0.809; p < 0.001), and non-linearly related to daily maximum VPD (R(2) = 0.837; p < 0.001), indicating that the xylem of Pinus sylvestris reacts like a transpiration-driven passive hydraulic system.