Sap flow in Scots pines growing under conditions of year‐round carbon dioxide enrichment and temperature elevation

Abstract

Starting in rrrr, individual trees of Scots pine (Pinus sylvestris L.) aged 30 years were grown in closed‐top chambers and exposed to normal ambient conditions (CON), elevated CO2 (Elev. C), elevated temperature (Elev. T) and a combination of elevated CO2 and temperature (Elev. C + T). Using the constant‐power heat balance method, sap flow was monitored simultaneously in a total of 16 trees, four for each treatment, over a 32 d period (after the completion of needle expansion and branch elongation in 1997). An overall variation in diurnal sap flow totals (Ft) was evident during the period of measurement (days 167–198, 1997) regardless of the treatments, with a range from 0·15 to 2·82 kg tree–1 d–1. Elev. C reduced Ft by 4·1–13·7% compared with CON on most days (P varies from 0·042 to 0·108), but slightly increased it on some days (P≥ 0·131), depending on the weather conditions. Although the decrease in Ft caused by Elev. C was statistically significant on only a few days (P≤ 0·042), the cumulative Ft for the 32 d decreased by 14·4% (P = 0·047), indicating that Elev. C may have an important influence on seasonal water use of the Scots pine. Analysis of the diurnal courses of sap flow combined with corresponding weather factors indicated that the CO2‐induced decrease in Ft could be largely attributed to an increase in stomatal sensitivity to vapour pressure deficit (VPD), whereas the CO2‐induced increase in Ft related to an increase in stomatal sensitivity to low light levels. Elev. T increased Ft by 11·2–35·6% throughout the measuring period and the cumulative Ft for the 32 d by 32·5% (P = 0·019), which could be largely attributed to the temperature‐induced increase in current‐year needle area and decrease in stomatal sensitivity to high levels of VPD. There were no significant interactive effects of CO2 and temperature on sap flow, so that Elev. C + T had approximately the same Ft as Elev. T and similar diurnal patterns of sap flow, suggesting that the temperature factor played a dominant role in the case of Elev. C + T.