THE RESPONSE OF PINUS-SYLVESTRIS TO DROUGHT - STOMATAL CONTROL OF TRANSPIRATION AND HYDRAULIC CONDUCTANCE

We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements we re made of the seasonal course of transpiration, canopy stomatal condu ctance, needle water potential, xylem water content, soil-to-needle hy draulic resistance, and growth. Comparison was made between drought-tr eated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volumetric water content (VW C) over the top 20 cm of soil reached a threshold value of 12%. Therea fter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle wate r potential declined from -0.54 to -0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declin ing below -1.5 MPa. Basal area, and shoot and needle growth were signi ficantly reduced in the drought treatment. In the year following the d rought, canopy stomatal conductance and soil-to-needle hydraulic resis tance recovered. Current-year needle extension recovered, but a signif icant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the deve lopment of substantial xylem embolism. Reduced growth in the year afte r a severe soil water deficit is most likely to be the result of reduc ed assimilation in the year of the drought, rather than to any residua l embolism carried over from one year to the next.