| Popis: |
An investigation of the impact of imposed drought on the physiology of 41 year old Scots pine (Pinus sylvestris) was made in southern Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Two new techniques were developed to aid these measurements; (i) the use of time domain reflectometry with short probes to detect changes in xylem embolism, and (ii) the continuous measurement of bole water potential based on relating the elastic deformation of the xylem to the internal water tension. Comparison was made between droughted plots and those receiving average precipitation. Under drought, transpiration rate declined once a threshold soil moisture of 12 % volumetric moisture content (VMC) over the top of 20 cm of soil had been reached, and transpiration was thereafter a near linear function of soil moisture. As the drought became intense, hydraulic resistance between soil and needles increased by a factor of three as pre-dawn needle water potential declined from -0.54 MPa to -0.71 MPa. A small but significant increase in xylem embolism was detected in year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, shoot and needle growth was significantly reduced in the droughted treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current year needle extension recovered, but a significant reduction in basal area increment was evident. Multiplicative non-linear regression models for relating the canopy stomatal conductance to environmental variables were developed. It was found that a simple model that included the effects of air vapour pressure deficit and soil moisture deficit could explain 56 % of the measured canopy stomatal conductance. Overall results suggest that in response to soil drought mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. The reduced basal area growth in the year after the drought is not likely to be a result of residual embolism from the previously droughted period. |
