Environmental and endogenous controls on leaf- and stand-level water conductance in a Scots pine plantation

Abstract

Measurements of leaf level gas exchange and conductance, tree transpiration via sapflow monitoring, soil moisture and water extraction, predawn water potential, and xylem abscisis acid (ABA) concentration were carried out over the course of the 1993 and 1994 summer seasons at the Hartheim Pinus sylvestris plantation on the Upper Rhein Plain, Germany. Periodic leaf level conductance determinations with porometry established a maximum value of ca 280 mmol m-2 s-1 (13.6 mm s-1). Half maximal conductance was attained at 40 μmol m-2 s-1 and 90 % of light saturation occurred at ca 500 μmol m-2 s-1 PPFD. Conductance decreased strongly with increases in vapor pressure deficit above 10 hPa, while the temperature optimum was 22°C at light saturation. Strong restrictions on maximum conductance at both leaf and stand levels were apparent below a soil moisture content of 16 volume percent. Although less strongly, conductance also decreased with initial drying of the upper soil layers and decreases in predawn water potential from -0.4 to -0.6 MPa. In this range of water potential change, xylem ABA increased to between 200 and 500 nmol L-1. Thus, an immediate leaf-level reaction to the onset of summer weather conditions is observed, i.e. leaf conductance and water use decrease. We hypothesize that ABA functions as a key control on water balance, transmitting information about soil water status and endogenously modifying canopy response in order to budget water and by avoid extensive cavitation damage in most years. Transpiration potential of the stand was reduced sapwood area. Simultaneous observations of sapflow and conductance have allowed us to view the effects of leaf conductance on whole plant water use, while thinning revealed the effects of stand level phenomena on conductance regulation.