Bayreuther Institut für Terrestrische Ökosystemforschung
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Soil water availability effects on transpiration and canopy conductance of Pinus sylvestris


From 01/1995 to 06/1996

Principal Investigator: John Tenhunen
Staff: Barbara Köstner, Nils Sturm
Grant: 0339476 B Vorhersage und Erklärung des Verhaltens und der Belastbarkeit von Ökosystemen unter veränderten Umweltbedingungen

Measurements of leaf level gas exchange and conductance, tree transpiration via sapflow monitoring, soil moisture and water extraction, predawn water potential, and xylem abscisic 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 avoid extensive cavitation damage in most years. Transpiration potential of the stand was reduced by thinning during autumn 1993 in approximate proportion to changes in leaf area index and 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.
(final report 1998)

List of publications of this Project

Sturm, N; Reber, S; Kessler, A; Tenhunen, JD: Soil moisture variation and plant water stress at the Hartheim Scots pine forest, Theoretical and Applied Climatology, 53, 123-133 (1996)
Oren, R; Zimmermann, R; Terborgh, J: Transpiration in upper Amazonia floodplain and upland forests in response to drought-breaking rains, Ecology, 77, 968-973 (1996)
Sala, A; Tenhunen, JD: Simulations of canopy net photosynthesis and transpiration in Quercus ilex L. under the influence of seasonal drought, Agricultural and Forest Meteorology, 78, 203-222 (1996)
Sturm, N; Tenhunen, JD; Hartung, W: Das Wirkungsgefüge Bodenwasserverfügbarkeit, Xylem-ABA-Konzentration und Bestandesleitfähigkeit - eine Freilanduntersuchung an Pinus sylvestris, Verhandlungen der Gesellschaft für Ökologie, 26, 79-83 (1996)
Sala, A; Tenhunen, JD: Site-specific water relations and stomatal response of Quercus ilex L. in a mediterranean watershed, Tree Physiology, 14, 601-617 (1994)
Castell, C; Terradas, J; Tenhunen, JD: Water relations, gas exchange, and growth of resprouts and mature plant shoots of Arbutus unedo L. and Quercus ilex L., Oecologia, 98, 201-211 (1994)
Sala, A; Sabate, S; Gracia, C; Tenhunen, JD: Canopy structure within a Quercus ilex forested watershed: variations due to location, phenological development, and water availability, Trees, 8, 254-261 (1994)
Tenhunen, JD; Hanano, R; Abril, M; Weiler, EW; Hartung, W: Above- and belowground controls on leaf conductance of Ceanothus thyrsiflorus growing in a chaparral environment: the role of abscisic acid, Oecologia, 99, 306-314 (1994)
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