© 2002 Heron Publishing—Victoria, Canada
Midday stomatal closure in Norway spruce—reactions in the upper and lower crown
R. Zweifel (1, 2), J. P. Böhm (3) and R. Häsler (4)
1. Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland / 2. Author to whom correspondence should be addressed (roman.zweifel@ips.unibe.ch) / 3. Giessenstrasse 11, CH-8606 Bubikon, Switzerland / 4. Swiss Federal Research Institute WSL, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland / Received September 18, 2001; accepted March 28, 2002; published online October 1, 2002
Summary
We used local microclimatic conditions and twig sap flow rates to interpret midday stomatal closure in the canopies of two
250-year-old Norway spruce (Picea abies (L.) Karst.) trees at a subalpine site in the Swiss Alps (1650 m a.s.l.). Both trees showed midday stomatal closure on most
clear summer days, despite the permanently wet soil. We used a modified Penman-Monteith formula to simulate potential transpiration
of single twigs (ETT) based on high-resolution temporal and spatial microclimate data obtained both inside and outside the crowns. Comparison
of calculated ETT values and measured twig sap flow rates enabled us to pinpoint the occurrence of midday stomatal closure and the microclimatic
conditions present at that time.
We found that vapor pressure deficit (and for upper-crown twigs, ETT) largely explained the timing of initial midday stomatal closure but gave no explanation for the different patterns of stomatal
behavior after initial closure in upper- and lower-crown twigs. After the initial stomatal closure, upper-crown twigs maintained
high transpiration rates by continuously regulating stomatal aperture, whereas stomatal aperture decreased rapidly in lower-crown
twigs and did not increase later in the day. Midday stomatal closure in lower-crown twigs occurred on average 1 h later than
in upper-crown twigs. However, the microclimate at the time of initial stomatal closure was similar at both crown locations
except that lower-crown twigs received significantly less solar radiation than upper-crown twigs both at the time of initial
stomatal closure and afterwards. High rates of sap flow in twigs did not always lead to stomatal closure and therefore could
not explain the phenomenon. We conclude that stomatal conductance can be modeled accurately only when both local microclimatic
conditions and tree water status are known. Further, we hypothesize that both the quantity and quality of light play an important
role in the reopening of closed stomata during the day.
Keywords:
evapotranspiration, Penman-Monteith, stomatal conductance, Picea abies, water relations.
