3. Results
3.1. Canopy conductance and its response to VPD
The two study periods with soil moisture and sapflow data
included wet growing seasons (e.g., 2003–2004 and 2009–2010
for the Piedmont and coastal sites, respectively) as well as dry
growing seasons (e.g., 2002 and 2008). The Piedmont sites experienced
an extreme drought during the 2002 growing season, with a
mean annual precipitation of only 945 mm, which was 71% of the
30-year mean for the area (Fig. 1). Consequently, at these sites, soil
water content of the top soil (0–45 cm) reached 0.12 m3 m3 for
several months and mean daily VPD ranged from 0.6 to 0.8 kPa.
At the coastal sites, 2008 was also a very dry year with 890 mm
of rainfall representing 65% of the 30-year mean. Seasonal fluctuations
in transpiration per unit leaf area (E) and reference
canopy-averaged stomatal conductance (at VPD = 1 kPa, Gs-ref)
were different in years with wet versus dry growing seasons. The
general trend for both P. taeda plantations was a sharp decrease
in E and Gs-ref during drought (P < 0.016).
3. Results3.1. Canopy conductance and its response to VPDThe two study periods with soil moisture and sapflow dataincluded wet growing seasons (e.g., 2003–2004 and 2009–2010for the Piedmont and coastal sites, respectively) as well as drygrowing seasons (e.g., 2002 and 2008). The Piedmont sites experiencedan extreme drought during the 2002 growing season, with amean annual precipitation of only 945 mm, which was 71% of the30-year mean for the area (Fig. 1). Consequently, at these sites, soilwater content of the top soil (0–45 cm) reached 0.12 m3 m3 forseveral months and mean daily VPD ranged from 0.6 to 0.8 kPa.At the coastal sites, 2008 was also a very dry year with 890 mmof rainfall representing 65% of the 30-year mean. Seasonal fluctuationsin transpiration per unit leaf area (E) and referencecanopy-averaged stomatal conductance (at VPD = 1 kPa, Gs-ref)were different in years with wet versus dry growing seasons. Thegeneral trend for both P. taeda plantations was a sharp decreasein E and Gs-ref during drought (P < 0.016).
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