4.3 Tree- and watershed-level estimates of transpiration
Sapwood area explained nearly 85% of the variability in daily
tree-level transpiration (Ts ) (in the monitored
trees) across all zones, with tulip poplar generating higher than
predicted transpiration for its diameter and sapwood area in the
riparian buffer and mid-hillslope (Figure 3). In contrast, loblolly pine
had lower than predicted transpiration for its sapwood area in the
mid-slope and upland-slope. Unlike species J sthat was related to zone, an effect test revealed thatTs rates were significantly correlated to sapwood
area and species but not to zone (sapwood – F ratio 122.8, P =
<0.0001; species – F ratio 9.2, P = 0.0076; zone – F ratio
2.1, P = 0.20). This is not surprising as Ts is
closely linked to and driven largely by sapwood area (Wullschleger and
Norby, 2001).
Annualized zone weighted watershed-level transpiration
(T w) was very similar from a dry year to a wet
year (361 mm vs. 370 mm, Table 4) in part becauseJ s was not significantly different for loblolly
pine, white oak, and tulip poplar between years. These three species
occupied 50% of the sapwood area and were responsible for almost 75%
of the water loss in both years (i.e., 73% in 2015) and (i.e., 76% in
2016). Oishi et al. (2010) also found similarities or small variations
in transpiration across wet and dry years, 329 mm to 349 mm over four
years. Hawthorne and Miniat (2018) also found that species composition
may explain the similarity between T w between wet
and dry years.
There were differences in scaled T w, depending on
which scaled T z was used to represent tree water
use (Table 4). We compared Tz based on scaledJs data from the buffer, mid-slope, and
upland-slope against Tw based on weightedTz from all three stations to determine their
relative differences. Riparian buffer zone estimates of annual water
loss were 24% higher in 2015 and 21% higher in 2016 thanT w based on weighted Tzfrom all three stations. Annual water loss from the mid-hillslope was
only 2% higher in 2015 and 2016 than T w based on
weighted Tz from all three stations while water
loss from upland-hillslope was 7% lower in both years thanT w. The riparian buffer and upland-hillslope
produced larger percent differences than mid-hillslope likely because
they did not fully capture the spatial changes in speciesJ s response to VPD as well as the range of
watershed soil moisture conditions and stand structure across the
watershed (Mitchell et al., 2012). In addition, the hydrologic controls
on tree J s in these two zones likely shifted from
climatic factors to soil moisture as VPD increased, and soil moisture
rapidly decreased (Emanuel et al., 2010). The riparian buffer and
upland-hillslope zones also had larger amplitudes for drainage and soil
moisture compared to the mid-hillslope. The coefficient of variation in
soil moisture was 24% in the mid-hillslope, 32% in the riparian
buffer, and 48% in the upland-hillslope. If we assume weighted
Tz across the three zones offered the best estimate ofT w, then it appears Tz from the
mid-hillslope was within a reasonable range to determine annualT w. J s-scaledT w from the buffer and upland-hillslope resulted
in broader estimates of T w than mid-hillslope and
should probably not be used as standalone zones to determineT w.