3.4. Canopy transpiration and physiological response
The change in daily transpiration rate (Td) over time
during the study period under different treatments is presented in
Figure 7. Overall, Td showed clear seasonal patterns
independent of year and treatment; Td progressively
increased with leaf development in late Spring, peaked from middle
Summer to early Autumn (late July to mid-September), and decreased
sharply with leaf fall in mid-autumn. The severe drought (dry spell I)
clearly reduced Td in spring and summer 2015; the
transpiration in the BYF period decreased by 22.6% on average compared
to that in 2014. However, the moderate drought in 2016 had negligible
negative effects on the time series for Td, although the
SWS obviously decreased (Figure 4). Nonetheless, this moderate drought
seemed to enhance the differences in Td between the
different treatments; the combined treatments had apparently higher
Td than the SHT and the control.
/Figure 7/
The amount of transpiration was calculated for the four growing periods
– SPR, BYF, FRS, and FRM (Figure 8). As expected (based on Figure 7),
the FRS period had the highest transpiration and accounted from 45.2%,
on average, of the whole growing season considering all treatments in
different years, while the SPR had the lowest, accounting for only
4.9%, on average. The combined treatments significantly enhanced the
transpiration during the four growing periods, with the exception of the
SPR in 2015; the FRS period had the highest level
(P <0.001) of significant difference whereas the SPR had
the lowest level. At the annual scale, the transpiration during the four
different growing stages was summed to derive the transpiration for the
whole growing season. As shown in Figure 8, 2016 had the highest amount
of transpiration, amounting to 206.5 mm on average, while 2015 had the
lowest, with 172.6 mm on average. Moreover, the combined terracing and
mulching also had significantly (P <0.01 or 0.001)
higher annual transpiration than the other treatments; the MST treatment
had the greatest value of 213.5 mm, whilst the control had only 152.3
mm.
/Figure 8/
The relationship between annual transpiration and the corresponding mean
SWS for different treatments and years is shown in Figure 9. Clearly,
the annual transpiration significantly (P <0.01) and
positively correlated with the mean SWS in both the shallow (0-100 cm)
and deep (100-280 cm) soil layers. This means that the higher
transpiration in the combined treatments could be due to the greater
soil water availability.
/Figure 9/
The mid-day leaf water potential (ΨL) during the growing
seasons in 2015 and 2016 is presented in Figure 10. Overall,
ΨL showed relatively moderate temporal variation during
the growing seasons, ranging from -1.04±0.007 to -1.67±0.068 MPa, even
in the extreme drought in July 2015 and the moderate autumn drought in
2016. This suggests that the jujube trees exhibited isohydric behavior
and a conservative water use strategy in response to extreme droughts.
In addition, the combined treatments generally had higher
ΨL than the SHT and control, especially during the dry
spells.
/Figure 10/