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/