Water Addition Prolonged the Length of the Growing Season of the Desert Shrub Nitraria tangutorum in a Temperate Desert

Abstract

Climate models often predict that more extreme precipitation events will occur in arid and semiarid regions, where plant phenology is particularly sensitive to precipitation changes. To understand how increases in precipitation affect plant phenology, this study conducted a manipulative field experiment in a desert ecosystem of northwest China. In this study, a long-term in situ water addition experiment was conducted in a temperate desert in northwestern China. The following five treatments were used: natural rain plus an additional 0, 25, 50, 75, and 100% of the local mean annual precipitation. A series of phenological events, including leaf unfolding (onset, 30%, 50%, and end of leaf unfolding), cessation of new branch elongation (30, 50, and 90%), and leaf coloration (80% of leaves turned yellow), of the locally dominant shrub Nitraria tangutorum were observed from 2012 to 2018. The results showed that on average, over the seven-year-study and in all treatments water addition treatments advanced the spring phenology (30% of leaf unfolding) by 1.29-3.00 days, but delayed the autumn phenology (80% of leaves turned yellow) by 1.18-11.82 days. Therefore, the length of the growing season was prolonged by 2.11-13.68 days, and autumn phenology contributed more than spring phenology. In addition, water addition treatments delayed the cessation of new branch elongation (90%) by 5.82-12.61 days, and nonlinear relationships were found between the leaves yellowing (80% of leaves) and the amount of watering. Linear relationships were found between the cessation of new branch elongation (90%), the length of the growing season, and amount of water addition. The two response patterns to water increase indicated that predictions of phenological events in the future should not be based on one trend only.

Keywords: branch phenology; desert species; foliar phenology; growing season; water addition.

Figure 1

Illustrations of Nitraria tangutorum nebkhas above-ground and below-ground structures, cited from Zhang et al., 2015 (A); distribution of the 20 plots and the water addition system (B); sprinkler on nebkhas (C).

Figure 2

Variations in air temperature (A–E) and precipitation (F–J) over the period of 2012–2018.

Figure 3

Relative changes of 0–20 cm soil water content (SWC) compared with control (Ctrl) in +25, +50, +75, and +100% water addition treatment plots in spring (May), summer (July), and autumn (September) in 2012 (A). Variation in 0–20 cm SWC during the growing season (May to September) of 2017 (B). SWC_Ctrl represents SWC in control plots. SWC_treat represents the SWC in water addition treatment plots. Arrows in (A) represent the water addition treatment application time.

Figure 4

Relative changes (△days; mean ± SE) of the two spring events of N. tangutorum after water addition treatments (+25% to +100%) compared with the control (Ctrl). Positive and negative values represent delayed and advanced days, respectively. A, C, E represent the 30% of leaves unfolded event. B, D, F represent the End of leaf unfolding event. * indicates significant differences at the P <0.05 level compared with values in control plots.

Figure 5

Relative changes (△days; mean ± SE) of the two autumn events of N. tangutorum after water addition treatments (+25% to +100%) compared with the control (Ctrl). Positive and negative values represent delayed and advanced days, respectively. A, C, and E represent the cessation of new branch elongation. B, D, and F represent 80% of leaves turned yellow. * indicates significant differences at the P <0.05 level compared with values in control plots.

Figure 6

Relative changes (△days; mean ± SE) in the growing season length of N. tangutorum after water addition treatments (+25% to +100%) compared with the control (Ctrl). Positive and negative values represent prolonged and shortened days, respectively. A–C represent the length of the growing season. * indicates significant differences at the P <0.05 level compared with values in control plots.

Figure 7

Correlations between the amount of water addition (Ctrl), +25, +50, and +100%) and the changes (△days) of the phenological events of N. tangutorum. The solid lines indicate P ≤0.05, and the dashed lines indicate 0.05< P <0.1. A, D represent cessation of new branch elongation. B, E represent 80% of the leaves turned yellow. C, F represent the length of the growing season.