An Experimental Investigation of the Precipitation Utilization of Plants in Arid Regions

Abstract

What represents a water source for the ecological restoration of a plant in an arid region is still up to debate. To address this issue, we conducted an in situ experiment in the Ulan Buh Desert of China, to study desert plants absorbing atmospheric water vapor. We selected Tamarisk, a common drought-salt-tolerant species in the desert, for ecological restoration as our research subject, used a newly designed lysimeter to monitor precipitation infiltration, and a sap flow system to track reverse sap flow that occurred in the shoot, branch, and stem during the precipitation event, and observed the precipitation redistribution process of the Tamarisk plot. The results showed that Tamarisk indeed directly absorbs precipitation water: when precipitation occurs, the main stem, lateral branch, and shoot all show the signs of reversed sap flow, and the reversed sap flow accounted for 21.5% of the annual sap flow in the shoot and branch, and 13.6% in the stem. The precipitation event in the desert was dominated by light precipitation events, which accounted for 81% of the annual precipitation events. It was found that light precipitation can be directly absorbed by the Tamarisk leaves, especially during nighttime or cloudy days. Even when the precipitation is absent, it was found that desert plants can still absorb water from the unsaturated atmospheric vapor; even the absorbed atmospheric water vapor was transported from the leaves to the stem, forming a reversed sap flow, as a reversed sap flow was observed when the atmospheric relative humidity reached 75%. This study indicated that the effect of light precipitation on desert plants was significant and should not be overlooked in terms of managing the ecological and hydrological systems in arid regions.

Keywords: Tamarisk; Ulan Buh Desert; arid area; atmospheric moisture; reverse sap flow.

Figure 1

(A) The map showing the species distribution of artificial forests in northern China [36]. Tamarisk, as a shrub with low water consumption, is widely planted in the arid areas of China. (B) This diagram shows precipitation and condensation water hanging on Tamarisk branches in the morning in the site; (C) this diagram shows the observed in situ Tamarisk, where sap flow sensors are wrapped at the main stem, lateral branches, and shoot, respectively.

Figure 2

(A) This picture shows the in situ Tamarisk, with sap flow probes wrapped on the main stem, lateral branches, and shoots. (B) The figure shows the experimental setup, with the Tamarisk placed in a semi-enclosed transparent controlled-climate room, where artificial precipitation experiments can be carried out; it was closed for the atmospheric water vapor absorption experiments. (C) This diagram of the new lysimeter and the depth of A and B.

Figure 3

Changes in in situ Tamarisk sap flow in the main steam, lateral branch, and shoot. SF stands for sap flow.

Figure 4

The canopy width (length, width, and height) of the target branch is measured in situ, based on the captured images.

Figure 5

The occurrence time of reversed sap flow and corresponding RH, the humidification process and the dehumidification process. Humidifying Tamarisk 1 in the control room and finding reverse sap flow when the RH reached 75% (A); humidifying the Tamarisk in the other two control rooms, (B) is the RH around 80%, with slight fluctuations from us manually controlling the humidifier, and (C) is high-intensity humidification, maintaining an RH at around 90%. The purple line is RH and the red dashed line is RH reached 75% (A), 75% (B) and 85% (C).

Figure 6

Effects of light and heavy precipitation events on reversed sap velocity (the ratio of the real-time sap velocity to the maximum sap velocity). (A) refers to a light precipitation during the day, (B) refers to a long precipitation during the night. SF stands for sap flow.

Figure 7

Relationship between the sap flow and water stress at different periods, where daytime (denoted as orange color □) and nighttime (denoted as blue color □) data were plotted separately. Daytime refers to the duration from 7:00 am to 7:00 pm at the same day, and nighttime refers to the duration from 7:00 pm to 7:00 am of the following day. (A) The mean values of branch sap flows from 18 to 28 July 2019 in relation to VPD; (B) the mean values of branch sap flows from 20 August to 2 September 2019 in relation to VPD. The green and red lines represent the trends of sap flow during the daytime and nighttime.

Figure 8

Relationship between the dry leaf mass and their absorption of atmospheric water. The red line represents the trend of relevance.

Figure 9

Water absorption and water consumption characteristics of Tamarisk during the growing season. (A) shows the relationship between RH and precipitation, the green line is RH and the blue line is precipitation. (B) demonstrates the sap flow. (C) represents the precipitation absorption of Tamarisk. (D) shows the water consumption during the growing season. (E) represents the change in DSR and the precipitation. And the red line with an arrow represents the trend in (B–D).