| Plant canopy temperature is an important physiological and ecological factor. It has been used to assess plants’ adaptability to their habitat. It is also a new method of assessing soil moisture. However, the study on canopy temperature of sand-fixation plants is still weak, and using canopy temperature to indicate plants water deficit is still hard to be used in practice. So, this study focused on five common sand-fixation plants, including Artemisia ordosica, Salix psammophila, Hedysarum mongolicum, Caragana korshinskii, and Populus alba. Field investigations and controlled experiments were conducted to measure and analyze the plants’ canopy temperature and stomatal conductance. Then the main meteorological factors that influence canopy temperature and stomatal conductance were identified and responses of canopy temperature and stomatal conductance to soil moisture are discussed. The regulation of stomatal conductance and canopy temperature response to soil moisture are summarized. Based on these results, stomatal conductance was used as an intermediate parameter and a plant water deficit model synthesizing meteorological factor and canopy temperature was established. The main conclusions are as follows:(1) The plant canopy temperature and stomatal conductance were found to follow the same rules over time in Artemisia ordosica, Salix psammophila, Hedysarum mongolicum, and Populus alba. On a scale of days, the change in the canopy temperature and stomatal conductance both showed a trend of first rising and then falling. Canopy temperature peaked around 10:00-14:00, and stomatal conductance around 8:00-10:00. Plant canopy temperature and stomatal conductance both showed visible spatial difference. In terms of space, the canopy temperature and stomatal conductance were higher on the sunny side of the canopy than on the night side for most of the day. The upper part of the canopy crown showed the highest value of temperature and stomatal conductance, followed by the central and lower parts. Canopy temperature and stomatal conductance both showed highly significant differences across species (P<0.01).(2) Canopy temperature and stomatal conductance were significantly influenced by meteorological factors. Air temperature was found to be the most impactful factor influencing canopy temperature: canopy temperature increased as air temperature increased. Humidity was the second most impactful factor, and canopy temperature decreased as humidity increased. Stomatal conductance tended to first rise then fall as these meteorological factors increased. Air temperature was the most impactful factor influencing stomatal conductance. The canopy temperature had a significant negative correlation with leaf area and leaf water content (P<0.05). Stomatal conductance showed a highly significant positive correlation with leaf water content (P<0.01).(3) The plant stomatal conductance showed a visible correlation to changes in soil moisture. Stomatal conductance reached maximum values of 3.08 mmol m-2 s-1,1.87 mmol m-2 s-1, and 1.92 mmol m-2 s-1 in Artemisia ordosica, Salix psammophila, and Caragana korshinskii, respectively, when these plants are well watered. Plant stomatal conductance was found to decrease with decreases in soil moisture content, even falling below 0.2 mmol m-2 s-1, after which it remained consistently low. In this way, stomatal conductance can be used directly to assess the soil moisture. Canopy temperature did not show any obvious relationship with soil moisture. It was found to be more profoundly influenced by meteorological factors than by soil moisture. In this way, canopy temperature cannot be used to assess soil moisture directly.(4) Two models of plant water deficit were established by synthesizing meteorological factors and canopy temperature data, CWSI and Ig model. First, the relationship between stomatal conductance and canopy temperature and that between stomatal conductance and soil moisture were summarized. Then, based on the results, stomatal conductance was selected for use as an intermediate parameter to establish a plant water deficit model. The degree of drought was divided and the value range of each degree was defined. Finally, the value range of the CWSI model index was set to (0,1), and the drought stress degree increased as the index value increased. The value range of the Ig model index was set to (0,∞), and the degree of drought stress increased as the index value decreased. The Ig model is more visual, and it is sensitive with respect to judging the degree of drought stress. For this reason, the Ig model may be more suitable for practical use than the CWSI model.This study delves into the basic characteristics of the plant canopy temperature and stomatal conductance and discusses the mechanism underlying the response to drought stress. A plant water deficit model was here established. The results of this study can provide a convenient and scientific way to assess plant water deficit and offer a reference for water management policy in the recovery of vegetation from desertification. |
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