Responses of water vapor and heat fluxes to environmental factors in a deciduous broad- leaved forest ecosystem in Beijing

doi:10.17521/cjpe.2021.0106

Abstract

Abstract:

Aims The responses of water and heat fluxes of temperate forest ecosystems to environmental factors are hypothesized to vary with time scales. This study aimed to examine the variations of water and heat fluxes in a typical natural deciduous broad-leaved forest ecosystem in Songshan and their response mechanisms to environmental factors at different time scales.
Methods The eddy-covariance (EC) method was used to continuously monitor the evapotranspiration (ET), sensible heat (H), latent heat (LE), soil heat flux (G), vapor pressure deficit (VPD), air temperature (T_a), photosynthetically active radiation (PAR), normalized difference vegetation index (NDVI), and soil water content at a depth of 10 cm (VWC) in a typical deciduous broad-leaved forest ecosystem in Songshan, Beijing in 2019. The wavelet analysis was used to examine the regulation mechanism of biotic and abiotic factors on energy distribution and water vapor exchange at different time scales.
Important findings The mean annual Bowen ratio (β) was 1.53 in 2019. ET had obvious seasonal dynamics, increasing gradually from day 100, peaking in July, and decreasing to the lowest level on day 300. The maximum daily evapotranspiration was 5.01 mm·d^-1, the cumulative annual evapotranspiration was 476.2 mm, and the cumulative annual rainfall was 503.3 mm. The main influencing factors of the water and heat fluxes varied with time scales, being mostly controlled by VPD at the daily scale, and by PAR at the seasonal scale. At the diurnal scale, water and heat fluxes lagged 3.36 h behind VPD. At the seasonal scale, water and heat fluxes lagged 8 days behind PAR. At the seasonal scale, PAR had an indirect impact on ET through its effects on VPD and a direct impact on β. The results indicate the time-delay relationships between water and heat fluxes and environmental factors at different time scales, which provides support for selecting the optimal input parameters of the models for quantitatively forecasting ecosystem processes at different time scales in northern temperate deciduous broad-leaved forests.

Key words: evapotranspiration, energy exchange, wavelet analysis, deciduous broad-leaved forest

LI Xin-Hao, TIAN Wen-Dong, LI Run-Dong, JIN Chuan, JIANG Yan, HAO Shao-Rong, JIA Xin, TIAN Yun, ZHA Tian-Shan. Responses of water vapor and heat fluxes to environmental factors in a deciduous broad- leaved forest ecosystem in Beijing[J]. Chin J Plant Ecol, 2021, 45(11): 1191-1202.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2021.0106

https://www.plant-ecology.com/EN/Y2021/V45/I11/1191

Figures/Tables 9

Fig. 1 Energy balance closure of the ecosystem based on the half-hourly data with and without energy balance correction at the deciduous broad-leaved forest ecosystem in Beijing in 2019. Numbers in parentheses show the 95% confidence intervals of the intercepts and slopes. G, soil heat flux; H, sensible heat flux; Hc, corrected sensible heat flux; LE, latent heat flux; LEc, corrected latent heat flux; Rn, net radiation.

Fig. 2 Dynamics in daily means of environmental variables in 2019 in the sample plot of the deciduous broad-leaved forest ecosystem in Beijing. A, Photosynthetically active radiation (PAR). B, Air temperature (Ta). C, Vapor pressure deficit (VPD). D, Precipitation (P) and soil water content (VWC) at a depth of 10 cm. E, Canopy conductance (gs). F, Normalized differential vegetation index (NDVI).

Fig. 3 Seasonal variation of daily mean values of water and heat fluxes in 2019 at the deciduous broad-leaved forest ecosystem in Beijing. A, Net radiation (Rn). B, Soil heat flux (G). C, Corrected sensible heat fluxes (Hc) and corrected latent heat fluxes (LEc). D, Bowen ratio (β). E, Evapotranspiration (ET).

Fig. 4 Wavelet coherence between the proportion of corrected sensible heat flux in net radiation (Hc/Rn) and photosynthetically active radiation (PAR)(A), air temperature (Ta)(B), soil water content (VWC)(C), vapor pressure deficit (VPD)(D), canopy conductance (gs)(E), and normalized difference vegetation index (NDVI)(F). The phase difference is shown by arrows. Arrows pointing down indicate environmental factors leading Hc/Rn by 90° or lagging Hc/Rn by 270°, while arrows pointing up indicate environmental factors lagging Hc/Rn by 90° or leading Hc/Rn by 270°. Arrows pointing left (right) indicate environmental factors and Hc/Rn vary anti-phase (in-phase). The area surrounded by black lines in the U-shaped represent the 0.05 significance level.

Fig. 5 Wavelet coherence between the proportion of corrected latent heat flux in net radiation (LEc/Rn) and photosynthetically active radiation (PAR)(A), air temperature (Ta)(B), soil water content (VWC)(C), vapor pressure deficit (VPD)(D), canopy conductance (gs)(E), and normalized difference vegetation index (NDVI)(F). The phase difference is shown by arrows. Arrows pointing down indicate environmental factors leading LEc/Rn by 90° or lagging LEc/Rn by 270°, while arrows pointing up indicate environmental factors lagging LEc/Rn by 90° or leading LEc/Rn by 270°. Arrows pointing left (right) indicate environmental factors and LEc/Rn vary anti-phase (in-phase). The area surrounded by black lines in the U-shaped represent the 0.05 significance level.

Fig. 6 Wavelet coherence between the evapotranspiration (ET) and photosynthetically active radiation (PAR)(A), air temperature (Ta)(B), soil water content (VWC)(C), vapor pressure deficit (VPD)(D), canopy conductance (gs)(E), and normalized difference vegetation index (NDVI)(F). The phase difference is shown by arrows. Arrows pointing down indicate environmental factors leading ET by 90° or lagging ET by 270°, while arrows pointing up indicate environmental factors lagging ET by 90° or leading ET by 270°. Arrows pointing left (right) indicate environmental factors and ET vary anti-phase (in-phase). The area surrounded by black lines in the U-shaped represent the 0.05 significance level.

Table 1 The lag relationship between the proportion of the corrected sensible heat flux in the net radiation (Hc/Rn), the proportion of the corrected latent heat flux in the net radiation (LEc/Rn), evapotranspiration (ET) and environmental factors at the daily scale

	PAR	T_a	VWC	VPD	g_s	NDVI
H_c/R_n	20.41 (14.36)	22.83 (14.13)	18.81 (13.67)	0.46 (1.44)	4.84 (10.54)	23.24 (14.06)
LE_c/R_n	9.47 (14.67)	22.17 (15.74)	19.94 (17.65)	7.87 (4.97)	4.13 (3.63)	23.77 (14.74)
ET	4.43 (11.23)	3.21 (10.47)	0.31 (0.20)	1.76 (9.70)	4.99 (1.05)	19.33 (15.43)

Table 2 The lag relationship between the proportion of the corrected sensible heat flux in the net radiation (Hc/Rn), the proportion of the corrected latent heat flux in the net radiation (LEc/Rn), evapotranspiration (ET) and environmental factors at the seasonal scale

	PAR	T_a	VWC	VPD	g_s	NDVI
H_c/R_n	10.41 (2.01)	-	20.64 (20.53)	11.17 (1.24)	13.10 (10.98)	20.36 (20.91)
LE_c/R_n	10.52 (8.79)	14.05 (6.30)	23.41 (8.40)	16.44 (9.86)	25.23 (0.67)	16.02 (13.36)
ET	3.06 (1.55)	9.55 (14.99)	15.09 (19.19)	10.67 (10.12)	-	27.81 (9.74)

Fig. 7 Partial wavelet coherence between Bowen ratios (β) and photosynthetically active radiation (PAR) after removing the effects of vapor pressure deficit (VPD)(A), between evapotranspiration (ET) and VPD after removing the effects of PAR (B), between β and VPD after removing the effects of PAR (C), and between ET and VPD after removing the effects of PAR (D). The area surrounded by black lines in the U-shaped represent the 0.05 significance level.

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