Transpiration of Swartzia tomentifera in response to microclimatic variability in the central Amazon: the net effect of vapor pressure déficit

Antezana-Vera, Saul Alfredo; Marenco, Ricardo Antonio

doi:10.1590/01047760202127012999

ABSTRACT

Background:

The distribution of precipitation in the Amazon region is changing in last decades, but it is uncertain if transpiration will decrease in response to changes in rainfall distribution. In this study we aimed to assess the effect of climatic variability on whole-tree transpiration (inferred from sap flow measurements). We collected climate data (precipitation, photosynthetically active radiation -PAR, relative humidity, vapor pressure deficit -VPD, and temperature) and measured sap flow rates (SFR) in trees of Swartzia tomentifera. The SFR was measured in the dry season and wet season using the heat ratio method.

Results:

Sap velocity was positively correlated with PAR and temperature, with partial correlation (rpart) of 0.33 to 0.57 (p < 0.001) during daytime, while at night rpart varied from 0.15 to 0.21 (p < 0.001). When the effects of PAR and temperature were controlled (i.e. after removing the effect of PAR and temperature on sap velocity and VPD), the net effect of VPD on sap velocity became slightly negative (rpart of -0.08 to -0.14, p < 0.01) during daytime. Maximum sap velocities were lower during the dry season (5.9 cm h^-1) than in the wet season (9.8 cm h^-1, p < 0.001).

Conclusion:

This study shows that tree transpiration can decrease in the dry season in the central Amazon, associated with an increase in temperature, PAR and vapor pressure deficit. A novelty of this study is to demonstrate that after adjusting for the effect of temperature and PAR, the effect of vapor pressure deficit on sap velocity can become negative. .

Keywords:
Sap flow rate; Sap velocity; Partial correlation; Transpiration

HIGHLIGHTS

Transpiration decreases in the dry season with increase in vapor pressure deficit (VPD).

Transpirational net VPD effect was found removing temperature and irradiance contribution.

Removing influences of temperature and irradiance reduces transpirational net VPD effect.

During daytime, an increase in net VPD greatly restrained transpiration.

[1] rmarenco@inpa.gov.br

Variable	Mean (± SE)	Dry season (± SE)	Wet season (± SE)	p value.
PAR (mol m^-2 day^-1)	25.69 ± 0.80	26.34 ± 1.980	25.55 ± 0.96	0.763
Rainfall (mm month^-1)	196.4 ± 31.3	95.10 ± 1.10	218.94 ± 35.79	0.008
Tmean (°C)	26.29 ± 0.27	26.90 ± 0.32	26.15 ± 0.33	0.182
Tmin (°C)	23.23 ± 0.19	22.65 ± 0.28	23.36 ± 0.21	0.157
Tmax (°C)	30.56 ± 0.44	31.12 ± 0.28	30.43 ± 0.56	0.302
RHmean (%)	78.32 ± 2.32	66.12 ± 1.17	81.04 ± 1.97	< 0.001
RHmin (%)	55.54 ± 2.58	42.27 ± 0.26	58.49 ± 2.27	< 0.001
RHmax (%)	91.74 ± 1.86	81.66 ± 2.57	93.98 ± 1.47	0.067
VPDmean (hPa)	7.59 ± 0.89	12.01 ± 0.69	6.60 ± 0.81	0.005
VPDmin (hPa)	2.35 ± 0.52	5.02 ± 0.66	1.75 ± 0.44	0.052
VPDmax (hPa)	19.96 ± 1.54	26.25 ± 0.33	18.55 ± 1.61	0.001
SWC (%, v/v)	43.81 ± 1.26	41.53 ± 0.09	44.31 ± 1.57	0.115

Variable	Climatic factor	Wet season (rpart)	Dry season (rpart)
		Daytime	Daytime
SV versus	PAR	0.35 (< 0.001)	0.46 (< 0.001)
	T (°C)	0.33 (< 0.001)	0.57 (< 0.001)
	VPD	-0.14 (< 0.001)	-0.08 (0.002)
		Nighttime	Nighttime
	T (°C)	0.21 (< 0.001)	0.15 (< 0.001)
	VPD	0.12 (< 0.001)	0.26 (< 0.001)

Parameters	Mean1 (± SD)
Tree characteristics
Tree height (m)	24.8 ± 1.6
Stem diameter (cm)	26.6 ± 8.9
Crown diameter (m)	8.1 ± 2.1
Wood density (g cm^-3)	0.82 ± 0.01
Sap wood depth (cm)	4.49 ± 2.04
Sap wood area, SWA (cm²)	302 ± 175
Sap flow parameters
Maximum inner SV (cm h^-1)	5.34 ± 5.06
Maximum outer SV (cm h^-1)	13.79 ± 5.75
Mean maximum inner-outer SV (cm h^-1)	9.56 ± 4.98
Total daily SFR (L day^-1)	22.15 ± 15.05
Daytime SFR (L day^-1)	20.08 ± 13.14
Nighttime SFR (L day^-1)	2.07 ± 1.43
Mean daily SFR (g cm^-2 h^-1)	2.64 ± 1.31
Nighttime to total SFR (100×2.07/22.15)	9.3%
Nighttime to daytime SFR (100×2.07/20.08)	10.3%
Kunert´s equation SWA (cm²)	300 ± 167
Kunert´s equation (total SFR L day^-1)	20.90 ± 12.4