Canopy temperature variability in a tropical rainforest, subtropical evergreen forest, and savanna forest in Southwest China

Abstract

Canopy temperature (Tc) measurements with infrared thermometry have been widely used to assess plant water status. Here, we evaluated Tc and its controlling factors in a primary tropical rainforest (TRF), subtropical evergreen broad-leaved forest (STF) and valley savanna forest (SAF) in southwestern China. We found differences between Tc and air temperature (Ta) of as much as 2.2 °C between the dry and wet seasons in the TRF. However, the canopy-to-air temperature difference (Tc-Ta) was only 0.3 °C between the dry and wet seasons in the STF. Solar radiation (SR) was the dominant factor in Tc-Ta variations during the dry and wet seasons at the three sites. The increased heating in the canopy leaves was likely the result of low stomatal conductance leading to low transpiration cooling. Changes in Tc-Ta in the TRF were highly sensitive to the degree of stomatal closure. The change in Tc-Ta was controlled by the climate, but inherent plant traits, such as stomatal conductance, also played an important controlling role.

Introduction

Plant canopy temperature (Tc) is a major driving factor in surface energy fluxes and provides insight into overall plant water status ([2]). Tc measurements with infrared thermometry have been widely used to manage deficit irrigation of field crops. For example, Tc depression has been shown to be significantly related to stomatal conductance across varieties of rice ([16]). Also, 12 infrared temperature sensors were installed in a mature peach orchard, which received different irrigation treatments ([19]). However, few studies have used infrared methods to measure Tc in natural forests. One exception is Leuzinger & Körner ([11]), who illustrated the importance of tree species composition for the local climate through assessments of canopy leaf temperature distribution over space and time for a mixed deciduous forest in NW Switzerland. Canopy architecture has a consistent influence on canopy foliage temperature in diverse, mixed deciduous forests: “dense canopy” species are warmer than “open canopy” species ([14]). Research has also shown that canopy foliage maintains a temperature close to Ta under normal conditions, but significantly warms up with ongoing drought ([14]). Another study demonstrated that Tc has great responsiveness to variations in water status of persimmon trees ([1]): the Tc response to water stress was different for each experimental season. In a year with normal water availability (2009), Tc did not allow detection of the existing differences in plant water status, but during a drought year (2011), water stressed trees had significantly higher Tc than control trees with differences of up to 1.7 °C ([1]).

IPCC-AR4 climate projections anticipated that climate change will increase temperatures, create greater variability in rainfall (both seasonal and annual), and generate more frequent and intense drought events worldwide ([6]). The higher air temperatures in combination with higher atmospheric vapor pressure deficits (δ_e) will lead to higher transpiration demand. However, under drought stress conditions, water uptake rates cannot match the transpiration demands and leaf stomata close to maintain plant water balance, which in turn leads to higher leaf temperatures ([3], [9]). Overall, different plant species in different vegetation communities are highly variable in functional traits, canopy architecture, rooting depth, and micrometeorological environment. The different responses of plant species to the expected climate changes might also be highly variable. There are still many uncertainties in relation to changes in Tc due to drought stress.

Primary tropical rainforests (TRFs), subtropical evergreen broad-leaved forests (STFs), and savanna forests (SAFs) are among the most important biomes in Southwest China because of their high primary productivity and unique characteristics ([17]). This area is exposed to Asian monsoon regimes from the southwest (Indian Ocean) and from the southeast (Pacific Ocean). Due to the predominant effect of the two monsoons, the climate in the study region is strongly seasonal with a wet season from May to October, and a dry season from November to April ([21]).

From late 2009 through 2010, southwestern China experienced the most severe drought since 1959, providing a unique opportunity to directly evaluate how Tc changed with drought stress in the different forest ecosystems. We know that components of the canopy energy balance, such as solar radiation (SR), wind speed (Ws), Ta, and air relative humidity (RH), affect Tc ([8]). The relative impact of each factor on Tc is likely to be different in different seasons and communities. However, little is known about Tc and its controlling factors in the TRF, STF, and SAF. Detailed studies of Tc are needed in these primary forests to improve our understanding of their potential sensitivity to drought stress.

The objectives of this study were to: (1) quantify the magnitude and seasonal dynamics of Tc and the relative contribution of different factors to Tc in the three forest ecosystems, and (2) evaluate the differences in Tc during normal and drought years.

Methods

Site information

Three typical primary forests (TRF, STF, and SAF) were selected in Yunnan province (Fig. 1) to evaluate Tc variability.

Fig. 1 - The locations of the three study sites (five stars). XSBN is a tropical rainforest (TRF); ALS is a subtropical forest (STF) and YJ is a savanna forest (SAF).

Tropical Rain forest (TRF)

The TRF study site is located in the Menglun Nature Reserve in Xishuangbanna. The site is located at the northern edge of tropical southwestern Asia, and is a transition area between the tropics and the subtropics. The climate is strongly seasonal with two air masses alternating during the year. Between May and October, the Asian monsoon from the Indian Ocean delivers most of the annual rainfall, whereas the dry and cold air of the southern edges of the subtropical jet streams dominates the climate between November and April. The dry season occurs between November and April, and includes a foggy-cool sub-season from November to February and a dry-hot sub-season from March to April. Dominant tree species are Pometia tomentosa, Gironniera subaequalis, and Terminalia myriocarpa. The soil is lateritic derived from siliceous rocks, such as granite and gneiss, with a pH from 4.5 to 5.5.

Subtropical evergreen forest (STF)

The STF site is in the Ailaoshan National Nature Reserve in the Xujiaba region. It lies within a protected section of a 5100 ha evergreen forest with a stand age of more than 300 years ([17]). The forest is primarily dominated by Lithocarpus hancei (Benth.) Rehder, Machilus bombycina, Castanopsis rufescens (Hook.f.et Th.) Huang et Y.T. Chang, and Lithocarpus xylocarpus. The soils are loamy Alfisols, and the 3-7 cm organic carbon horizon had a pH of 4.5.

Savanna forest (SAF)

The SAF site was located in the Yuan River region. The dry climate is due to the strong evapotranspiration in combination with a pronounced 6-month dry season (November-April). Vegetation is dominated by deciduous shrub species. The soil is a ferralic cambisol, containing moderate organic matter, higher N, P, K, Ca, and Mg concentrations, lower cation exchange capacity and moderate pH value, compared to soils of other tropical savannas

The key information of the three sites is shown in Tab. 1 and Fig. S1 (Supplementary material).

Tab. 1 - Site information. (MAT): Mean annual temperature (°C).

Site	TRF	STF	SAF
Location	21°55′ 39″ N 101°15′ 55″ E	24°32′ 17″ N 101°01′ 10″ E	23°28′ 40″ N 102°10′ 56″ E
Elevation	780	2502	481
MAT (°C)	22	11	24
Rainfall (mm year-1)	1487	1931	802
Predominant species	Pometia tomentosa, Gironniera subaequalis	Machilus bombycina, Lithocarpus chintungensis	Machilus bombycina
Canopy height (m)	33	26	5
Soil type	loamy Lateritic Oxisol	loamy Alfisol	ferralic Cambisol
Observation period	2009-2014	2009-2014	2014

Tc and meteorological factors

An infrared temperature sensor (Apogee Instruments Inc., Logan, UT, USA) was installed at each site to measure Tc. The sensors were installed on observational towers (Fig. 2) mounted on 4-cm diameter galvanized metal pipes 3 m above each forest canopy surface. The sensor was aimed at the canopy, with view half angles of approximately 22° from nadir. The absolute accuracy of the sensor is ± 0.2 °C. The radiation detected by an infrared radiometer included two components: (i) radiation directly emitted by the target surface; and (ii) radiation reflected from the background. The second component is often neglected. The magnitude of the two components in the radiation detected by the radiometer was estimated using the emissivity and reflectivity of the target surface ([2]).

Fig. 2 - Time series of daily micrometeorological variables for the TRF, STF and SAF sites: (a): shortwave incoming radiation (SR), (b): air temperature (Ta), (c): relative humidity (RH), and wind speed (Ws). (TRF): tropical rainforest site; (STF): subtropical evergreen forest site; (SAF): savanna forest site.

Results

Meteorological conditions

Fig. 2 shows average values of SR, Ta, RH, and Ws from 11:00-15:00 h for each day of measurements in the three forests. The values of SR for the whole year were scattered and there were no significant differences between the three forests (Fig. 2a). In general, Ta was highest in the SAF throughout the year and was lowest in the STF (Fig. 2b). In particular, Ta in late dry season in the SAF was almost 20 °C higher than in the STF. RH showed contrary patterns (Fig. 2c), and low Ws conditions persisted throughout the whole wet season in the three forests (Fig. 2d).

Seasonal patterns of canopy temperature (Tc)

In the TRF, the daily peak of the Tc-Ta appeared to occur at approximately 12:00 h in the dry season, while in the wet season, the maximum Tc-Ta value occurred at 14:00 h (Fig. 3a). The maximum Tc-Ta values in the dry and wet season in the TRF were 2.0 and -0.2 °C, respectively. In the STF, the daily peak values of Tc-Ta in the dry and wet season also appeared to occur around 11:30 h (Fig. 3b), while the maximum Tc-Ta value in the wet season occurred at 13:00 h. The difference of the maximum Tc-Ta value between the dry and wet season in the STF was small and the maximum values of the dry and wet season were 1.3 °C and 1.0 °C, respectively. In the SAF, the maximum Tc-Ta value was about 3.1 °C at 14:00 h in the wet season (growing season - Fig. 3c).

Fig. 3 - Mean diurnal time series of air to canopy temperature difference during dry and wet season at the TRF, STF and SAF sites. (TRF): tropical rainforest site; (STF): subtropical evergreen forest site; (SAF): savanna forest site.

To explore more deeply the influence of microclimate factors on Tc-Ta under the varying climatic conditions, we evaluated the correlations of environmental conditions with Tc-Ta on a statistical basis during the dry and wet seasons. Season-specific multivariable correlations between Tc-Ta and microclimate conditions are shown in Tab. 2. The analysis revealed the relative contribution of each parameter to Tc-Ta. SR is one of the primary factors correlated with Tc-Ta during the dry and wet seasons in the three sites. During the dry season, there was no single important variable in the TRF, but SR was still the most influential (β%=39%). The relative contribution of Ta and RH were almost equivalent at the TRF site. The two climate variables, SR and Ta, accounted for 80% of the variations in Tc-Ta in the STF during the dry season. During the wet season, Ws was the second most significant variable (β% = 21%) at the TRF site. Ws also influenced Tc-Ta significantly at the STF site during the wet season, while Ta and RH were not significant. RH accounted for 30% of the variations in Tc-Ta at the TRF site but was non-significant at the SAF site.

Tab. 2 - Coefficients (B) and relative contributions (β%) to Tc-Ta of the analyzed environmental factors. (TRF): tropical rainforest site; (STF): subtropical evergreen forest site; (SAF): savanna forest site. (**): p < 0.01; (*): p < 0.05 (t-test).

Factors	Dry season						Wet season
	TRF		STF		SAF		TRF		STF		SAF
	B	β%	B	β%	B	β%	B	β%	B	β%	B	β%
SR	0.003	39	0.003	49**	-	-	0.009	43**	0.003	55**	0.004	54**
Ta	-0.073	23	-0.119	31*	-	-	-0.351	19*	-0.053	10	0.074	14
RH	-0.021	25	-0.004	6	-	-	-0.069	17	-0.021	19	-0.035	30
Ws	0.896	13	-0.51	14*	-	-	-4.911	21**	-0.743	16*	-0.079	2

Interannual dynamics of Tc

Rainfall has decreased since the wet season of 2009, and the reduced rainfall continued until the end of the dry season in 2010 for the entire region ([18]). Therefore, we selected the middle of the wet season (September) in a drought year (2009) and a normal year (2011) to evaluate how Tc changes with drought stress at the TRF and STF sites.

The daily peak values of Tc-Ta among the normal and drought years at both sites appeared to occur at approximately 13:00-14:00 h, which was consistent with the period of maximum daily solar radiation but before the period in which Ta reached the daily maximum (Fig. 4a). There was virtually no difference in Tc-Ta values during the night when there was no solar heating. Tc-Ta values in the drought year (2009) were higher than in the normal year (2011) for both sites. The maximum Tc-Ta values for the drought year and normal year were 0.78 °C and -0.36 °C in the TRF; and 1.18 °C and -0.01 °C in the STF, respectively. Thus, the difference of Tc-Ta between the drought and normal years in both sites was about 1.1 °C.

Fig. 4 - Mean diurnal time series of SR, Ta, RH, and Ws on September 2009 and 2010 at TRF and STF sites. (TRF): tropical rainforest site; (STF): subtropical evergreen forest site.

Similarly, we also analyzed the relative contribution of each parameter to daylight (10:00-17:00 h) Tc-Ta during the drought and normal years at both sites. In the TRF, SR and Ws affected Tc-Ta both in drought and normal years. The relative contribution of SR in the normal year (2011) was higher than that in the drought year (2009, β% = 19% - Tab. 3). The relative contribution (β% = 32%) of Ws in the normal year (2011) was also higher than that in the drought year (2009, β% = 10%) in the TRF. In contrast, the RH contribution in the drought year (β% = 30%) was twice as much as that in the normal year (β% = 15% - Tab. 3).

Tab. 3 - The coefficients (B) and relative contributions (β%) of environmental factors are presented for each variable. (TRF): tropical rainforest site; (STF): subtropical evergreen forest site; (SAF): savanna forest site. (**): p < 0.01; (*): p < 0.05 (t-test).

Factors	TRF				STF
	2009		2011		2009		2011
	B	β%	B	β%	B	β%	B	β%
SR	0.004	19**	0.004	32**	0.004	32**	0.005	23**
Ta	-0.652	41	0.202	24	-0.611	47	-0.984	42
RH	-0.113	30	-0.035	15	-0.043	13	-0.202	33
Ws	2.744	10*	5.745	29*	1.477	8	-0.790	2

In the STF, SR also affected Tc-Ta in both years significantly. The relative contribution (β% = 32%) of SR in the drought year (2009) was higher than that in normal year (2011, β% = 23% - Tab. 3).

Tc-Ta variations in a changing climate

Based on the results described above, we collected data for the standard meteorological conditions at the three sites. For g_c of the sample tree species at the SAF site, we used the g_c values from the literature ([22]) for the same site.

Increasing drought frequency has the potential to affect the carbon and water cycle in forest ecosystems. Warmer global temperatures are expected to cause an intensification of the hydrologic cycle, with increased evaporative demand for ecosystems. As mentioned above, plants under drought stress decrease stomatal conductance, thereby reducing transpiration and increasing Tc. Therefore, we assumed a decrease to 25%, 50%, and 75% of maximum g_c values at the three sites in our examination of the differences in Tc between the three forest ecosystems in a changing climate.

Tc-Ta was very sensitive to all simulated meteorological conditions (Fig. 5). Tc-Ta increased linearly with increasing SR and RH. Tc-Ta also decreased non-linearly with increasing Ta and Ws (Fig. 5). Our simulation results were consistent with the research by Maes et al. ([12], [13]). Tc-Ta increased sharply with SR at all g_c levels and sites. At the same level of g_c, Tc-Ta had a steeper increase with increasing SR in the STF than the other two sites (Fig. 5a-c), on average, approximately 2 °C higher than the other two sites. However, the difference of Tc-Ta at three g_c levels with increasing SR in the STF was smallest, indicating that the change in Tc-Ta was relatively insensitive to the degree of stomatal closure. In contrast, the difference in Tc-Ta at the three g_c levels in the TRF became larger with increasing SR. In the TRF, the standard Ta was higher than at STF, while RH in the TRF was lower than in the STF, suggesting that standard δ_e in the TRF was higher than in the STF. Therefore, evaporative demand was significantly higher in the TRF than in the STF. Additionally, TRF had the highest g_c of the three sites, indicating that g_c could still maintain relatively high absolute values even with partial stomatal closure in the TRF. Consequently, canopy transpiration in the TRF corresponding to higher g_c was relatively higher than in the STF ([24], [23]), leading to high canopy transpirative cooling (lower Tc-Ta) at the same level of SR (Fig. 5a).

Fig. 5 - Influence of climate conditions on (Tc-Ta), including: shortwave incoming radiation (SR), air temperature (Ta), vapor pressure deficit (VPD), and wind speed (Ws), for three levels of gc at the rubber plantation (a, c, e, g) and the tropical rainforest sites (b, d, f, h).

Discussion

Seasonal variations in Tc-Ta

Tc-Ta in the different seasons exhibited distinct diurnal cycles at the three sites. Our study showed Tc-Ta differences of as much as 2.2 °C between the dry season and the wet season in the TRF. However, Tc-Ta differences were only 0.3 °C between the dry and the wet season in the STF. A combined analysis of the four controlling variables (SR, Ta, RH, and Ws) across the three sites indicated a clear distinction between the dry and wet season with respect to the control Tc-Ta. SR was the dominant factor in Tc-Ta variations during the dry and wet seasons at the three sites (Tab. 2). This clearly indicated that the input of radiation energy mostly drives Tc-Ta, a result that was also found by Maes et al. ([12]). All the effects of the micrometeorological factors were variable among the seasons and sites, which means that focusing on seasons was critical to better understand Tc patterns ([19]). This highlights the importance of large seasonal variability in the Asian monsoon region, which leads to a shift in the dominant factors between seasons.

Plants under soil water deficit decrease stomatal conductance, thereby reducing transpiration and increasing leaf temperature. At TRF and STF sites, measurements conducted with sap flow methods showed that canopy transpiration for dominant tree species in the dry season were lower than in the wet season at both sites (Fig. S2 in Supplementary material). The reduction in transpiration in the dominant canopy trees during the dry season (with high evaporative demand) could partly explain the increase in Ta differences of the canopy (Fig. 3).

Interannual variations in Tc-Ta

Tc-Ta variations do correspond with variations in SR, Ta, RH, and Ws in the different seasons. Hence, it is important to keep in mind that variations in Tc-Ta in the dry and wet season at the same site do not necessarily result solely from soil water availability. Tc-Ta variations can also be an indication of the relative changes in plant transpiration under similar micrometeorological conditions at the same site. To ensure that the changes in Tc-Ta were connected to changes in transpiration, we selected the same period (September) in 2009 and 2011.

In the TRF, there was no significant difference between all of the micrometeorological conditions in September during the drought (2009) and the normal year (2010) mean values in the TRF (paired t-test: p = 0.07 - Fig. 4b-d), indicating that micrometeorological conditions were similar during the study period in 2009 and 2011. Tc-Ta at noon in 2009 was higher by about 1 °C than that in 2011. The increased heating in the canopy leaves in 2009 was likely the result of low stomatal conductance, leading to low transpiration cooling. During sunny days, canopy transpiration was the main contribution to evapotranspiration (latent heat flux), because soil and canopy evaporation were exceptionally low in days without rainfall. The mean latent heat flux values in September in 2009 and 2011 in the TRF were 179 W m^-2 and 212 W m^-2, respectively. Therefore, the latent heat flux data by eddy covariance method proved that the higher Tc-Ta in 2009 was due to a reduction in canopy transpiration, as the latent heat flux was lower than in 2011. During the drought period, trees saved soil water by increasing the surface resistance and reducing the transpiration rate, and for this reason, the canopy maximum temperature exceeded the optimum temperature. We observed a Tc-Ta of 0.78 °C for the drought year in the TRF, which was consistent with the reported 0.6-0.8 °C for a temperate forest with “dense canopies” at the beginning of a drought period ([14]).

Wang & Gartung ([19]) found that the midday Tc-Ta in a water-stressed, deficit irrigation treatment of peach trees were in the range 5-7 °C. The maximum Tc difference between deficit-irrigated and control trees observed in persimmon trees was 4 °C ([1]), while the maximum Tc difference between drought year and normal year was about 1 °C in the TRF. The possible reasons for the lower Tc-Ta found in the tropical rain forest include: (i) tree species in the tropical rain forest are highly diversified in canopy architecture. The temperature of the branches and self-shaded leaves were not measured, resulting in an underestimation of the actual Tc. This might be a general problem in natural forests ([14]). (ii) Most studies dealing with drought effects on Tc on crops have worked with precise irrigation treatments for determining the Tc to the difference in drought stress level, such as irrigated 50% evapotranspiration and irrigated 25-33% ([3], [19], [1]). Therefore, the Tc of crops might be more sensitive to severe drought stress in the irrigation treatments.

To evaluate the effect of drought stress on soil water, we analyzed the water content in the soil profile at both sites (Fig. S3 in Supplementary material). We found that although the total rainfall in 2009 was 30% lower compared with the average value for the past 50 years in this region ([15]), deep soil moisture did not decrease significantly (Fig. S3b in Supplementary material). The dominant canopy tree species have deep roots and can use deep soil water ([10]). The deeper roots of the larger canopy dominant trees could be the reason for the lower impact of soil drought on Tc in larger trees, even in the drought environment. If there was not a sufficient amount of water in the entire soil profile for canopy cooling, Tc-Ta could be increased continually ([4]).

In the STF, the maximum difference in Tc between the drought year and normal year was also about 1 °C. However, the micrometeorological conditions, such as SR, RH, and Ws, were different between the drought and normal year (Fig. 4g-j). Consequently, the Tc difference should correspond with variations in the micrometeorological conditions.

Overall, infrared Tc is a result of the energy balance, micrometeorological conditions, and plant-controlled transpiration. Measurement methods provide a tool for assessment and planning for water stress in the different seasons.

Implications and uncertainties regarding Tc measurements in forests

Infrared Tc measurements provided a useful evaluation of plant response to environmental stress in different forest ecosystems. Infrared Tc measurements are automatic and effective, providing an alternative option to conventional methods such as stomatal conductance or stem water potential measurements in monitoring plant water status ([7], [9], [19], [1]).

However, monitoring Tc with an infrared temperature sensor in a fixed position only partially covers the canopy, and the data are too limited to account for variations in the entire canopy and differences between species, particularly for high diversity tropical and subtropical forests. For example, canopy architecture has a consistent influence on canopy foliage temperature, in that “dense canopy” species are warmer than “open canopy” species ([14]). In addition, the temperature of shaded leaves is lower than the sunlit leaves of the same trees ([1]). The use of Tc measurements to detect plant water stress appears to be more precise in tree species with larger leaf sizes than species with smaller leaves ([1]). Leaf adaptations of some species to drought, namely smaller leaf dimensions and more vertically oriented leaves, also result in lower Tc-Ta ([12]). Despite the fact that infrared Tc measurements can be used as a water stress indicator, there has yet to be a critical assessment of the relationship between Tc and plant species composition, canopy structure, and leaf functional traits.

Conclusions

The four meteorological controlling variables (SR, Ta, RH, and Ws) across the three sites indicated a clear distinction between the dry and wet season with respect to the control of Tc-Ta. SR was the dominant factor in Tc-Ta variations during the dry and wet seasons in all three sites. Tc-Ta at noon in the drought year (2009) was about 1 °C higher than Tc-Ta in a normal year (2011) in the TRF. The increased heating of canopy leaves in the drought year (2009) was likely the result of low stomatal conductance leading to low transpiration cooling. Changes in Tc-Ta in the TRF were highly sensitive to the degree of stomatal closure, while changes in Tc-Ta in the STF were relatively insensitive to the degree of stomatal closure. Changes in Tc-Ta were controlled by climate conditions, but plant inherent traits, such as stomatal conductance, also played important driving roles.

Acknowledgements

We thank Lei Yu, Meng-Nan Liu and Qi Luo for their assistance in the field. This work was supported by the Xishuangbanna Ecological Station for Tropical rain forest, the Ailaoshan Station for Subtropical Forest Ecosystem Research and the Yuanjiang Savanna Ecological station. This study was funded by the National Natural Science Foundation of China (41671209, U1602234, 41405143), the Yunnan Natural Science Foundation of Yunnan Province, China (2013FB077) and the National Key Research and Development Program of China (2016YFC0502105).

References

(1) Ballester C, Jiménez-Bello MA, Castel JR, Intrigliolo DS (2013). Usefulness of thermography for plant water stress detection in citrus and persimmon trees. Agricultural and Forest Meteorology 168: 120-129.

(2) Blonquist JM, Norman JM, Bugbee B (2009). Automated measurement of canopy stomatal conductance based on infrared temperature. Agricultural and Forest Meteorology 149: 2183-2197.

(3) Campbell GS, Norman JM (1990). Estimation of plant water status from canopy temperature: an analysis of the inverse problem. In: “Applications of Remote Sensing in Agriculture” (Steven MD, Clark JA eds). Butterworths, London, UK, pp. 255-271.

(4) Duffková R (2006). Difference in canopy and air temperature as an indicator of grassland water stress. Soil and Water Research 4: 127-138. Online | Gscholar

(5) Granier A (1987). Evaluation of transpiration in a Douglass-fir stand by means of sap flow measurements. Tree Physiology 3: 309-320.

(6) IPCC (2007). Climate Change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the IPCC (Parry ML, Canziani OF, Palutikof JP, Jvan P, Linden Hanson CE eds). Cambridge University Press, Cambridge, UK, pp. 976. Gscholar

(7) Jackson RD (1991). Relationship between normalized leaf water potential and crop water stress index values for acala cotton. Agriculture Water Management 20: 109-118.

(8) Jones HG (1992). Plants and microclimate: a quantitative approach to environmental plant physiology. Cambridge University Press, Cambridge, UK, pp. 115-120.

(9) Jones HG (1999). Use of infrared thermometry for estimation of stomatal conductance as a possible aid to irrigation scheduling. Agricultural and Forest Meteorology 95: 139-149.

(10) Kume TH, Takizawa N, Yoshifuji K, Tanaka C, Tantasirin N, Tanak Suzuki M (2007). Impact of soil drought on sap flow and water status of evergreen trees in a tropical monsoon forest in northern Thailand. Forest Ecology and Management 238: 220-230.

(11) Leuzinger S, Körner C (2007). Tree species diversity affects canopy leaf temperatures in a mature temperate forest. Agricultural and Forest Meteorology 146 (1-2): 29-37.

(12) Maes WH, Achten WMJ, Reubens B, Muys B (2011). Monitoring stomatal conductance of Jatropha curcas seedlings under different levels of water shortage with infrared thermography. Agricultural and Forest Meteorology 151: 554-564.

(13) Maes WH, Steppe K (2012). Estimating evapotranspiration and drought stress with ground-based thermal remote sensing in agriculture: a review. Journal of Experimental Botany 63: 4671-4712.

(14) Scherrer D, Karl-Friedrich BM, Christian K (2011). Drought-sensitivity ranking of deciduous tree species based on thermal imaging of forest canopies. Agricultural and Forest Meteorology 151: 1632-1640.

(15) Song QH, Lin H, Zhang YP, Tan ZH, Zhao JF, Zhao JB, Zhang XL, Zhou WJ, Yu L, YangLY, Yu GR, Sun XM (2013). The effect of drought stress on self-organisation in a seasonal tropical rainforest. Ecological Modelling 265: 136-139.

(16) Takai T, Yano M, Yamamoto T (2010). Canopy temperature on clear and cloudy days can be used to estimate varietal differences in stomatal conductance in rice. Field Crops Research 115: 165-170.

(17) Tan ZH, Zhang YP, Schaefer D, Yu GR, Liang N, Song QH (2011). An old-growth subtropical Asian evergreen forest as a large carbon sink. Atmospheric Environment 45 (8): 1548-1554.

(18) Tan ZH, Zhang YP, Deng XB, Song QH, Liu WJ, Deng Y, Tang JW, Liao ZY, Zhao JF, Song L, Yang LY (2015). Interannual and seasonal variability of water use efficiency in a tropical rainforest: tesults from a 9 year eddy flux time series. Journal of Geophysical Research: Atmosphere 120: 464-479.

(19) Wang D, Gartung J (2010). Infrared canopy temperature of early-ripening peach trees under postharvest deficit irrigation. Agricultural Water Management 97: 1787-1794.

(20) Wilson KB, Hanson PJ, Mulholland P, Baldocchi DD, Wullschleger S (2001). A comparison of methods for determining forest evapotranspiration and its components: sap flow, soil water budget, eddy covariance and catchment water balance. Agricultural and Forest Meteorology 106: 153-168.

(21) You GY, Zhang YP, Liu YH, Schaefer DA, Gong HD, Gao JB, Lu ZY, Song QH, Zhao JB, Wu CS, Yu L, Xie TN (2013). Investigation of temperature and aridity at different elevations of Mt. Ailao, SW China. International Journal of Biometeorology 57: 487-492.

(22) Zhang JL, Zhu JJ, Cao KF (2007). Seasonal variation in photosynthesis in six woody species with different leaf phenology in a valley savanna in southwestern China. Trees-structure and Function 21: 631-643.

(23) Zhang YJ, Holbrook NM, Cao KF (2014). Seasonal dynamics in photosynthesis of woody plants at the northern limit of Asian tropics: potential role of fog in maintaining tropical rainforests and agriculture in Southwest China. Tree Physiology 34 (10): 1069-1078.

(24) Zhang Y, Meinzer FC, Qi JH, Goldstein G, Cao KF (2013). Midday stomatal conductance is more related to stem rather than leaf water status in subtropical deciduous and evergreen broadleaf trees. Plant Cell and Environment 36: 149-158.