analysis of an oasis microclimate in china’s hyperarid zone

ORIGINAL ARTICLE

Analysis of an oasis microclimate in China’s hyperarid zone

Qi Feng Æ Zhuo Macuo Æ Xi Haiyang

Received: 13 January 2008 / Accepted: 24 September 2008 / Published online: 14 October 2008

� Springer-Verlag 2008

Abstract The microclimate of a desert oasis in China’s

hyperarid zone was monitored, analysed and compared to

that of nearby forested lands. Factors associated with dif-

ferences in photosynthetically active radiation (PAR)

between clear, cloudy and dust storm days are discussed.

Desert oases were shown to fulfill ecological functions

such as altering solar radiation, adjusting near-ground and

land surface temperatures, reducing temperature differ-

ences, lowering wind velocity, and increasing soil and

atmospheric humidity. Total solar radiation within the oasis

was roughly half that above the forest canopy. During the

growing season, air temperatures in Populus euphratica

Olivier and Tamarix ramosissima Ledeb. woodlands were,

on average, 1.62 and 0.83�C lower, respectively, than that

in surrounding woodlands. The greater the forest cover, the

greater was the difference in temperature. Air temperature

was higher at the upper storey than that at the lower storey

of the community, i.e., air temperature increased with

increasing height above the soil surface. During the

growing season, relative humidity was higher in woodlands

than in surrounding areas: relative humidity in P. euphra-

tica and T. ramosissima woodlands were, on average, 8.5

and 4.2% higher, respectively, than that in the surrounding

area. Mean wind velocity in the P. euphratica forest land

was 0.33 m/s, 2.31 m/s lower than that in the surrounding

area. On dust storm days PAR and total radiation, Q, were

significantly lower than that on cloudy or clear days. Their

ratio, gQ = PAR/Q, was larger and much more variable on

dust storm days than that on clear or cloudy days.

Keywords Hyperarid zone � Desert oasis �Photosynthetically active radiation (PAR) � Microclimate

Introduction

Microclimate is an important environmental factor in the

growth and development of plants. Conversely, plant

communities create different microclimatic environments,

which support different biotic communities (Zhang et al.

2002). Such communities are continuously exchanging

materials (water) and energy (heat) with the surrounding

environment, which, in turn, dictates both current and

future exchanges. While a forest ecosystem’s micromete-

orological conditions can limit trees’ physiologic activities,

including their growth and development, as well as their

rates of photosynthesis and transpiration, the vegetation, a

reflection of local surface conditions, influences the near-

surface microclimatic features of water and energy through

transpiration and the interception and reflection of incom-

ing light. Thus, the vegetation is responsible for differences

in heat balance and water balance in the system. These

differences are also important to human life and produc-

tivity (Ye and Zhou 2000). Therefore, the study of

microclimate is essential in coordinating the relationships

that exist between living things and their environment.

Since the beginning of the twentieth century, the envi-

ronmental characteristic of various forest communities,

including their microclimate have been the focus of

research in China and abroad (Federer 1968; Garrett 1978;

Kimmins 1977; Hirose 1995). While much fruitful forest

ecosystem research has recently been devoted to China’s

Q. Feng (&) � Z. Macuo � X. Haiyang

Cold and Arid Regions Environmental and Engineering

Research Institute, Chinese Academy of Sciences,

No. 260 West Duong Gang Road, 730000 Lanzhou,

People’s Republic of China

e-mail: [email protected]

123

Environ Geol (2009) 58:963–972

DOI 10.1007/s00254-008-1576-6

temperate broadleaved and coniferous forests, as well as to

its more southerly tropical monsoon evergreen-broadleaved

forests (Tan and Huang 1985; Sun and Chen 1995; Chang

et al. 1999), few reported on the microclimatic character-

istics of plant communities in the hyperarid zone.

Associated with wavelengths in the range of 400–

700 nm, PAR represents the fraction of energy of green

plants can absorb from total solar radiation through the

photosynthetic process. As a basic energy source for pho-

tosynthesis, PAR is clearly an important environmental

factor in plant growth (Zhang and Qin 2002; Britton and

Dodd 1976; Sivakumar and Virmani 1984; Sinclair and

Lemon 1974; Sinclair and Knoerr 1982; Alados and Olmo

2000; Yann and Agnes 2000; Zhang and Zhang 2000).

While many researchers, in China and abroad, have

measured PAR, and no monitoring of PAR has been done

during dust storms.

Desert riparian forests represent an important forest

resource in arid desert environments such as those existing

in arid zone of northwest China. The oases which occur on

both sides of desert rivers house ecosystems mainly veg-

etation populated by Populus euphratica Olivier and

Tamarix ramosissima Ledeb. These oasis ecosystems

interact closely with surrounding desert ecosystems (Fu

2000). The study of the microclimate in desert riparian

forest ecosystems will contribute to our understanding of

the degradation of these ecosystems under the influence of

environmental factors currently prevailing in the arid zone

(Cheng et al. 1991; Feng et al. 1997; Zhang et al. 2004).

This study will provide some basic data on the ecological

functions and benefits of riparian forest ecosystems in

the hyper-arid zone, as well as provide a scientific basis

for the rational management, reasonable utilization of

existing forests, and the establishment of artificial forest

ecosystems.

Study area and methods

Study area

The study area located in the lower reach of the Heihe

river, in the Ejin region of the Inner Mongolia Autonomous

Region of China. Situated in the interior the study area has

an extremely arid climate and is one of the driest regions in

China. This area is characterized by scanty precipitation,

strong evaporation, frequent wind, abundant sources of

sand, and long durations of sunshine. Based on meteoro-

logical data recorded at the Ejin weather station between

1957 and 2002, mean annual precipitation is 42 mm

(max = 103 mm), while the minimum annual of evapo-

ration is 4,035 mm, nearly 90-fold the precipitation. About

70–80 of total annual precipitation occurs between June

and September. Desert oasis vegetation dominated by

riparian trees (P. euphratica) and shrubs (T. ramosissima).

Both species are tolerant to salinity, water logging, and

wind- and sand-blast damage. Serving to maintain an

ecological balance, protect agricultural and livestock pro-

duction and provide timber and fuel wood, they provide

significant ecological, economic and social benefits.

Microclimatic gradients in the desert oasis were

monitored March 2002, October 2003. Observation plots of

P. euphratica forest were located in a 1,333 ha. P. eu-

phratica reserve at Qiaoqiao in the Ejin region of northwest

China (42�210N and 101�150N), with an elevation of

920.46 m AMSL. The young P. euphratica forest aged 20–

25 years, with a density of 500 plants per hectare, a mean

height of roughly 10 m, and a mean breast-height diameter

of 12 cm. It’s under story includes T. ramosissima and

Sophora alopecuroides L. The soil type in the region is a

P. euphratica forest soil and soil texture of the 0–1.5 m

layer ranged from clayey loam to sand. Organic matter

content was 0.724% (w/w) in the 0–0.30 m soil layer and

0.127% (w/w) in the 0.30–2.00 m soil layer. The water

table depth ranged from 1.5 to 3.5 m in depth.

Microclimatic observation plots of naturally arising

T. ramosissima forest were located on former grasslands

fenced-off in 1988. Located at Erdaoqiao in the Ejin region

of northwest China (41�580N and 101�060E), the site

showed a vegetation cover of 70% or more, a plant density

of 1,300 bunches ha-1, a mean plant height of 2 m, and the

T. ramosissima plants were aged at least 20 years. The soil

type was a T. ramosissima soil developed from the parent

material of river alluvium. Localized surface salt spots are

present, and the water table is 2.0–3.5 m in depth.

The measurement of PAR under dust storm, cloudy and

clear day conditions on 28 April, 30 July, 6 September

2002, and respectively.

Study method

A 6 m tall meteorological station, including an automated

micrometeorological station (ICT Co., Australia) and a

Bowen ratio station (COMPELL Co., USA) was located in

each of the forest plot locations. The main parameters

monitored included effective radiation (total radiation, net

radiation and effective radiation), air temperature and

humidity, wind velocity and direction, soil temperature,

soil heat flux, and CO2 concentration. The radiation probe

was located at a height of over 2 m for beneath-canopy and

unforested land measurements and at least 1 m above the

tallest portion of the canopy for above-canopy measure-

ments, while the air temperature and humidity probes were

placed at heights of 1, 2, and 4 m. Wind velocity probes

were mounted at heights of 1, 2, 3, 4, and 6 m, with the

wind vane at a height of 6 m. Ground temperature probes

964 Environ Geol (2009) 58:963–972

123

were placed at depths of 0, 0.05, 0.10, 0.15, 0.20 and

0.40 m. CO2 concentration was determined at heights

of 1.5 and 3.5 m. Two soil heat flux plates were buried at

0.05 and 0.10 m below the soil surface. All the sensors

were connected by a cable to an indoor data logger (Zeno

3200-A. D). Observations were taken at 10 min intervals

on a 24-h a day basis, under all weather conditions.

Meteorological data recorded by the Ejin Meteorological

Bureau in 2002–2003 were used as the weather data for the

control areas located outside the forests. The net radiation

is determined by the exchange degree of the radiation

between the atmosphere and plant association.

Results

Solar radiation

Owing to the strong weakening effect of forest canopy

(absorption and reflection) on total solar radiation, the mean

monthly total solar radiation ð �QtotRmonthÞ beneath the canopies of

P. euphratica and T. ramosissima forests for the whole

growing season were 341.72 and 345.14 MJ m-2, respec-

tively, compared to 681.26 MJ m-2 for nearby non-forested

lands (Table 1). Thus, for the whole growing season,

ð �QtotRmonthÞ in the T. ramosissima forest exceeded that of

P. euphratica forest by 3.42 MJ m-2. The monthly solar

radiation total, QRmonthtot , was lowest in October, with 132.71

and 140.13 MJ m-2 measured for P. euphratica and

T. ramosissima forests, respectively. Thus, QRmonthtot beneath

the canopies of P. euphratica and T. ramosissima forest was

equivalent to 50.2 and 50.7%, respectively, of that over the

surrounding area. This proportion was at a maximum in June

(68% for both forest types), when QRmonthtot was at a maxi-

mum, and dropped gradually to a minimum in October (28.4

and 30.0%, respectively). For occasions when total daily

radiation QRdaytot was roughly the same, the ratio of daily mean

net radiation (QRdaynet ) to QRday

tot were 37.9 and 25.7% for

P. euphratica and T. ramosissima forests, respectively. For

the P. euphratica forest, this ratio peaked in May, June and

July (43.4, 45.0 and 47.1%, respectively), while for

T. ramosissima peak ratios occurred in September and

October (73.7 and 49.6%, respectively (Table 1). While

in the summer the proportion of QRdaynet to QRday

tot radiation of

P. euphratica forest well exceeded that of T. ramosissima

forest, the contrary was the case in the fall.

Basic characteristics of PAR

In the oasis under study, both the total daily PAR radiation,

PARRdaytot , and QRday

tot were significantly lesser on the dust

storm day than that on either a clear day or cloudy day

(Table 2). This is attributable to the greatly incr-

eased quantity of dust particles in the air cutting down

PARRdaytot and QRday

tot values by absorbing and scattering

solar radiation. Although PARRdaytot on the cloudy day was

less than that on the dust storm day, it was itself distinctly

smaller than that on the clear day. This is attributable to the

fact that a portion of the solar radiation was absorbed and

reflected by clouds and vapor in the atmosphere. As a rule,

the weakening effect of dust storms on solar radiation was

much greater than that of cloud and vapor on the cloudy

day.

While PARRdaytot and QRday

tot were the largest on the clear

day, followed by the cloudy day and least on the dust storm

day, their ratio, gRday = PARRdaytot /QRday

tot was the largest for

Table 1 Variations in monthly cumulative solar radiation in P. euphratica and T. ramosissima oasis forests (MJ m-2 month-1)

Stand Radiation type* May June July August September October Mean

P. euphratica QRmonthtot 480.44 539.59 407.24 311.23 179.12 132.71 341.72

PARRmonthtot 353.17 385.52 328.72 212.26 141.22 108.22 254.85

QRmonthnet 208.70 242.56 191.61 58.57 41.99 33.97 129.57

T. ramosissima QRmonthtot 482.19 541.05 410.54 314.22 182.69 140.13 345.14

PARRmonthtot 280.25 312.18 256.53 182.69 106.22 81.47 203.22

QRmonthnet 85.48 91.81 72.55 77.29 134.55 69.50 88.53

Surrounding area QRmonthtot 730.35 797.66 779.68 720.11 593.04 466.73 681.26

*QRmonthtot , QRmonth

net , PARRmonthtot : mean monthly total radiation, mean monthly net radiation, mean monthly photosynthetically active radiation

Table 2 Daily cumulative PAR and solar radiation, their ratio gRday,

mean air temperature and relative humidity on different weather days

April 28

(Dust day)

July 30

(Cloudy day)

September

6 (Clear day)

PARRdaytot (MJ m-2 d-1) 4.09 12.4 21.4

QRdaytot (MJ m-2 d-1) 7.24 23.9 43.4

gRday (%) 56.5 51.9 49.3

Mean air temperature (�C) 2.92 27.8 22.7

Mean relative humidity (%) 80.4 30.8 22.5

PARRdaytot , QRday

tot , gRday: Daily cumulative PAR, daily cumulative solar

radiation, and their ratio

Environ Geol (2009) 58:963–972 965

123

the dust storm day (56.5%) and least on the clear day

(49.3%).

On the dust storm day, the mean air temperature of the

oasis was only 2.92�C and the mean relative humidity

(RH%) was 80.4%. The sudden reduction in air tempera-

ture caused by cold air in the dust storm led to vapor

condensation, results in an increase in RH% and increased

absorption of infrared radiation. Coupled with the weak-

ening of solar radiation by dust particles, was their

especially strong absorption in the near infrared band,

which was reflected by an increase in gRday on the dust

storm day. The gRday value was larger on the cloudy day

than that on the clear day, as a result of the quantity of

clouds and vapor being larger on the cloudy day. The

cloudy day conditions reduced the contribution of air

molecules and aerosol particles to the scattering of radia-

tion and increased the absorption of infrared radiation

(Ji and Ma 1993). The air temperature was greater on the

cloudy day than that on the clear day, but this is a result of

the cloudy day being selected in the summer while the

clear day was selected in the fall (Zhang and Qin 2002).

Dust storm weather clearly had a strong weakening effect

on PAR.

Comparison of daily variations in PAR

Plots of diurnal variations in hourly PAR and total radia-

tion totals (PARRhourtot and QRhour

tot , respectively) on dust

storm, clear and cloudy days (Fig. 1), show that daily

variation in PARRhourtot and QRhour

tot closely parallel each

other. On the clear day both PARRhourtot and QRhour

tot were

very large and closely tied to changes in solar latitude

angle. Their variations were stable, and their maximum

values occurred at noon, local time. Comparatively, daily

variations in PARRhourtot on cloudy and dust storm days were

quite variable, being closely tied to shifts in weather

regime, cloud cover, as well as air vapor and dust content.

On the whole, the PARRhourtot and QRhour

tot values were larger

on the clear day than that on either the cloudy or dust storm

day, with PARRhourtot being least overall on the dust storm

day.

The daily maximum PARRhourtot value on the clear day

was greater than that measured in other regions such as

Zhangye and Linze (Zhang et al. 2004). The range of

PARRhourtot values on the clear day was large, whereas on the

dust storm day it was very small, indicating that dust storm

weather can greatly cut down on the PAR reaching plants.

Overall, a good linear relationship existed between

PARRhourtot and QRhour

tot (Table 3), though this was stronger on

the clear day, when fewer factors were affecting gRhour

(i.e., slope of plots), than that on the cloudy or dust storm

days, when the relationship was weakened by the effects of

dust particles, clouds and vapor.

Analysis of gRhour

Many factors affected gRhour values, including dust parti-

cles, clouds, vapor and solar radiation intensify, etc.

Previous studies (Ji and Ma 1993; Zhang and Qin 2002)

have shown that daily variation in gRhour shows certain

regularity on a clear day, but that its variation on a dust

storm day is complex. Given the weight of multiple factors

(airborne dust and vapor content) affecting it is constantly

changing due to the influence of the unstable stratification

of strong winds in the atmosphere.

Variations in gRhour during the dust storm were indeed

large and inconsistent (Table 4). Values of gRhour [ 70%,

such as those measured for 7:00, 8:00 and 9:00 in the

morning, are rarely found. Such values are likely the result

6:000123456

0

1

2

3

4

5

0.0

0.3

0.6

0.9

1.2

Acc

umul

ated

PA

R

hour

ly (

MJ.

m-2 )

Time

PAR Q(c)

Acc

umul

ated

PA

R

hour

ly (

MJ.

m-2 ) PAR

Q(b)

Acc

umul

ated

PA

R

hour

ly (

MJ.

m-2 )

PAR Q

(a)

18:0014:0010:00

6:00 18:0014:0010:00

6:00 18:0014:0010:00

Fig. 1 Daily variations in PAR under different weather conditions

[PARRhourtot (y-axis) is in MJ m-2 h-1): a the day of 28th April, b the

day of 30th July, c the day of 6th September

Table 3 Relationships between the hourly cumulative PAR

(PARRhourtot ) and hourly cumulative solar radiation (QRhour

tot ) under

different weather conditions

Date Relationships R2

28 April 2002 (dust) PARRhourtot = 0.5005QRhour

tot ? 0.0391 0.8672

30 July 2002 (cloud) PARRhourtot = 0.5353QRhour

tot - 0.0611 0.8824

6 September 2002

(clear)

PARRhourtot = 0.5709QRhour

tot - 0.0870 0.9738

966 Environ Geol (2009) 58:963–972

123

of the influence of the dust storm, which led to very small


tot values generating a larger gRhour

value. Under dust storm conditions, total solar radiation

intensity over the dust storm day was very small.

According to Ji and Ma (1993), there exists a roughly

logarithmic relation between PARRhourtot and QRhour

tot in the

Linze region. Britton and Dodd (1976), working in Texas,

USA, showed that gRhour decreased with increasing solar

radiation. Dust particles reduce solar radiation by absorb-

ing strongly in the spectral range of 300–700 nm and have

even greater absorption in the near infrared range of 700–

2,000 nm (or even up to 4,000 nm). Thus, their presence

results in large values of g. On the dust storm day, cold air

caused vapor condensation and an increase in humidity,

which also contributed to increasinggRhour values. As a

whole, gRhour values were significant greater on the dust

storm day than that on the clear day.

The mean gRhour value between 7:00 and 11:00 on the

dust storm day exceeded 60% (Table 4). McCree (1976)

reported on instantaneous g value of 58.0–59.0% on an

overcast day. The present study has shown that high dust

concentrations resulting from a dust storm day have a much

greater impact on gRhour or gRday than that the high vapor

content associated with dense cloud.

Air temperature (2 m height) differences between the

open area surrounding the oasis and the oasis’ P. euphra-

tica and T. ramosissima woodlands were 1.62 and 0.83�C,

averaged over the main growing season of June–September

(Table 5). In the summer P. euphratica trees have dense

branches and leaves and thereby can block more solar

radiation than that T. ramosissima forest, hence its greater

ability to reduce monthly mean air temperatures between

June and September. Furthermore, in May P. euphratica

trees start to grow rapidly and hence have a strong ability to

check wind, whereas at this point T. ramosissima shrubs

are only starting to sprout, so their ability to check wind at

two meters’ height is less. As a result, air temperature was

lesser in P. euphratica forestland than that in T. ramo-

sissima forest. In October, the amplitude of daily air

temperature changes was decreasing. This amplitude was

lesser in the surrounding area than that in either the

P. euphratica or T. ramosissima forest, due to the lack of

shelter and its low heat absorbance of the open lands.

Overall, the oasis’ air temperature was lower than that of

the surrounding area during the growing season, indicating

that the forest served to reduce air temperature and

decrease temperature differences. The fact that air tem-

perature was lower in P. euphratica forest compared to

T. ramosissima forest shows that such forest functions are

tied to forest cover and ground surface conditions; the

greater the forest cover, the greater its temperature-

lowering ability.

Vertical variations in air temperature

During the growing season, the air temperature above the

oasis forest canopy was greater than that beneath the can-

opy in the interior of the oasis. For the P. euphratica forest

the temperature difference from top to bottom varied

between 0.97 and 5.74�C, while for the T. ramosissima

forest it varied between 0.61 and 3.49�C (Fig. 2). The air

temperature in the P. euphratica forestland increased rap-

idly from the ground surface to a height of 2 m and then

decreased from the height of 2–4 m. In contrast, for the

T. ramosissima forest the situation was the contrary, the air

temperature decreased rapidly from the ground surface to a

height of 1 m, but then increased from the height of 1–2 m,

with no changes occurring above a height of 2 m. The

vertical distribution of air temperatures in the forests has

the following features: (1) the air temperature over the

oasis’ forest canopy was greater than that under the canopy

and the beneath-canopy air temperature gradually

decreased from top to bottom, (2) in the P. euphratica

forest the largest vertical temperature variations occurred

Table 4 Hourly cumulative PAR (PARRhourtot ) and solar radiation (QRhour

tot ), and their ratio gRhour on a dust storm day

7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

PARRhourtot (MJ m-2 h-1) 0.22 0.15 0.32 0.53 0.48 0.42 0.43 0.51 0.40 0.34 0.20 0.09

QRhourtot (MJ m-2 h-1) 0.31 0.19 0.43 0.81 0.79 0.73 0.82 0.98 0.75 0.75 0.47 0.21

gRhour (%) 71.8 81.6 75.1 66.1 60.2 56.7 52.0 51.8 53.0 44.7 44.0 44.6

Table 5 Variations in monthly mean air temperature in P. euphratica and T. ramosissima forests (�C)

Stand May June July August September October Mean

P. euphratica forest 24.06 25.36 27.31 25.34 15.12 10.15 21.22

T. ramosissima forest 22.17 25.86 28.32 25.62 16.48 12.67 21.85

Surrounding area 20.30 26.5 28.90 27.30 16.90 8.90 21.47

Environ Geol (2009) 58:963–972 967

123

in the range from the ground surface to a height of 1 m,

while in the T. ramosissima forest the amplitude of varia-

tion above the height of 2 m was small, (3) under

extremely dry weather conditions the vertical variation in

air temperature was very much reduced.

The mean RH% was greater in the woodlands than that

in the surrounding area (Table 6). Averaged over the

growing season RH% was 8.5 and 4.2% (absolute) greater

in the P. euphratica and T. ramosissima woodlands,

respectively, than that in the surrounding area, and similar

trends were apparent for individual months. The forests

maintained a higher humidity due to the shading function

of the forest canopy. The humidity difference between the

woodlands and the surrounding area was smallest between

May and June, i.e. 4.9–6.3% for P. euphratica and 4.0–

4.7% for T. ramosissima. In contrast, the difference for the

P. euphratica woodland was at its highest (9.3–16%) in

September-October. The RH% peaked between August and

October. Its amplitude of variation was lesser in the

woodlands than that in the surrounding area, indicating that

forests play a unique role in maintaining conditions of

lower water requirements for plant survival in the extre-

mely arid zone. Different forest types have different effects

on the relative humidity: P. euphratica forest is tree forest,

so its canopy has a greater shading function than that the

shrubs of T. ramosissima forest. Therefore, it is not sur-

prising that the P. euphratica forest’s RH% was 4.33%

higher (absolute) than that of the T. ramosissima forest.

Vertical distribution of air humidity

In forested lands, the RH% above the forest canopy was

lower than that below (Table 4). The range of variation in

amplitude of RH% in the P. euphratica forest (25.8–

58.6%) was greater than in the T. ramosissima forest (24.5–

38.7%). The largest ranges of the relative humidity in the

P. euphratica forest occurred in May and October and

varied irregularly in between (Fig. 3). This may be related

to the tree species, which is broadleaved. The RH% at a

0

1

2

3

4

5

5

0

1

2

3

4

5

Air temperature

Hei

ght (

m)

May Jun Aug Sep Oct

(a)

Hei

ght

(m)

(b)

3025201510

5 3025201510

Fig. 2 Vertical air temperature variations in a P. euphratica and

b T. ramosissima forests

Table 6 Variation in monthly mean relative humidity in P. euphratica and T. ramosissima forests (%)





100

1

2

3

4

5

0

1

2

3

4

5

Hei

ght

(m)

Relative humidity (%)

(b)

Hei

ght

(m)

May Jun Aug Sep Oct

(a)

6050403020

10 6050403020

Fig. 3 Vertical humidity variations in a P. euphratica and


968 Environ Geol (2009) 58:963–972

123

height of 1 m in the T. ramosissima woodland changed

little with time, and the RH% was greater at the height of

2 m than that at 4 m, because the shrub canopy was at 2 m.

Soil temperature

Between May and September, the P. euphratica forest had

dense branches and leaves, and, thus, a stronger ability to

block solar radiation than that T. ramosissima forest

(Table 7). Consequently, during these months monthly mean

soil temperature ð �Tmthsoil Þ in the P. euphratica forest was

lower than that in the T. ramosissima forest. In October,

P. euphratica’s defoliation led to a large decrease in its

ability to block solar radiation, so that the P. euphratica

forest’s ð �Tmthsoil Þ rose above that of the T. ramosissima forest.

The ð �Tmthsoil Þ of the 0.20–0.40 m layer changed little over

the season, and was lower in the P. euphratica forest soil

than in that of the T. ramosissima forest (Fig. 4). Vertical

variation in ð �Tmthsoil Þ in the P. euphratica forest soil was not

significant between May and August, but varied greatly in

September and October, when the 0–0.10 m soil layer’s

ð �Tmthsoil Þ changed dramatically. The 0.10–0.40 m soil layer’s

ð �Tmthsoil Þ tended to decrease from May to August, but

increased from September to October. This was tied to the

P. euphratica tree’s growth regime, and it’s defoliation in

September through October, which led to a lessened ability

to block solar radiation. Vertical variation in ð �Tmthsoil Þ in the

T. ramosissima forest tended to be consistent across

months. In the hot summer, P. euphratica forest, unlike

T. ramosissima forest, significantly reduced the tempera-

ture at the ground surface and in the 0–0.20 m soil layer.

Variations in wind speed

The variation of wind force was mainly manifested in the

marked reduction of wind speed due to the forest canopy’s

blocking of the wind. The May–October mean wind speeds

in the P. euphratica and T. ramosissima forests were 0.33

and 0.72 m s-1, respectively, some 2.70 and 2.31 m s-1

lower than that of the surrounding area (Fig. 5). Clearly

P. euphratica forest had a much greater ability to block

wind than that does T. ramosissima forest. For individual

months, wind speed in the surrounding area is [T. ramo-

sissima forest [ P. euphratica forest. From this, it can be

Table 7 Variations in monthly mean soil (1.8 m) temperatures in P. euphratica and T. ramosissima forests (�C)




050

40

30

20

10

0

50

40

30

20

10

0

Hei

ght

(m)

Soil temperature (°C)

(b)

Hei

ght

(m)

May Jun Aug Sep Oct

(a)

5040302010

Fig. 4 Vertical soil temperature variations in a P. euphratica and


0.0

1

2

3

4

5

6

1

2

3

4

5

6

Hei

ght

(m)

Wind speed (m/s)

(b)

Hei

ght

(m)

May Jun Aug Sep Oct

(a)

1.51.00.5

Fig. 5 Vertical wind velocity variations in a P. euphratica and


Environ Geol (2009) 58:963–972 969

123

seen that forests in the hyper-arid zone can significantly

reduce wind speed (Table 8).

The common feature in the vertical variation of wind

velocity in the P. euphratica and T. ramosissima forests

was that wind velocity increased with increasing height

(Figure 5). The mean wind velocities at heights of 1, 2, 3,

4, and 6 m in the P. euphratica forest were 0.23, 0.24, 0.25

and 0.50 m s-1, respectively, while in the T. ramosissima

forest they were 0.24, 0.45, 0.72, 0.91 and 1.28 m s-1,

respectively. However, the wind velocity varied less with

height in the P. euphratica forest than that in the T. ra-

mosissima forest. The wind velocity at a height of 1–3 m in

the P. euphratica forest changed little, while the wind

velocity in the T. ramosissima forest continuously

increased from a height of 1 m to one of 6 m.

Comparison of different plants communities

Comparisons were made between a humid climate mon-

soon evergreen-broadleaf forest in the south Asian tropics,

a water conservation forest in the arid zone of a temperate

region, and the presently studied desert riparian forest from

an extremely dry climatic zone (Table 9). The latter forest

has been shown above to have a significant effect on

microclimate: it can diffuse solar radiation, reduce air,

ground surface and soil temperatures, decrease wind speed,

increase air and soil humidity, and, thus, plays even more

significant ecological rule than other forests.

The �QtotRmonth

under the canopy of the desert riparian forest

was about 50% of that of the bare land surrounding it,

while a corresponding proportion of 72.8% was found for

the monsoon evergreen-broadleaf forest. Thus, the desert

riparian forest can trap solar radiation more effectively than

that the monsoon forest. Similarly, the P. euphratica forest

has a stronger temperature lowering function than the

monsoon forests of Betula platyphylla Sukaczev and Pinus

tabulaeformis Carr. (Table 9).

Since the forest canopy prevents the exchange of gas

from inside to outside the forest, water in the interior of the

forest forms a small circulating system or high-humidity

environment. The RH% in the P. euphratica and T. ra-

mosissima forests in the hyper-arid zone were low, but

were nonetheless 8.5 and 4.2% higher, respectively, than

that outside the forest. This compares with values of 9% for

monsoon forest, and 7 and 4% for water conservation

B. platyphylla and Pinus tabulaeformis forests, respec-

tively. This shows that desert riparian forests in a hyper-

arid environment have a significant moisture-holding

function.

Soil temperatures in the monsoon, water-conservation

and desert riparian forests were 4–5, 6–8, and 7–10�C less,

respectively, that in the open areas surrounding them. The

desert riparian forest significantly reduced the temperature

at the ground surface and in the 0–0.20 m soil layer, and

buffered sharp variations in soil temperature. This is

favorable to soil microorganism activity, reduces evapo-

ration from the topsoil and enhances the effectiveness of

precipitation.

The mean wind velocities in the B. platyphylla, Pinus

tabulaeformis, P. euphratica and T. ramosissima forests

were 0.92, 1.01, 2.70, and 2.31 m s-1 lower than that in

their respective surrounding areas. This shows that the

desert riparian forests in the hyper-arid zone could signif-

icantly reduce wind velocity.

Conclusions and discussion

From the comparison of microclimatic characteristics of

the interior of a desert oasis in northwest China and the

surrounding area, we were able to draw the following

conclusions: total solar radiation and air temperature are

lower, but RH% higher, under forested portions of the oasis

than that in the surrounding area. Oasis plant communities

have ecological functions including diffusing solar radia-

tion, adjusting near-ground layer, land surface and soil

temperatures, reducing wind velocity, increasing soil and

atmospheric moisture, and decreasing soil erosion, etc.

Therefore, they can help conserve local water and soil

resources, improve the environment and aid in the main-

tenance of an ecological balance.

The microclimatic differences between P. euphratica

and T. ramosissima forest ecosystems include lower

growing season total solar radiation in P. euphratica than

that T. ramosissima forest, greater summertime net solar

radiation in P. euphratica forest than that in T. ramosissima

forest, and the contrary in the fall. Air temperatures in the

P. euphratica forest, which decreased from ground surface

to canopy, were lower than that in the T. ramosissima

forest, where, by contrast, the air temperature increased

Table 8 Monthly variations in monthly mean wind speed in the P. euphratica and T. ramosissima forests (m s-1)





970 Environ Geol (2009) 58:963–972

123

from the ground surface to the canopy. The P. euphratica

forest’s RH% was, on average, 4.3% (absolute) higher than

that in the T. ramosissima forest. The amplitude of varia-

tion in RH% was greater in the P. euphratica forest than

that in the T. ramosissima forest. Monthly mean soil tem-

peratures were lower in P. euphratica forest than that in

T. ramosissima forest and tended to decrease from the land

surface to the subsurface soil layer. The P. euphratica

forest had a greater ability to check wind and sand, than

that did the T. ramosissima forest. Vertical variations in

wind velocity in the P. euphratica and T. ramosissima

forests increased with height, but largely in the latter case.

On the dust storm day, oasis PARRhourtot was less than that

on either a clear or cloudy day. The dust storm reduced

PAR largely than that clouds or vapor. On the clear day


tot paralleled each other and showed a

strong linear relationship, whereas on the cloudy and dust

storm days the relationship was weaker. On the dust storm

day, gRhour (slope of relationship) values were very large

and varied in an irregular manner.

Desert riparian forest can diminish solar radiation inside

the forest, reduce temperature, increase humidity and

decrease wind velocity. Such functions conserve water and

soil resources, improve ecosystems, maintain an ecological

balance, protect agricultural and livestock production in

oases, provide better living and production conditions for

local people, and yield greater ecological, economic and

social benefits in a hyper-arid zone characterized by scanty

precipitation and high evaporation. It is urgent that desert

riparian habitats in China be restored and protected. Res-

toration of P. euphratica and T. ramosissima forests and

associated vegetation could employ artificial rejuvenation

and reproduction techniques to enlarge and diversify the

forests in such a manner as to optimize population and

community structures and so give full play to the forests’

primary function of protecting the environment of the arid

zone.

Acknowledgments This research was supported by a grant from

National Natural Sciences Foundation of China (No. 40671010;

40725001, 40501012), the Key Project of the Chinese Academy of

Sciences (KZCX2-XB2-04-02), National Key Technology R & D

Program (No. 2007BAD46B01).

References

Alados I, Olmo FJ (2000) Estimation of photosynthetically active

radiation under cloudy conditions. Agric For Meteorol 102:39–50

Britton CM, Dodd JD (1976) Relationships of photosynthetically

active radiation and shortwave irradiance. Agric Meteorol

17:1–7

Chang J, Pan XD, Ge Y, Chen ZH, Liu K, Chen QC (1999)

Microclimatic characteristics of evergreen Cyclobalanopsisglauca forest. Acta Ecol Sin 19(1):68–75 (in Chinese)T

ab

le9

Com

par

iso

no

fm

icro

clim

ate

ind

iffe

ren

tfo

rest

lan

ds

Item

sM

on

soo

nev

erg

reen

bro

adle

aved

fore

stW

ater

con

serv

atio

nfo

rest

inD

aqin

gM

oun

tain

Des

ert

rip

aria

nfo

rest

Cli

mat

iczo

ne

Su

btr

opic

alm

on

soo

nh

um

idcl

imat

eT

emp

erat

esu

bar

idzo

ne

Mid

-tem

per

ate

hy

per

-ari

dzo

ne

Stu

dy

area

Zh

aoq

in,

Gu

ang

do

ng

Bai

shit

ou

go

ub

asin

inD

aqin

gM

oun

tain

,In

ner

Mo

ng

oli

aE

jin

,In

ner

Mo

ng

oli

a

Lat

itu

de

and

lon

git

ude

23

�100 N

–4

0�2

00 –

42�3

00 N

11

2�3

40 E

–9

9�3

00 –

10

2�0

00 E

To

tal

ann

ual

sola

rra

dia

tio

n

(MJ

m-

2p

ery

ear)

3,4

88

.8–4

,792

.31

5,8

61

.52

6,5

10

–6

,72

0

Mo

nso

on

fore

stS

urr

oun

din

gar

eaB

etula

pla

typhy

lla

Pin

us

tabula

eform

isS

urr

oun

din

gar

eaP

.eu

ph

rati

caT

.ra

mo

siss

ima

Su

rro

un

din

gar

ea

Air

tem

per

atu

re(�

C)

19

.92

1.5

14

.28

14

.87

15

.52

7.8

8.5

21

.47

Rel

.h

um

idit

y(%

)8

77

86

86

56

13

7.5

33

.22

9

So

ilte

mp

erat

ure

(�C

)

0m

19

.62

4.7

13

.19

13

.79

19

.91

19

.97

20

.66

30

.12

0.0

5m

19

.82

4.8

10

.81

12

.66

18

.89

20

.34

18

.26

29

.21

0.1

0m

19

.72

4.2

10

.88

12

.63

18

.45

18

.61

16

.60

27

.99

0.1

5m

20

.12

4.1

10

.90

12

.38

18

.55

17

.94

16

.39

24

.00

0.2

0m

20

.02

4.2

10

.78

12

.21

18

.33

16

.80

15

.24

20

.03

Win

dv

elo

city

(ms-

1)

––

0.2

50

.16

1.1

70

.33

0.7

23

.03

Environ Geol (2009) 58:963–972 971

123

Cheng HS, Kang YH, Feng JC (1991) Preliminary study of plant

growth and water balance in Shapotou region of Tengger Desert.

J Desert Res 11(2):1–10 (in Chinese)

Federer CA (1968) Spatial variation of net radiation, Albedo and

surface temperature of forests. J Appl Meteorol 7:789–795

Feng JZ, Huang ZC, Zhang CL (1997) Several advances in the

quantitative study of environmental plant physiology. J Desert

Res 17(1):89–94

Fu XF (2000) Study of oasis development and environmental

coordination in arid zone. J Desert Res 20(2):197–200 (in

Chinese)

Garrett HE (1978) Spatial and temporal variation in carbon dioxide in

an oak-hickory forest ravine. For Sci 24(2):180–190

Hirose T (1995) Canopy structure and photon flux partitioning among

species in herbaceous plant community. Ecology 76(2):466–474

Ji GL, Ma XY (1993) Characteristics of photosynthetically active

radiation in Zhangye region. Plateau Meteorol 12(2):141–146 (in

Chinese)

Kimmins JP (1977) Forest Ecology, 2nd edn. Prentice Hall, Engle-

wood Cliffs, pp 596

McCree KJ (1976) A solarimeter for measuring photosynthetically

active radiation. Agric Meteorol 3:353–366

Sinclair TR, Lemon ER (1974) Penetration of photosynthetically

active radiation in corn canopies. Agron J 66: 201–205

Sinclair TR, Knoerr KR (1982) Distribution of photosynthetically

active radiation in the canopy of a loblolly pine plantation J Appl

Ecol 19:183–191

Sivakumar MVK, Virmani SM (1984) Crop productivity in relation to

interception of photosynthetically active radiation. Agric For

Meteorol 31:131–141

Sun XF, Chen LZ (1995) Preliminary study of radiation energy

environment of deciduous broadleaved forest in warm temper-

ature. Acta Ecol Sin 15(3):278–286 (in Chinese)

Tan SM, Huang JL (1985) Preliminary approach to the microclimatic

characteristics of mixed Eucalyptus tuoliensis forest. Acta Ecol

Sin 5(3):241–248 (in Chinese)

Yann NV, Agnes B (2000) PAR extinction in shortgrass ecosystems:

effects of clumping, sky conditions and soil albedo. Agric For

Meteorol 105:21–41

Ye DZ, Zhou JF (2000) Origin and control measures of dust storm

weather in North China. Acta Geogr Sin 55(5):513–521 (in

Chinese)

Zhang XZ, Zhang YG (2000) Measuring and modeling photosyn-

thetically active radiation in Tibet Plateau during April–October.

Agric For Meteorol (102): 207–212

Zhang YL, Qin BQ (2002) Basic characteristics of phytosynthetically

active radiation (PAR) and its climatological calculation in

Taihu Lake region, Agricultural Meteorology. J Solar Energy

23(1):118–123 (in Chinese)

Zhang YP, Liu YH, Ma YX, Wang JX, Dou JX, Guo P (2002)

Microclimatic characteristics in different growing stages of

tropic forest. J Nanjing For Univ (Nat Sci) 26(1):83–87 (in

Chinese)

Zhang L, Dong ZC, Huang XL (2004) Study of the model of the

relation between typical xerophyte growth and groundwater.

J Desert Res 1:110–113 (in Chinese)

972 Environ Geol (2009) 58:963–972

123

analysis of an oasis microclimate in china’s hyperarid zone

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