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Jan., 2017
Journal of Resources and Ecology
Vol. 8 No.1
J. Resour. Ecol. 2017 8(1) 50-56
DOI: 10.5814/j.issn.1674-764x.2017.01.007
www.jorae.cn
Declining Precipitation Enhances the Effect of Warming on
Phenological Variation in a Semiarid Tibetan Meadow Steppe
ZHAO Guangshuai1,2, SHI Peili1,2,*, ZONG Ning1, HE Yongtao1, ZHANG Xianzhou1,2, HE Honglin1, ZHANG Jing3
1. Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research,
Chinese Academy of Sciences, Beijing 100101, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. College of Global Change and Earth System Sciences, Beijing Normal University, Beijing 100875, China
Abstract: Vegetation phenology is a sensitive indicator of global warming, especially on the Tibetan Plateau.
However, whether climate warming has enhanced the advance of grassland phenology since 2000 remains debated and little is known about the warming effect on semiarid grassland phenology and interactions with early
growing season precipitation. In this study, we extracted phenological changes from average NDVI in the growing
season (GNDVI) to analyze the relationship between changes in NDVI, phenology and climate in the Northern Tibetan Damxung grassland from 2000 to 2014. The GNDVI of the grassland declined. Interannual variation of
GNDVI was mainly affected by mean temperature from late May to July and precipitation from April to August. The
length of the growing season was significantly shortened due to a delay in the beginning of the growing season and
no advancement of the end of the growing season, largely caused by climate warming and enhanced by decreasing
precipitation in spring. Water availability was the major determinant of grass growth in the study area. Warming increased demand for water when the growth limitation of temperature to grass was exceeded in the growing season.
Decreased precipitation likely further exacerbated the effect of warming on vegetation phenology in recent decades
due to increasing evapotranspiration and water limitations. The comprehensive effects of global warming and decreasing precipitation may delay the phenological responses of semiarid alpine grasslands.
Key words: NDVI; phenology; climate change; Qinghai-Tibetan Plateau; Damxung station
1
Introduction
The global climate has shown the greatest change at high
latitude and altitude areas (IPCC, 2007; Peñuelas et al.,
2002; Hansen et al., 2006; Piao et al., 2011b). Vegetation
phenology is a sensitive indicator of global warming in
terms of advancing of the start of the growing season and
lengthening of the season (Angert et al., 2005; Gao et al.,
2012; Pettorelli et al., 2005; Piao et al., 2006). Previous
studies have documented that climate change can lead to
changes in vegetation productivity and phenology (Nemani
et al., 2003; Walther et al., 2002). Parmesan and Yohe(2003)
showed that the global spring phenology of vegetation has
advanced. As the ‘third pole’ of earth, the Qinghai-Tibetan
Plateau is experiencing strong climate warming (Liu and
Chen, 2000) to be continued in this century (Piao et al.,
2010). Advance of spring phenology has been documented
widely in response to climate warming in the northern hemisphere, but debate remains about the effect of global
warming on timing of the growth season on the Qinghai-Tibetan Plateau (Qiu, 2008; Yu et al., 2012; Zhang et al.,
2012).
Since 2000, inconsistent extrapolations of spring
phenology and explanations have been presented for the
Qinghai-Tibetan Plateau using different resolution of remote
sensing data sources (Ding et al., 2013; Piao et al., 2011a;
Received: 2016-11-15 Accepted: 2016-12-20
Foundation: National Natural Science Foundation of China (41271067) and National key research and development program (2016YFC0502001).
*Corresponding author: SHI Peili. E-mail: [email protected].
Citation: ZHAO Guangshuai, SHI Peili, ZONG Ning, et al. 2017. Declining Precipitation Enhances the Effect of Warming on Phenological Variation in a
Semiarid Tibetan Meadow Steppe. Journal of Resources and Ecology. 8(1): 50-56.
ZHAO Guangshuai, et al.: Declining Precipitation Enhances the Effect of Warming on Phenological Variation in a Semiarid Tibetan Meadow Steppe
Song et al., 2011a; Yu et al., 2010; Zhang et al., 2013). Yu
et al. (2010) and Piao et al. (2011a) showed that the beginning of the growth season (BGS) was significantly delayed
after the mid-1990s on the Plateau, consistent with the pattern in northern high latitude areas (Delbart et al., 2006).
Conversely, Zhang et al. (2013) and others suggested that
warming in winter and spring resulted in a sustained advance of BGS (Ding et al., 2013; Song et al., 2011a). Cong
et al. (2013) drew similar conclusions for temperate steppe
in China. Yu et al. (2010) speculated that a delay in BGS
was due to winter and spring warming causing later fulfillment of chilling requirements. Piao et al. (2011a) posited
that the delay was caused by spring cold. However, all these
studies did not consider the effects of precipitation on phenology(Piao et al., 2011a; Yu et al., 2010; Zhang et al., 2013).
Grassland phenology is affected by changes in temperature and precipitation variation (Shen et al., 2011). Precipitation plays an important role in the temperature dependency of phenology in arid and semiarid regions (Shen et al.,
2015). Precipitation limits the growth and yield of grassland
in arid and semiarid regions, and therefore it is likely to affect NDVI and NDVI-calculated vegetation phenology
(Epstein et al., 2002; Jobbágy et al., 2002; Scanlon et al.,
2002). It is of significance to indentify the role of precipitation in mediating the effect of temperature on phenology
and understand their interactions on the vegetation phenology. In northern China, temperate grassland, green-up was
advanced in years with higher soil moisture (Liu et al.,
2013). But in semiarid and arid grasslands, warming may
enhance evapotranspiration and reduce water availability to
delay grassland green-up (Yu et al., 2003). In rapid climate
warming during the last decade on the northern Tibetan Plateau (An et al., 2016), the response of semiarid and arid ecosystems is of widespread concern, especially in areas with
higher warming and lower precipitation.
The Damxung Grassland Ecological Research Station is
situated in a transition zone between alpine humid meadow
and semiarid steppe, between the southern Tibetan Mountains and northern plateau. Therefore, vegetation, land use
and topology are characterized as typical of the Tibetan Plateau. In this study, we selected semiarid meadow steppe in
Damxung station and extracted the average NDVI in the
growing season (GNDVI, from April to October) and
phenological information about natural grassland from 2000
to 2014. We then analyzed the relationship between changes
in NDVI, phenology and climate. The study is aimed at analyzing phenological changes and responses to climate
change under natural conditions on the Qinghai-Tibetan
Plateau since the year 2000, and exploring factors leading to
inconsistent results amongst previous studies.
2
2.1
Materials and methods
Study area
This study was conducted at Damxung Grassland Ecologi-
51
cal Research Station (91°05′E, 30°25′N), covering an area
of 0.9 km2 and 15 NDVI pixels. Annual mean temperature
was 1.7℃, with a freezing period from November to January. Mean annual rainfall was 476.8 mm, with 85.1% rain
concentrated between June and August. Annual evaporation
was 1725.7 mm. The soil type is alpine meadow soil. The
vegetation type is steppe-meadow, comprising the dominant
species Kobresia pygeama, Stipa capillacea, Carex montis-everestii, and the accompanying plant species K. parva,
Anaphalis xylorhiza, and Potentilla bifurca Linn (Shi et al.,
2006).
2.2
NDVI data
We selected natural grassland (4328 m a.s.l.) as our focal
area. The grassland was fenced, and is located less than 4 km
from Damxung National Meteorological Station (4285 m
a.s.l.). We used MODIS NDVI datasets (MOD13Q1) provided by NASA Terra satellites to extract phenology from
2000 to 2014. The datasets included NDVI data and quality
control data with a resolution of 250 m. The NDVI data was
obtained based on the maximum synthesis method in 16
days, and was processed with geometric correction and atmospheric correction. To ensure data quality, we extracted
pixels with values of quality control data of 0 or 1.
2.3
Determination of vegetation phenological data
The ratio threshold method (RTM) (Yu et al., 2010) was
used to extract phenology, including the BGS, end of the
growing season (EGS) and length of growing season (LGS)
for natural grassland from 2000 to 2014. The reason why we
chose RTM is that our former work indicated RTM was the
best predictor of on-site observation of phenology at Damxung station. The NDVI ratio is the difference between the
NDVI value at a certain time and minimum NDVI value for
a specific time span, normalized by the total range of NDVI
values during maximum NDVI value and minimum NDVI
value. We selected an NDVI ratio threshold of 0.2 to indicate BGS, and a drop of the NDVI ratio below 0.6 to mark
EGS, as suggested by Yu et al. (2010) who used ground
observations and modeled growth season parameters for the
Qinghai-Tibetan Plateau.
Based on the RTM method, phenological metrics were
derived from NDVI time series using TIMESAT v.3.1
(Jönsson and Eklundh, 2004), a software package to best fit
temporal dynamics of vegetation. The asymmetric Gaussians smoothing filter was selected because it exhibited superior performance in the preservation of vegetation temporal dynamics on the Qinghai-Tibetan Plateau (Song et al.,
2011b).
2.4
Relationship between vegetation phenology and
climate
Daily temperature and precipitation data for 2000–2011
were obtained from the Damxung National Meteorological
52
Journal of Resources and Ecology Vol. 8 No. 1, 2017
Station. To explore the influence of climate change on
NDVI and phenology we calculated cumulative precipitation and average temperature with any continuous 16  n
(n = 1 to 23) days interval in steps of five days within a year,
matching the NDVI data cycle. Then we determined the key
phase of precipitation and/or average temperature to
phenology using stepwise regression, and then analyzed
trends in precipitation, temperature and its correlation with
phenology by linear regression (SPSS 17.0, SPSS Inc., Chicago, IL, USA).
3
3.1
Results
NDVI and relationship with climate change
The GNDVI decreased significantly (R2=0.44, P<0.01) in
the study period, at a rate of 0.05 per decade, although there
was a significant increase (R2=0.92, P<0.05) from 2000–
2004 (Fig.1). Further analysis showed that the GNDVI was
positively correlated with cumulative rainfall from April to
August and negatively correlated with temperature from late
May to July (Fig.2a, b).
3.2
Grassland phenology and relationship with
climate change
Based on the phenology results extracted using the RTM,
we analyzed variation in phenology from 2000 to 2014. The
BGS was significantly delayed (R2=0.32, P<0.05), with an
average delay of 2.1 d/a (Fig.3a). There was no significant
advance trend in EGS (R2=0.17, P=0.12) (Fig.3b). Therefore,
the LGS shortened significantly (R2 = 0.56, P<0.01), with
an average reduction of 2.9 d/a (Fig.3c), of which 72% was
caused by the delay in the BGS.
Fig.1 Interannual changes in average NDVI in the growing
season (GNDVI) from 2000–2014 in the Damxung alpine
meadow steppe
The BGS was negatively correlated with rainfall but positively correlated with temperature from late April to early
June (112th–160th Julian day of a year, JDOY) (Fig.4 a, b).
The EGS was positively correlated with rainfall from
mid-August to mid-October (224th–288th JDOY) (Fig.4c),
and with mean temperature from late September to October
(272th–304th JDOY) (Fig.4d).
4
Discussion
Our study showed that the BGS was delayed in response to
climatic warming in this semiarid grassland on the northern
Tibetan Plateau. Warming did not extend the date of the
EGS, and therefore LGS was shortened. Declining precipitation played an important role in the phenology. A decrease
in precipitation exacerbated the negative effect of warming
on green-up phenology and the length of the growth season.
4.1
Variation in NDVI in natural grassland
The GNDVI decreased from 2000 to 2014 due to the increase in temperature (R2 = 0.43, P<0.01) and decrease in
rainfall (R2=0.24, P=0.06). The GNDVIs in the studied
grassland were mainly affected by early growing season
temperature from late May to July and precipitation from
April to August. This suggests temperature and precipitation
in the early growing season are the dominant determinants
of vegetation productivity. Normally spring precipitation is
helpful to facilitate grassland green-up together with temperature.
Rainfall is the main factor limiting plant yield in arid and
semiarid areas around the world, while temperature is a
major factor limiting vegetation growth (Fensholt et al.,
2012). However, intense warming in recent decades on the
Qinghai-Tibetan Plateau has broken the limit of temperature
for the growth of vegetation in the early growth season (Liu
and Chen, 2000; Piao et al., 2010), thus humidity acts as a
decisive factor affecting grass growth (Ji and Peters, 2003;
Pennington and Collins, 2007). With rainfall reduction in
summer, rising temperatures may intensify the demand for
soil water (Angert et al., 2005; Zhao and Running, 2010;
Zhou et al., 2001), leading to a decrease in grass
GNDVI(Lotsch et al., 2005). Wang et al. (2007) found that
grassland biomass declines under increased temperature
when precipitation remains unchanged or even decreases in
Tibetan alpine meadows. Cong et al. (2017) found that
Fig.2 Relationships between GNDVI and a) April to August rainfall: cumulative precipitation of the 96th–240th Julian day of a
year (JDOY); b) late May to July temperature, average temperature of the 144th–208th JDOY.
ZHAO Guangshuai, et al.: Declining Precipitation Enhances the Effect of Warming on Phenological Variation in a Semiarid Tibetan Meadow Steppe
4.2
Fig.3 Interannual changes in a) beginning of the growing
season, BGS; b) end of the growing season, EGS; c) length
of the growing season, LGS during 2000–2014 in the
Damxung alpine meadow steppe
interannual variation in the accumulated growth degree-days
requirement on the Tibetan Plateau was driven by the number of chilling days. Changes in GNDVI reflect the influence of climate change on grassland growth, and indirectly
reflect variation in vegetation phenology. Phenology was
closely related to GNDVI (Piao et al., 2011a; Shen et al.,
2011).
53
Variation in phenology in natural grassland
The BGS of the study area has delayed significantly, especially after 2004. The GNDVI of the grassland area also
decreased, consistent with observed LGS shortening (Shen
et al., 2013). The shortening of LGS was mainly caused by
a delay in the BGS and little change in the EGS. The delay
of BGS was mainly affected by a significant reduction in
precipitation and warming in spring from late April to early
June; the EGS remained unchanged (Cong et al., 2016), as
did rainfall from mid-August to mid-October and temperature from late September to October.
Further analysis indicated that the threshold of precipitation was 80–120 mm for the BGS at Damxung station,
while the temperature before and after the BGS date showed
a significant upward trend (R2 = 0.55, P<0.01; R2 = 0.57, P <
0.01). In general, temperature is a limiting factor for grass
growth (Fensholt et al., 2012). However, affected by warming (Liu and Chen, 2000; Piao et al., 2010), temperature met
the requirements for grass growth and water conditions are
limited (Lotsch et al., 2005; Park and Sohn, 2010). Therefore, grass growth was initiated by spring precipitation (Shi
et al., 2006; Yu et al., 2003). There was little change in the
heat requirements for the BGS on the Tibetan Plateau over
the warming period from 1998 to 2012 (Cong et al., 2017).
When spring precipitation reaches the threshold the grass
begins to turn green and precipitation becomes the main
limiting factor (Ji and Peters, 2003; Pennington and Collins,
2007). The temperatures were basically more than 5℃
before and after the BGS date, and showed a dramatic increase. Shen et al. (2016) showed that increases in
Fig.4 Relationships between BGS and a) rainfall in early growing season, cumulative precipitation during the 112th–160th
JDOY; b) average temperature in early growing season during the 112th–160th JDOY; and relationships between EGS and c)
late growing season rainfall, cumulative precipitation of the 224th–288th JDOY, d) late season temperature, average temperature of the 272th–304th JDOY
54
Journal of Resources and Ecology Vol. 8 No. 1, 2017
preseason daily maximum temperature did not advance the
BGS date and higher summer daily maximum temperature
reduced greenness. The apparent negative effects of higher
daily maximum temperature on BGS are due to the accompanying decline in water availability (Shen et al., 2016).
Warming increases demand for water when the growth
limitation of temperature to grass is exceeded (Angert et al.,
2005; Zhao and Running, 2010; Zhou et al., 2001). Interannual variation of phenology and its relationship with climate
change at Damxung were different from previous studies
(Ding et al., 2013; Song et al., 2011a; Zhang et al., 2013;
Piao et al., 2011a; Yu et al., 2010). Regional heterogeneity
is an important reason (Gao et al., 2012; Jeong et al., 2011;
Piao et al., 2011b; Shen et al., 2011). Lioubimtseva et al.
(2005) considered that climate change shows different variation trends and intensity at different periods of time and
regions, and changes in precipitation are more complex than
for temperature. Meanwhile, the quality of NDVI data, such
as clouds, snow cover, atmospheric interference and bare
land, can make a difference (Piao et al., 2011a; Wang et al.,
2011). Different sources of remote sensing data may also
have an effect (Yi and Zhou, 2011; Zeng et al., 2011; Zhang
et al., 2013). The quality of the MODIS dataset is higher
than that of the GIMMS AVHRR and SPOT VGT datasets
(Zeng et al., 2011; Zhang et al., 2013). Phenology extraction methods may be another important source of variation
worth considering (Cong et al., 2012; Cong et al., 2013;
Shen et al., 2013). Different methods may lead to distinct
extracted results with bias.
5
Conclusions
The northern Tibetan semiarid meadow steppe has undergone declining precipitation and significant warming since
the turn of the century. The GNDVI of natural grassland
showed a declining trend. Interannual variation in GNDVI
was mainly affected by rainfall in the growth season and
summer temperature. The LGS was significantly shortened
due to warming and decreasing precipitation in spring,
largely caused by a delay in BGS, but no advanced trend in
EGS. Water availability is the major determinant of grass
growth in the study area. Warming will increase demand for
water when the growth limitation of temperature to grass is
exceeded in the growth season. Decreased precipitation has
likely exacerbated the effect of warming on vegetation
phenology in recent decades due to increasing evapotranspiration and water limitations. The comprehensive effects
of global warming and declining precipitation may delay the
phenological responses of semiarid alpine grasslands.
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Journal of Resources and Ecology Vol. 8 No. 1, 2017
降水减少加剧气候变暖对西藏半干旱草甸草原物候变化的效应
赵广帅 1,2，石培礼 1,2，宗宁 1，何永涛 1，张宪州 1,2，何洪林 1，张晶 3
1. 中国科学院地理科学与资源研究所生态系统网络观测与模拟重点实验室，北京 100101；
2. 中国科学院大学，北京 100049；
3. 北京师范大学全球变化与地球系统科学研究院，北京 100875
摘
要：植被物候被认为是全球变暖的一个敏感指标，特别是在青藏高原。然而，自 2000 年以来，对气候变暖是否能够促
进草地物候期的提前存在争议。升温以及生长季节早期降水如何与之交互对半干旱草原物候产生影响鲜为人知。本研究中，我们
提取藏北当雄草地 2000–2014 年生长季平均 NDVI(GNDVI)和草地物候变化信息，分析了 NDVI、物候变化和气候变化的关系。
结果表明天然草地 GNDVI 呈下降趋势。GNDVI 的年际变化主要受 5 月下旬至 7 月的气温和 4 月至 8 月的降水影响。由于生长季
起始期不断延迟，生长季结束期基本不变，天然草地生长季长度大大缩短，这大部分是由于春季变暖和降水减少造成。水分有效
性是研究区草地生长的主要决定因素，当草地生长的温度限制被打破后，升温将增加对水的需求。近 10 年来，由于蒸散和水分
限制不断增强，降水减少进一步加剧了气候变暖对植被物候变化的效应。全球变暖和降水减少的综合影响可能会延迟半干旱高寒
草地的物候响应。
关键词：归一化植被指数(NDVI)；物候；气候变化；青藏高原；当雄站