基于数字相机图像的西藏当雄高寒草地群落物候模拟

doi:10.3724/SP.J.1258.2012.01125

摘要/Abstract

摘要：

物候现象是环境条件季节和年际变化最直观、敏感的综合指示器, 其发生时间不仅反映了陆地生态系统短期的动态特征, 其微小的变化还会对陆地生态系统产生重要的反馈作用。高寒草地是青藏高原分布广泛、极具代表性的植被类型, 准确地获取高寒草地群落的物候特征, 对于理解和预测气候变化对青藏高原生态系统的影响具有重要意义。该文以西藏当雄高寒草地为研究对象, 探讨了近地面数字相机图像在高寒草地群落季相监测中的作用, 结果如下: 1)通过比较不同绿度指数的差别, 确定了准确表征高寒草地植被群落季相变化的绿度指数——绝对绿度指数(2G_RB); 2)结合土壤含水量数据, 通过线性回归分析得知高寒草地植被群落生长过程与表层(≤10 cm)土壤含水量的变化较为一致(R ² > 0.70); 3)通过对比分析, 发现降水在高寒草地群落季相“变绿”过程中具有“触发”作用。研究表明, 数字相机技术可作为物候监测手段, 实现高寒草地植被群落季相的实时、连续获取, 为更好地揭示气候变化影响下景观尺度季相演变特征, 诊断地方、区域和全球尺度上生态系统对气候变化的快速响应提供了有效的手段。

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关键词: 绝对绿度指数, 高寒草地群落, 数字相机, 季相

Abstract:

Aims Phenology refers to periodic appearances of life-cycle events. It is crucial for predicting plant phenological responses to climate change and for identifying the period of carbon-uptake. Tracking the real-time canopy status accurately, especially in harsh environments, is becoming a large challenge for understanding and modeling vegetation-climate interactions. Our objective focuses on how to obtain relatively accurate real-time canopy status in Qinghai-Xizang Plateau using digital camera images.
Methods A standard, commercially available webcam was mounted at the top of the eddy covariance tower at the Damxung Rangeland Station. Images were collected every half an hour from 9:30 a.m. to 5:00 p.m. local time each day. We extracted red, green, and blue color channel brightness data for a region-of-interest (ROI) from each image (ROI, the subset of image, can better describe the target’s characters). The size of ROI is [100:180] and [10:380]), and it composed the different greenness indices according to the equations. We confirmed the best one that can reflect the size of leaf area index and variations in chlorophyll content by comparing different indices.
Important findings The absolute greenness index (2G_RB) is able to describe the canopy status qualitatively and quantitatively and is powerful in tracking community phenological stages. This indicates that digital cameras can be used in monitoring real-time phenology of alpine grassland community. Linear regression analysis of soil moisture indicates greenness is best explained by surface soil moisture (≤10 cm). By comparing canopy phenological events with conventional meteorological data, we also speculate that precipitation plays a critical role in triggering the spring phenological response in semiarid alpine grassland.

Key words: absolute greenness index, alpine grassland community, digital camera, phenological phase

周磊, 何洪林, 张黎, 孙晓敏, 石培礼, 任小丽, 于贵瑞. 基于数字相机图像的西藏当雄高寒草地群落物候模拟. 植物生态学报, 2012, 36(11): 1125-1135. DOI: 10.3724/SP.J.1258.2012.01125

ZHOU Lei, HE Hong-Lin, ZHANG Li, SUN Xiao-Min, SHI Pei-Li, REN Xiao-Li, YU Gui-Rui. Simulations of phenology in alpine grassland communities in Damxung, Xizang, based on digital camera images. Chinese Journal of Plant Ecology, 2012, 36(11): 1125-1135. DOI: 10.3724/SP.J.1258.2012.01125

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链接本文: https://www.plant-ecology.com/CN/10.3724/SP.J.1258.2012.01125

https://www.plant-ecology.com/CN/Y2012/V36/I11/1125

图/表 7

图1 高寒草地植被冠层不同季相的数字图像示例。 A, 休眠期初期(年序日第25天)。B, 变绿期初期(年序日第167天)。C, 旺盛期初期(年序日第211天)。D, 凋落期初期(年序日第245天)。ROI, 感兴趣区域。

Fig. 1 Digital images samples of alpine grassland vegetation canopy in different phenological phase. A, Beginning of dormancy stage (DOY 25). B, Beginning of green-up stage (DOY 167). C, Beginning of maturity stage (DOY 211). D, Beginning of senescence stage (DOY 245). DOY, day of year; ROI, region-of-interest.

图2 感兴趣区域内各变量的时间序列。A, 各波段亮度值。B, 绝对绿度指数。C, 相对绿度指数。D, 比值绿度指数。

Fig. 2 Time series of variables for a region-of-interest (ROI). A, Digital values for different wave bands. B, Absolute greenness index. C, Relative greenness index. D, Ratio greenness index.

图3 绝对绿度指数(2G_RB)的实测值、拟合值的时间序列以及物候期的确定。

Fig. 3 Times series of observed absolute greenness index (2G_RB), fitted 2G_RB and determination of phenological phase.

图4 2011年4月1日到7月31日拟合绝对绿度指数与常规气象数据的时间序列。

Fig. 4 Time series of fitted absolute greenness index and conventional metrological data from April 1 to July 31, 2011.

表1 全年绝对绿度指数与各土层土壤含水量之间的线性回归分析

Table 1 Linear regression analysis between absolute greenness index and soil water content of different soil layers in the whole year

	5 cm土层土壤含水量 Soil water content in 5 cm soil layer	10 cm土层土壤含水量 Soil water content in 10 cm soil layer	50 cm土层土壤含水量 Soil water content in 50 cm soil layer
斜率 Slope	117.53	129.00	216.49
截距 Intercept	-12.33	-13.34	-21.51
p值斜率 p-value slope	≤0.01	≤0.01	≤0.01
p值截距 p-value intercept	≤0.01	≤0.01	≤0.01
决定系数 Coefficient of determination R²	0.74	0.71	0.64

表2 不同数据来源获取的群落季相初日的比较(单位: 年序日)

Table 2 Comparison of community phenological phase onset derived from different data sources (unit: day of year)

数据获取平台 Data acquisition platform	群落季相初日 Onset date of community phenological phase
数据获取平台 Data acquisition platform	变绿期 Green-up stage	成熟期 Maturity stage	凋落期 Senescence stage	休眠期 Dormancy stage
数字相机 Digital camera	161	192	238	278
卫星 Satellite	158	198	245	294

图5 绝对绿度指数(2G_RB)与增强型植被指数(EVI)的时间序列的比较。

Fig. 5 Time series comparison of absolute greenness index (2G_RB) and enhanced vegetation index (EVI).

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