2004-2013年珠江流域植被变化及其胁迫分析

植被对区域气候调节、水文循环等有着重要作用,在近年来中国南部地区极端气候频发的背景下,研究植被变化及胁迫意义重大。以珠江流域为研究区,利用MODIS EVI分析了植被的变化规律,并通过美国军事气象卫星DMSP灯光数据和气象数据探讨分析了人类活动和自然环境对植被变化的胁迫。结果显示：2004年到2013年期间珠江流域内年平均EVI介于0.33-0.38之间,EVI从高到底依次是常绿阔叶林 > 混交林 > 多树的草地 > 常绿针叶林 > 草地,不同植被类型的EVI变化趋势基本一致,同一植被类型EVI年际变化较小,其中混交林和草地年际最大变化量分别为0.07和0.04,而常绿阔叶林、常绿针叶林和多树的草地年际最大变化量均为0.06;在2004年至2013年年间,城市化水平增长了约71%,其年发展变化与EVI的年变化趋势相反;通过对比分析发现珠江流域人类活动对植被变化影响高于自然环境,即DMSP灯光变化与EVI变化的相关系数明显高于气温和降水。     

西南山区人口空间重组及其对植被的影响——以河流沿线为例

随着城市化的快速推进,山区人口迁出及空间重组成为影响中国山区人地关系的重要因素,这可能对山区植被恢复和生态改善产生巨大影响。基于人口空间数据、河流分布数据和MODIS数据,本文分析了河流沿线人口空间重组情况,以生长季EVI值为表征植被绿度的指标,采用基于像元的趋势分析方法和基于样本的相关分析模型,对2000—2010年间中国西南山区不同级别河流沿线的人口空间变化和植被变化作了系统性分析,并定量研究了人口空间重组与植被变化之间的关系。结果表明:(1)三级及以上河流出现人口往河流沿线聚集的趋势,人口在河流的影响区聚集程度大于对比区。其中,一级和二级河流沿线影响区人口密度增加量比对比区分别高75.9%和42.1%。(2)三级及以上各河流沿线影响区和对比区EVI均呈现出增加的趋势,且影响区增加趋势低于对比区。(3)植被EVI变化趋势与人口密度变化呈负相关关系,河流沿线人口密度增加不利于植被的恢复;河流级别越高,植被EVI变化趋势与人口密度变化的相关性越强。     

Relationships between phenology,radiation and precipitation in the Amazon region

In tropical areas, Dynamic Global Vegetation Models (DGVMs) still have deficiencies in simulating the timing of vegetation phenology. To start addressing this problem, standard Fourier‐based methods are applied to aerosol screened monthly remotely sensed phenology time series (Enhanced Vegetation Index, EVI) and two major driving factors of phenology: solar radiation and precipitation (for March 2000 through December 2006 over northern South America). At 1 × 1 km scale using, power (or variance) spectra on good quality aerosol screened time series, annual cycles in EVI are detected across 58.24% of the study area, the strongest (largest amplitude) occurring in the savanna. Terra Firme forest have weak but significant annual cycles in comparison with savannas because of the heterogeneity of vegetation and nonsynchronous phenological events within 1 × 1 km scale pixels. Significant annual cycles for radiation and precipitation account for 86% and 90% of the region, respectively, with different spatial patterns to phenology. Cross‐spectral analysis was used to compare separately radiation with phenology/EVI, precipitation with phenology/EVI and radiation with precipitation. Overall the majority of the Terra Firme forest appears to have radiation as the driver of phenology (either radiation is in phase or leading phenology/EVI at the annual scale). These results are in agreement with previous research, although in Acre, central and eastern Peru and northern Bolivia there is a coexistence of ‘in phase’ precipitation over Terra Firme forest. In contrast in most areas of savanna precipitation appears to be a driver and savanna areas experiencing an inverse (antiphase) relationship between radiation and phenology is consistent with inhibited grassland growth due to soil moisture limitation. The resulting maps provide a better spatial understanding of phenology–driver relationships offering a bench mark to parameterize ecological models.     

Strong links between teleconnections and ecosystem exchange found at a Pacific Northwest old-growth forest from flux tower and MODIS EVI data

Variability in three Pacific teleconnection patterns are examined to see if net carbon exchange at a low‐elevation, old‐growth forest is affected by climatic changes associated with these periodicities. Examined are the Pacific Decadal Oscillation (PDO), Pacific/North American Oscillation (PNA) and El Niño‐Southern Oscillation (ENSO). We use 9 years of eddy covariance CO₂, H₂O and energy fluxes measured at the Wind River AmeriFlux site, Washington, USA and 8 years of tower‐pixel remote sensing data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to address this question. We compute a new Composite Climate Index (CCI) based on the three Pacific Oscillations to divide the measurement period into positive‐ (2003 and 2005), negative‐ (1999 and 2000) and neutral‐phase climate years (2001, 2002, 2004, 2006 and 2007). The forest transitioned from an annual net carbon sink (NEP=+217 g C m^?2 yr^?1, 1999) to a source (NEP=?100 g C m^?2 yr^?1, 2003) during two dominant teleconnection patterns. Net ecosystem productivity (NEP), water use efficiency (WUE) and light use efficiency (LUE) were significantly different (P<0.01) during positive (NEP=?0.27 g C m^?2 day^?1, WUE=4.1 mg C g^?1 H₂O, LUE=0.94 g C MJ^?1) and negative (NEP=+0.37 g C m^?2 day^?1, WUE=3.4 mg C g^?1 H₂O, LUE=0.83 g C MJ^?1) climate phases. The CCI was linked to variability in the MODIS Enhanced Vegetation Index (EVI) but not to MODIS Fraction of absorbed Photosynthetically Active Radiation (FPAR). EVI was highest during negative climate phases (1999 and 2000) and was positively correlated with NEP and showed potential for using MODIS to estimate teleconnection‐driven anomalies in ecosystem CO₂ exchange in old‐growth forests. This work suggests that any increase in the strength or frequency of ENSO coinciding with in‐phase, low frequency Pacific oscillations (PDO and PNA) will likely increase CO₂ uptake variability in Pacific Northwest conifer forests.