Recovery of Vegetation Canopy after Severe Fire in 2000 at the Black Hills National Forest,South Dakota,USA

美国南达科他州Black Hills国家森林公园2000年森林火灾后植被恢复过程研究(英文)

Forest fires often result in varying degrees of canopy loss in forested landscapes. The subsequent trajectory of vegetation canopy recovery is important for ecosystem processes because the canopy controls photosynthesis and evapotranspiration. The loss and recovery of a canopy is often measured by leaf area index （LAD and other vegetation indices that are related to canopy photosynthetic capacity. In this study we used time series imagery from the Moderate Resolution Imaging Spectroradiometer （MODIS） sensor onboard the Terra satellite over the period of 2000-2009 to track the recovery of the vegetation canopy after fire. The Black Hills National Forest, South Dakota, USA experienced an extensive wildfire starting on August 24, 2000 that burned a total area of 33 785 ha, most of which was ponderosa pine forest. The MODIS data show that canopy photosynthetic capacity, as measured by IL,AI, recovered within 3 years （2001-2003）. This recovery was attributed to rapid emergence of understory grass species after the fire event. Satellite-based Normalized Difference Vegetation Index （NDVI） and Enhanced Vegetation Index （EVI） at the burned sites also recovered within 3 years （2001-2003）. Rapid recovery of LAI, NDVI, and EVI at the burned sites makes it difficult to use these variables for identifying and mapping burned sites several years after the fire event. However, the Land Surface Water Index （LSWI）, calculated as a normalized ratio between near infrared and shortwave infrared bands （band 2 and band 6 （1628 1652 nm） in MODIS sensor）, was able to identify and track the burned sites over the entire period of 2000 2009. This fmding opens a window of opportunity to identify and map disturbances using imagery from those sensors with both NIR and SWIR bands, including Landsat 5 TM （dating back to 1984）; furthermore, a longer record of disturbance and recovery helps to improve our understanding of disturbance regimes, simulations of forest succession, and the carbon cycle.

森林火灾在景观上往往造成不同程度的森林冠层损失,而冠层影响光合作用和蒸散,因此刻画灾后森林冠层恢复的轨迹对于了解生态系统过程具有重要意义。森林冠层的损失和恢复通常采用叶面积指数(LAI)或其它能够反映冠层光合能力的植被指数进行表征。本研究中,我们采用Terra卫星搭载的中分辨率成像光谱仪(MODIS)的长时间序列影像(2000-2009年)来重建火灾后森林冠层恢复的过程。以美国南达科他州布莱克山国家森林公园(The Black Hills National Forest, South Dakota)为例,该地区在2000年8月24日经历了一次大的自然火灾,烧毁了近33 785 ha森林,其中大部分是美国黄松林。基于LAI的研究表明,植被冠层光合能力在3年内(2001-2003年)基本恢复,这主要来自于林下未烧毁草地在灾后的快速生长;火烧迹地的NDVI和EVI在这3年内也呈现恢复的态势。可见,LAI、NDVI和EVI在火灾几年之后便难以有效地识别火烧迹地。然而,陆地表面水分指数(基于近红外和短波红外波段的遥感标准化指数,简称LSWI),能够有效地识别和追踪火烧迹地至今的整个过程(2000-2009年)。这一研究结果也使得采用其它具有近红外和短波红外波段的传感器研究森林火灾迹地恢复和干扰过程成为可能,其中包括Landsat 5 TM影像(可追溯至1984年)。更长时间序列的数据对于研究森林火灾灾后生态系统干扰和恢复过程、森林演替模拟以及碳循环具有重要的支撑作用,LSWI指标证明能够有效地刻画这一过程。