探討颱風和寒流對於臺北市兩個都市公園植被的影響

Abstract

都市綠地是都市地景的重要組成單元，也是提升都市生活品質重要的一環；其中，都市公園更是民眾休閒的重要場所。然而，一如自然森林綠地，都市公園綠地也經常受到許多自然擾動的影響，其中颱風和寒流即是臺灣常見的自然擾動；但相較於森林、海洋、紅樹林等自然環境，過去研究鮮少著重於颱風、寒流等自然擾動事件對都市公園綠地的影響。雖然都市公園綠地範圍較小，但因和民眾生活息息相關，一旦受到自然擾動的影響，很可能影響其所能提供生態系統服務質量。因此，研究極端天氣事件如何影響都市公園綠地，有助於了解都市公園綠地於未來全球暖化情況下，對於何種天氣事件較為敏感。本研究以Worldview-2、Worldview-3及Geoeye-1高空間解析度衛星影像作為研究材料，透過計算植生指標 (Vegetation Index, VI) ，比較2015年8月蘇迪勒颱風及2016年1月霸王級寒流對臺北市兩個都市公園綠地—大安森林公園和青年公園植被的影響。研究結果顯示，於颱風和寒流過境後兩個公園的增揚植生指標 (Enhanced Vegetation Index, EVI) 顯著下降，而常態化差異植生指標 (Normalized Difference Vegetation Index, NDVI) 則僅在颱風後下降，代表颱風會造成都市公園植生覆蓋立即下降，而罕見的寒流影響會因所使用的植生指數不同而有不同的結果；另外，整體來看，颱風擾動的影響與植生所在的位置有關，減損情況由外圍至內部遞減，而寒流擾動的影響則無明顯空間規律；在植生類型之間，都市公園兩類主要植生即樹木與草地，其中，樹木較草地容易受到颱風和寒流的影響；以區塊面積來看，若以NDVI作為判斷依據，受颱風影響的青年公園和受寒流影響的大安森林公園皆呈面積越大，受到的影響越小，但受寒流影響的青年公園則呈面積越大，受到的影響越大，若以EVI作為判斷依據，受颱風影響的青年公園則呈面積越大，受到的影響越小，而受颱風和寒流影響的大安森林公園則呈面積越大，受到的影響越大；在大安森林公園不同樹種間，NDVI在颱風後減損最嚴重的為艷紫荊，在寒流影響後減損最為嚴重的則為原生於熱帶的大王椰子，若以EVI評估，則分別為水黃皮和雀榕，而青年公園中受颱風和寒流影響後NDVI減損最為嚴重的樹種皆為楓香，若以EVI評估，楓香亦為受寒流影響後減損最為嚴重的樹種，而受颱風影響後減損最為嚴重的樹種則為阿勃勒。最後，研究結果亦指出，大安森林公園中的樹木，於受颱風和寒流影響後植生指數的變化，僅與樹高和DBH具顯著關係。本研究結果有助於評估未來氣候變遷造成颱風及寒流頻率和強度發生變化時，對都市公園綠地的衝擊，以作為都市公園經營管理的參考。Urban green space is an important component of urban landscape and plays a critical role in improving the quality of urban life. Among different types of green space, urban parks are among the most important leisure places to urban residents. Like forests, urban parks are also frequently influenced by natural disturbances, with typhoons and cold surges as the common natural disturbances in Taiwan. However, comparing with natural environments such as forests, oceans, and mangroves, few studies have examined the impacts of typhoons and cold surges on urban parks. Although the extent of urban parks is relatively small compared to many nature forests, once affected by natural disturbances, the quality of the ecosystem services they provide could be affected. Therefore, studying how extreme weather events affect urban parks helps our understanding of which types of extreme weather event have greater effects on urban parks. In this study, we used three types of high spatial resolution satellite images, WorldView-2, WorldView-3 and Geoeye-1, acquired prior to and after typhoon Soudelor of August 2015 and the cold surge of January 2016 of the Daan Forest Park and the Youth Park in the Taipei City together with vegetation indices (VIs) to assess how the two disturbances affect vegetation cover. The results showed that the enhanced vegetation index (EVI) of the two parks decreased significantly after the typhoon and the cold surge, while the normalized difference vegetation index (NDVI) only decreased after Typhoon Soudelor. The result indicates that the vegetation coverage of urban parks is more prone to the typhoon disturbance than to the cold surge.Overall, the typhoon disturbance pattern was directional, with increasing typhoon-induced vegetation loss from the center to the edges of the urban parks, while no such a pattern was observed for the cold surge disturbance. Trees were more susceptible to typhoon and cold surge disturbance than grassland. Based on NDVI, the impact of typhoon to the Youth Park and the impact of the cold surge to the Daan Forest Park were greater for larger patches, while the impact of cold surge to the Youth Park was smaller for larger patches. Based on EVI, the impact of the typhoon to the Youth Park was smaller for larger patches, while for the Daan Forest Park the larger the patches, the greater the impact for both the typhoon and the cold surge. Bauhinia blakeana D. and Roystonea regia suffered the most severe vegetation loss after the typhoon and the cold surge, respectively, in the Daan Forest Park based on NDVI, while the most severely affected tree species was Pongamia pinnata and Ficus subpisocarpa, respectively, based on EVI. In the Youth Park, Liquidambar formosana was the most severely affected tree species after the typhoon and the cold surge based on NDVI, while based on EVI, Liquidambar formosana also severely affected after the cold surge, and Cassia fistula L. was the most severely affected species after the typhoon. Furthermore, tree height and diameter in breath height had a significant relationship with vegetation loss associated with the typhoon and the cold surge in the Daan Forest Park。The results will contribute to evaluate the impacts of typhoons and cold surges on urban parks under a changing climate and thus provide useful information for urban park management.