2000-2020乾旱對副熱帶島嶼植被影響之探討

Abstract

陸域生態系為地球上重要的碳庫，而嚴重的乾旱會影響碳循環，受到氣候變遷的影響，乾旱發生的頻率以及嚴重程度正逐漸上升。臺灣因全球暖化加上降雨型態的改變，夏季的降水有減少的趨勢，未來中南部地區發生乾旱的機率上升。了解植被對於乾旱的反應，有助於評估在全球氣候變遷的情境下生態系功能的變化。透過遙測技術可以長時間且大規模的分析植被受環境變化的影響，本研究透過美國太空總署的遙測感應器MODIS (Moderate-resolution Imaging Spectroradiometer)衍生的增強植被指數(EVI; 250 m x 250 m)和常態化差異紅外指數(NDII; 500 m x 500 m)，以及3個月雨水累積的標準降水指數(SPI3)，評估臺20年來(2000-2020)乾旱對臺灣植被影響。結果發現，20年間臺灣共發生兩次極度乾旱，皆對植被生長有明顯負面影響，2002年極度乾旱造成植生指數減少約20%，特別是中、南部山區的植被受到的影響最大，經過4個月的時間才恢復至歷年平均值；2020年乾旱使植生指數下降約10%，並觀察到植物生長對乾旱的延遲反應。本研究也發現不同類群的植被對於乾旱的反應有所不同，乾旱期間草原的植生指數下降幅度最小，而闊葉林及混合林下降幅度較大；農田地雖受負面影響，但恢復速率較快。相關性分析結果顯示，植被的生長情形在不同程度的缺水壓力下不盡相同，且有明顯的空間分布差異及延遲反應。NDII指數與乾旱指數的相關性較高，顯著網格佔總植被網格數約54%，並隨時間延遲，相關性逐漸減弱；EVI指數則是在一個月的延遲下與乾旱指數有較高的相關性。本次研究的結果有助於了解生態系對於乾旱的反應，未來需要結合多方面且長期的研究，才能更深入了解以應對氣候變遷對生態環境的衝擊。

Terrestrial ecosystems are important carbon sinks on Earth but severe drought could affect carbon cycling. The frequency and severity of droughts are gradually increasing due to climate change. Taiwan is experiencing a trend of decreasing summer precipitation due to changes in rainfall patterns likely because of global warming, which increases the likelihood of droughts in central and southern Taiwan. Understanding the response of vegetation to droughts can help assess changes in ecosystem function under global climate change scenarios. Remote sensing technology can be used to analyze the impact of environmental changes on vegetation over a long period of time and over very broad spatial extent. In this study, the enhanced vegetation Index (EVI; 250 m x 250 m), normalized difference infrared index (NDII; 500 m x 500 m) derived from MODIS (Moderate-resolution Imaging Spectroradiometer) of NASA, and the standard precipitation Index for three-month accumulated rainfall (SPI3) were used to evaluate the impact of drought on Taiwan's vegetation over the past 20 years (2000-2020). The results showed that Taiwan experienced two extreme droughts during the 20-year period, both of which had a significant negative impact on vegetation growth. The extreme drought in 2002 caused a decrease in vegetation index of about 20%, especially in the central and southern mountainous areas, which took about four months to recover to the average value of the previous years. The drought in 2020 caused a decrease in vegetation index of about10%, and the lag response of plant growth to drought was observed. This study also found that different types of vegetation have different responses to drought. The grassland had the smallest decrease in vegetation index during the drought period, while the broad-leaved forest and mixed forest had larger decreases. Although the cropland was negatively affected, its recovery rate was faster. The correlation analysis showed that the response of vegetation growth to different degrees of water stress was different and showed significant spatial variation and lag response. Compared to EVI. NDII index had a higher correlation with the drought index, with about 54% of all vegetation pixels experiencing significant decreases, and the correlation gradually weakened over time. The EVI index had a higher correlation with the drought index with a lag of one month. The results of this study help to understand the response of ecosystems to droughts. In the future, multi-faceted and long-term research is needed to gain a deeper understanding of the impact of climate change on the ecological environment.