Trends in vegetation productivity related to climate change in China’s Pearl River Delta

In evaluating the impacts of climate change on ecosystems at a global scale, the remote sens- ing-based Normalized Difference Vegetation Index (NDVI) has been extensively used along with climate data [4–7]. The NDVI shows a high correlation with vegetation productivity; and the effectiveness of NDVI is based on the fact that chlorophyll in healthy green leaves strongly absorbs red radiation, and the spongy mesophyll layer in healthy leaves strongly reflects near infra-red (NIR) radiation [8]. Changes in vegetation phenology and productivity have become an important topic in global climate change and ecosystem analysis research. Changes in phe- nology are expressed as changes in leaf density and photosynthetic activity through growing seasons [9,10]. Seasonal characteristics of plants, such as emergence and senescence, depend- ing on the characteristics of the lower atmosphere, including the annual cycle of weather pat- terns and temperature and precipitation characteristics. For example, Jeong et al., (2011) [11] measured the start, end and length of vegetative growth from NDVI time series, compared to climate data over three decades, in the northern hemisphere. They observed that the growing season had advanced by 3.5 days and growing season length increased by 6.5 days per decade. De Jong et al., (2013) [6] used NDVI to examine the factors responsible for greening or brow- ning trends in vegetation activity globally, and found that half of the observed trends were induced by climatic changes.

Recent studies on climate change impacts on flora and fauna in subtropical and tropical areas of China have observed an increased incidence of damage from extreme climate events such as severe spring frosts following unseasonal warm weather which precipitates early tissue growth [12,13]. Ge et al., (2015) [13] concurred that a higher frequency of frost damage would favour deciduous species, which are more resilient to frost and in the long run would cause the disappearance of evergreen forests above 600 m in areas of central China. As one of the world’s emerging economies, China has experienced rapid development, industrialization, and urbanization. At present, China is dealing with conflicting issues of miti- gating climate change in the international arena, while protecting its resources and environ- ment during a domestic socioeconomic transition. Recently 10 megacities have emerged in China, among which the Pearl River Delta (PRD) region is the largest cluster of megacities. Whereas the average global temperature has increased by approximately ~0.8˚C since 1880, two-thirds of this since 1975 [14], in the largest cities, the urban heat island (UHI) effect has added 1.72˚C to the global increase [15]. In the PRD region temperature increase due to the UHI effect in the 1983 to 1993 decade of rapid industrialization and urbanization was 0.4˚C [16], and air quality deteriorated, with photochemical smog and NOx becoming semi-persis- tent over the region [17]. In addition, the regional heat dome circulation over PRD suggests

local temperatures will increase further, even without further local developments [18]. Accom- panying these climatic effects have been dramatic forest and vegetation disturbances, soil ero- sion, and loss of farmlands, natural ecosystems and biodiversity. Many site-specific ecological studies have been conducted in the PRD region, but the impacts of changes in meteorological parameters e.g. seasonal metrics of temperature, precipita- tion as well as landscape-wide climatic sensitivity, have not been studied. Most climate change impact studies address either natural or human-modified ecosystems. This study focusses on determining vegetation responses to different climatic factors across a large, dynamic and diverse region of rural-to-urban transition. The study determines the spatial patterns of tempo- ral trends in precipitation, temperature, and vegetation phenology and productivity. Study area The PRD region has grown at a breakneck pace, from farmland to urban-industrial in the last three decades. Although the PRD region contains eight megacities with populations over 10 PLOS ONE Climate and vegetation trends in the Pearl River Delta PLOS ONE | https://doi.org/10.1371/journal.pone.0245467 February 24, 2021 2 / 20 zones are available from http://koeppen-geiger.vu- wien.ac.at/present.htm and http://www.fao.org/ geonetwork/srv/en/metadata.show?id=30589 Administrative boundaries are available from https://www.diva-gis.org/gdata. Funding: This research is supported by grants from the Research Grants Council of Hong Kong SAR (Grant no. 15602619 and C7064-18GF to

MSW), and by another grant from the Research Institute for Sustainable Urban Development, the Hong Kong Polytechnic University (Grant no. 1- BBWD to SA and MSW). Competing interests: The authors have declared that no competing interests exist.