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

radiation, rather than temperature or rainfall anomalies, explains reversals in greenness in North Central Asia. Piao et al. (2013) [68] found that solar radiation limited productivity in the northern regions. In addition to increased cloud cover, our observed reduced sunshine hours from the late 1990s to early 2000s may be associated with the PRD region’s accelerated urbanization and industrial development especially during the 1995 to 2004 period [69] with low efficiency in energy use, and consequently increased aerosols/pollution [43,44]. Wang et al. (2017) [44] found that in China, the ratio of rural to urban dimming increased from 0.39 to 0.87 with increasing urbanization, and reached a maximum when urbanization reached 50% or population exceeded 250 persons/km2. In PRD region, air pollution produces a fog- like haze, which reflects and absorbs solar radiation, and this effect has been reported from other parts of China [70,71] and Asia [72,73]. Therefore, the reduction in vegetation productivity observed during the late 1990s and early 2000s observed here could, in addition to reduced sunshine hours due to increased cloud cover, result from dimming due to aerosols (Fig 5A). Aerosols act as condensation nuclei in cloud formation and may increase rainfall (Fig 5B). Aerosols also scatter sunlight, enhancing planetary albedo, thus reducing received radiation. Additionally, exposure to high surface ozone and SO2 concentrations common in the PRD [74] can stunt or damage plants [75]. Indeed, tree ring analysis of Pinus massoniana, a common tree in the PRD region indicates

growth reduction in the late 1990s to mid-2000s coincident with increased air pollution, spe- cifically at polluted sites compared to control sites [76]. An alternative, or parallel explanation for the reduced productivity in the 1995 to 2004 decade may be related to temperature stress, as He & Yang (2011) [77] observed that for the period 1996 to 2005, temperatures in PRD were above the 30-year average of 1981–2008 (and see also S1 Fig showing increased temperatures for Zhongshan and Zhuhai meteorological sta- tions in PRD). This is supported by our findings in section 3.4 of negative correlations between growing season productivity indicators and temperature. Studies in different parts of the world [58,59,78] and northern China [31] have reported an increase in growing season length due to an earlier SOS and /or delayed EOS, but our results did not indicate significant trends in SOS, EOS and LOS when the whole period 1982–2015 is considered. Most regions with increasing growing season length are non-tropical, where sea- sonal phenology is affected by freeze-thaw cycles, whereas nowhere in the PRD region is too cold for plant growth. Conclusion In this study, we examined a 34-year trend in vegetation productivity, precipitation and temper- ature as well as the response of vegetation to seasonal climate indicators in the Pearl River Delta region of southern China. An overall increasing trend in productivity metrics was observed over the study period (1982–2015) which is more related to increasing temperature in autumn

and winter, rather than to changes in the rainfall. This is contrary to the northern parts of China and other high latitude regions, where extension in growing season length due to advancement in green-up and/or delay in senescing dates is the primary factor for the higher seasonal productivity. In the PRD region, negative correlations between temperature and grow- ing season productivity indicators for croplands, suggest that rising summer temperature dur- ing the critical crop development stage, have negative implications for rice production in the region. Some forested regions in the north of the PRD also show negative correlation with tem- perature for the main growing season productivity indicators. Although longer-term research is required to understand the impacts of warming on tropical forests, results suggest that increased warming would have adverse impacts on forests in the PRD region. PLOS ONE Climate and vegetation trends in the Pearl River Delta PLOS ONE | https://doi.org/10.1371/journal.pone.0245467 February 24, 2021 15 / 20