sustainability Article Accumulation of Urban Insect Pests in China: 50 Years’ Observations on Camphor Tree (Cinnamomum camphora) Zhiyuan Xiang 1,2, Meifang Zhao 1,2,* and U. S. Ogbodo 2 1 Faculty of Life Science and Technology, Central South University of Forestry and Technology, Changsha 410004, China; [email protected] 2 National Engineering Laboratory for Applied Forest Ecological Technology in Southern China, Changsha 410004, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0731-8562-3868 Received: 2 December 2019; Accepted: 19 February 2020; Published: 20 February 2020 Abstract: Since China experienced a rapid and unprecedented process of urbanization and climate change from 1978 onwards, pest outbreaks were frequently reported on urban forests, which reflects a significant imbalance between natural regulation and human control. Based on information extracted from all journal articles and reports about insect pests on camphor tree (Cinnamomum camphora) in urban China, we characterized historical patterns and trends in pest outbreaks over large areas. Our results suggested that (1) most distribution areas of C. camphora in urban China had pest records (14 provinces) over the last 50 years, especially at the south-eastern coastal areas; (2) pests on camphor tree in urban China showed an accelerated growth since the 1990s; and (3) pests on camphor tree in urban China were characterized by native and leaf-feeding species. Urbanization seems to positively correlate with urban pest outbreaks. Changes of urban pest outbreaks could largely be described by synchronic changes of socio-economic indicators, of which CO2 emissions as metric tons per capita is the most significant predictor, followed by GDP and human population. Thus, managers and city planners should allocate resources to socio-economic-related pest outbreaks for a sustainable ecosystem. Keywords: Cinnamomum camphora; insect pest; outbreak; urban tree; climate change 1. Introduction The rapid change in pest population densities in forests, known as forest pest outbreaks, and their dynamics represent critical systematic evidence for ecosystem stability [1]. For example, pest outbreaks occur when trees are less phenotypically plastic in their ability to cope with conditional changes compared to insect pests [2]. In addition, pest outbreaks can reflect increased urbanization, uncertainties in socio-economic requirements, climatic change impacts, and insect pest sensitivities. Numerous studies have focused on pest outbreaks at community or local scales under climate impacts [3–5], which can contribute to forest dynamics modeling, theoretical validating, and calibrating [6]. Results can also be valuable reference information for informed prediction of potential future changes under global climate change, development of adequate management strategies [6], and characterization of life in changing environments, such as biodiversity [7] and adaptive evolution mechanisms [8]. Urban forests are essential green infrastructure, which are recognized for generating a range of benefits and providing valuable ecosystem services (e.g., controlling pollution). They are highly dynamic, inherently vulnerable, and commonly subject to high levels of stress and disturbances [9–11]. As a result, managing urban forests for sustainable urban development is becoming increasingly complex Sustainability 2020, 12, 1582; doi:10.3390/su12041582 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 1582 2 of 15 under accelerated urbanization, owing to large uncertainties in socio-economic requirements [12], climatic change impacts (higher temperature, higher CO2 concentrations, increased nitrogen deposition rates, and longer growing seasons), and insect pests’ sensitivities (e.g., natural enemies, competitors and vectors of taxa, feeding types, invasive intensities, etc.). In this context of rapid and complex change, baseline characterization of historical patterns and trends in pest outbreaks over large areas builds a necessary foundation for urban planning and urban forestry, public health risk identification, and integrated pest management, as well as for global change vulnerability monitoring, modelling, and mapping [6,13]. China has experienced a rapid and unprecedented process of urbanization since 1978 (see Appendix 1, Figure S1, Table S1 in Supplementary Material for maps and detailed descriptions of the tempo-spatial distribution of urbanization in China), which entails increasing urban tree cultivation [14]. Cinnamomum camphora, also known as camphor tree or camphor laurel, is a medium to large evergreen broadleaved fast-growing tree species with a dense crown. This species is a typical indigenous street and park tree in China and also a famous non-indigenous tree in the rest of the world that is widely cultivated for timber production, carbon sequestration, and provision of critical ecosystem services and economic benefits in urban areas (Figure1b, see Figure 1b, see Appendix 3, Table S3 in Supplementary Material for a detailed view of its global distribution). It is universally native to East Asia and Pacific West Bank, especially from coastal areas to the heartland in China (20 provinces and municipalities) and adjacent islands, including Taiwan and Hainan (Figure1c, see Figure 1c, see Appendix 3, Table S3 in Supplementary Material for a detailed view of its distribution in China). The area for cultivating camphor trees in Shanghai, the global financial center and transport hub, for example, accounts for 23.39% of all street/park trees according to the National Forestry Database from the China Forestry Science Data Center (http://www.cfsdc.org/). In Changsha, the core city of the Changsha–Zhuzhou–Xiangtan urban agglomeration, camphor trees comprise more than half of the total street/park tree populations. However, there have been frequent reports of pest outbreaks of C. camphora in China (see Appendix 2, Figure S2, Table S2 in Supplementary Material for maps and detailed descriptions of C. camphora-hosted insect pest damages). For example, 91.1% of C. camphora had experienced termite (e.g., Odontotermes formosanus) invasion in Wuhan, one of the central cities in China [15]. They were also seriously damaged by a newly discovered weevil pest in Shanghai [16]. Because environmental factors and insect outbreaks are distributed unevenly or randomly across a heterogeneous urban forest landscape. Detection of insect pest outbreaks is mostly made based on annual aerial surveys [17]. Since aerial overview survey coverage is limited in China (Figure1a), many forests that are infected by insect pests, especially those in urban landscapes, unfortunately go uncounted. There are also strong (though poorly studied) correlations among urban forest pest outbreak characteristics and socio-economic indicators. Which factors are responsible for the observed patterns of long-term insect pest outbreaks, and what possible drivers could be used to explain the magnitude of impact of forest insect pests, are also often difficult to determine. The aim of this study is to explore ecosystem stability of urban forests based on the characterization of historic patterns and trends in pest outbreaks. The objectives were to (1) determine the diversity of insect pests on C. camphora and (2) correlate and explain the urban forest pest outbreaks with indirect factors. Sustainability 2020, 12, 1582 3 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 3 of 16 Figure 1. GeographicalFigure 1. Geographical distribution distribution of of ((a) studies studies and andinsect insecttypes according types to according Senf et al. [17], to (b Senf) C. et al. [17], (b) C. camphoracamphorathroughout throughout the the world, (c) ( C.c) camphoraC. camphora in China,in and China, (d) reported and ( doutbreaks) reported of urban outbreaks insect of urban pests of C. camphora in China over past half-century (see Appendix 2, Figure S2, Table S2 in insect pests of C. camphora in China over past half-century (see Appendix 2, Figure S2, Table S2 Supplementary Material for photos and detailed descriptions of C. camphora-hosted insect pest in Supplementarydamages). Material Panel (a) represents for photos patterns and of the detailed current state descriptions of forest insect of disturbanceC. camphora reviewed-hosted from insect pest damages). Panelthe literature (a) represents on remot patternse sensing. ofDifferent the current insect types: state bark of forest beetle insect (olive triangles), disturbance broadleaved reviewed from the literature ondefoliator remote sensing.(green triangles), Different coniferous insect defoliator types: bark (orange beetle triangles) (olive and triangles), other fluid broadleavedfeeders (red defoliator triangles). Panel (b) shows C. camphora is a widely distributed urban tree species in the world (see (green triangles),Appendix coniferous 2, Table S2 defoliator in Supplementary (orange Material), triangles) i.e., the most and popular other fluidnative feederstree species (red in tropical triangles). Panel (b) shows C.and camphora subtropicalis aChina widely (green distributed circles), and has urban been introduced tree species by other in the countries world (cyan (see circles). Appendix Photo 2, Table S2 in Supplementaryin the lower Material), right corner i.e., is the C. camphora most popular in the urban native street ( treeChangsha, species Hunan in tropicalprovince, China and subtropical). Panel China (d) indicates native C. camphora has experienced rapid growth of