Biodiversity hotspots: Natural regeneration dynamics of threatened Dacrydium pectinatumcommunities along various environmental axes on Hainan Island, China
Haodong Liua, Qiao Chena, Yongfu Chena, Zhiyang Xub, Yunchuan Daic, Yang Liua, Yi Jianga, Xi Pengd, Huayu Lie, Juan Wange, Hua Liua*
a Research Institute of Forest Resource Information Techniques, Chinese Academy of Forestry, Beijing, 100091, China
b East China Inventory and Planning Institute of National Forestry and Grassland Administration, Hangzhou, 321000, Zhejiang, China
c Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing, 100091, China
d College of Forestry, Sichuan Agricultural University, Chengdu Sichuan 611130, China
e College of Forestry, Southwest Forestry University, Kunming Yunnan 650233, China
* Corresponding author, Hua Liu. Research Institute of Forest Resource Information Techniques, Chinese Academy of Forestry, Beijing, 100091, China
Tel: +86-010-6288-9258
Email: liuhua@caf.ac.cn
Biodiversity hotspots: Natural regeneration dynamics of threatenedDacrydium pectinatum communities along various environmental axes on Hainan Island, China
Abstract: Exploring the dynamics of natural regeneration facilitates the understanding of the mechanisms of community assembly and biodiversity formation in tropical forests. However, there is still a lack of convincing evidence related to regeneration, especially for threatened tropical plant communities. Dacrydium pectinatum is a constructive and an endangered species in the tropical mountain forests of Hainan Island. A total of 204 regeneration plots of 5 m × 5 m were investigated along environmental axes of temperature and precipitation in the northwest (Bawangling, 90 plots), southwest (Jianfengling, 90 plots) and southeast (Diaoluoshan, 24 plots) of Hainan Island. We examined the variation in community structure, mortality, density and species richness at the three sites and analyzed the key environmental drivers that affect regeneration. The results showed that the mortality of adults, seedlings and saplings was the lowest in Diaoluoshan, followed by Jianfengling and Bawangling. The peaks in the density and species richness of regenerating individuals were limited to mid-elevations. Elevation, soil total nitrogen, soil available phosphorus, canopy density and adult density were significantly correlated with density and species richness. All findings indicate that at a broad landscape scale, variation in precipitation and temperature due to latitude, longitude and elevation is the dominant cause for the formation of the regeneration dynamic patterns along distinct environmental axes and that the intermediate environmental conditions at middle elevations contribute to regeneration. At the community level, habitat preferences related to elevational factors, soil total nitrogen, soil available phosphorus and forest gaps play a key role in regeneration success. Biological mechanisms (negative density-limiting effects) also have an important effect. We recommend various actions to improve the protection of D. pectinatum , such as the prevention of habitat destruction, appropriate thinning of high-density stands, and strengthening of niche research, and increase biodiversity.
Key words: Biodiversity hotspots; Biodiversity conservation;Dacrydium pectinatum ; Natural regeneration; Hainan Island
1. Introduction
One of the oldest research aims in ecology is exploring the natural regeneration process of woody plants, which facilitates the understanding of species maintenance and assembly mechanisms in communities (Jarcuska 2009; Vizoso-Arribe et al. 2014). It is generally accepted that regeneration plays an important role in the composition, structure, and diversity of plant communities and ultimately determines the ecosystem service functions of forest environments (Miller et al. 2017; Zhu 2017). Nevertheless, the transition from seedling to sapling has been seen as a bottleneck in tree establishment (Yan et al. 2015), and many tree species may experience regeneration failures early in community succession (Collins and Carson 2004; Peña-Claros et al. 2008; Du et al. 2007). As a consequence, exploring the ecological processes that drive natural regeneration and their relative contributions, especially in highly diverse and threatened tropical forest ecosystems, remains a major challenge for ecologists.
In recent years, a number of hypotheses have sought to explain the mechanisms of regeneration. Resource-based niche partitioning is one of the widely discussed mechanisms contributing to regeneration (Satoh et al. 2015; Kitching and Roger 2013) and suggests that species undergo niche partitioning in association with specific abiotic environment combinations (light, soil nutrients, and water) to promote the coexistence and maintenance of species diversity (Chesson 2000; Adler et al. 2007; Jurinitz et al. 2013). Such available resource preferences among species also cause regeneration to exhibit different spatial change patterns along various environmental axes (Bai et al. 2012). Negative density dependence is also an important theory explaining regeneration dynamics (Suter et al. 2010). Under the assumptions of the Janzen-Connell (J-C) hypothesis, a decline in natural enemies and uneven competition for restricted resources might reduce the survival and recruitment of seedlings when they occur in localized areas with high conspecific density (Connell 1978; Janzen 1970). In general, the positive density dependence explained by environmental factors and the negative density dependence represented by the J-C effect may simultaneously play a role in regeneration dynamics. Thus, the contributions of biological factors simultaneously affecting seedling recruitment cannot be ignored when exploring the relationship between environmental factors and regeneration patterns.
Previous research has shown that the natural regeneration of plant communities is closely related to differences in environmental conditions and that saplings are more susceptible to biotic/abiotic factors than adults in old-growth tropical and subtropical forests (Queenborough et al. 2009; Jiang et al. 2016), where both a significant degree of variation and a large degree of overlap in microhabitat suitability among species exist (Kraft et al. 2008; Song et al. 2018). Large numbers of tree species might have specific light, nutrient and topography requirements for regeneration in diverse tropical and subtropical forests (Jin et al. 2018; Laurans et al. 2012). However, there are also studies supporting the idea of incomplete filtering, showing that tree species regeneration is not very sensitive to available resources (Dechnik-Vázquez et al. 2016). Given these contrasting results, there is still a lack of compelling data on the drivers contributing to regeneration, especially for threatened plant communities in tropical forests. Consequently, it is necessary to focus on the regeneration dynamics in plant communities and their driving factors along various environmental axes, particularly in tropical mountain forests on Hainan Island, China, where there has been no distinct conclusion regarding this topic.
Hainan Island in China is floristically rich and has been listed as an internationally significant “hotspot” area for biodiversity conservation (Myers et al. 2000). Based on current records, approximately 4,600 woody plants have been recorded, and 397 of these plants are endemic to the island (Francisco-Ortega et al. 2010).Dacrydium pectinatum is one of the constructive and endangered species in the tropical mountain forests of Hainan Island and belongs to the Podocarpaceae family (Keppel et al. 2011). According to our previous investigation, natural communities shaped by D. pectinatum as a canopy tree species are usually highly diverse. Currently, this species appears to have become extinct in Australia during the Miocene (Norton et al. 1988; Wu et al. 2019). D. pectinatum is the only species of this genus that exists in China, and it is found only in four original tropical rainforests on Hainan Island, namely, Bawangling, Jianfengling, Diaoluoshan and Wuzhishan (Huang et al. 2014). Here, we present a detailed investigation of the composition of seedlings and saplings and environmental factors in D. pectinatum communities along the environmental axes of air temperature and precipitation across three national nature reserves in Bawangling, Diaoluoshan, and Jianfengling on Hainan Island, China. Specifically, we hypothesize that in addition to abiotic factors, such as light and topography, biotic mechanisms, such as negative density dependence, can also affect the regeneration dynamics of the D. pectinatum community. We asked the following questions: (1) How do the dynamics of natural regeneration change along environmental gradients? (2) What drivers play a key role in contributing to regeneration, and can we find a direct or indirect effect on these drivers from niche partitioning and negative density dependence? We also hope that this study will highlight the gaps in biodiversity conservation knowledge regarding endangered plant communities in tropical forests.