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.