4.1 Changes in regeneration dynamics along various environmental
axes
The results showed that the adult and regenerating individuals in
Diaoluoshan had the lowest mortality, followed by those in Jianfengling,
while the highest mortality among the three D. pectinatumcommunities occurred in Bawangling. The peaks in the density and
richness of the regenerative individuals occurred at middle elevations,
and there was no obvious change in terms of an increase or a decrease
along the elevational gradient. Based on the geographical and climatic
factors at the three sites (Table 1), we found that the annual average
precipitation (1806.73 mm) and average temperature (21.89 °C) in
Diaoluoshan in the middle of the elevational gradient (935.97 m) were
significantly higher than those in Jianfengling (annual average
precipitation and annual average temperature of 1520.33 mm and 21.75 °C,
respectively) in the southwest and Bawangling (annual average
precipitation and annual average temperature of 1617.76 mm and 20.74 °C,
respectively) in the northwest. According to these findings, we
speculate that on a broad landscape scale, the distribution of
regenerative individuals in the D. pectinatum community is
related to differences in climatic conditions. Variation in
precipitation and temperature due to latitude, longitude and elevation
may be the dominant cause of the formation of the regeneration dynamic
patterns at the three studied sites. Additionally, Diaoluoshan is
located in the southeast of Hainan Island. It is affected by a marine
climate, which is relatively mild and humid. These superior climatic
conditions provide a greater opportunity for successful regeneration
than the conditions at other sites. In contrast, Jianfengling and
Bawangling, located in the forest hinterland in the southwest and
northwest of Hainan Island, respectively, experience lower precipitation
and higher air humidity, which may be limiting factors affecting
regeneration success; this is especially true for Bawangling, which is
higher in elevation than the other sites.
Our results confirm previous reports; for example, some studies have
shown that the distribution of vegetation is mainly affected by
temperature, precipitation and the combination of both, forming a
horizontal (latitudinal and longitudinal) and vertical distribution
pattern (Peñuelas et al. 2013). Along
meridional gradients, as the distance from the ocean toward inland areas
and the regularity of atmospheric circulation and ocean currents change,
the vegetation distribution is mainly controlled by water conditions
(Crimmins et al. 2011;
Vanderwel et al. 2013;
Vayreda et al. 2016). Precipitation is
considered to be the most important climatic factor affecting tree
regeneration. Relatively high precipitation might increase seedling
density, especially if future temperatures rise, and the evaporation of
the increased surface moisture from the earth may cause a decrease in
soil moisture, which may lead to regeneration failure
(Jump et al. 2006;
Silva et al. 2012). Latitude represents a
complex environmental gradient; across latitudes, there are differences
in photoperiod, temperature, growing season length, moisture and soil
nutrients, which in turn affect the community dynamics and distribution
patterns (McKown et al. 2014). In
addition, intermediate-elevation areas usually show a moderate expansion
phenomenon (Jones et al. 2010;
Alatalo et al. 2017;
Haider et al. 2011), where suitable
temperatures and precipitation are particularly conducive to plant
growth and species diversity. Thus, plant species are likely to show
aggregation phenomena near mid-elevation areas.