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.