The Spatial Pattern Distribution Of Four Species Of Apocynaceae In Tropical Montane Rain Forest Of Jianfengling,Hainan Island,China

There are four woody plants of Apocynaceae, including Alstonia rostrata C. E. C. Fisch., Tabernaemontana bufalina Lour., Tabernaemontana bovina Lour. and Rauvolfia verticillata(Lour.) Baill., at the 60 ha(1000 m×600 m) fixed sample plot in the tropical montane rainforest of Jianfengling. We mainly analyzed diameter class structure and point patterns of these four species, interspecific association, and effects of topographicalfactors and density dependence on the distribution of them by using modern point pattern analysis methods to detect the rule of individual distribution and their formation mechanism. The main results obtained are listed as followings:1. Diameter class structure of the four species:diameter at breast height(DBH) class structure of the all recorded individuals of the four woody plants of Apocynaceae generally showed inverse “J” shape, indicating that the whole Apocynaceae plants were successful regeneration and were under stable and normal growth condition. The DBH class structure of Alstonia rostrata showed inverse “J” shape and Tabernaemontana bufalina were “L”shape, both of them had many sapling trees, which indicated that both populations were successful regeneration and were increase types. Tabernaemontana bovina and Rauvolfia verticillata were less widespread and no their seeding in the sample plot, and they had sprouting and branches.2. Spatial patterns and DBH patterns of the two species:The analysis results of pair correlation function[g(r)] showed that spatial patterns of the Alstonia rostrata and Tabernaemontana bufalina were mainly aggregated at small scale, while at large scale the former's spatial pattern was aggregated and the latter became a random pattern. Marked correlation function[kmm(r)] analysis showed that DBH patterns of Alstonia rostrata was a uniform distribution at small scale, and became a random pattern at large scale, while DBH patterns of Tabernaemontana bufalina was a random pattern at 0-100 m scale.3. Spatial patterns of the two species' s different DBH class: The spatial patterns of Alstonia rostrata and Tabernaemontana bufalina changed similarly with the developmentof population. Small DBH class individuals showed aggregation distribution within the larger scale, as the DBH class grew the distribution became random ones at the studying scale. The results of point pattern analysis with heterogeneous Poisson null model ruling out the influence of environmental heterogeneity showed that:The spatial patterns of small DBH class of Alstonia rostrata and Tabernaemontana bufalina became random distribution at large scale. The results verified that environmental heterogeneity was the important factor to shape the species spatial distribution.4. The spatial association between different DBH classes individuals: the relationships between different DBH classes individual of Alstonia rostrata showed that: The relationship between small individuals and large individuals was negative association at small scale, but it became positive at large scale; The relationship between similar small DBH classes individuals was mainly positive association at small scale and become no significant association at large scale; while the relationship between similar large DBH classes individuals was no significant association at the studying scale. In summary, the greater the differences between DBH classes, the greater the impact of larger individuals on the smaller individuals. Positive association were found between the different DBH classes individuals of Tabernaemontana bufalina within the larger scale(0-65 m), which indicated that different DBH classes individuals were consistent with environmental selection.5. Interspecific association: The relationship between Alstonia rostrata and Tabernaemontana bufalina showed positive association at 4-49 m scale and no obvious association at 0-3 m and 50-100 m scale. Alstonia rostrata is a typical companion trees at tropical montane rainforests, while Tabernaemontana bufalina is mainly in shrub layer, the positive interaction between them at a certain scale was conducive to make full uses of the resources, so as to maintain the stable coexistence of species. Compared the relationship between adult tree and saplings or juvenile trees, the relationship between adult tree of Alstonia rostrata and saplings or junior trees of Tabernaemontana bufalina was positive interaction with a larger scale, while with their own saplings or junior trees was no association or negative association at small scale(<30 m), no association or positive association at larger scale(>30 m). This agreed with the Janzen-Connell Hypothesis.6. Topographic factors impact(elevation, slope and concavo-convex degree) on thepopulation distribution of two species: topographic factors also indirectly reflected other habitat factors, such as light, soil and water and so on. The spatial pattern of the Alstonia rostrata and Tabernaemontana bufalina were positively correlated with at least two of the three topographic factors, topographic factors influenced on the spatial patterns of their different DBH class individuals in varying degrees. That indicated topography influenced spatial pattern of Alstonia rostrata and Tabernaemontana bufalina, it means environmental heterogeneity is an important factor for the formation of the population distribution pattern.7. Density-dependent effect: Alstonia rostrata represented density-dependent self-thinning and thinning, while Tabernaemontana bufalina didn't. It indicated that density-dependent effect is also the factor to shape the species spatial distribution in the Tropical Montane Rainforest.In summary, we inferred that dispersal limitation, environmental heterogeneity and density-dependent effect played major roles in the formation of current spatial patterns of Apocynaceae in Tropical Montane Rainforest of Jianfengling.