Characteristics Of Plant Communities Across The Natural Tropical Coniferous Forest-broadleaved Forest Ecotones In Hainan Island, China

Although natural coniferous forests are mainly distibuted in the temperate and cold tremperateregions on earth,they occupy a proportion of specific areas in the tropics.Natural tropicalconiferous forests play a unique role in maintaining the biodiversity and heterogeneity oftropical forest ecosystems. In the tropical regions of southern China, most of the land isdominated by broadleaved trees and shrubs. However, in some specific places and habitatswhere environmental conditions are stressful for broadleaved trees, a few species of conifersare dominant. Due to their unusual environmental and floristic conditions, tropical coniferousforests often have a distinct community structure and contain flora and fauna that are differentfrom those of adjacent tropical rainforests. The widest distribution of natural tropicalconiferous forest is found on Hainan Island in China, and most of this type of forest is in theBawangling forest region (BFR). Most of the tropical coniferous forest on Hainan Island isdominated by the conifer Pinus latteri. A transition zone exists between conifers andbroadleaved trees of tropical lowland rain forest and tropical monsoon forest species, due toextensive shifting cultivation and logging of the coniferous forest and then invasion bybroadleaved trees into disturbed regions.No studies have explored the communitycharacteristics of the ecotone representing the transition between coniferous forest andbroadleaved forest. In this study, we conducted the field vegetation investigation and measuredthe plant fuctional traits and some environmental factors in19transects, spanning three standzones: the coniferous forest zone (CZ), the ecotone zone (EZ), and the broadleaved forest zone(BZ). We compared the varaitions in environmental condition, speices composition,community structure, community-wide plant fuctional traits, biodiversity (including speciesdiversity and functional diversity) and relationships between environmental factors and thecommunity features across the ecotones. The main results are as follows: 1. As the stand changed from coniferous forest zone to broadleaved forest zone acrossecotone,11environmental factors showed that,(1) soil pH value,soil organic matter content,total N, available N, available P and available K had a trend of increasing; total K, litter depthand canopy openness had a trend of decreasing; Soil water content and soil total P had nosignificant differences among the three stand zones.(2) Principal component analysis for themeasured environmental factors in the three stand zones showed that soil pH value,soil organicmatter content, litter depth, available P, total N and canopy openness were the maindeterminants for the distribution range of the three different stand zones.2. The compositional and structural features of the three stand zones were compared interms of stand factors, size class distribution, growth-form, and species, genus andfamilyrank-abundance distributions. Stem abundance and species richness increased as thevegetation zones changed from CZ through EZ to BZ. In each stand zone, stem abundance andspecies richness declined as the size class increased. The coniferous-broadleaved forest ecotonerevealed compositional and structural features between its adjacent forest stands.3. With the variation of vegetation from CZ through EZ to BZ,community plant functionaltraits of SLA, LDMC, WD, CC, LNC, LPC, LKC and Hmaxchanged in accordance with thechanges in soil and light regimes.(1) CZ had the highest values of Hmax, LDMC and LPC; BZhad the highes values of SLA, CC, LNC and LKC.(2) the low soil nutrients and high light inCZ were the major constraints for most lowland rain forest species with acquisitive traits tosurvive and grow, while high soil nutrients and low light in BZ were the major environmentalfilters for the conifer and tropical monsoon rain forest species with conservative traits. The EZcould afford environmental conditions for species of both strategies. The soil and lightconditions were the major determinants for the functional community structure of thevegetation types across the tropical coniferous-broadleaved forest ecotones.4. From CZ through the EZ to BZ, the eight diveristy indices including biodiveristyindicesand functional diversity indices changes.(1) there was significant difference in sevendiversity indices across three stand zones and showed gradually increasing trend except for Simpson’s diversity index.(2) multiple stepwise regression results showed that species andfunctional diveristy of CZ was strongly influenced by soil pH and available potassium, EZaffected largely by total nitrogen, canopy openness and litter thickness. Soil organic matter,available nitrogen and available potassiumwere the main factors significantly affecting speciesand functional diveristy in BZ.5.Assembly rulesof plant communities across the natural tropical coniferous-broadleavedforest ecotones were explored by using the null model approach.We tested the differencesbetween calculated and null simulated patterns of the distribution in plant functional traits andphylogenetic structure across the ecotones. The results showed that environment filtering wasthe dominant assembly rule in CZ and EZ,whilebiotic competition was the dominant assemblyrule in BZ. As the stand zones changed from CZ through EZ to BZ,community wide plantfunctional traits and phylogenetic structure transferred from cluster to overdispersion.Therelative effects of environmental filtering vs. biotic competition changed in accordance withthe variation of vegetations and their abiotic and biotic conditions across the ecotones.