Abstract
In South Asia, south-west Sri Lanka alone represents the sole surviving perhumid region where Mixed Dipterocarp Forests (MDF) harbour a number of relict signature taxa providing evidence of ancient plant migration routes from Gondwana to perhumid Far Eastern Sunda land, at least partially via southern Laurasia. Palynological and phylogenetic evidence now confirm that the Malesian rain forest flora overwhelmingly immigrated from tropical Africa/Madagascar and South Asia along an ever-wet equatorial corridor sometime before the collision of the Indian Plate with the Southern Laurasian coast in middle Eocene (c. 45 Ma). This ever-wet climatic and geological history has led to the evolution of a characteristic species composition and dynamics within SW Sri Lanka’s rain forest communities, which exhibit remarkable patterns of species distribution and habitat specialization, and which parallel those of Far Eastern sister taxa. We here examine the patterns of floristic variation in each of three MDF sites using classification and ordination methods to analyse tree data collected in 62 random plots, each 0.25ha, along an altitudinal gradient. In addition, a 25 ha forest dynamics plot (FDP) was established in the Sinharaja World Heritage Site, within the CTFS-ForestGEO network that addresses issues related to community ecology and phylogenetics incorporating large-scale biogeographic patterns of major clades of both plants and animals. In the above studies, ecologically distinct floristic assemblages were revealed, i) among the three MDF sites as well as, ii) among the ridge-, slope-, and valley habitats within each forest site. In the FDP too, >80% of the 125 species, with individuals >1 cm dbh and >100 individuals per species, are significantly more associated with one or more of eight topographic habitat categories than in others. These results suggest that ecological ranges and dispersion of tree species in SW Sri Lanka is primarily mediated by soil water and nutrient levels in topographically different catenal habitats mirroring those in the Far East. Many of these species are threatened relict endemics, and their distribution patterns have important conservation implications. Their spatial distributional features provide useful criteria in site-species matching or environmental filtering in forest restoration efforts.
Highlights
SouthAsia represents the surviving part of a continental plate which initially rifted from east Africa of the Gondwanan megacontinent as the Antarctica-India-SeychellesMadagascar plate in the late Jurassic period c.150 Ma
In the forest dynamics plot (FDP) too, >80% of the 125 species, with individuals >1 cm dbh and >100 individuals per species, are significantly more associated with one or more of eight topographic habitat categories than in others. These results suggest that ecological ranges and dispersion of tree species in SW Sri Lanka is primarily mediated by soil water and nutrient levels in topographically different catenal habitats mirroring those in the Far East
Shallow seas and freshwater peat swamps covered central areas, successively from 25 ̊ to 10 ̊N. Their fossil wood and pollen reveals a southern retreat of an already rich rain forest tree flora, similar to that of the modern hyper-diverse Sunda mixed dipterocarp forest, which is confined in South Asia to a tiny, 10,000 km2 patch in SW Sri Lanka, which itself must anticipate a final demise over future geological time (Ashton, 2014; Conti et al, 2002; Morley, 2000; Prasad et al, 2009, Rust et al, 2010) (Figure 1)
Summary
SouthAsia represents the surviving part of a continental plate which initially rifted from east Africa of the Gondwanan megacontinent as the Antarctica-India-SeychellesMadagascar plate in the late Jurassic period c.150 Ma. The nearest lowland regions with similar perhumid climate are found in the Malesian biogeographic region to the east and in the Neotropics (in a small area of the upper Amazon near the Andean foothills, the Choco region of the northwestern coast of South America, and in the narrow coastal regions of Central America facing the Caribbean) to the west Such a perhumid climate does not exist in present day lowland Africa, but prevails at higher altitudes in Central Africa (Ashton, 2014; Feddema, 2005). These perhumid climates in both Asian and Neotropical lowlands support the most species-rich terrestrial ecosystems of the world their tropical rain forests
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