Stemonoporus Thw. (Dipterocarpaceae), a genus endemic to Sri Lanka, contains as many as 26 species, all but one confined to the perhumid forests of the island. Several species reach 1800 m, the ...highest altitude recorded for any dipterocarp species in Sri Lanka. We report the first quantitative data on Stemonoporus-domirated stands in the montane zone (1500–1700 m). Stemonoporus cordifolius and 5. rigidus show clear single-species dominance of their respective stands. Stemonoporus gardneri is a co-dominant with Palaquium rubiginosum in its two stands. All Stemonoporus species appear to have a unique geographical and ecological range and no two Stemonoporus species co-exist in our stands.
Symbiotic nitrogen (N)‐fixing trees can provide large quantities of new N to ecosystems, but only if they are sufficiently abundant. The overall abundance and latitudinal abundance distributions of ...N‐fixing trees are well characterised in the Americas, but less well outside the Americas.
Here, we characterised the abundance of N‐fixing trees in a network of forest plots spanning five continents, ~5,000 tree species and ~4 million trees. The majority of the plots (86%) were in America or Asia. In addition, we examined whether the observed pattern of abundance of N‐fixing trees was correlated with mean annual temperature and precipitation.
Outside the tropics, N‐fixing trees were consistently rare in the forest plots we examined. Within the tropics, N‐fixing trees were abundant in American but not Asian forest plots (~7% versus ~1% of basal area and stems). This disparity was not explained by mean annual temperature or precipitation. Our finding of low N‐fixing tree abundance in the Asian tropics casts some doubt on recent high estimates of N fixation rates in this region, which do not account for disparities in N‐fixing tree abundance between the Asian and American tropics.
Synthesis. Inputs of nitrogen to forests depend on symbiotic nitrogen fixation, which is constrained by the abundance of N‐fixing trees. By analysing a large dataset of ~4 million trees, we found that N‐fixing trees were consistently rare in the Asian tropics as well as across higher latitudes in Asia, America and Europe. The rarity of N‐fixing trees in the Asian tropics compared with the American tropics might stem from lower intrinsic N limitation in Asian tropical forests, although direct support for any mechanism is lacking. The paucity of N‐fixing trees throughout Asian forests suggests that N inputs to the Asian tropics might be lower than previously thought.
Inputs of nitrogen to forests depend on symbiotic nitrogen fixation, which is constrained by the abundance of N‐fixing trees. By analysing a large dataset of ~4 million trees, we found that N‐fixing trees were consistently rare in the Asian tropics as well as across higher latitudes in Asia, America and Europe. The rarity of N‐fixing trees in the Asian tropics compared with the American tropics might stem from lower intrinsic N limitation in Asian tropical forests, although direct support for any mechanism is lacking. The paucity of N‐fixing trees throughout Asian forests suggests that N inputs to the Asian tropics might be lower than previously thought.
The pollination biology of two sympatric species,Polyalthia coffeoidesandPolyalthia korinti(Annonaceae), is described in detail. AnEndaeusspecies (Coleoptera: Curculionidae) is shown to be the major ...pollinator of both species, withCarpophilus plagiatipennis(Coleoptera: Nitidulidae) as the secondary pollinator ofP. coffeoides. BothPolyalthiaspecies show intrafloral dichogamy (protogyny) with a reproductively inactive phase between the pistillate and staminate phases, although there is no evidence of interfloral dichogamy. A pollination chamber is formed by the inner petals throughout the reproductively active phases. Thermogenesis occurs inP. korinti, with internal floral temperatures up to 6°C above ambient levels. The heat is presumably an energy reward for the beetles. Although most pollination systems are regarded as diversified and opportunistic, specialized pollination systems are typical of the Annonaceae. AlthoughP. coffeoidesandP. korintihave overlapping distributions, habitats, and flowering seasons and share the same pollinators, the extent of competition for pollinators is likely to be lessened due to the abundance and nonspecificity of the beetles.
The breeding systems of two sympatric species,Polyalthia coffeoidesandPolyalthia korinti(Annonaceae), are assessed using a range of approaches, including controlled pollination experiments and ...analysis of intersimple sequence repeat markers within and between populations. Natural (open) pollination resulted in similar levels of fruit formation as artificial cross‐pollination, suggesting that pollinator availability is not a limiting factor in reproduction. Both species possess facultatively xenogamous breeding systems, with 33%–36% fruit formation in artificially crossed experiments and 17%–19% fruit formation following geitonogamy. Reduced fruit set following geitonogamy suggests partial self‐incompatibility; this is supported by index of self‐incompatibility values of ca. 0.5 in both species. Analysis of population genetic structure supports the hypothesis of a mixed but largely xenogamous mating system. Genetic diversity within populations was estimated to be ca. 80% and 66% of total genetic diversity forP. coffeoidesandP. korinti, respectively. The levels of gene flow between populations were moderate (
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forP. coffeoidesand 0.970 forP. korinti), and genetic identity (I) values between populations within species were high. This possibly reflects the fragmentation of a previously more extensive population, correlated with the historical deforestation associated with crop cultivation and irrigation in Sri Lanka.
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