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Fungal decay of heartwood creates hollows and areas of reduced wood density within the stems of living trees known as heart rot. Although heart rot is acknowledged as a source of error in forest aboveground biomass estimates, there are few datasets available to evaluate the environmental controls over heart rot infection and severity in tropical forests. Using legacy and recent data from drilled, felled, and cored stems in mixed dipterocarp forests in Sarawak, Malaysian Borneo, we quantified the frequency and severity of heart rot, and used generalized linear mixed effect models to characterize the association of heart rot with tree size, wood density, taxonomy, and edaphic conditions. Heart rot was detected in 55% of felled stems > 30 cm DBH, while the detection frequency was lower for stems of the same size evaluated by non-destructive drilling (45 %) and coring (23 %) methods. Heart rot severity, defined as the percent stem volume lost in infected stems, ranged widely from 0.1-82.8 %. Tree taxonomy explained the greatest proportion of variance in heart rot frequency and severity among the fixed and random effects evaluated in our models. Heart rot frequency, but not severity, increased sharply with tree diameter, ranging from 56% infection across all datasets in stems > 50 cm DBH to 11% in trees 10-30 cm DBH. The frequency and severity of heart rot increased significantly in soils with low pH and cation concentrations in topsoil, and heart rot was more common in tree species associated with dystrophic sandy soils than with nutrient-rich clays. When scaled to forest stands, the percent of stem biomass lost to heart rot varied significantly with soil properties, and we estimate that 7% of the forest biomass is in some stage of heart rot decay. This study demonstrates not only that heart rot is a significant source of error in forest carbon estimates, but also that it strongly covaries with soil resources, underscoring the need to account for edaphic variation in estimating carbon storage in tropical forests. [ABSTRACT FROM AUTHOR]

Abstract: Mixed dipterocarp forests are perhaps the single most important rain forest type in the wet tropics. Only a few studies have purposefully examined differences in resource availability across mixed dipterocarp forest landscapes by simply measuring the abiotic variables of light, soil nutrition and soil water availability in relation to forest structure. We sought to directly measure the environment of canopy gaps across elevation and geology—from lowland mixed dipterocarp forest (100m amsl) to lower montane dipterocarp forest (1200m amsl) in southwest Sri Lanka. Middle elevation gap sites (300–900m amsl) were subdivided into valley, mid-slope and ridge topographic positions. Eighteen natural disturbances all of which were canopy openings caused by tree fall, were randomly selected within primary rain forest that ranged across 100–1200m elevation. Plots were placed in gap centers and in adjacent understories and measurements taken of forest structure (basal area, canopy height, canopy cover index, CCI), shade (light sensors—photosynthetically active radiation [PAR], canopy hemispherical photographs—global site factor [GSF]) and soil nutrition (pH, exchangeable Al, K, Mg and Ca; Total N; and plant available P). Soil moisture was measured at bi-weekly intervals for five years across middle elevation sites only (300–900m amsl). Stand basal area, mean canopy height, and canopy cover index all declined with increase in elevation. Understory PAR and GSF decreased with increases in canopy height, basal area and CCI. Size of canopy opening decreased with increase in elevation, but PAR and GSF increased. Valley sites had significantly greater levels of mean percent soil water content as compared to mid-slope and ridge sites of middle elevation sites. However, at the onset of the southwest monsoons in May all sites were similar. Differences were most pronounced during the dry season (December–April). No differences in soil moisture content could be found between gap and understory microsites. K and Ca in gap centers and adjacent forest understories increased with increase in elevation and change in associated geology. pH increased and Al decreased with elevation and associated geology but only for forest understory conditions. Results demonstrate strong differentiation in soil and light resources with elevation that appears related to size of tree-fall disturbance, stature of the forest, topographic position and underlying geology and soil-weathering environment. This suggests that forest management and conservation practices need to develop and tailor techniques and treatments (silviculture) to the forest that emulate and/or account for change in elevation, geology and topographic position. Further studies are needed to identify which are the primary underlying mechanisms (e.g. temperature, wind, soil nutrients, soil moisture availability) defining change in forest structure across elevation. [Copyright &y& Elsevier]

Dipterocarp forests of the Asian wet tropics have a long history of silvicultural research. This paper provides a review of this history and a summary of the ecological principles guiding the regeneration methods used. Dipterocarp forests are here defined as those of the seasonally wet regions of Thailand, Burma, and India, and those that are considered of the mixed dipterocarp forest type that dominate the aseasonal wet regions of Sri Lanka, Malaysia, and parts of Indonesia and the Philippines. Two silvicultural regeneration methods are described, shelterwoods and their variants, and selection systems. Both systems can be justified but emphasis is given to the development of shelterwood and selection regeneration methods that are tailored to the particular biological and social context at hand. The paper concludes with a call for improved land-use planning and stand typing to better integrate service and protection values with those values focused on commodity production. [ABSTRACT FROM AUTHOR]

In the last 10 years the Sri Lankan government has changed its policy regarding its remaining rain forest from one that promoted commercial exploitation to one of conservation. The growing importance of uplands as catchments for water production, biodiversity conservation and other downstream services has been recognized by the Sri Lankan government. It is therefore timely that we review 15 years of research investigating rain forest dynamics of southwest Sri Lanka with the objective of using this knowledge for forest restoration. We provide six common principles for understanding the integrity of rain forest dynamics in southwest Sri Lanka. The principles are: (i) disturbances provide the simultaneous initiation and/or release of a new forest stand; (ii) that disturbances are generally non-lethal to the groundstory vegetation; (iii) disturbances are variable in severity, type and extent across rain forest topography; (iv) guild diversity (habitat diversity) is dependent upon "advance regeneration"; (v) tree canopy stratification is based on both "static" and "dynamic" processes; and (vi) canopy dominant late-successional tree species are site specialists restricted to particular topographic positions of the rain forest. These principles are applied to determine effects of two rain forest degradation processes that have been characterized as chronic (continuous detrimental impacts) and acute (one-time detrimental impacts). Restoration pathways are suggested that range from: (i) the simple prevention of disturbance to promote release of rain forest succession; (ii) site-specific enrichment planting protocols for canopy trees; (iii) sequential amelioration of arrested fern and grasslands by use of plantation analogs of old field pine to facilitate secondary succession of rain forest, and plantings of late-seral rain forest tree species; and (iv) establishment and release of successionally compatible mixed-species plantations. We summarize with a synthesis of the... [ABSTRACT FROM AUTHOR]

The supra-annual mast fruiting of dipterocarps, dominant trees of Far Eastern lowland rain forests, has been considered an adaptation to reduce seed predation through satiation. However, this explanation raises four important questions. (1) What pollinates species that flower at intervals of more than one year? (2) How do mass-flowering species avoid competition for pollinators? (3) What is the environmental cue for floral induction that induces scores of species in several families to flower at irregular intervals of more than one year in an aseasonal climate? (4) And, most important, what factors could cause an initial phenological aggregation of species' fruiting times? Once such a phenological core of species exists, selection for predator satiation could entrain other species into the same fruiting schedule and perfect the synchrony of seed release. The essential question is how such a phenological core of species, flowering at irregular intervals in an aseasonal climate, could initially arise. The observations and analyses presented here indicate that the answers to these questions are intricately related to each other and to the El Nino-Southern Oscillation phenomenon. (1) Several mass-flowering dipterocarps are pollinated by thrips, which persist at low levels between mass flowerings and can explode in density as the dipterocarps come into flower. (2) Mass-flowering dipterocarp species in Shorea section Mutica significantly reduce the overlap in flowering times by staggering their flowering periods, thereby reducing competition for pollinators and/or the clogging of stigmas with foreign pollen. The new technique used to analyze whether a particular pattern of resource use significantly reduces overlap overcomes an error inherent in previous techniques, and it should be used whenever the total flowering season or range of resources is determined not by external conditions, but by the range of flowering times or resources used by the species in question. (3) The time of floral induction in Shorea section Mutica is derived using the linear relationship between the duration of each species' flowering period and mean flowering date. An analysis of meteorological records suggests that induction is caused by a drop of roughly 2$^\circ$C or more in minimum nighttime temperature for three or more nights. This extraordinary trigger is probably the most reliable signal associated with an invasion of the aseasonal tropics by a dry air mass. (4) Such a trigger does not appear to be directly adaptive in the aseasonal tropics, but in the seasonal tropics it would result in flowering at the onset of dry season and ripening fruits at the beginning of rainy season. This suggests that the Dipterocarpaceae may have arisen initially in the seasonal tropics, even though their center of species diversity is now in the aseasonal tropics. Invasion of the aseasonal tropics by trees having such a floral trigger, adapted to the seasonal tropics, could help explain the origin of a phenological core of species flowering and fruiting more or less synchronously at intervals of several years. The droughts causing these mass flowerings may, in some cases, be driven by the climatic effects of the El Nino-Southern Oscillation phenomenon in the western Pacific, as evidenced by the temporal association of El Nino years with drought and mast fruiting on windward slopes of tropical Malesia. [ABSTRACT FROM AUTHOR]