Deforestation in the tropics is an important source of carbon C release to the atmosphere. To provide a sound scientific base for efforts taken to reduce emissions from deforestation and degradation (REDD+) good estimates of C stocks and fluxes are important. We present components of the C balance for selectively logged lowland tropical dipterocarp rainforest in the Malua Forest Reserve of Sabah, Malaysian Borneo. Total organic C in this area was 167.9 Mg C ha⁻¹±3.8 (SD), including: Total aboveground (TAGC: 55%; 91.9 Mg C ha⁻¹±2.9 SEM) and belowground carbon in trees (TBGC: 10%; 16.5 Mg C ha⁻¹±0.5 SEM), deadwood (8%; 13.2 Mg C ha⁻¹±3.5 SEM) and soil organic matter (SOM: 24%; 39.6 Mg C ha⁻¹±0.9 SEM), understory vegetation (3%; 5.1 Mg C ha⁻¹±1.7 SEM), standing litter (<1%; 0.7 Mg C ha⁻¹±0.1 SEM) and fine root biomass (<1%; 0.9 Mg C ha⁻¹±0.1 SEM). Fluxes included litterfall, a proxy for leaf net primary productivity (4.9 Mg C ha⁻¹ yr⁻¹±0.1 SEM), and soil respiration, a measure for heterotrophic ecosystem respiration (28.6 Mg C ha⁻¹ yr⁻¹±1.2 SEM). The missing estimates necessary to close the C balance are wood net primary productivity and autotrophic respiration.Twenty-two years after logging TAGC stocks were 28% lower compared to unlogged forest (128 Mg C ha⁻¹±13.4 SEM); a combined weighted average mean reduction due to selective logging of -57.8 Mg C ha⁻¹ (with 95% CI -75.5 to -40.2). Based on the findings we conclude that selective logging decreased the dipterocarp stock by 55-66%. Silvicultural treatments may have the potential to accelerate the recovery of dipterocarp C stocks to pre-logging levels.
Overhunting reduces important plant-animal interactions such as vertebrate seed dispersal and seed predation, thereby altering plant regeneration and even above-ground biomass. It remains unclear, however, if non-hunted species can compensate for lost vertebrates in defaunated ecosystems. We use a nested exclusion experiment to isolate the effects of different seed enemies in a Bornean rainforest. In four of five tree species, vertebrates kill many seeds (13-66%). Nonetheless, when large mammals are excluded, seed mortality from insects and fungi fully compensates for the lost vertebrate predation, such that defaunation has no effect on seedling establishment. The switch from seed predation by generalist vertebrates to specialist insects and fungi in defaunated systems may alter Janzen-Connell effects and density-dependence in plants. Previous work using simulation models to explore how lost seed dispersal will affect tree species composition and carbon storage may require reevaluation in the context of functional redundancy within complex species interactions networks.
Stable carbon isotope (δ(13)C) series were developed from analysis of sequential radial wood increments from AD 1850 to AD 2009 for four mature primary rainforest trees from the Danum and Imbak areas of Sabah, Malaysia. The aseasonal equatorial climate meant that conventional dendrochronology was not possible as the tree species investigated do not exhibit clear annual rings or dateable growth bands. Chronology was established using radiocarbon dating to model age-growth relationships and date the carbon isotopic series from which the intrinsic water-use efficiency (IWUE) was calculated. The two Eusideroxylon zwageri trees from Imbak yielded ages of their pith/central wood (±1 sigma) of 670 ± 40 and 759 ± 40 years old; the less dense Shorea johorensis and Shorea superba trees at Danum yielded ages of 240 ± 40 and 330 ± 40 years, respectively. All trees studied exhibit an increase in the IWUE since AD 1960. This reflects, in part, a response of the forest to increasing atmospheric carbon dioxide concentration. Unlike studies of some northern European trees, no clear plateau in this response was observed. A change in the IWUE implies an associated modification of the local carbon and/or hydrological cycles. To resolve these uncertainties, a shift in emphasis away from high-resolution studies towards long, well-replicated time series is proposed to develop the environmental data essential for model evaluation. Identification of old (greater than 700 years) ringless trees demonstrates their potential in assessing the impacts of climatic and atmospheric change. It also shows the scientific and applied value of a conservation policy that ensures the survival of primary forest containing particularly old trees (as in Imbak Canyon and Danum).
Sustainable forest management (SFM), which has been recently introduced to tropical natural production forests, is beneficial in maintaining timber resources, but information about the co-benefits for biodiversity conservation and carbon sequestration is currently lacking.
One of the main environmental threats in the tropics is selective logging, which has degraded large areas of forest. In southeast Asia, enrichment planting with seedlings of the dominant group of dipterocarp tree species aims to accelerate restoration of forest structure and functioning. The role of tree diversity in forest restoration is still unclear, but the 'insurance hypothesis' predicts that in temporally and spatially varying environments planting mixtures may stabilize functioning owing to differences in species traits and ecologies. To test for potential insurance effects, we analyse the patterns of seedling mortality and growth in monoculture and mixture plots over the first decade of the Sabah biodiversity experiment. Our results reveal the species differences required for potential insurance effects including a trade-off in which species with denser wood have lower growth rates but higher survival. This trade-off was consistent over time during the first decade, but growth and mortality varied spatially across our 500 ha experiment with species responding to changing conditions in different ways. Overall, average survival rates were extreme in monocultures than mixtures consistent with a potential insurance effect in which monocultures of poorly surviving species risk recruitment failure, whereas monocultures of species with high survival have rates of self-thinning that are potentially wasteful when seedling stocks are limited. Longer-term monitoring as species interactions strengthen will be needed to more comprehensively test to what degree mixtures of species spread risk and use limited seedling stocks more efficiently to increase diversity and restore ecosystem structure and functioning.
Hyperdiverse tropical rainforests, such as the aseasonal forests in Southeast Asia, are supported by high annual rainfall. Its canopy is dominated by the species-rich tree family of Dipterocarpaceae (Asian dipterocarps), which has both ecological (e.g., supports flora and fauna) and economical (e.g., timber production) importance. Recent ecological studies suggested that rare irregular drought events may be an environmental stress and signal for the tropical trees. We assembled the genome of a widespread but near threatened dipterocarp, Shorea leprosula, and analyzed the transcriptome sequences of ten dipterocarp species representing seven genera. Comparative genomic and molecular dating analyses suggested a whole-genome duplication close to the Cretaceous-Paleogene extinction event followed by the diversification of major dipterocarp lineages (i.e. Dipterocarpoideae). Interestingly, the retained duplicated genes were enriched for genes upregulated by no-irrigation treatment. These findings provide molecular support for the relevance of drought for tropical trees despite the lack of an annual dry season.