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Aims: Evergreen and deciduous forests can coexist at different spatial scales in the seasonally dry tropics of Southeast Asia. At present, the main drivers for their differences in soil carbon (C) stock remains unclear. We aim to characterize the patterns of C and nitrogen (N) isotope ratios in soil profiles of two forest types to get a better understanding of how soil organic matter (SOM) pools are maintained under different types of forest ecosystem. Methods: We analyzed the coupled trends in the natural abundances of C and N isotopes (δ13C and δ15N) in leaf litter, and mineral soil and its density fractions, collected from eight sites in Cambodian tropical forests. Results: The mean value of the beta coefficient, which was calculated as the slope of the regression line for δ13C values against the logarithm of the C concentration and can serve as a natural indicator for SOM turnover, was − 1.9, while the Δ15N, which was the difference in δ15N between the leaf litter and mineral soil at 15 − 30 cm depth, was 6.2‰. Contrary to our expectations, these indexes did not differ significantly between evergreen and deciduous forest types. Conclusions: The absence of clear difference in beta coefficients and Δ15N values between forest types was considered as derived from counteracting effects of several factors, e.g., tree biomass, precipitation and soil mineralogy, associated with multiple processes of C and N cycles. We especially highlighted the significant role of soil acidity in characterizing SOM cycles in seasonally dry tropical forests. [ABSTRACT FROM AUTHOR]

We have reviewed publications on the physiological and ecological features of the growth and regeneration processes of Juniperus sabina L. which grows in semiarid sandy land in the Ordos plateau in northern China where desertification has progressed over time. J. sabina is a key native plant species used for ecological restoration in this region. The life history of J. sabina in this sandy land that has been revealed through this review includes several unique features: (1) both vegetative and seed propagations are observed, but seed propagation is not successful in the location where the mature J. sabina stands. Instead, seed propagation can occur at a different place with different landscapes from the mature stands. (2) Nurse plants play an important role in providing the microclimatic environment necessary for the growth of J. sabina seedlings and young plants. (3) During the horizontal and vertical growth processes of the J. sabina patch, the root system was affected by burial in shifting sand and consequently acquired greater access to the water supply in deeper soil horizons, which could support larger growth. These characteristics suggested that the regeneration by seed propagation and growth strategy of J. sabina in this region was strongly affected by sand movement and the landscape that is generated by sand movement. [ABSTRACT FROM AUTHOR]

The genus Aquilaria (Thymelaeaceae) is widely used for the production of agarwood, and includes several valuable plantation species in Southeast Asia. To understand appropriate planting conditions, it is necessary to characterize species-specific ecological traits such as photosynthesis and drought tolerance among Aquilaria species. To this end, we measured leaf morphological and photosynthetic traits among three Aquilaria seedlings (A. hirta, A. malaccensis, A. subintegra) in Malaysia. We also monitored changes in growth and survival in a shaded nursery and 7 and 18 months after transfer to open conditions. To avoid transplant stress, the seedlings were kept in polybags. The highest leaf cuticle and epidermis layer ratio, leaf mass per area, and long-term water use efficiency indicated by δ13C were recorded in A. hirta, suggesting that this species had relatively high drought tolerance. By contrast, the high maximum photosynthetic rate and high stomatal conductance observed in A. subintegra explained their high growth rate, although their drought tolerance was the lowest among the studied species. A. malaccensis was considered an intermediate species in terms of photosynthesis and drought tolerance. Interspecific differences in growth and survival were present after transfer to open conditions. A. hirta had the lowest growth rate and highest survival rate (50% survival after 18 months), whereas A. subintegra had the highest growth rate and lowest survival rate (9% survival after 18 months). A. malaccensis was intermediate between the two. In summary, the studied Aquilaria species were susceptible to stress under the open and dry conditions of our study. This includes Aquilaria hirta, the most drought-tolerant species in this group, and A. subintegra, the most vulnerable species to drought, which may require careful management when planted in open conditions. [ABSTRACT FROM AUTHOR]

Key message : Large variations in leaf δ N in Bornean tropical rainforest trees may indicate that various tropical species have species-specific strategy for nitrogen uptake under low soil nutrient conditions, including root symbiotic microorganisms such as ectomycorrhiza. Abstract: Lowland tropical rainforests in Southeast Asia are characterized by high species diversity despite limited soil nutrient conditions. The plant nitrogen isotope ratio (δN) reflects plant uptake of soil nitrogen. We analyzed δN values and nitrogen content (N %) in leaves and roots of 108 woody species with different types of symbiotic microorganisms, of different life forms (emergent, canopy, sub-canopy, understory, and canopy gap species), and from different families in a Bornean lowland dipterocarp forest to gain more insight into the diversity of nitrogen uptake strategy in the rhizosphere. Leaf δN values in the species studied varied largely from −7.2 to 5.0 ‰, which is comparable to the values of known Asian trees including temperate, sub-tropical, and tropical mountain forests. Leaf δN also varied significantly among both life forms and families, though the phylogenetically independent contrast (PIC) relationships were not statistically significant among life form, family, and symbiotic types. Some families showed specific leaf δN values; Dipterocarpaceae, the dominant family in the canopy layer with symbiotic ectomycorrhiza in Southeast Asia, had small intraspecific variation and higher leaf δN values (0.03 ‰) compared with species exhibiting arbuscular mycorrhiza, whereas several families such as Burseraceae, Euphorbiaceae, and Myrtaceae showed large interspecific variation in leaf δN (e.g., from −7.2 to 5.0 ‰ in Euphorbiaceae). These variations suggest that tropical species may have family- or species-specific strategy, such as root symbiotic microorganisms, for nitrogen uptake under low-nutrient conditions in tropical rainforests in Southeast Asia. [ABSTRACT FROM AUTHOR]

Knowledge of variations in morphophysiological leaf traits with forest height is essential for quantifying carbon and water fluxes from forest ecosystems. Here, we examined changes in leaf traits with forest height in diverse tree species and their role in environmental acclimation in a tropical rain forest in Borneo that does not experience dry spells. Height-related changes in leaf physiological and morphological traits [e.g., maximum photosynthetic rate ( A), stomatal conductance (gs), dark respiration rate ( R), carbon isotope ratio ( δC), nitrogen (N) content, and leaf mass per area (LMA)] from understory to emergent trees were investigated in 104 species in 29 families. We found that many leaf area-based physiological traits (e.g., A, R, gs), N, δC, and LMA increased linearly with tree height, while leaf mass-based physiological traits (e.g., A) only increased slightly. These patterns differed from other biomes such as temperate and tropical dry forests, where trees usually show decreased photosynthetic capacity (e.g., A, A) with height. Increases in photosynthetic capacity, LMA, and δC are favored under bright and dry upper canopy conditions with higher photosynthetic productivity and drought tolerance, whereas lower R and LMA may improve shade tolerance in lower canopy trees. Rapid recovery of leaf midday water potential to theoretical gravity potential during the night supports the idea that the majority of trees do not suffer from strong drought stress. Overall, leaf area-based photosynthetic traits were associated with tree height and the degree of leaf drought stress, even in diverse tropical rain forest trees. [ABSTRACT FROM AUTHOR]

Macaranga myrmecophytes (ant-plants) provide their partner symbiotic ants (plant-ants) with food bodies as their main food, and they are protected by the plant-ants from herbivores. The amount of resource allocated to food bodies determines the plant-ant colony size and consequently determines the intensity of ant defense (anti-herbivore defense by plant-ants). As constraints in resource allocation change as plants grow, the plant-ant colony size is hypothesized to change with the ontogenesis of Macaranga myrmecophyte. To determine the ontogenetic change in the relative size of the plant-ant colony, we measured the dry weights of the whole plant-ant colony and all of the aboveground parts of trees at various ontogenetic stages for a myrmecophytic species ( Macaranga beccariana) in a Bornean lowland tropical rain forest. Ant biomass increased as plant biomass increased. However, the rate of increase gradually declined, and the ant biomass appeared to reach a ceiling once trees began to branch. The ant/plant biomass ratio consistently decreased as plant biomass increased, with the rate of decrease gradually accelerating. We infer that the ontogenetic reduction in ant/plant biomass ratio is caused by an ontogenetic change in resource allocation to food rewards for ants related to the physiological changes accompanying the beginning of branching. [ABSTRACT FROM AUTHOR]