Abdul Rahim Nik

The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula

The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula is revealed through field survey and meteorological data analyses. The regions of general flowering coincide with those which experienced a low night-time temperature (LNT) c. 2 mo before flowering. This supports the hypo- thesis that low air temperature induces the development of floral buds of diptero- carps. LNT was found to be caused by radiative cooling during dry spells in winter when the northern subtropical ridge (STR) occasionally migrates southwards with a dry air mass into the equatorial region. LNT events usually occur in La Nifia episodes, not in El Nifio episodes as believed previously. This is because the south- ward migration of the STR is associated with the intensification of local meridional Hadley Circulation in the western Pacific, which is strengthened in a La Nifia episode. Results suggest that El Nifio-like climate change in increased atmospheric carbon dioxide concentrations may be critical for the tropical rain forest biome in south-east Asia.

Soil physical properties and preferential flow pathways in tropical rain forest, Bukit Tarek, Peninsular Malaysia

Soil physical properties and water movement within soil were investigated using dyes in a tropical rain forest, the Bukit Tarek Experimental Watershed of Peninsular Malaysia. The saturated hydraulic conductivity (Ks) decreased with increasing soil depth. TheKs values were higher than those reported for other tropical soils. The geometric means of theKs values ranged from 4.69×10−3 (80 cm) to 4.07×10−2 cm s−1 (10cm). This suggests saturation overland flow may not be dominant but that subsurface flow must play an important role in stormflow generation. The shapes of the soil moisture characteristic curves resembled those of forest soils which have large changes in volumetric water content at pressure heads <30 cmH2O. The relatively high conductivities were due to the presence of a porous zone of decomposed root channels which existed continuously in vertical direction. Besides decayed roots, living roots also encourage preferential flow in vertical and lateral (downslope) directions. Termite activities may also form water flow pathways in tropical regions. These detailed results help us analyze water flow within the soil in tropical rain forests.

Rainfall-runoff responses and roles of soil moisture variations to the response in tropical rain forest, bukit tarek, peninsular Malaysia

Field observations were conducted at Bukit Tarek Experimental Watershed in Peninsular Malaysia to investigate the relationship between rainfall-runoff responses and variation in soil moisture in a tropical rain forest. Stormflow depended strongly on the antecedent wetness as represented by the initial runoff rate. Though heavy rains fell in almost every month, the soil moisture decreased when fair weather was sustained. The soil moisture depleted and became dry at 160 cm depth during occasional dry spells. During dry conditions, streamflow responded quickly to rain events but declined rapidly after the rain stopped, and the soil moisture of surface soil (≤20 cm) increased but remained dry at lower depths (≽80 cm). This suggests that the rain water was mostly retained in the soil and only small proportions appeared as stormflow. As soil moisture conditions became wetter, the recession limb of the storm hydrograph was more gradual. Stormflow volume increased with increasing soil moisture. During wet conditions, the soil profile was moist at all parts of the slope. The hydraulic gradient was around 1.0 and there was downward soil water flux, which followed the pressure gradient. This suggests that subsurface flow from the upper part of the slope might also be important for stream-flow production. Positive pressures were observed at 10 cm and 160 cm depths during large storms. The behavior of the subsurface flow might be an important determinant of stormflow.

Patterns of root respiration rates and morphological traits in 13 tree species in a tropical forest

The root systems of forest trees are composed of different diameters and heterogeneous physiological traits. However, the pattern of root respiration rates from finer and coarser roots across various tropical species remains unknown. To clarify how respiration is related to the morphological traits of roots, we evaluated specific root respiration and its relationships to mean root diameter (D) of various diameter and root tissue density (RTD; root mass per unit root volume; gcm(-3)) and specific root length (SRL; root length per unit root mass; mg(-1)) of the fine roots among and within 14 trees of 13 species from a primary tropical rainforest in the Pasoh Forest Reserve in Peninsular Malaysia. Coarse root (2-269mm) respiration rates increased with decreasing D, resulting in significant relationships between root respiration and diameter across species. A model based on a radial gradient of respiration rates of coarse roots simulated the exponential decrease in respiration with diameter. The respiration rate of fine roots (<2mm) was much higher and more variable than those of larger diameter roots. For fine roots, the mean respiration rates for each species increased with decreasing D. The respiration rates of fine roots declined markedly with increasing RTD and increased with increasing SRL, which explained a significant portion of the variation in the respiration among the 14 trees from 13 species examined. Our results indicate that coarse root respiration in tree species follows a basic relationship with D across species and that most of the variation in fine root respiration among species is explained by D, RTD and SRL. We found that the relationship between root respiration and morphological traits provides a quantitative basis for separating fine roots from coarse roots and that the pattern holds across different species.

Rainfall Characteristics of Tropical Rainforest at Pasoh Forest Reserve, Negeri Sembilan, Peninsular Malaysia

We investigated the rainfall at the Pasoh Forest Reserve (Pasoh FR), Peninsular Malaysia. Pasoh FR is located in the Southwest rainfall regime, in which the average annual rainfall (1,500–2,000 mm) is less than that in other regions of Peninsular Malaysia. Monthly rainfall in 1996 and 1997 ranged from 2.2 to 206.7 mm with a mean of 115.6 mm. The rainfall in 1997 was much smaller due to the El Nino Southern-Oscillation (ENSO) event. The longest period of dry days was 49 days. Dry periods as well as fluctuation in rainfall are major factors affecting the growth of vegetation. A distinct diurnal cycle in rainfall, in which 52% of the rainfall occurred between 13:00 and 19:00 h, was apparent. The frequency of the amount of rainfall in each event was an inverse J-shaped type distribution. The amount of rainfall in one event ranged from 1.2 and 93.1 mm with a mean of 11.4 mm and a median of 5.6 mm. The rainfall was characterized by a short duration (range:1.0-22.0 h, mean: 3.8 h) and high intensity. The maximum hourly rainfall intensity during a rain event ranged from 0.6 to 63.8 mm h-1 with a mean of 7.8 mm h-1 and a median of 3.8 mm h-1.

Characteristics of Energy Exchange and Surface Conductance of a Tropical Rain Forest in Peninsular Malaysia

Energy exchange above tropical rain forest was studied using micro-meteorological monitoring from a 52 m tower established in the Pasoh Forest Reserve (Pasoh FR) in Peninsular Malaysia. The meteorological conditions were comparatively drier during the first half of the year and wetter toward the end of the year due to the seasonal variation of rainfall. The five-year observational period from 1995 to 1999 included a low rainfall duration due to the El Nino from 1997 to 1998. The latent heat flux estimated by the Bowen ratio method occupied a dominant portion of the energy exchange even in the driest condition in early 1998. Although evapotranspiration from the dry canopy tended to be smaller in this period than in a wet period during the end of 1998, the surface conductance estimated using the Penman Monteith Equation was consistently controlled by the same function of solar radiation and specific humidity deficit. This suggests the evaporation did not suffer from severe stress of soil water even in the driest condition.

Midday depression of leaf CO2 exchange within the crown of Dipterocarpus sublamellatus in a lowland dipterocarp forest in Peninsular Malaysia

We observed diurnal and seasonal patterns of leaf-scale gas exchange within the crown of a Dipterocarpus sublamellatus Foxw. tree growing in a lowland dipterocarp forest at Pasoh, Peninsular Malaysia. Observations were carried out nine times over 6 years, from September 2002 to December 2007. Observation periods included both wet and mild-dry periods, and natural and saturated photosynthetic photon flux density (PPFD) light conditions. In situ measurements of the diurnal change in net photosynthetic rate and in stomatal conductance were carried out on canopy leaves of a 40-m-tall D. sublamellatus tree, which was accessed from a canopy corridor. A diurnal change in electron transport rate was observed under saturated PPFD conditions. The maximum net assimilation rate was approximately 10 micromol m(-2) s(-1). There was a clear inhibition of the net assimilation rate coupled with stomatal closure after late morning and this inhibition occurred year-round. Although the electron transport rate decreased alongside this inhibition, it sometimes followed on. Numerical analysis showed that the main factor in the inhibition of the net assimilation rate was patchy bimodal stomatal closure, which occurred in both mild-dry and wet periods. The midday depression occurred year-round, though there are fluctuations in soil moisture during the mild-dry and wet periods. The magnitude of the inhibition was not related to soil water content but was related to vapor pressure deficit (VPD): that is, whether the days were sunny and hot or cloudy and cool. On cloudy, cool days in the wet period, the net photosynthesis was only moderately inhibited, but it still decreased in the afternoon and was coupled with patchy stomatal closure, even in quite moderate VPD, leaf temperature and PPFD conditions. Our results suggest that patchy stomatal closure signaled by the increase in VPD, in transpiration and by circadian rhythms, was the key factor in constraining midday leaf gas exchange of the D. sublamellatus canopy leaves.

Vertical profiles and storage fluxes of CO2, heat and water in a tropical rainforest at Pasoh, Peninsular Malaysia

Ambient CO2 concentration, air temperature and relative humidity were measured intermittently for a 3-year period from the floor to the canopy top of a tropical rainforest in Pasoh, Peninsular Malaysia. Mean diurnal CO2 storage flux (Sc; μmol m-2 s-1) and sensible and latent heat storage fluxes (Qa and Qw; W m-2) ranged from -12.7 to 3.2 μmol m-2 s-1, -15 to 27 W m-2 and -10 to 20 W m-2, respectively. Small differences in diurnal changes were observed in Sc and Qa between the driest and wettest periods. Compared with the ranges of mean diurnal CO2 eddy flux (-14.7 to 4.9 μmol m-2 s-1), sensible eddy flux (-12 to 169 W m-2) and latent eddy flux (0 to 250 W m-2), the contribution of CO2 storage flux was especially large. Comparison with summertime data from a temperate Japanese cypress forest suggested a higher contribution of Sc in the tropical rainforest, probably mainly due to the difference in nighttime friction velocity at the sites. On the other hand, differences in Qa and Qw were smaller than the difference in Sc, probably because of the smaller nighttime sinks/sources of heat and water vapour.

Effects of soil water status on the spatial variation of carbon dioxide, methane and nitrous oxide fluxes in tropical rain-forest soils in Peninsular Malaysia

To assess the effects of soil water status on the spatial variation in soil carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) fluxes, we examined these gas fluxes and environmental factors in a tropical rain forest in Peninsular Malaysia. Measurements of soil CO 2 , CH 4 and N 2 O fluxes were taken ten, nine, and seven times, respectively over 30 mo at 15 or 39 sampling point within 2-ha plot. Mean (± SE) value of spatially averaged CO 2 flux was 4.70 ± 0.19 μmol CO 2 m −2 s −1 and observed spatial variation in CO 2 flux was negatively related to the volumetric soil water content (VSWC) during the dry period. Over the wet period, extremely high CO 2 emissions were positively correlated with VSWC at some locations, suggesting that no spatial structure of CO 2 flux was because of such hot-spot CO 2 emissions. Flux of CH 4 was usually negative with little variation, with a mean value of –0.49 ± 0.15 mg CH 4 m −2 d −1 , resulting in the soil at our study site functioning as a CH 4 sink. Spatial variation in CH 4 flux was positively related to the VSWC throughout the entire study period (dry and wet). Some CH 4 hot spots were observed during dry periods, probably due to the presence of termites. Mean value of spatially averaged N 2 O flux was 98.9 ± 40.7 μg N m −2 h −1 and N 2 O flux increased markedly during the wet period. Spatially, N 2 O flux was positively related to both the VSWC and the soil N concentration and was higher in wet and anaerobic soils. These findings suggest that denitrification is a major contributor to high soil N 2 O fluxes. Additionally, analysis by adjusting confounding effects of time, location and interaction between time and location in mixed models, VSWC has a negative effect on CO 2 flux and positive effects on CH 4 and N 2 O fluxes. We found that soil water status was related temporally to rainfall and controlled greenhouse gas (GHG) fluxes from the soil at the study site via several biogeochemical processes, including gas diffusion and soil redox conditions. Our results also suggest that considering the biological effects such as decomposer activities may help to explain the complex temporal and spatial patterns in CO 2 and CH 4 fluxes.