Search

Estimates of biomass integrated over forest management areas such as selective logging coupes, can be used to assess available timber stocks, variation in ecological status and allow extrapolation of local measurements of carbon stocks. This study uses fifty 0.1 ha plots to quantify mean tree biomass of eight logging coupes (each 450–2500 ha) and two similarly sized areas in un-logged forest. These data were then correlated with the spectral radiance of individual Landsat-5 TM bands over the 15 km x 15 km study area. Explanation of the differences in radiance between the ten forest sites was aided by measurements of the relative reflectance of selected leaves and canopies from ground and helicopter platforms. The analysis showed a marked variation in the stand biomass from 172 t ha-1 in coupe C88 that was disturbed by high-lead logging to 506 t ha-1 in a similarly sized area of protection forest. A two parameter linear model of Landsat TM radiance in the near-infrared (NIR) band was able to explain 76% of the variation in the biomass at this coupe-scale. The local-scale measurements indicated that the differences in the mean radiance of each coupe (in cloud-free areas) may relate to a change in the proportion of climax tree canopy relative to a cover of either pioneer trees or ginger/shrubs; the canopies of climax trees have the lowest NIR radiance of the vegetation characteristic of selectively logged forest. The coupe harvested following ‘Reduced Impact Logging’ guidelines had a residual biomass and NIR radiance more like that of undisturbed lowland dipterocarp forest than coupes disturbed by ‘conventional’ selection felling. The predictability of tree biomass (at the coupe-scale) by such a parsimonious model makes remote sensing a valuable tool in the management of tropical natural forests.

The influence of single trees on saturated hydraulic conductivity (Ks) was investigated for six isolated oak trees (Quercus robur) growing on a Dystric Gleysol in an area of parkland in northwest England. The Ks was measured within the A soil horizon over a 0.10–0.25 m depth using a borehole permeameter. A dataset of 119 Ks values was obtained and comprises of 55 values from around 1 oak tree at distances of 1–13 m from the trunk, 45 tests around 5 other oak trees, and 19 tests in open grassland. For the intensively sampled tree, Wilcoxon rank sum tests showed a significant difference (p

Existing approaches to modelling the impacts of reforestation on tropical hydrology only simulate one or two changes, thereby limiting our ability to quantify the balance between complex positive and negative changes, even for a single micro-basin. To initiate a more holistic and multiscale approach, we develop a new simulation model structure within the Matlab-Simulink systems environment that firstly, illustrates quantifiable interrelationships between reforestation-related hydrological changes in the component systems of evapotranspiration, runoff generation, sediment delivery and nutrient processes. Secondly, the model structure allows us to highlight basin-scale time-series observations needed to quantify reforestation-related changes in the component hydrological processes. The dynamic model developed is called forSIM and comprises of component models that are derived by means of the data-based mechanistic (DBM) philosophy. Such a modelling approach is required to constrain the large uncertainties that can arise from whole system modelling. The review of the hydrological processes and controlling characteristics that change following reforestation and, therefore, need to be simulated, has highlighted thelack of basin-scale time-series observations of the potentially positive impacts of ‘protective’reforestation onsediment and nutrient delivery, and the need to utilise more macro-scale data. The next phase of the modelling process is to derive estimates of systems parameters and simulation scenarios for specific macro-basins in the tropics undergoing extensive reforestation. # 2007 Elsevier B.V. All rights reserved.

Within tropical rainforests, most rainfall infiltrates the soil to then return to the surface within stream channels or from streamside soils. The wet zones created are very sensitive to disturbance and should be protected from forestry (or agricultural) operations. This study identifies the location of returning subsurface waters using measurements of the topsoil moisture content and first-order, stream-head location. The ability of a topographically based index to predict the location of both channel heads and wet soils is then evaluated to assess its utility for streamflow generation modelling and the designation of buffer or protection zones. Analysis of the location of perennial stream-heads in the 0.44 km2 Baru catchment (East Malaysia) indicated that these could be predicted by the Kirkby topographic index (λ spatial distribution) to within two terrain pixels (each 20 m × 20 m). Within this catchment, topography, therefore, played a key role in the generation of perennial streams, and the λ spatial distribution may be useful in locating the perennial stream network on digital maps for objective ‘hydrological buffer zoning’. The spatial structure of 468 soil moisture content measurements within the 0.21 km2 Huai Pacha catchment (Northern Thailand), was identified by variogram analysis and then used to interpolate the data. A patchy distribution of near-saturated soils was observed along the perennial streams. The λ spatial distribution was poorly correlated with the whole moisture data-set, showing the need to characterise soil and geological variability at this locality in addition to topographic factors. The wet patches did, however, coincide with areas of high topographic convergence and with values of the λ spatial distribution greater than 9 ln(m). Use of this threshold in the λ spatial distribution, rather than a fixed-width riparian buffer defined smaller protection zones with a higher proportion of the sensitive saturated or near-saturated soils. Consequently, the λ-based buffers would be more efficient, easier to justify scientifically, and allow access to more of the timber growing on less sensitive, drier soils. More studies on return-flow are needed in tropical rainforests to add to our understanding of runoff processes, validate distributed hydrological models and provide a hydrological basis for the objective definition of forestry buffer or protection zones within this critical environment.

The impact of different forestry practices on headwater streams in the wet tropics is a serious environmental concern. Despite this, there are very few experimental catchment studies that quantify the hydrological impacts of specific tropical forestry operations. While new field studies are imperative, more could be gained from existing catchment studies by extracting information from the available time-series. Data Based Mechanistic (DBM) models can be used to extract hydrological information from such time-series by fitting a range of Transfer Function models using the simplified recursive instrumental variable (SRIV) algorithm and without a priori assumptions about the water pathways. An optimal model and its associated hydrological system parameters are then identified from objective statistical measures (e.g. R t 2 , YIC) and only then by selection of that model with the most plausible physical explanation. While the DBM method has been applied to the relationship between rainfall input and streamflow output in tropical rainforests, it has never been used to simultaneously examine the relationships within rainfall–streamflow data and the component water pathways of overland flow, subsurface flow and transpiration. Modelling these component pathways is important to the understanding of likely changes in the rainfall–streamflow behaviour resulting from tropical forestry. We aim to show the value of our multiple component DBM models to simulate the sensitivity of stream behaviour to different densities of skidder vehicle trails within a managed rainforest in Borneo. The first application of DBM modelling to the component water pathways of an equatorial rainforest catchment shows that the overall rainfall–streamflow response is flashy, with a residence time of 36.25 ± 0.19 min, when compared to other small catchments, but on minor aquifers. The overall response is, however, much less flashy in comparison to the infiltration-excess overland flow pathway which has a residence time of only 5.50 ± 0.09 min. The overland flow pathway is also less non-linear in its response in comparison to the subsurface pathway and overall response. A new DBM model of transpiration was found, and was similarly shown to give a rapid dynamic response to the input variable, in this case air temperature. The DBM approach indicated that higher order models (e.g. multiple subsurface pathways) were not statistically identifiable within any of the high frequency, 12-month time-series modelled. The first application of DBM modelling to land use/change scenarios in the same equatorial rainforest catchment suggested that the majority of the stream behaviour, in a period 7-years after selective logging, is insensitive to the skid trail densities associated with reduced impact logging (RIL) or clearfell systems in Malaysia. This is largely because of the relative insignificance of the overland flow pathway and the fact that changes in this pathway are predicted to occur on the rising stage of stream hydrographs, rather than at the more critical peak. While controversial, this result is consistent with our previous findings which show that some forestry impacts on hydrology are exaggerated in the popular perception.