Topsoil Moisture Research Articles

Gaseous emissions and soil fertility of homegardens in the Nuba Mountains, Sudan

AbstractIntensification of homegardens in the Nuba Mountains may lead to increases in C and nutrient losses from these small‐scale land‐use systems and potentially threaten their sustainability. This study, therefore, aimed at determining gaseous C and N fluxes from homegarden soils of different soil moisture, temperature, and C and N status. Emissions of CO2, NH3, and N2O from soils of two traditional and two intensified homegardens and an uncultivated control were recorded bi‐weekly during the rainy season in 2010. Flux rates were determined with a portable dynamic closed chamber system consisting of a photo‐acoustic multi‐gas field monitor connected to a PTFE coated chamber. Topsoil moisture and temperature were recorded simultaneously to the gas measurements. Across all homegardens emissions averaged 4,527 kg CO2‐C ha−1, 22 kg NH3‐N ha−1, and 11 kg N2O‐N ha−1 for the observation period from June to December. Flux rates were largely positively correlated with soil moisture and predominantly negatively with soil temperature. Significant positive, but weak (rs < 0.34) correlations between increasing management intensity and emissions were noted for CO2‐C. Similarly, morning emissions of NH3 and increasing management intensity were weakly correlated (rs = 0.17). The relatively high gaseous C and N losses in the studied homegardens call for effective management practices to secure the soil organic C status of these traditional land‐use systems.

Using Temporal Changes in Drought Indices to Generate Probabilistic Drought Intensification Forecasts

Abstract In this study, the potential utility of using rapid temporal changes in drought indices to provide early warning of an elevated risk for drought development over subseasonal time scales is assessed. Standardized change anomalies were computed each week during the 2000–13 growing seasons for drought indices depicting anomalies in evapotranspiration, precipitation, and soil moisture. A rapid change index (RCI) that encapsulates the accumulated magnitude of rapid changes in the weekly anomalies was computed each week for each drought index, and then a simple statistical method was used to convert the RCI values into drought intensification probabilities depicting the likelihood that drought severity as analyzed by the U.S. Drought Monitor (USDM) would worsen in subsequent weeks. Local and regional case study analyses revealed that elevated drought intensification probabilities often occur several weeks prior to changes in the USDM and in topsoil moisture and crop condition datasets compiled by the National Agricultural Statistics Service. Statistical analyses showed that the RCI-derived probabilities are most reliable and skillful over the central and eastern United States in regions most susceptible to rapid drought development. Taken together, these results suggest that tools used to identify areas experiencing rapid changes in drought indices may be useful components of future drought early warning systems.

Assessing a dry surface layer‐based soil resistance parameterization for the Community Land Model using GRACE and FLUXNET‐MTE data

AbstractThe Community Land Model (CLM) exhibits biases in evapotranspiration (ET) and total water storage (TWS) in semiarid regions. The TWS seasonal cycle amplitude is too low, while evapotranspiration is too strong and variable. These biases are consistent with excessive soil evaporation when the canopy is sparse or absent, which reduces moisture inputs into the ground. Here we improve the simulation of soil evaporation by replacing CLM's existing empirical soil resistance parameterization with a more mechanistically based formulation in which soil evaporation is controlled by the rate of diffusion of water vapor through a dry surface layer (DSL). The thickness of the DSL is parameterized as a soil‐type dependent function of top layer soil moisture. Soil resistances are calculated from the DSL thickness combined with a soil tortuosity factor. Compared to the existing CLM soil resistance parameterization, the DSL‐based soil resistances for a given soil moisture value are generally larger, especially for moister soils. CLM simulations using the DSL‐based soil resistance expression have reduced biases of ET relative to the FLUXNET‐MTE (Model Tree Ensemble) data set and TWS relative to Gravity Recovery and Climate Experiment (GRACE) observations. Averaged over global semiarid regions, the CLM mean annual amplitude of TWS increases from 26.3 mm to 32.5 mm when the new soil resistance parameterization is used, in good agreement with the GRACE mean annual amplitude for these regions of 32.2 ±2.1 mm. CLM mean annual ET, averaged over global semiarid regions, decreases from 27.8 mm/month to 23.5 mm/month, closer to the FLUXNET‐MTE mean annual ET of 21.1 ± 0.9 mm/month. The seasonal amplitude of CLM ET also decreases, from 23.3 mm/month to 18.3 mm/month compared to 14.9 ± 1.1 mm/month for FLUXNET‐MTE.

Urban air temperature anomalies and their relation to soil moisture observed in the city of Hamburg

The spatial variability of the urban air temperature for the city of Hamburg is analyzed based upon a one-year dataset of meteorological and pedological measurements. As local air temperature anomalies are subject to land-use and surface cover, they are monitored by a network of measurement stations within three different urban structures. Mean annual temperature deviations are found to be +1.0 K for inner city sites and +0.25 K to −0.2 K for suburban sites compared to a rural reference. The nocturnal urban heat island (UHI) is identified and averages +1.7 K at the inner city stations, +0.7 K at a suburban district housing area and +0. 3K at a nearby green space. The observed UHI effect is most prominent when the wind speed is low (≤ 2m s −1 )a nd the sky is only partly cloudy (≤ 6/8 th ). In spring 2011 an average inner city UHI of up to +5.2 K is observed during situations matching these conditions, while the extraordinary dry fall of 2011 lead to remarkably high air temperature differences at all observed stations. As expected, no evidence for a significant impact of topsoil moisture on nighttime UHI effect is found. The analysis of air temperature anomalies during daytime results in an annual mean deviation of −0.5 K above unsealed, vegetated surfaces from a sealed site during days with a turbulent mixing induced by wind speed > 2m s −1 . Here, there is an indication for a relation between the water content of upper soil layers and the warming of air: 11 to 17 % of the variance of the diurnal air temperature span is found to be explained by the soil water content for selected relevant days.

Drivers of a riparian forest specialist (Carex remota, Cyperaceae): it is not only a matter of soil moisture.

• Plants respond to the prevailing conditions in the surrounding environment, but since they are dynamic systems this response may vary during their life. Thus, the identification of key aspects for the maintenance of plant populations requires the consideration of plant performance across environmental gradients and along life stages. This study examines how abiotic conditions and biotic interactions and processes determine the spatial distribution of two life-story stages that play a key role in the functioning of a representative population of Carex remota.• We used structural equation modeling (SEM) to test for direct and indirect influences of abiotic and biotic factors on seedlings and adults of Carex remota. The variables used in the analysis were number of seedlings, cover of adults, soil moisture, leaf litter cover, relative light, and topographic position.• Population patterns partially depend on direct and indirect effects of abiotic conditions. Whereas adult individuals were only affected by topsoil moisture, seedling emergence was largely affected by multiple environmental conditions. The number of seedlings increased with high topsoil moisture, low leaf-litter values, high light values as well as in low parts of the study area. The importance of adult individuals in determining seedling success is also highlighted: higher abundance provides seed rain in the surroundings and modifies the microenvironmental conditions favoring high seedling establishment.• As hypothesized, adults and seedlings responded to the environmental conditions differently. Seedling emergence was a critical aspect in C. remota performance, and abrupt changes in the environment during this stage may strongly influence population performance.

A comparative analysis of the impacts of climate change and irrigation on land surface and subsurface hydrology in the North China Plain

Few studies have investigated the similar/different characteristics of regional environmental changes induced by climate change and human activities (e.g., irrigation, groundwater pumping). In this study, three climate change scenarios and one irrigation scenario were simulated using the Community Land Model 4.0 to investigate and compare the water-related environmental changes induced by climate change and irrigation in the North China Plain (NCP). Compared to future change climate scenarios, extensive groundwater-fed irrigation in the NCP could have similar magnitude of effects on land surface fluxes and states, but with much larger effects on subsurface water fluxes/states. For example, it was found that groundwater-fed irrigation has led to the decrease of annual mean water table depth by 1 m in major agricultural areas of the NCP while climate change has negligible impacts on water table depth. That is, human water use tends to dominate the subsurface water balance in the NCP. Moreover, climate change and irrigation exhibited different effects on the vertical profile of soil column. That is, irrigation appears to have much larger effects on the top layer soil moisture (SM) whereas increase in precipitation associated with climate change exerts more influence on lower layer SM. Through indentifying the similar/different characteristics between climate change and irrigation, our results highlight the importance of exactly accounting for the effects of human water use and could provide guidance for determining effective measures for adapting to environmental changes induced by climate change and human water use for this region.

Effects of Tillage Practices on Top Layer Soil Moisture Content and Soil Porosity in Banana Plantation

Soil moisture content and soil porosity were measured in banana plantation under three tillage practices, including deep-tillage (DT), no-tillage (NT), and no-tillage plus straw cover (NTSC). In the experiments, the top layer soil was separated into three depth including 0~15 cm, 15~30 cm and 30 ~ 45 cm. The soil sampling were performed at four plant growth periods, including the stage of banana seedling (SBS), metaphase of banana (MB), the stage of pregnancy (SP) and pumping stage (PS). The results showed that: Among three depth of top layers, soil moisture content in 0~15 cm at four plant growth periods was lower than that in other two depth by three tillage practices. In 0~15 cm top layer soil, all the soil moisture at four plant periods by NTSC practice were highest. And all the soil moisture in soil treated by three tillage practices were increased with the plant growth. The soil porosity in this layer at SBS and MB stages by DT practice was significantly higher than that by NT and NTSC practices. In 15~30 cm layer soil, the soil moisture content and soil porosity at SBS stage was highest in the soil treated by DT practice compared to the that in soil treated by other two tillage practices. In 30~45 cm layer, the soil porosity at SBS stage by DT and NTSC practices were significantly lower than that by NT practice. At MB stage by DT practice it was significantly higher than that by other practices.

Effects of harvest residue management on soil carbon and nitrogen processes in a Chinese fir plantation

Plantation management can affect ecosystem soil carbon (C) and nitrogen (N) cycles. However, how different harvest residue management strategies impact soil C and N processes over the long term is largely unclear. In this study, we examined effects of harvest residue management on soil C and N concentrations, labile soil C and N pools and soil CO2 efflux (Rs) at different stages after Chinese fir (Cunninghamia lanceolata) was replanted in subtropical China. The residue management treatments were slash and burning, whole-tree harvesting, stem-only harvesting and stem only harvesting with double residue retention. Our results showed that the harvest residue management treatments did not differ significantly in their effect on soil C and N, mineral N (NH4+-N plus NO3−-N), dissolved organic C or total dissolved N concentrations, except for soil N concentrations in surface soil (0–10cm) at year 3 and soil total dissolved N concentrations at year 12, which were significantly lower where the slash was burnt than in the double residue retention treatment. Similarly, Rs did not differ significantly among the four residue management strategies at year 15. Topsoil temperature and topsoil moisture were also unaffected by harvest residue management treatment. Soil temperature was found to be the most important factor controlling the temporal pattern of Rs, accounting for 65.8% of seasonal variation of Rs. There was no significant difference in temperature sensitivity of Rs (Q10) or annual Rs among the four treatments. These results indicated that harvest residue management may not significantly cause long-term effects on soil C cycling and N availability in subtropical Chinese fir plantations.

Soil moisture response to environmental factors following precipitation events in a small catchment

Moisture variation in upper soil layers following a precipitation event is recognized as an important process in the conversion of rainfall to soil water. To understand the variability of topsoil moisture following a period of precipitation, we explored changes in soil moisture after 12 precipitation events at 38 sampling locations in the Wulongchi catchment (1.92km2) located in the Danjiangkou Reservoir Area, China. Our results indicated that variation in soil moisture increased with decreasing soil moisture following each of the 12 precipitation events. Using a two-way indicator species analysis (TWINSPAN), the soil moisture measurement periods following each event were classified into three discrete groups: humid periods, moderate periods, and dry periods. For the humid periods, the total variance of soil moisture that could be explained by redundancy analysis was more consistent than those of the moderate and dry periods. For the humid periods on the first axis, soil thickness was the main significant factor, and the topographic wetness index, slope position and aspect were also significant environmental factors. For the moderate and dry periods, however, the effect of aspect on soil moisture decreased, whereas the effect of relative elevation increased. Each environmental factor that was tested for its influence on topsoil moisture variation presented different effects throughout all three of the studied soil moisture periods.

Effects of straw mulch and buried straw on soil moisture and salinity in relation to sunflower growth and yield

Salt accumulation in the root zone can be controlled by reducing the upward movement of salts and evaporation in arid areas of China. This paper aimed to examine the effects of the combined application of straw mulch and buried straw layer on soil moisture, soil salinity and growth of sunflower (Helianthus annuus L.) plants. A three-year field experiment was conducted in the Hetao Irrigation District, Inner Mongolia, China. Three field management practices were studied: deep tillage with no mulch (CK), deep tillage with straw mulch (SM) and combined application of straw mulch and burying of a maize straw layer (12tha−1) at a depth of 40cm (SM+SL). Except in the second half of the first growing season, soil moisture at the 0–40cm depth was higher with SM+SL and SM than CK. Also the topsoil (0–20cm) moisture during the early growth period under SM+SL was higher than that under SM by 1.6–9.9% in 2011 and 1.6–3.4% in 2013, but the value for SM+SL was 2.1–10.4% higher than that for SM during the whole growth period of 2012. Compared with SM, the topsoil salinity under SM+SL decreased by 5.4–23.0% in 2011, 0.7–19.8% in 2012 and 4.5–31.6% in 2013 but these two mulch treatments moderately increased the soil salinity in the subsoil (20–40cm) layer compared with CK. Furthermore, SM+SL promoted sunflower growth, as indicated by taller plants and greater leaf area index. The highest sunflower shoot biomass was always obtained from the SM+SL treatment. Averaged across the three years, SM+SL increased the shoot biomass by 4.8% compared to SM and 20.8% compared to CK. The SM+SL may be an effective saline soil management practice in the Hetao Irrigation District and other similar ecological areas.

Hydrological links in Southeastern South America: soil moisture memory and coupling within a hot spot

ABSTRACTSoutheastern South America has been identified as a hot spot of soil moisture and evapotranspiration coupling efficiency during austral summer in a previous study. Here, hydrological processes such as coupling and memory of soil moisture, evapotranspiration and precipitation and the links between these variables are discussed on the daily time scale over this region. The correlations between surface variables, rainfall persistence and soil moisture memory are discussed over three subregions selected on basis of their coupling efficiency and mean daily intensity of precipitation. The relationship between surface climate and land cover is qualitatively assessed. The memory, or statistical persistence, is longer and has a more robust spatial pattern for the root zone than for the top soil moisture. Where the coupling efficiency between soil moisture and evapotranspiration is high, the evapotranspiration is regulated by soil moisture conditions independently on the intensity of precipitation, whereas in a region with low coupling efficiency and high intensity, the evapotranspiration is regulated by the atmosphere. The coupling efficiency is in general related to the memory of the root‐zone layer, since the soil state is modified when the soil moisture and the atmosphere interact, resulting in an anticorrelation between these metrics. The persistence of rainfall is another factor that modulates the memory. Nevertheless, there are some areas around the La Plata River where both the coupling efficiency and the memory are relatively high, such as Uruguay and the northeast of Argentina, where an improvement of soil moisture initial conditions could improve predictability of surface variables on a monthly timescale.

Preferential flow as a potential mechanism for fire-induced increase in streamflow

After vegetation fires, discharge of streams and rivers is often higher than before. This is usually attributed to decreased canopy interception and evapotranspiration caused by vegetation removal, and to increased overland flow resulting from increased soil water repellency. In this paper we examine whether fire-induced changes in preferential flow can reinforce this postfire streamflow response. We studied five recently burned soils and adjacent unburned soils in Portugal and found that by reducing topsoil moisture and increasing soil moisture variability, fire increased the propensity for preferential flow. This was confirmed by 2-D soil moisture and repellency profiles that showed preferential paths in burned soil that were more distinct, wetter, and slightly narrower than in unburned soil. Since water infiltrating along preferential flow paths bypasses the dry soil matrix, we suggest that narrow flow paths promote deep infiltration– which effect size varies with soil depth, (effective) rainfall, and overland flow. We pose that the resulting increase in infiltration increases drainage and interflow because the excess water cannot stay in the soil, and incorporate fire-induced or -enhanced preferential flow into a conceptual model of flow routing that explains the commonly observed increase in stream flow postfire.

Impact of the new common agricultural policy of the EU on the runoff production and soil moisture content in a Mediterranean agricultural system

Soil moisture variability and the depth of water stored in the arable layer of the soil are important topics in agricultural research and rangeland management. In this study, we use the distributed rainfall-runoff (DR2) model to perform a detailed mapping of topsoil moisture status (SMS) in a mountain Mediterranean catchment. This model, previously tested in the same study area against the Palmer Z-index, is run at monthly scale for the current scenario of land uses and under three scenarios that combine the land abandonment and the application of the new common agricultural policy (CAP) of the European Union. Under the current conditions, runoff yield is scarce and presents a high spatial variability when monthly rainfall intensity and depth are low, and infiltration processes mainly lead to water storage in the soil. When rainfall intensity is high, runoff accumulation along the hillslopes controls the depth of available water in the soil, and SMS is more homogeneous. On average, scrublands and pasture have the wettest values, crops of winter cereal and abandoned fields have intermediate conditions, and areas of bare soil and forest have the driest conditions all the year around. The abandonment and no revegetation of the low productive fields located in steep areas and the collapse of their landscape linear elements (LLEs) produce not only an increase of 2.3 % of the overall SMS in the catchment in comparison with the current scenario but also an increment of the effective runoff that cross the cultivated areas of the lowlands and the runoff depth that reach the wetlands, increasing the soil erosion risk and compromising the conservation of the lakes. When the new green areas of the CAP are installed in the upper part of the fields of the lowlands and around the lakes, the runoff depth and thus siltation risk clearly decreases but also SMS decreases 1.7 and 1.1 % considering the current land uses and adding revegetation practices in the abandoned fields, respectively. Hence, a management scenario where: (1) abandoned fields are covered with a dense cover of shrubs, (2) the LLEs are preserved, (3) the green areas of the PAC are created, and (4) runoff harvesting practices are applied to partially compensate the water deficit, will help to preserve the humidity of the soil and will be of interest to keep the agricultural land use around the protected lakes of the study area.

Brightness Temperature and Soil Moisture Validation at Different Scales During the SMOS Validation Campaign in the Rur and Erft Catchments, Germany

The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite was launched in November 2009 and delivers now brightness temperature and soil moisture products over terrestrial areas on a regular three-day basis. In 2010, several airborne campaigns were conducted to validate the SMOS products with microwave emission radiometers at L-band (1.4 GHz). In this paper, we present results from measurements performed in the Rur and Erft catchments in May and June 2010. The measurement sites were situated in the very west of Germany close to the borders to Belgium and The Netherlands. We developed an approach to validate spatial and temporal SMOS brightness temperature products. An area-wide brightness temperature reference was generated by using an area-wide modeling of top soil moisture and soil temperature with the WaSiM-ETH model and radiative transfer calculation based on the L-band Microwave Emission of the Biosphere model. Measurements of the airborne L-band sensors EMIRAD and HUT-2D on-board a Skyvan aircraft as well as ground-based mobile measurements performed with the truck mounted JULBARA L-band radiometer were analyzed for calibration of the simulated brightness temperature reference. Radiative transfer parameters were estimated by a data assimilation approach. By this versatile reference data set, it is possible to validate the spaceborne brightness temperature and soil moisture data obtained from SMOS. However, comparisons with SMOS observations for the campaign period indicate severe differences between simulated and observed SMOS data.

Water-carbon coupling modeling of summer maize at the leaf and canopy scales

Transpiration and photosynthesis are two closely related and intercoupled processes that dominate the physiological activities and yield of crops. Therefore, there is a need to study water-carbon coupling modeling at various scales to increase water use efficiency (WUE). Using a summer maize field in North China as an example, the variations in leaf and canopy photosynthesis and transpiration (or evapotranspiration) were analyzed. The synthetic model of photosynthesis-transpiration based on stomatal behavior (SMPT-SB) was then calibrated and validated at the two scales. The leaf photosynthesis and transpiration, as well as the canopy photosynthesis and evapotranspiration, have a consistent diurnal trend. However, the canopy evapotranspiration is affected more by topsoil moisture content. The regression coefficient between leaf photosynthesis, transpiration, and WUE estimated by the SMPT-SB and the measured values was found to approach 1, with a coefficient of determination of more than 0.74. The relative error between the two sets of values is less than 11%. Therefore, the SMPT-SB could express fairly well leaf photosynthesis, transpiration, and WUE. The estimated canopy-scale photosynthesis by the SMPT-SB is also in good agreement with the measured values. However, this model underestimates the canopy evapotranspiration when the topsoil has high moisture content and therefore overestimates, to a certain extent, the canopy WUE.

Fine‐scale variability of topsoil moisture in a first‐order stream of a temperate forest

ABSTRACTThis work explains the effect of microtopography on the spatio‐temporal gradient of topsoil moisture in a first‐order stream in a forested mountainous area of northern Spain. This gradient was also related with the availability of suitable microsites for a forest riparian sedge (Carex remota).Topsoil moisture, presence of C. remota and height and distance from the stream edges were measured in 385 points along 35 transects perpendicular to the stream. Soil moisture measurements were repeated in three different dates.Topsoil moisture showed a sigmoid trend that defined the limits of a wet riparian zone at 1·25 m of distance from and 0·55 m in elevation above stream banks. Our riparian zone was narrower than other studies because of the steep slopes (25%) of the mountainous area studied. Elevation above stream banks was more influential than distance in defining the limits of the riparian zone.In the riparian zone, values of soil moisture were high and constant even at the end of a dry period due to the continuous water flow. In the adjacent upland forest, soil moisture varied according to rainfall. These high and constant soil moisture values defined the suitable microsites for C. remota. Copyright © 2013 John Wiley & Sons, Ltd.

Surface energy balance and actual evapotranspiration of the transboundary Indus Basin estimated from satellite measurements and the ETLook model

The surface energy fluxes and related evapotranspiration processes across the Indus Basin were estimated for the hydrological year 2007 using satellite measurements. The new ETLook remote sensing model (version 1) infers information on actual Evaporation (E) and actual Transpiration (T) from combined optical and passive microwave sensors, which can observe the land‐surface even under persistent overcast conditions. A two‐layer Penman–Monteith equation was applied for quantifying soil and canopy evaporation. The novelty of the paper is the computation of E and T across a vast area (116.2 million ha) by using public domain microwave data that can be applied under all weather conditions, and for which no advanced input data are required. The average net radiation for the basin was estimated as being 112 Wm−2. The basin average sensible, latent and soil heat fluxes were estimated to be 80, 32, and 0 Wm−2, respectively. The average evapotranspiration (ET) and evaporative fraction were 1.2 mm d−1 and 0.28, respectively. The basin wide ET was 496 ± 16.8 km3 yr−1. Monte Carlo analysis have indicated 3.4% error at 95% confidence interval for a dominant land use class. Results compared well with previously conducted soil moisture, lysimeter and Bowen ratio measurements at field scale (R2 = 0.70; RMSE = 0.45 mm d−1; RE = –11.5% for annual ET). ET results were also compared against earlier remote sensing and modeling studies for various regions and provinces in Pakistan (R2 = 0.76; RMSE = 0.29 mmd−1; RE = 6.5% for annual ET). The water balance for all irrigated areas together as one total system in Pakistan and India (26.02 million ha) show a total ET value that is congruent with the ET value from the ETLook surface energy balance computations. An unpublished validation of the same ETLook model for 23 jurisdictional areas covering the entire Australian continent showed satisfactory results given the quality of the watershed data and the diverging physiographic and climatic conditions (R2 = 0.70; RMSE = 0.31 mmd−1; RE = –2.8% for annual ET). Eight day values of latent heat fluxes in Heibei (China) showed a good resemblance (R2 = 0.92; RMSE = 0.04 mm d−1; RE = 9.5% for annual ET). It is concluded that ETLook is a novel model that can be operationalized further—especially after improving the preprocessing of spaceborne soil moisture data. This preprocessing includes (1) downscaling of topsoil moisture from 25 to 1 km pixels, and (2) translation of topsoil moisture into subsoil moisture values.

Overstorey and juvenile response to thinning and drought in a jarrah (Eucalyptus marginata Donn ex Sm.) forest of southwestern Australia

Aims: Forest thinning is expected to affect tree water use and carbon assimilation, but the related influence from climate variability is little known. Recent forest thinning in the Wungong catchment coincided with a record dry year following the thinning, which provides a rare opportunity to understand the climate influence on the thinning effect. Methods: A field experiment was conducted to examine changes before and after thinning, especially the rainfall, soil moisture, leaf water status, tissue isotope signature (13 C and 15 N) and N concentration of overstorey and understorey juvenile trees of Eucalyptus marginata (Donn ex Sm.). Results: Despite the post-thinning drought, surface soil was moister and juvenile jarrah plants were less water stressed, attributable to reduced rain interception and transpiration as a result of less canopy cover. The overstorey was under stress but mainly due to drought rather than by thinning. The concentration of N declined in both tree stems and juvenile leaves along with available N in soil, suggesting a soil N limitation. No treatment effects were detected from leaf relative water content and tissue isotope signature (13 C and 15 N). Conclusions: The drought effects were superimposed over the thinning effects on overstorey growth, with stemwood δ13C being a major indicator of water stress. The water relations and carbon assimilation of understorey juveniles were however dependent more on topsoil moisture, and the wetter soil during the year following thinning enhanced growth activity and hence the depletion of 13 C (more negative δ13C) in juvenile leaves.

Does water stress, nutrient limitation, or H-toxicity explain the differential stature among Heath Forest types in Central Kalimantan, Indonesia?

To investigate the causes of the reduced stature of heath forest compared to lowland evergreen rain forest (LERF), the quantity and quality of small litterfall (LF), the standing crop of litter on the forest floor (LSC), and the annual rates of litter decay were determined over a period of 12 months in three contrasting lowland rain forest types in Central Kalimantan, Indonesia. In addition, a litterbag experiment monitored the mass loss of leaves from three dominant tree species in two heath forests (HF) of contrasting stature. Soil water and shallow groundwater dynamics in the two HFs were monitored as well. LF in the LERF was higher compared to both tall heath forest (THF) and relatively stunted heath forest (SHF), but did not differ between the two HFs. Stand-level nutrient-use efficiencies for nitrogen and phosphorus were greatest for the SHF, followed by the THF and the LERF, respectively. The observed differences in nutrient-use efficiency between the two HFs did not result in different LF totals, LSC or decomposition rates and hence cannot explain the difference in HF stature. Neither could phenolic concentrations in leaf LF, which were very similar for the two HFs. Top-soil moisture levels were consistently higher in the SHF compared to the THF and never reached wilting point in either forest type whereas shallow groundwater levels in the SHF were both closer to the surface and more persistent than in the THF. Thus, severe water stress is not thought to be a factor of importance determining HF stature. Rather, considering the much lower pH of the topsoil in the SHF compared to the THF it is hypothesized that different degrees of H-toxicity to fine roots may ultimately prove responsible for the contrast in HF stature.

Effect of local species composition on topsoil properties in mixed stands with silver fir (Abies alba Mill.)

This study was carried out in three stands with fir and spruce (Picea abies L. Karst.), pine (Pinus sylvestris L.) and beech (Fagus sylvatica L.). The objectives were to characterize the spatial pattern of topsoil variation; to determine the contribution of local stand density and local species composition to small-scale variation in topsoil morphology, pH and gravimetric water content during mild midsummer drought; and to compare the effect of spruce, pine and beech vs fir. Although local stand density was related to soil properties, only in the case of single soil characteristics could this variable explain more than 10 per cent of their variation. Compared with fir, increasing density of pine exerted a stronger effect on an increase in organic horizon thickness and a comparable effect on a decrease in pH, but a weaker effect on decrease in topsoil moisture. Irrespective of the soil characteristics, the effect of spruce was approximately twice as strong as that of fir. Beech, however, had a much weaker effect on a decrease in topsoil moisture than did fir and the influence of this species on topsoil pH was the opposite.