Upslope Positions Research Articles

Slope gradient and shape effects on soil profiles in the northern mountainous forests of Iran

In order to evaluate the variability of the soil profiles at two shapes (concave and convex) and five positions (summit, shoulder, back slope, footslope and toeslope) of a slope, a study of a virgin area was made in a Beech stand of mountain forests, northern Iran. Across the slope positions, the soil profiles demonstrated significant changes due to topography for two shape slopes. The solum depth of the convex slope was higher than the concave one in all five positions, and it decreased from the summit to shoulder and increased from the mid to lower slope positions for both convex and concave slopes. The thin solum at the upper positions and concave slope demonstrated that pedogenetic development is least at upper slope positions and concave slope where leaching and biomass productivity are less than at lower slopes and concave slope. A large decrease in the thickness of O and A horizons from the summit to back slope was noted for both concave and convex slopes, but it increased from back slope toward down slope for both of them. The average thickness of B horizons increased from summit to down slopes in the case of the concave slope, but in the case of convex slope it decreased from summit to shoulder and afterwards it increased to the down slope. The thicknesses of the different horizons varied in part in the different positions and shape slopes because they had different plant species cover and soil features, which were related to topography.

Nitrogen Management Effects on Spring Wheat Yield and Protein Concentration Vary with Seeding Date and Slope Position

Efficient N fertilizer can improve economic returns and reduce environmental risk. This study evaluated effects of fertilizer rate, source, timing and placement combinations on spring wheat (Triticum aestivum L.) yield and protein concentration under varying environmental conditions created by differing slope position and seeding dates. Grain yield and protein responses to N management were evaluated for 3 yr at two locations near Brandon, MB, Canada. Grain yield was generally higher by up to 0.9 t ha−1 and protein lower by up to 20 g kg−1 on lower vs. upper slope positions. Grain yield was higher by up to 0.7 t ha−1 and protein concentration lower by 20 g kg−1 with early vs. late seeding. While grain yield and protein both usually increased with N application, by up to 25 and 20%, differences among fertilizer treatments were infrequent. In 2 site‐years, grain yield was lower with fall‐ than spring‐banded urea and the use of fall‐banded controlled release urea (CRU) reduced N losses and led to yields intermediate between fall‐ and spring‐banded urea. Where differences occurred among fertilizer treatments, yield was generally highest with spring side‐banded urea. However, protein occasionally increased with use of CRU vs. urea fertilizer. Spring side‐banded urea, although less costly than enhanced efficiency fertilizers or split N applications, produces both yield and protein concentrations as high or higher than a range of enhanced efficiency fertilizer products or split applications of N, and is a good N management option under environmental conditions in southwestern Manitoba.Core Ideas Benefits of controlled release fertilizers are low where N losses are low In‐soil banding of N is an effective management practice Use of urease inhibitors may not overcome loss of efficiency for surface application as compared to in‐soil banding Spring in‐soil bands are as or more effective than split applications of N in increasing spring wheat yield under common prairie conditions When banded in the fall, controlled release urea can be more effective than uncoated urea where environmental conditions promote N losses

Morphology and chemical properties of plough horizons of soils in various slope positions

Studies on morphological features and chemical composition of Ap horizons of soils located on gentle Narew valley-sides were carried out. The Luvisols with a texture of loamy sands and sandy loams are developed on the slope shoulder while the Mollic Gleysols of a similar texture with deposits to 110 cm thick are located on the footslope. The soil particle size distribution and total nitrogen (TN), total porosity (TP) and KH2SO4 H2O2 _ content in both slope positions are similar. The mean TN, TP and KH SO H O 2 4 2 2 _ content on the slope shoulder is 0.94, 0.36 and 1.26 g kg-1, respectively, while soils on the footslope are characterized with a mean content of total nitrogen, phosphorus and potassium of 1.15, 0.35, and 1.17 g kg-1, respectively. Plough horizons of soils located on the footslope are more acidic with higher Corg content. The mean exchangeable acidity in deluvial soils (4.20 cmol(+) kg-1) is higher than in soils located in upper slope positions (2.92 cmol(+) kg-1). Similar texture of soils in both slope positions as well as thick deluvial deposits on the footslope is evidence of tillage erosion as the main factor of soil downslope translocation on these gentle slopes.

Spring-Thaw Nitrous Oxide Emissions from Reed Canarygrass on Wetness-Prone Marginal Soil in New York State

In temperate climates, a significant fraction of annual emissions of nitrous oxide (N2O) from agricultural land can occur during soil thaw in late winter and early spring. The objective of this study is to determine the impact of land use change from long-term fallow grassland to managed perennial grass crops on these thaw-related N2O emissions, and to identify field-scale drivers that influence emissions. Using static chambers, we monitored mid-afternoon N2O fluxes during the 2013 spring thaw from March 27 to April 7, observing fallow grassland and second year reed canarygrass (Phalaris arundinaceae L., v. Bellevue) over a short topographical gradient. Soil temperature, soil moisture and residual above-ground biomass were also observed, as were hourly air temperature and precipitation. Fluxes of N2O were generally low (-9.8 to 21.3 μg N2O-N m-2 hr-1) except for one observation of 77.6 μg N2O-N m-2 hr-1. Hot-moment analysis, non-parametric statistical tests, and ANOVA results showed that downslope positions converted to managed and fertilized grass had significantly higher N2O emissions compared to the fallow and upslope positions. We found that these downslope managed grass sites had mean soil moisture of 75.0 % water filled pore space (WFPS) and less insulating residual above-ground biomass than the fallow grassland. Our results suggest that converting fallow grassland to managed perennial grass cropping systems for bioenergy or other uses could increase springthaw N2O emissions in wetness-prone areas.

Influences of intensive tillage on water-stable aggregate distribution on a steep hillslope

Few studies have examined direct impacts of intensive tillage on soil structural stability in hillslope landscapes. In this experiment, we performed simulation tillage within the steep hillslope landscape of the Sichuan Basin, China, i.e., 20-time consecutive tillage by hoeing in a short time interval (3 days) under no rainfall. The 137Cs tracing was used to demonstrate tillage erosion/deposition in soil profiles and landscape positions. A wet-sieving method was used to determine mean weight diameter (MWD) and geometric mean diameter (GMD) as indices of soil aggregate stability. Soil organic matter (SOM) was also determined just before and after hoeing tillage implementation. The greatest changes in 137Cs inventory due to 20-operation tillage were observed at 1 and 20m slope positions, with a decrease of 100% at the 1m position and an increase of 50.58% at the 20m position compared with pre-tillage. The <0.25mm water-stable aggregates after 20-operation tillage increased by 32.62–42.14% (mean 36.98%) for all slope positions compared with those of pre-tillage. After 20-operation tillage, the great differences in MWD and GMD values were found at the upper slope positions (1m), with decreases of 35.01% and 39.24%, respectively, and at the lower slope positions (15 and 20m), with decreases of 14.17–24.79% and 20.37–29.63%, respectively, as compared to pre-tillage. However, MWD and GMD values remained unchanged at the middle slope position (10m). This result suggests that apparent changes in soil structural properties occur close to the upslope and downslope boundaries of the field and the middle slope acts as a conveyor belt. On the whole, simulated tillage markedly increased the CV values of MWD and GWD by 96.80% and 86.15%, respectively, compared with those of pre-tillage, implying that intensive tillage causes a great within-field spatial variability of soil aggregate stability. Soil aggregate stability can be affected by both the disruption caused by the act of tillage and the loss of surface soil and incorporation of subsoil through intensive tillage. Our results indicate that intensive tillage has a twofold influence: aggregate redistribution due to the soil transfer process and the mechanical breakage of macroaggregates. It is suggested that the use of reduced tillage is not only an effective alternative for diminishing soil erosion, but also a viable strategy for improving soil structure on the hillslopes.

Hydromorphic Soil Development in the Coastal Temperate Rainforest of Alaska

Predictive relationships between soil drainage and soil morphological features are essential for understanding hydromorphic processes in soils. The linkage between patterns of soil saturation, reduction, and reductimorphic soil properties has not been extensively studied in mountainous forested terrain. We measured soil saturation and reduction during a 4-yr period in three catenas of the perhumid coastal temperate rainforest of Alaska and compared these measurements to soil morphological features of Spodosols and Histosols. Soil saturation and anaerobic conditions indicated by redox potential corresponded to low-chroma colors, Fe concentrations, and accumulation of organic matter. Hue changes from 7.5YR to 10YR, 2.5Y, and 5GY were observed, at depths corresponding to water table position and Fe depletion, in Spodosol B horizons of backslope landscape positions. The depth to the features was dependent on the distance from the top of the catena, with a consistent pattern of near-surface soil saturation evident below the topographic break of 10% slope. The mean annual water table position was 34 cm below the surface in upper slope positions and 14 cm below the surface in lower slope positions. The association of color changes confirms the influence of saturation and reduction on the soil morphology of Spodosols. The consistent patterns in soil saturation, reduction, and soil morphological features offers guidance for hydric soil identification, wetland delineation, and ecological processes related to near-surface soil saturation.

Soil Fertility Status and Productivity Trends Along a Toposequence: A Case of Gilgel Gibe Catchment in Nadda Assendabo Watershed, Southwest Ethiopia

The Impact of landscape positions and land use type on properties as well as land productivity are commonly observed in Ethiopia. The study was therefore conducted to quantify soil fertility and productivity trends under three land uses along the gradient (lower, middle and upper slope positions) of Nadda Assendabo watershed, Southwest Ethiopia. Thus field survey was covered a total of 54 composite soil samples using split plots (for soil physical properties) and split-split factorial arrangements with randomized complete block design from woodlot, grazing and crop land following topographic positions then different parameters were analyzed. The highest mean soil moisture content (56.68%) and porosity (71.09%) were observed for woodlot at the lower slope while the highest soil bulk density (1.74 g/cm³) was observed for soil crop land at the upper slope where as the lowest (0.94 g/cm³) was for woodlot at the lower slope. Sand, silt and clay fractions were significantly affected by interaction effects of slope and land uses (P≤0.01). Soil pH, electrical conductivity, organic carbon, cation exchange capacity, total nitrogen, available phosphorus, base saturation percentage and exchangeable basic cations for all land uses were increased down slope while they were affected significantly by slope steepness, land use and soil depth (P≤0.01). Therefore, electrical conductivity at middle and upper slope in grazing land and woodlot; total nitrogen, available phosphorus in crop land at lower slope was significantly different (P≤0.05) due to interaction effect. Furthermore, above ground biomass and grain yields of sorghum, maize and teff were also calculated on farm fields using split plots arrangements. Thus both yields were significantly affected by interaction effect of slope and crop type. Finally, based on the results woodlot, grazing and crop land were recommended for upper, middle and lower slopes respectively.

Is tropical montane forest heterogeneity promoted by a resource‐driven feedback cycle? Evidence from nutrient relations, herbivory and litter decomposition along a topographical gradient

SummaryRidges of tropical mountains often differ strikingly from neighbouring ravines in terms of forest structure, productivity and species composition. This heterogeneity is poorly understood despite its critical role in biodiversity maintenance, carbon and nutrient budgets.We examined measures of tree biomass and productivity, foliage and litter quality (nutrient concentrations, specific leaf mass, phenolics), herbivory and leaf litter decomposition in each six plots laid out in upper and lower slope position in a tropical montane moist forest in southeastern Ecuador.Productivity, quality of foliage and litter as well as herbivory were significantly lower in upper slope position, and closely correlated with soil nutrient concentrations and accumulated humus. The decomposition of upper slope leaf litter (decomposition ratek) was substantially lower than in litter from lower slope forest, whereas the site of decomposition (slope position) only had a marginal effect on the decomposition rate.Our results suggest that the differences in stand structure, productivity, foliar quality, herbivory and decomposition between slope positions are ultimately due to stronger nutrient limitations in upper slope forest. We propose a general conceptual model that explains origin and maintenance of contrasting forest types along topographical gradients through down‐slope fluxes of nutrients and water, and a nutrient‐driven positive feedback cycle.

Pedological characteristics and classification of soils along landscapes at Abobo, southwestern lowlands of Ethiopia

Soils of the lowlands in southwestern Ethiopia are under intensive cultivation, however, very little is known about them. The objective of this study is to provide basic information on how pedogenic properties along the landscape relate to their use, which could assist better management to establish sustainable crop production system. Four soil profiles from varying slope positions and land management were described in-situ, and sampled horizon-wise for physico-chemical analysis. Accordingly, soils at upper-slope position (Pedons 2 and 4) were characterized by shallow depth with the upper boundary of C-horizon 47/52 cm from the soil surface, whereas those at lower-slope position (Pedon-3) exhibited thick sola with a deep C-horizon (>200 cm). The C-horizon of all Pedons, except Pedon-3, had high coarse fragments, ranging from gravelly (48%) to extremely gravelly (69%). The soil consistence (wet) showed various grades of stickiness and plasticity along the profile depth. Soil organic carbon (OC), total N, exchangeable bases (Ca, Mg, K) and cation exchange capacity (CEC) generally decreased with profile depth, whereas available P content showed increasing trend with depth of Pedons and along the slope, ranging from 8.7 to 33.7 mg kg-1. According to WRB legend, Pedon-1 and 2 with cambic horizons were classified as Haplic Cambisols and Fluvi-Mollic Cambisols, respectively, whereas Pedon-3 was classified as Leptic Regosols soil unit. Overall, this study revealed that the effects of land management on soil properties as a function of landscape is profound, suggesting the need for developing appropriate management options for sustainable use of the land. Key words: Landscape, Pedon, slope, soil characteristics, soil classification.

Pattern of cumulative soil erosion and redistribution pinpointed through magnetic signature of Chernozem soils

An experimental approach for estimation of cumulative soil erosion by using magnetism of Chernozem soils is presented. An agricultural land of 4.5ha, utilized for crop production, has been mapped at 10×10m grid interval by using magnetic methods. Field measurements of magnetic susceptibility show high sensitivity to the present soil surface conditions (crop residue cover, soil compaction, sealing) in addition to the main response to erosional removal of the uppermost soil horizons. Laboratory measurements of magnetic susceptibility (χ), frequency dependent magnetic susceptibility (χfd) and Isothermal Remanent Magnetization on soil samples allow construction of surface maps of these magnetic characteristics. Their systematic depth variations along non-disturbed reference soil profiles have been established, which lead to the construction of empirical predictive curves of cumulative soil loss according to the model, proposed by Royall (2001). Using the predictive curves of χ and χfd, maps of soil magnetic parameters are transformed into the distributed pattern of predicted soil loss. Comparison of the maps with terrain slope reveals that in the upper slope positions (having slopes of 1–5°) tillage redistribution plays the main role. In contrast, at toe slope sites, characterized by higher slopes (11–15°), water erosion caused significant removal of soil material. Mean annual erosion rates of 8–6.8t/ha/y have been calculated, which corresponds to about 50years of agricultural land use.

Carbon Dioxide in Arable Soil Profiles: A Comparison of Automated and Manual Measuring Systems

Carbon dioxide (CO2) concentrations in arable soil profiles are influenced by autotrophic and heterotrophic respiration as well as soil physical properties that regulate gas transport. Although different methods have been used to assess dynamics of soil CO2 concentrations, our understanding of the comparability of results obtained using different methods is limited. We therefore aimed to compare the dynamics in soil CO2 concentrations obtained from an automated system (GMP343 sensors) to those from a manually operated measurement system (i.e., soil gas sampled using stainless steel needles and rods). In a winter wheat field in Denmark, soil CO2 concentrations were measured from 29 November 2011 to 14 June 2012 at upslope and footslope positions of a short catena (25 m). Carbon dioxide was measured at 20- and 40-cm soil depths (i.e., within and below the nominal plow layer) using the two measurement systems. Within the measurement range for the GMP343 sensors (0–20,000 ppm), mean results from the two systems were similar within the plow layer at the upslope (P = 0.060) and footslope (P = 0.139) position, and also below the plow layer at the upslope position (P = 0.795). However, results from the two systems deviated for the soil from the footslope position below the plow layer (P = 0.001). These results were partly attributed to larger variation in soil parameters below than within the nominal plow layer. The data suggested that generally the application of either system may be adequate; however, differences may occur in response to soil spatial variability. A better coverage of spatial variability is more easily addressed using manually operated systems, whereas temporal variability can be covered using the automated system. Depending on the aim of the study, the two systems may be used in combination to enhance both spatial and temporal data coverage.

Physiochemical Characteristics and Lead Concentrations in the Inland Valleys of Ishiagu, Southeastern Nigeria

The research work was conductedat two locations in 2012, 100 meters away from mining pitsto determine the effects of lead, slope position and depth on the variation of soil chemical properties.Random sampling method was used to collect soil samples from the study area.Samples were collected from upper-slope, middle-slopeand bottom slope positions at depths 0 �15 cm and 15 �30 cm. Results indicated that the soils are generally clayey to clay loam and poorly drained in all the depths. The upper slope position in thetwo locations gave the highest contents of sand, while the highest percent of clay and silt were obtained from the bottom slope position in both Amaeze and Ihietutu locations.Results showed a significant difference among the chemical properties of upper,middle and bottom slope soils. Soil pH of the upper -slope (6.2) was the highest OriginalResearch Article

Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of China

Land-use change from one type to another affects soil carbon (C) stocks which is associated with fluxes of CO2 to the atmosphere. The 10-years converted land selected from previously cultivated land in hilly areas of Sichuan, China was studied to understand the effects of land-use conversion on soil organic casrbon (SOC) sequestration under landscape position influences in a subtropical region of China. The SOC concentrations of the surface soil were greater (P\0.001) for converted soils than those for cultivated soils but lower (P\0.001) than those for original uncultivated soils. The SOC inventories (1.90–1.95 kg m-2) in the 0–15 cm surface soils were similar among upper, middle, and lower slope positions on the converted land, while the SOC inventories (1.41–1.65 kg m-2) in this soil layer tended to increase from upper to lower slope positions on the cultivated slope. On the whole, SOC inventories in this soil layer significantly increased following the conversion from cultivated land to grassland (P\0.001). In the upper slope positions, converted soils (especially in 0–5 cm surface soil) exhibited a higher C/N ratio than cultivated soils (P = 0.012), implying that strong SOC sequestration characteristics exist in upper slope areas where severe soil erosion occurred before land conversion. It is suggested that landscape position impacts on the SOC spatial distribution become insignificant after the conversion of cultivated land to grassland, which is conducive to the immobilization of organic C. We speculate that the conversion of cultivated land to grassland would markedly increase SOC stocks in soil and would especially improve the potential for SOC sequestration in the surface soil over a moderate period of time (10 years).

Impact of erosion and tillage on the productivity and quality of selected semiarid soils of Iran

Abstract This greenhouse research was carried out to study the effects of water and tillage erosion on agricultural productivity and soil quality in soil samples from a semiarid region of Iran. A factorial experiment of complete randomized block design was used to compare the effects of soil erosion (eroded and non-eroded soils), slope position, water stress and fertilizer (N-P-K) on yield and yield components of wheat as soil productivity index. The results showed that erosion ie water and tillage erosion has a significant effect (p&lt;0.01) in decreasing soil productivity due to its negative impact on soil organic matter, nutrients (N and K) and hydraulic conductivity. Complete N-P-K fertilization and water stress had significant effects on increasing and decreasing of wheat yield, respectively. The effect of water stress in particular was so high that it could eclipse the erosion impact on yield reduction. Wheat dry matter and grain mass on foot and mid slopes were significantly higher than that on upslope positions where total N and available K were the lowest and equivalent calcium carbonate the highest. Saturated hydraulic conductivity and total nitrogen were found to be the most important soil properties as far as their correlations to wheat yield are concerned.

The impact of mulch type on soil organic carbon and nitrogen pools in a sloping site

Three mulch treatments were tested for their ability to control erosion on a sloping site. Additionally, the choice of mulch can also enhance revegetation success and improve soil organic matter input. This study aimed to investigate the effects of three mulching treatments, hydro-seeding, granite mulch and forest mulch, on soil C and N pools at different positions on highly erodible slope with approximately 30 % gradient. Soil moisture, total C (TC), total N (TN), hot water-extractable organic C (HWEOC), hot water-extractable total N (HWETN), microbial biomass C and N (MBC and MBN), inorganic N and potentially mineralisable N were measured. All variables were significantly higher in soils amended with forest mulch than those with hydro-seeding and granite mulch, for the same slope positions. Soil moisture was significantly higher in the lower slope position than middle and upper slope positions in hydro-seeding and granite mulch treatments, whereas no slope effect was observed on soil moisture under the forest mulch application. In the forest mulch treatment, the upper slope position had higher soil TC, TN, HWEOC, HWETN, MBC, MBN, NO3−-N and total inorganic N than the middle and lower slope positions. Five years following mulch application, forest mulch still exerted a significant influence on soil fertility compared to the other treatments and the influence on soil moisture suggests that this treatment would be the most effective in the control of water-driven soil erosion on this steep site.

Temperatures Beneath Bark of Dead Trees used as Roosts byMyotis volansin Forests of the Pacific Northwest, USA

Few studies have examined temperatures inside bark roosts of tree-roosting bats. We measured temperatures beneath plates of exfoliating bark of six species of dead trees used for roosting by long-legged myotis (Myotis volans) from June to July 2003–2006 in Idaho and Oregon, USA, and compared these across tree species and with ambient temperature (TA). Temperature profiles beneath bark oscillated within the range of TA, demonstrating the ability of bark cavities on dead trees to insulate against daily extremes in maximum and minimum TA. Slope position affected the difference in bark cavity temperature from TA during daytime periods, with bark cavities on dead trees in upper slope positions being cooler than TA compared to bark cavities on dead trees in lower slope positions. Differences in bark cavity temperatures from TA varied among dead tree species during nighttime periods, with white fir (Abies concolor) and ponderosa pine (Pinusponderosa) warmer than TA compared to other dead tree species. Mean daily...

Soil redistribution and organic carbon accumulation under long‐term (29 years) upslope tillage systems

AbstractFew studies have demonstrated soil redistribution under upslope tillage (UT) rather than downslope tillage (DT) and its impact on soil organic carbon (SOC) redistribution in long‐term agricultural practices in hillslope landscapes. We selected two neighbouring sites from the Sichuan Basin, China, one under DT and the other under UT, to determine the pattern of soil and SOC redistribution under a long‐term UT practice. DT caused soil loss at upper slope positions and soil accumulation at lower slope positions. However, UT resulted in soil accumulation at upper slope positions and soil loss at lower slope positions. The total erosion rate decreased by 60.5% after 29 years of UT compared with DT. Having the same direction of soil movement by tillage and water exaggerated total soil loss, whereas having the two movements in the contrasting direction of soil for the two reduced it. SOC stocks at positions from summit to downslope were much larger (33.8%) and at toe‐slope positions were only slightly greater (4.5%) in the UT soils than comparable values for the DT site. The accumulation rate of SOC at the UT site increased by 0.26 Mg/ha/year compared with that at the DT site. It is suggested that soil movement by water and tillage erosion occurred in the same direction accelerates the depletion of SOC pools, whereas the opposite direction of soil movement for the two can increase SOC accumulation. Our results suggest that UT has significant impacts on soil redistribution processes and SOC accumulation on steeply sloping land.

The performance of grass filter strips in controlling high‐concentration suspended sediment from overland flow under rainfall/non‐rainfall conditions

ABSTRACTThere is little information on the performance of vegetative filter strips (VFS) in filtering high‐concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1‐m‐wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min‐1 m‐1) and sediment concentrations of 100–300 kg m‐3 under simulated rainfall and non‐rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D50) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D50 at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D50 of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D50 decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley &amp; Sons, Ltd.

干热河谷主要造林树种气体交换特性的坡位效应

植物光合与水分生理特征是植物逆境适应能力评价的重要参考指标。极端生境下,坡位的选择有时成为造林成败的关键。探讨了元谋金沙江干热河谷9个主要造林树种光合与蒸腾作用在不同坡位间的空间差异及干热季向湿润季转换的时序差异。结果表明,低的坡位有助于树种维持相对高的净光合速率,且树种不同,由坡位引起的光合增益效应亦具有较大差别;低坡位的光合增益效应在干热季更为明显,而在湿润季,干热胁迫解除,低坡位的光合增益效应具有较大程度的降低;在不同坡位间发生的光合限制主要受非气孔因素主导。无论在干热季或湿润季,与净光合速率的变化情形一致,低的坡位均促进了树种的蒸腾速率。水分利用效率受坡位的影响较为复杂,因树种、季节而异。;Plant photosynthesis and transpiration are important reference indices for evaluating adaptable ability in adverse habits. In extreme growth environment, the selection of slope position is often becoming the key factor determining the success or failure of reforestation. Valley-type savanna is a unique type of savanna that develops in deep-incised river valleys, such as those of the dry-hot valleys in the Hengduan Mountains, southwest China, and the total area of this type in China is approximately 3.0×10⁶ hm². In the dry-hot valleys of China, the dry-hot valley of Jingsha River is typical, where the environment shows the changes from dry-hot to wet weather in one year, and because of its long dry-hot season, tree species often are difficult to survive and grow well. In this study, based on the consideration that different slope positions have different soil and atmosphere condition in the extreme habitat, the effects of slope position on photosynthesis and transpiration of nine main tree species for vegetation restoration in dry-hot season and wet season of the dry-hot river valleys were analyzed. The trial tree species include nine species, there are <em>Azadirachta indica</em> A.Juss, <em>Leucaena glauca</em> L. Benth., <em>Tephrosia candida</em> DC., <em>Eucalyptus camaldolensia</em> Dehn., <em>Acacia auriculiformis </em>A. Cunn., <em>Melia toosenden</em> Sieb.et Zucc., <em>Dodonaea viscosa</em> L. Jacq., <em>Phyllanthus emblica</em> L. and <em>Jatropha curcas</em> L. These tree species all grow in fields. The results showed that: comparing with upper slope position, lower slope position was in favor of keeping leaf net photosynthetic rate (<em>Pn</em>) a relative high level, but this favorable effect of lower slope position on <em>Pn</em> had obvious difference among different tree species. The effect of slope position on <em>Pn</em> was more obvious in dry-hot season than in wet season, while in wet season the favorable effect of lower slope position on <em>Pn</em> decreased at great extent because the dry-hot stress got relieved. Photosynthesis restriction, induced by raise of slope position, was mainly controlled by non-stomatal factor. Whether in dry-hot season or wet season, lower slope position both promoted transpiration rate (<em>Tr</em>) of the tested tree species, which was accordant with the change of <em>Pn</em>. The change of water use efficiency(WUE) with different slope positions was relatively complicated, the effects were different among different tree species and between the two seasons. It is obvious that the lower slope position benefits the photosynthesis of the tested tree species, and which helps trees to spend the dry-hot season. Because water deficit is one of the main factors that restrict the growth of tree species for vegetation restoration in the dry-hot valley of Jingsha River, and the leaf water potential of trees in lower slope position is more higher than in upper slope position, it can be knowed that the difference of water condition may be the main factor that cause the difference of photosynthesis in different slope positions.

Early melt season snowpack isotopic evolution in the Tarfala valley, northern Sweden

AbstractThis study investigated the stable water isotopic (δ18O and δD) evolution of two snowpacks located at the same elevation on a north-facing (Nf) and a south-facing (Sf) slope within the Tarfala research catchment, northern Sweden, and the potential impact of pre-freshet snowpack melt on streamflow. Our results indicate that over the study period the Sf snowpack showed a more enriched isotopic composition, especially in the top of the profile, and contributed more snowmelt to streamflow than the Nf slope. The Sf snowpack also showed a significantly higher variability in snowpack δ18O levels and snowpack snow water equivalent (SWE) over time. Comparing snowpack and snowmelt isotopic values it was seen that the Sf slope experienced earlier snowmelt from upslope positions due to greater insolation that subsequently enhanced the meltwater flux at the base of downslope snowpacks. In contrast, the Nf slope primarily underwent changes within the snowpack and experienced relatively minimal melt. Detailed field-based isotopic snowmelt studies such as this highlight the potential importance of incorporating spatio-temporal runoff generation concepts into distributed energy-balance models, which could allow for more accurate prediction with regard to the spatio-temporal dynamics associated with the snowmelt ion pulse.