South-facing Hillslopes Research Articles

Effects of hillslope aspect on erosion rate of alpine meadows in the Three-River Headwater Region, China

Soil erosion causes serious grassland degradation in the Three-River Headwater Region (TRHR) of the Qinghai-Tibet Plateau. Identifying source areas of erosion is important, including understanding the influences of hillslope aspect on soil erosion. However, the effect of hillslope aspect on soil erosion is a much-debated process, wherein contrasting results were previously reported. We hypothesized that aspect-induced differences in environmental variables will be related to differences in erosion magnitude and spatial pattern, and that these differences will impact soil physiochemical characteristics as well. To test this hypothesis, soil samples were collected at conjoined south- and north-facing hillslopes along elevational profiles in a small watershed in Yushu County, with measurements of soil water content (SWC), soil organic carbon (SOC), soil organic matter (SOM), soil particle size distribution, soil total nitrogen (STN) and phosphorus (STP). Soil erosion rate was estimated using 137Cs technology. The results showed that soil erosion rates of the two hillslopes ranged from 0.073 to 10.83 t ha-1 yr-1, and were 3 times greater on the south-facing hillslope. Soil water content ranged from 7.6% to 76.8% on the two hillslopes with lower values on the south-facing hillslope due to the greater solar radiation. By Spearman correlation analysis and path analysis, we concluded that lower soil water content on the south-facing hillslope restricts vegetation growth, and hence resulting in greater soil erosion under the case where vegetation growth is dominantly controlled by water availability. Greater soil erosion on the south-facing hillslope caused greater surficial accumulation of coarser materials and greater loss of soil nutrients, which are expected to complicate the hydrological erosion process. Moreover, the downslope increased soil erosion rate should be fully considered in conservation practices. These results will improve our understanding of the interaction and feedback mechanisms between soil characteristics, vegetation function, and soil erosion in the TRHR.

Geophysical imaging of regolith in landscapes along a climate and vegetation gradient in the Chilean coastal cordillera

Many studies have recently shown the potential of geophysical tools in bridging the information gap between individual point-scale measurements. Here, we upscale and extend the point-scale layering information from pedons (excavated pit of 1 m2) using geophysical methods. We applied multi-frequency ground-penetrating radar (GPR) in four study areas in the extreme climate and vegetation gradient of the Chilean Coastal Cordillera. The main goals of this study were to understand how granitic based regolith material varies depending on climate, vegetation cover, aspect, and topography.GPR was successfully used in all four study areas. Reflections, which were imaged up to a depth of 8 m, could be associated with boundaries visible in the pedons. The main recognizable reflections were linked with the interface between the mobile soil and the immobile saprolite. This boundary is characterized by hyperbolic-shape features, probably connected to heterogeneities (e.g. pebbles). A deeper GPR penetration depth in south-facing hillslopes was observed than in north-facing hillslopes. This is probably due to less sun exposure in the south facing slopes, which results in higher soil water content and denser plant growth, facilitating weathering processes. Furthermore, thicker layers in the GPR profiles are visible going from north to south along the latitude. Most of these observations were in agreement with the soil pedons.These results demonstrate the utility of the GPR technique for characterizing subsurface variations in regolith properties (e.g. thickness, boundaries). Additional soil pedons should be excavated based on GPR results. Applying noninvasive geophysical methods could improve the understanding of the interactions between soil formation, vegetation, and other environmental parameters.

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings.

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate-growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long-term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate-growth response. The negative radial growth response to warm and dry early- to mid-summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against 13 C was reduced in warm, dry summers and further diminished on south-facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced 13 C discrimination, and increased intrinsic water-use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate-growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.

Trampling routes modified size distribution of Sarcopoterium spinosum in semiarid rangelands

The presence of livestock creates a “three-phase-mosaic” pattern of dominant surface patches: shrubs, trampling routes, and intershrub areas. These patches differ in soil properties, runoff generation, and sediment production; runoff yield is highest in the routes. In light of knowledge that the productivity of arid and semiarid ecosystems is controlled primarily by water dynamics, the spatial correlation between flock trampling routes and the size of the Sarcopoterium spinosum shrubs was studied on north- and south-facing hillslopes in the northern Negev of Israel. The shrubs were divided into two categories, according to their location relative to the trampling route above them: close to the route (CR), i.e., within 50cm, and far from the route (FR), i.e., further than 50cm. More shrubs per unit of area (FR and CR together) were located on the north-facing hillslopes, and they had lower height and canopy area than those on the south-facing ones. Sarcopoterium spinosum shrubs adapt to the mesoclimatic conditions, which are determined by the aspect: on the wet north aspect they grow in much greater numbers but are smaller than those on the dryer south. On the former aspect, the soil water content is high, which accounts for the large number of shrubs; however, their small size possibly results from competition for soil nutrients. On the latter aspect, the more arid conditions promote the activity of water-enriching mechanisms which accounts for their small number and large size. On both south- and north-facing hillslopes most of the shrubs were located close to and downslope from a trampling route, and were bigger than more distant ones. Flocks of sheep and goats engineer water conditions that favor enhancing the growth of shrubs close to and downslope from the route; and this is manifested in a strip-like shrub pattern along the routes with predominantly horizontally oriented lines of bigger shrubs.

Coevolution of soil and topography across a semiarid cinder cone chronosequence

Soil evolution and the development of surface and subsurface diagnostic horizons affects hydrologic partitioning of precipitation to infiltration and runoff, and the vegetative carrying capacity of landscapes, all of which affect rates of hillslope erosion. Rates of erosion, in turn, feedback on soil development by removing or preserving soil horizons. This coevolution is difficult to investigate because landscape age and initial conditions are often poorly constrained. In this paper we investigated the coevolution of the soils and hillslope topography by exploiting differences in vegetation type and density as a function of slope aspect across a semiarid basaltic cinder cone chronosequence, spanning cone ages from 1.065 to 1000kyr, in the San Francisco volcanic field (SFVF) of northern Arizona, USA. We document that soils on south-facing hillslopes exhibit systematically more aeolian-derived dust despite having higher rates of erosion. We attribute this to the fact that south-facing slopes likely had more dust-trapping vegetation cover during the glacial climates that dominated the Quaternary. The higher dust contents of soils on south-facing slopes was associated with formation of argillic horizons, lower saturated hydrologic conductivity and increased water holding capacity. Greater water retention, in turn, likely increased rates of erosion by bioturbation and freeze-thaw-driven creep in a positive feedback. Over time, dust accumulation at the hillslope point of inflection increased with age up to several hundred thousand years, then decreased with time as the cones degraded by erosion. Data suggest that approximately 200kyr of time was required before the soils developed sufficient water-holding capacity to drive in situ weathering of the basalt cinders. These results further demonstrate the importance of feedbacks among soil development, hydrology, and geomorphology in the evolution of hillslopes.

The Staglieno Monumental Cemetery (Genoa, Italy): microenvironment characterization through weathering indicators

The site of the Monumental Cemetery of Staglieno (Genoa, Northern Italy, about 0.33 km2 wide) was addressed by a systematic analysis of decay factors for monuments, to identify potential management concerns. Geologic (structural and stratigraphic), geomorphologic, hydrogeological, hydrologic, meteorological (between 1947 and 2007), botanic and urbanistic (land use) analyses were addressed, and thematic cartography edited for each issue. The decay was observed on ten monuments in white Carrara marble placed in four different areas of the Cemetery (highly vegetated alluvial plain, mildly steep hill slope, south-facing hill slopes, humid creek banks, sheltered galleries, etc.). At the mesoscale, the monuments were addressed for detailed survey, rendered as a weathering map, coupled with decay quantification by the Fitzner indexes. The large- and mesoscale analysis was associated with in situ observation and microinvasive ablation of the surface deposits. These were then analysed by plane polarized light optical microscopy and scanning electron microscope with microprobe (SEM + EDS). Finally, to enhance field observations, all decay evidences were gathered into a protocol inspired to the UNI 11182 weathering descriptions grouped in (1) chromatic alteration (2) ablation weathering (3) accretional weathering. By calibrating the influence of each factor and agent, we attained a weathering zonation into subareas (e.g. Cimitero Protestante, Porticato Inferiore, Boschetto Irregolare, Galleria Pontasso).

Sustainability of Hillslopes in Semiarid Rangelands: Effects of Rock Fragments

Rock fragments (RFs) affect micro-environmental conditions in underlying and surrounding soil, and thereby stimulate abiotic and biotic processes that interact with the surrounding environment. Microenvironmental conditions are of great importance in arid and semiarid landscapes, where surfaces feature high percentages of RFs. On south-facing hillslopes in the northern Negev region of Israel, soil was sampled from beneath small, medium, and large RFs – i.e., 4-6, 8-10, and 13-16 cm, respectively – that lay on the surface or were partially embedded in the soil. Control samples were taken from nearby bare soil. Rock fragment characteristics affected various soil properties with differing intensities. Under large and medium RFs soil moisture contents were higher than under the small ones; and embedded RFs promoted higher moisture contents than those lying on top of the soil (designated as

The influence of vegetation on debris-flow initiation during extreme rainfall in the northern Colorado Front Range

Abstract We explored regional influences on debris-flow initiation throughout the Colorado Front Range (Colorado, USA) by exploiting a unique data set of more than 1100 debris flows that initiated during a 5 day rainstorm in 2013. Using geospatial data, we examined the influence of rain, hillslope angle, hillslope aspect, and vegetation density on debris-flow initiation. In particular we used a greenness index to differentiate areas of high tree density from grass and bare soil. The data demonstrated an overwhelming propensity for debris-flow initiation on south-facing hillslopes. However, when the debris-flow density was analyzed with respect to total rainfall and greenness we found that most debris flows occurred in areas of high rainfall and low tree density, regardless of hillslope aspect. These results indicate that present-day tree density exerts a stronger influence on debris-flow initiation locations than aspect-driven variations in soil and bedrock properties that developed over longer time scales.

Soil quality and aggregation in runoff water harvesting forestry systems in the semi-arid Israeli Negev

Water availability is a major limiting factor for dryland afforestation. Earthworks that modify natural landforms for the formation of runoff harvesting systems are prevalent in the Israeli drylands, with the aim of establishing afforestation projects. However, serious concerns alarm that such earthworks have detrimental effects on the geo-ecosystem functioning. Therefore, the objective of this study was to examine the impact of the establishment of contour bench terrace (also called shichs/shychs/shikim) runoff harvesting systems on selected soil properties, with particular focus on soil aggregation. Thus, we assessed the effect of the establishment of contour bench terraces in a multi-aged forestry land, comprised of 2-year-old and 9-year-old afforestation areas, and compared them to ‘natural’ hillslopes as a reference treatment. The study was implemented in the Ambassadors' Forest, located in the semi-arid northern Negev of Israel, where we sampled the surface soil (at a depth of 0–5cm) in north- and south-facing hillslopes. Considerable differences were recorded for the afforestation systems between the source (inter-terrace area) and sink (terrace-bottoms) areas. Data normalizing according to the relative cover percentage of the terraces and inter-terrace areas showed that the mean values of mean weight diameter (MWD), stable aggregate content, and particulate organic carbon in the natural hillslopes were 1.4%, 32.4%, and 20%, respectively, greater than in the 9-year-old afforestation systems, and 12.1%, 28.9%, and 31%, respectively, greater than in the 2-year-old systems. Means of clod content, aggregate slaking index, and clay dispersion index in the natural hillslopes were 62.3%, ~twofold, and 35.0%, respectively, smaller than in the 9-year-old systems, and almost threefold, nearly twofold, and 46.2%, respectively, smaller than in the 2-year-old areas. The soil calcium carbonate content was similar in soils of the natural hillslopes and 9-year-old afforestation lands, which was ~17% smaller than in the 2-year-old afforestation systems. Considerable differences among the land-uses were also recorded for the soil texture. Mean coarse root biomass, despite revealing only a marginally-significant effect (p-value=0.0765), was 40.0% greater in the natural hillslopes than in the 9-year-old systems, and more than twofold greater than in the 2-year-old systems. Hillslope aspect affected some of the measured properties, revealing only slightly better soil conditions in the (mesic) north- than in the (xeric) south-aspects. The data revealed that forestry-related earthworks degrade the soil quality and geo-ecosystem functioning in the short term. Yet, despite some discrepancies, the data also showed the occurrence of self-restoration processes of the geo-ecosystem over the long term.

Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits

Abstract. We combine ecohydrological observations of sap flow and soil moisture with thermodynamically constrained estimates of atmospheric evaporative demand to infer the dominant controls of forest transpiration in complex terrain. We hypothesize that daily variations in transpiration are dominated by variations in atmospheric demand, while site-specific controls, including limiting soil moisture, act on longer timescales. We test these hypotheses with data of a measurement setup consisting of five sites along a valley cross section in Luxembourg. Both hillslopes are covered by forest dominated by European beech (Fagus sylvatica L.). Two independent measurements are used to estimate stand transpiration: (i) sap flow and (ii) diurnal variations in soil moisture, which were used to estimate the daily root water uptake. Atmospheric evaporative demand is estimated through thermodynamically constrained evaporation, which only requires absorbed solar radiation and temperature as input data without any empirical parameters. Both transpiration estimates are strongly correlated to atmospheric demand at the daily timescale. We find that neither vapor pressure deficit nor wind speed add to the explained variance, supporting the idea that they are dependent variables on land–atmosphere exchange and the surface energy budget. Estimated stand transpiration was in a similar range at the north-facing and the south-facing hillslopes despite the different aspect and the largely different stand composition. We identified an inverse relationship between sap flux density and the site-average sapwood area per tree as estimated by the site forest inventories. This suggests that tree hydraulic adaptation can compensate for heterogeneous conditions. However, during dry summer periods differences in topographic factors and stand structure can cause spatially variable transpiration rates. We conclude that absorption of solar radiation at the surface forms a dominant control for turbulent heat and mass exchange and that vegetation across the hillslope adjusts to this constraint at the tree and stand level. These findings should help to improve the description of land-surface–atmosphere exchange at regional scales.

Aspect-dependent soil saturation and insight into debris-flow initiation during extreme rainfall in the Colorado Front Range

Hydrologic processes during extreme rainfall events are poorly characterized because of the rarity of measurements. Improved understanding of hydrologic controls on natural hazards is needed because of the potential for substantial risk during extreme precipitation events. We present field measurements of the degree of soil saturation and estimates of available soil-water storage during the September 2013 Colorado extreme rainfall event at burned (wildfire in 2010) and unburned hillslopes with north- and south-facing slope aspects. Soil saturation was more strongly correlated with slope aspect than with recent fire history; south-facing hillslopes became fully saturated while north-facing hillslopes did not. Our results suggest multiple explanations for why aspect-dependent hydrologic controls favor saturation development on south-facing slopes, causing reductions in effective stress and triggering of slope failures during extreme rainfall. Aspect-dependent hydrologic behavior may result from (1) a larger gravel and stone fraction, and hence lower soil-water storage capacity, on south-facing slopes, and (2) lower weathered-bedrock permeability on south-facing slopes, because of lower tree density and associated deep roots penetrating bedrock as well as less intense weathering, inhibiting soil drainage.

Geodiversity, self-organization, and health of three-phase semi-arid rangeland ecosystems, in the Israeli Negev

Source–sink, two-phase mosaic-like ecosystems are widespread throughout the world's drylands. Such ecosystems are composed of woody vegetation patches and intershrub spaces and have been characterized as having high flexibility and survivability. Recent studies from the semi-arid Negev drylands of Israel reported that livestock grazing has resulted in the modification of two-phase mosaic-like shrublands into three-phase mosaic rangelands, with livestock trampling routes encompassing a separate, and the most degraded phase, while the shrubs encompass the most improved phase. The objective of this study was, therefore, to reassess this theory through the investigation of patch-scale (spatial scale of one to several decimeters) geodiversity and self-organization of these ecosystems. In terms of the effect of type of surface cover (microhabitat), the soil hygroscopic moisture content and stable aggregate content of the uppermost layer (0–5cm depth) were significantly affected by this factor, and revealed the highest, intermediate, and smallest values for the shrubby patches (3.06% and 77%), intershrub spaces (2.81% and 68%), and the trampling routes (2.63% and 55%), respectively. An opposite effect was recorded for the sand content, revealing 23.9%, 25.3%, and 26.0%, respectively. The clay dispersion index was also significantly affected by microhabitat, and revealed a higher value for the trampling routes (0.83) than for the intershrub spaces and shrub patches (0.37 for both). At the same time, other soil characteristics were not significantly affected by microhabitat. Overall, some differences were recorded between north- and south-facing hillslopes, proposing somewhat better soil quality in the northern aspects. A conceptual model is proposed, in which moderate livestock pressure increases ecosystem geodiversity at the patch scale, modifying the ecosystem's self-organization to encompass a new (dynamic) equilibrium of a tri-modal pattern, and increasing ecosystem health. Also, a simple numerical simulation is proposed, modeling the effect of livestock trampling routes on the redistribution of water at the patch scale, with the resultant modifications in distribution of vegetation cover. Yet, it is proposed that functioning of three-phase mosaic rangelands is more complex than previously suggested, encompassing several simultaneous effects, of which some may have offsetting impacts.

Meteoric 10Be, clay, and extractable iron depth profiles in the Colorado Front Range: Implications for understanding soil mixing and erosion

Analyzing meteoric 10Be in soil profiles along with soil measurements such as pedogenic Fe and clay content permits better understanding of meteoric 10Be transport in soils, soil formation, and hillslope geomorphology. This study presents meteoric 10Be depth profiles from saprolite-derived soil catenas sampled in the Gordon Gulch catchment, Colorado Front Range and from nearby c. 130 and 15ka glacial moraines. We compare meteoric 10Be data with more classically used soil analyses of clay and extractable iron (Fed) content. Meteoric 10Be and Fed do not show consistent trends in the hillslope profiles. Meteoric 10Be and clay concentrations, however, generally decrease with depth and highest concentrations of 10Be in horizons coincide with highest clay concentrations. Soils at moraine sites are better developed than those on hillslopes, and correlations of 10Be, Fed, and clay are stronger. In Gordon Gulch, south-facing hillslopes display meteoric 10Be bulges at depth and lower near-surface concentrations compared to the declining profiles and higher near-surface concentrations of north-facing hillslopes. The aspect differences imply that south-facing hillslopes experience greater vertical mixing than north-facing hillslopes, and that greater lateral transport and erosion has occurred over the last 15–20ka on south-facing hillslopes relative to north-facing hillslopes. Meteoric 10Be depth profiles at moraine sites display bulge profiles with highest meteoric 10Be concentrations in B-horizons, reflecting landform stability and pedogenic processes. Overall, our data demonstrate that signatures of chemical weathering in soils (Fed and clay) may or may not correlate to the meteoric 10Be added through atmospheric deposition, and that profile shape of meteoric 10Be can provide insight into the relative contribution of vertical mixing and surface erosion in relation to catena location and aspect during hillslope evolution.

Seasonal changes in the soil hydrological and erosive response depending on aspect, vegetation type and soil water repellency in different Mediterranean microenvironments

Abstract. Mediterranean areas are characterized by a strong spatial variability that makes the soil hydrological response highly complex. Moreover, Mediterranean climate has marked seasons that provoke dramatic changes on soil properties determining the runoff rates, such as soil water content or soil water repellency (SWR). Thus, soil hydrological and erosive response in Mediterranean areas can be highly time- as well as space-dependant. This study shows SWR, aspect and vegetation as factors of the soil hydrological and erosive response. Erosion plots were set up in the north- and the south-facing hillslope and rainfall, runoff, sediments and SWR were monitored. Soil water repellency showed a seasonal behaviour and it was presented in three out of four microenvironments after the summer, disappearing in the wet season. In general, runoff rate was higher in shrubs patches (0.47 ± 0.67 mm) than in inter-shrub soils (1.54 ± 2.14 mm), but it changed seasonally in different ways, depending on the aspect considered, decreasing in the north-facing hillslope and increasing in the south-facing one. The main factor determining the hydrological and erosive response was the rainfall intensity, regardless of the rainfall depth of the event. This response was modulated mainly by SWR in the north-facing hillslope and the vegetation pattern in the south-facing one.

Aspect and soil textural controls on snowmelt runoff on forested Boreal Plain hillslopes

Plot studies were conducted on a jack pine forest with sandy soil and aspen forests with sandy and loam soils to examine the controls of slope aspect, soil texture and fall soil moisture content on near-surface snowmelt runoff and infiltration. It was hypothesized that near-surface runoff would be greater from north-facing slopes on loam soils with increased fall soil moisture content. Fall soil moisture had no measurable effect on spring snowmelt runoff. Infiltration of snowmelt dominated (drainage coefficients 53–100%, median 87%) over near-surface runoff (runoff coefficients 1–65%, median 7%) for most plots. Runoff was related to concrete frost at the mineral soil surface. In contrast to the processes hypothesized, south-facing hillslopes with sandy soils generated greater runoff than north-facing slopes or sites with finer-textured soils. These results were due to greater concrete frost development resulting from periodic spring snowmelt and re-freezing in the upper soil. South-facing hillslopes with sandy soils featured lower canopy cover, allowing greater solar radiation to reach the snow surface which led to the formation of concrete frost and faster melt rates resulting in near-surface runoff. Where hillslopes are connected to receiving surface waters by continuous concrete frost, snowmelt runoff at the watershed scale may be enhanced.

Spatial variability of steady-state infiltration into a two-layer soil system on burned hillslopes

Rainfall–runoff simulations were conducted to estimate the characteristics of the steady-state infiltration rate into 1-m 2 north- and south-facing hillslope plots burned by a wildfire in October 2003. Soil profiles in the plots consisted of a two-layer system composed of an ash on top of sandy mineral soil. Multiple rainfall rates (18.4–51.2 mm h −1 ) were used during 14 short-duration (30 min) and 2 long-duration simulations (2–4 h). Steady state was reached in 7–26 min. Observed spatially-averaged steady-state infiltration rates ranged from 18.2 to 23.8 mm h −1 for north-facing and from 17.9 to 36.0 mm h −1 for south-facing plots. Three different theoretical spatial distribution models of steady-state infiltration rate were fit to the measurements of rainfall rate and steady-state discharge to provided estimates of the spatial average (19.2–22.2 mm h −1 ) and the coefficient of variation (0.11–0.40) of infiltration rates, overland flow contributing area (74–90% of the plot area), and infiltration threshold (19.0–26 mm h −1 ). Tensiometer measurements indicated a downward moving pressure wave and suggest that infiltration-excess overland flow is the runoff process on these burned hillslope with a two-layer system. Moreover, the results indicate that the ash layer is wettable, may restrict water flow into the underlying layer, and increase the infiltration threshold; whereas, the underlying mineral soil, though coarser, limits the infiltration rate. These results of the spatial variability of steady-state infiltration can be used to develop physically-based rainfall–runoff models for burned areas with a two-layer soil system.

The implications of geology, soils, and vegetation on landscape morphology: Inferences from semi-arid basins with complex vegetation patterns in Central New Mexico, USA

This paper examines the relationship between land surface properties (e.g. soil, vegetation, and lithology) and landscape morphology quantified by the catchment descriptors: the slope–area (S–A) relation, curvature–area (C–A) relation, and the cumulative area distribution (CAD), in two semi-arid basins in central New Mexico. The first site is composed of several basins located in today's desert elevations with mesic north-facing and xeric south-facing hillslopes underlain by different lithological formations. The second site is a mountainous basin exhibiting vegetation gradients from shrublands in the lower elevations to grasslands and forests at higher elevations. All three land surface properties were found to have significant influences on the S–A and C– A relations, while the power-law exponents of the CADs for these properties did not show any significant deviations from the narrow range of universal scaling exponents reported in the literature. Among the three different surface properties we investigated, vegetation had the most profound impact on the catchment descriptors. In the S–A diagrams of the aspect-controlled ecosystems, we found steeper slopes in north-facing aspects than south-facing aspects for a given drainage area. In elevation-controlled ecosystems, forested landscapes exhibited the steepest slopes for the range of drainage areas examined, followed by shrublands and grasslands in all soil textures and lithologies. In the C–A diagrams, steeper slopes led to a higher degree of divergence on hillslopes and a higher degree of convergence in the valleys than shallower slopes. The influence of functional types of vegetation detected on observed topography provided some initial understanding of the potential impacts of life on the organization of topography. This finding also emphasizes the critical role of climate in catchment development. We suggest that climatic fluctuations that are capable of replacing vegetation communities could lead to highly amplified hydrological and geomorphic responses.

Long-Term Effects of High Intensity Prescribed Fire on Vegetation Dynamics in the Wine Spring Creek Watershed, Western North Carolina, USA

We examined the long-term effects of a prescribed fire in a southern Appalachian watershed in Nantahala National Forest, western North Carolina, USA. Fire was prescribed in 1995 on this site by forest managers to restore a degraded pine (Pinus spp.)-hardwood community, specifically to stimulate forage production, promote pine and oak (Quercus spp.) regeneration, and increase plant diversity. Before and after the prescribed fire, permanent plots were sampled across a south-facing hillslope, which corresponded to three community types: mesic, near-stream cove (riparian); dry, mixed-oak (mid-slope); and xeric, pine-hardwood (ridge). In an earlier paper, we reported the first two years of post-burn vegetation response from this prescribed burn. In our current study, we compared the pre-burn (1994) forest condition with 10 years post-burn (2005) vegetation measurements to determine the effects of fire on the mortality and regeneration of overstory trees, understory shrubs, and herbaceous-layer species. Overstory mortality was high immediately after the burn at the ridge location and ten years after the fire. Mortality of pitch pine (Pinus rigida Miller) (91.8 %) and hickory (Carya spp.) (77.5 %) reduced overstory basal area from 26.97 m2 ha−1 pre-burn to 18.86 m2 ha−1 post-burn in 1995 and to 9.13 m2 ha−1 in 2005. At the mid-slope and riparian locations, no significant overstory mortality occurred over time. Understory density was significantly higher 10 years after the burn (2005) than pre-burn, and basal area had returned to pre-burn levels. Density of mountain laurel (Kalmia latifolia L.), black huckleberry (Gaylussacia baccata [Wang.] K. Koch), and blueberry (Vaccinium spp.) had increased due to prolific sprouting. The prescribed fire had varying effects on diversity across the hillslope gradient over time. On the ridge, overstory diversity declined following the fire (H’basal area = 1.14 in 1994, H’basal area = 0.75 in 1995, and H’basal area = 0.80 in 2005). Diversity significantly increased in the herbaceous layer and remained higher than pre-burn conditions through 2005 (H’cover = 1.02 in 1994, H’cover = 1.97 in 1995, and H’cover = 2.25 in 2005). For the mid-slope and riparian positions, no change in diversity was observed in the overstory, understory or herbaceous layer.

The effect of rock fragment size and position on topsoil moisture on arid and semi-arid hillslopes

Rock fragments on arid hillslopes affect rainwater redistribution and overland flow, through various hydrological processes. The present study focuses on the evolution of topsoil moisture content under rock fragments following rain events. Measurement was taken under rock fragments in various sizes (large and small) and positions (‘on top’ of the soil surface and 'partially embedded' within the soil surface) at north- and south-facing hillslopes in arid and semi-arid areas. The main findings: (1) the topsoil moisture content under rock fragments was higher over time than that of bare soil areas; (2) rock fragments affect the topsoil moisture content for a longer time in semi-arid areas than in arid areas; and (3) large rock fragments and ‘partially embedded’ ones are favorable micro-environments for accepting and retaining rainwater and overland flowing water. This may have eco-geomorphic implications regarding the mosaic-like patterns of source and sink areas on arid hillslopes.

HOLOCENE FIRE HISTORY OF A COASTAL TEMPERATE RAIN FOREST BASED ON SOIL CHARCOAL RADIOCARBON DATES

The long-term role of fire in coastal temperate rain forest is poorly under- stood. To determine the historical role of fire on western Vancouver Island (British Co- lumbia, Canada), we constructed a long-term spatially explicit fire history and examined the spatial and temporal distribution of fire during the Holocene. Two fire-history parameters (time-since-fire (TSF) and fire extent) were related to three landscape parameters (landform (hill slope or terrace), aspect, and forest composition) at 83 sites in a 730-ha low-elevation (less than ;200 m) area of a mountainous watershed. We dated fires using tree rings (18 sites) and 120 soil-charcoal radiocarbon dates (65 sites). Comparisons among multiple radiocarbon dates indicated a high probability that the charcoal dated at each site represented the most recent fire, though we expect greater error in TSF estimates at sites where charcoal was very old (.6000 yr) and was restricted to mineral soil horizons. TSF estimates ranged from 64 to ;12 220 yr; 45% of the sites have burned in the last 1000 yr, whereas 20% of the sites have not burned for over 6000 yr. Differences in median TSF were more significant between landform types or across aspects than among forest types. Median TSF was sig- nificantly greater on terraces (4410 yr) than on hill slopes (740 yr). On hill slopes, all south-facing and southwest-facing sites have burned within the last 1000 yr compared to only 27% of north- and east-facing sites burning over the same period. Comparison of fire dates among neighboring sites indicated that fires rarely extended .250 m. During the late Holocene, landform controls have been strong, resulting in the bias of fires to south-facing hillslopes and thus allowing late-successional forest structure to persist for thousands of years in a large portion of the watershed. In contrast, the early Holocene regional climate and forest composition likely resulted in larger landscape fires that were not strongly controlled by landform factors. The millennial-scale TSF detected in this study supports the distinction of coastal temperate rain forest as being under a fundamentally different disturbance regime than other Pacific Northwest forests to the east and south.