Effects Of Topographic Position Research Articles

Effects of Land Use and Topographic Position on Soil Organic Carbon and Total Nitrogen Stocks in Different Agro-Ecosystems of the Upper Blue Nile Basin

Soil organic carbon (SOC) and total nitrogen (TN) are key ecological indicators of soil quality in a given landscape. Their status, especially in drought-prone landscapes, is associated mainly with the land-use type and topographic position. This study aimed to clarify the effect of land use and topographic position on SOC and TN stocks to further clarify the ecological processes occurring in the landscape. To analyze the status of SOC and TN, we collected 352 composite soil samples from three depths in the uppermost soil (0–50 cm) in four major land-use types (bushland, cropland, grazing land, and plantation) and three topographic positions (upper, middle, and lower) at three sites: Dibatie (lowland), Aba Gerima (midland), and Guder (highland). Both SOC and TN stocks varied significantly across the land uses, topographic positions, and agro-ecosystems. SOC and TN stocks were significantly higher in bushland (166.22 Mg ha−1) and grazing lands (13.11 Mg ha−1) at Guder. The lowest SOC and TN stocks were observed in cropland (25.97 and 2.14 Mg ha−1) at Aba Gerima, which was mainly attributed to frequent and unmanaged plowing and extensive biomass removal. Compared to other land uses, plantations exhibited lower SOC and TN stocks due to poor undergrowth and overexploitation for charcoal and firewood production. Each of the three sites showed distinct characteristics in both stocks, as indicated by variations in the C/N ratios (11–13 at Guder, 10–21 at Aba Gerima, and 15–18 at Dibatie). Overall, land use was shown to be an important factor influencing the SOC and TN stocks, both within and across agro-ecosystems, whereas the effect of topographic position was more pronounced across agro-ecosystems than within them. Specifically, Aba Gerima had lower SOC and TN stocks due to prolonged cultivation and unsustainable human activities, thus revealing the need for immediate land management interventions, particularly targeting croplands. In a heterogeneous environment such as the Upper Blue Nile basin, proper understanding of the interactions between land use and topographic position and their effect on SOC and TN stock is needed to design proper soil management practices.

Locally high, but regionally low: the impact of the 2014–2016 drought on the trees of semi-arid savannas, South Africa

Drought is thought to be a factor limiting the abundance of shrubs and trees in savannas, but this has rarely been tested empirically. The severe drought of 2014–2016 provided an opportunity to document the impact of drought on the mortality of woody plants in semi-arid savannas of South Africa. Annual data from long-term tree monitoring plots were combined with opportunistic sampling in areas of high mortality to determine the extent and severity of mortality across an area of approximately 10 000 km2. Although mortality was not extensive, certain landscapes suffered mortality similar to the highest values reported for semi-arid ecosystems. Rainfall deficit was identified as a key determinant of spatial variation in mortality at the regional scale, which varied from 3% to 49% over two years. Spatial variation at the landscape scale was also high, and affected primarily by species composition and height of the woody plants, with higher mortality for taller individuals. An effect of topographic position and unknown edaphic factors was also evident. Species that suffered high mortality included a common bush encroachment species, but also many non-encroachers. Additional study is needed to confirm that the observed mortality will cause a long-term reduction in woody plant density.

English

English

Snowpack influences spatial and temporal soil nitrogen dynamics in a western U.S. montane forested watershed

AbstractDeclines in winter snowpack have increased the severity of summer droughts in western U.S. forests, with the potential to also impact soil available nitrogen (N). To understand how snowpack controls spatiotemporal N availability, we examined seasonal N dynamics across elevation, aspect, and topographic position (hollow vs. slope) in a forested watershed in the northern Rocky Mountains. As expected, peak snow‐water equivalent (SWE) was generally greater at higher elevations and on north‐facing aspects. However, the effects of topographic position and snowdrift led to variability in snow accumulation at smaller spatial scales. Spatial patterns of the snowpack, in turn, influenced soil moisture and temperature, with greater SWE leading to generally higher soil moisture levels during the summer and smaller temperature fluctuations throughout the year. Wetter conditions in spring or fall generally supported greater inorganic N pools, but at the driest locations (low‐elevation slope), pulses of N mineralization in summer may have played important roles in overall N dynamics. More importantly, soil moisture during the summer appeared to be more influenced by antecedent snowpack from the previous year than by current‐year summer rain. Subsequently, N mineralization under snowpack may be strongly influenced by soil moisture and temperature conditions from the previous fall, before snowpack accumulation. Together, our results indicate that snowpack strongly influences N dynamics beyond the current growing season in western coniferous forests through mediation of soil moisture and temperature, and suggest that further decline in winter snowpack may affect these forests through constraints in both water and N availability.

Elevation and topography influence community structure, biomass and host tree interactions of lianas in tropical montane forests of southern Ecuador

AbstractQuestionsHow does the structure of liana communities (diameter, density and biomass) in tropical montane forests vary along elevation and topographic gradients? How do patterns of tree infestation vary with elevation? Is tree diameter growth reduced by lianas in tropical montane forest?LocationTropical Andean forests (1000–3000 m a.s.l.) in the San Francisco Reserve and in the Podocarpus National Park, southern Ecuador.MethodsAll lianas (DBH ≥ 1 cm) were censused in 54 permanent plots (20 × 20 m) equally distributed between three study sites (1000, 2000 and 3000 m a.s.l.) and three topographic positions (lower, mid and upper slope) per site (six replicate plots at each site by position combination). The DBH and number of lianas hosted was recorded for all trees (DBH ≥ 10 cm). Liana biomass was estimated using allometric equations. ANOVAs were used to test for effects of elevation and slope positions on liana parameters and proportion of trees infested. The relationships between liana biomass and tree parameters and environmental parameters were analysed with partial least squares regression. We used the available literature data to perform a regression analysis of liana biomass in response to elevation in humid tropical old‐growth forests between sea level and 3000 m a.s.l.ResultsLiana diameter, density and biomass all decrease with elevation. The decreasing liana biomass agrees with results from previous studies of liana biomass in other humid tropical forests, indicating a decrease of 0.18 Mg·ha−1 liana biomass per 100 m of elevation gain. Topographic variation leads to thinner but more abundant stems upslope; there was no effect of topographic position on liana biomass. Liana biomass and liana infestation are both positively correlated with host tree DBH at every elevation. Tree diameter growth is reduced by liana infestation; the proportion of infested trees is lower in Andean montane forests than in tropical lowland forests.ConclusionsLiana biomass distribution and tree infestation vary significantly with elevation. Biomass of lianas and relative contribution of lianas to total above‐ground biomass both decrease with elevation. Topographic effects likely result from higher soil fertility at lower slope positions.

Microorganisms in Small Patterned Ground Features and Adjacent Vegetated Soils along Topographic and Climatic Gradients in the High Arctic, Canada

In this study, we determine differences in total biomass of soil microorganisms and community structure (using the most probable number of bacteria (MPN) and the number of fungal genera) in patterned ground features (PGF) and adjacent vegetated soils (AVS) in mesic sites from three High Arctic islands in order to characterize microbial dynamics as affected by cryoturbation, and a broad bioclimatic gradient. We also characterize total biomass of soil microorganisms and the most probable number of bacteria along a topographic gradient within each bioclimatic subzone to evaluate whether differences in topography lead to differences in microbial dynamics at a smaller scale. We found total microbial biomass C, the most probable number of heterotrophic bacteria, and fungal genera vary along this bioclimatic gradient. Microbial biomass C decreased with increasing latitude. Overall, microbial biomass C, MPN and the number of fungal isolates were higher in AVS than in PGFs. The effects which topographic position had on microbial biomass C varied across the bioclimatic gradient as there was no effect of topographic position in Isachsen (subzone A) and Mould Bay (subzone B), when compared to Green Cabin (subzone C, warmer site).There was no effect of topographic position on MPN counts at Mould Bay and Green Cabin. However, in Isachsen, MPN counts were highest in the wet topographic position as compared to the mesic and dry. In conclusion, PGFs seem to decouple the effect climate that might have on the total biomass of soil microorganisms along the bioclimatic gradient; and influence gets ameliorated as latitude increases. Similarly, the effect of topography on the total microbial biomass is significant at the warmest bioclimatic zone of the gradient. Thus, climate and topographic effects on total microbial biomass increase with warmer climate.

Establishment of Alleycropped Hybrid Aspen “Crandon” in Central Iowa, USA: Effects of Topographic Position and Fertilizer Rate on Aboveground Biomass Production and Allocation

Hybrid poplars have demonstrated high productivity as short rotation woody crops (SRWC) in the Midwest USA, and the hybrid aspen “Crandon” (Populus alba L. × P. grandidenta Michx.) has exhibited particularly promising yields on marginal lands. However, a key obstacle for wider deployment is the lack of economic returns early in the rotation. Alleycropping has the potential to address this issue, especially when paired with crops such as winter triticale which complete their growth cycle early in the summer and therefore are expected to exert minimal competition on establishing trees. In addition, well-placed fertilizer in low rates at planting has the potential to improve tree establishment and shorten the rotation, which is also economically desirable. To test the potential productivity of “Crandon” alleycropped with winter triticale, plots were established on five topographic positions with four different rates of fertilizer placed in the planting hole. Trees were then harvested from the plots after each of the first three growing seasons. Fertilization resulted in significant increases in branch, stem, and total aboveground biomass across all years, whereas the effects of topographic position varied by year. Allocation between branches and stems was found to be primarily a function of total aboveground biomass.

Effects of microsite conditions and early pruning on growth and health status of holm oak plantations in Central-Western Spain

The aim of this study was to examine the influences of early pruning and microsite conditions on the growth and health status of holm oak plantations on former agricultural land in Central-Western Spain. With that purpose, we designed a cross-sectional study to examine the effects of topographic position, gravel surface cover and accompanying vegetation cover on holm oak plantations density, growth (height and basal diameter) and health status. We surveyed three technically identical and multi-patched holm oak plantations dating from 1995, 1998 and 2003, aged 15, 12 and 7 respectively, covering an homogeneous climate and soil area of about 500 ha located in Central-Western Spain. Six hundred and seventy-two pruned and unpruned trees were sampled in 107 systematically sampled plots covering a wide range of site conditions in the studied area. At the tree level, two linear mixed effect models were adjusted for each year under study to explore the influence of pruning and microsite factors on height and diameter. Also at a tree level, a linear mixed effect model was fitted in order to analyze the influence of microsite factors and pruning on the health status of trees. At plot level, we performed a Multifactor Analysis of variance model to explore the influence of the microsite predictor variables and of the age of plantations on stocking. The results revealed that both basal diameter and height were increased by early pruning in all plantations under study. The effects on height and basal diameter of the three microsite parameters analysed varied with the age of plantations. The oldest plantations (15 years old), were affected by the surface gravel cover, while vegetation cover was significant in 12 years old plantations. Slope position was near relevant (p < 0.1) only in the youngest plantations. When significant, a high surface gravel cover was directly related to higher trees and larger basal diameters. Plantations density was significantly affected by the topographic position and, in all the plantations studied, pruned trees were associated with a better health status. Overall, our results show that early pruning and microsite conditions significantly influence the growth of holm oak seedlings. These results suggest that the management and growth modelling of holm oak should take into consideration the spatial distribution of the different microsites represented. Moreover, early- pruning should be considered as a positive practice affecting not only final tree form but also the growth and health status of trees. Finally, our results should lead to for further studies and help for improving future projects in areas with similar conditions.

Effects of Topographic Position and Geology on Shaking Damage to Residential Wood-Framed Structures during the 2003 San Simeon Earthquake, Western San Luis Obispo County, California

A statistical evaluation of shaking damage to wood-framed houses caused by the 2003 M6.5 San Simeon earthquake indicates that both the rate and severity of damage, independent of structure type, are significantly greater on hilltops compared to hill slopes when underlain by Cretaceous or Tertiary sedimentary rocks. This increase in damage is interpreted to be the result of topographic amplification. An increase in the damage rate is found for all structures built on Plio-Pleistocene rocks independent of topographic position, and this is interpreted to be the result of amplified shaking caused by geologic site response. Damage rate and severity to houses built on Tertiary rocks suggest that amplification due to both topographic position and geologic site response may be occurring in these rocks, but effects from other topographic parameters cannot be ruled out. For all geologic and topographic conditions, houses with raised foundations are more frequently damaged than those with slab foundations. However, the severity of damage to houses on raised foundations is only significantly greater for those on hill slopes underlain by Tertiary rocks. Structures with some damage-resistant characteristics experienced greater damage severity on hilltops, suggesting a spectral response to topographic amplification.

Environmental characteristics affecting Helianthus annuus distribution in a maize production system

Population density and seedling growth stage were measured over three years along transects passing through patches of feral Helianthus annuus in fields of maize. The effect of topographic position on seed re-dispersal via erosion was investigated, and density of H. annuus was significantly correlated ( P < 0.01) to percent soil organic carbon (SOC) (positive) and relative elevation (negative), but not to soil pH. Mean seedling growth stage did not change with distance from the patch center. Seedling density was always highest at the patch center with no reduction in mean seedling size. Marked seed dispersed at various topographic positions were recovered two weeks after dispersal. A small percentage of H. annuus seeds was recovered, most of which were found at lower topographic positions – their movement being facilitated by water flow down slope. Seeds at the lowest topographic positions were not moved to other topographic positions. Prevailing wind moved seeds to a lesser degree. The association of weedy H. annuus with SOC and the dispersal to lower topographic positions appear to be useful factors in explaining its distribution and might be generalized to other weed species and systems.

Effects of topographic position, leaf litter and seed size on seedling demography in a semi-deciduous tropical forest in Panamá

This study examined whether topography-induced gradients in water potential and leaf litter depth contribute to species coexistence in tropical forests through species-specific effects on seedling emergence and mortality. Seedling emergence and mortality were followed for a period of 12 months in 36 (1 × 2 m) plots on Barro Colorado Island (BCI), Panama. Plots with and without litter were distributed on slope and plateau sites in three catchments. In the absence of manipulations, the lower litter depth on slopes resulted in approximately four times as many emergent seedlings than on plateau sites. However, litter depth had little effect on seedling community composition. By the end of the first dry-season, post-emergence, there were no significant differences in surviving seedling numbers between any treatments. There were differences in the emergent seedling community between slope and plateau sites within the same catchment as well as differences in composition between catchments, suggesting that both niche partitioning and dispersal limitation might play a role in structuring seedling community composition. During the wet-season seedling mortality was highest on slope sites although this pattern was reversed during the dry-season. In both seasons mortality was higher for small-seeded species. These results demonstrate that gradients in water potential related to topography impact on patterns of seedling emergence and mortality although processes in the first year after emergence may be insufficient to explain observed habitat preferences of adult plants.

Topographic position affects the water regime in a semideciduous tropical forest in Panama

The effects of topographic position on water regime in a semideciduous tropical forest on Barro Colorado Island in Panama were assessed by measuring soil matric potential using the filter paper technique and by using measured soil water release characteristics to convert a long-term (20 years) gravimetric water content data-set to matric potential. These were also compared against predictions from a simple water balance model. Soil matric potentials on slope sites were significantly higher than on plateau sites throughout the measurement interval and slopes experienced a shorter duration of drought during the annual dry-season. Measured values of matric potential agreed with those predicted from converting the gravimetric measurements using water release characteristics. Annual duration of drought predicted by the simple water balance model agreed with values determined from the converted long term water content data-set and was able to predict the annual duration of drought on plateau sites. On slope sites, the water balance systematically and significantly overestimated the duration of drought obtained from the water content data-set, suggesting that slope sites were supplied with water from upslope. Predictions of annual drought duration from sites with higher annual rainfall than Barro Colorado Island (BCI), suggest that while plateau sites on BCI experience a water regime consistent with annual rainfall, slopes experience a water regime more similar to that of forests with much higher rainfall. We conclude that such large variations in water regime over small spatial scales may play a role in maintaining high species richness through providing opportunities for niche specialisation and by buffering slopes against possible climate change.

Landscape-scale variability of N mineralization in forest soils

Our understanding of the controls on N-cycling and availability in forest soils following disturbance is limited. A comparative study was conducted to examine the spatial distribution of soil N in forest soils and assess the effects of site disturbance. Sampling grids were established within a 120 × 120 m representative area at a native site, and at recently (i.e. 4-year-old) burned and clear-cut sites. A three-dimensional classification of landscape form was used to stratify each landscape into distinct landform elements. The spatial distribution of inorganic-N was not related to landform element, irrespective of site disturbance, indicating an absence of topographic control at the scale studied. However, a narrowing of the NH 4 +-to-NO 3 − ratio at the clear-cut site compared to the native site suggests that N-cycling was influenced by site disturbance. Similarly, an increase in the size of the microbial biomass at the clear-cut site, coupled with a widening of the microbial biomass C-to-N ratio, suggest that disturbance altered both the size and composition of the microbial biomass. Potential N and C mineralization, and net nitrification in the forest floor and surface mineral horizons representing two distinct landform complexes were studied in a controlled aerobic 8-wk incubation experiment. Accumulation of NH 4 + and NO 3 − differed markedly between sites although the effects of topographic position were generally non-significant. Inorganic-N accumulated principally as NH 4 + in soils from the native site due to an extended lag in nitrification. In contrast, NH 4 + accumulation in soils from the recently disturbed sites remained limited, whereas NO 3 − accumulation predominated. Thus, although topography did not markedly influence N distribution at the scale studied, site disturbance had a direct effect on N-cycling processes in these forest soils.

Effect of Topographic Position and Fire on Species Composition in Tallgrass Prairie in Northeast Kansas

Effect of Topographic Position and Fire on Species Composition in Tallgrass Prairie in Northeast Kansas

El efecto del fuego y las características topográficas sobre la vegetación y las propiedades del suelo en la zona de transición entre bosques y pastizales de las sierras de Córdoba, Argentina

En el contexto actual de cambio climático, el estudio de la dinámica de las zonas de transición entre bosques y pastizales tiene un papel fundamental, tanto a nivel científico como en la planificación del manejo de los ecosistemas. En este trabajo evaluamos el efecto combinado del fuego, la exposición de ladera y la posición topográfica sobre la vegetación y los suelos en una zona de transición entre bosques y pastizales en las montañas de Córdoba. Para ello elegimos cuatro áreas con diferentes edades post-fuego. En cada sector seleccionamos todas las combinaciones posibles de exposición y posición topográfica. En cada sitio realizamos un relevamiento florístico completo y analizamos una muestra compuesta de suelo. El tiempo transcurrido después del último incendio fue el principal factor determinante de las distintas características analizadas (fisonomía, composición florística, riqueza y propiedades del suelo), diferenciando sitios quemados recientemente de los no quemados. La exposición de ladera también tuvo un rol importante condicionando la fisonomía, la composición florística y las propiedades de los suelos. Sin embargo, no encontramos una relación directa entre la recuperación de la cobertura arbórea y el tiempo transcurrido después del último incendio, indicando que otros factores como las características geomorfológicas, historia de uso y herbivoría, estarían condicionando la recuperación post-fuego de estos sistemas.