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Soil Saturation (soil + saturation)
Selected AbstractsEvaluation of soil saturation, soil chemistry, and early spring soil and air temperatures as risk factors in yellow-cedar declineGLOBAL CHANGE BIOLOGY, Issue 3 2006D. V. D'AMORE Abstract Yellow-cedar (Callitropsis nootkatensis (D. Don) Oerst.) is a valuable tree species that is experiencing a widespread decline and mortality in southeast Alaska. This study evaluated the relative importance of several potential risk factors associated with yellow-cedar decline: soil saturation, soil aluminum (Al) toxicity or calcium (Ca) deficiency, and air and soil temperature. Data were collected from permanent vegetation plots established in two low-elevation coastal forests exhibiting broad ranges of cedar mortality. Measurements of each risk factor were contrasted among classified forest zones to indicate if there were strong links with decline. Hydrology alone is weakly associated with yellow-cedar decline, but could have a predisposing role in the decline by creating exposed conditions because of reduced forest productivity. Yellow-cedar decline is not strongly associated with soil pH and extractable Al and Ca, but there appears to be Ca enrichment of surface soils by feedback from dead yellow-cedar foliage. Air and soil temperature factors are strongly associated with decline. Based on these results, an hypothesis is presented to explain the mechanism of tree injury where exposure-driven tree mortality is initiated in gaps created by soil saturation and then expands in gaps created by the tree-mortality itself. The exposure allows soils to warm in early spring causing premature dehardening in yellow-cedar trees and subsequent freezing injury during cold events. Yellow-cedars growing in the protection of shade or snow are not preconditioned by this warming, and thus not as susceptible to cold injury. Yellow-cedar decline appears to be associated with regional climate changes, but whether the cause of these changes is related to natural or human-induced climate shifts remains uncertain. Management implications, the possible role of climate, and recommended research are discussed. [source] The cycle of instability: stress release and fissure flow as controls on gully head retreatHYDROLOGICAL PROCESSES, Issue 1 2001A. J. C. Collison Abstract Gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation, sapping or piping. The empirical evidence to substantiate these conceptual models is sparse, however, and often contradictory. This paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology, stability and elastic deformation of a marl gully complex in Granada Province, south-east Spain. The hydrological and slope-stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head, and that vertical or subvertical heads will be stable unless saturation is achieved. Owing to the high unsaturated strengths of marl measured in this research, failure in unsaturated conditions is possible only where the gully head wall is significantly undercut. Head retreat thus requires the formation of either a tension crack or an undercut hollow. Finite-element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base, and a second stress anomaly just upslope of the head wall. Although tension cracks on gully heads have often been attributed to slope unloading, this research provides strong evidence that the so called ,sapping hollow' commonly found in the gully headwall base is also a function of stress release. Although further research is needed, it seems possible that ,pop out' failures in river channels may be caused by the same process. The hydrological analysis shows that, once a tension crack has developed, throughflow velocity in the gully headwall will increase by an order of magnitude, promoting piping and enlargement of this weakened area. It is, therefore, possible to envisage a cycle of gully expansion in which erosion, channel incision or human action unloads the slope below a gully head, leading to stress patterns that account for the tension crack and a stress-release hollow. The tension crack promotes faster throughflow, encouraging hollow enlargement and piping, which undercut the gully head. The tension crack permits the development of positive pore-water pressures behind the gully head, leading either to failure or contributing to toppling. Finally the debris may be eroded by fluvial action, unloading a new section of slope and completing the cycle of gully head retreat. Copyright © 2001 John Wiley & Sons, Ltd. [source] Arbuscular mycorrhizal fungi and water table affect wetland plant community compositionJOURNAL OF ECOLOGY, Issue 5 2006BENJAMIN E. WOLFE Summary 1Most studies of the community-level effects of arbuscular mycorrhizal fungi (AMF) have been conducted in upland grassland plant communities where a majority of the plant species are colonized by AMF. Here, we examine the effects of AMF on plant community composition in experimental wetland plant communities, where the dominant plant species are non-mycorrhizal and subordinate plant species are colonized by AMF. We also assess how an important abiotic soil variable, depth to water table (soil saturation), might mediate the community-level effects of AMF. 2In the low water table (un-saturated) treatment, above-ground plant biomass increased in the presence of AMF relative to the controls, while in the high water table treatment, biomass decreased with the presence of AMF. Contrary to predictions, plant diversity was unaffected by the presence of AMF in the low water table treatment, but significantly decreased in the presence of AMF in the high water table treatment. Changes in biomass and composition were driven by the interactions between the dominant non-mycorrhizal species Carex hystercina, and the remaining mycorrhizal plant species. 3Our results indicate that AMF have the potential to influence plant community composition in calcareous fens and that these effects can be mediated by soil saturation. 4This study has implications for understanding how established principles of above-ground/below-ground interactions from upland communities translate to wetland plant communities and for understanding how AMF function can be mediated by abiotic soil properties. Contrary to previous thought, AMF may be important drivers of plant community composition in wetland plant communities. [source] Control of woolly aphid, Eriosoma lanigerum (Hausmann) (Hemiptera: Pemphigidae) on mature apple trees using insecticide soil-root drenchesAUSTRALIAN JOURNAL OF ENTOMOLOGY, Issue 1 2003Adrian H Nicholas Abstract Woolly aphid, Eriosoma lanigerum, is an important pest of apples that infests both the aerial and root parts of the tree. Root colonies are protected from the pesticide sprays applied during the growing season and the climatic effects of winter. Consequently, root colonies are a major source of aerial re-infestation in the following spring. Imidacloprid, the first of a new group of insecticides from the chloronicotinyl family, is known to provide excellent control of woolly aphid on trees up to 7-years-old when applied as a root soil drench. This study compared the effects of a single application of chlorpyrifos, imidacloprid, pirimicarb or vamidothion, applied as a root drench over four growing seasons. A soil wetting agent was added to each chemical to improve soil saturation and penetration. Imidacloprid provided excellent control of woolly aphid on the trees that were 17-years-old at the start of the study and continued to do so for four seasons. Pirimicarb appeared to offer some suppression of woolly aphid during the first season but not in subsequent seasons, while chlorpyrifos and vamidothion failed to control woolly aphid in any season. The potential role for imidacloprid in IPM programs is discussed. [source] | ||||||||||