Persistence Of Repellency Research Articles

Effects of differing wildfire severities on soil wettability and implications for hydrological response

Fire-induced or enhanced soil water repellency is often viewed as a key cause of the substantial increases in runoff and erosion following severe wildfires. In this study, the effects of different fire severities on soil water repellency are examined in eucalypt forest catchments in the Sandstone Tablelands near Sydney, burnt in 2001 and 2003. At sites affected by different fire severities and in long-unburnt control sites, repellency persistence was determined in situ and in the laboratory for surface and subsurface soil samples ( n=846) using the Water Drop Penetration Time (WDPT) test. All long-unburnt samples were found to be water repellent, with severe to extreme persistence (>900 s) being dominant for surface (0–2.5 cm) and slight to moderate persistence (10–900 s) for subsurface (2.5–5 cm) soil, indicating naturally very high ‘background’ levels of repellency. In contrast to the generation or enhancement of repellency usually reported following forest fires of similar severity in previous studies, burning caused widespread destruction of repellency. The mineral soil depth to which repellency was destroyed (0.5–5 cm) was found to increase with burn severity. Below this charred wettable layer, persistence of pre-existing water repellency increased. Two years after the fire, the frequency of extreme repellency persistence was reduced in the surface and subsurface. However, recovery to pre-fire repellency levels had not been achieved. The associated hydrological impacts of these fire effects are more complex than simply the enhancement of overland flow, runoff and soil erosion with increasing fire severity. For forest fires sufficiently severe to remove foliage and ground litter above already repellent soil, a more severe burn, in which there is destruction of surface soil repellency, would result in lower runoff response compared to a burn insufficiently severe to destroy surface repellency. During storms intense enough to saturate the wettable surface rapidly, this layer may, however, be removed by overland flow, with potentially severe implications for soil fertility and seedbed survival, post-fire ecosystem recovery, and downstream sedimentation and water quality. The results demonstrate that existing fire severity classifications are not well suited to predicting fire impacts on soil hydrological responses and highlight the need for a new fire severity evaluation scheme. A scheme encompassing not only foliage and ground cover status, but also changes to surface and subsurface soil hydrological properties, would provide a better prediction of the immediate hydrological effects of wildfires on catchments such as flash flooding and erosion, and also of their time-to-recovery than current classifications allow. Such a scheme could prove invaluable given the future increase in fire frequency and severity predicted for many regions.

Water repellency in sandy luvisols under different forest transformation stages in northeast Germany

Soil water repellency can adversely affect soil hydrological properties, reduce infiltration capacity and induce preferential flow, surface runoff and erosion. Although primarily described from semiarid and Mediterranean climates, it is also a concern in forest sites under the humid climatic conditions of central Europe. The objective of this study was to characterize the water repellency on geologically comparable forest sites with different stand ages and tree species in terms of the effects of forest transformation upon soil physical properties. The investigated forest site called “Kahlenberg” (50 km northeast of Berlin) contains populations of Pinus sylvestris and Fagus sylvatica of different ages and proportions of single tree species. For the characterization of water repellency, the water drop penetration time (WDPT) test, for determining the persistence of water repellency, and the ethanol percentage (EP) test, which measures the severity or degree of water repellency, were carried out using soil samples from four forest plots and different soil depths (0–160 cm). “Potential” water repellencies were determined after 3-day oven-drying at 45 °C. For two plots, the “actual” water repellency with field moist soil samples was also measured using the WDPT test. The WDPT test revealed a significant proportion of severely and extremely hydrophobic samples in the upper 10 cm of the soil profile for all plots, whereas the persistence of repellency decreased with increasing soil depth. Soil organic matter contents and measured WDPT gave a positive linear correlation with r=0.73. Maximum measured ethanol percentages were 30% and restricted to the upper 5 cm of the soil profiles. The correlation of the organic matter content with EP was r=0.83. For all plots, the EP exhibited a shallower depth distribution than the WDPT. The two plots with pure pine and beech stands exhibited relatively low water repellencies, compared to the plots with mixed tree populations. This corresponds to a higher proportion of mor-type humus and a greater thickness of the humose topsoil at mixed stands compared to the pure pine and beech sites.

Oviposition attraction and repellency of Aedes aegypti (Diptera: Culicidae) to waters from conspecific larvae subjected to crowding, confinement, starvation, or infection.

As the biomass of Aedes aegypti (L.) larvae increased in relation to the volume of rearing waters, oviposition attraction of these waters to conspecific, gravid females first rose to a peak and then declined. Further increases in biomass rendered waters strongly repellent. Comparable responses were elicited by a decrease in the volume of rearing waters or an increase in the relative size or number of mosquito larvae. Low volumes of water reduced oviposition attraction and increased repellency, whereas larger volumes increased attraction. Excessively large volumes diluted attraction to neutrality. Constraints imposed by the physical dimensions of the larval environment which interfered with the normal postural movements and behaviors of the larvae also induced repellency, independent of rearing volume. Titration of repellent waters revealed that infection with the digenean Plagiorchis elegans (Rudolphi) generated the most powerful repellent effect, whereas crowding or starvation induced significantly weaker responses. At no time did dilution of repellent waters restore attractive properties. Repellents, even at minute concentrations, overrode attractants. The density-dependent action of oviposition attractants and repellents may help to maintain larval populations near optimal levels through their influence on recruitment. Oviposition repellency induced by sublethal infections with P. elegans may maintain population levels below the carrying capacity of the environment. Persistence of oviposition attraction and repellency varied inversely with temperature. The magnitude of the repellent effect induced by larvae appears to be a good indicator of their probability of survival and may be of selective significance. Repellency may deflect ovipositing females away from sites close to human habitation and may lead to new, and perhaps more effective, method, to control mosquitoes and the diseases they transmit.