Forested catchments throughout the world are known for producing high quality water for human use. In the 20th Century, experimental forest catchment studies played a key role in studying the processes contributing to high water quality. The hydrologic processes investigated on these paired catchments have provided the science base for examining water quality responses to natural disturbances such as wildfire, insect outbreaks, and extreme hydrologic events, and human-induced disturbances such as timber harvesting, site preparation, prescribed fires, fertilizer applications, pesticide usage, rainfall acidification, and mining. This paper compares and contrasts the paired catchment approach with landscape-level water resource monitoring to highlight the information on hydrologic processes provided by the paired catchment approach that is not provided by the broad-brush landscape monitoring.

Core Ideas: Wildfire is a major cause of forest soil disturbance. Fire severity, scale of fire, slope, infiltration rates, and post‐fire rainfall are major factors. Soil management has limited potential for mitigating negative effects. Fire area and severity are on the increase. Climate change is causing wildfires to be hotter, windier, drier, and larger. Wildfires and prescribed fires cause a range of impacts on forest soils depending on the interactions of a nexus of fire severity, scale of fire, slope, infiltration rates, and post‐fire rainfall. These factors determine the degree of impact on forest soils and subsequently the need for post‐fire soil management. Fire is a useful tool in landscape management but it can be benign or set off serious deteriorations in soil quality that lead to long‐term desertification. If parts of the nexus are absent or not inherently risky, forest soil impacts can be relatively minor or nonexistent. A low severity prescribed fire on a small landscape unit with minimal fuel loading, slopes less than 10%, and no water repellency is unlikely to damage soil condition and functions with all but heavy rainfall. On the other hand, a high severity wildfire in a substantial area of heavy fuels with slopes >100% and water repellency may undergo serious soil damage with even moderate rainfall. Soil management is not likely to be needed in the former case but virtually impossible in the latter scenario. This paper examines thresholds in the nexus factors which can raise the risks of wildland fire from low and moderate to high. It documents the interactions of the fire nexus using several case histories in North America and elsewhere to demonstrate different degrees of soil impact.

In Australia, farmers and natural resource managers are striving to enhance environmental outcomes at farm and catchment scales by planting streamside management zones (SMZs) on farms with trees and other perennial vegetation. Lack of sound information on and funding for establishing and managing trees in SMZs is hindering wide-scale adoption of this practice. Australian Codes of Forest Practice discourage or prevent harvesting of trees in SMZs of perennial streams. One concern is the potential effect of tree harvesting in SMZs on delivery of sediment to adjacent streams. The aims of this paper were to summarize the literature relevant to this concern and, in one case study in an agricultural context, to determine the effects on turbidity of harvesting a SMZ plantation. Information was sourced from published studies that reported on impacts of tree harvesting inside SMZs. In addition, a study was conducted in Tasmania, Australia, to evaluate the water quality benefits of a SMZ and the effects of tree harvesting in this zone. This case study consisted of a 20-year-old Eucalyptus nitens pulpwood plantation in a SMZ of an intermittent stream that was harvested according to the state Code of Forest Practice. A machinery exclusion zone immediately adjacent to the stream limited machinery traffic. Ground cover and water quality pre- and postharvesting were measured to identify the major sources of sediment in this headwater catchment, and to determine the effect of tree harvesting. Literature indicates that tree harvesting in SMZs is an accepted practice in the USA, New Zealand, and Germany, if conducted carefully, i.e., using best management practices (BMPs). Negative effects of this practice on water quality, in- and near-stream habitat, and biodiversity have been recorded, but these effects were generally minor or transitory. Tree harvesting in the Tasmanian study resulted in minimal mineral soil exposure and increased surface roughness. Postharvesting turbidity levels in streamflow were similar to preharvest levels (<2.5 nephelometric turbidity units exiting the catchment). Much more significant sources of sediment were a road, a dam that was accessible to cattle, and a cultivated paddock. These sources led to turbidities of c. 300 NTUs in a dam immediately below these points and above the harvested stream reach during a storm in late June 2009. In-stream dams, installed many years earlier to store water for stock and irrigation, acted as very effective sediment traps. The SMZs and other BMPs used in agroforestry landscapes are effective at protecting water quality. Forest harvesting operations can be conducted in SMZs without increasing stream turbidity, if existing BMPs are followed.

Forest ecosystems play an important role in hydrological processes as surface and subsurface runoff, as well as the storage of water at the catchment scale. Therefore, it is important to have a greater understanding of the effects of forests in the long-term water balance of Mediterranean catchments. In this sense, this study evaluates the effect of native forests, forest plantations, and the combination of both, on long-term streamflow variations in central Chile, an unusual area of Mediterranean climate characterized by a well-marked annual cycle with dry summers and wet winters. Thus, the temporal pattern of monthly streamflow was evaluated for mean flow (Qmean), maximum flow (Qmax), and minimum flow (Qmin) in 42 large-scale (>200 km²) Mediterranean catchments. Each series of monthly streamflow data was QA/QC, and then evaluated using the Mann–Kendall’s non-parametric statistical test to detect temporal variations between 1994 and 2015. In addition to the previous analysis, the monthly series were grouped into wet seasons (April–September) and dry seasons (October–April), to determine if there were any significant differences within the annual hydrological cycle. The areas covered with native and forest plantations and their relative changes were evaluated for each catchment through streamflow variations and forest cover indicators. Results revealed that streamflow variations are positive and significant when more forest cover exists. The intra-catchment relationships assessed for both species revealed the significant role of native forests and mixed masses as key ecosystems for the long-term conservation of summer streamflow in Mediterranean catchments of central Chile. These findings encourage an urgent need to create highland afforestation programs on degraded areas of central Chile, to maximize water storage in a region that is quickly drying out due to unsustainable water and land use management practices and the effects of global warming.

This study examined the effects of two types of parent material, sandstone and limestone, on the response of vegetation growth after the 1990 Dude Fire in central Arizona. The operating hypothesis of the study was that, given the right conditions, severe wildfire can trigger vegetation type conversion. Overall, three patterns emerged: (1) oak density increased by 413% from unburned sites to burned sites, with the highest densities occurring on sandstone soils; (2) weeping lovegrass (Eragrostis curvula Nees), a very aggressive non-native grass species seeded after the fire, now makes up 81% of the total herbaceous cover in the burned area; and (3) bare ground cover is 150% higher and litter cover is 50% lower in the burned area. Soil analysis was not definitive enough to differentiate impacts between parent materials however it was useful in quantifying the long-term impact of the fire on soils. The results of this study support the idea that catastrophic fire events can trigger vegetation type conversion and that perennial, non-native species used in rehabilitation efforts can persist within the ecosystem for long periods of time. Hence, the recovery period needed for the Dude Fire site to revert back to a pine-oak dominated forest could be on the scale of many decades to centuries.