This paper reviews the importance of large continuous openings (macropores) on water flow in soils. The presence of macropores may lead to spatial concentrations of water flow through unsaturated soil that will not be described well by a Darcy approach to flow through porous media. This has important implications for the rapid movement of solutes and pollutants through soils. Difficulties in defining what constitutes a macropore and the limitations of current nomenclature are reviewed. The influence of macropores on infiltration and subsurface storm flow is discussed on the basis of both experimental evidence and theoretical studies. The limitations of models that treat macropores and matrix porosity as separate flow domains is stressed. Little-understood areas are discussed as promising lines for future research. In particular, there is a need for a coherent theory of flow through structured soils that would make the macropore domain concept redundant.

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Macropores and water flow in soils

Information on primary production, decomposition, hydrology, and element cycling was intergrated in annual budgets of accumulation and flux among components of a mature Douglas-fir forest ecosystem. Annual N input in precipitation and dust was 2.0 kg/ha, and an estimated 2.8 kg/ha were fixed by cyanophycophilous lichens in the canopy. Annual N loss to groundwater was 1.5 kg/ha. Microparticulate litterfall provided a large input of N to the forest floor. Total annual loss to ground water was 9400 eq/ha and, because of little cation accumulation, loss exactly balanced input. Net transfers of P were small. Total annual input was 0.5 kg/ha, total loss was 0.7 kg/ha, and net accumulation was -0.2 kg/ha. Input of elements in precipitation and dryfall was small compared with that in the Eastern United States. Water chemistry profiles showed that the biologically important elements N, P, and K increased in concentration as water passed through the canopy and litter layer but decreased as water passed through the rooted part of the mineral soil. In contrast, Na increased by a factor of 20 as water passed through the rooted soil. Concentrations of all elements except Mg were lower in the stream water than in solution at 2.0-m depthmore » in the subsoil. Total return to the forest floor in litterfall was greater than that reported for other Douglas-fir stands mainly because of plentiful microparticulate forms and coarse woody debris. Leaf fall accounted for less than half of the total litterfall input of N to the forest floor. Fluxes of hydrogen ions (H/sup +/) resulting from water flow were negligible compared with H/sup +/ release during carbonic acid dissociation and H/sup +/ removal accompanying cation release in weathering. Uptake of metalic cations by vegetation and release during decomposition exceeded uptake and release of sulfur and phophorus anions, resulting in a net H/sup +/ flux of approximately 1 x 10/sup 3/ eq x ha/yr.« less

The Internal Element Cycles of an Old‐Growth Douglas‐Fir Ecosystem in Western Oregon

Many species of detritivorous invertebrates in small streams depend almost entirely on inputs of leaf litter for their nutritional requirements, however the concentration of this resource varies considerably seasonally. An experiment designed to test the hy- pothesis that productivity of coarse particle detritus feeders (shredders) is seasonally food limited was performed using replicate streamside channels that received one of three input rates of whole leaf detritus (rates equal to those naturally falling into montane streams, and two levels of increased inputs). Seven of the nine common shredder species attained significantly higher adult mass, higher densities, or both when food was supplemented. Larval densities of Malenka spp. and Brillia retifinis were significantly higher when additional food was added. Brillia retifinis densities were more than 10 x greater in high food additions than in the "natural" treatment; B. retifinis apparently fills a role as a fugitive in this system and is better able to track shifts in resource abundance by virtue of its short generation time. Zapada cinctipes and Z. haysi were more dense in high and intermediate food input treatments during the last half of the summer. Other taxa had nonsignificant trends to higher densities when extra food was provided. Most of the change in benthic densities can be attributed to decreased rates of emigration with increased food supply. Six of the eight taxa for which adult mass at emergence was measured were significantly more massive (4-46%) when food was added. Both sexes exhibited this increase in mass, but females gained proportionally more in most species. There were no detectable changes in the timing of adult emergence due to food manipulations. The biomass of most taxa increased in proportion to the overall increase in biomass of the common coarse-detritus consumers. The main exceptions to that pattern were a disproportionate increase in the percentage of the biomass represented by Brillia retifinis and a decrease in the representation of Zapada cinctipes. The responses of this community to food supplementation demonstrate food limitation of detritivores. The exponential increase in benthic biomass under food addition shows the scope for productivity if food were not limiting. These coexisting species benefitted from enhanced food supply on a spatial scale that is relevant for population level processes. The large seasonal variation in resource abundance under natural conditions creates bot- tlenecks during periods of low food supply, which constrain subsequent production even during periods when food is abundant.

Seasonal Food Limitation of Detritivores in a Montane Stream: An Experimental Test

Summary 1. Riparian trees regulate aquatic ecosystem processes, such as inputs of light, organic matter and nutrients, that can be altered dramatically when these trees are harvested. Riparian buffers (uncut strips of vegetation) are widely used to mitigate the impact of clear-cut logging on aquatic ecosystems but there have been few experimental assessments of their effectiveness. 2. Forests along 13 headwater stream reaches in south-western British Columbia, Canada, were clear-cut in 1998, creating three riparian buffer treatments (30-m buffer, 10-m buffer and clear-cut to the stream edge), or left as uncut controls, each treatment having three or four replicates. 3. We predicted that periphyton biomass and insect consumers would increase as buffer width decreased, because of increased solar flux. We used two complementary studies to test this prediction. 4. In one study, we compared benthic communities before and after logging in all 13 streams; a second study focused on periphyton and insect colonization dynamics over 6-week periods in each of four seasons in four streams, one in each treatment. 5. Photosynthetically active radiation, and mean and maximum water temperature, increased as buffer width narrowed. 6. Periphyton biomass, periphyton inorganic mass and Chironomidae abundance also increased as buffer width narrowed, with the largest differences occurring in the clearcut and 10-m buffer treatments. 7. Photosynthetically active radiation, water temperature, periphyton biomass and periphyton inorganic mass were significantly greater in the 30-m buffer treatment than in controls during some seasons. 8. Synthesis and applications. We have shown that a gradient of riparian buffer widths created a gradient in light and temperature that led to non-linear increases in periphyton biomass and insect abundance. For example, Chironomidae abundance was generally greater in the 10-m and 30-m buffer treatments than in controls, whereas this was not always the case in the clear-cut treatment. This pattern may be due to the high sediment content of the periphyton mat in the clear-cut treatment, which potentially limited the response of some insects to increased food resources. Overall, our results indicate that uncut riparian buffers of 30-m or more on both sides of the stream were needed to limit biotic and abiotic changes associated with clear-cut logging in headwater, forested watersheds.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2664.2003.00855.x

Responses of periphyton and insects to experimental manipulation of riparian buffer width along forest streams

Forest management activities may substantially alter the quality of water draining forests, and are regulated as nonpoint sources of pollution. Important impacts have been documented, in some cases, for undesirable changes in stream temperature and concentrations of dissolved oxygen, nitrate-N, and suspended sediments. We present a comprehensive summary of North American studies that have examined the impacts of forest practices on each of these parameters of water quality. In most cases, retention of forested buffer strips along streams prevents unacceptable increases in stream temperatures. Current practices do not typically involve addition of large quantities of fine organic material to streams, and depletion of streamwater oxygen is not a problem; however, sedimentation of gravel streambeds may reduce oxygen diffusion into spawning beds in some cases. Concentrations of nitrate-N typically increase substantially after forest harvesting and fertilization, but only a few cases have resulted in concentrations approaching the drinking-water standard of 10 mg of nitrate-NIL. Road construction and harvesting increase suspended sediment concentrations in streamwater, with highly variable results among regions in North America. The use of best management practices usually prevents unacceptable increases in sediment concentrations, but exceptionally large responses (especially in relation to intense storms) are not unusual.

Forest Practices As Nonpoint Sources of Pollution in North America

The hydrological, thermal, and chemical characteristics of two small streams flowing through relatively undisturbed, low-elevation mountain watersheds in southwestern British Columbia were investigated. All observations and chemical analyses of ecosystems were consistent with the hypothesis that stormflow originated mainly from flow of water through soil macrochannels to groundwater and thence to streams. Water budgets indicated unmeasured groundwater losses. The streams exhibited annual chemical cycles for most parameters, with maximum values in late summer and early autumn and minimum values in winter and early spring. Nitrate concentrations displayed no consistent seasonal variation, whereas potassium and sulphate concentrations were relatively uniform throughout the year. Most chemical parameters decreased with increasing discharge, whereas dissolved oxygen concentrations increased. Potassium concentrations exhibited some increases and some decreases, and chloride, nitrate, and sulphate concentrations were generally not significantly related to discharge. Concentration-discharge relationships were used to infer the origin of stormflow water. Differences in the chemistry of the two very similar streams have important ramifications for the design of watershed nutrient studies. Nutrient budgets were very similar to those of other watersheds in humid temperate regions, with net losses of calcium, sodium, magnesium, potassium, chloride, and sulphur. Nitrogen and phosphorus exports in dissolved or particulate organic form were not measured. Based on dissolved inorganic measurements, nitrogen was accumulated, while any gains or losses of phosphorus were extremely small.

M. C. Feller

Papers

Chemical characteristics of small streams near Haney in southwestern British Columbia