MODULE 3: PROTECTION AGAINST FLOODING SUBMODULE 3.1: FORESTS AND FORESTRY IN WATER MANAGEMENT AND FLOOD PROTECTION 1
Concept
A few decades ago, forestry and water policies used to be based on the assumption that under any hydrological and ecological circumstances, forest is the best land cover for maximizing water yield, regulating seasonal flows and ensuring high water quality. The public consider water to be one of the most important forest “products”. The influence of forests to water cycle is believed to be always positive. However, some part of above-mentioned “facts” are now taken as obsolete myths. Forest hydrology research conducted during the last decades unveiled different picture. While the important role of the watersheds’ forest cover in protection of water quality has been confirmed, assumptions about the importance of these forests in flood protection and securing the water supplies during dry seasons has been proven to be misleading. Recent research has shown that, especially in arid areas, forests are not the best land cover for increasing downstream water yields. Moreover, the data suggest that the flood-protective effects of watersheds’ forests have often been overestimated. Especially in case of major events affecting large-scale watersheds or river basins, the role of forests is unimportant and the floods can be explained by other causes. Of course, forests are still a significant component of water cycle and their proper management can help to solve water related problems. However, it is necessary to realise that decision-making many times means “something for something”. For instance, better flood protection (e.g. by afforestation) usually means less water available for hydroelectric dams, irrigations or decreased groundwater recharge. Decision maker has to understand the mechanisms of forest influence on water regime and to balance them properly. This material provides the basic facts for such decisions and suggests some further sources of information.
2
Forest ecosystem and water
From the hydrology viewpoint, forest consists of two main components: trees and soil. Soil, in general, acts as a “sponge” storing water, while trees are similar to “pumps” pumping the soil moisture into the air 1. Besides that, eroded soil particles (possibly together with attached chemicals) represent the commonest water pollutant. Trees have also another mechanism to increase water evaporation and to reduce the streamflows and groundwater levels: interception. On the other hand, trees can significantly reduce soil erosion and thus improve the quality of surface waters, as well as to protect the thickness of soil layer and thus increase its water capacity. Forests can also, through the improvement some soil properties, accelerate the water infiltration and thus reduce overland flow. The following picture depicts main constituents of water balance with their framework parameters: Figure 1 The hydrologic cycle
TRANSPIRATION + evaporation 200 – 800 mm/yr
PRECIPITATION 400 – 1600 mm/yr
Interception 60 – 500 mm/yr
Subsurface inflow 100 – 500 mm/yr
Capillary rise N/A
Soil water
Overland flow usually avoidable
Groundwater recharge 50 – 500 mm/yr
Groundwater
1
Subsurface outflow 100 – 500 mm/yr
Groundwater discharge
Streamflow
FAO, 2008: Forests and water: A thematic study prepared in the framework of the Global Forest Resources Assessment 2005, FAO Forestry paper 155, Rome, 2008
2.1
Sources of water
For the vast majority of landscapes, the precipitation, whether in form of rain or snow, represent the only source of water. Floodplains have an additional source of water: uptake of groundwater, which is recharged from rivers, and can become, especially in dry climate, the main source of water. However, such areas are restricted to the flat surfaces near rivers. In general, forests do not influence precipitation or, at least, their influence is rather limited and regional: “On the whole, it seems that the idea that forests importantly affect precipitation is rejected by most meteorologists, except where fog drip and rime occur frequently.” 2 The quoted exception refers to so-called “cloud forest”, where the condensing fogs substantially contribute to water balance. Such forest can be found in the mountains near the alpine tree limit or in some coastline areas with frequent fogs.
2.2
Water losses
The yearly precipitation (rarely combined with other sources) has to be divided among several “water loses”, which are described in the following chapters. 2.2.1
Interception
Not all of the precipitation, which falls to an ecosystem, reaches the soil surface to form streamflow or infiltrate to groundwater resources. “Interception is the process by which falling precipitation interacts with the vegetation. The result of this interaction depends on vegetation and precipitation characteristics.” 3 During this process, the canopy (tree crowns, shrubs, herb layer or litter) intercepts a part of water, which falls on an ecosystem, and this water is detained there long enough to evaporate back to the atmosphere. The volume of precipitation, which can be stored this way, is defined as an interception capacity. From the short-term point of view, the interception seems to be unimportant as the interception capacity of forest canopy during single precipitation event reaches only a few millimetres (usually 1 – 3 mm). However, one single rainfall many times also not exceeds several millimetres, which is comparable with an interception capacity. For example, assume that interception capacity of a watershed is 1 mm. If 2 mm of rain falls, interception loss will be about 50%, but if 10 mm of rain falls, interception loss will be about 10%. Depending on the regional climate (precipitation characteristics such as average intensity, average duration and average wind speed during rains), interception losses can reach 15 – 30% of yearly precipitation (hundreds of millimetres per year). Interception losses in coniferous forests are usually higher than losses in broadleaved forests. As for manageable properties of forests, an alteration of tree species composition is probably the best and the most sustainable way to change the existing interception.
2
Satterlund, D.R. and Adams, P.W. 1992. Wildland watershed management. 2nd edition. Wiley, New York. 436 pp. 3 Tate K. W. 1996: Interception on Rangeland Watersheds; Rangeland Watershed Program, Factsheet, No. 36
Table 1 Rough estimates of interception losses Tate (1996) - USA 4 Vegetation type Interception losses Throughfall % % Coniferous forest 12 - 30 67 - 85 Coniferous litter 5 Broadleaved forest 13 - 21 75 - 82 Broadleaved litter 3 46 Evergreen oak 71 (including litter) Grassland 10 - 20 Prairie 14 - 24 Tall grasses 57 - 84 (Andropogon gerardii)
Stemflow % cca 3 1,5 - 5 -
Calder ex Nisbet (2005) - GB Interception losses % 25 - 45 10 - 25 (ash 11, beech 15) -
cca 3
-
-
-
-
Conifers with their needle-shaped leafs usually prevent the merge of single raindrops, while broadleaves support such merge creating larger drops that fall from crowns easier. In addition, the litter of conifers has slightly higher water capacity. Interception capacity of broadleaves significantly decreases during winter season, while the interception capacity of conifers is almost constant throughout the year. However, the differences between the trees of the same species can be larger than the differences between two specimens of the different species. Denser crowns intercept more water than sparse ones. Scabrous leaves or rougher bark can also increase the interception capacity. Some tree species allow for denser herb and brush layer than others, which also influence the interception. Table 2 Interception by tree species in % of annual precipitation (Augusto et al. 2002) 5 Abies Betula Carpinus Fagus Picea Pinus Pseudotsuga alba spp. betulus sylvatica abies sylvestris menziesii
36
17
27
22
35
40
41
Quercus petraea
Quercus robur
23
24
There is also a correlation between the age of forest stand and its interception capacity. In general, even-aged stands reach the near-the-full interception capacity in rather young age and it culminates at the age about 40 years 6. Older stands usually have slightly lower interception capacity. 2.2.2
Evapotranspiration
Evapotranspiration is the common term for two different processes results of that are difficult to distinguish one form another. Evaporation usually refers to evaporation of the water stored in soil directly to the air; transpiration refers to water vapour released through stomata of plants. In fact, evaporation includes also interception but, due to different measurement methods, they are often distinguished. Transpiration is sometimes called “productive evaporation� as the transpired water is used for physiological processes in plants. Evapotranspiration (without interception) can exceed 50 % of yearly precipitation and most of that is transpiration. It is due to the different speed of water transport in soil and in plants. Evaporation from soil surface depends on capillary rise of soil water, which is generally slow (from centimetres to metres per day). Transpiration depends on the needs of plants and is much faster, because plant roots takes up the water from various depths of soil at the some time, and do it actively. Because of above-mentioned mechanisms, the traditional myth that trees protect soils against drying is busted. The opposite is true; trees are huge consumers of water and make soils drier than most other plants 7, and much drier than they would be without vegetation cover. 4
Nisbet T. 2005: Water Use by Trees, Forestry Commission, Information Note, Forestry Commission, Edinburgh, 8 pp. 5 Augusto, L., Ranger, J., Binkley, D., Rothe, A. 2002: Impact of several common tree species of European temperate forests on soil fertility, Annals of Forest Science, Vol. 59 No. 3 (April 2002) p. 233 6 Yu Yannian, 1990: Hydrological effects of forests, The Hydrological Basis for Water Resources Management (Proceedings of the Beijing Symposium, October 1990), IAHS Publ. no. 197
While evaporation from soil surfaces or interception depends on many variable factors, transpiration rates are much more constant as they reflect the physiological needs of trees. Expressed as an equivalent of precipitation, transpiration rates usually fall into a relatively narrow range of 300– 390 mm per year, regardless of precipitation in particular year. There are only few exceptions. Some tree species of flood-plain forests can transpire more water, up to 500 mm per year, because they have special mechanisms to get rid of surplus water. Some plant species of arid regions have an ability to stop transpiration (to close stomata) and thus their annual transpiration can be lower in dry years. Therefore, transpiration of such plants, expressed in percents, varies form year to year significantly, being larger in dry years than in wet ones. Forestry measures, which are able to change the existing transpiration rates, include an alteration of tree species composition and modification of the ratio between age classes of forest stands (if even-aged). Table 3 Typical range of annual evaporation losses (mm) for different land covers. Note that there are various approaches to interception and their inclusion to evapotranspiration (e.g., interception of herb layer is not recognised – please compare to the table 1) Nisbet 2005 Strebel et Renger ex Šály (1998) 8 Total Evapotranspiration Land cover Transpiration Interception Land cover evaporation (without interception) Sprucewood 550–750 Conifers 300–350 250–450 550–800 Pinewood 400–600 Broadleaves 300–390 100–250 400–640 Beechwood 450–500 Grass 400–600 – 400–600 Grass 400 – 700 Bracken 400–600 200 600–800 Arable Arable 370–430 – 370–430 350 - 550 (not irrigated) (not irrigated)
In even-aged forest stands, transpiration reaches its maximum at the age of 25-50 years. Overmature stands usually have much lower transpiration rates than young stands, in case of some species, the transpiration of their over-mature stands is comparable to clearings or very young plantations, as shows, for example, well-known Kuczera curve for Eucalyptus regnans 9. For spruce woods, the evapotranspiration of clearing equals to 45% of evapotranspiration of mature 90-years-old forest, but the 40-years-old forests’ evapotranspiration is by 15% higher than in mature forest 10.
7
Nisbet T. 2005: Water Use by Trees, Forestry Commission, Information Note, Forestry Commission, Edinburgh, 8 pp. 8 Šály, R.1998: Pedológia, vysokoškolské skriptá, Technická univerzita vo Zvolene, 177 pp 9 Kuczera, G., 1985. Prediction of water yield reductions following a bushfire in ash-mixed species eucalypt forest. Melbourne and Metropolitan Board of Works Report MMBW-W-0014, Melbourne. 10 Fedorov S.F., Marunich S.V.,1989: "Forest cut and forest regeneration effects on water balance and river runoff" in Roald L., Nordseth K. & Hassel K. A. 1989: FRIENDS in Hydrology, IAHS Publication No. 187, International Association of Hydrological Sciences, IAHS Press, Institute of Hydrology, Wallingford, Oxfordshire OX10 8BB, UK, pages 291-297
Figure 2
Source: http://melbournecatchments.org/learn-more/impact-of-logging-on-melbourne-water/
This relationship shows the relation: the higher proportion of young stands, the lower runoff from the area. The proportion of such stands is manageable through the rotation lengths. 2.2.3
Infiltration, subsurface flow and overland flow
Precipitation, that finally reach soil surface, can infiltrate to soil and continue flow under ground, or cannot infiltrate and continue to move as an overland flow. Whether the water infiltrate or not depends on several factors, however, forest management measures cannot change the majority of them. Soil texture, structure and the volume of soil pores result from pedogenic process and they can be (negatively) changed only through soil compaction or using soil improvement techniques such as tilling, adding sand or clay, etc, which is expensive and of limited use. Forest cover has positive impact on soil properties; unfortunately, it takes a lot of time to improve them by the afforestation or by the improvement of forest management. However, one soil property is changeable: to what extent the soil has already been saturated. It is necessary to realise that not all water in soil can be removed by the same force. A part of soil water is bound to soil particles so firmly that it is unavailable even for plants. This is so-called hygroscopic water that cannot be separated from the soil unless it is heated over 105 °C, which means that it is almost ever-present in any soil. Capillary water is held in the capillary pores and is retained on the soil particles by surface forces. It is available for plants, however, it is bound so firmly that gravity cannot remove it; on the contrary, capillary water can rise upward to soil surface. It evaporates easily at common temperatures; however, it moves too slowly to have a direct effect on infiltration rate. If capillary water is connected with water table, it becomes an important “inexhaustible� source of water, but usually it is not the case, and capillary water exists as so-called hanging water, which is exhaustible. Gravitational water occupies the largest soil pores and moves down easily due to gravity. This water do not sustain in soil too long. It is subject to subsurface flow (see bellow) and, subsequently it returns to the surface on the foothill as so-called return flow, or it flows to groundwater stores and form subsurface water table. It is only partially available for plants (in smaller pores) but it can recharge the capillary water if plant roots deplete this.
The content of water that occupies all capillary pores and the smallest non-capillary pores is called field capacity. Significant surface runoff does not occur until soil moisture is near field capacity, except during extremely intense rainfall. Forest soils usually have slightly larger field capacity than non-forest soils 11. Abovementioned facts mean that transpiration, which removes the water available for plants, at the same time, creates the space (free pores) for the infiltration of new precipitation. “For example, by late summer, a single mature pine tree in the northern Sierra Nevada depleted soil moisture to a depth of about 6 m and to a distance of 12 m from the trunk. This single tree transpired about 88 m3 more water than the surrounding logged area. This summer evapotranspiration use by one tree is equivalent to about 180 mm of rainfall over the affected area.” 12 This effect of the tree transpiration on peak flow (compared to non-forest land) is best observable in a case of the first storm in the autumn. During the second storm in the autumn, we can observe only half the effect of forest to peak flow than in the first storm. On top of that, the interception capacity of forests, which is usually higher than that of nonforest vegetation, causes that lesser water reaches the soil surface (see above). Forest fires can cover the soil surface by the water-repellent layer of ashes, which can prevent infiltration for some time and thus to increase runoff as well as soil erosion. 2.2.3.1
Losses by subsurface flow
It is not commonly known that precipitation infiltrated to soil on a slope largely do not continue to groundwater, quite the contrary, majority of this water flows downhill shallowly under a surface. Dependent on soil texture, only 17 to 59 % of the water that falls on the surface reaches the depth of 1 meter8 (more in sandy soils, less in loamy soils). A large part of this water returns to surface in permanent water bodies or in natural drainage channels with occasional streams. The subsurface flow is not very fast (1 – 200 mm/hour), however, if the soil on the entire slope is, after long-lasting rains or snowmelt, fully saturated, the reaction on new rain is very fast as the water flowing from upper section of a slope expel the water stored in soils downwards. Majority of floods is a result of this flow, not of overland flow. The water that reached groundwater can also quickly contribute the floods (through springs) if expelled by newly infiltrated water. 2.2.3.2
Overland flow
Overland flow occurs when the precipitation cannot infiltrate fast enough to a soil and excessive volume have to flow on surface. This type of overland flow,cold Hortonian overland flow, is rather rare, occurring mainly in dry areas. More commonly, infiltration-excess overland flow is combined with saturation-excess overland flow and/or return flow (subsurface flow that returns to surface on foothills). Even such a combined overland flow usually creates less than 5% of total runoff and is restricted to special parts of forest land, such as roads or places with waterproof litter. 13. Despite its rarity, overland flow outside common pathways (natural drainage channels) is generally unwanted. However, it is impossible to avoid it completely. If occurs, it causes soil erosion, increases sediment load in water bodies (water quality) and deposits sediments to terrain depressions.
11
Wahren, A., Feger, K.-H.,Schwärzel, K. , and Münch, A. 2009. Land-use effects on flood generation – considering soil hydraulic measurements in modelling, Advances in Geosciences, 21, pages 99–107, www.adv-geosci.net/21/99/2009/. 12 Ziemer, Robert R. 1998. Flooding and stormflows. In: Ziemer, Robert R., technical coordinator. Proceedings of the conference on coastal watersheds: the Caspar Creek story, 6 May 1998; Ukiah, California. General Tech. Rep. PSW GTR-168. Albany, California: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture; 15-24 13 Chappell, N. A. 2005: Water pathways in humid forests: myths vs. observations, Suiri Kagaku (WATER SCIENCE) No. 6 Vol. 48 2005
3
Forestry impact on water quantity
Thanks to abovementioned mechanisms, forests and forestry measures have an ability to influence groundwater recharge and streamflows. However, the needs of human society are not always identical and thus there is no universal solution of them. In general, society can have problems with surplus water (floods, too wet soils) or lack of water (problems with water supply, water transport and hydroelectric dams’ performance). These two groups of problems cannot be solved by the same measures, it is necessary to set up priorities for each particular watershed.
3.1
Runoff decrease and flood protection
Forests, as consumers of water, can decrease runoff from the watershed and flows in downstream watercourses, and thus reduce the risk of floods. Unfortunately, the processes managing this decrease behave in a rather unsuitable way. In principle, the streamflow is reduced by pretty the same volume of water, whether we are talking about peak flows after long rainy period or about low flow after drought period. Expressed in percents, the decrease of peak flows is negligible compared to decrease of average flows or even base flows. “Any small gains the plantations achieved through reducing peak flows were therefore obtained at the expense of very serious reductions (for hydropower generation) in low flows.”14 On top of that, flood protective function of forests is traditionally overestimated. The positive influence of forest cover is observable mainly at the micro-level (small watersheds up to 100 square kilometres) and in association with short-duration and lower intensity rainfall events. As rainfall duration or its intensity increases and the distance down the stream becomes greater, other factors will prevail and forest cover of the watershed is becoming less important. If, despite the above-mentioned weak points, it is for some reason important to improve flood protection influence of forests, there are several possibilities how to do it: • Increase of the forest area (afforestation of non-forest land) – forests usually have higher interception capacity and higher transpiration, which make soils dryer and more “prepared” for storm events. Soils under forest usually have higher water infiltration rates than the same soils under other vegetation covers. Forest have also positive influence on soil texture and, subsequently, on water capacity of soils. However, as for abovementioned parameters, non-forest ecosystems do not differ too much from forests. Therefore, afforestation is not miraculous cure for floods. Floods can naturally occur even in fully forested small-scale watersheds. • Conversions to coniferous stands – conifers (needle-leaved trees) have higher interception capacity, slightly higher transpiration rates and are effective all year round, while deciduous broadleaves are effective only in vegetation growing season and even if fully leafed, they usually have lower interception capacity. • Manipulation with rotations – interception capacity and transpiration culminates in premature or pole-sized stands. Rotations between 50-90 years means higher proportion of such stands at the expense of over-mature stands or clearings. • Paying attention to forest protection measures – healthy and vital forests are larger consumers of water. • Ban on cattle grazing in forests – cattle grazing compacts soils, creates erosion rills serving as drainage channels and suppress herb and brush layers decreasing thus interception. • To avoid soil compaction – use of proper machinery and road network can help to do this. • Non-forestry measures – construction of silt or debris dams can contribute to water retention (besides their main function that is the reduction of sediment load). 14
Calder I. R., Aylward B.,2006: Forest and Floods: Moving to an Evidence-based Approach to Watershed and Integrated Flood Management, International Water Resources Association, Water International, Volume 31, Number 1, Pages N/A
•
3.2
Indirect measures – the prevention of soil erosion can decrease sediment deposition in dams and certain sections of watercourses, which improve their efficiency. See the chapter “Forestry impact on water quality” for more details.
Runoff increase and water supply
In watersheds, where the water supply is the main goal, it is necessary to lower the water consumption by trees as well as interception and other water losses. There is no question that forest removal, even partial, increases overall runoff (water yield) or groundwater recharge. Both natural and planted forests use more water than most of alternative land uses, for instance agriculture and grazing1. First-year increases in water yield following forest clearance range can reach hundreds of millimetres, depending on local rainfall. “Transpiration losses are greater from evergreen than from deciduous forest, and the drier and less windy the climate, the less the evapotranspiration losses, because leaves in dry climate are usually narrower and smaller” 15. Of course, it does not necessarily mean that trees or forests should be removed because of their excessive water consumption. Such an approach would impair or eliminate the many forest ecosystem services such as flood protection (see above), forest related biodiversity, erosion minimisation, improved water quality, carbon sequestration, recreational and aesthetic services as well as sustainable forest production. A major dilemma is that although water quantity can be increased by forest removal, the land uses replacing forest can impair the quality of the water acquired and make it less usable. Grassland is a good watershed cover for water yield, but it has two weaknesses: worse protection against landslips and a tendency to allow overgrazing of grassland, resulting in soil compaction and erosion, which increase peak flows and sediment discharge into watercourses. For these reasons, the deforestation of the watershed is optional but very controversial measure, which should be planned extremely carefully. However, to some extent, several less sensitive measures can increase water yields: • Conversions to deciduous broadleaved stands – deciduous broadleaves (or deciduous connivers) have lower interception capacity, slightly lower transpiration rates and allow recharge of groundwater and/or water reservoirs during winter season when without leaves. During summer storms, they are equally effective to coniferous stands. Note that evergreen broadleaves are less suitable for this purpose. • Manipulation with rotations – interception capacity and transpiration are the highest in premature or pole-sized stands, while older (over-mature) stands have usually more open canopy, thinner tree crowns and lower interception capacity and transpiration (see Kuczera curve). Longer rotations, above 90 years, means higher proportion of over-mature stands at the expense of young stands with high water consumption. • Decrease of stand density (stocking level) – stronger selective cuts and thinning as well as wider spacing of transplants in artificial regeneration can decrease transpiration a interception capacity. • To avoid the establishment of intensive forest plantations – forest plantations, because of their vigorous growth and high stand densities (thanks to short rotations), usually have higher water consumption 16 and thus they are not suitable for catchment areas of water reservoirs or areas with groundwater resources dependent on regular recharge. • To minimise afforestation of non-forest land – the reason were mentioned above.
15
Nisbet, T.R. & McKay, H. 2002. Sustainable forestry and the protection of water in Great Britain. In Proceedings, International Expert Meeting on Forests and Water,pp. 101–112. Shiga, Japan. Tokyo, International Forestry Cooperation Office, Forestry Agency. 16 Keenan R.J., Gerrand A., Nambiar S. & Parsons M. 2006: Plantations and Water: Plantation Impacts on Stream Flow, SCIENCE for DECISION MAKERS, Commonwealth of Australia
3.3
Maintenance of existing water balance
In many cases, the actual management and land use is adapted to the actual water regime. In that case, the main afford should focus on the prevention of unwanted changes. It requires: • Monitoring of age and species structure of the forests • Land use planning, monitoring of land use changes • The improvement of age structure of forests (if necessary) – the proportion of age classes should fit the model for the preferred rotations
4
Forestry impact on water quality
Forests’most significant contribution to water management is in maintaining high water quality1. They do this through minimising soil erosion and subsequent reducing sediment load in water bodies and, to some extent, through trapping and filtering other water pollutants. The soil particles are the most common pollutant in surface waters. Though they mainly represent mechanical pollution, they can also bear some chemicals, for example the residues of pesticides and fertilisers. Forest soil and litter on riverbanks can filter some chemical pollutants from overland and subsurface flow. A part of pollutants (residues of fertilisers) can be even “consumed” by tree roots 17. In addition, the shade of riparian trees has a positive impact on water quality. Transported and deposited soil particles are called sediment, and their content in water is called sediment load. Sediment deposition can be beneficial if it occurs in the right place, but there are usually many unwanted effects related to deposition1. Sediment can reduce reservoirs’ and dams’ capacity, raise riverbeds and block channels, resulting in floods, adversely alter aquatic habitat in streams or wear down turbine blades in power installations. Therefore, there are many links between the water quality protection and flood protection. Forest can also protect the water against atmospheric pollutants (e.g. dust), however, they can be washed to the water by rain 18. From the viewpoint of soil erosion prevention, forest represents the most suitable land use. Their management is, thanks to long rotations, less intensive than in other land uses. It results in less frequent disturbances of soil surface and almost no use of fertilizers and pesticides. In addition, the risk of fossil fuel runoff is lower than in agriculture1. The most wanted property of forest is untouched ground cover (litter, grass, herbs, mosses, etc) without erosion rills and gullies. The development of forest road network represents the most important soil disturbance factor in forestry. Forest road cuts and fills act as permanent sources of eroded particles, however, during the road construction, the pollution is usually much more serious 19. Other forestry activities causing soil erosion and ground cover damage include skidding, logging and movement of off-road vehicles. However, the frequency of such disturbances is lower than in other land uses. The measures for the protection of water quality include: • Long rotations – the longer rotation, the less frequent soil disturbances • Careful maintenance of forest roads • Proper forest zoning – designation of protective forests and riparian buffer zones whenever necessary • Proper timing of forestry interventions – they should be carried out, if possible, in months with the lowest incidence of storms, in months with snow cover, frosts, etc. • Use of proper machinery 17
Chesapeake Bay Program, 1997, Riparian Forest Buffers in the Chesapeake Bay Watershed", The Chesapeake Bay Program, 410 Severn Ave., Suite 109, Annapolis, MD 21403., www.chesapeakebay.net/content/publications/cbp_12188.pdf 18 IUFRO (International Union of Forest Research), 2007, Research Spotlight: How do Forests Influence Water?, IUFRO Fact Sheet No 2, Vienna, Austria, http://www.iufro.org/science/taskforces/water/publications 19 Toy T. J. , Foster G. R. , Renard K. G., 2002: , Soil Erosion: processes, predicition, measurement, and control, Edition: 3, Vydal: John Wiley and Sons, 338 strán, str. , 200, 227
• • • •
Ban on cattle grazing in forests – cattle grazing compacts soils and suppress herb and brush layers, which increases soil erosion Control of ungulates population – game can significantly contribute to soil erosion To avoid recreational or military overuse of forests Paying attention to forest protection measures – dead or damaged forests can become the sources of nitrates due to their leaching from decaying timber, salvage loggings are usually more damaging to soils than normal loggings