Indicators of SQ
3. Indicators of SQ
The use of indicators to assess the impact on SQ has been proposed for developing a sensitive and dynamic system to document the SQ and how it responds to management [186]. Indicators of SQ must
be sensitive to changes in soil function (Moffat 2003; 186?) and they should be defined as quantifiable physical, chemical and biological parameters and processes. The indicators should be related to a defined level or rate to determine whether SQ is improved or degraded [186?]. Johnson and Todd (1998) [94] furthermore emphasise that SQ indicators should correlate well with ecosystem processes, integrate soil properties and processes, be accessible to many users, responsive to management and climate, and, whenever possible, be a part of existing databases [7]. For our purpose, soil indicators responsive to forest management are especially of interest.
In literature, a list of indicators on SQ has been proposed through the last 15 years (e.g. Karlen et al. 1997; [186]). Among these indicators, we chose to work with SQ indicators connected to a forest- production perspective (Table 1).
2.1. Reduced nutrient input and availability In the long-term perspective, wood production capacity is influenced by the inputs and outputs of nutrients. Hence, input-output nutrient budgets may provide a picture of the direction of change in SQ and therefore seem to be adequate indicators of SQ. Here we present two approaches of calculating such budgets; 1) the soil nutrient store method, 2) the nutrient balance method.
In method 1, the soil nutrient store is determined by normal soil sampling on two occasions to assess potential changes. In method 2, inputs to and outputs from the system are accumulated over a time period to calculate a balance for individual elements. Inputs are wet and dry deposition, weathering of soil minerals, N-fixation and fertilisation. However, the weathering rate is difficult to determine [174]. Outputs from the system take place through harvesting, leaching, erosion and vaporisation. If the budget is balanced, nutrient stores are neither increased nor decreased. A positive budget means that the considered element is accumulated in the system while a negative budget suggests that export exceeds import and as such the studied nutrient is depleted from the system causing the wood production capacity to be unsustainable over time. A thorough description of the method and the calculations involved is given by Ranger and Turpault (1999). The use of method 2 as a diagnostic tool for sustainable forest management is well described and used, e.g. by [172], Ranger and Turpault (1999) and [176].
The time interval in methods 1 and 2 must cover all relevant forestry operations as well as the entire stand development, since the distribution of nutrients within the forest alters gradually as the stand develops [133, 146]. Thus, for forests which are largely used for wood production a time interval of preferably one rotation period or more is appropriate.
2.2. Soil acidification Soils become acid when the acid production rate exceeds the acid neutralisation rate. The most important acidifying processes in nearly all forest soils in humid climates are i) biomass growth and subsequent harvesting, ii) nitrification and subsequent leaching of nitrate and base cations, iii) production and leaching of organic acids during decomposition, and iv) deposition of N and S and subsequent leaching of nitrate and sulphate together with base cations. Soil acidification is counteracted by weathering [211, 174]. The intensity of the acidity can be measured by the pH.
A neutral pH indicates a sufficiently high weathering rate to compensate ongoing production of acidity in the system. At low pH, acidity produced or deposited from the atmosphere is accumulated in the soil indicating an insufficient acid neutralisation by chemical weathering. Soil acidification is, however, a natural process, e.g., the podzols in the boreal forests are naturally acidic and have low buffering capacities. The rate of soil acidification can be accelerated by human activities. Forest A neutral pH indicates a sufficiently high weathering rate to compensate ongoing production of acidity in the system. At low pH, acidity produced or deposited from the atmosphere is accumulated in the soil indicating an insufficient acid neutralisation by chemical weathering. Soil acidification is, however, a natural process, e.g., the podzols in the boreal forests are naturally acidic and have low buffering capacities. The rate of soil acidification can be accelerated by human activities. Forest
The pH is assessed at two occasions to assess potential changes in soil acidification. As for the nutrient budget determination above, a time interval of preferably one rotation period or more is appropriate.
2.3. Compaction
Soil compaction takes place as a consequence of off-road driving in the forest [59B], for instance in conjunction with harvesting, and on forest roads and skid trails. The pressure this excerts on the soil may lead to increased bulk density and loss of soil porosity. The root environment will deteriorate and roots will have difficulties of extending during dry summers and during wet winters because of lack of oxygen [70, 224, 47, 218, 58] leading to lower production rates. An estimate of the compaction degree can thus be made from measurement of bulk density and soil porosity. A macropore volume <10% has been observed to restrict root growth [76, 171, 193]. This critical value for macro-pore volume seems to be a valuable indicator across a wide range of soils, whereas the use of bulk density as an indicator is more difficult since the bulk density varies widely [193].
The soil porosity and bulk density are measured at two occasions to assess potential changes in compaction. Preferably the measurements should be performed before and after harvesting or other forest operation that might affect compaction considerably. The recovery time could be estimated by follow-up measurements at different times after the forest operation.
2.4. Soil erosion
Erosion is often a local phenomenon and a natural process where wind and water work together moving soil fragments from one place to another. The rate of soil loss (erosion) forms equilibrium with the rate of soil formation. The soil formation begins with the weathering of rock by physical or chemical processes and is a slow process. When soil loss is higher than soil formation, erosion is taking place.
Soil loss will often begin with splash erosion where rain drops hit the soil which creates a net downslope movement of particles. A flow of water and soil particles will move over land surfaces and the greater the flow velocity the more soil loss. Steep slopes will cause a faster flow. Vegetation slows down flow and hereby reduces the soil loss. Channelized flow into rills, gullies and canyons will further erode material downward through the soil. Soils mostly affected by erosion include sandy and loamy soils on steep slopes in upland sites and in high rainfall areas [41] where bare soil is most vulnerable.
Erosion processes can become accelerated when humans interfere. Forest operations like site preparation, harvesting, soil damages near surface water due to off-road driving and construction of skid trails and forest roads speed up erosion [230] so that soil loss is faster than soil formation. Soil horizon disappears and the eroded material sediments in lakes and streams and clogs them. Soil loss and soil formation before and after management operations will tell more about the amount of erosion. However, these processes are difficult to determine and can only be estimated inaccurately.
Theoretically, the rate of soil formation sets the upper limit for an acceptable rate of soil loss by erosion. Knowledge of soil’s age together with information about changes in climatic and biotic factors enables rates of soil formation to be estimated in tons per hectar per year. The estimation of soil formation then needs to be referenced to a former state of the art, e.g. before and after a forest operation. Different equations for soil loss have been developed like the Universal Soil Loss Equation (USLE) by Wischmeier and Smith (1965) or the Revised Universal Soil Loss Equation (RUSLE) by Renard et al. (1997).