Nội dung

MEASUREMENT OF ATMOSPHERIC DEPOSITION UNDER FOREST CANOPIES: SOME RECOMMENDATIONS FOR EQUIPMENT AND SAMPLING DESIGN ANNE THIMONIER Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland (Received 18 March, 1996; accepted 25 February, 1997) Abstract. Quantification of the forest water flux provides valuable information for the understanding of forest ecosystem functioning. As such, throughfall (and stemflow to a lesser extent) has been frequently measured. Although throughfall collection may seem relatively simple, the requirements to obtain reliable estimates are often underestimated. This review addresses the criteria to take into account when working out the sampling procedure, from the selection of equipment to implementation in the field. Sound sampling of the forest water flux is difficult due to its high spatial and temporal variation. The high costs entailed by the ideal sampling design often prohibit its implementation. Different procedures are available, some of which are compromises between the aim of the study (monitoring or experimental study, short or long term objectives, absolute or relative estimates, quality of the assessment to be achieved) and the available means. Key words: atmospheric deposition, methodology, sampling, spatial variation, temporal variation, throughfall, stemflow, water chemistry 1. Introduction Precipitation under forest canopies is frequently measured in forest ecosystem studies. Terms and definitions used to describe it differ, but Parker’s (1983) designations have been most commonly used. Two components of the forest water flux are distinguished. Throughfall consists of the water dripping from the canopy as well as the portion of precipitation reaching the forest floor without having being intercepted by the crowns. Stemflow is the water running down the branches and the trunk and depositing at the base of the tree (Parker, 1983). Throughfall usually makes up the major portion of precipitation under the canopy and, as such, is the most commonly measured component of the forest water flux. Stemflow can represent a substantial fraction of the total water input in stands of smooth-barked species with upright branches, but it makes a negligible contribution to the water flux in forests of rough-barked species (Parker, 1983; Brechtel, 1989). Throughfall and stemflow are two major pathways in forest nutrient cycling, and their quantification is necessary to establish both water and nutrient budgets. Although they may supply less material than litterfall, they constitute a source of dissolved minerals readily available for plant uptake (Parker, 1983). Water and nutrient inputs via throughfall and stemflow influence all soil chemical and biological processes, including pedogenic transformations, turnover of nutrient Environmental Monitoring and Assessment 52: 353–387, 1998. c 1998 Kluwer Academic Publishers. Printed in the Netherlands. 354 A. THIMONIER pools, accumulation and mobilisation of possibly toxic substances, and buffering reactions (Mayer, 1987). Throughfall and stemflow sampling is also useful in assessing and monitoring the pollution climate to which forest ecosystems are exposed (e.g. Johnson and Lindberg, 1992; Matzner and Meiwes, 1994; Meesenburg et al., 1995). Throughfall and stemflow composition does not readily differentiate between the origin of the elements reaching the forest floor, but parallel sampling of the incident precipitation in the open field or above the forest canopy helps discriminate the influence of vegetation (filtering effect of dry and occult deposition and exchange processes) from wet deposition. Derivation of dry deposition from throughfall measurements has been attempted, although not always successfully (Ulrich, 1983; Lovett and Lindberg, 1984; Bredemeier, 1988; Puckett, 1990; Potter et al., 1991; Beier et al., 1992; Joslin and Wolfe, 1992; Draaijers and Erisman, 1993; Brown and Lund, 1994; Neary and Gizyn, 1994; Rustad et al., 1994; Cappellato and Peters, 1995; Reynolds, 1996). However, as the throughfall method has the advantage of being relatively inexpensive and simple compared to other methods directed towards the measurement of more specific pathways of atmospheric deposition (Erisman et al., 1994), it has been used extensively in studies dealing with deposition measurements. Throughfall can provide a valuable quantification of the total inputs to the forest floor of critical chemicals involved in acidification or eutrophication processes, such as nitrogen and sulphur compounds. It is thus crucial to obtain a representative measurement of throughfall and stemflow, or at least to be aware of the limitations of the estimates. This review focuses on the criteria that should be considered when selecting the type of collector, its design, and the siting in the field. It is more specifically directed towards the requirements in monitoring studies and reviews some of the manuals which have been written to harmonise the sampling procedures at national and international levels. This contribution concentrates on the sampling of precipitation in the wet form; problems related to snow collection are not addressed. 2. Sampling Equipment 2.1. TYPE OF COLLECTOR 2.1.1. Incident Precipitation and Throughfall Wet-only collectors against continuously open collectors. Deposition of atmospheric compounds by rain (wet deposition) is theoretically best measured with specially designed collectors, which are closed by a lid during dry periods and open whenever raindrops (or snowflakes) are detected by a sensor. Such a system prevents the deposition of particles and gases on the walls of the collector during dry periods, as occurs in continuously open collectors. Downwash of the dry-deposited compounds can significantly affect the composition of the sample collected in continuously exposed collector (bulk precipitation) (Erisman et al., 1994; Draaijers et volume Ca Mg K Na Cl NH4 NO3 SO4 H 1.07 1.13 1.31 1.38 1.00 1.03 1.11 1.06 1.12 1.08 0.98 rain : rain+snow : 25 7 2 10 12 52 43 80 46 0.98 1.06 0.87 1.22 1.18 1.14 1.13 1.03 1.02 0.95 wet-only mean bulk/ collector wet ratio mean bulk/ wet ratio wet-only collector 148.9 4.5 8.7 3.8 97.6 125.2 57.8 52.0 26.5 64.9 Daily collection Three day-collection interval Event-based collection 1142 1.7 4.1 2.3 63.0 79.0 22.1 19.3 10.3 27.0 1.15 1.29 1.05 1.11 1.08 1.07 0.87 0.93 0.98 0.89 wet-only mean bulk/ collector wet ratio Stedman et al., 1990 Slanina et al., 1979 Mosello et al., 1988 Eskdalemuir (United Kingdom) Den Helder (The Netherlands) Pallanza (Italy) 387 3.6 2.5 2.8 34.8 47.4 67.1 35.7 17.2 25.0 1.22 1.55 1.33 1.09 1.36 1.29 0.80 1.06 1.05 1.16 wet-only mean bulk/ collector wet ratio Daily collection Stedman et al., 1990 Stoke Ferry United Kingdom) – – – – – – – – – – 1.07 2.00 1.50 1.10 1.20 1.40 1.10 1.10 1.00 0.92 wet-only mean bulk/ collector wet ratio Variable collection intervals (from 7 days to 24 days) Galloway and Likens, 1976 Ithaca (United States) Table I Examples of bulk/wet concentration ratios taken from the literature (only case studies where the collection interval was the same for both wet and bulk collectors are presented). The given volume corresponds to the cumulative precipitation height in mm over the sampling period. Volume weighted mean concentrations are given in eq l 1 MEASUREMENT OF ATMOSPHERIC DEPOSITION UNDER FOREST CANOPIES 355 356 A. THIMONIER al., 1996; Table I). Differences in chemical composition of precipitation collected by wet-only and bulk collectors have been assessed in a number of comparative studies (Galloway and Likens, 1976; Galloway and Likens, 1978; Slanina et al., 1979; Soederlund and Granat, 1982 in Slanina, 1986; Dasch, 1985; Mosello et al., 1988; Richter and Lindberg, 1988; Stedman et al., 1990; Bredemeier and Lindberg, 1992). The usually higher precipitation volumes collected by bulk collectors (ratio bulk/wet >1 for precipitation amount, Table I) may be related to the higher aerodynamic blockage by the wet-only collector, reducing catch efficiency (Stedman et al., 1990). The sensitivity of the sensor driving the opening of the lid on wet-only collectors may also influence the precipitation amount that is collected, especially at low precipitation rates. Calcium (Ca), magnesium (Mg), and potassium (K) concentrations are often higher in bulk samples than in wet-only samples, because of the deposition of soil-derived particles on the walls of the collectors during rain-free periods. Differences for nitrogen compounds (nitrate NO3 , ammonium NH4 + ) and sulphate (SO4 2 ) are usually smaller, but local or regional sources of emissions can significantly influence the composition of bulk samples (Stedman et al., 1990). Part of the differences between wet-only and bulk concentrations may also be the result of delayed opening of the lid at the onset of precipitation, when the concentrations of compounds may be highest: below-cloud scavenging of aerosols and gases in the atmosphere (washout) leads to substantially higher concentrations in rain drops in the early stage of an event (e.g. Hansen et al., 1994; Minoura and Iwasaka, 1996; Burch et al., in press). Wet-only collectors may then underestimate wet deposition (Slanina et al., 1979; Claassen and Halm, 1995a). A number of studies have been carried out to assess the collection efficiency of various wet-only or wet/dry collectors (collectors with an additional bucket collecting deposition during dry periods) (Galloway and Likens, 1976; Slanina et al., 1979; Bogen et al., 1980; De Pena et al., 1980; Schroder et al., 1985; Graham et al., 1988; Graham and Obal, 1989; Hall et al., 1993; Claassen and Halm, 1995a and 1995b). These studies showed that the performances of the collectors could be quite variable according to the robustness of the device, the tightness of the lid, and the sensitivity of the sensor. Most of all, however, wet-only collectors have the drawback of being expensive and of requiring a power supply. An exception may be the low-cost wet-only device developed by Glaubig and Gomez (1994), involving a counter-weighted cover held in place over the collector by a piece of water-soluble paper; but as the system must be re-installed after the end of each rain event, this collector would be only suitable in regions characterised by heavy rainstorms interrupting long dry periods. The following review concentrates on continuously open collectors only. Funnels against troughs. Funnel-type gauges are generally used in the open field to measure rain amount and chemistry. Although it has been recommended that collectors of the same design as open-field collectors should be employed for throughfall measurements (e.g. Environmental Data Centre, 1993), no general consensus over this has been reached. A review of studies involving throughfall collec- MEASUREMENT OF ATMOSPHERIC DEPOSITION UNDER FOREST CANOPIES 357 Figure 1. Bird’s-eye view of trough-type collectors used in the French network of forest ecosystem monitoring (after Ulrich and Lanier, 1993) Figure 2. Examples of funnel-type rainwater collectors. (a) Collector proposed by EMEP (1977) and Environmental Data Centre (1993). (b) Collector used in the ‘Swedish wet deposition measurement network’ (after The Working Group for Environmental Monitoring, 1989). (c) ‘Münden 100’ collector used in the Hessian Research Programme ‘Forest Damage by Air Pollution’ (after Brechtel, 1989). (d) Collector used at the Klosterhede research site in Denmark (after Beier and Rasmussen, 1989). tion reveals that two types of collectors, troughs (Figure 1) and funnels (Figure 2), are in common use. Troughs are believed to collect more representative volumes, as this type of gauge integrates a larger area and thus a variety of canopy conditions (e.g. Reigner, 1964, in Helvey and Patric, 1965; Kostelnik et al., 1989; Draaijers 358 A. THIMONIER Figure 3. Examples of throughfall collectors: spiral-type and collar-type (after Rasmussen and Beier, 1987). et al., 1996). The two types of gauge have been compared against each other in a few studies: Reynolds and Leyton (1963, in Crockford and Richardson, 1990) and Hogg et al. (1977, ibid.) found that troughs and rain gauges yielded similar mean volumes. Kostelnik et al. (1989) obtained significantly larger throughfall amounts in troughs relative to funnels. Crockford and Richardson (1990) also sampled higher volumes with troughs than with standard rain gauges. Conversely, Reynolds and Neal (1991) observed a small bias toward a lower catch in troughs. Troughs were shown to slightly reduce the variance of the estimates (Reynolds and Leyton (1963, in Crockford and Richardson, 1990) and Hogg et al. (1977, ibid.), but increasing the collection area by using troughs rather than funnels does not reduce the number of gauges necessary in the same proportions (Helvey and Patric, 1965). Stuart (1962, in Kostelnik et al., 1989) reported that an increase in sampling area of throughfall gauges only slightly reduced the variance of throughfall volume estimates. Potter et al. (1991) still needed at least 12 randomly selected 1.0  0.1 m trough collectors to stabilise the coefficient of variation for base cation canopy exchange and dry deposition values estimated from throughfall measurements. Generally speaking, although sampling efficiency can vary according to the collector type, the sampling strategy (number and location of collectors) is more important than the type of gauge. Yet, in very heterogeneous canopies inducing a large variability in throughfall distribution, troughs might collect a more representative sample (Weihe, 1976; Crockford and Richardson, 1990). MEASUREMENT OF ATMOSPHERIC DEPOSITION UNDER FOREST CANOPIES 359 Figure 4. Stemflow amount collected per stem versus height of incident precipitation for three tree species (after Cepel, 1967). 2.1.2. Stemflow Stemflow is traditionally sampled with gutter-like collectors coiled in spiral or collar around the stem of individual trees, and connected to a storage bottle by a tube (Figure 3). As large amounts of stemflow can be collected (Figure 4), the collection vessel must either have a high capacity or consist of several containers of smaller capacity connected in series. An automated tipping-bucket system, allowing continuous recording of volumes and sampling of representative proportional fractions, is probably preferable over the long-term when the sampled species yield large amounts of water. 2.2. DESIGN OF THE COLLECTORS AND SAMPLING ACCURACY 2.2.1. Incident Precipitation and Throughfall Sources of errors. The accuracy required for the measurement of both precipitation amount and chemistry is difficult to achieve using a single type of gauge (Hall et al., 1993). To avoid contamination of the sample by splashing and by windraised material from the ground, the collector must be set at a sufficient height, but it then creates an obstacle to the windflow, resulting in a lower catch of the falling precipitation (Rodda et al., 1985; Rodda and Smith, 1986; Sevruk, 1989; Sevruk et al., 1994). The collection is especially biased against snowflakes and fine rain (Rodda et al., 1985). The consequences for precipitation chemistry might be substantial as fine rain drops are more concentrated than drops with greater radii (Bächmann et al., 1993). Wind-field deformation due to a funnel-type gauge can 360 A. THIMONIER account for 2–10% of water losses for rain and up to 15% for snow according to WMO (1971, in The Working Group for Environmental Monitoring, 1989). Sevruk et al. (1994) stated that losses due to aerodynamic blockage could be as large as 3–25% for rain and up to 100% for snow. Windshields are usually not regarded as a satisfactory solution to the problem in precipitation chemistry sampling, as they can also be a source of contamination. Studies have been dedicated to improve the aerodynamic performance of the collector itself. The value of two parameters describing the change in windflow over the opening of a collector should be reduced (Hall et al., 1993): the relative increase in wind speed measured above the collector inlet (acceleration) and the height of maximum wind speed above the inlet opening relative to the diameter of the inlet opening (called displacement by Hall et al., 1993). Comparing different shapes of collectors with equivalent depth to diameter ratios, Hall et al. (1993) showed that funnels induced comparable or greater acceleration, but lower displacement than cylinder-type collectors. Rodda et al. (1985) also tested various shapes of gauge and established that a simple funnel yielded rainfall depths which most closely matched those measured by a gauge at ground level. Beside shape characteristics, the aerodynamic performance of a collector depends on its depth and the ratio of depth to diameter (Hall et al., 1993). Reducing the collector depth reduces the aerodynamic blockage caused by the collector. Shallow collectors however are less efficient in draining the collected sample into the storage vessel, and are much more susceptible to splashing losses. Wind-driven circulation inside the collector may also cause the ejection of collected precipitation, especially in the form of snow or fine rain droplets, as well as increased evaporation from the wetted collector walls. Experiments conducted on cylindrical collectors showed that internal air circulation was highest for a ratio of depth to diameter around unity. With increasing ratios (deeper collectors for a same diameter), ejection of material became increasingly difficult (Hall et al., 1993). Other sources of errors in the deposition estimates are due to wetting (adhesion of water on the walls of the collector) and evaporation, accounting for 2–10% and 0–4% water losses, respectively, for funnel-type gauges. Wetting and evaporative losses are likely to be higher for troughs due to their larger surface area. The collector should also be designed to prevent rain from splashing in and out. WMO (1971, in EMEP, 1977) recommends that precipitation gauges should comply with the following: – the area of the aperture should be known to the nearest 0.5% and the construction should be such that this area remains constant; – the rim of the collector should have a sharp edge and should fall away vertically inside and should be steeply bevelled outside. Sevruk (1989) showed that increasing the thickness of the rim led to an increasing wind speed increment above the opening of a gauge; – the vertical wall of the collector should be sufficiently deep and the slope steep enough (at least 45 ) to prevent loss by splashing and to allow good MEASUREMENT OF ATMOSPHERIC DEPOSITION UNDER FOREST CANOPIES 361 drainage. According to Crockford and Richardson (1990), troughs should similarly contain a V-section close to that of the ideal funnel-type gauge; – the receiver should have a narrow neck and should be sufficiently protected from radiation to prevent loss of water by evaporation. Diameter of the opening. In the case of funnels, manuals often recommend rather large diameter openings (20–40 cm). When the sampling interval is short, large diameters have the advantage of providing enough solution for analysis (Lewis and Grant, 1978). In forest stands, preference for large diameter funnels additionally results from the reasoning that a larger area will sample a broader variety of canopy conditions (see the above discussion on trough- and funnel-type collectors). However, when the collection frequency is low and when rainfall is potentially high over the defined sampling interval, the large volumes collected by larger openings require high capacity containers, which can be difficult to handle. Some studies have concluded that the sampling area of the collectors had actually a minor influence on the precision of rainfall quantification, as already mentioned in the previous discussion on the type of collector. A few investigations more specifically dealt with the comparison of collectors of the same design but with various collection areas: in forest stands. Weihe (1985) found no significant differences between throughfall amounts collected by 100 cm2 and 200 cm2 surface area funnel-type gauges. In the open, Huff (1955) successfully tested several sizes of smaller surface area gauges against standard rain gauges; the results showed that the small orifice gauges could be used in place of the standard gauge without loss of accuracy. These studies were not concerned with the influence of the sampling area on water chemistry; however, these few results support the use of relatively small diameters when the rainfall depth over the sampling period would otherwise require high capacity containers. The volume of the vessel connected to the funnel or to the trough should be large enough to contain the maximum precipitation amount expected at the sampling location during the defined sampling interval. Commonly, for funneltype collectors, the diameter of the funnel and the sampling frequency are such that the bottle connected to the funnel has a 2 to 5 l capacity. Figure 2 shows different examples of gauges in use. Use of a standard rain gauge for more accurate volume estimates. It would be valuable to measure precipitation in the open with both a standard rain gauge and the chosen device so that comparisons can be made of the volumes collected. Such an exercise is useful in the open, where the influence of wind is more critical than in forest stands. The amount of precipitation recorded by the standard rain gauge enables the correction of the water flux. The use of the values of the standard rain gauge to compute fluxes of elements may however be inappropriate. Concentrations in the collector might be enhanced due to evaporation, and the water amount and concentrations from the same collector should then be used in order to offset 362 A. THIMONIER this bias. On the other hand, as collection efficiency of non aerodynamicallyshaped gauges is biased against more concentrated fine rain droplets, concentrations measured in the collector may be lower than if the collector had the same catch efficiency as a standard rain gauge. Positioning in the field. In the open field, the opening of the rain gauges must be set horizontal above the ground level rather than parallel to the ground surface. There is no consensus over the height at which the collecting surface should be positioned. The manual of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP-Forests) (Programme Coordinating Centres, 1994) recommends that the height should be approximately 1.5 m above ground level. The Working Group for Environmental Monitoring (1989) advocates a height of between 1.5 m and 2 m. The manual of the International Co-operative Programme on Integrated Monitoring of Air Pollution Effects (Environmental Data Centre, 1993) recommends 1.20 m. ISO/DIS 4222 (in The Working Group for Environmental Monitoring, 1989) standardised the height at 1.80.2 m. In forest stands, when funnel-type gauges are used, the opening area must be set horizontal, as in the open field. Conversely, troughs must be tilted (25 according to Draaijers et al., 1996) to allow drainage towards the container. This might be an additional factor affecting the water amount sampled (Sevruk, 1989). In the monitoring sites of the Nordic countries, collectors have been set directly on the ground or on a short pole (0.5 m) (The Working Group for Environmental Monitoring, 1989). The ICP-Forests manual (Programme Coordinating Centres, 1994) recommends however that the opening area should be raised to a height of approximately 1 m over the ground level to avoid contamination by soil. 2.2.2. Stemflow High volumes of stemflow are usually collected from each sampled tree (Figure 4). Rasmussen and Beier (1987) suggested that the wide opening of some collecting devices led to an overestimation of the amounts of stemflow by including a fraction of throughfall. It might be advisable to adjust the very small diameter slit (2 mm) they recommend (Figure 3) to the species sampled. The opening should also not be blocked too easily. The stemflow collectors should be placed around the stem of the trees between 0.5 m and 1.5 m above ground level (Programme Coordinating Centres, 1994). Care should be taken not to damage the bark, as stem exudates may contaminate the sample. 2.3. MATERIAL Whatever the type of collector chosen, all components should be made of chemically inert material. Quality Teflon (with smooth surfaces) is ideal but is expensive. Alternatively, polyethylene is recommended for analyses of macro-ions in most