OULANKA REPORTS

27 ·2007 SCIENTIFIC STUDIES TO STRATA SPECIFIC INTERCEPTION OF BOREAL FOREST STANDS IN THE OULANKA NATIONALPARK IN THE NORTHERN FINLAND

Marco Langer Dipl.-Geogr. M. Langer, University of Bremen, Institute of Geography,

P.O. Box 330440, D-28334 Bremen, Germany (E-mail-address: mlanger@uni-bremen.de )

 OULANKA RESEARCH STATION

UNIVERSITY OF OULU

ISBN 978-951-42-8740-4

ISSN 0358-3651

OULUN YLIOPISTOPAINO

Oulu 2009

Abstract Scientific studies on forest stand climatology of boreal forests in the Oulanka Nationalpark region, Northern Finland, with it regard its function as a control and a filter in landscape ecosystems and with special reference to strata specific interception and leaching by drop and seeping water were carried out during spring, summer and autumn 2006. As a part of these scientific works differences between an Empetrum-Myrtillus-old growth forest (OGF) and a Pinus sylvestris-reforested area (RA) concerning to interception in tree-, shrub-, dwarf-, moss- and litter layer and soil depths of 10, 30 and 50 cm (seeping water) were studied. The first results show a clear depence of interception to different rainfall features and rainfall intensities. Concerning the predominantly long rainfall events, those particularly took place during springtime, is shown, that the bulk of throughfall (> 90 %) both in OGF and in RA is dropping down to the dwarf layer. In contrast to that, the amount of rainfall in moss- and litter layer achieved nearly 20 %. During drizzle-rainfall events these layers remained to be dry. Depending of whether such an event took place after a long rainfall event or after a long dryness period, differences concerning to wetting capacity were carried out in the tree layer. Thus, in RA 20 % of drizzle rainfall events after long dryness periods have reached layers below the tree layer, in contrast to the OGF, in which nearly 60 % have achieved the dwarf layer. 1. Introduction The global increase of air temperatures since the Late-Glacial and during Holocene is well-documented by means of proxy-data and meteorological monitoring. In addition a global warming is documented meanwhile by sustainable models (CGCM2, CSM_1.4, ECHAM4/OPYC3, GFDL-R30_c, HadCM3) (KÄLLÉN & KATTSOV 2005, LECKEBUSCH et al. 2006). These model calculations show a more intensive global warming in high latitudes in comparison to the rest of the world: While the global average for increasing air temperatures is arranged between 3 und 5 Kelvin in 2100; the amount of global warming in areas northern of 60° N will ascend to 7 Kelvin in 2100. Thus, due to a change of conditions for evapotranspiration as soon as for different forms and distribution of precipitation an atmospheric warming will increasingly take influence on the global and regional water supply. As a result of growing population, industry and agriculture there may be a new force to the ressource „water“. Hence, the economical interest of water is high as never had been before (see LOZÁN et al. 2005). The research of the components that have an influence on the regional water cycle is very important. By means of that knowledge an exactly and providently water cultivation is guaranteed (see GOLDBERG & BERNHOFER 2005). The influence of forest-vegetation to the water balance is of that importance that only a part of a rainfall-event is falling down to the forest soil via canopy. That part of rainfall which is not directly falling to the ground is holding by different vegetation layers and can evaporate from them. Thus, the needed energy for evaporation in the canopy is not available for soil depths, however the water amount that evaporates from the upper tree-layer doesn’t normally achieve the soil- and ground water and discharge respectively (see BARRY & CHORLEY 1992).

For a long time several scientific studies have existed to the problem of rainfall-retention through strata-specific interception as one effected element to the water supply (see early work reports written by BAUMGARTNER 1979, BRECHTEL 1969, DELFS 1955, DELFS et al. 1958, MITSCHERLICH 1971 or WEIHE 1959, 1970 and 1976 as soon as AUSSENAC 2000, CARLSON & GROOT 1997, CHEN et al. 1993, CROCKFORD & RICHARDSON 2000, HALL 2003, HASHINO et al. 2002, HEDSTROM & POMEROY 1998, HERWITZ & SLYE 1995, HUANG et al. 2004, LUNDBERG & KOIVUSALO 2003, NAKAI et al. 1999A and 1999B, PECK & MAYER 1996, POMEROY et al. 1998, POMEROY et al. 2002, SERRATO & DIAZ 1998, ZHENG et al. 2000). Thereby interception not only depends on meteorological parameters as solar radiation, soil- and air-temperature, relative air humidity, wind and precipitation but also on vegetation structure (plant species, layer composition, height and density of cover of vegetation, age of plant cover). Therefore it is astonishing that strata-specific interception in general is not included. CLARK (1937), STÅLFELT (1944) and MÄGDEFRAU & WUTZ (1951) have admittedly referred to the importance of shrub-, dwarf- , moss- and lichen-layer to the strata specific interception. Adding to last-mentioned studies scientific research to strata specific interception in two different boreal forest stands were carried out in spring 2006, summer 2006 and autumn 2006.

2. Study areas The study areas lie in the northwestern part of Finland, approximately 20 km south of the Arctic Circle (66°20’ N) close to the Finnish-Russian border (29°22’ E) and are administratively allocated to the province Oulun Lääni (Fig. 1). As part of the „Kuusamo Upland“ the study area are located east of the Maanselkä watershed between Gulf of Bothnia in the West and the White Sea in the East, in which all streams of that area drain (KOUTANIEMI 1983).

Fig. 1. Location of the study areas. Referring to the geomorphology the study area is divided into the “Kuusamo Upland” and “Oulankajoki Valley”. The first mentioned area is characterized by glacial hilltops, partly forested

(finn. vaara), partly treeless (finn. tunturi). Embedded between several hilltops vast Aapamire-complexes (finn. suo) in WNW-ESE direction are proceeded here (AARIO et al. 1974). In contrast to that the valley of the Oulankajoki lies in an average height of 135 - 160 m. The typical young tertiary fracture zone of Oulanka-Valley (continued in Paanajärvi) was filled with glacial sediments of retreating glacier in 9500 BP. In the following Postglacial the riverbed of Oulankajoki has intensively eroded by nearly 30 m (KOUTANIEMI 1979). The vegetation of that area belongs to the north boreal zone (AHTI ET AL. 1968). Whereas the Upland is characterized of boreal forests, peatlands and ponds, the WNW-ESE-directed valley of Oulankajoki offers an ideal corridor for typical step plants from dry regions in Southeast-Euope (i.e. Silene tatarica from Ukraine; see also SÖYRINKI & SAARI 1980). 2.1. Climate The study area belongs after KÖPPEN & GEIGER (1936) to the cold snow forest climate Dfc. The annual average air temperature for Kuusamo is -0.6 °C and for Oulanka -0.9 °C. This area is also characterized by high seasonal thermal contrasts; the amplitude in air temperature attains 70 K for Kuusamo, nearly 80 K for Oulanka. The lowest and highest measured air temperature in Oulanka achieved +31.1 °C and -48.0 °C, respectively (KOUTANIEMI 1999). The annual amount of precipitation is for Kuusamo 571 mm and for Oulanka (Kiutaköngäs) 554 mm. The highest amount of rainfall normally appears during the months july and august. The snowcover averages 80 cm; whereby Oulanka is one the most snow region in Finland (SOLANTIE 1975, KOUTANIEMI 1983). Due to heavy ice- and snow-packs on trees the timberline lies in a height of 450 m. The study area lies in a transition zone between a westerly-maritime characterized climate zone and an easterly-continental characterized climate zone. Whereas arctic, in the most cases dry cold air masses enter the eastern part of that region and provide a longer frost-period, in contrast westerly air masses from the Gulf Stream are often caused for humid weathering conditions. Therefore due to high temperature-anomalies the research area belongs to one of the most continental and due to the relatively high amount of rainfall-events in the summertime to one of the most maritime areas in Finland (KOUTANIEMI 1983). 2.2. Study areas For the determination of the strata specific interception one old-growth-forest and one reforested area were carried out. For ensuring relatively homogeneous conditions in local climate both field areas are located very close to each other. Furthermore similarly conditions concerning height, exposition and declination were assured. Accordingly to the initial question these field areas essentially only differ in vegetation structure.

Fig. 2. Study area. Old-Growth-Forest (OGF). The study area “Old-Growth-Forest” (OGF) (Fig. 2) is characterized by an average sea level height of 308 m and an average declination of 0.5° (from NE to SW). On this area a typical Empetrum-Myrtillus forest (see SÖYRINKI & SAARI 1980) with almost 250 years-old trees has developed. The tree population achieves 1100 stems per hectare. The oldest trees reach height up to 16 m; the average height of the tree-layer is 9.8 m. The main species in the tree-layer consist of Betula pubescens (59 %), Picea abies (34 %) and Pinus sylvestris (7 %). The comprehensive dwarf-layer is characterized by Vaccinium myrtillus, Vaccinium vitis-idaea, Vaccinium uliginosum, Empetrum nigrum. Therefore this area is sporadic covered by Carex spec, Ledum palustre and Betula pubescens. The moss-layer is almost completely filled by Sphagnum spec. The study area of “Reforested Area” (RA) (Fig. 3) is located 300 m north of OGF. This area lies in a height of 305 m and is characterized by an average declination of 1.0° (from NE to SW). In contrast to GOF this area is distinguished by a drainage system that runs parallel. These drainage lines were aimed to dehumidify former mire-complexes. Thus, better conditions for fast-growing trees were managed. After drainage this area was covered with one year old pines. The tree-layer is signified by an average density of 1775 stems per hectare and primarily consists of Pinus sylvestris (73 %) with an average height of 5.0 m (max. height: 8.6 m), Betula pubescens (21 %) with an average height of 2.8 m (max. height: 4,2 m) and Picea abies (6 %) with an average height of 2.6 m. The rich shrub-layer is covered among last-mentioned plant species with Sorbus aucuparia, Juniperus communis, Ledum palustre and sporadic Salix-species. The dwarf-layer is essentially composed of Vaccinium myrtillus, Vaccinium vitis-idaea, Vaccinium uliginosum, Empetrum nigrum. On dry habitats Calluna vulgaris, Ledum palustre and Salix spec. were identified. The moss-layer is dominated by different Sphagnum species. The soil conditions in both field areas are characterized by substrates of silt and sand. The typical podzol is marked by a huge amount of litter on the one hand and a formation of thick eluvial horizons in the upper soil-layer on the other hand.

Fig. 3. Study area: Reforested Area (RA). 3. Methods In several periods during spring 2006 (20.05. to 11.06.), summer 2006 (17.07. to 04.08.) and autumn 2006 (13.09. to 03.10.) scientific studies to strata specific interception on both study areas were held. In determining interception in dependence on meteorological parameters (temperature, solar radiation, wind, precipitation, wind, relative humidity and suction power) climate stations on both study areas were installed. The amount of solar radiation were recorded in a height of 2 m. Temperature measurements were carried out in 2 different heights (2 m, 5 cm) and 3 different soil depths (10 cm, 30 cm and 50 cm). Tensiometers for measuring soil water content were put in soil depths of 10, 30 and 50 cm. The daily amount of evaporation was measured by three Piché-evaporimeters in 0.1 m, 1.0 m and 2 m, respectively. For collecting meteorological data on both field areas a climate station (company: Thies) with a measuring time of one minute was installed. Data memories were carried out for average values (every 10 minutes) and for extreme values (every 30 minutes), respectively. Thus a more detailed analysis of special weathering conditions in both field areas were enabled. Interception is not directly measurable but rather determinable from the difference of open field precipitation and precipitation that is falling through different vegetation layers. Thus on an open field area (200 m northern of RA) a reference climate station was installed. For determining the throughfall underneath the tree layer on both field areas two rain gauges (each 6400 cm²) were installed. In addition for estimation of the precipitation amount underneath the tree-, shrub- and dwarf-layer 43 randomly-distributed rain gauges (100 cm²) were used. The part of the precipitation that normally falls through the moss- and litter-layer respectively was caught in two vessels (each 1600 cm²) per field area. For concrete conclusions of seeping water in the upper soil layer the use of tensiometers in different soil depths were needed (10 cm, 30 cm and 50 cm). The stemflow received by three trees per field area is also measured in three tipping buckets of the same design. The stemflow collectors are polyurethane foam rings sealed with silicone rubber, forming a watertight junction between the ring and the tree bark. The calculation of stemflow was

aquired by means of stemcounting-method. Thereby the stemflow of single trees will be multiplied by the amount of stems according the same average stadium. The sum of stemflow water was converted into square units. Finally the results were expressed in percent of open field precipitation (comp. DELFS et al. 1958). 4. Results The results presented here include data from 23.05.06 to 11.06.06 (spring 2006), from 18.07.06 to 04.08.06 (summer 2006) and from 14.09.06 to 03.10.06 (autumn 2006). 4.1. Throughfall After each rainfall event measurements to strata specific interception were carried out. The daily sum of precipitation during the measuring time is shown in fig. 4.

Fig. 4. Course of daily open field precipitation during the measuring times 2006 (dotted lines: measured times). During the study period the main part of open field precipitation took place during the springtime 2006. The summer period 2006 was extremely dry (3.1 mm open field precipitation). The autumn 2006 has placed in an intermediate position. As well as in RA the relative amount of throughfall in OGF in springtime was much higher than in summer and autumn period (see tab. 1). The amount of stemflow in all measuring periods was quite low. The interception values during the springtime 2006 attained 8 % in the RA; 19 % in OGF. During the summer these values got higher. As well as in OGF nearly 40 % of open field precipitation in RA was already intercepted in the canopy layer (see tab. 1).

Table 1. Amount of Throughfall, stemflow and canopy interception in the tree layer.

stem ugh flow thro fall + stemflow interception

t

hroughfall

season % mm m % m % % m m mm

spring 9 48,1 ,4 92, 48 81,2 0,8 0 0 ,5 ,0 4,2

sum 0,0 61 1, 3mer 61,3 1,9 0,0 ,3 9 8,7 1,2

RA

autu 0,2 81 19 1mn 80,6 19,1 0,8 ,4 ,3 8,6 4,4 spring 8 42,7 ,0 81, 42 11,0 0,0 0 0 ,7 9,0 10,0

sum 0,0 61 1, 3mer 61,3 1,9 0,0 ,3 9 8,7 1,2

OGF

autu 0,0 79 18 2mn 79,7 18,9 0,0 ,7 ,9 0,3 3,4

The results presented in shrub- , dwarf-, moss- and litter layer down to the upper soil layer show a gradual decrease in the amount of througfall precipitation (see tab. 2). On RA during summer period 2006 32.3 % of throughfall precipitation in shrub- and 35.5 % in dwarf layer respectively were caught (see tab. 2). In addition there was no seeping water distinguished because the whole amount of open field precipitation was already caught by the litter layer (see tab. 2). In contrast during spring and autumn period 2006 a small amount of throughfall was achieved underneath the moss- and litter layer (see tab. 2) Table 2. Strata specific amount of throughfall (in % of open field precipitation) during spring 2006, summer 2006 and autumn 2006.

shrub rf l llayer dwa layer moss ayer litter ayer spr 90,3 93,0 19,0 12,0 ing

summer 32,3 35,5 0,0 0,0

RA

autumn 65,0 76,8 27,9 4,6 spr 92,0 20,1 11,4 ing

summer 61,0 0,0 0,0

OGF

autumn 74,3 12,2 2,5

4.2. The Rainfall-Interception in relationship of different types of rainfall and rainfall intensities Precipitation normally prevails in liquid or in solid form. Depending on amount of rain per catchment area and measuring time range rainfall events will normally classified into drizzle rain events (0.1 mm/h), land rain (1 -2 mm/h; > 6 h), rain shower (8 – 10 mm/h), thunderstorm (10 – 50 mm/h) and heavy shower (50 – 100 mm/h) (see DYCK & PESCHKE 1989). In relationship at different amount per hour rainfall events can additionally classified into weak (< 0.5 mm/h), medium (0.5 – 4 mm/h) and

strong rainfall events (> 4 mm/h). From the estimated values during the study period 2006 drizzle rainfall and land rainfall events were emphasized at best. 4.2.1. Land rain Land rain, typical for middle-european weather conditions, took only place in Oulanka during spring 2006 (24.05., 28.05., 31.05.; see tab. 3). It is apparently, that a high amount of throughfall has reached the areas underneath the dwarf layer. Concerning the interception values in the tree layer in GOF and RA clearly differences between them have appeared (see tab. 3). Whereas in RA the amount of throughfall achieved more than 90 %, in OGF nearly 15 % of that same amount intercepted (see tab. 3).That could be a matter of a relatively high amount of birches in OGF. On that time they are normally lingering in a regressive phenological phase. Due to their distinctively foliage they are respond for a higher caught of rain. In contrast to the upper vegetation layers 20 % reached the moss- and litter layer (see tab. 3). Table 3. Strata specific amount of throughfall (in % of open field precipitation) during land rain events (24.05.2006, 28.05.2006, 31.05.2006).

RA OGF tree layer 96,8 87,9

shrub layer 90,9 dwarf layer 97,0 91,0 moss layer 21,5 19,1 litter layer 18,3 14,6

4.2.2. Drizzle rain In contrast to land rain events during the study period 2006 more drizzle rain events were emphasized. But you have to consider if the strata specific amount of throughfall during a drizzle rain occurred after a wet phase or after a long dryness period. In both cases throughfall was estimated until the dwarf layer, except RA after a long dryness period. In addition there is no throughfall measured in moss- and litter layer (see tab. 4). Drizzle rainfall events in a wet milieu show that in OGF nearly 95 % of open field precipitation achieved the dwarf layer (see tab. 4). That could be interpreted from dropping water from the tree layer after the tree layer has reached the maximum wetting capacity. For the estimation of drizzle rain after a long dryness period a special condition has aquired. Whereas on the pines-populated RA 20 % of open field precipitation fell through the tree layer on OGF nearly 60 % fell down to the dwarf layer. That could be interpreted with the different wetting capacity of needles and foliages of plant structures, too. The more densely structured needles afford a longer binding of water drops. The process of dripping down is firstly admitted if the wetting capacity is saturated. On leaves water can easier drip down.

Table 4. Strata specific amount of throughfall (in % of open field precipitation) during drizzle rainfall events after a wetting period and after a dryness period.

after wet condition after dry condition

RA OGF RA OGF tree layer 55,0 53,8 20,0 57,7

shrub layer 60,0 0,0 dwarf layer 95,0 86,4 0,0 68,2 moss layer 0,0 0,0 0,0 0,0 litter layer 0,0 0,0 0,0 0,0

4.3. Seeping water The amount of seeping water can comprehend by use of soil suction power beyond the matrix-potential. Due to soil saturation after snow thawing in spring 2006 and long dryness periods during summer 2006 the course of the seeping water was only apprehended during the study period in autumn 2006. In the following the course of throughfall on a rainy day (20.09.2006) through all above-mentioned layers will be presented (see fig. 5). On this day occurred several rainfall events with different length and intensity (medium rainfall event between 01:00 and 02:00 a.m., a weak rainfall event 08:30 a.m., a long rainfall event between 01:00 and 08:00 p.m.; see fig. 5). On that day all rain bulks and measuring fixtures at 08:00 a.m., 02:00 p.m. and 06:00 p.m. were checked and analyzed. During the first check 40 – 60 % of throughfall reached the dwarf layer (see fig. 6). 6 hours later the amount of throughfall in the dwarf layer in OGF and in RA respectively has risen (see fig. 7). It could refered to an increase of dropping water from the tree layer due to a higher wetting capacity after several rainfall events (starting 01:00 p.m., see fig. 6). The amount of throughfall that reached the moss layer has been achieved during the third check at 06:00 p.m. after long rainfall events (see fig. 8). The amount of throughfall that fell down to the upper soil layer will be apparently by considering the suction power courses in the different soil depths (10 cm, 30 cm and 50 cm, see fig. 9). At 05:00 p.m. (4 hours after the long rainfall event has started) a significant decrease in suction power, especially in the upper 10 cm soil depths would be recognized (see fig. 9). This figure also shows that OGF is subdued to a more moistly milieu than RA does.

Fig. 5. Course of hourly rainfall events during september the 20

th, 2006.

Fig. 6. Strata specific amount of throughfall (in % of open field precipitation) at 08:00 a.m. on September the 20

th, 2006.

Fig. 7. Strata specific amount of throughfall (in % of open field precipitation) at 02:00 p.m. on September the 20

th, 2006.

Fig. 8. Strata specific amount of throughfall (in % of open field precipitation) at 06:00 p.m. on

September the 20th

, 2006.

Fig. 9. Course of suction power (in hPa) in 10 cm, 30 cm and 50 cm soil depths (OGF, RA) during September the 20

th and 21

st.

5. Discussion The first results of the study period in spring, summer and autumn 2006 show a different strata specific interception in an old-growth forest and in a reforested pine-plantation respectively from the tree layer to the upper soil layer. Thereby rainfall-interception in relationship at different types of rainfall and rainfall intensities were carried out. The similar weather conditions during the spring and autumn period admitted by use of classification of different types of rainfall in land rainfall events and drizzle rainfall events. On last-mentioned in OGF as well as in RA the major part of throughfall has aquired until the dwarf layer, whereas the amount of throughfall in moss- and litter layer has to be occurred during sufficient wetting conditions. On weak or medium rainfall events the amount of throughfall is yet decreasing through the canopy. Different values in vertical distribution of strata specific interception after rainfall events were estimated depending on whether wet or dry conditions occurred before them. In a hot and dry summer strong rainfall events or thunderstorms no seldom appears in that region. On that summer no rainfall event of that types were unfortunately recognized. How far the strata specific interception depends on meteorological parameters as energy budget, saturation vapour pressure and different wind conditions is a component part of furthermore study periods as well as the reference of interception to different phenological stages of vegetation. With the financial help of LapBIAT (Lapland BiosphereATmosphere) this studies will be continued during the growing season in 2007.

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