Hyytiälä Science Trails

The Science Trail offers everyone interested in science a comprehensive overview of Hyytiälä's versatile and multidisciplinary research topics related to forests, soil, lakes, rivers, swamps and climate. One part of the path goes through an economic forest and other part through a Natura area with valuable nature and landscape.

The trail consists of 18 information boards. Some of the boards contain tasks for students, ask for equipment that is needed for some of the tasks from the station's office.

A quick tour takes about two hours (approx. 4 km), but you can easily spend several hours by diving deeper into the topics. Part of the Science Trail is in difficult terrain and the path is not fully accessible. Be careful not to don't stray from the path, there are valuable measuring devices in the forests.

Pins on the map
  1. Rain measuring field
  2. Measurement of the base area and length of the timber
  3. Commercial forest management
  4. Effects of environmental factors on tree growth
  5. SMEAR II measuring station
  6. Carbon exchange
  7. Tick mapping
  8. Collectable forest products
  9. Soil and nutrients
  10. Small particles in the atmosphere
  11. Climate effects of aerosols
  12. Forest smells – volatile hydrocarbons
  13. Old forest and decaying wood
  14. Greenhouse gases in the swamp
  15. Mustikkakorpi
  16. Measuring board
  17. Game management
  18. Makkarakallio landscape

The starting point marked on the map

1. Precipitation gauge site

The precipitation gauge site is used to study the properties of rain and snow in a collaboration involving various universities and research organisations. The data from the various measuring instruments complement one another and help improve the accuracy of the measurements. The measuring equipment setup is adjusted as the research questions change, so the information sign may not include all instruments used at the moment.

Inside the windbreak, there are first two laser distrometers measuring the drop size and velocity of rain on the left, then a video distrometer in the middle, a rain gauge inside its own small windbreak and finally an anemometer measuring wind speed on the far right.

The electric field strength meter is used to monitor the effects that weak natural radioactivity has on the electric charge in the atmosphere.

The rain gauge is used to measure the amount and intensity of rainfall. The atmospheric fallout collectors collect samples of impurities carried by rain, such as heavy metals, mercury and combustion products.The meteorological station measures temperature, humidity and wind. The weather radar monitors the motion and type of precipitation in the atmosphere. The webcam records the scene.

The images recorded by the precipitation imaging devices are used to measure things like the diameter and falling speed of raindrops and snowflakes. The radiometer measures temperature, humidity and other things at different heights in the various layers of the upper atmosphere. Large and small windbreaks are used to reduce the effects of air vortices so that snowflakes in particular would not swirl past the gauges but land in them.

2. Measuring basal area and tree height

Basal area is the sum of the cross-sectional areas of all tree trunks measured at 1.3 meters above the ground per hectare. It is used to estimate the amount of wood in a forest stand and the need for forest management practices, which are of interest to the forest owner. It is also used to describe the forest volume in research.

As basal area increases, so does forest volume – but this also slows down growth because nutrients and light become more scarce. In commercial forests, the aim is to strike a good balance between forest volume and growth in order to produce a maximum amount of high-quality wood. One way to achieve this is through thinning, a practice in which some of the trees are removed to allow the remaining trees to grow faster and increase in value.

A stand’s basal area is easy to assess using a relascope. The relascope is used to calculate how many tree trunks are ‘IN’, i.e., fill the slot in the relascope when sighted at 1.3 metres above the ground. The trees are tallied by standing in one sample point and turning a full circle. A forest compartment’s basal area is typically calculated using an average of several sample points.

A more detailed idea of the volume and quality of wood can be gained by also measuring tree height and using it to assess stem volume. Timber trade is mostly based on the value of stem volume or the cubic volume of the stand. When both the basal area and mean height have been assessed, the forest compartment’s stem volume can be checked from a table drawn up for this purpose.

Tree height is easy to measure using a hypsometer. The hypsometer has two scales for measuring tree height at different distances: one is used to measure height from a distance of 15 metres and the other from a distance of 20 metres of the tree. The distance from the tree is measured either using a tape measure or a combination of a levelling rod and hypsometer. In the latter case, look through the hypsometer’s prism to see when the 0 line on the levelling rod fixed on the tree aligns with the line indicating the desired distance in the hypsometer. Tree height can also be measured using infrared devices or calculated from laser scanning data taken from a plane.

There are also various, often commercial, mobile apps available for measuring forest volume. These can be searched in application stores using search terms such as ‘forest measurement’.

3. Forestry in a commercial forest

An example history of a young spruce forest

2004: The stand was clear-cut in November, with a total outturn of 300 m3/ha. Of the felling volume, 75% was logs and 25% pulpwood. The logs were used to produce sawn timber, paper and energy, and the pulpwood to produce paper and energy. The logging residues and tree stumps were also used to produce renewable energy. The effects of harvesting logging residues were a hot topic at the time, so a small-scale experiment was carried out in the area. The area was divided into four compartments: in one compartment, logging residues were harvested for energy production, in the second, tree stumps were harvested, in the third, both were harvested, and in the fourth, neither were harvested.

2005: In May, the soil was scarified by an excavator and one-year-old spruce seedlings from southern Finland were planted using the Pottiputki planting tool at a density of 1,800 seedlings per hectare.

2009: The sapling stand was managed using a selective cleaning-thinning method where any broadleaved trees growing near enough spruce saplings to harm sapling growth were removed with a brushcutter.

2013: The sapling stand was thinned out to reach the goal density of 2,000 trees per hectare. Extra trees were removed with a brushcutter.

2019: The brushwood and damaged trees following the thinning were removed with a brushcutter.

2020: The mean height of the stand was 8 m and volume 50 m3.

An estimate of future forest management practices:

2032–2037: First thinning (outturn 50–70 m3)

2047–2052: Second thinning (outturn 70–100 m3)

2067–2087: Clear cutting (outturn 450–550 m3)

4. Environmental factors affecting tree growth

Trees use the glucose they produce through photosynthesis as the fuel for their growth. Trees use about half of the glucose they photosynthesise to grow their stem, branches, roots and leaves or needles. The growth speed of a tree depends on its photosynthetic efficiency, but also on environmental factors such as temperature and the availability of water and nutrients.

The picture shows a slice of discs, which were sawed off the trunks of two trees of the same size. The number of annual rings can be used to determine the age of the trees and the width of the annual rings to estimate the growth rate of the trees.

Each year’s growth depends particularly on the conditions of the previous year’s late summer, because this is when the tree forms the new buds that already contain every part of the new branches or shoots, which then reach their full size the following summer.

5. SMEAR II measuring station

The SMEAR II (SMEAR = Station for Measuring Ecosystem-Atmosphere Relations) experimental forest stand is used to study how the forest ecosystem and its components work and how they interact with the atmosphere. The site has hundreds of measuring instruments that continuously monitor the soil, trees, other vegetation and the atmosphere to study the mechanisms, pathways, storage and concentrations of compounds related to the forest stand both in the forest and in the atmosphere. By simultaneously measuring both the weather and other atmospheric properties, researchers can uncover how they affect phenomena observed in the stand and, conversely, how the phenomena that take place in the stand affect the atmosphere.

Thanks to long-term data from the experimental forest, our understanding of the changing weather conditions and events resulting from the forest stand’s growth is becoming increasingly detailed. The results help estimate the effects of climate change on the boreal forest zone and concurrent atmospheric phenomena.

The SMEAR II is part of an international measuring network. By combining the data collected through the network, researchers can gain a better understanding of global phenomena related to the atmosphere and vegetation. At Hyytiälä, similar measurements are also conducted in nearby lake and peatland ecosystems. Large proportion of the measurements are part of ACTRIS, Integrated Carbon Observation System ICOS (https://www.icos-ri.eu/) and Long-Term Ecosystem Research in Europe LTER-Europe (http://www.lter-europe.net/lter-europe).

The research targets all aspects of the relationship between the atmosphere and ecosystem.

The equipment installed in the mast collect data from an area wider than the experimental range itself, thanks to a method based on measurement height and wind data.

The measurement towers allow measurements to be conducted at the tree canopy height.

Numerous aerosol measurement campaigns with topical scientific breakthroughs have been done at SMEARII from 1996 and they continue. Those campaigns have strong base because of the well-qualified continuous aerosol precursor concentrations, physical, optical and chemical properties measurements at SMEARII.The aerosol instruments are located in Hitu-hut, small-hut, main-hut, tower (35 m), mast (127 m), hall roof,  cloud-radar field and container-area. All aerosol measurements are qualified by the continuous audits, calibrations at site and comparison campaigns by European Center for Aerosol Calibration (ECAC).These continuous aerosol measurements at SMEARII are part of the European Aerosols, Clouds, and Trace gases Research Infra Structure ACTRIS, http://www.actris.eu/).

The possibility to combine measurement material multidisciplinary sheds like to complex scientific questions such as feedback loops between ecosystems, atmosphere and climate. List of the measurements can be found here: List of the measurements

6. Carbon cycle

Forests absorb carbon from the atmosphere through photosynthesis and release it back through respiration and decomposition. Forests are powerful carbon sinks, meaning that, over the course of a year, they capture more carbon than they release.

Carbon cycle = Carbon fixation – Carbon release

Carbon cycling can be measured at the forest stand level from above the stand, which is what the mast pictured on the right does. This involves measuring wind speed, wind direction and the carbon dioxide concentration in the air and then using these measurements to calculate how much carbon the forest stand is assimilating or releasing at the time.

To understand how specific parts of a tree or a forest stand capture and release carbon, they can be sectioned out to containers whose carbon dioxide concentration is then measured. If the tree or other plants are photosynthesising, the carbon dioxide concentration in the container decreases. If the tree or soil is releasing more carbon than capturing it, the carbon dioxide concentration increases.

The measurement stand’s carbon exchange in 2016. When the figure is higher than 0, the forest stand captures more carbon than it releases (spring, summer, early autumn). When the figure is lower than 0, the forest stand releases more carbon than it captures (late autumn and winter). Note that summertime carbon fixation is much greater (about 5 g of carbon/m2/day) than wintertime carbon release (about 0.5 g of carbon/m2/day). The measurements are conducted with the measuring instruments located in the mast shown in the top left picture. You can spot the mast peeking above the forest canopy at least when you are near the Hyytiälä buildings.

7. Tick mapping

The Hyytiälä Forestry Field Station participates in a collaborative study conducted by several Finnish university research stations monitoring annual variation in the number of ticks. The study was sparked by observations about the growing abundance of ticks and concerns about the increase in tick-borne diseases. By biting people, ticks can transmit dangerous diseases, such as Lyme disease and tick-borne encephalitis (TBE).

In Finland, two tick species are considered as health threats to humans: the castor bean tick (Ixodes ricinus) and the taiga tick (Ixodes persulcatus). The two species look superficially similar, and they may occur in the same areas and environments. They also share the same hosts: wild mammals, pets, domestic animals and humans.

The tick study coordinated by the University of Turku is a continuation of the tick data collected through citizen-science efforts in 2014, in which samples provided by citizens revealed the current tick and tick-borne pathogen situation in Finland.

The network of participating research stations spans the entire country. The ongoing long-term monitoring study will offer a better understanding of how the number of ticks varies annually and during individual summers, how weather conditions affect the abundance and activity of ticks, and how the distribution of ticks changes over the course of the study. The study will also provide more samples of tick-borne pathogens.

8. Non-timber forest products

Foraging for various non-timber forest products turns regular forest walks into adventures. In Finland, everyman’s rights allow everyone to pick wild berries, mushrooms, herbs and other herbaceous plants, as long as they are not protected species. In addition to these, forests have a host of other non-timber forest products that can only be collected with the landowner’s permission. These include, for example, dwarf shrubs, clubmosses, moss, lichen and anything collected from a living tree.

At the University of Helsinki, research on non-timber forest products is clustered at the Ruralia Institute, which has units in the university consortia of Mikkeli and Seinäjoki. The University also collaborates with the Natural Resources Institute Finland (Luke), whose research includes non-timber forest products as a major natural resource.

Luke’s Marjahavainnot.fi website (in Finnish) allows everyone to contribute to the monitoring of the crop yield of three key wild berries: bilberry, lingonberry and cloudberry.

9. Soil and nutrients

Plants use their roots to absorb water and nutrients from the soil. The roots of different plants and trees have adapted to different habitats. For example, Scots pines have taproots that go deep into the ground, whereas spruces have a wide but fairly shallow root system. Thanks to their deeper roots, Scots pines fare better in dry places than spruces or broadleaved trees. Very wet places favour plants specialised in wet conditions, including trees such as the downy birch and the black alder.

In Finnish forests, the main nutrient for growth is nitrogen. Nitrogen fertilisation increases forest growth on mineral soils (i.e., everywhere but peatlands) almost always. Growth response of a spruce stand at different nitrogen fertilisation levels compared to a non-fertilised stand.

In Finland, the most common soil type is podzol, which has clearly distinguishable layers: plant litter layer, humus layer, horizon or topsoil, horizon or subsoil, and horizon or parent material.

10. Atmospheric aerosol particles

The numerical majority of atmospheric aerosol particles or aerosols are generated in the air as various vapours condense into solid or liquid matter. Aerosol formation can take place in slightly different ways in different places, but the process generally requires sulphuric acid (H2SO4), stabilising bases such as ammonia (NH3) and amines, and oxidised volatile organic compounds emitted by vegetation (see the sign ‘The smell of the forest – volatile organic compounds).

Vapours first form small molecule clusters (about 1 nanometre or nm in size), which then grow into larger particles (about 100 nm in size) when vapours condense on their surface under favourable conditions. 1 nm = 10-9 m = 0.000,000,001 m. In comparison: the diameter of human hair is about 70,000 nm!

The figure illustrates how aerosol formation is observed at Hyytiälä. The horizontal axis shows the time of the day and the vertical axis the aerosol diameter. The colour represents aerosol numbers. The first small aerosols are detected at around eight in the morning. Over the next hours, the aerosols grow larger, forming a curve in the graph.

Aerosols typically begin to form in the morning and before noon, when there is enough sunlight available. Sunlight is needed to form oxidising agents (OH and O3), which in turn are needed to form condensable vapours.

Aerosols are also emitted into the atmosphere directly in particle form, for example as dust, pollen and soot. Some of these aerosols travel to Hyytiälä over long distances, all the way from Central and Eastern Europe.

The second figure shows that between 1 June and 31 August 2010, aerosols travelled to Hyytiälä from two directions. The aerosols coming in from the northwest were generated through secondary organic aerosol formation, whereas the aerosols coming in from the southeast were generated from forest fires.

In general, one cubic centimetre (a cube the size of a sugar cube) contains about one thousand aerosols in Hyytiälä, a few hundred aerosols in Värriö in eastern Lapland, and about ten thousand aerosols in Helsinki . Leino et al. 2014 (BER)



11. The climatic effects of aerosols

Large aerosol particles, about 100 nm and larger, affect climate in many ways. Aerosols affect the course of sunlight. Some aerosols reflect sunlight back to space, while other, darker aerosols can absorb it. The same principle also explains why dirty snow melts faster than clean snow. The blue haze over mountains and the vibrant colours of the sunset are also caused by rays of light scattering from aerosols.

Aerosols are also needed to form clouds, because inside every cloud droplet is an aerosol upon which water begins to condense. Clouds also affect radiation: they reflect sunlight and emit heat from the Earth back into space.

Because aerosols are needed to form cloud droplets, they also affect cloud properties. In places with plenty of suitable aerosols, water begins to condense on more numerous aerosols, forming smaller cloud droplets than in places with fewer suitable aerosols.

This makes for brighter clouds that reflect more sunlight back into space, cooling off the climate.

Small cloud droplets stay longer in the atmosphere than large ones, meaning that it takes longer for the clouds to precipitate. This is because small cloud droplets take longer to grow into rain drops (about 100 μm = 100,000 nm and larger in size). Clouds formed of many small cloud droplets will thus have more time to reflect solar radiation back into space than clouds that rain quickly.

12. The smell of the forest – volatile organic compounds

Some of the carbon that plants absorb from the atmosphere through photosynthesis (number 1 in the figure on the right) is released back into the atmosphere as various biogenic volatile organic compounds (BVOCs).

In the atmosphere, BVOCs oxidise quickly and form vapours, which can then both form new atmospheric aerosol particles or increase the size of aerosols as they condense (see sign 10, ‘Atmospheric aerosol particles’). Hydroxyl radicals (OH), ozone (O3) and nitrate radicals (NO3) serve as oxidising agents in the process.

For example, the smell of freshly mowed grass and the surrounding forest are caused by BVOCs. There are tens of thousands of different known compounds. Volatile organic compounds are also released into the atmosphere through various industrial and combustion processes, but biogenic volatile organic compounds outnumber anthropogenic volatile organic compounds globally by ten times.

BVOCs are created in plants as metabolic by-products, but they also protect plants against various stress factors. For example, if caterpillars or aphids attack a tree, the tree releases BVOCs that repel these pests. The same BVOCs can also attract the parasites and predators of these pests, such as birds.

Dry spells, long hot spells and other unfavourable weather conditions as well as air pollution also cause stress to plants. It has been shown that plants emit BVOCs to reduce their oxidative stress. When this happens, oxidising agents (3) in the atmosphere react with the BVOCs instead of consuming compounds vital to the plants.

BVOCs also allow plants to communicate with each other. This allows a tree attacked by pests to warn the trees around it, giving them enough time to start producing pest-repelling BVOCs before they themselves are attacked.

13. Old-growth forests and decaying wood

One of the characteristics of old-growth forests is coarse woody debris, or decaying wood. Many forest species are dependent on dead wood, and the lack of it is one of the key factors endangering species living in the forest. In Finland, forests in their natural state have about 60–100 m3 of coarse woody debris per hectare, whereas commercial forests only have about 5 m3 of coarse woody debris per hectare. Decaying stumps are not included in these figures.

Various polypores thrive on the trunks of dead trees. Different polypores favour different tree species and decay types.

14. Greenhouse gases in peatlands

Peatland vegetation is dictated by the degree of its wetness and nutrient levels. The greenhouse gas cycle is more complex in peatlands than on mineral soil. Peatland plants fix carbon from the air through photosynthesis, just like forest plants do, but the rate of photosynthesis depends on the vegetation. Plants release carbon dioxide through respiration. Decomposition releases more carbon dioxide from peat than from the humus layer in the forest. The decomposition rate depends on the water level and temperature.

Methane is an important factor in the peatland greenhouse gas balance. It is released as a result of decomposition under anoxic conditions. Methane is a much more potent greenhouse gas than carbon dioxide.

The peatland surrounding our Siikaneva measuring station releases annually an average of 150 g CO2 in carbon dioxide equivalents (meaning that both carbon dioxide and methane are considered) per square metre. Cf. the SMEAR forest in Hyytiälä absorbs 1,100 g CO2/m2 annually. When discussing the carbon balance of peatlands, it is important to bear in mind that peat stores an enormous amount of carbon. The amount of carbon stored depends on the thickness of the peat layer.

15. Bilberry spruce mire

The bilberry (Vaccinium myrtillus) spruce mire is an important mire type and habitat in terms of biodiversity, but it has grown increasingly rare as peatland forests have been drained to promote forestry.

In bilberry spruce mires, the tree layer typically consists of spruce and downy birch. The shrub layer consists of willows. The field layer’s dwarf shrubs consist of bilberry and lingonberry, and herbaceous plants of cloudberry, chickweed wintergreen, narrow buckler fern and the sedge Carex globularis. The most common moss is common green peat moss, and common haircap moss in hummocks.

The peat layer is less than one metre thick and consists of decomposed Sphagnum and wood peat

16. Measuring raft

The measuring raft anchored at the deepest point of Lake Kuivajärvi is used to study the lake’s role in the carbon cycle and the effect the transfer of various gases and heat between the lake and the atmosphere has on the lake’s physical and biological properties. Physical properties include the amount of heat stored in the water and the rate at which heat is transferred between the lake and the atmosphere. Biological properties include the absorption or release of substances created through plant photosynthesis and the activity of other living organisms.

The measurements also speak of the lake’s interaction with the surrounding forest, because also measured are the carbon and nutrient concentration and temperature of the water that flows from the catchment area’s streams into the lake and out of it.

The star indicates the raft’s location on the lake.

The raft’s instruments measure, among other things, the transfer of carbon dioxide, water vapour and heat between the lake and the air. The amount of carbon dioxide in the water is also measured at different water depths. The temperature measurements are used to monitor the amount of thermal energy stored in the water at the time.

Like all ecosystem measurements conducted in the near vicinity of Hyytiälä, the raft measurements are also conducted continuously over several years. Long-term data offers new insights into the interactions between the lake and the atmosphere.

The raft is also part of a global measurement network of ecosystem–atmosphere relations, which allows researchers to study how the atmosphere and ecosystems work and interact with each other.

17. Game management

Game management is aimed at ensuring good living conditions for game animals, but many game management practices also help preserve biodiversity.

Game management can take the form of feeding or the management or improvement of game animal habitats. The objective of feeding game animals is to help them survive the winter when food is scarce. Game animal habitats can be improved by favouring broadleaved trees in mixed stands because they offer game animals food.

Alder/spruce pair. Alders offer food and spruces shelter for hazel grouses. Studies have shown that hazel grouses feed more often in alders that are standing next to a spruce.

18. Makkarakallio views

To many Finns, forest lakes surrounded by lakeside cliffs and ancient Scots pines are iconic Finnish landscapes, just like those pictured in Akseli Gallen-Kallela’s many paintings. For many, lakes offer not just pretty views, but also food and recreational opportunities.

Hyytiälä’s Makkarakallio (‘Sausage cliff’) is named after the forestry students’ campfire site.

Image: Albert Laukkanen