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Hartheim Forest Research Site

  

Objectives and goals of the research site

The Hartheim Forest Research Site of the University of Freiburg (Fluxnet ID: "DE-Har") is operated by the Chair of Environmental Meteorology (previously "Meteorological Institute") since 1969. At this field facility we conduct scientific research on interactions between soil, forest and atmosphere at an unusually dry location. The aim of the Hartheim Forest Research Site is to scientifically record long-term and intensive meteorological, hydro-meteorological and forest-growth parameters related to the interactions between soil, forest and atmosphere, in order to study processes and to develop or test models. The focus of the research is on water use of forests, processes of forest growth, processes leading to storm damage and the exchange of trace gases (such as greenhouse gases) between the forest and the atmosphere. Since its inception, more than 40 peer-reviewed scientific publications on measurements and modeling studies were carried out at the site or using data from the site.

The drought-damaged forest with the tall  tower (in foreground) and the small canopy access tower (back) in autumn 2018 (Photo: D. Redepenning)

Location

Canopy structure and stand development

The Hartheim Forest Research Site (fenced area: 0.7 ha) is a stand that consists mainly of Scots pine (Pinus sylvestris) with scattered European black pines (Pinus nigra) (2019: 17.3 m high). The stand is characterized by a homogeneous structure. The forest has been repeatedly thinned in the last 50 years:

The development of stand height (green, left axis in m) and stand density (blue, right axis in trees / ha) at the Hartheim Forest Research Site from 1969 to 2019.

Climate

A particular characteristic of the Hartheim Forest Meteorology Site are regularly reoccurring summer dry periods:

 

Standorteigenschaften und Klima an der Forstmeteorologischen Messstelle Hartheim

Longitude
(WGS-84)
7.59814°E Average air temperature (1978 -2018) 10.6°C
Latitude
(WGS-84)
47.93391°N Annual precipitation total (1978 - 2018) 641 mm / year
Height a.s.l. 201 m Average shortwave irradiance (1978 - 2018) 4.25 MJ / (year m2)

 

Annual average air temperatures at the Hartheim Forest Meteorology Site from 1978 to 2018 relative to the 41-year mean 1978 - 2018. Red years were above average (warmer), blue years were cooler than average.

Development of  annual precipitation at the Hartheim Forest Meteorology Site from 1978 to 2018 relative to the 41-year average 1978 - 2018. Blue years received more precipitation, yellow years less than the average.

The anomalies in the year 2018 (in red) with respect to annual mean air temperature (10.6°C) and annual precipitation (641 mm) at Hartheim Forest.

Development of solar irradiance at the Hartheim Forest Meteorology Site from 1978 to 2018 relative to the 41-year average 1978 - 2018. Blue years received less solar radiation, yellow years more than the average.

In recent years (2014, 2015 and 2018), the pine forest has suffered from extreme drought stress and elevated temperatures, resulting in the death of many pine trees. This situation provides an exceptional opportunity to scientifically monitor the impact of regional climate change on forest ecosystems at an extreme dry location for Germany. This research provides an important basis for an assessment of the future development of forest land use under future extreme conditions in Germany and Central Europe, and therefore the data collected is important not only for the local context, but also for the planning of adaptation measures of other forest ecosystems.

Soil conditions

Schäfer (1977) identifies the type of soil at Hartheim Forest Research Site  as an anthropogenic "Kalkpaternia-Pararendzina". Due to the relatively low precipitation, resulting in a limited solution weathering, and because of the high carbonate content the site experiences a stagnating soil development . The pH of the mineral soil ranges between 7.6 and 8.3; the nitrogen content in the top layer varies between 0.23 and 0.14% (Hädrich and Stahr, 1992).

The edaphic conditions already suggest that fine and middle roots are mainly to be found in the top layer. Although the soil has a soil-related high water capacity, it can store only small amounts of water due to the flatness of the top layer. Despite high nutrient element stocks, the nutrient supply is relatively low due to the limited chemical weathering (Schäfer, 1977).

Measurement systems

At Hartheim Forest Meteorology Site we operate two towers (height: 18 m and 30 m), which serve as platforms for meteorological, hydrological and ecophysiological measurements. More than 100 measured quantities are recorded continuously in and above the pine forest. The measured variables include air pressure, precipitation, radiation in different wavebands, air temperature, soil temperature, soil moisture, humidity, wind speed, wind direction and gas concentrations. The measuring point has line power and a telephone connection. Line power ensures a permanent operation of the site. Due to the available infrastructure (including two large huts), the Hartheim Forest Research Site is regularly used for carrying out research projects.

Hartheim Main Tower

The tall  tower (30 m) allows profile measurements of temperature, humidity, and wind, as well as radiation and flux measurements in and over the forest canopy (Photo: A. Christen)

Precipitation

The total precipitation is measured above the forest with a precipitation sensor HP (Ombrometer). A CS125 sensor records the intensity and type of rainfall sensor and  fog. The canopy through fall (dripping and falling precipitation from needles, branches ect. on the forest floor) is sampled with four gutters. Spiral-shaped PVC sleeves with collection vessels collect the stem flow on seven trees with weekly readings.

Radiation

At the top of the 30m-tower, radiation measurements of the incoming and outgoing radiation are  measured separately for short- and long-wave radiation components. With a photocell, the photosynthetically active radiation is detected. Two phenocams record the seasonal development of the forest canopy in the visible and near infrared range.

  

The latest pictures of the two phenocams at 30 m (left) and at 8 m in the trunk space (right). Data from the phenocams is  integrated into the global PhenoCam network.

Profiles of air temperature / humidity

Profiles of temperature and humidity in and above the forest are measured on the 30m-tower. Active ventilated psychrometers according to Frankenberger (Pt 100) are used at 5 heights.

Wind, turbulence, tree movements

Vertical profiles of wind and turbulence are recorded with a profile of ultrasonic anemometers. The ultrasonic anemometers measure the three-dimensional wind vector at 6 heights twenty times per second. Special  accelerometers record the movement of the trees.

Energy- and trace-gas fluxes

An ultrasonic anemometer-thermometer (CSAT3B Campbell Scientific Inc., Logan, USA) combined with a carbon dioxide and water vapor gas analyzer (Li-7200 Closed Path Analyzer, Licor Inc., Lincoln, USA) determines the tubulent fluxes of sensible and latent heat and the exchange of carbon dioxide with the eddy-covariance method. Dendrometers are also used on several trees to record long-term tree growth.

Eddy Covariance System Hartheim

This eddy-covariance system above the forest continuously measures the exchange of heat and trace gases, including a sensor for determining the uptake / release of methane (Photo: A. Christen)

Soil climate and hydrology

Soil temperatures are measured at 1, 3, 5, 10, 20, and 40 cm depths. Soil moisture is determined by means of TDL (Time Domain Reflectomerty). Buried ground heat flux plates are used to measure the soil heat flux into the forest soil.

Selected research publications on Hartheim Forest Research Station

Peer reviewed journal articles

Dissertations and Theses

Selected book chapters, reports and conference contributions