Parameters ¶

Since the long-term Pfynwald irrigation experiment was set up in 2003, it has facilitated the measurement of over 120 parameters at various time and space scales and across various research projects. Below, you'll find details to those specific to the VPDrought project.

Click on the text tags to see the definition of the respective parameter, the explanations of the measurement methods and the significance of the parameter for the Scots pines in the VPDrought experiment.

Solar radiation refers to all electromagnetic radiation emitted by the sun. It covers a wide range of wavelengths, including visible light, ultraviolet (UV) radiation and infrared (IR; heat radiation) radiation.

Photosynthetically active radiation (PAR; μmol/m/s) is the part of solar radiation that can be used by plants for photosynthesis. Solar radiation increases foliage temperature, which in turn enhances transpiration (evaporation of water through leaves or needles). This helps the plant absorb nutrients from the soil and cool the leaves to avoid overheating.

In the VPDrought experiment, we measure PAR on a scaffold above the treetops. This allows us to detect when plants are under water stress.  

• Real-time data

Plants reflect, absorb, and emit light energy differently at various wavelengths depending on their vitality and the environmental conditions they face. During dry heat, just when water is scarce, sunlight is often at its strongest. The leaves or needles are then in an unbalanced state: they absorb more energy than they need for photosynthesis, as the stomata close to prevent water loss through transpiration (evaporation of water through the leaves or needles).

In the VPDrought experiment, we monitor canopy temperature with 16 infrared sensors (IR; °C) above the tree canopy. This allows us to determine when the Scots pines are under water stress.

• Real-time data

Precipitation is the process by which water from the atmosphere reaches the earth's surface. This includes water in various forms such as rain, snow, hail, sleet and dew.

Precipitation ensures that plants can absorb nutrients from the soil and have sufficient water for transpiration (evaporation of water through the leaves or needles). Transpiration cools the leaves and needles and helps to transport nutrients to the various parts of the plant. Precipitation is mainly responsible for ensuring that the soil is sufficiently moist and that metabolic processes can take place. Thus, precipitation directly influences the growth and development of plants via water availability in the soil.

In the VPDrought experiment, we measure precipitation with a rain gauge above the tree canopy. In addition, we determine the interception (rainfall retention capacity of trees).

• Real-time data

Air temperature (T; °C) is a measure of the thermal energy of the air. The air temperature influences many meteorological and climatic processes and is an important factor for biological processes and ecosystem processes.

In plants, the air temperature influences the rate of photosynthesis and other metabolic processes and regulates their growth and development. Transpiration (evaporation of water through the leaves or needles) increases as the temperature rises. Temperatures below freezing can lead to frost damage. Extremely high temperatures can cause leaf scorching, growth disorders and, in the worst case, plant death.

In the VPDrought experiment, we measure air temperature and relative humidity with 59 sensors at different heights, below, in and above the tree canopy, on 16 scaffolds in all treatments.

• Real-time data

Relative humidity (RH; %) is a measure of the moisture content of the air. It indicates how much water vapor the air contains compared to the maximum amount it can hold at a certain temperature. As the temperature increases, this amount increases disproportionately.

Relative humidity plays an important role in the transpiration (evaporation of water via the leaves or needles) of plants. Through transpiration, plants can absorb water and nutrients from the soil and cool their leaves and needles. If humidity is too low, the plants' stomata close to conserve water, which reduces CO₂ uptake and impairs photosynthetic performance.

In most cases, the relative humidity below the canopy is slightly higher than above. In the VPDrought experiment, we measure the air temperature and relative humidity with 59 sensors at different heights, below, in and above the canopy, on 16 scaffolds on all treatments.

• Real-time data

Vapor Pressure Deficit (VPD; kPa) is a measure of the difference between the actual vapor pressure of the water vapor in the air (e_a) and the saturation vapor pressure (e_s) at a given temperature. The VPD calculation is based on the relative humidity (RH) and the air temperature (T) as detailed in the following equation.

VPD = e_s – e_a  =  0.611 (17.27 x T / (237.3 + T)) – RH x e_s/100

Simply put, VPD describes how “thirsty” the air is and indicates how much more water vapor the air can absorb at a given temperature before it is saturated.

VPD is a decisive factor for transpiration (evaporation of water via the leaves or needles) and thus for water supply and cooling in plants. Transpiration creates a suction in the water pipe system of the plant, which allows the roots to absorb water and nutrients dissolved in it from the soil. Adequate VPD promotes efficient water absorption. However, too high VPD values can lead to excessive water release, which puts the plant under drought stress. To counteract this, the plant can close its stomata to save water. However, this has the disadvantage that CO₂ uptake is reduced and photosynthetic performance is impaired.

In the VPDrought experiment, we reduce the VPD in the canopies of 3-5 Scots pines on 6 scaffolds to investigate the influence of VPD on the metabolic processes of the trees.

• Further information
• Real-time data

The soil matric (or matrix) potential (Ψ; kPa), often referred to as soil water tension or soil moisture potential, is a measure of the force (suction) necessary for plant roots to extract water from the soil.

It reflects the attraction of the soil to the water, which is exerted by the matrix of the soil (i.e. the solid particles and their structure). This attraction is caused by capillary forces and adsorption forces. A low soil matrix potential (very negative values) means that the water is strongly bound to the soil particles and it is more difficult for plants to extract it. A high soil matric potential (less negative values) means that the water is less strongly bound and more readily available for plant roots. Unlike soil water content, soil matric potential is directly related to plant drought stress.

In the VPDrought experiment, we measure the soil water content with 45 sensors, at 15 locations and at 3 different depths (10, 80, 120 cm) continuously on the control, irrigated and dry plots.

• Real-time data

Soil temperature (T; °C) is influenced by a variety of factors, including air temperature, solar radiation, soil cover, soil moisture and soil properties. Soil temperature varies throughout the day and year and plays a crucial role in many biological and biogeochemical processes in the soil.

Root growth and development depend strongly on temperature. Warm soil temperatures generally promote root growth, while cold or excessively high soil temperatures can slow down growth. The activity of soil microorganisms, which are important for the decomposition of organic substances and the release of nutrients for plants, is controlled by soil temperature. Warm temperatures promote microbial activity, while cold temperatures slow it down.

In the VPDrought experiment, we measure soil temperature with 45 sensors, at 15 locations and at 3 different depths (10, 80, 120 cm) continuously on the control, irrigated and dry plots.

• Real-time data

Tree stems shrink during the day and expand again at night. These daily fluctuations are a consequence of the current water saturation of the tree. This water saturation is determined by transpiration (evaporation of water via the leaves or needles) and the water absorption by the roots. Water-related daily variations in stem radius have a direct influence on when a tree can form new wood and bark cells, i.e. when it grows. Continuous long-term measurements of the stem radius therefore provide information about the water balance, timing and rate of growth of a tree.

In the VPDrought experiment, point dendrometers measure the variations in trunk radii on 45 trees at breast height (1.3 m) every 10 minutes with a resolution in the micrometer range (μm). These fully automatic devices deliver a measuring point every 10 minutes and cover all treatments. The measurement points are processed on the TreeNet server to calculate the growth and tree water deficit components.

• Further information
• Real-time data

Tree water deficit (TWD; μm) refers to the state in which a tree absorbs less water than it loses through transpiration (evaporation of water via the leaves or needles). The resulting negative water balance causes the tree to suffer. Tree water deficit can be caused by various factors, such as drought, insufficient rainfall, high temperatures, strong winds or poorly permeable soil.

An increased tree water deficit reduces photosynthesis, as the stomata close to minimize water loss. This limits the entry of CO₂ and therefore the tree's growth and biomass production in the medium run. Many trees react to water deficit by shedding leaves or needles to minimize water loss. This reduces the photosynthetic area and therefore sugar production, which further weakens the health and growth of the tree.

If the water deficit is severe, hydraulic failure can occur, in which the water conduction pathways (xylem) are blocked by air bubbles. This phenomenon called cavitation prevents the transport of water from the roots to the leaves and needles and can lead to the death of parts of the plant or even the entire tree. Prolonged water shortage can lead to chronic stress, which affects the long-term health and survival of the tree. Repeated periods of drought stress can weaken the vitality of the tree and reduce its life expectancy.

In the VPDrought experiment, we derive tree water deficit from stem radius variations. These are measured by point dendrometers on 45 tree stem at breast height (1.3 m). The fully automatic devices provide a measuring point every 10 minutes and cover all treatments. The measurement points are processed on the TreeNet server to calculate the components growth and tree water deficit.

• Real-time data

Sap flow (L/h) refers to the movement of water and dissolved nutrients through the vascular system of plants, particularly in the xylem vessels. This process enables the transport of water and nutrients from the roots to the leaves or needles, where they are used for photosynthesis and other vital processes. Sap flow data thus provides insight into water movement in the tree trunk, which is mainly controlled by transpiration (evaporation of water via the leaves or needles) in the crown and water uptake via the roots. Thanks to these data, we can calculate how much water the trees use and how they control water loss by closing the stomata.

In the VPDrought experiment, we apply the Heat Ratio Method (HRM) to measure sap flow. This involves sensors inserted into the sapwood of the tree trunk on 45 trees on all treatments. These sensors consist of a heating needle and two temperature needles positioned above and below the heater. By periodically heating the middle needle, the sensors measure the temperature differences caused by the heat as it moves with the sap flow. By analyzing the speed at which the heat spreads, we can quantify the volume of water transported.

• Further Information
• Real-time data

Wind speed (m/s) and wind direction are measured with an anemometer, a device that detects the movement of the air. Wind direction is given in degrees (from 0° to 360°) or by means of cardinal directions (N, NE, E, SE, S, SW, W, NW). For example, a wind direction of 90° means that the wind comes from the east.

Wind speed influences the evaporation rate of water at the Earth's surface and the release of water into the atmosphere by plants. The Pfynwald, like the entire Valais main valley, is characterized by a mountain and valley wind circulation. At night, the air flows down the valley, while during the day it flows up the valley. The highest wind speeds are reached in the late afternoon.

In the VPDrought experiment, we measure wind speed and wind direction with 11 anemometers on 7 scaffolds at different heights from 2 m above the ground to above the canopy as well as at least one point in the canopy of all trees where vapor pressure deficit (VPD) is reduced. Wind speed influences the amount of water required to reduce the VPD. Wind direction is critical to ensure that the air with a reduced vapor pressure deficit actually reaches the canopy of the target trees.

• Real-time data