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Water regime group
Stand water balance – can be negative, balanced, positive or highly positive according to the water inputs and outputs in/out the stand; the water inputs can be surface or underground inflows, the main water inputs into the stand are vertical precipitation, as well as horizontal in the mountain areas; the water outputs can be surface runoff or underground drainage and evapotranspiration, the largest part of the total water evaporation from the forest stand is formed by transpiration; e.g. in the case of a spruce stand the important part of precipitation (about 30%) is caught by leaves and again evaporated into the atmosphere (interception), about 1-2% of water forms the stem flow and the rest of the precipitation falls through the canopy down to the soil surface, where the small amount (about 1%) directly flows off and the rest soaks into the soil, the water from the soil environment will then be used by plants for transpiration (about 60% of total precipitation).
Simplified water balance equation:
Precipitation + inflow  interception + evaporation + transpiration + outlet
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Measurements of the precipitation above the stand (the red circle) and on the free plot. From the left side: typical precipitation gauge (CHMI), collecting vessel, calibrated snow bar (for the snow cover measurement), collecting vessel (for the determination of rain water chemical composition), collecting vessel, automatic precipitation gauge (ENVITECH, AMET, CZ), calibrated little snow bar, horizontal precipitation collector and hydrometeorological box (CHMI).
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Transpiration – the physiologic process in which the water absorbed by the plant is transpired into the atmosphere through the stomatal apertures as a water vapour. The small part of water could be directly transpired through leaf cuticle and stem bark (about 10%); only the small part of absorbed water remains permanently in plants (about 2%), plant organs (e.g. leaf, wood) contain about 60% of the water.
- Transpiration enables not only uptake and transport of nutrients, but especially it reduces potentially harmful temperature fluctuations. Since, only 1-5% of absorbed solar radiation is used in the photosynthetic processes, whereas, 95-99% is thermally dissipated. Exchange of heat between plants and their environment is affected by heat conduction and convection (sensible heat exchange) and by evaporation or condensation (latent heat exchange). Transpiration, water uptake and conduction of water from the roots to the transpiring surfaces are inseparably linked processes in water balance. The vapour-pressure deficit of the air and the difference of water potential gradients in the continuum soil-plant-atmosphere are the driving forces for water transport through the plant and evaporation.
- Techniques available for the measurements of the transpiration rate: gas exchange by infra-red analysers, lysimeters, heat pulse velocity method, aerodynamic and eddy covariance techniques.
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Sapflow Meter SF 300 (Greenspan Technology, Australia) – an instrument for the transpiration measurements via the monitoring of sap flow rates by heat pulse velocity method. The system consists of energy source, two pairs of probes, and data-logger.
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Heat pulse method – is based on the idea that all transpired water passes through the stem. The sensor consists of three needles placed in different depths of sapwood under the canopy. Upper and lower needles have sensors for temperature detection; the middle one generates heat pulses. Water passing through the stem is marked by these pulses. Data-logger records the response lag-time necessary for the balancing of temperatures after the heat pulse. On the base of these results obtained on one tree level, it is possible to calculate the whole stand transpiration using the following dendrometrical characteristics:
- crown envelope surface area
- leaf area
- sapwood area
- stem diameter at 1.3 m
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Transpiration losses of young spruce stand during two summer days. The curves were estimated by heat pulse method (SF 300) and by the eddy-covariance system (Edisol). The figure documents an effect of water refilling in the afternoon hours. |
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