Realtime Dendrometry Data
Stem diameter is one of the most commonly measured attributes of trees. Dendrometers are used to measure the diameter of fruits, plants and trees. High resolution dendrometers are used to monitor the diurnal swelling and shrinkage of stems. During the day stems “shrink” as stomata open and the tree transpires. At night the stem “swells” due to cessation of transpiration and trunk refilling of moisture. Maximum Daily trunk Shrinkage (MDS), the calculated difference in daily minimum and maximum stem diameter, is a commonly used parameter in irrigation scheduling. Significant crop research has been undertaken in this field to explore the correlation of MDS to physiological and abiotic parameters including soil moisture and water potential, vapor pressure deficit (VPD) and stem water potential.
Seasonal datasets can be used to compare fertilisation treatments, pruning, thinning or drought treatments. In forestry dendrometers are used for long term data collection in the study of growth dynamics, biomass allocation and carbon uptake. In horticulture Dendrometers are used to monitor MDS for irrigation management.
Dendrometer bands are a long accepted and widely used method of measuring tree circumference and can provide changes in tree diameter at breast height (DBH), basal area, and basal area increment. The DBS60 Band Dendrometer is a high resolution (1μm [0.001mm]), non-invasive sensor capable of measuring a wide range of diameters (50mm>). The stainless-steel band has a very low linear thermal co-efficient. Thermal variations caused by daily or seasonal changes in temperature have no measurable impact on the measurement accuracy. The DBS60 is IP66 rated and is designed to be installed in the harshest field conditions for years at a time.
Pivot dendrometers are designed for simple, error free installation, being fastened on the stem by a spring-based lever clamp. Adherence pressure is set as a compromise between the influence on plant tissues and installation stability. The DPS40 Pivot Stem Dendrometer is a high-resolution pivot-based sensor for measurement of small stems, from 5mm to 40mm, the bearing of the position sensor is carefully shaped for minimal effect of temperature and axial forces.
The SFM1 Sap Flow Meter is a discrete standalone instrument based upon the Heat Ratio Method. This measurement principle has proven to be a robust and flexible technique to measure plant water use; being able to measure high, low, zero and reverse flows in a large range of plant anatomies & species from herbaceous to woody, and stem sizes > 10 mm in diameter.
The theoretical basis and ratiometric design of the Heat Ratio Method makes possible the measurement of high, low, zero and reverse flows.
The SFM1 Sap Flow Meter consists of two temperature sensing needles arranged equidistance above and below a central heater. These needles are inserted into the water conducting tissue of the plant by drilling 3 small parallel holes. Heat is then pulsed every 10 minutes into the water conducting tissue of the plant. The heat is used as a tracer to directly measure the velocity of water movement in the plant stem. The Heat Ratio Method does not require insulation.
The SFM1 Sap Flow Meter is a dedicated self-contained data logger, with a heater and two temperature sensing needles, that provides power to the heater and logs sap flow in litres per hour of water used by the plant. This is the water actually used by the plant in litres, completely independent of any water that may have been lost to evaporation from bare soil, run off or through drainage.
Photosynthetically Active Radiation (PAR)
Light intercepted by a leaf may be absorbed, reflected, or transmitted; the fraction absorbed depends on the spectral content of the radiation and the absorption spectrum of the leaf.
Photosynthetically active radiation (PAR) is light of wavelengths 400-700 nm and is the portion of the light spectrum utilised by plants for photosynthesis. Photosynthetic photon flux density (PPFD) is defined as the photon flux density or PAR. If PAR is low for a given species of plant, growth and carbon assimilation is limited, while too much PAR may damage the photosynthetic apparatus.
The spectral responses of all quantum sensors deviate from the ideal response to some degree. Spectral error occurs when measuring a light source that has a different spectral output than the light used to calibrate the sensor. This error occurs because no quantum sensor can perfectly match the ideal quantum response, which is defined as an equal response to all wavelengths of light between 400 and 700 nm. The Apogee SQ-500 Full Spectrum Quantum Sensor has a response closer to that of an ideal quantum sensor than the SQ-110.
Plants sense light using photoreceptors, such as phytochrome, and use wavelengths outside of the PAR range – mainly within the UV and far-red light spectrums to sense and respond to their environment. The plant canopy selectively absorbs red wavelengths (approximately 660 nm) more than far-red wavelengths (approximately 730 nm) resulting is a decrease in the red: far-red ratio of light toward the base of the canopy, such changes in light quality result in photomorphogenic changes in plant growth. In agricultural production systems an understanding of these responses is central to optimising planting density and canopy management.
Canopy Light Inception
Plant light interception efficiency is a key determinant of carbon uptake by plants; plant productivity over seasonal time-scales is approximately proportional to intercepted light. Canopy architecture, leaf area, leaf angle distribution, and leaf dispersion are determinants in the light distribution and interception within the canopy. In horticultural crops pruning strategies can optimise tree structure and drive higher productivity and increase plant health and longevity.
The measurement of fraction of intercepted PAR (f) is an indicator of a plant’s light use efficiency or its ability to convert sunlight into biomass. The simple method requires at least one PAR sensor above the canopy to measure direct beam and one or more PAR arrays beneath the canopy.
A PAR array is necessary beneath or within a canopy because it samples a larger area and considers sunlight variability caused by the canopy. Plotting f over a growing season against some measure of yield or biomass indicates the light use efficiency of crops. The MFR-NODE and AD-NODE can be configured with LINPAR and PAR sensors to measure, monitor and calculate intercepted PAR (f), and hence biomass and yield.
Plant Reflection of Light
NDVI and PRI are calculated from measurements of electromagnetic radiation reflected from plant canopy surfaces.
NDVI (Normalized Difference Vegetation Index) is a standardized index use to measure the state of plant health. Leaf chlorophyll absorbs red light (approximately 680mm), and the cellular structure of the leaves strongly reflect near-infrared light, approximately 730mm. When the plant is water stressed or diseased the spongy layer deteriorates and the plant absorbs more of the near-infrared light, rather than reflecting it.
By observing how NIR changes compared to red light provides an accurate indication of the presence of chlorophyll, which correlates with plant health. PRI (Photochemical Reflectance Index) originally defined as an index of the diurnal xanthophyll cycle activity, provides a measure of photosynthetic light-use efficiency (LUE) which can be used as an indicator of stress. PRI bands are centred at 532nm and 570nm.
Leaf wetness refers to the presence of free water on the canopy, and is caused by intercepted rainfall, dew, or guttation. The duration of the time period during which the leaves are wet is generally referred to as leaf wetness duration (LWD). Leaf wetness is a concern for the development of disease and the dispersal of pathogens; LWD is an important input (along with temperature) in many crop disease models which are used for determining the appropriate time for the use of preventative measures, such as fungicide application.
Infrared Radiometry – Canopy Temperature
An infrared thermometer measures radiant energy. This radiation is simply “light” that is slightly outside the human eye’s sensitive range. All objects radiate infrared energy. The intensity of infrared radiation is proportional to the temperature of the object. Infrared thermometers produce no “intrusion error.” A hot object “target” is radiating its infrared radiation in all directions. The object’s radiation characteristics, and hence its temperature, are not disturbed by the presence of the infrared thermometer.
The infrared thermometer optics collect a sample of infrared radiation from the hot object (soil & plant) being measured and focus it on the tiny infrared detector. The detector, in turn, converts it to a proportional electrical signal, which is the exact electrical analog of the incoming infrared radiation, and hence the hot object’s temperature. This minute electrical signal is then amplified, converted to a digital signal, and digitally linearized and the resultant temperature either displayed or data logged.
Low temperature infrared thermometry (IRT) is technically quite difficult especially when measuring temperatures of crop canopies which have a very weak infrared signal. Temperature needs to be resolved to 0.1°C to make meaningful irrigation and management decisions. Continuous measurement of the transducer temperature and sky reflectance of infrared light must be undertaken. Accurate measurements of plant canopy temperature, which, along with other environmental variables, allows estimation of canopy transpiration and crop stress using a calculation such as Crop Water Stress Index (CWSI).
The THERM-MICRO Leaf Temperature Sensor is a very small thermistor that can be adhered to a leaf surface for the measurement of absolute temperature of the leaf at the surface. The THERM-MICRO’s small size means that it has almost no thermal mass, resulting in minimal boundary layer influence and measurements which are highly responsive to changes in leaf temperature.
Frost (Leaf & Bud Temperature)
Frost damage to plants can have large impacts on crop yield and quality. The SF-421-SS is a combination of two temperature sensors (precision thermistors) designed to mimic a plant leaf and a flower bud. Protection of crops during frost events is dependent on the accuracy of plant temperature predictions.
Often, air temperature is not a reliable predictor of timing, duration and severity of frost events because plant canopy temperatures can be significantly different than air temperature under certain environmental conditions. On clear, calm nights, plant leaf and flower bud temperatures can drop below freezing even if air temperature remains above 0°C. This is called a radiation frost and is due to the lack of air mixing (wind) near the surface, and a negative net longwave radiation balance at the surface.