Photo courtesy of halbergman/iStock.

Urban environments present a multitude of environmental stressors that negatively impact tree biology. These stressors include soil deoxygenation due to waterlogging from poor or impeded drainage, soil compaction from human activities, increased frequency of drought and heat waves due to climatic changes, aerial pollution from elevated atmospheric pollutants caused by traffic and industry and the application of de-icing salts during subzero temperatures.

Over time, these stressors can limit carbohydrate availability for growth, as leaf photosynthetic rates generally decline in response to stress and reduce soil nutrient uptake, ultimately leading to diminished tree health. Symptoms of poor health include leaf yellowing, crown or branch dieback, bark loss and the ingress of pests and diseases, which arborists use as visible indicators to evaluate and quantify tree health. However, visual assessments can be highly subjective, relying on individual knowledge and interpretation, resulting in significant variability between assessors. In the forestry and crop industries, the value of physiological tests to identify low vigor or damaged plants before visible symptoms of deterioration is well documented. Despite this, the application of such tests by arborists has received limited attention.

A mature tree displaying symptoms of decline, including sparse canopy. All photos courtesy of Glynn C. Percival.

Visual assessment

Tree health is typically assessed by arborists through visual cues such as leaf size and color. Smaller, stunted or yellowing/necrotic leaves often indicate stress, usually due to reduced photosynthesis. Twig growth rates also can reflect a tree’s health, with slower growth suggesting stress. Twig and branch dieback, starting at the tips and moving inward, is a clear sign of progressive stress. Pest infestations and diseases, like canker, are more common in stressed trees, especially under drought and heat conditions.

Trees, like humans, can be stressed or in poor health without showing any visible symptoms. However, the sequence of stress responses in trees tends to follow a fixed pattern:

Reduced photosynthesis > stomatal closing > cell growth > wall/protein synthesis > protochlorophyll formation > nitrate reductase > ABA accumulation > respiration > accumulation of proline and sugars > cell leakage > necrosis > decline > death.

Importantly, visible symptoms of poor tree health, such as necrosis and branch dieback, appear at the end stages of stress. Ideally, detection of stress at the earlier stages is crucial for timely intervention. In humans, for example, medical checkups involve several tests to diagnose health issues before symptoms become visible. These tests include hemogram, blood sugar, pulmonary function, vitamin levels, lipid profile, liver and thyroid profiles, CT calcium score, ECG, chest X-ray, BMI, 2D-ECHO and cardiac tests. Early medical diagnosis allows prompt treatment, such as antibiotics or blood thinners.

The following physiological tests can be used to evaluate tree health.

Shoot and root electrolyte leakage

Stem and leaf tissue samples to evaluate electrolyte leakage as a measure of damage to leaf cell membranes.

Electrolyte leakage (EL) operates on the principle that damage to leaf, stem or root tissue compromises cell membrane semi-permeability, causing solutes within the plant cell to leak into the cytoplasm. This solute leakage can be quantified by placing leaf, twig and/or root samples in 50 ml Universal bottles containing 30 ml of distilled water. The samples are stored at room temperature for 24 hours before measuring conductivity with a conductivity probe. Total solute leakage is then determined by autoclaving (boiling the samples under pressure) for 20 minutes at 120°C. Results are expressed as the percentage of solute leakage after 24 hours.

For example:

Healthy plant: Conductivity of leaf/twig after 24 hours = 10. Conductivity after autoclaving = 200. Percent conductivity after 24 hours = (10/200) × 100 = 5%.

Damaged or low-vigor plant: Conductivity of leaf/twig after 24 hours = 100. Conductivity after autoclaving = 200. Percent conductivity after 24 hours = (100/200) × 100 = 50%.

Values exceeding 8% typically indicate cell membrane damage, while values of more than 20% are associated with reduced tree survival. Shoot electrolyte leakage has been used to quantify leaf damage and first-year survival of conifer and fruit trees following heat and/or freezing damage. EL was highly correlated with leaf damage, subsequent growth and survival rates of these tree species.

Advantages of using electrolyte leakage to measure tree health and vitality include the simplicity of the procedure, which requires only 48 hours to complete. However, practical disadvantages include the need for laboratory facilities and the destructive nature of the assay. Additionally, collecting root samples from urban trees can be challenging due to roots often growing beneath roads and pavements. However, stem tissue can be obtained at any time, allowing for year-round health assessments if required.

Root growth potential

Root growth potential (RGP) is a performance test that evaluates the ability of trees to initiate and elongate roots in a controlled environment conducive to root growth, such as a greenhouse or polytunnel. This test is primarily used to assess the survival potential of young trees before planting them in urban landscapes. Numerous studies have reported positive correlations between RGP and the growth and survival of young trees post-planting.

RGP is based on two components of root growth: the initiation of new roots and the elongation of existing roots. New roots are easily identifiable by their white color. The number of new white roots formed is counted to determine the RGP, with an RGP of five or more being associated with higher survival rates following planting into the landscape. RGP has been extensively used to evaluate the effects of environmental and physiological stress on trees, particularly conifers for forest plantings, and serves as a useful indicator of young tree vitality. It is one of the most widely used methods to assess tree viability and vigor before large-scale plantings.

The test is cost effective if a controlled growth environment is available. However, RGP testing is destructive and typically requires 28 days to obtain results, although a 14-day test has provided meaningful results in some cases. Additionally, RGP varies by species, necessitating a database of acceptable performance levels for individual species to use this test operationally.

Chlorophyll fluorescence

Measurement of leaf chlorophyll fluorescence on a semi-mature tree helps quantify potential damage to the leaf photosynthetic system following prolonged drought.

Chlorophyll fluorescence operates on the principle that environmental stress reduces leaf photosynthetic capacity, either directly or indirectly. Early detection of decreased photosynthetic capacity, before physical signs of deterioration appear, allows for remedial action to prevent tree decline. Reduced photosynthetic capacity can be detected by measuring chlorophyll fluorescence responses using a plant efficiency analyzer (PEA).

Chlorophyll fluorescence has been regularly used to provide a rapid and nondestructive diagnostic system for detecting and quantifying injury in tree leaves and needles in response to heat waves, freezing, salinity, drought and waterlogging. The Fv/Fm ratio (variable to maximum fluorescence) is particularly useful, as it has been shown on occasion to detect the effects of environmental stress before visible signs of tree deterioration appear. Previous studies have shown that Fv/Fm measurements following heat damage correlated closely with stem volume increment over time in spruce trees, suggesting that Fv/Fm measurements can forecast field growth performance prior to planting. Research also has demonstrated that chlorophyll fluorescence values are highly predictive of growth, foliar damage and survival rates in white and red pine, white and black spruce, silver birch, evergreen oak and holly following exposure to elevated storage temperatures, freezing stress and salinity prior to planting.

The practical advantages of using chlorophyll fluorescence include the use of a light, portable piece of equipment, nondestructive and noninvasive measurements and the ability to obtain readings in less than five seconds, allowing for the evaluation of many trees in a single day. Minimal training is required for its use and application, and no detailed scientific background is necessary. Operationally, leaves are adapted to darkness for 30 minutes by attaching light-exclusion clips to the leaf surface, and chlorophyll fluorescence is measured using a HandyPEA portable fluorescence spectrometer. Measurements are recorded for up to one second, with fluorescence responses induced by a red light (peak at 660 nm) of 1500 μmol m-2 s-1 photosynthetically active radiation (PAR) intensity provided by an array of six light-emitting diodes.

However, there are limitations to using chlorophyll fluorescence as a physiological test of tree health. These include the expense of the machine ($2,000 to $3,000) and the absence of leaves during the dormant season on deciduous trees. While the use of the fluorimeter is straightforward, interpreting the data requires experience and knowledge of leaf photosynthetic processes. Recent work has shown that chlorophyll fluorescence values can be obtained from stem tissue by gently removing the peridermal layer on each side of the terminal branch with a scalpel and measuring the exposed green chlorophyll-containing wood underneath; these values were correlated with freeze-induced tissue injury.

SPAD meter

A SPAD meter is used to measure the leaf-chlorophyll content of young containerized trees.

Environmental stress affects the content and ratio of leaf photosynthetic pigments, which in turn can limit carbohydrate availability for growth and reduce nutrient uptake, resulting in leaf chlorosis and necrosis, which manifest as leaves yellowing. Assessing leaf photosynthetic pigments is crucial for indicating premature senescence, for example, as the breakdown of leaf chlorophyll is one of the initial responses to prolonged stress. Consequently, knowledge of foliar chlorophyll concentrations, or “greenness,” provides a useful estimation of tree health.

The SPAD, or chlorophyll content meter, is a commercially available, portable device used to measure leaf greenness based on optical responses when a leaf is exposed to light by estimating leaf chlorophyll concentrations. The meter provides instantaneous, nondestructive readings by quantifying light intensity (peak wavelength: approximately 650 nm: red LED) absorbed by a leaf. A second peak (peak wavelength: approximately 940 nm: infrared LED) is emitted simultaneously with the red LED to compensate for leaf thickness.

Past research has shown a close link between leaf chlorophyll concentration and leaf nitrogen content in crops such as rice, maize and wheat, as most leaf nitrogen is contained within chlorophyll molecules. Consequently, chlorophyll meters are widely used to detect nitrogen deficiencies and improve nitrogen management systems (fertilizer application) in agriculture. Using a SPAD meter to assess the health of maple (Acer pseudoplatanus), European beech (Fagus sylvatica) and English oak (Quercus robur) showed that SPAD values lower than 25 indicated impaired leaf photosynthetic processes and correlated with a foliar nitrogen content of less than 1.5%, a value associated with critical nitrogen deficiency.

Plant nitrogen status is important for landscape managers, as foliar nitrogen content influences tree aesthetics, vigor, pest and disease susceptibility and the ability to tolerate environmental stress. Using this tool, it may be possible to synchronize fertilizer application with tree demand. However, extending SPAD technology to detect nutrient levels, particularly nitrogen content in woody plants, presents challenges. Issues such as leaf physiological age, position, sampling time, interactions with other mineral content and complex source-sink relationships in woody plants are likely sources of variability.

Twig starch concentration

Seen here is differing starch content of twigs following iodine staining to measure tree health.

Starch testing in twigs is a well-established chemical technique used to assess the living status of woody plants. This method operates on the principle that healthy trees accumulate starch within their leaves and woody tissues as a short-term energy reserve. Consequently, healthy trees contain more starch than unhealthy ones. Starch in woody plant tissue can be visually observed by staining with iodine, which turns starch violet.

Although considered a crude test, starch status can be determined by dipping the cut end of a twig into an iodine solution. A dark violet staining reaction indicates ample starch within the twig, while no color change suggests insufficient or no starch, indicating poor tree vigor.

Advantages of this test include its simplicity and quick response time, providing arborists with an immediate assessment of tree vitality. However, there are disadvantages, such as the variability of starch content between species, necessitating baseline data for individual tree species. Additionally, other organic substances, such as fungal metabolites and spores, also may react with iodine, causing color changes. Occasionally, dead woody tissue from felled trees has been shown to contain starch, which could misleadingly indicate that the tree is alive based on the iodine test.

Infrared thermometry

Infrared thermometry is a technique with potential applications for both stress and decay detection in plants. As this article focuses on its use for stress detection, readers are directed to Catena and Catena (2008) and Catena (2003) in the Arboricultural Journal for information on decay detection. The principle behind infrared thermometry is that one of the initial tree responses to stress is leaf stomatal closure. Prolonged stomatal closure can lead to plant starvation and death due to the loss of fixed carbon. More immediate toxicity results from the cessation of light energy conversion into photochemical energy, leading to the production of high-energy reactive oxygen species and subsequent heat buildup within the tree.

An infrared thermometer measures tree temperatures by detecting the infrared energy emitted by materials above absolute zero (0 K). The basic design includes a lens to focus the infrared energy onto a detector, which converts the energy into an electrical signal displayed as temperature. This setup allows for temperature measurement from a distance without direct contact.

Initially, infrared thermometry was used to identify “hot spots” in crops via aerial photography, indicating pest or disease buildup, aiding in pesticide application decisions. Similarly, it has been used to monitor canopy temperature in response to water stress, providing crucial information for scheduling orchard irrigation. It also has identified stress in plants due to weed competition. Infrared thermometry is noninvasive and nondestructive, enabling the evaluation of many trees in a single day.

With climate-change models predicting hotter and drier conditions over the next 20 years, irrigation management and water-use efficiency will become increasingly important for arborists. Infrared thermometry is a valuable tool for water-use management, supported by ample scientific evidence. However, the technique has disadvantages, including the high cost of the equipment (ranging from $15,000 to $30,000), although cameras can now be rented daily. Successful application requires proficiency in capturing suitable images and processing them correctly. Thermometry values also can be unreliable if the bark surface is obscured by moss, lichens, epicormic shoots or undergrowth. Additionally, values can be affected if the tree trunk is wet (due to rain, melting snow or frost) or overexposed to direct sunlight, as water absorbs infrared radiation.

Remote sensing

Satellite remote sensing offers a robust tool for the long-term monitoring of urban vegetation, spanning from individual leaves to entire tree canopies and ecosystems. Satellite data are now extensively utilized worldwide for monitoring and quantifying the impacts of climate change on bio-climatologic and bio-meteorologic phenomena. Remote sensing technology detects the chemical and physical properties of objects by recording, measuring and interpreting imagery and digital representations of energy patterns from noncontact systems. This provides a rapid, nondestructive method for detecting biotic and abiotic stress conditions. For example, heat waves induce complex physiological and biochemical changes in trees, and the health status of trees can be inferred from alterations in plant electromagnetic radiation. Remote sensing technology has been employed to study the effects of climate change on the annual phenology of rangelands and broad-leaved forests in the Alpine region of Italy.

According to researchers, the advent of the Google Earth Engine (GEE) has revolutionized the processing of Earth observation data, facilitating analyses and potential solutions for deforestation, drought disasters, water management and climate monitoring.

Remote sensing has been applied to map the urban heat island (UHI) effect in Turin, Italy, enhancing the city’s resilience against climate change. Remote sensing identified areas with high UHI, low vegetation and water content and widespread concrete and asphalt structures. This information was used to help mitigate summer heat waves and regulate the high energy demand for cooling buildings.

However, remote sensing technology may be relatively unspecific and not always sensitive enough to detect early or mild stress stages. The sensitivity of these spectral tools also can be limited by the opacity of thick leaves and dense canopies, leading to signal saturation.

Conclusion

The strong correlation between the physiological tests discussed and the health and survival of both deciduous and conifer tree species has significant implications for professionals in urban tree management. These tests can assist in the decision-making process by quantifying the likelihood of survival following prolonged drought, heat, salt, pollution or waterlogging episodes. This, in turn, helps determine the economic feasibility of applying remedial measures. The ability of these physiological tests to predict damage and future tree growth supports their continued use as operational tools for assessing tree viability.

Glynn C. Percival, Ph.D., is the senior arboricultural researcher at the Bartlett Tree Research Laboratory based in Charlotte, North Carolina, a division of Bartlett Tree Experts, a 51-year TCIA member company.

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