Pollutants and Street Tree Health

G.M. Moore
University of Melbourne, Burnley College, 500 Yarra Boulevard, Richmond, Australia 3121.

Abstract

Trees vary in their responses to air and soil borne pollutants; some are sensitive while others are tolerant of some or many pollutants. The age and physiological state of the plant, as well as the prevailing environmental conditions, can influence the tree’s responses to pollution.

Although Australia does not suffer from the widespread pollution damage to trees that is common in other countries, pollution damage to trees at a local level is not uncommon, especially in larger cities where population densities are high. Particulate matter, chlorides, natural gas, petroleum products and horticultural chemicals can be problematic urban pollutants that effect street trees. Microclimatic effects can influence the effects that pollutants might have on urban trees.

Identification of pollution damage is often difficult because the symptoms can be confused with other causes of plant injury. However, the identification of the pollutant is often the first step to returning damaged plants to normal health and vigour.

Introduction

Inadvertent damage to trees as a result of unwanted chemicals in the soil, air or water has been known for centuries and in 1969 pollution was formally recognised as a cause of tree injury and death on a global scale (Andrews 1972; Ormrod 1978; Nowak et al 2006).). Definitions of pollution vary (Moore 1983), but one which is both comprehensive and indicative of human involvement is: Pollution is the unfavourable alteration of our surroundings wholly or largely as a by-product of human actions, through direct or indirect effects of changes in energy patterns, radiation levels, chemical and physical constitution and abundances of organisms (Presidents Science Advisory Committee, 1965). It is also difficult to adequately define a pollutant, but in this paper it is defined as any substance that adversely affects something that human’s value, provided it is in concentrations high enough to do so.

Trees vary in their responses to and tolerances of pollution and certain species or cultivars have been developed for their pollution resistance. Such plants often come from those environments where potential pollutants are at naturally high levels (Nowak et al. 2006). The list of potential pollutants is almost limitless, but there are relatively few substances which represent a major and common threat to urban trees. This paper reports a number of cases studies involving pollution to urban street trees in Melbourne over the last twenty years.

The effects of any pollutant on a tree depends on its concentration, the duration of exposure and the number of times exposure occurs as well as the species of tree and environmental conditions (Moore 1983). Pollutants may also affect tree health and vigour indirectly by altering the physical properties of the soil such as aeration, or the biological components of the soil, like mycorrhizal fungi. Pollutants may be present in air or water, where they can come into direct contact with trees, but for many of the pollutants that affect street trees, their final destination is in soils.

Globally the major pollutants that impact upon street trees are sulphur dioxide (SO2), oxides of nitrogen (nitric oxide NO and nitrogen dioxide NO2), carbon monoxide (CO), ozone (O3), particulate matter and in developing nations, heavy metals (lead, cadmium, zinc, copper, iron and nickel). Several of these combine to form peroxyacetyl nitrate (PAN), which is often called photochemical smog (Table 1) (Ormrod 1978; Moore 1983). Within Australia, there are relatively few pollution issues related to street trees, apart from localised occurrences within inner city regions, and pollution damage to trees is occasionally acute but may be subtle or chronic. The coastal positions of major cities, their relatively small population densities and prevailing winds combine to minimize wide-scale air-borne pollution damage, while pollution spills into waterways are rare and pollution of soils is generally localised. When damage does occur, however, the causes are often difficult to identify due to the length of time taken to diagnose the cause and remedies can be difficult to prescribe.

Table 1: Major pollutants causing damage to street trees. The highlighted pollutants are those that occur more commonly in urban Australia (after Moore 1983).

Pollutant	Medium affected
	Air	Soil
Sulphur Dioxide (SO2)	ü	ü
Ozone (O3)	ü
Peroxyacetyl nitrate (PAN)	ü
Nitrogen dioxide (NO2)	ü
Fluorides	ü
Chlorines	ü	ü
Ammonia	ü	ü
Ethylene	ü
Particulates	ü	ü
Heavy metals (Cd, Mg, Pb, Zn, Cu, Fe, Ni)	ü	ü
Agrochemicals:
Fertiliser		ü
Herbicides	ü	ü
Pesticides	ü	ü
Detergents:
Natural Gas		ü
Petroleum		ü
Salt (usually, but not always NaCl)		ü

Because pollution damage to street trees is considered rare in Australian cities, it is often not considered at all when trees exhibit symptoms of poor health and vigour. However, it does occur and fortunately there is a relatively small list of likely causal agents in Australia as many or the more significant pollutants have been controlled by environmental pollution controls over the last thirty years. Australian legislation, for example, that banned lead from petrol has meant the heavy metal pollution from lead has all but been eliminated and the uses of ozone and fluorides have also been restricted. This leaves a relatively short list of suspects as sources of pollution damage to urban trees with the most commonly encountered source of pollution being particulate matter from vehicular traffic. This is followed by spills of petroleum products, especially diesel fuel, natural gas from leaking pipes, accidental spraying with horticultural or agricultural chemicals and chlorines

The sensitivity of trees to pollutants varies. Some trees are sensitive to most pollutants while others are quite tolerant (Nowak et al. 2006; Alkalaj and Thorsteinsson 2014). However, many trees show seasonal variation in their pollution sensitivities and many species are more sensitive to pollutants when they are young or when they are actively growing (Harris 1983). Evergreen species are generally regarded as being better at removing pollutants from the air than deciduous species, probably because they do so all year round, but deciduous trees may have higher tolerances as they dump pollutant loads with each leaf fall (Nowak et al 2006; Alkalaj and Thorsteinsson (2014).The sensitivities and tolerances of many Australian native trees to pollutants are still unknown.

Pollution Damage to Street Trees

Particulate Matter

Trees may be damaged by particulate matter such as dust, soot, smoke, grease, oil or soil present in the air. This is a major problem in developing countries such as China and India (Andrews 1972, Pirone 1978). Many of the particulates are not toxic, but they impair plant function by coating leaf surfaces which may reduce light penetration to the mesophyll and so reduce photosynthesis. They may also clog stomata which interferes with gaseous exchange reducing photosynthesis, respiration and impairing transpiration which may result in higher leaf temperatures.

Under Australian condition, particulate matter influence on street trees is likely to occur in two contrasting environments. Particulate matter levels in the air are often highest at major controlled traffic intersections where vehicles are stopped for increased periods of time. Trees growing close to these intersections are likely to experience much higher levels of pollution than trees growing further from the intersections (Chen et al. 2015). This often results in trees that are showing chronic or subtle symptoms of pollution damage such as slower and/or stunted growth, early leaf shed or delayed bud burst. Such trees often have shorter life spans than other trees of the same species growing in the same street.

Another common occurrence is that trees exposed to fine dust particles that are churned up by vehicles moving on unsealed roads grow more poorly than trees of the same species and age growing in paddocks. This phenomenon is also common in cities where roads to industrial sites are unpaved or in poor condition and where the traffic consists primarily of trucks.

Trees with smaller compound leaves are more efficient in removing particulate matter than trees with larger leaves (Kumar et al. 2013) and particulate matter concentrates on the leaf tip and margins (Kumar et al. 2013) and leaves of complex shapes with large circumference tend to be more efficient in removing particulates (Ingold 1971). Trees with stickier, rougher and hairier accumulate more particulate matter on their leaf surfaces (Beckett et al. 2000; Kumar et al. 2013).

In a recent study by Guo of particulate matter on the leaves of trees growing on the intersection of Grattan Street and Royal Parade, Melbourne, particulates were measured using a simple wipe technique. Leaves on trees at various distances from the intersection were wiped with dry filter paper, then water-moistened paper and finally alcohol soaked papers. Using the three wipes removes most of the particulate matter from the leaf surfaces. The collected filter papers, as well as the leaves, were scanned into a computer (as shown in Fig8) and processed in Photoshop. Using Photoshop, the images were converted into binary colour (white and black) where the particulate matter appears as black spots and recorded as the number of pixels of black. However, because leaf surface areas differed they were also calculated as a number of pixels. With the area of filter paper constant, the particulate matter present was expressed as the ratio of black pixels on filter paper to pixels of leaf surface area:

It is also of interest that street trees can have a role in the removal of particulate matter and dust from roads (Beckett et al. 2000; Mori et al. 2015), which is one of the reasons behind the massive tree planting that is taking place along highways and urban roads in China – mitigating the urban heat island (UHI) effect is another. Different species of trees have different capacities both for the uptake of pollutants into leaf tissues and for the adsorption of particulate matter onto the leaf surfaces (Blanusa et al. 2015). In the Melbourne study, it was found that the hairy surface of plane tree leaves was more effective in accumulating particulate matter of the leaf surface than either elm or eucalypt leaves (Figure 2).

Particulate pollutants can be removed from the surfaces of the plant, but it is a tedious and expensive operation, using compressed air or high pressure water jets. Not all particulates do damage through physical changes alone, some are absorbed by the plant causing chemical and toxicological changes to structure and metabolism.

Chlorine

Chlorine is quite a quite common pollutant that affects urban trees as it is a common constituent in chemicals that are used for household swimming pools and cleaning products or for industries where chlorine is used as a disinfectant. The symptoms of chlorine damage, which include a yellow mottling of the leaves or rapidly induced chlorosis are variable making it difficult to establish chlorine as the cause of damage (Ormrod 1978), but the source of the chlorine is often close to the injured trees. Confusion with insect damage can usually be avoided as the agents are not present (Ormrod 1978, Davis 1979).

Generally, chlorine damage to trees is restricted as spills are small and damage only occurs to trees in the immediate vicinity of the spill or leak. However, an incident occurred in Melbourne in December 2007 in which a fire caused the decomposition of chlorine swimming pool chemicals that had been poorly stored. Significant chlorine was released into the atmosphere which affected trees growing along four streets close to the storage site. In trying to assess whether and the extent to which any trees had been affected, the following symptoms were sought (Table 2):

• mottling, chlorosis and/or necrosis of leaves typical of chlorine damage (Figures 3 and 4)
• evidence of an uneven mottling, chlorosis or necrosis across the canopy. Depending on the canopy density, gaseous pollutants usually have a greater impact on the windward side of the tree while damage to the leeward side may be less extensive or absent.
• evidence of a rapidly induced chlorosis – specifically where older and new leaves were healthy but other leaves showed mottling, chlorosis or necrosis consistent with chlorine
• where symptoms consistent with chlorine damage to plants were established, attempt to eliminate other causes of similar symptoms. These are usually due to insect pests or diseases, which can be detected by a leaf inspection using a hand lens (x10).
• under some circumstances soil nutritional deficiencies can cause symptoms similar to chlorine damage. If plants were generally chlorotic or showed a longer history of chlorosis indicating a long term problem no attribution to chlorine damage can be made.

Table 2: Street trees affected by chlorine exposure and the symptoms shown

SPECIES	SYMPTOMS	PROGNOSIS
Peppercorn	General loss of foliage and any older foliage remaining on the tree   necrotic.   New   shoots   green   and   should   persist. Symptoms consistent with chlorine exposure (Figure 6).	Severe, sub-lethal exposure to chlorine from which the tree should recover
Poplar	Several specimens showing significant necrosis of all leaves on some trees. Other specimens showed necrosis and chlorosis of most leaves. For specimens further way most severe damage is on the side closest the source. Some specimens showed minor mottling, chlorosis and necrosis of some leaves. Most severe damage on the side of the tree closest to the source, but new shoots were green and should persist (Figure 5).All being consistent with chlorine exposure	A   severe   but   sub-lethal exposure to chlorine from which the trees should recover or a relatively minor exposure to chlorine from which the tree should recover
English Oak	Moderate mottling, chlorosis and necrosis of leaves. Most severe damage on the side of the tree closest to the source with little or no damage on the leeward side of the tree. All consistent with chlorine exposure. New shoots green and should persist (Figures 3).	A relatively minor exposure to chlorine

 

Elm (U procera)	Significant damage due to heat injury and chlorine on the part of the tree closest the source. Significant chlorosis and necrosis of leaves on the side of the tree away from the source. Most severe damage on the side of the tree closest to the source. All consistent with chlorine exposure. New shoots green and should persist.	A relatively minor exposure to chlorine
Eucalypt	Some chlorosis noticeable from the ground, consistent with chlorine damage.	A relatively minor exposure to chlorine
Melaleuca ericifolia	Minor chlorosis of leaves visible under magnification. Most severe damage on the side of the tree closest to the source with no damage on the leeward side of the tree.	A     minor     exposure     to chlorine   from   which   the tree should fully recover
Apple (Malus)	Significant mottling, chlorosis and necrosis of some leaves. A likely reduction in fruiting capacity.	Damaged   but   the   plant should recover
Pittosporum	Some  significant mottling, chlorosis and  necrosis of  some leaves, consistent with chlorine exposure.	A minor exposure and the tree should fully recover
Pine	Mottling, chlorosis and necrosis of some leaves, consistent with chlorine exposure on windward side of tree.	A minor exposure and the tree should fully recover
Ash	Some significant, chlorosis and necrosis of leaves, consistent with chlorine exposure. New growth is green and healthy	A minor exposure and the tree should fully recover
Cupressus	Some  chlorosis and  necrosis of  some  leaves, new growth healthy and green. Consistent with a single brief exposure to chlorine (Figures 4)	A     minor     exposure     to chlorine   from   which   the tree should fully recover
Brushbox	Some chlorosis and necrosis of some leaves, consistent with chlorine exposure. Relatively minor damage on the windward side of the plant.	A     minor     exposure     to chlorine   from   which   the tree should fully recover

 

 

 

 

 

 

 

 

 

The pattern of affected plants was consistent with a point source of chlorine from the storage site, with the chlorine spread by a wind blowing from a westerly direction. Once a point is reached at a distance from the source, the concentration of chlorine is insufficient to cause symptoms. The health and growth of plants growing outside the boundaries of the drifting chlorine is normal, which is consistent with the symptoms observed not being caused by soil conditions or the presence of pest or diseases.

 

 

 

 

 

 

 

 

 

 

 

 

Chlorine is also a vital ingredient in the creation of hydrochloric acid, which is the destructive element of acid rain. However, this rarely has impact in Australia and when it does it is almost always a local event centred upon a major chemical spill or manufacturing plant malfunction.

Natural Gas

Both natural and manufactured gas (coal gas) can cause damage to trees. Manufactured gas can cause direct damage as it may contain hydrogen cyanide, carbon monoxide and other contaminants (Grey and Deneke 1978) that are toxic to the plant. However, it use in Australia has largely, if not wholly ceased, as natural gas dominated the market (Moore 1983). Natural gas is not toxic to plants (Pirone 1978), but can cause injury by indirect means usually by the displacement of oxygen in soils that makes them anoxic. It is the methane present in the gas that contributes to many of these effects. Gas may also compete with other chemicals for enzyme binding sites and so interfere with plant cellular metabolism. Natural gas may also impact plants by drying soils (Grey and Deneke 1978, Pirone 1978, Davis 1979) and upsetting the balance of the soil micro-flora and faunas and reducing the growth and efficacy of mycorrhizal fungi.

Natural gas leaks can be a major source of pollution, as they often go undetected for some time and so their effects can be widespread. The symptoms of gas injury are varied. Growth may decline, leaves and young branches may die-back and in some cases the bark may crack and lift and rupture (Figure 7) (Davis 1979). Often when the bark lifts, a pulpy material develops under the bark which is indicative of natural gas as the causal agent. The lifting of the bark often allows subsequent insect or fungal attack upon the tree. When gas leaks occur several trees will usually show symptoms not just a single plant.

Methane can be produced by organic materials deposited in landfill sites (Davis 1979). The organic materials decompose over many years, releasing methane. High levels of methane have prevented filled sites from being vegetated in Melbourne for up to three decades after capping. The natural gas produced from such decomposition can travel for considerable distances under capped sites and injure plants up to 1km away. With old landfill sites on the edges of waterways, the methane may be trapped under the capping and flow under it to emerge on the banks of a river or creek and cause damage to riverine vegetation. Because the river is at a distance from the landfill and has never been disturbed, the cause of the damage may remain a mystery.

Both methane and natural gas can leak through cracks in the cap which can result in severe but localized damage. It is not uncommon that when old landfill sites are to be revegetated, planting pits penetrate the cap and act as vents through which the gas emerges killing planted advanced specimens. Often horticulturists are blamed for these planting failures until the site history is revealed.

Petroleum Products

Spills of petrol or oil based products often cause damage to trees. Street trees are more likely to be impacted from petrol tanker accidents and rollovers. Their effects are usually localized (Grey and Deneke 1978), and attract media attention and so the cause of damage is easily found. However, seepage from small spills can cause damage over wide areas and the cause of the damage is not detected due to the volatile nature of the fuel (Davis 1979). Spills usually give localized plant mortality, but the more volatile materials may cause leaf chlorosis and necrosis.

Diesel fuel is a powerful phytotoxin and even small spills can prove deadly (Moore 1983). A common arboricultural situation involving trees and fuel is the accidental or deliberate spill of diesel on construction sites. The diesel rapidly kills tree roots and trees can wilt and die with 48 hours (Figure 8). Soil analysis will often reveal the presence of diesel fuel if the tests are done within a week or two of the tree decline. Fuel, but particularly diesel should not be stored on development sites in the vicinity of trees or their root systems and the application of Australian Standard, AS4970 Protection of Trees on Development Sites should be applied to such situations.

Some other petroleum based products can also be problematic. Kerosene is very toxic to plant tissues and root tissues in particular. It is occasionally encountered on domestic properties, especially in rural parts of Australia where kerosene was used as a source of fuel for lighting, refrigeration and for other household appliances. When these were replaced, tanks with kerosene were often just left standing and when they rust, the fuel leaks into the soil. Such leaks will kill trees and the kerosene can persist in the soil at high levels for at least three decades, if not longer, making planting in the sites impossible. The soil has to be removed and replaced if planting is to occur earlier.

Horticultural Chemicals

The use of chemicals in horticulture and agriculture as fertilisers, herbicides and pesticides has meant that their abuse, accidental spraying and spilling is inevitable. The most damaging of these chemicals that injure plants are non-selective herbicides (Grey and Deneke 1978, Davis 1979) and the extent of injury may range from a few chlorotic spots on a leaf to the death of the whole plant.

A common group of chemicals causing damage to plants belong to the group of auxin mimics, which includes 2,4-D and MCPA (Pirone 1978, Davis 1979). Fortunately, their mode of action leads to quite distinctive symptoms of damage (Figure 9), which include the recurving of leaf petioles, twisting of shoot tips and leaves losing colour and becoming chlorotic. Such distinct symptoms often means that the causal agent can be easily and quickly identified. If an auxin mimic herbicide affects plants via drift or is diluted in a mist or rainfall event, the symptoms may be more subtle and hormone-like and include the development of root growth at stem nodes, development of epicormic buds and changes in leaf morphology.

Accidental spraying by of glyphosate and the effect of drift is also common. Because it is so widely used, careless use of glyphosate (often as Roundup or Zero in domestic contexts) has caused considerable damage to non- targeted trees. Depending on the species affected and concentration of the dose of chemical applied, symptoms range from the death of whole specimens to the killing of branches and leaf necrosis (Figure 10).

 

 

 

 

 

 

 

 

 

 

 

 

Conclusion

While pollution damage to trees in Australian cities is not common, pollutants can cause significant injury to street trees. Often the damage is subtle or chronic and so may not become evident for many years by which time the damage has been done (Moore 1983). The damage done to street trees by particulate matter, petroleum product spills, accidental spraying with pesticides and natural gas leaks should not be under- estimated and the possibility of pollution damage should not be discounted when street trees perform badly without other apparent causes such as poor soils or insect or fungal attack.

As is often the case with plant responses to stress, the alleviation of other environmental stresses may reduce the effects of pollution damage (Harris 1983). Trees that are otherwise healthy and that are growing in good environments often show higher tolerance of pollutants – prevention is always better than cure. Knowledge of pollutants and their symptoms when affecting trees is important to those managing street trees. Pollution can reduce the amenity value and life spans of street trees, and the competent urban forest manager needs to be aware of pollution as a real and potential risk.

Acknowledgements

The assistance of Dr E Moore, linguist, for her helpful comments on the manuscript is greatly appreciated, as is the permission of Ms Jingyi Guo to use two figures on particulate matter pollution in Melbourne from her Master in Environmental Science at the Office of Environmental Programs at the University of Melbourne.

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