- Dust from unpaved roads is a forgotten but major health hazard
- Road side tree planting has many co-benefits beyond dust control: productive asset, reduced crop damage, reduced soil erosion, improved visibility, wind break, shade, carbon sequestration, beautification
- In planning road side tree planting one has to consider ownership of road reserve, plans of future road widening, economic value of tree species, shape of tree barrier, root development, road vision, road safety, access to water.
- Given the multiple benefits of road side tree planting, it is surprising that it is not more widespread. Its application extends to almost all geographies
- Promoting road side tree planting and road side forest can off-set part of the carbon dioxide emissions that come with new roads.
This section provides guidance on how to plan, implement and monitor roadside vegetation activities without compromising road longevity and safety. Roadside vegetation is any vegetation growing along the roads. Planting trees, shrubs and grasses along the road can create a productive asset and can alleviate the negative effects of roads on the local environment. Negative effects include erosion, loss of fertile soils, gully formation that undermines road foundations, heavy dust, and more. In particular, dust lifted by vehicles, especially along unpaved roads, has a direct effect on the health of people and livestock living near the roads and on crop production. In a survey conducted in Ethiopia, close to 44 percent of the respondents said that the occurrence of dust had increased after road construction (Agujetas et al. 2016). Road dust is composed of coarse particles that can worsen heart- or lung-related conditions when inhaled through the nose and mouth (Greening 2011). High levels of dust can cause skin irritations and diseases, eye irritations, shortness of breath, chronic obstructive pulmonary disease (COPD), asthma, interstitial lung disease, lung fibrosis, lung emphysema, and increased risk of lung and skin cancer (Krzyżanowski et al. 2005). The human body can handle particles larger than 10 µm, but it becomes more difficult for the body to block smaller particles (Nordstrom and Hotta 2004). Dust can have a physical and chemical impact on crops and lead to yield reduction. In the survey cited above, 11 percent of respondents recorded such a decline. Dust from unpaved roads settles on the flowers of crops, impeding them from producing fruit. Moreover, dust affects photosynthesis, respiration and transpiration and therefore interferes with plant growth (Leghari et al. 2013). Dust on plants can smother the leaves, block stomata, and obstruct photosynthetic activities (Rahul and Jain 2014).
The scope for roadside plantation is enormous. The benefits of roadside vegetation are multiple (see Box 12.1). It can create barriers against road dust. Trees and shrubs especially can trap the dust with their leaves, minimizing the amount of dust reaching farms and houses. There is little research on how effectively roadside tree lines intercept dust. However, Maher et al. (2013) estimated that more than 50 percent of particulate matter is reduced by roadside tree planting, even more so if the leaves are hairy. Moreover, trees provide shade and contribute to the beautification of the area. This is a significant service because in many countries a large number of people walk alongside the road.
Roadside vegetation can also protect the road. Grasses can especially help to reduce runoff velocity and trap sediments, thus reducing roadside erosion. In some waterlogged areas, trees that use large quantities of water (such as eucalyptus) can be used to dry the road subgrade and help protect the road.
Furthermore, roadside plantations will not only check the deterioration of roads and the environment but will also create productive assets for local communities. This can be from the direct benefits of trees—timber, fruits, and bee pollination—or by acting as windbreaks that reduce desiccation and wind erosion.
Roadside plantations can also improve road visibility and break the monotony. Care should be taken not to disturb views. A very special use of roadside tree planting comes from the Netherlands. On some roads in flat areas, where speeding was common, trees were planted at a slightly closer distance in the direction of the traffic. The idea is that this gives drivers the impression they are speeding up and causes them to slow down.
Box 12.1. Advantages of roadside vegetation
- Removes dust and other pollutants from the air, protecting crops, roadside communities, and livestock.
- Reduces soil erosion: holds soils in place.
- Wind breaks that reduce desiccation and wind erosion.
- Flood control: slows and absorbs road runoff.
- Carbon dioxide sequestration.
- Direct benefits: timber, fodder, fuelwood, fruits, pollinator habitat.
- Provides shade and keeps the road cool for road users.
- Improves visibility.
- Can be used to guide road speed.
In spite of the considerable benefits it brings, roadside vegetation is still exceptional. India is a main exception: it launched the Green Highways Policy in 2015. This includes the provision to set aside one percent of all road investments for a roadside tree development fund.
However, it is recommended that roadside vegetation everywhere be systematically integrated into road-building programs, in particular for unpaved roads. Detailed planning for roadside vegetation should be developed as part of road development programs, showing the main objective of the roadside vegetation (dust control, beautification, improved visibility, erosion control, etc.) and the preferred planting for different roadside stretches. Site characteristics, such as rock content, soil depth, accessibility, steep slopes, and access to water resources, as well as road visibility, expected speeds, and impact risks, need to be assessed. It is also important to actively engage the local community for purposes of management and to identify tree species that will provide economic and environmental benefits. In developing roadside vegetation, the following are the recommended practices with respect to the different steps in developing roadside vegetation.
- Site selection
- Species selection
- Site preparation
- Design of roadside vegetative barriers
- Combining water harvesting and tree planting
What matters is not how many trees are planted along roads, but how many trees survive and thrive. Maintenance is the most important component for the establishment of roadside plantations: it requires careful planning and preparation. All necessary resources and arrangements for maintenance should be arranged in advance. Nursery seedlings often die because of animal damage, high surface temperatures, high evapotranspiration rates, lack of soil moisture, and competition with other vegetation.
Water is the main element in the establishment of new plantations. Trees and shrubs should be watered systematically at the time of planting and several times during the first two years. The use of irrigation bags or a large container that will trickle water into the soil is convenient for irrigating large plants. Sandy or rocky soils have low water-holding capacities, causing wetting fronts to travel deeper and in a narrower band. Less water but more frequent irrigation is recommended for these soils. On the other hand, finer textured soils, such as loams and clays, have a higher water-holding capacity and wider wetting fronts. More water can be applied in these soil types and at less frequent intervals than in sandy soils. It is important not to wet the leaves or needles to help prevent disease.
Seedling quality will influence the amount of water that the seedling needs. Healthy seedlings grow new roots faster and can access deeper soil moisture. Poor-quality seedlings are slow to develop roots and must be irrigated more frequently. Seedlings also have to adapt to a new location: while in the nursery they were watered daily, they now must be hardened to sustain their new environment. To make them adapt and survive over time under harsh conditions, it is important to reduce the amount and rate of watering slowly until the tree has fully adapted and can survive on its own.
Moreover, when weeds and other undesirable vegetation are growing near planted seedlings, soil moisture is depleted sooner, requiring more frequent irrigation than if seedlings were free from competing vegetation.
The question is who will take care of the maintenance and management of the roadside vegetation. Numerous modalities can work. The question is not so much which modality is best, but whether there is a clear arrangement in place. Given the highly distributed nature of roadside tree planting, local management is generally best. Three other factors that contribute to effective management are: (i) restrictions of free movement of cattle and ruminants; (ii) clearly assigned ownership and usufruct rights to the roadside plantation; and (iii) the ability to economically use the plantations, even if it means harvesting and replanting.
The returns to roadside tree stands, particularly if native species are used, are generally not immediate. Returns from planting may take several years to materialize, during which time and money may be spent to look after the plantations. There are several arrangements to overcome this income gap, such as:
- Making a payment to local caretakers based on the survival rate and health of the roadside vegetation.
- Joint tree ownership shared between caretakers and investors, whereby investors annually compensate the tree caretaker for his or her efforts, and upon tree maturity proceeds are shared.
Box 12.2. Roadside vegetation maintenance practices
Mulch. Mulch is a protective material placed on the soil surface to prevent evaporation, decrease surface temperatures, avoid weed establishment, enrich the soil, and reduce erosion. Applying mulch immediately after planting and maintaining it for several years helps hold moisture in the soil and suppresses weed germination. Several materials can be used as mulch, such as wood fiber, erosion mats, hay, straw and compost.
On sites where vegetation is expected to take several years to establish, such as arid or high elevated sites, it is important to apply mulch that will last more than one year. Materials with the highest durability are most long-fibered wood mulches, as well as erosion mats made from polypropylene. Straw, hay, and short-fibered wood products are less likely to be present after the first year. Mulching around seedlings is specially recommended for hot and dry sites and those with competing vegetation. It is less important to mulch around seedlings on sites that have a low potential for establishing competing vegetation the first several years after planting. Mulching is also less critical on sites that have low evapotranspiration rates or high rainfall.
Pruning. It is important to develop well-spaced structural branches early in the life of a tree. Branches that grow close together when the trees are young will grow into each other with age, and they will not be able to develop their full structural strength. Once the structural branches have been established, little pruning should be needed. It is advisable to examine the trees yearly and prune or cut branches for reshaping, if necessary. Uncontrolled growth of trees and shrubs could cause problems for vehicles, such as reduced sight distance, and vehicle or personal injury. Trees also need to be pruned to remove dangerous hanging branches or to prevent lower branches from blocking a path or obstructing visibility.
Protecting the seedlings. Fencing will be necessary in free-grazing areas and places subject to damage. Social fencing is sometimes considered an alternative to a physical fence. If all the residents of the area agree to keep their cattle off the plantation, and if there is no risk of cattle from other villages encroaching upon it, it is possible to establish the plantation without a physical fence. However, social fencing is particularly challenging in roadside plantations that often cross several districts.
There is a range of methods to protect seedlings, including rigid and non-rigid netting, fencing and animal repellents. When selecting fencing materials, it is preferable to use materials that allow sufficient sunlight for photosynthesis. Stone or brick fences are not advisable because they block the sunlight and impede plant growth. Individual trees can be fenced by surrounding them with sticks made with small branches from nearby trees.
Plastic netting can be installed to protect seedlings from animals browsing over each seedling. The netting acts as a barrier to foraging for foliage, stems, and even root systems, without impeding plant growth. There are two general types of netting: rigid and non-rigid. Non-rigid netting is a soft, fine-mesh plastic material. When installed on a seedling, it fits perfectly around the seedling. Rigid netting has larger mesh openings and keeps its shape when installed. Rigid netting, while typically more expensive, is usually preferred over non-rigid because it is easier to install and seedling growth inside the netting is less restricted.
Netting must be installed as soon as possible after planting to ensure immediate protection.
Tree shelters are translucent plastic tubes placed around seedlings after planting. They create a favorable growing environment while protecting the seedling against animal damage. Tree shelters enhance plant growth by creating a microclimate, which has lower light intensities, higher temperatures, and higher humidity. Tree shelters should be considered for sites where the potential for animal damage is very high. Tree shelters are not suitable for all species or site conditions. Tree shelters must not be removed until a portion of the seedling crown has grown out of the shelter. If the tree shelter is removed while the seedling is still growing inside the shelter, it will not be capable of supporting itself. Tree shelters are more effective than other methods, but they are also costlier.