With tree planting commitments rising globally as a means of tackling both the climate and biodiversity loss crises, the methods used by the tree planting industry are coming under closer scrutiny. The UK government is currently falling 86% short of its tree planting target of 30 million trees per year and there is increasing pressure to improve this performance significantly. Concerns have been raised about whether the tree planting industry has the capacity to meet these targets, with bottlenecks caused by the number of available tree planters, and shortages in the provision of saplings from nurseries. The cumulative effect of these pressures means that the survival of each individual planted sapling becomes more critical to achieving planting targets, and protecting the growing trees from herbivore damage becomes essential.
Plastic tree shelters have a myriad of benefits for saplings, but historically have either not been collected or sent to landfill, littering the landscape with millions of discarded plastic tubes. A recent initiative from one of the tree shelter manufacturers seeks to address this, providing a simple recycling programme to incentivise tube collection. Biodegradable tree tubes are also now being widely tested and some organisations such as the National Trust and the Woodland Trust have announced this year that they will be extensively trialling sustainable alternatives to plastic. A recent study even concluded that tree shelters could be avoided altogether to reduce the carbon footprint of planting trees and eliminate plastic pollution. With initiatives to reduce plastic increasing, is there still an argument for using plastic tree shelters? Can recycling offer an effective way to close the loop and reduce plastic waste whilst increasing sapling survival rates?
The primary reason to use tree shelters is to protect growing saplings from browsing animals such as deer, rabbits, and voles. A recent report concluded that 40% of woodland is in unfavourable condition due to herbivore damage, and with deer numbers in particular increasing year on year, protection of woodland is critical to improving its condition. There has also been a deer population explosion as a result of reduced demand for venison related to the Coronavirus pandemic. Deer browse tree seedlings and the ground vegetation of woodlands extensively, reducing the density of seedlings in naturally regenerating woodland. Deer can cause devastation in new forest plantations, decimating the survival rates of saplings. Browsing also slows the growth of the surviving saplings, affecting the height and shape of the developing tree and having a significant effect on how quickly a forest can regenerate. It is difficult to gauge the economic cost of deer browsing on forestry but the cost of deer management and preventative measures in 2018 – 2019 were £8.7 million in Scotland alone.
The measures to reduce or eliminate deer browsing include culling, fencing, encouraging weed growth to protect saplings, and using individual tree shelters. One of the great advantages of tree shelters is that they protect growing saplings from browsing damage by other mammals as well as deer. Indeed tree shelters can significantly improve the survival rates of saplings, with unprotected trees having survival rates of 2 - 90%, and protected trees having a survival rate of 67 to 100%. They do need to be of sufficient height to avoid deer browsing on the emerging tree at the top, and taller tree shelters can eliminate deer browsing altogether. A recent meta-analysis of mitigation techniques concluded that fencing was the most effective way of reducing deer browsing and increasing sapling growth, followed by cages and then shelters. Tree shelters also provide protection from land operations such as mechanical weeding, and the application of herbicides or pesticides.
Tree shelters were originally used for species such as oak that are slower to establish, but are now commonly used for many species. One of their unique properties is that the polypropylene tubes create a microclimate within the shelter that accelerates tree growth. The microclimate is an interaction between light, heat, carbon dioxide, and moisture, which regulates the balance between photosynthesis, respiration, and transpiration. The microclimate created by tree shelters can significantly increase the height of protected saplings by promoting apical dominance, and helping the sapling to outcompete any surrounding vegetation. This can be at the expense of establishing extensive root structures, however, although an extensive root structure is less useful to the individual tree in competing for light with surrounding trees.
There are alternatives to using plastic tree shelters to protect trees from herbivore browsing or rubbing damage. These include biodegradable shelters made of a variety of materials including wool, cardboard, and other plant-based materials, all of which are being extensively tested. Many of the biodegradable alternatives still require removal because the shelter degrades and leaves debris, or requires industrial composting to break down fully. They do not create the microclimate that is associated with polypropylene shelters, so do not have the benefits of accelerating tree growth.
Other alternatives to tree shelters include fencing, which has the advantage of protecting woodland ground flora that deer browse extensively, increasing biodiversity. In a recent study fencing was found to be the most effective way of reducing deer browsing and increasing sapling survival, but it can be costly and is more economical on larger plots of land. It has the disadvantage of not excluding other herbivores such as voles and rabbits, although this can be mitigated for by using smaller fencing mesh to exclude rabbits, and mowing margins in surrounding vegetation to deter voles.
Planting techniques can aid with reducing herbivore browsing, although it is less effective according to the mitigation study. The Miyawaki Method is a technique that involves planting seedlings very densely, making it difficult for the deer to access them for browsing. Allowing weeds to grow up between seedlings is another technique that can protect the young trees, although that does favour voles and could increase vole damage. Culling of deer is another option but again, only targets one species. Tree shelters are the only option that protects the trees against all browsing species.
By far the biggest disadvantage with traditional polypropylene tree shelters is that a significant proportion are not collected at the end of life, leaving the countryside littered with plastic. It is estimated that 13 million tree shelters are used every year just in the UK, and although they may contain 30% of recycled plastic, the manufacture of each tube has a carbon footprint of 0.44kg CO2. A recent Life Cycle Analysis study advocated avoiding the use of tree shelters altogether due to the environmental impact caused by degrading plastics left onsite. Even if the tree shelters are collected and recycled, the brittle plastic can shatter and leave microplastic particles behind. However, the researchers concluded that tree shelters are essential in some situations and that polypropylene tubes can have a lower environmental impact than those made from natural biodegradable materials due to the production processes. Their preferred option of replacing trees that are damaged or killed does raise issues of increasing the need for new saplings, putting additional pressure on nursery capacity.
Retrieving and recycling tree tubes is critical to reducing their environmental impact and keeping plastic out of the wider landscape. Of the 13 million tree shelters estimated to be used annually, 150 thousand were collected in the last year by one manufacturer, indicating that a significant expansion of the scheme may be required. The Tubex collection scheme is a not-for-profit initiative that arranges for the collection and recycling of the tubes once they have been removed from the trees. The scheme delivers dumpy bags to store the collected tubes and then arranges transport of the dumpy bags to the recycling facility. The tubes are processed into pellets, some of which are made back into more tree shelters, closing the manufacturing loop. Even after 8 to 10 years in situ, almost all of the polypropylene Tubex tree shelters can be recycled. The scheme has been carefully designed to minimise transportation and to keep the carbon footprint as low as possible. The scheme has a minimum order of 12 dumpy bags, each of which can hold 350 tree shelters, to ensure that the process remains as efficient as possible.
It is evident that recycling plastic tree shelters is critical to reducing their environmental impact, allowing forest creation schemes to benefit from increased sapling survival without causing pollution. The government woodland creation grant schemes in the UK usually include a stipulation that individual tree protection must be removed after 10-15 years, which applicants must adhere to if they are to receive £200 per hectare annual maintenance payments. There is no requirement that any removed plastic shelters must be recycled and no additional allowance for recycling costs, however. Including extra provision for one-off recycling costs on top of the maintenance costs would greatly incentivise applicants to build recycling into the scheme from the outset and reduce the environmental impact of plastic tree shelters.
The recycling schemes exist and are designed to make the process as smooth as they can for people retrieving shelters, although the process of removal can be difficult and laborious. It may be that an additional one-off cost for the labour of tube removal should be considered too, particularly in inaccessible areas where removal could be challenging and expensive.
Plastic tree shelters are an invaluable tool for the creation of woodland and used responsibly can offer a multitude of benefits. It is absolutely essential that they are retrieved and recycled, however, and the forestry industry should be applying pressure to government to include provision for this in the grant system.