Crown retrenchment (mass reduction) can also occur in younger urban trees – it is not exclusive to older trees. Triggered by the onset of disease, insect damage, root disturbance, pollution, or other stressor, retrenchment will usually occur once a tree has been strained (Read, 2000; Shigo, 1986).
Ultimately, a tree will have to halt mass increase if it cannot supply its existing mass with the energy that is required for maintenance and subsequent retention. Put simply by Shigo (1991) in Modern Arboriculture: “trees planted in the wrong places are committed to an early death at the time of planting” and “no system can increase in mass beyond the energy available to power it“. “To survive, the system must then reduce its mass to maintain order within the limits of the amount of energy available”. Coined ‘The Demons of D‘ by Shigo, discolouration, dieback, decay, decline, depletion, dysfunction and death may manifest as a result of poor urban conditions – the tree simply cannot maintain a high level of functionality.
Trees need eight things to survive: energy, space to grow, water, essential elements, concentrations of factors, time, temperature, and genetic code. As trees cannot just get up and walk away from a site and choose where they want to go, they must adapt or die. When one of the eight factors is compromised, the others suffer too – to varying extents (Shigo, 1986).
In today’s post, we’ll briefly look at rooting space as a cause of mass reduction. In following posts, which will come over the rest of the week, we will look at other causes.
Limited rooting space
The size of the rooting environment is directly attributable to root growth. Once tree roots have reached a point where they can no longer grow, additional required water and nutrients that are necessary to aid in providing the energy for growth cannot be uptaken, as root growth is confined to a closed soil environment with a limited ‘carrying capacity’ (Day & Bassuk, 1994; Grabosky et al., 2001). This is rather common in urban environments, particularly where a tree is bordered by built structures on two or more sides (such as roads or footpaths). At times, the available soil is less than the drip line of the tree – even soon after planting. Unless the roots can break free of the area that the root zone is confined to and can find a source of water and nutrients elsewhere, dynamic mass will be reduced and / or growth will halt (Day et al., 2010; Jim, 2001; Sanders et al., 2013).
Day, S. & Bassuk, N. (1994) A review of the effects of soil compaction and amelioration treatments on landscape trees. Journal of Arboriculture. 20 (1). p9-17.
Day, S., Wiseman, P., Dickinson, S., & Harris, J. (2010) Tree root ecology in the urban environment and implications for a sustainable rhizosphere. Journal of Arboriculture. 36 (5). p193-205.
Grabosky, J., Bassuk, N., Irwin, L., & van Es, H. (2001) Shoot and root growth of three tree species in sidewalks. Journal of Environmental Horticulture. 19 (4). p206-211.
Jim, C. (2001) Managing urban trees and their soil envelopes in a contiguously developed city environment. Environmental Management. 28 (6). p819-832.
Read, H. (2000) Veteran Trees: A Guide to Good Management. UK: English Nature.
Sanders, J., Grabosky, J., & Cowie, P. (2013) Establishing maximum size expectations for urban trees with regard to designed space. Arboriculture & Urban Forestry. 39 (2). p68-73.
Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.
Shigo, A. (1991) Modern Arboriculture. USA: Shigo and Trees Associates.