Tree’s are a very complex organism and vital to keeping the Earth healthy. Beside creating oxygen for us to breath, they regulate the Earth’s temperature1.
Understanding how and why Trees cool the environment can provide an interesting framework for better thermal regulation in buildings. As well as increasing the performance of existing passive cooling and heating strategies.
Tress and other vegetation increases shading, insulation (wind barriers), cooling through evapotranspiration and increases soil’s ability to perform as a thermal mass. These factors can decrease air temperatures on average 2oC in rural areas compared to urban areas3 and decrease soil temperatures by 7°C lower and surface temperatures by 6°C4.
“One degree is enough to push up air-conditioning use by up to 20%” (source)
These effects however are only due to the fact trees need to maintain an optimal temperature in order to photosynthesis. Trees like buildings can not move in order to regulate their temperature. Hence they require certain mechanisms to maintain an optimal temperature.
Most trees can not survive if internal temperatures reach above 40oC2
In order to maintain the optimal temperature to perform photosynthesis trees have evolved and adapted to specific climates.
Typically trees protect themselves from the sun heat via:
Shading– from the trees canopy.
Evaporative cooling– through transpiration from the leave.
Insulation– provided by the bark of the tree.
Conductive Cooling– providing cool water from the ground to the leaves.
These mechanisms provide different protection against heat transfer processes. An investigation into the different mechanisms will provide a better understanding which part of the Tree does what.
Heat Transfer Processes
Radiation: heat transfer through direct and indirect solar radiation. Tree area affected: Leaves and trunk.
Conduction: heat transfer through direct contact. Tree area affected: Leaves and trunk.
Convection: heat transfer through the motion of different temperature fluids. Tree area affected: Leaves and trunk
Evaporation: heat transfer through the loss of water. Tree area affected: Leaves
The Main Focus: Tree Bark
Water is a scarce commodity in many cities and climates. Tree bark manages to regulate its temperature without the use of evaporative cooling (through evapotranspiration).
Understanding the different types of bark and the mechanism to regulate its temperature could provide a solution the heat regulation in arid climates where water preservation is vital.
Even in direct sunlight tree trunks remain cool enough to touch(50oC) unlike most construction materials that can reach in excess of 80oC2.
This is important as the outer bark protects the cambium layer which transport nutrients and water to the leaves which must stay cool.
Fissured and Textured Bark
Mechanism– The rough texture greats peaks and valleys that create self shading which also increases the airflow around the trees2.
What it does best– Decrease heat gain from solar radiation. Increase heat loss through convection. Decrease heat gain through conduction.
What it doesn’t do- Although the thick bark provides insulation this can be problematic if it needs to lose internal heat quickly.
Mechanism– Thin layers of bark flake off creating air gaps that are highly insulating. The thin surfaces also accumulate heat quickly which means it can emit the heat away just as quick2.
What it does best- Decrease heat gain through conduction (insulating air gaps). Decrease heat gain from solar radiation (flaky bark shades trunk). Increase heat loss through conduction and convection (thin structure of bark doesn’t retain heat for long)
What it doesn’t do- Although there is insulation in the air gaps, this mechanism does not protect the tree from extremely cold conditions where too much heat could be lost.
Mechanism- Generally smooth bark trees are protected from solar radiation from the trees canopy. The round profile of a tree is to minimize surface area to reduce solar heat gain2. This is more prevalent in smooth tree barks which are can also me more slender. However the bark performs poorly when exposed to direct solar radiation6.
What it does best- All though the bark provides no real protection from solar radiation the bark temperatures match the air temperature when not exposed to solar radiation6. However this is only useful in certain climates.
What it doesn’t do- Provide protection from conduction, convection and solar radiation.
Mechanism- White bark prevents overheating by reflecting as much solar radiation as possible6.
What it does best- Decrease heat gain from solar radiation
What it doesn’t do- Prevent heat transfer through conduction
The Quiver tree from the Namib desert could provide a very interesting framework for sustainable design and passive cooling and heating strategies. The quiver tree has adapted to have 3 of the bark types shown here. Showing that to survive the desert heat it requires more than one strategy.
- V. Novák, Encyclopedia of Earth Sciences. Springer Netherlands, 2014, pp. 280-283. [Online]. Available: Springer Online
- W. Henrion, H. Tributsch, Optical solar energy adaptations and radiative temperature control of green leaves and tree barks, Solar Energy Materials and Solar Cells, Vol. 93, no. 1, pp. 98-107, 2009 http://dx.doi.org/10.1016/j.solmat.2008.08.009.
- L. Bounoua, P. Zhang, G. Mostovoy, K. Thome, J. Masek, M. Imhoff, M. Shepherd, D. Quattrochi, J. Santanello, J. Silva, R. Wolfe and A. M.Toure, Impact of urbanization on US surface climate, Environmental Research Letters, vol. 10, no.8, 2015 doi:10.1088/1748-9326/10/8/084010
- E. B. Peters, J. P McFadden. Influence of seasonality and vegetation type on suburban microclimates, Urban Ecosystems, vol. 13, no.4, pp. 443-460, 2010 doi:10.1007/s11252-010-0128-5
Akbari, H.(2009). Cooling our Communities. A Guidebook on Tree Planting and Light-Colored Surfacing. Lawrence Berkeley National Laboratory. Lawrence Berkeley National Laboratory: Lawrence Berkeley National Laboratory. 2009 Retrieved from: http://escholarship.org/uc/item/98z8p10x
- V. Nicolai, The Bark of Trees: Thermal Properties, Microclimate and Fauna, Oecologia, vol. 69, no. 1, pp. 148-160, 1986. http://www.jstor.org/stable/4217921