The Balancing Act and the Treeshelter Microclimate
A key benefit of the treeshelter is enhanced growth and subsequent early establishment of the tree. Why is this?
A different microclimate
Within the treeshelter, a microclimate is generated that is somewhat different from the open environment. The factors that compose this microclimate are warmth, varying levels of moisture and CO2, reduced air movement, and reduced and modified light. All these factors have an impact on the growth of the tree and on its biomass distribution through the processes of photosynthesis, respiration, transpiration and photomorphogenesis. The microclimate has therefore an important impact on the metabolism or growth of the tree and affects the assimilation rate of carbon dioxide, the transpiration rate, the energy budget of the leaves, etc.
The various factors composing the microclimate can be measured and can be related to the tree’s well-being. Whereas heat can be measured by temperature, moisture in the air can be measured by relative humidity (% RH) or vapour pressure. In particular, the vapour pressure deficit (VPD), the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated, is very relevant for growth of seedlings. Indeed, as the VPD increases as the air dries, the plant needs to draw more water from its roots. Another measure is the Leaf-to-Air Vapour Pressure Deficit (LAVPD). These factors affect the transpiration rate as measured by the transpiration flux density.
An intricate balance
In order for a plant to grow and develop properly, it must balance photosynthesis, respiration, and transpiration. Left to their own devices, plants do a good job of managing this intricate balance. If a plant photosynthesises at a high rate, but its respiration rate is not high enough to break down the photosynthates produced, photosynthesis will either slow down or stop. On the other hand, if respiration is much more rapid than photosynthesis, the plant won't have adequate photosynthates to produce energy for growth. Hence, growth either will slow down or stop altogether.
The transpiration rate of a tree depends on both climatic demand and its physiological capacity to meet this demand. Light radiation, the water vapour gradient between the leaf and its surrounding air, and the wind contribute to create the climatic demand. The development stage of the tree, the leaf surface, and the stomatal resistance modify the physiological capabilities of responding to the microclimatic demand.
Reduced transpiration rates may induce high leaf temperatures. This may be lethal for the foliage as temperatures inside the shelter may be 10oC higher than outside temperatures. On the other hand, such low transpiration rates may help conserve water reserves of the soil and delay the occurrence of a water deficit for the tree during the dry season. While this could be a useful attribute in dry areas, weeds and soil evaporation may use a significant amount of water saved by the tree. It is therefore strongly recommended that weeding is carried out around sheltered trees. It is noteworthy that weeds growing inside shelters are less problematic because they experience the same transpiration limitations as the tree.
Continuous low transpiration may not be a problem as long as the shelter remains sealed. Indeed, a sudden air movement inside the shelter will induce very large transpiration rates, with no physiological control. In such circumstances, the tree may desiccate. It is therefore important that the treeshelter is well installed and the bottom in contact with the soil is sealed thus preventing a lifting by wind, animal or accident.
Note that the impact of the treeshelter microclimate will be different in different climatic environments. See the Tubex work on the Ventex product for an analysis of these effects in hotter climates. Although the effect on tree species has been beneficial in all cases, there are some species (e.g. beech) that require special consideration (see tree species).
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