Trees absorb water through their roots and use it to transport nutrients and sugars throughout the tree which is essential for tree growth,
Sufficient moisture is necessary for the tree to perform its functions such as:
- Photosynthesis, the process by which trees convert light energy into chemical energy.
- Transport of nutrients and sugars throughout the tree, which are necessary for growth and survival.
- Maintenance of turgor pressure, which helps the tree maintain its shape and rigidity.
A lack of moisture can have a number of negative effects on tree growth, including:
- Reduced photosynthesis, which can slow down the tree’s growth.
- Reduced turgor pressure, which can cause the tree to wilt and reduce its ability to photosynthesize.
- Increased risk of damage from pests and diseases, as a dry tree is more susceptible to attack.
Excessive moisture can lead to oxygen deprivation in the roots, and can promote the growth of pathogens and pests, which can damage the tree.
Individual tree species all have unique moisture requirements and tolerance with some species well adapted to growth in specific moisture conditions and climates, such as dry or wet forests. The availability of moisture may vary depending on the season, and a tree’s ability to cope with drought or flooding varies depending on the species and stage of growth. It’s important to note that to optimize the tree’s growth, the availability of moisture should be balanced with other environmental factors such as temperature, light, and nutrient availability.
A trigger for plant growth is moisture availability. Plants need water to grow, and the primary source of this is rainfall, often referred to as precipitation. It is classified into Mean Annual Precipitation (MAP) classes (for example, 900mm p.a.), which is the average total rainfall measured at a location. However, temperature plays a role in the amount of moisture that is available to a plant due to evaporation.
The higher the temperature the more the evaporation, which means that a plant requires a higher precipitation level in hotter areas than in cooler areas. For example, a plant that requires 800mm per year in an area with an MAT of 14ºC might require 1000mm per year in an area with an MAT of 16ºC to achieve the same level of growth. Therefore, a more accurate indicator of moisture demand for a species is provided by using Moist Availability Zones, which are derived from the ratio between MAP and Potential Evapotranspiration (ETp).
The higher the ratio the more humid an area is. Conversely the lower the ratio, the more arid the area. Climatic data such as MAT, MAP and ETp have been derived for the whole world using climatic models and are available for use in deriving site suitability maps. Examples of such datasets are the WorldClim dataset (Fick and Hijmans, 2017) and the Aridity Index dataset (Trabucco and Zomer 2018).