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### Forest canopy height

Importance of forest canopy height

 Forest canopy height h (m), the height of the highest vegetation components above ground level, is essential for studying micrometeorogical phenomena over forests and forest-atmosphere interaction. For example, if we want to compare the vertical distribution of scalars such as temperature, humidity, and wind speed within and above the forest canopy between different forest sites, the measurement height z (m) of these scalars should be normalized by the forest canopy height as z/h, i.e., the relative height to the canopy height. Forest canopy height is also required in estimating forest biomass and carbon pools.

Previous definitions of forest canopy height

According to Forest Mensuration (van Laar and Akça, 1997,2007), the following definitions are commonly used.

1. Arithmetic mean height
The average of the height of all trees in a stand, regardless of their size.

2. Lorey’s mean height
The weighted mean height whereby individual trees are weighted in proportion to their basal area.
where gi (m2) is basal area of each tree sample, and hi (m) is each tree height.

3. Top height
The mean height of tall trees selected according to a certain criterion, such as:
• The arithmetic mean of the 100 tallest trees per hectare
• The arithmetic mean of the 20 % thickest trees in DBH (diameter at breast height) per unit area

However, these definitions have uncertainties in determining forest canopy height in that:

• Estimated canopy height can be smaller than the actual height of upper-canopy trees due to the effect of presence of shorter trees.

• If we adopt the taller trees only in calculating canopy height, we have to determine the threshold, which necessarily involves an arbitrariness.

• Even though the threshold is fixed, estimated canopy height can be changed depending on the forest structure such as stand density and size structure.

• The differences of the measures between different definitions can sometimes reach several meters even if they are for the same stand (Prodan, 1965)