2.4. Plant height

Plant height is the shortest distance between the upper boundary of the main photosynthetic tissues (excluding inflorescences) on a plant and the ground level, expressed in metres. Plant height, or maximum height (Hmax), is the maximum stature a typical mature individual of a species attains in a given habitat. Hmax is associated with growth form, position of the species in the vertical light gradient of the vegetation, competitive vigour, reproductive size, whole-plant fecundity, potential lifespan, and whether a species is able to establish and attain reproductive size between two disturbance events (such as e.g. fire, storm, ploughing, grazing).

What and how to measure?

Healthy plants should be sampled that have their foliage exposed to full sunlight (or otherwise plants with the strongest light exposure for that species). Because plant height is quite variable both within and across species, there are three ways to estimate Hmax, depending on species size and the number of plants and time available, including the following: (1) for short species, measurements are taken preferably on at least 25 mature individuals per species; (2) for tall tree species, height measurements are time-consuming, and for these, the height of the five tallest mature individuals can be measured; and (3) for trees, when more time is available, measure ~25 individuals that cover the entire range of their height and diameter. Use an asymptotic regression to relate height to diameter, and derive the asymptote from the regression coefficients, or use the formula to calculate the height of the thickest individual in the stand.

The height to be measured is the height of the foliage of the species, not the height of the inflorescence (or seeds, fruits), or the main stem if this projects above the foliage. For herbaceous species, this is preferably carried out towards the end of the growing season. The height recorded should correspond to the top of the general canopy of the plant, discounting any exceptional branches, leaves or photosynthetic portions of the inflorescence.

For estimating the height of tall trees, some options are

(1) a telescopic stick with decimetre marks; and

(2) trigonometric methods such as the measurement of the horizontal distance from the tree to the observation point (d) and, with a clinometer or laser, the angle between the horizontal plane and the tree top (α) and between the horizontal plane and the tree base (β); tree height (H) is then calculated as H = d × [tan(α) + tan(β)]; height estimates are most accurate if the measurement angle is between 30 degrees (easier to define the highest point in the crown) and 45 degrees (a smaller height error caused by inaccuracy in the readings); the horizontal distance between the observer and the stem should preferably equal 1–1.5 times the tree height.

Special cases or extras

(1) Rosettes. For plants with major leaf rosettes and proportionally very little photosynthetic area higher up, plant height is based on the rosette leaves.

(2) Herbaceous.   For herbaceous species, vegetative plant height may be somewhat tricky to measure (if the plant bends, or if inflorescence has significant photosynthetic portions), whereas reproductive plant height can be ‘safer’ in this sense. Additionally, some authors have suggested that the projection of an inflorescence above the vegetative part of the plant may be a useful trait in responses to disturbance, so both of these heights should be useful to measure. Others, while recording maximum canopy height, arbitrarily use a leaf length of two-thirds of the largest leaf as the cut-off point to estimate the position of a transition between vegetative and reproductive growth.

(3) Epiphytes. For epiphytes or certain hemi-parasites (which penetrate tree or shrub branches with their haustoria), height is defined as the shortest distance between the upper foliage boundary and the centre of their basal point of attachment.

(4) Large spreading crowns. For trees with large spreading crowns, it is difficult to estimate the height above the tree stem. For such individuals, it is easier to measure (with an optical rangefinder or laser) the vertical height as the distance from eye to a location at the crown margin that is level with the tree top; multiply this by the sine of the sighting angle to the horizontal (as measured with a clinometer) and add the vertical height from eye level down to tree base (a subtraction if eye level is below tree base level).

(5) Dense undergrowth. For vegetation types with dense undergrowth that makes the measurement of Hmax difficult, there are modified versions of the equation above; they involve the use of a pole of known height that must be placed vertically at the base of the tree.
References on theory, significance and large datasets: Gaudet and Keddy (1988); Niklas (1994); Hirose and Werger (1995); Thomas (1996); Westoby (1998); Kohyama et al. (2003); King et al. (2006); Poorter et al. (2006, 2008); Moles et al. (2009).

More on methods: Korning and Thomsen (1994); Thomas (1996); Westoby (1998); McIntyre et al. (1999); Weiher et al. (1999).