Individual Wind Risk of Trees

Individual trees in a forest stand can differ significantly from each other regarding their properties even in plantations. Differences are the result of lifelong competition for resources including light, water, and nutrition. At the same time mechanical stimulation is known to affect plant growth. Since the main mechanical stimulus in natural conditions is the loading exerted by the wind, differences in tree growth are pronounced where sheltered and non-sheltered conditions are found in close proximity.

Forest canopies are very efficient in terms of wind energy absorption. The vast majority of momentum is absorbed in the upper part of the canopy. Small suppressed trees benefit from a sheltered environment, that is created for them by their taller neighbours, which absorb most of the wind energy. Increased height growth is normally correlated with diameter growth, so that the taller individuals also have bigger trunks and higher root mass which results in higher stiffness and improved anchorage.

This poses the question as to which individuals are the most vulnerable in a forest stand. Does the higher amount of biomass of the dominant trees fully compensate for the increased wind exposure? Or does their dominant and favourable position come at the cost of higher risk of failure. Do small trees accept the risk of mechanical failure and invest more biomass into height growth to reach higher light levels?

"Wind loading" - Model

In the Figure below is the measured 10 min maximum turning moment of the nine experimental trees plotted against the mean wind speed at 30.8 m height. n is the number of data points used for the analysis. The red lines are the best fit of a quadratic model (f(u) = a * u²). The grey lines are identical to the red ones, except that they refer to the y-axis on the right hand side of the plots, which is the same for all plots and allows direct comparison of the models. Data points and red lines refer to the y-axis on the left hand side, which is adjusted for each graph.

Individual critical wind speed

For estimating the individual critical wind speeds, the models need to be extrapolated over a wide range. For the purpose of inter tree comparison, the predicted mean turning moments are normalised by the tree's critical moment. This allows the representation of all nine models in a single plot. The critical wind speed is reached, when the curves reach unity.

Estimated mean critical wind speeds for breaking, overturning, and general (critical turning moment) tree failure are 19.8 ms^-1, 18.8 ms^-1, and 18.7 ms^-1. The standard deviations for the three values are 1.56 ms^-1, 2.20 ms^-1, 2.06 ms^-1, indicating that the values for the breaking moments are more similar than for the two other.

Individual risk for wind damage

In the Figure below is the "best" predicted critical mean wind speed plotted versus several tree characteristics. The plots show that bigger trees are at higher risk of tree failure than smaller ones and are more likely to fail during a gale event despite their better anchorage and stiffness. The relationship suggest that the dominant position in a forest stand comes at the cost of increased risk of wind damage.