Farm Forestry New ZealandThe wind in the wildings
Nick Ledgard, Ian Harman and John Finnigan, New Zealand Tree Grower February 2020.
New Zealanders, particularly those in rural areas, are generally very aware of the climatic environment in which they live – mainly concerning temperature, rainfall and wind. They can confidently relate details concerning temperature and rainfall, but they are not nearly so knowledgeable about wind.
It is a similar situation with wilding spread. We know the species involved, the age at which they produce seed, and the direction in which the seed is disseminated. We also know that seed spread is almost solely by wind, but we know little detail about wind-flow patterns in the field and how they are affected by topography. Such knowledge is important as it determines how often and how far conifer seed is disseminated. Fortunately, that lack of knowledge is changing due to a ‘Winning against Wildings’ research programme. Within that programme, some of the world’s leading wind researchers are involved in determining the role of wind in wilding seed dispersal.
Wind not constant
Most of us think of wind as simply a flow of air from point a to point b. In reality, this flow is not a single constant current – it is one made up of many turbulent eddies or gusts which vary considerably in size, intensity and, importantly, vertical uplift. We can appreciate how topography − hills and valleys − might influence the wind flow, but most would be unaware that ground surface roughness also has a major effect. The turbulent eddies in a wind flowing over a smooth grassy surface are less intense than in one flowing over a rough forest canopy. In other words, there is more likelihood of seed being lifted high and dispersed far in the forest flow.
Much of the current research uses a sophisticated wind tunnel, within which are slopes and hills covered with a simulated tree cover. It is revealing interesting facts. One is that just above tree height, wind velocity slows down slightly as it approaches a forested hill. We all know that there is a slow-down/reversal in wind direction behind a hill, but the slow-down on the front side of the hill is also important. The researchers found that, although the wind speed on the front side of a hill reduces above the trees, there was a slight increase within the trees. This creates an internal flow which could potentially carry seeds towards the top of the hill and out into the faster winds over the hill top.
They also found that within the wake zone of wind slow-down/reversal behind a hill, the turbulent eddies are very strong and can carry seeds upwards out of that zone and into the high velocity winds which are cresting over the hill top – therefore potentially sending any seed off into the far distance. When considering this, note that the rough surface of the hill will have created more intense turbulent eddies, which can carry seed higher than if the hill had a smooth outer surface.
Turbulence
Another unappreciated effect is that, if they are covered with trees, even quite gentle hills can create intense turbulent wake zones with reversed flow, whereas this would not happen if they were covered with grass. It could well be incorrect to believe that trees planted on the back ‘sheltered’ slopes of a hill forest are unlikely to be the source of distant seed dispersal. To complicate things even more, seeds from trees growing on the sides of hills can be ‘flicked’ around into the lee of the hill – where any strong upward eddies in the wake can carry them up into the high velocity winds flowing over the top.
The research is continuing for a few years yet. Findings are being used with new 3D wind models which can contribute to better seed spread prediction. This in turn would be incorporated into the currently used wilding tree spread decision support systems, thereby minimising the risk of planting the wrong species in the wrong site.
Nick Ledgard is a retired Scion researcher, Ian Harman and John Finnigan are from CSIRO, Australia.
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