Plotting the deep secrets of our forests

A researcher measures a stinkwood tree.
A researcher measures a stinkwood tree.

Forest trees grow slowly and can live for hundreds of years. In the forests of the Garden Route National Park, this growth is measured in permanent sample plots (PSPs), and forest ecologists are currently preparing for the 30-year remeasurement of these trees. Taking into consideration that this happens only once every decade, there is reason to take notice.
The 967 plots are scattered over more than 40 000 hectares of forest under SANParks management. Established from 1984 to 1991, they have been remeasured every 10 years. The forest varies from place to place, and the plots were established in places that provide an estimate of the number and sizes of various tree species growing in different areas.

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The data were used to calculate sustainable timber harvesting levels for various areas and the plots were kept for long-term monitoring of natural forest stand dynamics and responses to harvesting.
Plots are located at the intersections of grids randomly superimposed on the forest map. Plot lines are usually 100m apart and orientated along the true north-south line, with plots laid out 80m apart on these lines.
The plots were established long before GPS devices which could mark their location in the dense forest canopy, and the approximate location of each one was indicated on a map with a description of how to find it. Finding them again is not always easy.

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It entails starting at a clearly defined landmark, such as the intersection of two slipping paths, and measuring along one of the paths for a specific distance to reach the plot line. The first plot on the line is reached by bashing through the dense undergrowth for a specific distance while following a compass bearing.
The distance is usually measured by dragging a 20m measuring chain or cable. The plots are marked unobtrusively so that their presence should not affect normal activities in the area. The centres, for example, are marked by a length of PVC pipe that will hopefully remain protruding about 10cm above the ground.
However, these are sometimes obscured by undergrowth or fallen trees, pulled out by inquisitive bushpigs, or uprooted by management activities. The only other indication of the presence of a plot is a stainless steel pin in the base of each tree within it. Don’t remove any of these if you do happen to find them while out in the forest. Although they are difficult to find, scientific services have never yet lost a plot.

WHAT GROWS THERE?

The fastest grower in the PSPs is hard pear (Olinia ventosa) with a mean diameter growth rate of about 3,8mm per year. Yet, when conditions are favourable, some trees grow almost 10mm in diameter a year. However, the mean diameter growth rate for all species and size classes is a mere 1,091mm per year. Many trees have grown by only a few millimetres over the past 20 to
30 years, and some trees have even become thinner! This sometimes occurs when trees are under moisture stress during periods of drought, or when they are busy dying.

Plots are circular with a radius of 11,3m and an area of approximately 400m². All trees with a diameter at breast height (DBH) of at least 10cm are measured within each plot and are identified by true bearing and distance from the centre of the plot. This can be used to locate the centre if the PVC pipe is missing. The point of diameter measurement is fixed at 1,3m above the stainless steel pin driven into the base of the tree. The species, DBH and condition are recorded for each tree. Dead trees are recorded, and those now larger than 10cm are added.
This gives scientists information on the tree species and size class composition for each area, and data on increments, mortality rates and in-growth rates for each tree, species and size class.
These plots complement various other long-term forest dynamics monitoring projects that have been gathering valuable data on the Garden Route forests for more than 40 years.

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