As some others have mentioned, this is part of continual forest management inventory where the plots are remeasured on periodic basis. The state of the forest is determined from permanent plots of a fixed radius. Growth of trees, recording of trees taken in harvest, death of trees, progression of rot and volume loss are part of determining the health of the forest over time.
Aluminum tags and nails are used to avoid / minimize damage to saws as aluminum is soft and supposedly cuts like wood. I am not so sure that is the case, but aluminum is certainly better than iron.
As mentioned above, tags are to be placed at the DBH point and on the stump below where the felling cut is made. The tag / nail at the DBH point is there so that a remeasure of the tree will take place at the exact same place each time the tree is measured. The tag is the ID for the tree. The stump tag is there in case the tree is harvested. In that case the board food volume of the plot will be reduced by volume of the harvested tree and reflected in the overall growth statistics for the forest.
Interesting problems occur with this activity. Trees push out the nails or wood grows over the tags even when space was left on the nail to allow room for growth. These are supposed to be replaced, but assigning a tag to the tree with the correct ID is not always easy. Imaging the problem when you return to the office and find that the tree you relabeled with an existing ID was for a pine and you recorded the species as a fir because there where just too many lost tags and deciding what number goes with what tree gets confusing when you're out in the woods. Or some goof ball has come into the plot and pulled out all the tags. Also, new young trees get added to the plot as they reach a minimum size. These young trees are easy to miss on re-visits to the plots.
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