An email about setting up 1 Ha plots


This is an excerpt from an email I sent to a colleague when asked if I had any tips on how to set up 1 Ha square permanent survey plots in miombo woodlands. The plots will have every tree >5 cm trunk diameter tagged with a metal numbered tag so they can be resurveyed in future years.

For corner markers I normally buy long lengths of iron re-bar at a hardware store in town then hire a boy with a hacksaw to cut them to 1.5 m lengths. During the initial measurement of the plot the markers can be kept at about 1 m height above the ground to make it easier to tie string around, then at the end of the survey bury them so only about 30 cm is showing. This will minimise the chances of a corner marker going missing due to animals or humans.

Iron re-bar is also good for marking out intermediate points like the ends of 10 m lines that subdivide a plot. We worked out that for a single plot it’s good to have 24 lengths of iron re-bar at a minimum. 4 for plot corners, 18 (9 each side) to mark out the 10 m lines, and 2 extra to run a line down the middle of a plot (at 50 m) perpendicular to the 10 m lines. We did this further subdivision so that if we got to the end of a day and didn’t have time to finish a whole 10 m strip, we could at least stop half way as a convenient place to stop. This also means that you need approximately 1400 m of string per plot at a minimum, ideally with another ~200 m as spare. Obviously if you’re hammering in re-bar you will need a hammer, but hopefully you’ll have one of those for tree tags anyway.

I don’t know how effective it was but we tried to lower the height of any strings so they were nearly on the ground if we were leaving them set up overnight, to try and prevent animals catching themselves on them and potentially hurting themselves or ripping the strings to pieces.

I tried to keep the GPS unit turned on and with me all day, hanging off my belt, tracking my location so that I could later produce a diagram of the order of tree tags in the plot to aid re-measurement. In a similar vein I always tried to make sure our plots were orientated so the corners were NW,NE,SW,SE rather than N,E,S,W, and that we always started tagging from the NW corner and finished in the SE corner, snaking back and forth, up and down the 10 m strips on the way.

We made sure that tags were always on the North side of the tree. Tags should be nailed in so the nail head is pointing down, to allow the tag to slide away from the tree, minimising the chance that the tag will be swallowed up by the tree as it grows.

When marking out the corners of a plot the diagonal lengths are really important to make sure the plot is square rather than rhombic. I’ve attached a diagram of the order I think the various strings should be set up. Bear in mind though that in practice, trying to co-ordinate six people at once, it very rarely worked this smoothly, almost never. The diagonal length of a 100x100 m square plot should be 141.4 m I think, if I remember rightly. Cutting an extra length of string (ideally of a different colour so it doesn’t get mixed in with the normal string) to 141.4 m is a good way of measuring these diagonal lengths, as it’s awkward faffing around with a 100 m and a 50 m tape measure.

A hat waving on the end of a tall stick is a good way to indicate your location in long grass to somebody trying to run a string out to your location. Much better than yelling “Agui, aqui, um pouco a esquerda”.

easuring compass orientations for the corners to get 90 degrees helps to keep things square. Do it with a real compass rather than the shitey compass on a GPS unit.

When marking the GPS of plot corners, set the GPS to average waypoint to try to improve the accuracy, and leave it at the plot corner for a few minutes to equilibrate.

For setting up our plots I find that five people is an ideal minimum number to have on a team to allow things to progress efficiently:

  1. Scribe, GPS, mortality notes and photos
  2. DBH measurement and species ID
  3. Tag hammering
  4. Height measurement
  5. Organising tags and providing new nails, carrying extra equipment, moving ahead of the group to locate the next tree.

You will already know that having a stick marked off at 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 m is very useful to assess what height the DBH measurement should be. The 1.3 m mark should be made especially clear as this is the default DBH height. We made one of these sticks out of a length of PVC waste pipe (~40 mm) and made the gradations by wrapping the pipe in Duct tape, which in our case was exactly 10 cm wide. We bought the PVC pipe from the same builders' merchants as the iron -rebar. A 20 cm stick on a lanyard which can hang around the neck of the tag hammerer is also useful so they can measure 20 cm above the DBH line. I made one of these when we were at the meeting in Mozambique last year.

Each tree should have a ‘base stem tag number’, normally the first stem which is tagged on the tree. All other stems can then be grouped into being from that tree by writing the base stem tag number alongside the other stem measurements.

If you choose to mark the location of each stem using a handheld GPS (though I’d still recommend marking the tree locations with x,y plot coordinates), wiping the GPS each time you start a new plot means that you can give stems the same GPS number and tag number, which minimises confusion when it comes to typing up data. This means that some stems will have the same GPS location and you might have lots of identical GPS points per tree, but who cares.

Strong magnets, if you can get them and import them, can be invaluable as corner markers to aid future measurements. You can dig a hole and pop a few of them in at each corner then cover it over with earth. Magnetic locators will clearly detect magnets even if they are a foot or more underground. This helps if the undergrowth gets really thick or your corner markers go missing for whatever reason, especially because conventional handheld GPS units can only get down to about 2-3 m accuracy. There are special magnet detectors that we have used in the past. A colleague from Hamburg had one called a Heli-flux GA-52Cx Magnetic locator. Getting one of those before fieldwork might be asking a bit much though.

Diagram of plot setup