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Entries in tree growth (5)




Am working on a little primer for communities about measuring the growth of tropical trees, and I spent most of yesterday playing with spreadsheets. The book is largely based on the growth data collected by forestry ejidos in the Selva Maya (see Selva Maya III and Reading the Bands). Data is a bit noisy, and the relationships between size and diameter growth are not as clear as I would like (see jumble of points in the diameter/growth scattergram shown at the lower right of the computer screen). That said, the data do provide a strong estimate of the average annual growth rate of each species (inset shows that the standard errors around the mean are less than 2.0%, i.e. Mean=0.38043006 and Std. Error = 0.00620184). Mean growth estimates with standard errors as large as 10.0% are routinely used for management planning, so I am actually quite pleased with this result.

Am hoping that I can finish the primer before the end of the year. Will be in Spanish (thx, Silvia).


Growth Bands

Growth data is an important component of sustainable forest use, but collecting these data can be tricky for timber trees in the tropics that don't have clear growth rings. What I usually do is put dendrometer bands on the trees (see How To Make Dendrometer Bands and Selva Maya III). We went to the forest at Tikon one day to learn how to make these bands. Everybody learned how to make them, but the foresters in the group (the ones in the brown hats) really took an interest in putting the bands on the trees. I just stepped back and marveled at what good students they were.

Image above shows Kyaw Thin Latt (WCS Myanmar) scratching the zero point on the band with the tip of the caliper. We banded about fifteen trees that day. I couldn't get them to stop.


Tools of the Trade

We gave each of the field crews in the Selva Maya growth study (see Selva Maya III) a little plastic lunch box (available at OfficeMax in Chetumal) full of stainless steel strapping and extension springs. These items are what you need to make dendrometer bands (as shown here) to precisely measure tree growth.

A dendrometer band is custom-made for each sample tree in the field. Lot of opening and closing of the plastic lunch box, but they held up very well (thx, OfficeMax).


How to Make Dendrometer Bands

Silvia Purata and I made this video (in Spanish) for foresters in the Selva Maya (see Selva Maya III and Selva Maya Interrupted) to show them how to make dendrometer bands for measuring tree growth.  The bands provide a very precise estimate of radial increment. When they are properly fitted, you can actually see the tree swell after a rain storm. [NOTE: The video was filmed in the Parque Zoológico of Chetumal in Quintana Roo and edited in a local hotel room using Final Cut Express and my Powerbook. Silvia did the voice-overs].


Selva Maya III

Fitting a dendrometer band around a Manilkara tree.


Reading the band with a caliper to see how much the tree has grown.


The forest management systems used by communities in the Selva Maya are very good - but there is always room for improvement.  The growth data used to calculate harvest volumes and rotation lengths, for example, are based on government estimates.  This could cause big problems.  If the growth estimates are too high, you'd cut more wood than you should. If the growth estimates are too low, you'd be leaving valuable timber in the forest.  Better to actually measure how fast the trees are growing.

In 2005, with support from the Overbrook Foundation, we started an extensive study of tree growth in the Selva Maya using stainless-steel dendrometer bands.  The research included 21 timber species (in addition to mahogany) and invoved 7 forestry ejidos.  The communities banded almost 3,000 sample trees.  As far as I can tell, this is the largest community-based study of tree growth ever initiated in the tropics.