Field Performance of Planted Trees

All tree-planting projects should be monitored, but there are many different approaches to monitoring, ranging from basic photo-monitoring and assessment of tree survival rates to complex field trial systems, designed to investigate species performance, the effects of silvicultural treatments and biodiversity recovery. A current trend is to use drones to monitor tree survival and growth.

Simple monitoring using photography

The simplest way to assess the effects of tree planting is to take photographs before planting and at regular intervals (once per season or annually) by locating points with a clear view of both the planted sites and notable landmarks. Mark the position of the points with a metal or concrete pole or paint an arrow on a large rock. Record the date, point number, location (co-ordinates if you have a GPS), and age of plot and use a compass to measure the direction in which the camera is pointed.

Sampling trees for monitoring

The minimum requirement for adequate monitoring is a sample of 50 or more individuals of each species planted. The larger the sample is, the better. Randomly select trees to include in the sample; label them in the nursery before transporting them to the planting site. Plant them out randomly across the site, but make sure that you can find them again. Place a coloured bamboo pole by each tree to be monitored; copy the identification number from the tree label onto the bamboo pole with a weather-proof marker pen and draw a sketch map to help you find the sample trees in the future.

Labelling planted saplings

Soft aluminium strips, used to bind electrical cables, can easily be formed into rings around tree stems. Use metal number punchers or a sharp nail to engrave an identification number on each label and bend them into a ring around the stem above the lowest branch. This will prevent the label from being buried when the tree is planted. Such labels are also fire resistant. Once the trees have grown to a girth of 10 cm or more (measured 1.3 m above the ground, GBH=girth at breast height), more permanent labels can be nailed to the trunks (made from sliced-up drinks cans), marking the girth measuring point at 1.3 m.

Monitoring tree performance

To monitor tree performance, work in pairs with one partner taking measurements and the other recording data on pre-prepared record sheets. One pair can collect data on up to 400 trees per day. Prepare record sheets that include a list of the identification numbers of all labelled trees in advance. Take along the sketch maps made when the labelled trees were planted to help you find them. In addition, take a copy of the data collected during the previous monitoring session. This can help you sort out tree identification problems, especially for trees that might have lost their labels.

• When to monitor

Measure the trees 1–2 weeks after planting, to provide baseline data for growth calculations and to assess immediate mortality, which might result from transplantation shock or rough handling during the planting process. After that, monitor the trees annually; in seasonal forests, this task should be undertaken at the end of each rainy season. The most important monitoring event, however, is at the end of the second rainy season after planting (or after about 18 months), when field performance data can be used to quantify the suitability of each tree species to the prevailing site conditions

• What measurements should be made?

Assign a simple health score to each tree and record descriptive notes about any particular health problems observed. A simple scale of 0 to 3 is usually sufficient to record overall health.

Score zero if the tree appears to be dead. For deciduous tree species, don’t confuse a tree with no leaves in the dry season with a dead one. Do not stop monitoring trees just because they score zero on one occasion. Trees that appear dead above ground could still have living roots, from which they might sprout new shoots.
Score 1 if a tree is in poor condition (few leaves, most leaves discoloured, severe insect damage etc.).
Score 2 for trees showing some signs of damage but retaining some healthy foliage.
Score 3 for trees in perfect or nearly perfect health.

If you cannot find a tree, record it as "NF" = not found (do not assume it has died unless you find the corpse). You may find the tree alive and well in a subsequent survey, in which case you can correct the survival % of the previous survey.

Monitoring of tree performance involves measuring tree height and crown width.

Measure the tree height from the root collar to the highest shoot tip. For trees up to 10 m, telescopic measuring poles can be used, to calculate growth rates of girth at breast height (GBH), are easier.

Measure root collar diameter (RCD) or GBH provide a more stable assessment of tree growth.

For small trees, use callipers with a Vernier scale to measure RCD at the widest point. Once a tree has grown tall enough to develop a GBH of 10 cm, measure both the RCD and the GBH the first time and only GBH thereafter.

Suppression of weed growth can also be quantified. Measuring crown width and using a scoring system for weed cover. Use tape measures to measure the width of tree crowns at their widest point and imagine a circle of about 1 m in diameter around the base of each tree.

Score 3 if weed cover is dense over the whole circle;
Score 2 if weed cover and leaf litter cover are both moderate;
Score 1 if only a few weeds grow in the circle
Score zero for no (or almost no) weeds.

For further details of how to monitor tree survival and growth and analyse the data, please refer to Chapter 7 and Appendix 2 of "Restoring Tropical Forests". The same information in Thai can be downloaded here.