Quantifying forest disturbance using indicator plant species
Secondary forests now play a vital role in the conservation of tree species diversity in Southeast Asia because of the continuing fragmentation and decreasing extent of undisturbed forests in this region. To be able to determine the structural and species compositional integrity of secondary forests, a ground based rapid assessment method was developed, using indicator plant species from the genera Macaranga and Mallotus (Euphorbiaceae). Here we explain how this method works and how it can be applied in the field.
Forest disturbance and pioneer plant species
One of the most obvious changes that takes place after disturbance in tropical
forests is the sudden appearance of pioneer plant species. The increase in
pioneers in disturbed habitats is closely related to the increase in light
levels in the forest understorey. Because pioneers increase in numbers after
disturbance, they are, in principle, suitable for detecting and quantifying
tropical forest disturbance. Here we focus on two common closely related, easy
to recognize and species rich genera of Southeast Asia (Macaranga and
Mallotus, Euphorbiaceae). Many species of these two genera can be classified
as typical pioneers, and their abundances are closely related to general level
of disturbance in the studied forests, the time since disturbance took place,
and number of disturbances.
Classification into pioneer and climax species
Since ecological field data for most tropical
plant species is lacking, we had to develop methods to determine whether species
belonging to the genera Macaranga and Mallotus could be classified
as either pioneer or climax species. For this purpose we used the
length/width-ratio of the leaves, seed size and wood density (which were
strongly correlated with the light establishment preferences of a subset of 33
Macaranga and Mallotus species). Each of these characters was
subdivided into three classes, the order of which represented the change from
pioneer to climax habit (Table 1).
| Class | Wood density (g cm-3) | Leaf shape (l / w) | Seed diameter (mm) |
| 1 | ≤ 0.48 | ≤ 1.61 | ≤ 3.2 |
| 2 | 0.49 – 0.66 | 1.62 – 2.48 | 3.3 – 5.4 |
| 3 | ≥ 0.67 | ≥ 2.49 | ≥ 5.5 |
Table 1. Class ranges used to classify species into pioneers and non-pioneers. The class order reflects the change from a pioneer to a climax life history strategy.
The average of these three values for each species was used to classify them as possessing either the pioneer (average ≤ 2) or the climax (average > 2) life history strategy.
Pioneer species as indicators for forest disturbance
The rapid assessment method is based on plots of
900 m length consisting of 30 quadrates of 10 x 10 m at regular intervals. All
Macaranga and Mallotus species taller than 30 cm found in these
quadrats have to be identified and counted. The number of pioneers found in each
plot (sum of the 30 quadrates) gives an estimate of the disturbance level in the
plots. However, for a reliable disturbance level estimate, at least 5 such plots
have to be established within the forest area under study. The average values
found for several common types of forest disturbance are listed in Table 2.
| Disturbance type (time since disturbance) |
Pioneers (n ± sd) |
Non-pioneers (n ± sd) |
Pioneers (%) | Plots (n) |
| Undisturbed | 23.5 ± 36.4a | 250.8 ± 143.4a | 8.6 | 20 |
| Logged (21) and thinning (12) | 11.6 ± 9.6a | 51.8 ± 37.5bc | 18.3 | 5 |
| Logged (20-30) | 35.3 ± 21.6b | 82.3 ± 69.9bc | 30.0 | 6 |
| Logged (10-20) | 58.4 ± 44.4bc | 80.3 ± 110.9bc | 42.1 | 7 |
| Logged (0-10) | 138.4 ± 130.0cd | 110.0 ± 78.8ab | 55.7 | 7 |
| Once burned (15) | 168.2 ± 82.1de | 45.6 ± 20.0bcd | 78.7 | 5 |
| Once burned (3) | 507.0 ± 273.8ef | 57.0 ± 39.7abcd | 89.9 | 3 |
| Once burned (1) | 476.0 ± 229.3ef | 16.2 ± 12.6cde | 96.7 | 5 |
| Thrice burned (3) | 1124.6 ± 662.0f | 35.0 ± 29.2bcd | 97.0 | 5 |
| Twice burned (3) | 1671.7 ± 1275.3f | 8.3 ± 4.9de | 99.5 | 3 |
| Logged (15) and burned (1) | 1264.2 ± 866.3f | 6.0 ± 7.3e | 99.5 | 5 |
Table 2. The relation between pioneer and non-pioneer abundance (n per plot of 0.3 ha) and disturbance type. Disturbance types are ordered according to increasing dominance of pioneers. Significant differences in pioneer and non-pioneer numbers (log transformed) between disturbance types (indicated with different characters) were tested using a General Linear Factorial Model with a Fishers’ Least Significant Difference test.
Instructions for using the indicator method
When you enter the interactive forest disturbance page you will be asked to
enter the number of plots that you have surveyed. This is necessary to determine
the reliability of your survey. Each plot should consist of 0.3 ha, preferably
based on 30 subplots of 10 x 10 m positioned regularly (every 30 m) along a 900
m transect. However, make sure that your plots are all located within the same
forest disturbance type.
After you have entered the number of plots you can enter the total number of individuals (the sum of all the plots) for each species of Macaranga and Mallotus that you have encountered. When you have done that you can submit your query by clicking 'Give disturbance info' and you will get an estimate of the disturbance level in your plots.
If you like, you can also submit your data to a database of Macaranga and Mallotus density records by clicking 'click here to save this data for research purposes'. Your data will than be used to determine current disturbance levels on the island of Borneo. If you submit your data, make sure that you fill in your address, exact location of the plots and the additional information on plot layout and known disturbance history of the plots.