4. Discussion and recommendations

If the results summarized in Table 6 are considered in the light of knowledge gathered in the industrialized countries (Chapter 2), then the conclusion is unavoidable that possibilities exist today for application and further development of biological monitoring in tropical and subtropical countries as well. Nevertheless, a great deal remains to be done in the way of preliminary work before practical application can commence.

When making decisions on the methods which have the greatest likelihood of success in given instances, the principal criteria which must be examined are the ecological and sociological conditions in the countries where studies are being planned. This refers above all to site factors, but also has to do with the level of knowledge and expertise on the part of those who will be conducting the studies, laboratory capacities and the behavior of local populations. The last-mentioned aspect can have a crucial bearing on the success of some active monitoring procedures, since theft and destruction of test stations can occur out of ignorance. For instance, there is always a certain risk involved in setting out self-watering pot installations, although these are without a doubt indispensable during dry periods. Yet, it is precisely such active monitoring methods deployed in source-oriented grids that yield the most reliable data when collecting evidence against environmental polluters or investigating the environmental compatibility of planned installations. Only such approaches can eliminate uncertainty caused by unknown soil and nutrient conditions and competition from other vegetation. By arranging the stations in a checkerboard pattern or in radiating lines it is sometimes even possible to partially overcome unfavorable topographic reliefs.

Table 6 lists only one plant species which could be used for active monitoring relatively unproblematically and without major preliminary studies. This is the tobacco variety BEL-W-3, which responds to photochemical oxidants, especially ozone, and as such can be classified as a response indicator. Since photochemical oxidants are secondary atmospheric pollutants, this plant can be used to perform a kind of background measurement. In addition, young eucalypts would be suitable for use as response indicators for SO₂, although the heterophyllous nature of these trees would have to be taken into account. The same holds for various lichens, if it is possible to transplant them onto exposure installations and adapt them to different sites. One accumulative indicator which has already been successfully used for active monitoring is the lichen Ramalina duriaei. It would also be possible, following the example of the standardized grass culture, to test suitable species and varieties native to tropical and subtropical countries. However, the required investigations, above all testing of tolerances, accumulation abilities, suitability for being cut back, etc., would be extensive and require a long period of time.

A number of organic and anorganic materials may also be suitable for active monitoring, since they react to pollution with physically measurable responses. Sheet steel, for example, exhibits an accelerated corrosion rate when exposed to SO2 or other acidic gases, manifesting itself in the form of increased weight. Trials and/or modifications of this approach would definitely be required in especially dry regions, since the formation of iron oxide depends on a certain minimum atmospheric humidity.

The expansion of natural rubber can be measured simply by taking readings with a millimeter scale, without the need for any laboratory tests. It is influenced by the concentration of ozone in the ambient air. Preliminary trials still remain to be performed on this method, however, so that it will not be ready for use until sometime during 1988.

Little interference from local populations need be feared in the case of passive monitoring. In this approach, either standardized methods are used for examining naturally occurring vegetation for pollution injury in situ, or else apparently undamaged plant organs are removed and their pollutant concentrations analyzed in the laboratory. The reliability of the data which can be obtained in this way increases with the number of points sampled around a source. Compared to active biological monitoring, therefore, this requires a more closely spaced grid in order to permit conclusions to be drawn on the atmospheric pollutants that are causing injury to the plants.

In general, with passive biological monitoring it is essential to take the vegetation at the site of study and surrounding region into account. The only alternatives are therefore to exclusively address such plant species that occur widely within a given biome, like certain domesticated plants, or else to perform assessments of each individual site on the basis of thorough familiarity with local conditions. The latter possibility will not be discussed any further here, since at this time only a handful of ecotoxicologists from the industrial nations are likely to possess the required knowledge.

Great care is required when undertaking routine use of wild plants as bioindicators within the scope of a passive monitoring program. Only a few studies have been performed on the effects of pollutants on such organisms, but they show that even varieties of one and the same species can respond very differently. Particularly where herbaceous plants are concerned, experience has shown that several different varieties can occur within an area spanning just a few hectares, thus potentially leading to completely wrong assessments. It should be mentioned in this connection, however, that "mosses" of the genus Tillandsia have been used quite successfully as accumulative indicators for passive monitoring (SCHRIMPFF, 1987, 1988).

Of the species listed in Table 6, it is particularly the eucalypts which react sensitively to SO2 that appear to hold promise for passive monitoring. The fact that during the last 20 years their silvicultural utilization has spread rapidly in the subtropics outside of Australia, where they originated, ensures that they would have a large validity range and importance (cf. Chapter 2). Their degree of representation for other tree species would have to be investigated. The low-growing citrus trees which have proven to be sensitive to photochemical smog in California may well also play a useful role. In addition, the mango tree has also expanded far beyond its native range, namely the Malay Archipelago, and is now cultivated nearly everywhere in the tropics. Because of its wide distribution it can also be regarded as a potential bioindicator species.

Various varieties of grape plants have repeatedly been observed to respond to fluoride pollution and ozone with visible symptoms. The degree of sensitivity is dependent on the variety, however, a fact which would make it necessary to conduct preliminary tests.

Finally, certain lichens could also be used as response indicators, although it must be taken into account that in hot, dry regions these plants are only metabolically active for a few hours each day. Here, too, preliminary studies would be a must.

Lichens can also be utilized as accumulative indicators within the scope of passive monitoring. However, an essential prerequisite for this type of approach is the availability of staff and equipment for effective laboratory analysis, an aspect which even in the industrial countries does not always attain the required standards. However, appropriately dried and prepared samples can be relatively easily transported and thus centrally processed in specialized laboratories. Besides lichens, Panicum maximum could also hold promise as an accumulative indicator, as studies at my own institute have shown (WUSTEMANN, 1982).

From what has been said above, a few conclusions can be drawn regarding future work aimed at use of plants as bioindicators in tropical and subtropical countries. Accordingly, the following steps must be carried out:

1. Review and evaluation of the studies which have already been carried out in tropical and subtropical countries on the effects of pollutants. Centrally coordinated compilation of documentation on various relevant fields at appropriate specialized institutes.

2. Compilation of an atlas containing plates illustrating the effects of pollution on selected agricultural and silvicultural plants from the tropics and subtropics as a basis for development of response indicators.

3. Development and/or adaptation of response indicators for tropical and subtropical conditions. Cooperation with institutions in industrial and threshold countries (South Africa, Brazil, Australia) of these regions.

4. Identification and development of accumulative indicator plants together with appropriate analytical techniques.

5. Preparatory studies and development work in specialized German laboratories within the scope of a feasibility study.

6. Implementation of a selected model biological monitoring project in a tropical or subtropical country.

On the whole, it can be ascertained that, although much remains to be learned, our present level of knowledge nevertheless represents a firm basis for continued work. If activities are appropriately coordinated by a national working group, it should be possible to concentrate on a few promising goals. In cooperation with institutes in the countries where these biological monitoring procedures could potentially be applied, preparations could then be carried out for field studies.

We would like to conclude by reiterating our conviction that biological monitoring is highly suited for use in the developing countries. This conviction is based above all on two factors. One is that, in general, biological monitoring is technologically less demanding than physical sampling and analysis of pollutants, a fact which responds well to the situation in these countries. The second, even more important, aspect has to do with the inherently different type of information provided by biological monitoring (cf. Chapter 2). If use of bioindicators and response-oriented monitoring approaches in the tropical and subtropical countries begins soon, it may well be possible to set a development in motion which is based on obtaining evidence of and preventing pollution injury, and which is not limited to implementation of cost-intensive pollution monitoring networks.