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Plant diversity and pollutant removal efficiency in constructed wetlands
J. Brisson
Institut de recherche en biologie végétale, Département de sciences biologiques, Université de
Montréal, 4101 est, rue Sherbrooke, Montréal (Qc) H1X 2B2, CANADA.
There are good theoretical reasons suggesting that increased plant richness would result in
increasing pollutant removal efficiency in constructed wetlands. Root partitioning
maximizes the full exploitation of the wetland belowground space. Complementary nutrient
uses among plant species should increase total nutrient removal through assimilation. Plant
diversity also promotes increased bacterial diversity and activity. Yet, there is still little
supporting empirical evidence for the positive effect of plant diversity on pollutant
removal. In addition, plant composition in wetland polycultures is difficult to maintain due
to community dynamics and the progressive dominance of the most competitive species.
Not surprisingly, constructed wetlands are still mostly planted with a single species.
There is an increasing number of studies comparing removal in monocultures and
polycultures in experimental constructed wetlands (Qiu et al. 2011, Liang et al. 2011,
Zhang et al. 2007, Fraser et al. 2004, Karathanasis et al. 2003,
Engelhardt and Ritchie 2002,
Coleman et al. 2001
). But convincing results from any individual experimental studies are
uncommon due to strong methodological limitations. Diversity studies ideally require a
large number of treatments (different monocultures and polycultures). Thus, for economical
reasons, the researcher must often make a compromise between the size and number of
experimental units. There experiments often suffers from being done in small experimental
units (questioning their application in real-size constructed wetlands) or they lack sufficient
replication, or often both. Finally, the interpretation of the results needs careful analysis.
The ecological literature on how species richness influences ecosystem functions and
processes in a wide range of ecosystems (but mostly grasslands) provides a rich
methodological framework that should inspire constructed wetland scientists.
Although results form individual experiments comparing pollutant removal between
monocultures and polycultures should be interpreted with care, some overall generalities
are slowly starting to emerge. First, there is little evidence that diverse polycultures of
plants or outperform their most efficient species. However, diverse polycultures are often as
efficient as the most efficient species it contains for most pollutants. Plant diversity may
have other benefits than pollutant removal alone, such as increased resilience to
perturbation or diseases, esthetical value, better habitat. Therefore, there could be a benefit
of having a diverse plant community in constructed wetlands without compromising on
efficiency. Of course, these preliminary generalities requires to be confirmed under a much
larger range of situations. The relation between diversity and wetland effect on water
quality remains an open question that provides promising research avenues.
