The forests of western Amazonia are famed for being among the most biologically diverse ecosystems on Earth. However, most ecological studies in this region have been principally concerned with upland terra firme and seasonally flooded forests, largely neglecting the wetland forests,or swamps, i.e. those forests which remain waterlogged year-round. As a result, vast areas have remained essentially undescribed and undervalued.
The largest area of contiguous wetland forests in Amazonia is found in the Pastaza Maranon Foreland Basin in Northern Peru. This 120,000 km2 subsiding basin contains a mosaic of wetland ecosystem types and has recently been shown to include 33,000 km2 of peatlands – ecosystems whose thick accumulations of organic matter make them the most carbon-dense ecosystems in Amazonia. Our new study was the first to describe systematically the composition and diversity of the tree communities that occur in these peatland ecosystems.
Our work focussed on the two broad classes of forested peatlands: palm swamps (known locally as aguajales) and peatland pole forests. Drawing on a rich forest inventory dataset from the surrounding region, we compared the tree community composition of these peatland forests with that of surrounding forest types, including upland terra firme, white sand forests, and seasonally flooded forests. We found the two peatland forest types to be compositionally distinct from each other and also distinct from all the other forest types.
Our most striking result concerned peatland pole forests, which we describe in the paper for the first time. We found that peatland pole forests are the least diverse tree communities to have been described in Amazonia, and typically have a level of diversity closer to that of typical temperate forests than to other tropical forests. The diversity of these forests contrasts particularly strongly with surrounding upland forests, which are among the most diverse forests in the world.
What makes these peatland forests so different from other tropical forest types? In part, we can attribute the low diversity of peatlands to their isolation, resulting from a patchy distribution pattern, and extremely low soil fertility. However, other forest types in Western Amazonia that also have a patchy distribution and occur in nutrient poor environments, such as white-sand forests, can maintain much higher levels of diversity, suggesting that low fertility and isolation are not entirely responsible. Similarly, peatland forests in other tropical regions (e.g. Southeast Asia), with equally extreme environmental conditions, can sustain communities with significantly higher diversity indicating that another mechanism is contributing to the extraordinarily low diversity found in peatland pole forests.
In our paper, we suggest that this low diversity is due to the short time that patches of peatland pole forests are likely to have existed in the landscapes of the Pastaza Maranon Foreland Basin. This landscape is extremely dynamic as a result of lateral river migration which repeatedly erodes peat deposits. As a result, peatland forest types only occupy any given location for a short period of time (on the order of a few hundred to a few thousand years according to our palaeoecological work at these sites). Patches of peatland pole forests are likely to have a particularly brief existence, only a few hundred years, as they appear to be restricted to rain-fed peat domes, which require several thousand years to develop.
The transient nature of peatland pole forests would mean that their composition and diversity is determined not only by those species able to survive within these extreme environments, but also by those species that have been able to disperse to these sites over relatively short timescales. Consistent with this prediction, our results demonstrate stronger patterns of distance decay in the similarity of the composition of peatland forests compared with other forest types. In addition, analysis of preserved pollen records in more examples of domed peatlands will enable us to test whether the recent appearance of the peatland pole forests studied so far is typical across the basin.
It is remarkable that the species composition and diversity of an entire ecosystem has remained undescribed until now. Through our analyses we have demonstrated that Amazonian peatlands have a unique species composition and are the least diverse forest type documented in Amazonia. By exploring the context of this extremely low diversity, we suggest that temporal environmental dynamism may have resulted in these exceptionally species poor communities. The role of processes operating over centennial/millennial timescales in determining species composition and diversity are often overlooked: peatland pole forests represent a fascinating tropical laboratory to study how the age of a specific landscape influences its composition today.
By Freddie Draper @FCDraper