by Malaka Rodrigo  Courtesy Mongabay.com

• A new type of forest ecology has been discovered from the ancient kingdom of Polonnaruwa in north-central Sri Lanka, centered around the irrigation canals abandoned there more than 700 years ago.
• The unique new forest type closely resembles the riverine forests that are found alongside rivers and other bodies of water, but has a different species composition and vegetation structure.
• Sri Lanka boasts a legacy of extensive ancient irrigation systems scattered within the island’s dry zone, raising the prospect of the discovery of more of these dry canal-associated evergreen forests.”

POLONNARUWA, Sri Lanka — For centuries, Sri Lanka’s ancient civilization diverted river water to large reservoirs and man-made canals for agriculture. After the collapse of the island’s hydraulic civilization about 700 years ago, these canals were abandoned. But water still flows through them today, nurturing a different type of vegetation.

That’s what plant ecologist Magdon Jayasuriya discovered when he examined one such abandoned ancient irrigation canal built between the 11th and 14th centuries, historically known as the Kingdom of Polonnaruwa. The canal was built to divert water from the Mahaweli River to a cascade of small reservoirs and farmlands scattered near what is today Somawathiya National Park.

It’s here that Jayasuriya has identified a new type of forest vegetation, described in a new study in the Ceylon Journal of Science: dry canal-associated evergreen forest,” or a canal forest that joins an already diverse list of forest types found within Sri Lanka’s dry zone.

The canal forest closely resembles a riverine forest or dry riverine evergreen forest, but differ in plant species diversity and characteristics. These differences may be because of factors ranging from the water flow, to the availability of groundwater, to the steepness of the banks, Jayasuriya told Mongabay.

Riverine forests

Riverine forests, found adjacent to bodies of water, are natural habitats with unregulated natural water flow. The canal forest, by contrast, would have experienced regulated and maintained water flow when the canal was in use, and a less uniform flow after its abandonment. The banks of the canal are also shallower than riverbanks due to seasonal strong and unregulated water flow creating more habitats in the latter and adding more diversity, according to the study.

The canopy of the canal forest appears shorter, too, at about 10-12 meters (33-40 ft), while river forests can grow to heights of about 20-25 m (65-82 ft).

The new forest type is dominated by Vitex leucoxylon, a tree known locally as nebada and which accounts for more than half of the vegetation.  Terminalia arjuna, or kumbuk, a tree generally found in greater abundance within river forests, makes up only a fifth of the canal forest vegetation, the study says.

Based on detailed analysis of the plant species found within this forest ecology, Jayasuriya said the Polonnaruwa area may have had savanna forests, though there is no direct evidence of such today. Hinting at the possibility of savanna forests in the past, Jayasuriya cites the presence of Tamilnadia uliginosa and Antidesma ghaesembilla trees, considered savanna species.

It is possible that over 700 years ago during pre-Polonnaruwa, this canal would have flowed through a savanna forest though it is not present today,” Jayasuriya said.

Jayasuriya said he had to take an uncomfortable tractor journey across Samawathiya National Park, lying in the flood plain of the Mahaweli River, to study this new vegetation type. Due to time constraints, I couldn’t fully study the forest. As Sri Lanka has lots of ancient irrigational canals and tanks, I’m sure canal forests would exist elsewhere as well, but would have been overlooked,” Jayasuriya said.

He called for further study of this vegetation type in Sri Lanka, with the support of both archaeologists and hydrologists.

The discovery of this new forest vegetation type after a long period is evidence of the possibility of many ecologically important discoveries waiting to be made,” Jayasuriya said, adding that young researchers in particular should delve into the field.

Citation:

Jayasuriya, A. H. M. (2019).A new forest vegetation type in Sri Lanka: Dry canal-associated evergreen forest. Ceylon Journal of Science, 48(4), 375-381. doi:10.4038/cjs.v48i4.7679