The possible role of 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 sp. on regeneration of die-back forests of Horton Plains National Park

Abstract

Some natural forest communities including the Upper Montane Rain Forest (UMRF) of Horton Plains, Pidurutalagala ridge, Kobonilgala near Corbett's gap in Knuckles range and the summits of Hakgala Strict Nature Reserve in Sri Lanka are seriously deteriorating due to stand level dieback. The extent of forest die-back in the Horton Plains National Park (HPNP) was estimated as 654 ha of forest, representing 24.5 % of the forests and 17.2 % of the total area of the national park. Die-back is severe in the Totapolakanda and Kirigalpotta areas, where more than 75 % of the canopy trees were dead and the remaining trees showing signs of die-back. Larger trees were more prone to die-back than smaller ones. Forest regeneration in these two areas was very slow. Although, some previous reports attribute the die-back to the severity of the weather conditions, there is little information available on the causes of dieback, the process of and factors controlling the regeneration of die-back areas. It has been reported that damage caused to tree barks browsing by Sambar deer in the Horton Plains is significant. In this research, study sites were selected using a remote-sensed map of HPNP and two die-back forest types were recognized based on the presence or absence of 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 spp. beside the composition of the vegetation. Eight, 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 free or 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 dominated 50 m X 20 m transects (further divided in to five 20 m X 10 m segments) were selected randomly. Saplings (< 5 cm dbh) in those segments were recorded. Specimens were taken from unidentified individuals and were identified using the specimens in the national herbarium. 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 cover in 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 dominated die-back sites were above 70% 80%. In the sites dominated with 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 spp, 6 - 13 forest tree saplings were recorded, and the light intensity of the ground was 15-30 μmol.m-²s-1. In contrast, 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 free regenerating - forest die back sites consists of 28 - 70 tree saplings and receive 50-70 μmol.m²s-1 to the ground level. The number of tree saplings observed in the 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 free sites is significantly higher (p = 0.0006) than the number of saplings found in the 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 dominated sites. Further, the number of trees > 1 m height was higher in the 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 free sites (32-57 individuals) than in the 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 dominated sites (12-17 individuals). Our results revealed that there is an impact of 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 cover on availability of saplings of the other forest species. Further, it seems that the growth and development of the tree saplings were also affected by the 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 cover. 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 in the dieback forest in HPNP seems to act as a barrier and limits necessary conditions such as light, soil surface and nutrients for the growth of some of the tree species, and may result in the delay of regeneration of the die-back forest. However, further observations are required to determine whether 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 vegetation is reestablished and acts as a barrier periodically in the same area or remains as 𝘚𝘵𝘳𝘰𝘣𝘪𝘭𝘢𝘯𝘵𝘩𝘦𝘴 free, regenerating forest stands.