Changes in tree species diversity and dominance across a disturbance gradient in Nepalese Sal (Shorea robusta Gaertn. f.) forests

Abstract

Disturbance is often touted as a management tool, as moderate disturbance is believed to enhance diversity; thus an understanding of frequent and fluctuating disturbance regimes in forests and their effects on stand structure, dominance and diversity is very crucial. Here, the effects of different disturbance regimes, along a gradient, on diversity and dominance of five Shorea robusta Gaertn. f.-dominated forests were investigated in 25 one-ha plots in Nepal. A total of 67 tree species were recorded; of which 41 species were encountered in least disturbed and 10 species in heavily disturbed forest. Significant variations among forests were observed for all measures of alpha diversity. Alpha diversity measures declined linearly along a disturbance gradient while dominance increased linearly. Relative basal area of S. robusta increased as the level of disturbance increased, which in turn produced more than two-fold higher important value index in heavily disturbed forest than the least disturbed forest. All alpha diversity measures declined in an order of three with increasing relative basal area of S. robusta. The similarity in species composition between each pair of disturbed forests was generally low (Jaccard’s similarity index < 57%), suggesting a higher Beta diversity. It can be concluded that diversity of Sal forests declines with increasing magnitude of disturbance, which in turn favors a higher dominance of S. robusta. Controlling the population of the dominant species, mainly S. robusta, is recommended to enhance diversity and to achieve multiple-use forest management objectives.

Keywords

Human pressure / disturbance ecology / species richness / alpha diversity / important value index

References

[1] Anonymous. 1994. Operational Forest Management Plan for Nawalparansi District (1995–2000). Forest Research and Survey Centre, Kathmandu.

[2] Brown N.D., Whitmore T.C.. Do Dipterocarp Seedlings Really Partition Tropical Rain Forest Gaps?. Royal Society of London Biological Science, 1992, 335: 369-378.

[3] Cannon C.H., Peart D.R., Leighton M., Kartawinata K.. The structure of lowland rainforest after selective logging in West Kalimantan, Indonesia. Forest Ecology and Management, 1994, 67: 49-68.

[4] Collins S.L., Glenn S.M., Gibson D.J.. Experimental Analysis of Intermediate Disturbance and Initial Floristic Composition: Decoupling Cause and Effect. Ecology, 1995, 76: 486-492.

[5] Connell J.H.. Diversity in tropical rain forests and coral reefs. High diversity of trees and corals is maintained only in a nonequilibrium state. Science, 1978, 199: 1302-1310.

[6] Fulbright T.E.. Disturbance effects on species richness of herbaceous plants in a semi-arid habitat. Journal of Arid Environment, 2004, 58: 119-133.

[7] Gautam K.H., Devoe N.N.. Ecological and anthropogenic niches of sal (Shorea robusta Gaertn. f.) forest and prospects for multiple-product forest management - a review. Forestry, 2006, 79: 81-101.

[8] Hubbell S.P.. The unified neutral theory of biodiversity and biogeography. Monographs in population biology 32. 2001, Princeton, New Jersey: Princeton University Press

[9] Hubbell S.P., Foster R.B., O’Brien S.T., Harms K.E., Condit R., Wechsler B., Wright S.J., de Lao S.L.. Light-Gap Disturbances, Recruitment Limitation, and Tree Diversity in a Neotropical Forest. Science, 1999, 283: 554-557.

[10] Jackson J.K.. Manual of afforestation in Nepal, 1994 second edition Kathmandu, Nepal: Forest Research and Survey Center

[11] Kennard D.K., Gould K., Putz F.E., Fredericksen T.S., Morales F.. Effect of disturbance intensity on regeneration mechanisms in a tropical dry forest. Forest Ecology and Management, 2002, 162: 197-208.

[12] Lamb D., Erskine P.D., Parrotta J.A.. Restoration of Degraded Tropical Forest Landscapes. Science, 2005, 310: 1628-1632.

[13] Lindenmayer D.B., Franklin J.F.. Conserving forest biodiversity, a comprehensive multiscaled approach. 2002, Washington DC, USA: Island Press

[14] Nagaike T., Kamitani T., Nakashizuka T.. The effect of shelterwood logging on the diversity of plant species in a beech (Fagus crenata) forest in Japan. Forest Ecology and Management, 1999, 118: 161-171.

[15] Onaindia M., Dominguez I., Albizu I., Garbisu C., Amezaga I.. Vegetation diversity and vertical structure as indicators of forest disturbance. Forest Ecology and Management, 2004, 195: 341-354.

[16] Pande P.K.. Comparative vegetation analysis and sal (Shorea robusta) regeneration in relation to their disturbance magnitude in some sal forests. Tropical Ecology, 1999, 40: 51-61.

[17] Pandey S.K., Shukla R.P.. Regeneration strategy and plant diversity status in degraded sal forests. Current Science, 2001, 81: 95-102.

[18] Ramirez-Marcial N., Gonzalez-Espinosa M., Williams-Linera G.. Anthropogenic disturbance and tree diversity in Montane Rain Forests in Chiapas, Mexico. Forest Ecology and Management, 2001, 154: 311-326.

[19] Sagar R., Raghubanshi A.S., Singh J.S.. Tree species composition, dispersion and diversity along a disturbance gradient in a dry tropical forest region of India. Forest Ecology and Management, 2003, 186: 61-71.

[20] Sagar R., Singh J.S.. Tree density, basal area and species diversity in a disturbed dry tropical forest of northern India: implications for conservation. Environmental Conservation, 2006, 33: 256-262.

[21] Sapkota I.P., Tigabu M., Oden P.C.. Species diversity and regeneration of old-growth seasonally dry Shorea robusta forests following gap formation. Journal of Forestry Research, 2009, 20: 7-14.

[22] Sapkota I.P., Tigabu M., Oden P.C.. Spatial distribution, advanced regeneration and stand structure of Nepalese Sal (Shorea robusta) forests subject to disturbances of different intensities. Forest Ecology and Management, 2009, 257: 1966-1975.

[23] Sheil D.. Tropical forest diversity, environmental change and species augmentation: After the intermediate disturbance hypothesis. Journal of Vegetation Science, 1999, 10: 851-860.

[24] Sheil D., Burslem D.F.R.P.. Disturbing hypotheses in tropical forests. Trends in Ecology and Evolution, 2003, 18: 18-26.

[25] Swamy P.S., Sundarapandian S.M., Chandrasekar P., Chandrasekaran S.. Plant species diversity and tree population structure of a humid tropical forest in Tamil Nadu, India. Biodiversity and Conservation, 2000, 9: 1643-1669.

[26] Tárrega R., Calvo L., Marcos E., Taboada A.. Forest structure and understory diversity in Quercus pyrenaica communities with different human uses and disturbances. Forest Ecology and Management, 2006, 227: 50-58.

[27] Timilsina N., Ross M.S., Heinen J.T.. A community analysis of sal (Shorea robusta) forests in the western Terai of Nepal. Forest Ecology and Management, 2007, 241: 223-234.

[28] van Gemerden B.S., Olff H., Parren M.P.E., Bongers F.. The pristine rain forest? Remnants of historical human impacts on current tree species composition and diversity. Journal of Biogeography, 2003, 30: 1381-1390.

[29] Vetaas O.R.. The effect of canopy disturbance on species richness in a central Himalayan oak forest. Plant Ecology, 1997, 132: 29-38.

[30] Webb E.L., Sah R.N.. Structure and diversity of natural and managed sal (Shorea robusta Gaertn.f.) forest in the Terai of Nepal. Forest Ecology and Management, 2003, 176: 337-353.

[31] Whittaker R.H.. Communities and Ecosystems. 1970, New York: Macmillan

[32] Zhu J.J., Mao Z.H., Hu L., Zhang J.X.. Plant diversity of secondary forests in response to anthropogenic disturbance levels in montane regions of northeastern China. Journal of Forest Research, 2007, 12: 403-416.