Expansion of pine into mid-elevation Himalayan oak forests: Patterns and drivers in a multiple-use landscape

https://doi.org/10.1016/j.foreco.2021.119491Get rights and content

Highlights

  • Quercus leucotrichophora dominated forest cover decline in Western Himalaya.

  • Simultaneous expansion of Pinus roxburghii forests in the study area.

  • Degraded oak stands transitioned to pine forest between 1991 and 2017.

  • Degraded oak-pine forest transitions more likely in drier, exposed sites with lower winter precipitation and more pine forest in the neighborhing landscape.

Abstract

Oak species worldwide are experiencing declines, with negative consequences for hardwood forests and their associated biodiversity, but the causes of these declines vary across species and habitats. We examine changes in extent of banj oak-dominated (Quercus leucotrichophora) moist temperate hardwood forests in a Western Himalayan landscape between 1991 and 2017. Using classified satellite imagery from 1991, 2001 and 2017, we found that dense banj oak forests have undergone substantial degradation and loss of area, while chir pine (Pinus roxburghii) forests have expanded, largely by replacing degraded banj oak stands. This transition was most likely to occur at sites that had lower levels of winter precipitation and Topographic Wetness Index (TWI) values and more pine forest in the surrounding landscape. Our findings are consistent with known requirements of shade and moisture for the recruitment of banj oak. Loss of banj oak forests is likely to reduce biodiversity and a number of ecosystem services linked to the well-being of local communities.

Introduction

Oak species (Quercus spp.; Family Fagaceae) are of great ecological and economic importance, with more than 300 species known worldwide, and constitute dominant species in old-growth hardwood stands globally. Across their global range, several oak species are threatened by climate change, deforestation, over-exploitation and disease, factors that adversely affect regeneration, recruitment and tree survival (Tyler et al., 2006, Siwkcki and Ufnalski, 1998, Xia et al., 2016). For instance, large-scale die-offs of oak stands have been reported in several parts of Europe, attributed to extended summer drought and root fungal infestation (Siwkcki and Ufnalski 1998). Demographic studies in Californian forests show recruitment limitation in commonly occurring oak species (Tyler et al. 2006). Other sites such as Ozark Mountains in south-eastern United States have recorded widespread oak mortality due to insect infestations (Heitzman 2003). Sichou oak (Q. sichourensis) faces extinction in the wild due to desiccation sensitivity of its seeds, which impacts seed germination (Xia et al. 2016). In the oak forests of the Western Himalaya in India, chronic extractive disturbances affect structural and physiological attributes of some oak species, causing long-term forest degradation (Thadani 1999).

While the extent and quality of oak-dominated forests is widely perceived to have declined over the last few decades, most studies are snapshot in nature and limited to temperate sites (e.g. Siwkcki and Ufnalski, 1998, Badano et al., 2015). Further there is little understanding of the relative importance of anthropogenic, climatic, habitat and topographical drivers of oak forest change at the landscape level. Landscape changes related to climate change, degradation and habitat fragmentation are likely to affect patterns of oak species germination and recruitment through effects on microclimate and soil (Singh et al. 1984). Oak species are likely to be vulnerable to desiccation under unusual drought conditions due to their large seed sizes (Siwkcki and Ufnalski, 1998, Xia et al., 2016).

Banj oak (Quercus leucotrichophora) occurs in the Western Himalayan Broadleaf Forest biome, a Global 200 Ecoregion and Endemic Bird Area prioritized for conservation attention, due to its high biodiversity and increasing threat levels (Malcolm et al., 2002, Olson and Dinerstein, 2002, Birdlife International, 2016). Occurring in the mid-altitudes, banj oak-dominated forests are considered to be Pleistocene relicts resembling cloud forests in structural and functional attributes and harbouring high floral diversity (Singh and Singh, 1987, Shahabuddin et al., 2017, Menon et al., 2019). Banj oak forests provide critical ecosystem services including support for agrarian livelihoods (Singh, 1997, Thadani, 1999, Naudiyal and Schmerbeck, 2017), as well as hydrological modulation (Pathak et al., 1985, Qazi et al., 2017), nutrient cycling and soil conservation (Singh and Singh, 1986, Singh, 1997).

In the Western Himalaya, banj oak-dominated forests are perceived to be threatened by various anthropogenic processes and possibly climate change (Singh and Rawat, 2010, Shahabuddin and Thadani, 2018). Chronic extraction of forest products is considered to be a major driver of oak forest degradation through its role in opening up the forest canopy, simplifying structure and reducing biomass and diversity (Singh et al., 1997, Singh, 1998, Makino, 2011). Another purported cause of decline in extent of banj oak forest stands is the spread and proliferation of chir pine (Pinus roxburghii) into former oak stands, encouraged by repeated fires (Singh et al. 2016) and anthropogenic disturbances, particularly chronic extraction (Singh et al., 1984, Singh and Singh, 1987, Shahabuddin and Thadani, 2018). This supposition is supported by observations of pine germination and rapid recruitment in oak stands degraded by intense local use (Singh, 1997, Pandey et al., 2020). Disease vulnerability, linked to climate warming, is also likely to affect the health of oak forests (Siwkcki and Ufnalski 1998). Lastly, as elsewhere in the tropics, increased urbanization and infrastructure development, can cause forest fragmentation, with known negative effects on remnant fragmented forests, including deleterious edge effects and losses of forest specialist species (Murcia 1995).

In this study, we assess the extent of banj oak forest degradation at the landscape-scale, as well as its loss in the Western Himalaya between 1991 and 2017. In particular, we quantify the conversion of banj oak forests to pine forest over the last twenty-six years. We also investigate the relative importance of bioclimatic, topographic, anthropogenic and local habitat factors associated with the observed patterns of conversion of degraded banj oak forest to pine forest between 2001 and 2017.

Section snippets

Study area

The study was conducted over a 1285 km2 landscape, in the mid-elevational zone (1500–2400 m ASL) of the Western Himalaya, in the state of Uttarakhand, India (29.521–29.388°N and 79.586–79.548°E; Fig. 2). The area falls within Nainital and Almora districts and is under the jurisdiction of the Nainital and Almora Forest Divisions. Banj oak is the dominant species in late-successional, dense-canopied forest, with associates such as Rhododendron arboreum, Lyonia ovalifolia, Alnus nepalensis, Myrica

LULC change in the study area 1991 to 2017

The LULC classification for the 2017 image had an overall accuracy of 86.9% (kappa hat = 0.87). For 2001, the overall classification accuracy was 88.3% (kappa hat = 0.88). The 1991 image had an overall accuracy of 85.9% (kappa hat = 0.86). All classes had accuracy >77% (Fig. 2; Table A.3a–f).

The area under dense oak showed a net decline of 51.3 km2 (22.2% of original area) between 1991 and 2017, while that of degraded oak declined by a net 61.1 km2 (29.2% of original area). Area of pine showed

Decline of oak forest

While numerous scholars have suggested a trend of decline in oak-dominated forests in the Western Himalaya (Singh et al., 1984, Somanathan, 1991, Singh, 1997, Singh and Rawat, 2010), ours is the first study to quantify the extent and spatial pattern of such decline at the landscape scale and analyse its drivers. We find substantial and widespread loss and degradation of banj oak forest since 1991, and simultaneously, expansion of pine stands, which were found to replace degraded oak stands over

Conclusions

We found substantial reduction of banj oak forest habitats in a Western Himalayan landscape between 1991 and 2017, while pine forests expanded considerably. Several degraded oak stands transitioned into pine forests. Spatial variation in winter precipitation was a key factor driving degraded oak to pine forest transitions, as was area under pine forest in the surrounding landscape. Local habitat and topographical factors related to soil moisture also significantly influenced the probability of

CRediT authorship contribution statement

Arundhati Abin Das: Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Tarun Menon: Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Jayashree Ratnam: Resources, Visualization, Writing - review & editing. Rajesh Thadani: Resources, Visualization, Writing - review & editing, Supervision. Gopalakrishnan Rajashekar: Methodology, Resources, Visualization, Writing - review & editing. Rakesh

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Uttarakhand Forest Department for permission to conduct research, and Survey of India for digital toposheet data. GS was funded by Department of Science and Technology (Women Scientists’ Scheme). AD was funded by CEDAR, and additional financial support from the DBT-RA programme in Biotechnology & Life Sciences is gratefully acknowledged. TM was funded by CEDAR, Tata Trusts and NCBS-TIFR. Vishal Singh and Anvita Pandey provided logistical support. We thank S.P. Singh for his valuable

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