Introduction 

Forests occupy around 31% of the Earth’s surface and are among the most studied ecosystems ([82], [25]). It is estimated that forests harbor more than 60% of all terrestrial animal and vascular plant species ([82]) and provide shelter to more than 75% of amphibians and birds. Therefore, the existence of these organisms directly depends on forests ([25]). The world’s tropical regions have the most widespread forests and diverse ecosystems. Half of the tropical land (1.7 billion hectares) is covered with forests ([74]). The occurrence of 60% of vascular plants indicates this region is an eminent reservoir of biodiversity ([25]). Several studies have been carried out in the tropics on tree diversity and composition of forests, addressing questions like spatial patterns ([15]), dominance and distribution ([73]), beta diversity ([16]), structure and composition ([30]) and vegetation classification ([39]). Studies have shown that tropical tree diversity, floristic composition, and stand structure are influenced by environmental and geographic variables at local, regional, and global scales ([71], [8], [49]).

Tropical regions of India have 22% of its geographical area occupied by forest and tree cover ([26]). India is home to diverse forest types due to its wide range of climatic and geographical variations. They are classified according to climate, biogeography, and tree species composition ([13], [67], [3]). India is home to four world biodiversity hotspots, i.e., regions with the highest diversity and number of endemisms, and are exposed to various anthropogenic disturbances, with the Western Ghats (WG) being one amongst them ([56]).

Forests in the WG have been extensively studied for their floristic diversity ([36], [61]), tree composition and diversity across spatial ([34], [4]) and temporal scale ([2], [38]), forest dynamics ([23], [6]), the effect of anthropogenic disturbance ([66], [57]), diversity across altitudinal and latitudinal gradients ([81]), endemism ([37], [68], [31]) and effect of biotic and abiotic factors on vegetation ([63], [64]). The characteristic feature of the Western Ghats is a decrease in annual precipitation and an increase in the length of the dry period from the south to the north ([67]). Based on the geology and climatic variables, WG escarpments have been divided into northern (NWG), central (CWG), and southern (SWG) parts ([89], [67]). The NWG stands out due to its four months of rainfall, eight months of dry period, and lower elevations compared to CWG and SWG. The region is characterized by a unique mosaic of forested and open landscapes, with hills adorned by laterite and basaltic outcrops, creating a seasonal environment that sustains a diverse endemic herbaceous flora ([51]). These open habitats are interspersed with patches of forest located in valleys, ravines, and plains. These forests are highly fragmented due to several anthropogenic activities. They are often only found in legally protected areas like sanctuaries and national parks and in community-protected areas like sacred groves and private lands. Despite their fragmented distribution, these forests act as repositories for medicinal plants, endemic species, and wild relatives of crop plants ([35]). Except for a few sporadic ecological works in the forest fragments of NWG, it remains understudied mainly in terms of forest tree diversity, endemism, phytosociology, and influence of environmental and physical factors on vegetation compared to its counterparts (CWG and SWG - Fig. 1). This review is aimed to gain a comprehensive understanding of the forest ecology of NWG by compiling the various works carried out in the region. By doing so, it seeks to identify the research gaps in order to plan future studies in NWG. Thus, this review serves as an essential step in better understanding the fragmented and understudied tropical region in the biodiversity hotspot of WG and in aiding the conservation of its already fragmented patches.

Fig. 1 - Map representing forest ecology studies of the Western Ghats, NWG-Northern Western Ghats, CWG-Central Western Ghats, and SWG-Southern Western Ghats (prepared based on literature).

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  Study area: Northern Western Ghats 

The NWG lies between the latitudes 15° 48′ - 21° 00′ N and is about 750 km long, i.e., one-third of the total length of the WG escarpment. The north-south boundaries of NWG are marked by the Tapti River, Gujarat, in the north, and the Kali River in Uttar Kannada of Karnataka state in the south ([67]). NWG occupies about 62,270 km² (33.75%) of the area and differs in geology, topography, and climate from its counterparts ([87]). Geologically, the entire NWG is composed of a massive Deccan basalt trap with a narrow coastal belt on the west and plateaus and plains on the east. The crest of NWG rises to 1600 m a.s.l., with the highest peak (Kalsubai - 1646 m) in Maharashtra. Low elevated Konkan coasts and certain high elevated areas in the latitude below 18° N have laterite caps that were weathered from the duricrusts ([51]). NWG has four months of rainfall (July-October) and a prolonged eight months dry period. It receives rainfall of up to 5000 mm year^-1 in the high-elevation mountain regions. The seasonal and rainfall variations influence the vegetation of NWG.

  Literature search 

A comprehensive literature survey was carried out to retrieve information about the past and present works on the forest ecology of NWG. Online resources (Web of Science^® and Google Scholar^®), books, research papers, review articles, and various reports were used. More emphasis was given to the studies conducted at a regional and local scale in NWG.

The Web of Science database ([99]) was used to find relevant literature. A keyword search was conducted using the terms “Forest”, “India”, “Western Ghats”, and “Northern Western Ghats” at different levels of literature search (Tab. 1). The keywords “Forest AND Northern Western Ghats” resulted in 85 studies focusing mainly on faunal investigations, geoscience, agronomy, soil science, and zoology. “Forest AND Northern Western Ghats AND Plants” resulted in 27 papers, which were on topics like forestry, environment science, mycology, limnology, ornithology, and new species discovery. After further scrutiny of forest vegetation studies, only eight studies related to the scope of the present review were found using WOS.

From the extensive literature search with Google Scholar and the Web of Science, a total of 159 publications were found for WG with an emphasis on NWG. A heat map of the 159 publications was prepared using the software QGIS to visualize the spatial distribution of literature (Fig. 1). The publications were selected with priority topics related to forest or tree vegetation, diversity, structure, composition, and driving factors (climatic, physical, and disturbance) from protected areas, sacred groves, and private forests across the Western Ghats. As the focus was exclusively on tree and forest ecology, the floristic studies (flora, checklists, inventories) were excluded. Further, to understand the scope of forest ecological studies, we reviewed a total of 34 exclusive publications from NWG, ten from NWG-WG, and 115 from WG. We classified those 34 publications into four categories: quantitative, qualitative, reports/general articles, and remote sensing/GIS (Tab. 2).

  Early attempts to understand NWG forests 

The literature on ecological studies of forests of NWG is sporadic, beginning with Brandis ([10]) describing how moisture and rain influence the forests and their distribution across India and Burma. Brandis’s later work ([11]) mainly focused on forest cutting for timber, silviculture, and the beginning of the conservation of forests by the British Indian government. The vegetation of hill stations and pristine forest areas from regions like Mahabaleshwar, Matheran, Bhimashankar and Kolhapur were studied first ([86], [78], [28]). Other studies from the Pune region and around Pune city in NWG have documented the vegetation of dry tropical forests using quantitative methods ([77], [79], [9]).

Classifying forests in NWG

Champion & Seth ([13]) provided an exhaustive classification of 16 major type groups and 221 sub-type groups based on criteria such as physiognomy, structure, function, floristic composition, climate-moisture, rainfall, temperature, geography-elevation, location, and aspects. Out of these types, NWG shows seven major types and 11 sub-types (see Tab. S1 and Tab. S2 in Supplementary material). Additionally, each type was further classified based on dominant or characteristic species of that forest type, known as the “floristic series”. The floristic series of a given plant community has a specific order of species, beginning with dominant and co-dominant species, followed by characteristic or climax species. However, many of these floristic series were created with limited data and a lack of confirmation from quantitative surveys, leading to confusion regarding forest management.

Bahuguna et al. ([3]) revised the forest classification system by Champion & Seth ([13]) that had been in place for over six decades. This study validated each forest type by conducting field observations that considered factors such as species composition, biotic pressure, management intervention, and climate influence. As a result, the survey modified the forest types to 10 major groups and 44 sub-groups, with NWG containing seven types (see Tab. S2 in Supplementary material).

Puri & Mahajan ([78]) conducted a comprehensive local level study in the Mahabaleshwar forests of NWG, and identified four types of communities based on the percentage of occurrence of the species (community type mentioned in Tab. S3 - Supplementary material). However, no other quantitative information was considered in this categorization which was based solely on the percentage of occurrence of the species. Ghate et al. ([34]) later, while classifying forests of WG, proposed a new type of “stunted semi-evergreen forest” for NWG, based on samples from 11 sites, of which four were semi-evergreen forests.

A few researchers have attempted to analyze the vegetation structure and composition at the entire WG level ([67], [34], [68]). Pascal ([67]) classified wet evergreen forests into various vegetation types based on their composition and floristic series. He used species or groups of species from the same genus, taking into account their abundance, ecological importance, and characteristic species to differentiate wet evergreen forests. He designated Memecylon umbellatum-Syzygium cumini-Actinodaphne angustifolia type for the mid-elevation forest of NWG based on northernmost sampling at Goa (16° N). However, vegetation further north of Goa was assumed to be the same type based on two hill station studies Mahabaleshwar (18° N - [78]) and Matheran (19° N - [86]) for the semi-evergreen forest. This type is compatible with “Montane sub-tropical forests” and “Western sub-tropical broad leaved hill forests” described by Champion & Seth ([13]).

Recent studies examining the semi-evergreen forests of the NWG at local levels have identified various types and sub-types based on the Pascal ([67]) classification ([96], [98], [47]). Watve ([96]) and Watve et al. ([98]), while investigating the fragmented forest in the Mulshi region, identified the vegetation types as Dimpcarpus-Aglaia-Ficus nervosa and Memecylon-Xantolis-Actinodaphne apart from Memecylon umbellatum-Syzygium cumini-Actinodaphne angustifolia (M-S-A) type. Similarly, vegetation studies in Chandoli National Park ([47]) and Koyna Wildlife Sanctuary ([43]) from NWG have reported a poor representation of Actinodaphne angustifolia, leading to the suggestion of a new sub-type of M-S-A as Memecylon-Syzygium-Olea. A recent study by Tadwalkar et al. ([90]) proposed a new type for the Amboli reserve forest area, Memecylon-Syzygium-Diospyros. Contrary to the standard definition of the floristic series, a few studies above considered only dominant and co-dominant species when determining the sub-types. The dominant and co-dominant species were obtained from the Importance value index (IVI) of quantitative studies, which is the combined relative value of density, frequency, and dominance (basal area) of each species.

The vegetation/community type studies conducted in NWG showed that floristic series are different at the local scale. However, many fragments, valleys, and other protected areas occupied by semi-evergreen and deciduous forests are yet to be described in the NWG region. A more holistic approach to all other forest types in the NWG can lead to the precise identification of various vegetation types and subtypes.

Tree diversity, community structure and composition

Earlier floristic records, such as checklists and floras were not intended to collect quantitative data and did not assess the diversity, community structure and composition of local or regional areas. One of the early quantitative tree ecological works by Satyanarayan & Mudaliyar ([86]) carried out at Matheran, showed that Syzigium cumini and Syzigium heyneana were dominant species. Memecylon umbellatum was ranked fifth in terms of its dominance and was known as a fast-growing secondary species in open canopy areas.

Watve et al. ([98]) investigated forest fragments in the Mulshi region of NWG using the transect cum quadrat method for studying the structure and composition. They recorded 52 species and a Shannon diversity index of 2.1 to 3.83; however, the species richness was low (12-20) at a local level. Though species diversity and dominance were comparatively lower in the other climax forests, a fair composition of evergreen species was observed in those forest pockets. To summarize, the quantitative studies carried out from 1960 to 2020 in NWG used various sampling methods, including random quadrats and transects of various sizes; the studies recorded 49-107, 444-4200 individuals with a Shannon diversity of 0-3.83. The basal area ranged from 6-72 m² ha^-1 (Tab S3 in Supplementary material).

Studies on the regeneration of forests are limited in NWG. Some of them are species-specific or qualitative observations ([93]) on the regeneration of secondary forests. However, regeneration status across different forest types of NWG is currently understudied.

Protected areas

In India, various protected areas and other forest types have undergone extensive studies to evaluate their vegetation diversity, composition, dynamics, and impact of human disturbances ([59], [85], [94], [58]). The northern part of the Western Ghats and Konkan region alone harbour 27 Protected Areas (PA) spanning 4804 km² (Tab. 3). While qualitative studies classified forest types in some PAs, quantitative studies are fewer and more scattered compared to the Southern Western Ghats. A study using a belt transect at Chandoli National Park reported around 109 woody species and a new forest subtype ([47]). Similar research was also conducted in Koyna Wildlife Sanctuary ([43]) and Amboli Reserve Forest ([90]). Comparative account are given in Tab S3 (Supplementary material).

A few recent studies have explored a variety of ecological questions related to woody species in the Western Ghats. Watve et al. ([97]), using the transect cum quadrat method, investigated the seed dispersal mechanism in tree species from Mulshi and showed the dominance of zoochory in 108 species that were documented. Tadwalkar et al. ([91]) examined the dispersal mode of 185 woody species from various protected areas, such as Koyna WLS, Chandoli NP, Radhanagari WLS, Phansad WLS, and Mahabaleshwar Ecological Sensitive Area and observed that the dispersal of zoochorous species is affected by disturbance.

Community protection or Sacred groves

Sacred groves (SG) are traditionally conserved patches of forests managed by local people and dedicated to a deity ([27]). These groves have been protected for long periods of time. They are subjected to various taboos prohibiting the cutting of trees or hunting of animals, as it is believed that it may incur the deity’s wrath upon the village. These forests often contain rare and endangered species of plants, as well as giant lianas and trees ([27], [35]).

SGs are found across many parts of India, from Meghalaya in the northeast to Rajasthan in the west and various regions of WG ([27]). The WG is an abode of SGs, mainly distributed in mosaics of human habitation or agricultural lands or attached to long tracts of forest. Studies have been conducted to investigate the diversity of these sacred groves ([62]), as well as their socio-economic implications ([14]). Comparisons of the diversity of sacred groves to other landscapes in SWG have also been explored ([1]).

Maharashtra is home to a number of sacred groves, ranging from 233-2800 in an area of 3570 hectares, mostly concentrated in the NWG and Konkan regions ([29], [20]). There has been considerable research conducted on the socio-ecological ([7], [70]), floristics, conservation, vegetation, and ethnobotanical aspects of these sites ([35]). There have also been some local studies conducted on their phytosociology ([32]), carbon-biomass ([40], [50]), and degradation ([52]).

  Anthropogenic disturbances and forests of NWG 

Anthropogenic disturbances in tropical forests around the world have become a pressing concern, posing significant threats to biodiverse ecosystems, which are diverse and impactful across the world. One of the major threats of tropical forests is selective logging, resulting in structural degradation, leading to alteration of ecosystems, and depletion of biodiversity ([22], [54]). Additionally, mining activities, including gold mining ([88]), open-pit mining activities ([75]), and the development of plantation and crop monocultures ([21]) have been shown to drive extensive deforestation, posing significant and potentially underestimated risks to tropical forests worldwide.

Despite affecting biodiversity, these human-caused activities have substantial consequences for the carbon cycle and climate change. For example, tropical forests that have undergone deforestation are recognized as a continuous net carbon source to the atmosphere, thereby contributing to carbon emissions, despite their initial status as carbon sinks ([55]). In addition, there is a lack of documentation regarding the carbon deposited in deadwood along a landscape-scale degradation gradient in tropical forests, suggesting that carbon stocks in degraded tropical forests may be underestimated ([72]).

The majority of threats identified on a global scale align with observations in the Western Ghats. Recognizing the Western Ghats as biodiversity hotspots emphasizes both their rich diversity and the detrimental impact of human activities on the environment ([56]). These activities, including shifting cultivation, cutting, lopping, periodic fires, construction (road and dam) for hydroelectric projects, and mining, have led to the significant destruction of forest cover in the region ([81], [33], [19]). In the Southern Western Ghats, Parthasarathy ([66]) documented a moderate degree of anthropogenic influence on forest structure and species richness along elevation and slope. Muthuramkumar et al. ([57]) identified disturbances as a major factor responsible for the negative influence on floristic structure and composition in the fragments of tropical wet evergreen forests. Anthropogenic disturbances have a profound effect on evergreen forests in the coastal areas of the Central Western Ghats, resulting in the local extinction of evergreen species and vegetation changes in secondary forests, and such forests also show less carbon sequestration potential ([81]).

Though NWG is a natural mosaic of forest fragments and human-dominated open habitats like outcrops and grasslands, limited studies have been conducted to investigate the effects of degradation on vegetation composition. One such isolated study in Tansa Wildlife Sanctuary reported a decrease in biomass, tree height, and density due to human disturbance ([95]). However, this study was based solely on observations and did not quantify the disturbance. A local study in the Mulshi area of NWG showed that despite the fragmentation, a patch of forest could harbor a large diversity of plant species ([48]). A study investigating the effect of disturbance on the dispersal mode of woody species showed a decreasing trend in zoochory in disturbed areas ([91]). Watve ([96]) and Watve et al. ([98]) studied forest fragments in the Mulshi region of NWG to analyze the impact of fragmentation on species richness, community dominance, and dispersal. In a recent study, Page & Shanker ([63]) investigated the effect of the environment on the species composition of woody evergreen plants across the latitudinal gradient of WGs, including a few forest sites from the NWG. However, the influence of the environment on woody species from other forest types in the NWG is yet to be explored.

Community-protected areas like sacred groves are more prone to human intervention, yet very scanty research is available on tree species and disturbance in NWG. Kulkarni et al. ([52]) showed that diversity decreases with increasing disturbance in a study conducted on 15 sacred groves of NWG.

  Remote sensing and GIS studies 

Leading-edge methodologies such as satellite remote sensing images and Geographical Information Systems (GIS) have revolutionized vegetation mapping studies worldwide. This rapid and cost-effective technique proves invaluable for mapping vegetation types and monitoring changes ([60]). In India, various attempts have been made to classify vegetation, monitor land use patterns, and estimate carbon sequestration potential ([44], [83], [84], [46], [80]). Joshi et al. ([44]) used phenological physiognomic vegetation type and multi-temporal satellite data to identify 35 vegetation classes, including 14 forest cover types in India. Similarly, Reddy et al. ([83]) characterized 29 land use/land cover classes with 14 forest types and presented the percentage of vegetation cover across forest types and biogeographic zones. Roy et al. ([84]) studied vegetation mapping for nearly one and a half decades using remote sensing. This research resulted in the classification of around 100 vegetation types, which were compared with vegetation maps from global studies and made publicly available ([84]).

In another GIS-based study focusing on the WG, Ramachandra & Bharath ([80]) studied carbon sequestration and revealed that the forests of the WG region hold an estimated 1.20 MGg of carbon. There have been several other regional and local GIS-based studies with similar objectives, such as vegetation mapping, forest cover change, biomass, and fragmentation, across the WG ([45], [65], [18], [46], [41], [42]). Using remote sensing data, Ramachandra & Bharath ([80]) found that there had been a 5% and 10% loss of evergreen and intact forests, respectively.

Panigrahy et al. ([65]) conducted a forest cover change investigation in the NWG of Maharashtra over the period 1985-2005 and reported a yearly reduction of 0.5% in dense forest cover due to anthropogenic pressures. Kale et al. ([45]) investigated the potential corridors and forest fragments adjoining protected areas in the nine protected areas of NWG. Das & Singh ([18]) focused on forest diversity in four districts of NWG, estimating a total of 95.2 M tons of biomass from some vegetation types. More recently, Jaybhaye et al. ([41], [42]) conducted local studies in the Kas-Panchgani plateaus and the Harishchandragad Wildlife Sanctuary, respectively, and documented the loss of forest cover in the NWG over time.

Privately protected forests are an important source of refuge for biodiversity. Private forest refers here to “revenue lands that are privately owned, not used for agriculture, horticulture, construction, water, or other non-forest activities, and support some form of standing tree growth of native species or are capable of doing so with adequate protection and/or rehabilitation” ([53]). Studies on private forests in the NWG are few and far between. Patel & Agoramoorthy ([69]) highlighted their importance in the Ratnagiri region of the NWG, while Kulkarni & Mehta ([53]) conducted a comprehensive study on private forests in five districts of the NWG, using remote sensing and GIS to track the extent, distribution, and dynamics of land use, and provide strategies for conservation and identify biodiversity corridors.

  Future prospects and conclusion 

In conclusion, the forest fragments of the northern Western Ghats are understudied compared to the southern and central regions. Although there have been a few studies on woody species diversity, composition, structure, and dispersal mode from some specific locations and regions, there is a need for further research into other unexplored regions of NWG. In particular, quantitative methods are needed to study the tree diversity and composition, and a large-scale investigation into all forest types of NWG is necessary to validate the earlier classification. By doing so, we can gain a better understanding of the floristic series of NWG. Additionally, studies should focus on forest regeneration, carbon sequestration, effect of physical and climatic factors on vegetation across forest fragments of NWG and compare the tree diversity and disturbance across different protection regimes, such as Protected Areas, Reserved Forests, Community-protected areas (sacred groves), and Private Forests. By doing so, we can better manage and conserve these forests for the continued benefit of mankind.

  Acknowledgements 

MD and BS conceptualized the work. BS, AK and SV carried out manuscript writing. All authors reviewed and edited the manuscript.

BS, AK, SV, and MD are thankful to Director, Agharkar Research Institute, Pune, India for providing facilities for research. The authors are also thankful to Dr. Karthick Balasubramanian, Dr. Aparna Watve, Dr. Radhika Kanade, and Dr. Utkarsh Ghate for fruitful discussions on the current topic. This work was supported by ARI in-house project BD-01. BS is grateful for fellowship support from Chhatrapati Shahu Maharaj Research, Training, and Human Development Institute (SARTHI), Government of Maharashtra, and SV to the Council for Scientific and Industrial Research (CSIR), Government of India. The support from the Maharashtra Forest Department is also acknowledged.

  References

  Supplementary Material

Tab. S1 - Forest types occurring in Northern Western Ghats (NWG) and their details according to Champion & Seth (1968) with addition of latest literature and own observations.
Tab. S2 - Vegetation type applicable to forests of NWG according following studies.
Tab. S3 - Overview of quantitative studies from NWG.