Abstract
Restoration of forest landscapes is a significant worldwide concern, and in recent years natural regeneration has been promoted as a practical means of accomplishing such goals. This study evaluates natural regeneration as a forest restoration strategy in the central Himalayan region of India, which has a history of forest disturbance. The study targeted four dominant forest communities (Sal, Chir-pine, Banj-oak, and Mixed-oak) distributed over an elevation gradient of 300–3000 m MSL and comprising over 62% of the total forest cover. First, we conducted a field investigation of forest stands to determine landscape structure and forest restoration outcomes, followed by an assessment of the regeneration status of individual trees, stand-level abiotic and biotic factors influencing forest regeneration, and the survival pattern of seedlings. Second, to present a pan-regional picture, we completed a thorough review of recent studies on the regeneration of other forests with a comparable structure. With mature tree, sapling, and seedling densities of 652 to 884 ha–1, 140 to 368 ha–1, and 176 to 3086 ha–1, respectively, across all stands, we recorded a total of 9,166 individual trees of 49 different tree species, which is comparable to similar stands. An assessment of regenerating individuals revealed that all stands maintained heterogeneity of native and new species, thus conferring good ecosystem resilience. The dominant species contributed significantly to forest regeneration and should sustain a similar structure into the near future. We examined seedling survival and possible causes of mortality, which indicated a need to control biotic and abiotic stressors across stands to maintain biodiversity and restoration outcomes. The study suggests that encouraging the conservation of forests for better regeneration, educating residents, and encouraging widespread forest stewardship might result in cost-effective landscape restoration.
- girth-class distribution
- landscape restoration
- natural regeneration
- policy recommendations
- seedling survival
Restoration Recap
The Himalayan region is of global interest, but the restoration of its forests poses a significant challenge because of the region’s steep terrain and inaccessibility. Based on a detailed investigation of central Himalayan forests, this study highlights the potential for natural regeneration to serve as a cost-effective strategy for land restoration.
This study provides field-based evidence on the dynamics of natural regeneration related to different forest types, distribution of native and new species, climatic and edaphic conditions, biotic disturbances, and ecological resilience of forests that could lead to the development of location-specific restoration strategies across the Himalayan landscape and India’s central Himalayan region in particular.
The findings are of importance to policy makers, land managers, practitioners, applied researchers, students, and volunteers seeking to understand the social and ecological aspects of natural systems to inform land management.
Forests are crucial for maintaining biodiversity, sequestering carbon, reducing climate change, and enabling ecosystems to function properly (Criddle et al. 2003, Brown et al. 2007, Diaz et al. 2018). Deforestation and forest degradation, however, have been accelerated by using forests for economic and subsistence purposes (FAO 2020). Forest restoration requires a wide range of costly resources, thereby presenting a difficult challenge (Shi et al. 2014, FAO and UNEP 2020). Maintaining native species composition and natural regeneration is desirable to meet sustainable forest management goals (Zhang et al. 2015, Hanief et al. 2016, Chen et al. 2018, Siminski et al. 2021). Recent studies have shown that fostering natural regeneration is a cost-effective strategy for ecological restoration (Chazdon and Guariguata 2016, Uriarte and Chazdon 2016), and a better understanding of natural forest regeneration can inform decisions related to forest protection and restoration (Chokkalingam et al. 2018). However, our knowledge of natural regeneration as a possible management tool is limited (Winter 2012, Crouzeilles et al. 2017). To enable cost-effective landscape restoration, practitioners need a better understanding of forest regeneration and seedling establishment (Lof et al. 2019).
India ranks as the tenth most forested country in the world, with a forest cover of 767,419 km2, or 23.34% of the land area (FSI 2019). The country also has 964,000 km2 of degraded land, or 29.32% of its land area (SAC 2016). India has made global commitments to restore deforested and degraded landscapes (Sundriyal and Dhyani 2015). The government has launched the Green India Mission as part of its National Action Plan on Climate Change. The mission’s goals are to protect and enhance existing forested areas, restore degraded land and declining woodlands, and address climate change (MoEF 2012). The country plans to increase forest cover on 50,000 km2 of forested and non-forested lands along with improving the quality of forest cover on an area of equal size to improve biodiversity and ecosystem services; however, this approach involves significant costs (MoEF 2016).
The Indian Himalayan Region (IHR) comprises 9% of the land area, 36% of the forested land, and 44% the biodiversity of India (Sundriyal et al. 2019). The region generates vital ecosystem services to communities both in the region as well as further downslope (Singh 2007). However, studies conducted from 2017–2019 indicated that significant areas in many states are being degraded and need immediate restoration. For example, Uttarakhand state includes nearly 11% of the total forest cover of IHR. Of the total forest cover (24,303 km2) nearly 20% is very dense forest, 53% is moderately dense forest, and 27% is open forest. Furthermore, 383 km2 is scrubland, while 13,313 km2 of forests have poor cover (FSI 2019). The data revealed that there is significant pressure on the forests of Uttarakhand (Singh and Singh 1992, Joshi and Negi 2011). In the Himalayan region, ecological restoration of degraded forests poses a significant challenge in view of intense human disturbance, rough terrain, inaccessibility, and high costs (Mir et al. 2017, Upadhaya et al. 2018, Paul et al. 2019).
Ecological restoration of degraded lands is a vital step toward conserving biodiversity and mitigating climate change (Chazdon and Uriarte 2016, Brancalion et al. 2019, Rozendaal et al. 2019, Crouzeilles et al. 2020). There are four major approaches to landscape restoration: active restoration, passive restoration (natural regeneration), rehabilitation, and reclamation (Crouzeilles et al. 2017). Active restoration, the preferred approach across the Himalayan landscapes, involves direct seeding, raising tree seedlings in nurseries and planting on targeted sites, and undertaking management and thinning operations to accelerate recovery (Sundriyal and Sharma 1996, Sharma et al. 2014, Joshi and Chandra 2020). In comparison to active restoration, natural restoration may be a cost-effective approach that not only promotes recovery of native tree species but also restores ecological processes, biodiversity, and ecosystem services (Chazdon and Guariguata 2016, Mir et al. 2017, Upadhaya et al. 2018, Paul et al. 2019). The success of restoration, however, is constrained by a lack of understanding regarding the regeneration of natural systems in connection to elevation, slope, aspect, and previous restoration efforts in the Himalayan area. To accelerate natural regeneration, it is crucial to assess species’ potential in terms of their diversity, provenance, growth, survival and mortality at the seedling and sapling stages (Crouzeilles et al. 2017, Tiwari et al. 2018, Chazdon et al. 2020, Siminski et al. 2021).
With a better knowledge of the dynamics of natural regeneration in various social, economic and biophysical contexts, we argue that forest restoration based on natural regeneration could lead to a cost-effective reforestation strategy in areas undergoing deforestation and degradation (Menz et al. 2013, Latawiec et al. 2016). The present study, which relates to natural regeneration dynamics in four disturbed forest stands of the central Himalayan region of India, was conducted in this context. We addressed three basic questions: 1) what is the natural regeneration status of dominant forest communities, 2) could recovery be enhanced through assisted natural regeneration (ANR) methods, and 3) in what ways could information on natural forest regeneration be applied to large-scale restoration? We divided the study into two parts to address these questions. First, we conducted a thorough field investigation of four forest communities in the area dominated by Sal (Shorea robusta), Chir-pine (Pinus roxburghii), Banj-oak (Quercus leucotrichofora), and Mixed-oak (Q. lanuginosa and Q. floribunda). Together, these communities constitute 62% forest cover of the region. The study measured vegetation structure, regeneration status, seedling survival patterns, abiotic and biotic factors, forest floor phytomass, litter accumulation, invasive species, and restoration history of these stands. Second, we completed a thorough review of recent research on the regeneration of comparable forest types to present a pan-regional picture of regeneration. The study’s findings are an important addition to the existing knowledge on the subject, and provide guidance regarding situations where it may be necessary to compare costly active restoration with natural regeneration.
Location of the forest stands investigated in the central Himalaya, India. A) Location of Uttarakhand state in northwestern India, B) Nainital District of Kumaon Division, C) Elevation gradient map and location of forest study sites in Nainital District.
Methods
The present study was conducted in Uttarakhand state in the central Himalayan region of India encompassing an area of 53,483 km2. Of that area, 63.41% is forested, 11.68% net area sown, 0.96% is current fallow (agricultural land kept fallow during the current year), 1.44% is fallow land other than current fallow (temporarily out of cultivation for a period of not less than one year and not more than five years), 5.29% is culturable wasteland, 6.47% is land under miscellaneous tree crops and groves, 3.21% is permanent pasture and other grazing lands, and 7.54% is land not available for cultivation (FSI 2019). The forest stands we evaluated are located in the Nainital district (28°58′31.84″ N to 29°36′45.19″ N; 78°51′11.34″ to 79°58′23.06″ E longitude).
The Indian Forest Act 1927 classified forests into three broad categories. Of the forested land in Uttarakhand state, nearly 70% is categorized as reserve forests (forests under full control of the government and safeguarded to protect the abundant resources therein), 26% as protected forests (all forest-related activities are permitted except those expressly prohibited), and 4% as unclassed forest (e.g., community forests, and privately and publicly owned properties).
Over 15 million people reside in the state; they are supported by an agrarian and pastoralist economy. There is a significant dependence on forests for firewood (4,075,981 tons/annum), fodder (32,118,736 tons/annum), small timber (38,801 m3/annum), and bamboo (2,427 tons/ annum) (FSI 2019). Nearly one-third of the total forest area, mainly dominated by Chir-pine, is fire-prone. Government policy dictates that nearly two-thirds of each mountainous state should be under forest cover. However, the forest cover in Uttarakhand declined by 205 km2 between 2013 and 2019. The decline was accompanied by a significant increase in open forest and scrubland (FSI 2013, 2019).
For this study we investigated the four prominent forest types of Uttarakhand state in central Himalaya (Figure 1) characterized by the predominant tree species, viz., Sal (Shorea robusta Gaertn.), Chir-pine (Pinus roxbughii Sarg.), Banj-oak (Quercus leucotrichophora A. Camus), and Mixed-oak (Quercus lanuginosa Lam. and Q. floribunda Lindl. ex A. Camus). These forests are distributed over an elevation range of 300 to 3000 m and collectively comprised 62% of total forest area of the state (Thadani and Ashton 1995, Baduni and Sharma 2001, Rawat and Vishvakarma 2011, Joshi and Chandra 2020). All selected forest stands were managed by the Forest Department. The Sal and Chir-pine stands are located in a sub-tropical zone, whereas the Banj-oak and Mixed-oak stands occur in a temperate zone. The Sal forest dominates the Siwaliks and Tarai hills (elevation 300 to 900 m above MSL) and covers over 4,175 km2 (17.86% of the total forested area of the state). The Chir-pine forests dominate low-mid hills (elevation 800 to 1600 m above MSL) and cover an area of 6,900 km2 (28.17% of the total forest cover of the state). Banj-oak dominates mid-elevation hills and encompasses 13.86% area of the forest, while Mixed-oak woodlands cover < 2% of the forest area (FSI 2013, 2019).
All the stands we investigated experience varying degrees of persistent disruption, from which they are constantly recovering (Joshi 2023). The Sal stand has a history of logging over 50–60 years during which the stand was cleared and replanted with Sal to generate revenue. Over the years, activities related to thinning and natural regeneration were implemented to achieve the desired species density. The forest is mature now and will be subjected to commercial harvesting in the near future. The Chir-pine stand has a history of intense disturbance from recurring forest fires, grazing, logging, and firewood and grass collection because of its proximity to villages. Several attempts have been made in the past to restore the site through plantings, but success has been limited. The Banj-oak stand has been subjected to wood collection for firewood, charcoal, and making agricultural implements, minor timber harvesting and grazing. Despite these disturbances, forest growth is good. In the past, some efforts to plant additional species generated mixed results. Because the Mixed-oak stand is located away from villages, there has been minimal pressure on this stand. However, the soils of both the Banj-oak and Mixed-oak stands are characterized by thick layers (3–6 cm) of litter and humus that hinder seed germination.
Climatic data during the study period for the subtropical region (Sal and Chir-pine stands) indicates an average annual rainfall of 1,728 ± 182 mm. Minimum temperature ranges between 7 to 23°C (mean 15.53 ± 5.56°C), and maximum temperature ranges between 20 to 38°C (mean 29.11 ± 5.69°C). For the temperate region (Banj-oak and Mixed-oak stands) mean annual rainfall is 2,100 ± 158 mm, minimum temperature ranges from 5 to 21°C (mean 14.47 ± 5.90°C), and maximum temperature ranges between 16 to 32°C (mean 24.25 ± 4.61°C). Between 78–84% of total rainfall occurs during the rainy season (June to September). The soil is an acidic sandy loam in the Krol series (Valdiya 1980) with varying chemical composition across various sites. Geologically all sites fall in the lesser Himalayan region characterized by low-quality metamorphosed and pyrogenous rocks.
Assessing Landscape Structure for Forest Restoration Outcomes
We undertook a structural analysis of four forest stands to elucidate key landscape features in terms of tree species composition, distribution, dominance, and diversity. For this, we used the random quadrat sampling method. We prepared forest grid maps of each stand, assigned a random number to each quadrat, and used a random number generator to select quadrats for sampling. We sampled fifty 10 × 10 m quadrats in each stand to cover broad physiognomy, tree species combinations, slopes, and aspects. We tallied the tree species and number of individuals occurring in each quadrat as well as the trees’ circumference at breast height (dbh, 1.3 m above ground level). All individuals with dbh > 10.1 cm were categorized as trees. All quadrat data were further analyzed for detailed phytosociological parameters (species richness, frequency, density, basal area, importance value index) (Sundriyal and Sharma 1996). Furthermore, to depict and interpret population dynamics we also measured girth class distribution of different tree species by measuring tree circumference (Sundriyal and Sharma 1996) and then distributing all individuals in the relative proportion of seedlings (height < 20 cm), saplings (height > 20 cm and dbh < 10 cm), and adult trees in different diameter size classes (i.e., dbh 10.1–20.0; 20.1–30.0, 30.1–40.0, 40.1–50.0, 50.1–60.0, 60.1–70.0, and >70.1 cm) (Saha et al. 2016). The data present a comparison of the relative distribution of the entire range of individuals in distinctive girth classes.
Regeneration Status of Individual Trees
We undertook an investigation of the natural regeneration of tree species across all stands. All individuals < 10 cm dbh were considered regenerating individuals (Khan et al. 1987, Deb and Sundriyal 2008). To inventory regeneration potential (i.e., number of seedlings and saplings), each 10 × 10 m quadrat was further divided into four 5 × 5 m sub-quadrats. One sub-quadrat in each quadrat was sampled randomly to assess regeneration potential. Individuals with a dbh < 10 cm and height > 20 cm were categorized as saplings, while those with a height < 20 cm were considered seedlings (Saha et al. 2016, Singh et al. 2016). Regeneration status was classified into the following five categories (Shankar 2001): 1) “Good,” if seedlings > saplings > adults; 2) “Fair,” if seedlings > saplings < adults; 3) “Poor,” if the species was present only as saplings but not seedlings; 4) “No regeneration,” if a species occurred only in mature form; and 5) “New,” if there was an absence of adult trees and the species was present only as seedlings and/or saplings.
The diversity of forests was assessed using two common species diversity indices: the Shannon Index (Shannon and Weaver 1963) and the Simpson Index (Simpson 1949). The Shannon diversity index favors rare species, while the Simpson diversity index favors more abundant species. We also assessed species richness using Margalef ’s Index and Menhinick’s Index; the former articulates a functional relation between the number of species and the total number of individuals, while the later states the ratio of the number of taxa to the square root of sample size (Magurran 2004). Margalef ’s Index is calculated as (S – 1) / ln N, where S = total number of species, N = total number of individuals in the sample, and ln = natural logarithm. Menhinick’s Index is calculated as S / √(N), where S = number of species recorded, and N = total number of individuals in the sample.
Seedling Survival and Mortality
For investigating seedling survival, we placed three random plots (50 × 50 m) in every stand and tagged the seedlings of dominant and commonly found tree species with aluminum foil (Khan et al. 1986, Deb and Sundriyal 2008). A total of 1,646 seedlings from nine dominant species were tagged across all stands to evaluate seedling survival. These sites were repeatedly visited at 2-month intervals for one year to assess the status of all tagged seedlings. Possible causes of mortality were also assessed for dead seedlings. To assess recruitment status of native species, we also calculated the species turnover rate:
Comparison of Tree Density and Regeneration Capability
To develop a pan-Himalayan picture of tree density, total basal area, and sapling and seedling density across diverse Himalayan forests, a comparative analysis of these parameters was undertaken with published literature on Sal, Chir-pine, Banj-oak and Mixed-oak forests of the central Himalaya encompassing 16, 15, 40, and 18 forest stands, respectively (Champion and Seth 1968, Chaturvedi and Singh 1987, Rawat and Singh 1988, Rana et al. 1989, Singh and Singh 1992, Bargali et al. 2014, Gosain et al. 2015, Adhikari et al. 2017, Manral et al. 2018, Verma and Garkoti 2019, Mittal et al. 2020). (For more details on published literature, see Supplementary Material Table S1).
We also assessed the effectiveness of ongoing active restoration (i.e., tree planting) activities in the state of Uttarakhand. The state’s Forest Department has been the nodal department for undertaking all forest restoration activities; therefore, we interviewed and consulted with ten select officials. We gathered information about how restoration activities are prioritized and planned, operating strategies, nursery management (e.g., plants raised), site preparation, and planted sites monitoring. The costs involved in such operations were also assessed based on the governmental scheduled rate.
Results
Forests Characteristics, Natural Regeneration, and Biotic and Abiotic Features
Details of the four forest stands we investigated, their abiotic and biotic characteristics, and other parameters are provided in Table 1. To assess the restoration potential of various stands, an analysis of species richness and structure was undertaken by comparing the tree layer with the sapling and seedling strata. A detailed profile of tree species richness and density in different forest stands was also estimated (Supplementary Material Tables S2 and S3). Analysis of species richness revealed a total of 9, 14, 15 and 15 tree species in Sal, Chir-pine, Banj-oak, and Mixed-oak stands, respectively (Figure 2). It was interesting to note that all stands maintained considerable similarity in species richness between tree species and regenerating species (seedlings and saplings). The species richness and taxonomic group similarity between the tree stratum and the sapling layer was estimated to be 86% for the Sal stand, 53% for the Chir-pine stand, 76% for the Banj-oak stand, and 78% for Mixed-oak stand. All stands maintained a reasonable turnover of native species, which was 67%, 43%, 61%, and 64% for the Sal, Chir-pine, Banj-oak, and Mixed-oak stands, respectively (Figure 3). The lower value for the Chir-pine stand was the result of intense biotic pressure and ongoing active restoration measures at this stand. We recorded 11%, 14%, 8% and 7% of plants as new species in the Sal, Chir-pine, Banj-oak, and Mix-oak stands, respectively. We recorded no regeneration for 22%, 43%, 31% and 29% of the tree species at the same forest stands, respectively, (Figure 3, Supplementary Material Table S2).
In terms of structure, the percentage distribution of densities in the seedling, sapling, and tree strata was 75, 7 and 18 at Sal; 68, 5 and 27 at Chir-pine; 17, 20, and 67 at Banj-oak; and 17, 24 and 59 at Mixed-oak stand, respectively (Figure 3). The fact that the subtropical forests (Sal and Chir-pine) supported a fair number of seedlings may be due to year-round better growing conditions at these sites (Figure 3).
Biotic and abiotic characteristics vary with stands (Table 1). All stands had a disturbance and management history. The stands also differed in accumulation of forest floor phytomass and litterfall. The temperate sites contained more organic matter and humus than the sub-tropical sites, which may have limited seed germination and seedling growth. All stands contained invasive species (especially the Chir-pine stand), further contributing to biotic challenges to the forests. Stands also varied in soil texture, moisture, and chemical composition. All these factors could affect regeneration.
The regeneration performance of individual species varied remarkably with sites (Figure 4). For all stands, 4% of species recorded good, 52% fair, and 2% poor regeneration. In the Sal stand, S. robusta, Mallotus philippensis, Terminalia alata, Syzygium cumini and Tectona grandis showed fair regeneration; Bauhinia variegata was present as a newly regenerating species, while Pterocarpus marsupium and Ehertia laevis showed no regeneration (Supplementary Material Table S3).
In the Chir-pine stand, P. roxburghii, Myrica esculenta, Toona ciliata, Cedrus deodara, and Q. leucotrichophora were recorded in both the sapling and seedling strata, while Syzizum cumini, Bauhinia variegata, Acacia spp. and Coculus spp. were present either as saplings or seedlings only. In contrast, Lyonia ovalifolia, Symplocos ramosissima, Pyrus pashia, Aesculus indica and Acer oblongum were present in the tree and/or sapling level. In the Banj-oak stand, Q. leucotrichophora, M. esculenta, L. ovalifolia, C. deodara, A. oblongum, P. roxburghii, Machilus duthei and Debregeasia hypoleuca exhibited fair regeneration, while there was no regeneration of Rhododendron arboreum, Cupressus torulosa, A. indica and Viburnbum cotonifolium. In the Mixed-oak stand, Quercus lanuginosa, Q. floribunda, R. arboreum, L. ovalifolia, Carpinus viminea, M. duthei, A. indica, Litsea umbrosa, and C. deodara showed fair regeneration, while M. esculenta was a newly regenerating species. Quercus leucotrichophora, Ilex dipyrina, V. cotonifolium and C. torulosa were not regenerating (Supplementary Material Table S3).
Stands details, and biotic and abiotic characteristics of four investigated forests in the Central Himalaya, India.
Tree species richness (number of tree species) in the seedling, saplings, and tree strata in the forest stands investigated in the central Himalaya, India.
Density of seedlings, saplings and mature trees in forest stands investigated in the central Himalaya, India.
Regeneration status of the four forest stands investigated: (A) Sal stands, (B) Chir-pine stand (C) Banj-oak stand, and (D) Mixed-oak stand.
Girth class structure of forest stands investigated: Girth class represents A = seedlings, B = saplings, C = small trees (dbh 10.1–20 cm), D = young trees (dbh 20.1–30 cm), E = mature trees (dbh 30.1–40 cm), F = old trees (dbh 40.1–50 cm), G = (dbh 50.1–60 cm), H = (dbh 60.1–70 cm), and I = (dbh > 70.1 cm).
Tree Girth Class Structure
The girth class structure devised for all the stands showed the relative distribution of the total number of individuals in different classes (Figure 5). Sal, Chir-pine, and Banj-oak stands exhibited reverse J-shaped distributions showing a higher number of individuals in the seedling and sapling strata that continuously decreased in dbh class 10.1–20.0 to > 70.1 cm. However, the Mixed-oak stands displayed a bell-shaped girth class distribution with a higher proportion of individuals in the middle girth classes (Figure 5). Trees with a higher percentage of recruits in smaller to moderate dbh classes than large dbh classes (reverse J-shaped distribution) showed that these populations are more stable and can regenerate into mature trees under favorable conditions. Contrarily, a J-shaped curve indicates an unstable population because species are not represented in the smaller dbh classes.
Diversity and Dominance Index
An analysis of stand diversity showed the highest species diversity in Mixed-oak and Banj-oak stands (Table 2). The diversity for different strata revealed seedlings > saplings > trees for Sal and Mixed-oak stands, and saplings > seedlings > trees at the Banj-oak stand (Table 2). Data suggested resilience and stability in these stands. However, the Chir-pine stand showed a diversity trend of trees > saplings > seedlings, indicating that forest requires immediate management to conserve biodiversity. The concentration of dominance was estimated to be greater for the Chir-pine stand than to that in all other sites. The concentration of dominance increased from seedlings to saplings to trees for all sites except for Chir-pine, where it demonstrated an inverse trend (Table 2). A higher value for the concentration of dominance for a stand with low species diversity is expected because species diversity behaves inversely to the index of dominance. Species evenness also demonstrated a pattern similar to the Shannon and Simpson indices across sites and plant growth stages (tree, sapling, and seedling) because evenness takes into account the number of species and relative abundance of species in a stand (Table 2). The Menhinic Index for various stands ranged between 0.291 to 0.470, while the Margalef Index ranged between 1.056 to 1.769 (Table 2). Higher Menhinic Index and Margalef Index values indicate greater species richness or diversity, considering the total number of individuals observed.
Diversity indices for seedling, sapling and tree strata in four investigated forest stands in the Central Himalaya.
Seedling Survival and Growth Pattern
Seedling survival varied considerably among species and forest stands. In the present study, a total of eight species were tagged for assessing seedling survival and mortality: Shorea robusta, M. philippensis and S. cumini in Sal; P. roxuurghii in Chir-pine; Q. leucotrichophora in Banj-oak; and Q. lenuginosa, Q. floribunda, Lyonia ovalifolia and L. umbrosa in Mixed-oak. Altogether 907, 350, 178 and 211 seedlings were tagged in these stands, respectively. The dominant species of each stand comprised 59%, 100%, 100%, and 63% of total seedlings tagged in the four stands, respectively. Of those, 351, 0, 95 and 87 seedlings survived after one year, a survival rate of 0 to 71% for the different forest stands. Of the three species investigated in the Sal stand, a total of 37 to 61% seedlings survived after one year: C. cumini > M. philippensis > S. robusta (Figure 6A). In the Chir-pine stand, Pinus roxburghii exhibited 74% survival through 10 months; all seedlings subsequently were lost to a forest fire (Figure 6B). In the Banj-oak stand, seedling survival for Q. leucotrichophora was 49% (Figure 6C), while in the Mixed-oak stand, maximum seedling survival was recorded for L. umbrosa (71%), followed by Q. floribunda (67%), Q. lanuginosa (64%) and L. ovalifolia (20%) (Figure 6D).
Factors leading to seedling mortality varied with stands. In Sal, the seedlings of S. robusta suffered from disturbance from selective logging, fungal infection, soil dryness, herbage collection, and herbivory (Table 3). In Chir-pine, P. roxburghii seedlings succumbed to a forest fire; such forests are prone to wildfire during winter and summer months. The Banj-oak seedlings were lost due to grazing. In the Mixed-oak stand, insect herbivory, litter collection, and browsing by wild animals caused seedling mortality for Q. lanuginosa and Q. floribunda.
The annual growth rate of seedlings of selected tree species varied from 6 cm to 15.95 cm year–1 in terms of height, and 0.87 to 2.29 mm year–1 in terms of collar diameter. Shorea robusta showed maximum growth in terms of height, and P. roxburghii in terms of collar diameter (Figure 7). The peak growth rate (height as well as collar diameter) of most of the species was recorded during rainy seasons; the greatest growth was recorded for P. roxburghii during rainy seasons (June through September). The growth of three oak species (i.e., Q. leucotrichophora, Q. floribunda, and Q. lanuginosa) did not vary significantly among species.
Relationship between Tree Density with Total Basal Area and Sapling Density
We also completed a comparative analysis of the relationship between tree density with total basal area (TBA) and saplings of all similar forests of the central Himalayan region (Supplementary Material Table S1). In Sal stands, the tree density ranged between 273 and 911 trees ha–1 (mean 582 ± 232 trees ha–1) and TBA ranged from 6.82 to 77.60 m2 ha–1 (mean 43.79 ± 26.27 m2 ha–1) (Supplementary Material Table S1, Figure 8A). Mean sapling and seedling density was 449.43 ± 334.65 and 4809.85 ± 3405.70 individuals ha–1, respectively. Table S1. In this study, the tree density and TBA values were higher than those in other similar stands, showing that the stand we investigated was more mature, but the sapling and seedling densities were lower than the average values for other similar stands.
Seedling survival percentage by tree species: A) Sal stands, B) Chir-pine stand, C) Banj-oak stand, and D) Mixed-oak stand.
Seedling growth (per year increment in height and collar diameter) pattern of selected species across all sites.
Relationship between tree density (trees/ha) with sapling density (saplings/ha) and total basal area (m2/ ha) in the central Himalaya. The solid line indicates the relationship between tree density and sapling density, while the dotted line indicates the relationship between tree density and total basal area: A) Sal stands, B) Chir-pine stand, C) Banj-oak stand, and D) Mixed-oak stand.
Causes of seedling mortality (%) as observed in field conditions across different forest stands in the Central Himalaya.
In the Chir-pine stands, tree density ranged between 300 and 935 trees ha–1 (mean 649.32 ± 369 ha–1), and TBA ranged from 27.95 to 90.85 m2 ha–1 (mean 53.49 ± 25.81 m2 ha–1) (Supplementary Material Table S1, Figure 8B). Tree density was higher and TBA was lower than the mean values of these parameters in other stands. Also, mean sapling density was estimated as 478.94 ± 290 ha–1 and seedling density as 508.17 ± 213.39 ha–1 (Supplementary Material Table S1, Figure 8B). Seedling density was higher while sapling density was lower than average values for other similar stands.
For the Banj-oak stand, tree density was 210 to 1550 trees ha–1 (mean density 721.38 ± 285 ha–1) and TBA ranged from 19.1 to 90.83 m2 ha–1 (mean 65 ± 18.66 m2 ha–1). Tree density was lower while the TBA value was higher than the average values of other similar stands. The mean sapling density of 506 ± 571 ha–1 and seedling density of 1628 ± 808 ha–1 for other similar stands was higher than the values recorded in the present study (Supplementary Material Table S1, Figure 8C). The data showed that this Banj-oak stand is an old-growth stand.
A comparison of Mixed-oak stands disclosed that the tree density ranged between 49 to 1152 trees ha–1 (mean 742.52 ± 418 ha–1), and TBA between 8.94 to 110.5 m2 ha–1 (mean 51.46 ± 42.21 m2 ha–1). Our value of tree density was higher while the TBA value was lower than that of similar forest stands. The mean sapling and seedling density for other similar stands was calculated as 516 ± 341.45 ha–1 and 1,637.36 ± 1167 ha–1, respectively (Supplementary Material Table S1, Figure 8D). Our values of sapling and seedling density were lower than the average value for similar stands.
Contemporary Restoration Approaches
An appraisal of diverse approaches for landscape and forest restoration in Uttarakhand state revealed that agencies and associated partners use active restoration, assisted natural regeneration, monoculture, and mixed tree plantings. Currently, afforestation activities are being funded under the Compensatory Afforestation Fund Act in lieu of forest land diverted for non-forest purposes to mitigate the impact of diversion of such forest land. The main actor for these activities is the state’s Forest Department. In reserve forests that are under the direct control of the Forest Department, local communities are not involved in tree planting programs, although such stands are visited by community members for collecting diverse forest resources. The Department promotes mainly commercial species that provide timber, logs, and resin to earn government revenue. There are other planting programs, such as agroforestry, agro-silvi-horticulture, farm forestry, bamboo planting, silvipasture, community forestry, etc. that are undertaken for diverse purposes and stakeholders. Joint forest management also occurs in some areas.
An analysis of the funds required for enrichment, new tree plantings, and rehabilitation of degraded areas revealed a high monetary cost (Supplementary Material Table S4). Forest restoration and rehabilitation require continuous monitoring for at least three years. Moreover, the costs do not include the expenditures for nursery propagation and plant-material transportation. In view of the hilly terrain and remote locations, the planting costs are high and, at times, monitoring of such sites is difficult. This makes planting a cost-intensive approach. The discussions with officials also revealed diverse risk factors that impact restoration. For example, high dependence of local communities on forest resources across all Himalayan regions, frequent conversion of forest land to non-forest use, natural calamities, land encroachments, forest fires, and forest-related conflicts have a high impact on restoration activities. As a result, only a small portion of degraded land is actually restored annually through active planting. Therefore, stimulating natural regeneration would not only lessen the economic burden but also support large-scale restoration.
Discussion
This study explored the significance of natural regeneration in landscape restoration by analyzing the ecological and social dynamics of four significant forest communities in the central Himalaya: Sal, Chir-pine, Banj-oak, and Mixed-oak stands. Species richness, composition, density, and girth class distribution were some of the ecological variables influencing natural regeneration. Rainfall, temperature, geology, and edaphic characteristics were among the abiotic variables. Human reliance on forest resources also influenced stand character and regeneration. The diversity and composition of the tree species varied among stands, but the dominant species control the structure of the forest in all strata. Differences in elevation, soil, age structure, successional stage, forest types, and maturity of the trees may be the cause of site-specific variance in structure, composition, density, and basal area among the sites (Saxena and Singh 1982, Ralhan et al. 1982, Rana et al. 1989, Sundriyal and Sharma 1996, Kumar and Ram 2005, Deb and Sundriyal 2011, Joshi and Tiwari 2011, Maren et al. 2014, Sarkar and Devi 2014, Sharma et al. 2014, Prasad et al. 2015, Khaine et al. 2017, Joshi and Chandra 2020).
The natural regeneration status offers crucial information regarding the potential future composition and structure of a stand. The dominant tree species, biophysical conditions, and human activities at each stand significantly affect these variables, and there was an increasing trend in species richness in the seedling and sapling strata with elevation (Rawal et al. 2012, Prasad et al. 2015). There was a considerable taxonomic and floristic similarity as well as turnover of early and late successional species; we observed between 43–67% of native species regenerating across all sites, and between 8–14% of all species were new. New species in a stand increase levels of diversity and provide a wider array of ecosystem services (Siminski et al. 2021). The percentage distribution of seedlings, saplings, and trees was consistent with past findings for the Sal (Rana et al. 1989, Kapkoti et al. 2016) and the Chir-pine stands (Chaturvedi and Singh 1987, Singh et al. 2014, Mittal et al. 2020). However, both the temperate forests (Banj-oak and Mixed-oak) had fewer seedlings and saplings (Singh et al. 2009, Singh et al. 2016, Saha et al. 2016, Mittal et al. 2020, Dhyani et al. 2020). Our findings agree with published data. Litter and humus accumulation in the temperate forests makes it difficult for seeds to reach mineral soil (Singh and Singh 1992, Humphrey and Swaine 1997, Harmer et al. 2005). Furthermore, due to decreased light and a depleted soil seed bank, the dense canopy has an influence on regeneration (Dupuy and Chazdon 2006). Except for Q. floribunda, which is tolerant of shading, oaks are outcompeted by more shade-tolerant species under intact oak canopies (Bargali et al. 2005). The severe physical environment and other factors also hinder Q. leucotrichophora from regenerating (Singh and Rawat 2012, Singh et al. 2016).
The girth class distribution pattern of various species reveals the capacity of species to reproduce and survive under a range of environmental circumstances and biotic stresses. The dominant species in each stand were represented in all girth classes. A few species were represented with discontinuous regeneration and showed their presence either in trees or in a regeneration stage only; such species should be monitored for their presence in stands soon.
Stand diversity ranged from 1.5 to 3.5, quite comparable to other forests in the central Himalaya (Pokhriyal et al. 2010, Bargali et al. 2013, Malik and Bhatt 2016), though lower than what has been reported for tropical forests (Sarkar and Devi 2014). The Simpson diversity index values for the various strata are consistent with those reported for various tropical forests (Deb and Sundriyal 2011, Kushwaha and Nandy 2012, Sarkar and Devi 2014). The concentration of dominance is inversely related to diversity; therefore, forests with high diversity showed low dominance values (Bargali et al. 2013, Sarkar and Devi 2016). Species evenness followed a pattern similar to the Shannon and Simpson diversities across sites and plant growth stages (i.e., tree, sapling, and seedling). A higher evenness value shows more consistency in species distributions. It is reported that species richness, abundance, and seedling populations decline with tree canopy closure and maturity of forest stands (Harmer et al. 2005). Forests with higher species richness, density of trees and regenerating individuals, diversity, and dominance of lower girth classes are good indicators of forest health, and such stands exhibit greater resilience and stability.
The survival strategy of a species is directly responsible for the future composition of a forest, which was monitored by assessing the survival of seedlings over a limited period. Both biotic and abiotic factors influence seedling survival and mortality at different stands. For example, seedlings and saplings in the Chir-pine stand were susceptible to frequent forest fires. Other common external stresses that impacted the survival of seedlings included trampling, gathering of litter and fodder, and grazing. Early seedling mortality is also brought on by microsite effects (e.g., forest gap size, light availability, humidity, temperature), infection, and competition (Singh and Singh 1992, Holz and Placci 2005, Tyler et al. 2006, Tripathi and Khan 2007, Malik and Bhatt 2016).
Seedling survival patterns help in identifying speciesspecific recovery patterns in forest stands. For example, the dominance of saplings and seedlings of dominant tree species like S. robusta (Sal stand), P. roxbughii (Chir-pine stand), Q. leucotrichophora (Banj-oak stand), and Q. lanuginosa and Q. floribunda (Mixed-oak stand) indicated that these species will continue to dominate the stands in the near future, although they could be affected by site-specific biotic and abiotic processes such as soil aridity, winter frost, and herbivory (Singh and Singh 1992, Kushwaha and Nandy 2012). Despite having fewer seedlings and saplings, the data showed that the forest stands would continue to generate enough seedlings and saplings when gaps develop, thus maintaining similar forest structure into the near future as well.
Implications for Landscape Management
The data on species composition, forest structure, and diversity of various stands depict the status of various forest types along with the effect of natural regeneration, providing relevant information on future forest dynamics. Such information offers guidance for managing the sites and encouraging natural regeneration-based restoration. This study reveals that all stands should maintain native genetic diversity, which will sustain their functional traits, ecosystem resilience, and services (de Rezende et al. 2015). There were enough mature trees that could produce seeds that could germinate when canopy gaps become available, so even a low number of seedlings and saplings in a forest stand is not a major concern. In addition to colonizing forest gaps, sal and oak species also thrive under the closed canopy through root sprouts (Singh and Singh 1992). Even at low tree density and after being lopped for fodder, oak species may still produce a lot of seeds and seedlings (Upreti et al. 1985, Bargali et al. 2014). By reducing biotic pressure, such forests can regenerate, although the recovery process may differ depending on the degree of degradation. We suggest that forests with moderate densities need protection from human interference to optimize community structure. However, in high-density forests, appropriate thinning, manual regeneration and open gaps are needed to improve the light environment and nutrient availability, thereby promoting natural regeneration.
The knowledge acquired from this study and from our literature review supports natural regeneration as a possible and practical landscape restoration technique. By promoting natural regeneration, stands with high seedling production can be regenerated without assistance. To enable forest recovery, areas with sufficient rootstock (such as sal and oak stands) can simply be conserved and protected. Active restoration based on the soil seed bank may be beneficial in stands with limited regeneration or inadequate seed sources, although it may take more time (Löf et al. 2019). However, restoration efforts may be hampered in places without forests or in locations that have been severely degraded. An active restoration strategy on such sites, which includes soil treatment and the planting of pioneer and secondary species, can have positive effects (Chazdon 2008, Chazdon et al. 2020). Restoration outcomes may be enhanced by choosing appropriate species that promote rural economies and livelihoods.
It is important to remember that the Himalayan region still has forests, even though in many places they are constantly disturbed. To facilitate natural regeneration as an effective technique for landscape and forest restoration, we suggest the following research-and-development and policy-related recommendations.
First, information on the regeneration status of various tree species should be utilized in selecting the type of restoration procedure. This requires a better understanding of the dynamics of natural regeneration in relation to forest types, cover, composition, biotic and abiotic factors, etc. The colonization of native species and the opportunistic emergence of new species are encouraged by natural regeneration. Natural regeneration will support the maintenance of forest stands’ biodiversity and species composition. Learning from past restoration projects, the species planted, their successional status, and the strategies employed will further improve our understanding of restoration actions.
Second, because site-specific biotic variables may pose a risk to forest restoration and have an influence on the restoration process, it is highly desirable to address these risk factors. All biotic variables, such as forest fire; grazing; trampling; extraction of fodder, firewood and litter; land conversion; encroachment; and forest-related conflicts need to be managed and regulated in order to promote natural regeneration as a way of restoration. The restoration process can be aided by selective weeding, the spatial distribution of species, and enrichment planting. Many of these problems can be resolved by monitoring and constructing enclosures at certain restoration locations.
Third, a wide range of interested parties are involved in restoration efforts (e.g., members of the public, NGOs, representatives of commercial interests, and government agencies). These participants need to be trained in the potential value of natural regeneration as a method of restoration. These partners need to understand how site characteristics, climate, physiography, soils, species composition and dominance, restoration approach, and management and monitoring can affect outcomes on a given site.
Fourth, developing forest stewardship among the populace is a realistic strategy to enhance natural regeneration. In the area, forest management has a long history. Women have been the main custodians because they are primarily responsible for forest resource collection. They need to be educated in forest management. However, we noted that community members were not participating in any management initiatives in the reserve forests because these forests are directly under the supervision of the Forest Department. Nevertheless, field research showed that people frequented these stands to gather a variety of forest products (Joshi 2023). Community members might be more willing to take part in restoration efforts if they are given secure tenure and resource rights. Furthermore, involving communities in decision processes and making them aware of the value of natural regeneration in forest management and conservation are necessary to achieve better restoration outcomes. It may motivate the community to get involved in restoration efforts.
Fifth, for extensive land restoration, governance and planning are extremely desirable. There are no restoration projects underway right now that include restoration via promoting natural regeneration in the central Himalayan region. Instead, active tree planting is the preferred method for land rehabilitation by agencies. Active planting is helpful for locations that have difficulty regenerating naturally such as abandoned lands, severely deforested lands, regions with little rainfall, and areas under heavy biotic pressure with deteriorated soil, weed infestations, and a lack of a seed bank or rootstocks. However, active reforestation requires a lot of effort, money, planting supplies, and monitoring on a regular basis.
Sixth, additional research and long-term monitoring of natural regeneration is needed to elucidate the factors influencing the recruitment of seedlings and saplings, as well as their mortality in different types of forests. More research should be undertaken to increase understanding of the site-specific abiotic and biotic variables influencing natural regeneration. Selecting species determined by the character of a specific stand would be beneficial. Additionally, research results can provide recommendations for how to manage biotic and abiotic elements such that natural forest regeneration is more successful.
All countries need to make major efforts to preserve forests and repair damaged forests to meet environmental goals, management objectives, community needs and aspirations, and financial considerations. This research strongly suggests matching forest restoration objectives to the state of natural regeneration. Planting in forested areas, though effective, involves high costs and may act as a barrier to achieving global forest restoration. Promoting natural regeneration instead could be more cost effective and allow for treatment of larger landscapes.
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