In our study in SNP, the plots invaded by bamboo (Y. maling) showed the low species richness and density of the shrubs, herbs, and tree seedlings as compared to the non-bamboo plots. These results indicate that the abundance of bamboo restricts the richness and density of the shrubs and herbs species and hinders tree regeneration in the forests. A supportive study on the expansion of dwarf bamboo in Japan9 showed the negative relationship between bamboo density and herb species richness. Also, studies have suggested altering forest structure and dynamics, revealing lower density of trees and lower species diversity, by the overabundance of bamboo in Brazilian Atlantic forest15. Such evidences signify that the high degree of bamboo dominance over a long period may effectively alters the forest composition and structures.
Our analysis of the effect of the bamboo overdominance on the forest composition in SNP and the results drawn from GAM vividly estimated the significant decline in the richness of shrubs and herbs and further predicted that, with increasing 1000 shoots of bamboo per 100 m2, there would be an adverse change in the species richness of shrubs (2-3 species). Also, changes in the richness of tree seedlings, (1-2 species) were estimated with 1000 shoots of bamboo. Similarly, disadvantageous changes of density of shrubs (28-30 individual) and herbs (1943-1969 individual), tree saplings (23-25 individual) and tree seedlings (141-145 individual) were estimated with increasing the 1000 shoots of bamboo in 100m2 area significantly. The GAMs predictions indicate that the abundance of bamboo has negatively influenced the richness of shrubs, herbs and tree saplings and tree seedlings. Concurrently, the density of shrubs, herbs, tree saplings and tree seedlings in SNP, as the physical and physiological stresses of bamboo, may cause the reduction of the tree seedlings’ richness15. Further, it is suggested that the dominance of bamboo reduces the tree regeneration and the undergrowth vegetation, and their thick layer of litter affects the tree seedling regeneration negatively causing alternations in the plant community composition and species diversity32. Another supportive study from Southwest China suggested that the high density of bamboo Fargesia nitida greatly decrease understory species richness especially shrubs33 (Tao et al. 2012). However, the expansion of a dominant native species may threaten other native forest species and enables invasion of non-native species in herbs and shrubs layers7.
The rapid overdominance of bamboo species (Y. maling) in SNP has consequently indicated the alteration in forest composition when compared with non-bamboo plots. Here, we recorded the widespread extension of bamboo reaching upto 3300 m asl, which is commonly observed in the Temperate broadleaved forests, and expanded across the Temperate Coniferous broad-leaved forests. We observed excellent bamboo growth under tree species like Quercus spp., Rhododendron spp., Eurya spp., Symploccus spp., Vitex heterophyllum and Abies densa accompanying various shrub species because of its tremendous adaptability and excellent resource use (light) capability in different habitats34. They expand through their rhizomes and survive over several decades in many forest types and thus have raised concerns about the regeneration of tree seedlings and the diversity of shrubberies and herbaceous species13, 35,36,37. Consequently, some studies have suggested controlling and managing the invasion of bamboos by applying moderate grazing. That can control forest structures to some extent thereby reducing the density and height of the bamboos and creating favourable conditions for the regeneration of other species35,38,39.
Additionally, our results reveal a positive alteration in species richness of shrubs, herbs, and tree seedlings and in density of tree, shrubs, herbs and tree seedlings while, negative change in tree saplings richness and density of tree saplings and bamboo (stems). Previous study from similar region resulted negative correlation between species richness of woody species and elevation40 however, contradictory results as adverse change in species richness along the elevation emerged from the mountains of China41. The change in species richness and density may be due to difference in climatic, physiographic and edaphic factors 2,42.
In general, bamboos offer major ecosystem services, providing livelihood opportunities and supplying food resources to humans and wild fauna. These uses are wide-ranging, like tender shoots eaten as vegetable, the leaves for fodder to cattle and as important food to the wild animals especially for the endangered animal Red panda (Ailurus fulgens) 21,22 and the Himalayan black Bear (Selenarctos thibetanus) in SNP. The bamboo seeds and young shoots are the good food resources for wild herbivores43,44. Also, the species offers various uses in constructive purposes, linking directly to the livelihood of the forest fringe communities. Simultaneously, bamboo may provide opportunities for carbon farming and carbon trading16,45 and supports the increasing organic matters in soil46. Nonetheless, as per the National Park rule and regulations, the community people around SNP are restricted to use bamboo and other resources for the commercial purposes. The invasiveness of native bamboo species (Y. maling) in SNP might have resulted, due to, i) ban on grazing and shifting shepherd communities outside the park area, ii) restriction on harvesting bio-resources for the forest fringe communities, iii) global climate change, and iv) limited management measures, etc.
The management of bamboos by controlling their expansion in the areas of high bamboo density could be an appropriate forest restoration approach. It is understood that the bamboos have great potential for the rapid growth and forming complex dominant structures in the forests. The sustainable utilization of bamboos by the local communities would be suggestive especially in the PAs, not only in Darjeeling Himalaya but also in the entire eastern Himalayan forests. Such uses of bamboos can be encouraged, in order to, i) limit its uncontrolled extension further inside the forests, ii) decrease extraction pressure in other forest wood species, iii) revitalize the local bamboo-based traditional handicrafts and linking with livelihood alternatives and iv) remove bamboo clumps form affected areas, which can be used as a planting material in waste land restoration purpose. Besides, the growing ecotourism in the region demands the supply of various ethnic products for which the bamboo resources become the best option as an alternative livelihood. The bamboo should be used in such a way that the negative impacts of its current expansion on the other species could be limited.
Based on the information gathered from our case study from Khangchendzonga Landscape-India, appropriate management plans need to be developed, focusing on its sustainable harvesting system and identifying the threshold levels of extraction for conservation and sustainable utilization approaches. Furthermore, we suggest in-depth further studies on, i) bamboo-based forest community structures and dynamics, ii) changing the intensity of bamboos community growth with respect to climate change, iii) monitoring ecological resilience with respect to the bamboos plantation and cultivation, and iv) integrating mechanism of sustainable utilization of bamboo by the communities around PAs.