Assessment of plant diversity , regeneration status , biomass and carbon stock in a Central Himalayan cypress forest

The present study assessed population structure, biomass, carbon stock, phytosociociological and regeneration status of a Central Himalayan cypress forest of India. A total of 36 plant species (07 trees, 08 shrubs and 21 herbs) were recorded from the study site. Total tree density ranged from 460 to 600 tree ha -1 and total basal area ranged from 19.11 to 58.20 m 2 ha -1 . The total biomass of trees across all the sites ranged between 178 and 431 t ha -1 while carbon stock ranged between 89.07 and 206 t ha -1 . Cupressus torulosa was the only tree species showing fair regeneration at all the sites while most of the species were represented by seedling and sapling indicating new regeneration.


INTRODUCTION
Cupressus torulosa D. Don, commonly known as Himalayan cypress is an evergreen conifer tree species distributed throughout the Himalayan region at elevations of 1800 to 2800 m (Shahni, 1990).It is an important species forming Himalayan moist temperate forest (12/E 1 ) in the western Himalayan region and occurs as open forest of scattered trees on steep rocky ground with xerophytic shrubs and little grass (Champion and Seth, 1968).
Cypress forest most frequently grows on limestone, which provides relatively dry soil conditions on limestone cliffs and shale, and occasionally on other rock types (Troup, 1921).This species generally occur in stands of varying extent, sometimes pure, sometimes associated with deodar (Cedrus deodara), spruce (Picea smithiana), silver fir (Abies pindrow), blue pine (Pinus wallichiana) or oaks (Quercus spp.).These forests are of immense significance from the environmental conser-vation and sustainable development view point as they provide a diverse range of resources; they store carbon, aid in regulating the planetary climate and purify water (Baduni and Sharma, 1996).Of the different types of forest vegetation, the coniferous forests have played an important role in human culture as they have been the subject of various uses including folklore and mythology.
In last four decades many studies (Singh and Singh, 1987;Rawat and Singh, 1988;Singh et al., 1994;Bargali et al., 2013Bargali et al., , 2014Bargali et al., , 2015) ) concentrated on the structure and function of major forests types and adjacent tree plantations of the region.*Corresponding author.E-mail: surendrakiran@rediffmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License The plant diversity and regeneration status of a particular forest provide valuable information for the management and conservation of biodiversity.Therefore, in the present study species diversity, population structure, regeneration status, biomass and carbon stock of tree species in C. torulosa forest of Indian Central Himalaya were carried out.

Study area
The study sites were selected between 2100 to 2400 m above mean sea level (at 29°19' to 29°28' N and 79°22' to 79°38' E) in Nainital, Central Himalaya in an area dominated by C. torulosa forest.As altitudinal gradients influence tree species diversity and distribution; the study site was therefore, divided into three sub sites, hill base (low altitude; 2,120 m), hill slope (mid altitude; 2,220 m) and hill top (high altitude 2,325 m).

Climate
Nainital has long cold and often snowy winter and short summer.The climate is determined by the monsoon rhythms and the year can be divided into three main seasons: winter, usually cold and relatively dry (mid-December to mid-February); summer, warm and dry (April to mid-June); and a rainy season, which is warm and wet (mid-June to mid-September).The period of transition occur between summer and winter and between winter and summer are autumn (October to November) and spring (February to March), respectively.The rainy season accounts for about three-fourths of the annual rainfall (Figure 1).Mean minimum monthly temperature ranges from 2.3°C (January) to 18°C (June) and mean maximum monthly temperature varies from 11.2°C (January) to 28.4°C (June).Mean annual rainfall is 2,195 mm.Average humidity fluctuates near the saturation point during the monsoon and is lower during summer, and ranging between 36.5% (May) to 88.7% (July).The study sites are located within the krol formation and Blaini formations (Valdiya, 1980).Soil is thin due to topography (mainly slope) and usually acid in nature.Soil moisture and water retention capacity ranged between 14.1 to 27.4% and 30.9 to 40.7%, respectively.

Vegetation analysis
For the phytosociological study, the quadrat method was used.The number and size of the quadrats were determined by the running mean method (Kershaw, 1973) and species area curve (Misra, 1968).Ten (10) plots of 10 × 10 m at each site were randomly established at hill base, hill slope and hill top for determination of species richness and other vegetation parameters.Trees and saplings were sampled in 10 × 10 m quadrats, shrubs in 5 × 5 m quadrats, and seedlings and herbs in 1 × 1 m quadrats within each plot (Curtis and McIntosh, 1950;Phillips, 1959).Circumference at breast height (CBH at 1.37 m from the ground) of individual tree and sapling was measured in each quadrat.Individual trees having ≥ 30 cm girth were considered as trees, 30 to 10 cm was considered as saplings and ≤10 cm at the base were considered as seedlings.
Index of similarity (IS) between forests was calculated following (Muller-Dombois and Ellenberg 1974) using species richness in different forests as:

Max Temp Min Temp Total rain RH
Where C is the number of common species shared between compared sites, and A and B are the number of species in each site.

Population structure and regeneration pattern
To describe population structure and to understand species regeneration, individuals were measured for CBH with a girthing tape at each site for each tree species.In addition to seedling and sapling classes (Good and Good, 1972), six or more classes based on CBH were arbitrarily established.The total number of individuals belonging to these size classes was calculated for each species at each site.Regeneration status of individual tree species was determined on the basis of the population size of seedlings, saplings and trees (Khumbongmayum et al., 2006).If the population size is seedlings > saplings > trees the regeneration status of tree species is "good".If the population size is of seedlings > or ≤ saplings ≤ trees, seedlings ≤ saplings > trees, seedlings ≤ saplings and the species had no adult trees, the regeneration status of tree species is "fair".If the species survives only at sapling stage, but not as seedlings (saplings may be <, > or = trees), the regeneration status of tree species is "poor".If a species is absent both in sapling and seedling stage, but present only as trees it is considered as "not regenerating".A species is considered as "new" if the species are found only either in seedling or sapling stage without any tree.

Biomass and carbon stock estimation
The biomass for each tree component was estimated using the regression equation of Rawat and Singh (1988) and Adhikari et al. (1998): Where, ln = natural log, Y= dry weight of component (kg), X= CBH (cm), a = the y intercept and b = slope of regression.
The carbon stock was assumed to be half of the total estimated biomass of each tree species (Brown, 2002;Jhariya et al., 2014).

Species composition and forest structure
A total of 36 plant species were recorded in the study area, of which 7 were trees, 8 were shrubs and 21 were herbs.At hill base, 26 species (5 trees, 6 shrubs and 15 herbs) were recorded; at hill slope 25 species (5 trees, 8 shrubs and 12 herbs) were recorded while at hill top 28 species (4 trees, 6 shrubs and 18 herbs were recorded.Among trees, C. torulosa was dominant (IVI 205 to 300) at all three sites (Table 1) whereas, Cedrus deodara was co-dominant.Highest tree density (360-600 tree ha -1 ) was also reported for C. torulosa (Table 1).Total density of trees ranged from 460 to 600 trees ha -1 and within the range (420 to 1640 trees ha -1 ) was reported for temperate forest of Kumaun Himalaya by Saxena and Singh (1982).Similar ranges for density were reported (Adhikari et al., 1998, Baduni and Sharma, 1996, Pande et al., 2014) for other cypress and cypress-mixed oak forests in the region.Tree density increased with increasing elevation from hill base to hilltop which could have been due to less anthropogenic disturbances at hill top compared to hill base.Total basal area of trees was higher at hill top (58.20 m 2 ha -1 ) than at hill base (19.11m 2 ha -1 ) of which C. torulosa contributed from 70% (at hill base) to 100% (at hill top).Baduni and Sharma (1996)  and it was maximum at hill top (Table 1).Baduni and Sharma (1996) reported maximum density of saplings at hill slope in a C. torulosa forest, possibly due to the suitable moisture conditions.Maximum density of saplings (80 to 400 ha -1 ) was shared by C. torulosa at all the three sites.Seedling density ranged from 300 to 400 individuals ha -1 and it was greatest at hill base (Table 1).Adhikari et al. (1998) reported density of 90 to 420 seedlings ha -1 and 100 to 810 seedlings ha -1 for saplings in moist temperate cypress forest.
A total of 8 shrub species were reported from the study area and Reinwardtia indica was dominant at hill base and slope while Colquehonia coccinea was dominant at hill top.The density values of shrubs ranged from1303 ind ha -1 to 5380 ind.ha -1 (Table 1).For other cypress forest of the region a lower range of 1280 to 4170 ind ha -1 was reported by Adhikari et al. (1998).Numbers of herbs species ranged from 15 at hill base to 18 at hill top (Table 1).A higher number of herbs (22 to 25) were reported by Rawat and Singh (1988) for oak forest and by Pande et al. (2014) for mixed oak forests of the region.

Plant diversity
A higher Shannon diversity index (0.96) for tree species was recorded in hill base (Table 1).At hill top, only one species was present in tree layer so diversity indices were not calculated.In all the three sites, herb diversity was found to be higher than trees and shrubs.Devi and Yaddava (2006) reported that for Indian forests Shannon diversity Index (H') ranged from 0.83 to 4.1.The value of the Shannon diversity Index in the present study lies within the range reported for temperate forests (Singh et al., 1994, Bargali et al., 2013).The Simpson index (Cd) was 0.67 in hill slope and 0.64 in hill slope for tree species.The evenness index was maximum (1.07) in hill slope and minimum (0.88) in hill base.
For shrub species diversity was maximum (0.75) at hill slope and minimum (0.52) at hill base.The Simpson and evenness index were maximum at hill slope and minimum at hill top.Herb layer also followed the same pattern as recorded for shrub layer (Table 1).
The similarity index value was maximum for herb layer ln Y = a+ b ln X (48.8) and minimum for tree and sapling layer (33.3) indicating that the species were more common in the herb layer as compared to other layers (Table 1).βdiversity was maximum (1.91) for seedling layer and minimum (1.20) for shrub layer (Table 1).

Population structure and regeneration pattern
According to the criteria given by Saxena and Singh (1984), four types of population structure were reported at hill base (Figure 2A).The dominant species, C. torulosa showed higher number of population in the middle size class than lower and higher size classes.This indicates that seedlings were not sufficient to replace trees.Benton and Werner (1976) suggested that such type of population could become extinct if such tendency continues.In Q. floribunda more individuals were recorded in the middle size class (saplings), indicating a poor generation.In C. deodara most trees belonged to higher girth classes while populations of lower girth classes including seedlings and saplings were completely absent.This indicates that this species did not reproduce well in the past and seedlings were not establishing.In P. cerasoides and Q. leucotrichophora, individuals belonged to lower size classes (seedlings and saplings) while higher girth classes were completely absent, indicating that these species were new to the area and on maturity they could reproduce well and expand their populations.At this site the maximum tree species (60%) were included in the category of "Fair" regeneration while 20% were included in "new" and 20% in "not regenerating" category.At hill slope, C. torulosa indicated good regeneration followed by a phase of poor regeneration.Fewer established seedlings compared to saplings indicated poor regeneration.It reproduced well in the immediate past and continued to do so at present but at a lower rate.Occurrence of only seedlings and saplings in Q. floribunda and P. cerasoides indicated that these species has recently invaded the area and might become canopy (upper most strata in the forest) or sub canopy species later on.Absence of established seedlings of C. deodara indicated that this species reproduced well at first but at present its regeneration has been stopped.Q. leucotrichophora was represented by saplings only and reflect that this species was new to the area.At this site, 60% species showed "fair" regeneration, 20% showed "poor" regeneration and 20% were included in "new" (Figure 2B) At hill top, C. torulosa had high density in intermediate size classes and decreasing number toward higher and lower size classes.According to Saxena and Singh (1984) and Kittur et al. (2014), the population is on the way to extinction if such a trend continues.Knight (1975) referred to the species showing such population structure as infrequent reproducer.P. cerasoides, F. numoralis and R. purpurea, were represented by seedlings and saplings only indicating that these species were new to area and may become established in due course of time to become canopy and sub-canopy species.At this site all the species showed fair regeneration (Figure 2C).

Biomass and carbon stock
The total biomass of trees across all three sites ranged from 178.1 t ha -1 at hill base to 431 t ha -1 at hill top.Above-ground biomass contributed from 68 to 85%.Among the tree components, bole contributed maximum biomass of 36% at hill base and 54% at hill top.Leaves contributed about 4-6%.Fine root contributed the minimum value (1.1 to 1.5%) of biomass (Table 2).
The contribution of the foliage to total above ground biomass was 4.9 to 7.5% and was similar to that reported for P. roxburghii forest (Chaturvedi and Singh, 1987), oak forest (Rawat and Singh, 1988) and P. patula forest (Bargali and Singh, 1997)  .These value are comparable to the value (47.6 to 95.9 t ha -1 ) reported for oak forest (Rawat and Singh, 1988).Root biomass contributed 15-32% of total biomass.The carbon stock in C. torulosa forest ranged from 89.07 to 206.14 t ha -1 (Table 3).Rana et al. (1989) reported a higher range of carbon stock values (166.8 tree ha -1 C for P. roxburghii mixed broad leaf forest to 440.1 tree ha -1 C for Quercus floribunda dominated mixed oak forest).The total carbon storage of C. torulosa forest was comparable with that of P. roxburghii forest.

Conclusions
Phytosociological analysis of the present study indicate the dominance of C. torulosa at all three sites.However, co-dominant species and composition varied from the base to the top of the hill.These results probably indicate a wide ecological amplitude and tolerance of C. torulosa.Most species showed random or contagious distribution patterns.
The population structure of a tree species reflects its biological and ecological characteristics and it describes the distribution of individuals by size class.In the present study, C. torulosa was the only tree species showing fair regeneration at all three sites.The variation among the population size of seedlings, saplings and adults at the three study stands might be due to the variation in climatic as well as edaphic factors required by the species for growth and survival.Seedling and saplings of

Table 1 .
Ecological indices of tree, shrub and herb in cypress forest of Central Himalaya.

Table 2 .
Componentwise biomass (t ha -1 ) of tree layer in a Central Himalayan cypress forest .deodara were absent both at hill base and hill slope, indicating that this species reproduced well in the immediate past but its regeneration hadd stopped.Most of the species, viz.Q. floribunda, Q. leucotrichophora, P. cerasoides, F. numoralis and R. purpurea were represented by only seedlings and/ or saplings.These species could be recent invadders. C

Table 3 .
Carbon stock (t ha -1 ) of different component of tree species in C. torulosa forest Department of Botany, DSB Campus, Nainital for providing necessary lab facilities.