Structural parameters measurements were converted into estimates of coarse wood biomass, using allometric equations for moist forest stands, which is most
widely accepted and recommended by Intergovernmental Panel on Climate Change (Chave et al. 2005). The wood density for the tree species was adopted from Manual of Afforestation in Nepal (Jackson 1994). The measured biomass was converted to the C content by multiplying with the IPCC (2006) default carbon fraction of 0.47. Most commonly used Shannon-Wiener Index (H’) was used as the measure of diversity indices (Magurran 2004).
Statistical analysis was completed in R (R-CoreTeam 2013). Data exploration was used to study the nature and distribution of the forest stand characteristics and to explore the relationship between AGB, species richness and environmental variables. Regression and Generalized Linear Model (GLM) were used for the spatial variation in forest AGBC. Analysis of Variance (ANOVA) was used to test the models that better explained the relationship of the variables. The Karl Pearson Correlation was used to correlate edaphic factors and forest AGBC.
Results
The mean dbh distribution of trees was positively skewed in the northern aspect with mean dbh range of large number of trees ranged within 15 to 35 cm.
Whereas, in the southern aspect reversed J shaped distribution of mean dbh for most of the trees ranged within 15 to 25 cm.
Figure 2 : Altitudinal variation in mean dbh and its distribution in different aspects
The mean dbh and altitude relationship was explained better by the polynomial regression model. Mean dbh of the trees was found to increase significantly (P < 0.001, R2= 0.421) by the rate of 0.0000291 cm with each metre rise in altitude.
Figure 3 : Altitudinal variation in tree abundance
The relationship of tree abundance along the altitudinal gradient was observed through Generalized Linear Model because the relationship didn’t fit to any other regression model. The number of trees was found to increase significantly (P < 0.001) after an altitude of 2025 m by the rate of 0.0002778 individual with each metre rise in altitude.
Table 1 : Observed and estimated species richness and diversity in different aspects
Overall, the species diversity was found to be high in both the north (2.46) and south (2.62) aspects. Furthermore, as shown in (Table 1) the observed species richness was found to be 24 in the north and 25 in the south aspects. However, the jackknife estimator of species richness showed 26 and 30 species richness in the north and south respectively. The jackknife estimator reduces bias and estimates species richness independent of abundance of the species (Smith and Pontius 2006).
Species Richness
Aspect Observed Estimated (Jack2) Shannon-Weiner Diversity Index (H')
North 24 25.998 2.463
South 25 29.918 2.625
Figure 4 : Altitudinal variation in total AGBC in different aspects The total above ground biomass carbon of the forest constitutes the aboveground biomass carbon of trees and biomass of LHG. Overall relationship was defined by polynomial regression (Figure 4). In the northern aspect, total AGB was found to increase significantly with the increase in altitude (P < 0.001, R2 = 0.378) by the rate of 0.000267 t/ha with each metre rise in altitude. Similarly, in the southern aspect total AGB was also found to increase significantly with the increasing altitude (P < 0.001, R2 = 0.812) by the rate of 0.000664 t/ha with each metre rise in altitude. Overall AGB was found to rise starting from an altitude of 1950 m in northern aspect whereas from 2,100 m in southern aspect.
Table 2 : Correlation analysis of AGB and soil parameters in North Soil temperature(°C) Soil pH Soil Moisture (%)
AGBC (ton/ha) -0.982 0.998* -0.080
*Correlation significant at 0.05 level
Table 3 : Correlation analysis of AGB and soil parameters in South Soil temperature(°C) Soil pH Soil Moisture (%)
AGBC (ton/ha) -0.903 -0.517 -0.135
In the northern aspect, AGBC as positively and significantly correlated with soil pH (r =0.998, P < 0.05) only. In the southern aspect, soil pH, moisture and temperature were found to have negative and insignificant relation with the AGBC.
Discussion
In this study, it should be noted that altitude is treated as a way of presenting data along a gradient, not as a direct controlling factor (Girardin et al. 2014).
Vegetation responds to varied changes in environmental factors such as precipitation, moisture, solar radiation, temperature, nutrient availability along rising altitude which in turn alters the microclimatic condition of forest (Kumar et al. 2013). Most of the studies have showed pattern of decreasing aboveground biomass with increase in altitude (Sundqvist et al. 2013). However, this study found an opposite pattern along 1000 m altitudinal gradient. Total AGBC was found to increase from lower to higher altitudes (Figure 4). The mean dbh was also found to have significant increasing trend from lower to higher altitudes (Figure 2). This might be due to the dominance of old growth temperate Oak forest characterized with large dbh at higher altitudes. The dbh distribution in studied forest patch was found to increase significantly after an altitude of 1900 m.
The increase in total AGBC could be related with increasing dbh because dbh stands as an important predictive parameter of biomass (Chave et al. 2005). The positive relationship between altitude and AGBC might also be due to the increase in walking distance from the nearest human settlement as the altitude rises because it has already been reported that villages occur between 1400 to 2000 m bordering the forest of Panchase (Maren et al. 2013). The Panchase Protected Forest Area (PPFA) within the vicinity of settlements and villages is buffered fringe areas designated as ‘intensive use zone’ (GoN 2013).
It is likely that forest in higher areas and conservation areas are less impacted by anthropogenic factors due to the difficulty in its access. Strong positive relationship was also found for AGBC and altitude in the study of conserved forests of Borneo supports this result (Laan et al. 2014). Furthermore, the PPFA also embrace multitude of cultural and aesthetic values as a sacred religious site.
This notion of Panchase as a sacred site might have contributed to the conservation ethics of the local people for protecting forest patches which is supported by the findings from formal protected areas and 25 sacred grooves of Indian sub continent (Bhagwat et al. 2005). Thus, the increasing trend of AGB within altitudinal range of 1500-2500 m showed consistency with the above mentioned findings in different protected forests.
Tree abundance was found to decrease at mid altitude and increase significantly in temperate climatic zone along higher altitudes (Figure 3). This might be due to the past history which revealed that some patches at the mid lands in southern aspect of Government Managed Forests, suffered deforestation and were
converted to tree less openings dominated by grasses and bushes (Berberis spp.) due to its free access (Maren et al. 2013) and anthropogenic pressure was also reported high at low and mid-altitudes (1750-2250 m) of Himalayan gradient in Eastern Nepal (Carpenter 2005). Tree abundance was also found higher at higher altitudes along Eastern Himalayan altitudinal gradient of India (Acharya et al.
2011). Thus, the increase in number of trees in higher altitudes might be due to the zoning of core protection in higher altitudes and the conservation ethic driven by cultural values.
Species diversity and species richness was found higher in this study (Table 1).
Shannon Weiner’s index values ranging from 2.476 to 2.624 is in accordance with the values reported for other temperate forests (Sharma et al. 2009b; Sharma and Raina 2013). Little changes have been found in species richness and diversity between 1500-2500 m in Nepalese Himalayas (Grytnes and Vetaas 2002). Thus, the results for the altitudinal variation in forest stand structure, richness and biomass are in parallel with other findings within altitudinal range of 1500-2500 m (Acharya et al. 2011; Bhagwat et al. 2005; Culmsee et al. 2010; Laan et al. 2014;
Maren et al. 2013; Sharma and Raina 2013).
Aspect has been found to exert significant influence on soil moisture and spatial distribution characteristics of vegetation of forest ecosystem by forming a range of microclimates in multifaceted landscapes (Astrom et al. 2007; Gallardo-Cruz et al. 2009; Olivero and Hix 1998; Sharma et al. 2011; Sharma et al. 2010; Sternberg and Shoshany 2001).
Distribution of trees under different dbh classes was skewed to right in both aspects of Panchase Mountain which was comparable with the peak at dbh class of 15 to 25 cm (Figure 2). Relatively small number of trees can be expected to have high dbh. In comparison, large number of trees with high dbh range indicated that the forest is dominated by the large trees and somewhat at matured stage in the northern aspect. Whereas in the southern aspect, the pattern of most of the trees in smaller dbh range revealed that the forest is dominated by relatively small girth trees and the forest is still in developing phase.
This might be due to reported notion of local people of Panchase that there used to be severe forest deforestation and degradation in the past during open access GMFs and the consequence was even more intensified along steep terrain in southern aspect (Maren et al. 2013). This could be the reason behind the regenerative phase of forest in southern aspect after the designation of forest as protected forest. In general, trees growing on southern aspect are also exposed to harsh and drier climatic conditions and are prone to various natural disturbances which hinders their growth (Sharma et al. 2010).
This differences of dbh in different aspects might be attributed to the occurrence of moisture and more favorable opportunities for growth of tree species on the northern aspect as reported by (Sharma et al. 2010; Sharma and Raina 2013).
Steep slopes considerably limits the plant growth (Gallardo-Cruz et al. 2009). This observation is in parallel with results of previous study in Panchase (Maren et al.
2013) as well as with the findings of (Sharma et al. 2011) which reported higher values of stem density on southern aspects.
Based on the differences in dbh, aspect wise variation in total AGBC could be expected. AGBC was found to comparatively higher in northern and southern aspect (Figure 4). Distribution and presence of relatively tall and high girth trees in northern aspect might have resulted in high AGBC content. Decrease in AGB was found to be driven by decrease in tree height and increase in stem density in Andean forests (Girardin et al. 2014). Studies in temperate forests of Himalayas have reported significant higher values of tree biomass on northern aspects compared to southern aspects which were attributed with relatively cooler and favorable climate for tree growth and facilitates accumulation of large amount of biomass on these aspects (Sharma et al. 2011; Sharma et al. 2010). Total AGB was also found higher in north facing slopes than the south facing slopes in Mediterranean (Sternberg and Shoshany 2001).
Maren et al. (2013) reported the prevalence of tree less openings dominated by bushes in some patches at the mid altitudes in southern aspect as a consequence of deforestation in the past history. Despite its gregariousness and exceptionally hard wood of Oak species, persists in the form of scrub when exposed to adverse disturbances (Sharma et al. 2009a). This might be the reason behind lower AGBC in the southern aspect.
Ample literatures have acknowledged that environmental heterogeneity has been recognized as one of the explanations for variation in species diversity (Panthi et al. 2007; Sharma et al. 2009b; Sharma and Raina 2013). Species diversity and both observed and estimated species richness was found to be higher in southern aspect comparative to northern aspect (Table 1). This might be due to the dominance of Daphniphyllum himalense from lower to higher belts of north facing slopes. Stainton (1972) reported Daphniphyllum himalense, a successional species likely to colonize fallow lands near settlements and appeared to be replacing Oak-Rhododendron forest to Daphniphyllum- Rhododendron forest at higher altitudes (Maren et al. 2013).
The findings of aspect variation in species richness in this study is in contrast with the findings of Panthi et al. (2007) from trans-Himalayan inner valley of Central Nepal. However, the occurrence of comparatively high species richness
in warmer southern aspect has been supported by findings from north western Himalayas (Sharma and Raina 2013).
Although ample evidences have been observed for diversity-productivity relationship (DPR), however various factors are likely to affect the relationship (Jacob et al. 2010; Morin et al. 2011; Vilà et al. 2005; Waide et al. 1999; Zhang et al.
2012; Zhang et al. 2011). High diversity has been reported to cause high production of AGB (Vilà et al. 2005; Zhang et al. 2012).
Relating these findings with the observed findings of this study, overall high species richness and diversity in both aspects might also be the reason for high AGBC. However, while comparing DPR in two different aspects, higher AGBC has been found for northern aspect despite its low species richness and diversity relative to southern aspect. This contrast might be due to the past disturbance and management regime of the forest in different aspects, some of which have already been discussed above and is in parallel with other studies in temperate and sub-alpine forests (Jacob et al. 2010; Zhang et al. 2011).
Studies in Asia have found relatively weak relationship between soil temperature and AGB and are governed by varying cofactors (Lewis et al. 2014). Soil pH was found to have strong and significant positive correlation with AGBC (r < 0.5) in northern aspect (Table 2) whereas, negative and insignificant correlation with AGBC in southern aspect (Table 3). This finding is supported by (Panthi et al.
2007) which relates soil pH with availability of soil nutrients. The north aspect was characterized by the sufficient soil temperature and the humidity which might aid the decomposition rate and resulted in high nutrient content.
Furthermore, hundreds of non-milk-yielding buffaloes are found grazing in the forest of Panchase all year round (Maren et al. 2013). The occurrence of their faecal matter could also have aided in high organic matter in the forest.
As Lewis et al. (2014) stated that the impact of various soil properties on AGB is highly complex and since we do not fully understand the mechanisms that create and maintains AGBC with soil parameters, based on the correlation analyses alone, it is difficult to jump into concrete conclusions.
Conclusion and Recommendation
Middle mountain forests represent an ideal laboratory for detecting the changes in forest ecosystem structure and various environmental parameters. The forest stand in the north represented a good forest stand with even aged trees whereas the forest stand in south represented a regenerative forest. This study as a baseline research in Panchase has documented the biomass production status of
the forest in different aspects. Continued measurements of the biomass over a long period could provide a potential basis for future forest management aspect and modeling of forest production under different varying climate and environment. The dominancy of Daphniphyllum himalense in the forest of Panchase has been found to shift upwards to replace native Rhododendron-Oak forests and could possibly have negative consequences on the species diversity of the forests. Further, the observed decrease in tree number and low AGB in the southern aspect supports the previous findings of degraded forest. Thus, based on the findings of this study and the literatures reviewed, higher species diversity needs to be maintained for the promotion of resilient forest ecosystem to buffer out the impacts and disturbances associated with changing environmental conditions.
This study somehow presents an understanding of the AGBC along altitudinal gradient and different aspects. In addition, multitude of ecosystem services provided by the forests needs to be sustained through integration of biomass production estimates in changing environmental conditions and maintenance of diversified species which might buffer out the impacts and associated changes in the environment in forest management strategy which would form the natural solution for the ecosystem adaptation. A comprehensive study of all carbon fluxes of the forest ecosystem and in temporal scale would unarguably increase the understanding of current patterns and predicting the future trends of forest productivity.
Acknowledgements
We would like to acknowledge the Enhance knowledge on Ecosystem based Adaptation through research and integrate them in curriculum Project for the research fellowship. We would like to thank the Panchase Protected Forest Committee for providing the permission to carry out this research in Panchase and the people of Panchase for their benign co-operation. We also are thankful to the Central Department of Environmental Science for providing the platform for the research.
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