Characteristics of Forests
Almost a dozen parameters were considered for the comparative study between two types of forests. The characteristic of such variables are illustrated in the following table (Table 1)
Table 1
The characteristics of natural and plantation forest forests from the study area [MAI is mean annual increment; other variables have their usual meaning]
Variables | Plantation forest | Natural forest | Remarks |
Mean age (years) | 24 | 16 | Age of natural forest has estimated from Jackson, (1994) by interpolation of mean DBH and height of trees) |
Mean height (m) | 15.73 | 6.7 |
Mean DBH (cm) | 23.13 | 10.47 |
Mean wood density (kg/m3) | 0.65 | 0.83 |
Biomass (t/ha) | 409.76 | 133.87 | Used root-shoot ratio 1:5 |
MAI (t/ha) | 17.07 | 8.37 | Pinus MAI was obtained from the coring sample and natural forests' MAI obtained from Jackson (1994) by interpolation and adjustment |
Carbon Stock (t/ha) | 189.77 | 62.82 | |
Stocking (number/ha) | 1080 - trees 100 - reg. | 1557 - trees 35217 - reg. | Reg. = regenerations |
Undergrowth | Nil or nominal | Profound | Source: field survey |
Fire signs | Present | Absent | Source: field survey |
Natural regeneration | Absent | Present | Source: field survey |
The result showed higher mean age, DBH, height, biomass density, carbon stock density and mean annual increment (MAI) in plantation forest but the forest had very few undergrowths, signs of frequent fire, lower wood density and absence of regeneration. Meanwhile, the higher mean density of wood, stock per hectare (both trees and regeneration), under-growth biomass, absence of fire sign, and good condition of natural regeneration was found in natural forest (Table 1).
The mean value of all continuous variables (Table 1) between plantation and natural forest were significantly different (Table 2) for a given site. The test result showed that the significant (p < 0.05) difference in biomass production concerning the type of forests (Table 2).
Table 2
Comparison between variables using t-test and their respective standard deviation for plantation and natural forests from the study area [SD forests for forested deviation; other variables have their usual meaning]
Variables | p-value | SD- plantation forest | SD- natural forest |
Base girth at 15 cm | < 0.05 | 23.66 | 26.08 |
Girth at breast height | < 0.05 | 20.65 | 21.18 |
DBH | < 0.05 | 6.57 | 6.74 |
Total height | < 0.05 | 2.52 | 6.74 |
Wood density | < 0.05 | 0 | 0.08 |
Biomass per tree | < 0.05 | 170.24 | 246.83 |
Carbon per tree | < 0.05 | 80.01 | 116.01 |
The highest variability between plantation and the natural forest was found in biomass content per plant in the study area whereas the lowest was in wood density as indicated by the standard deviation of the respective variables. More interestingly, the study found that almost similar variation in DBH distribution variation between plantation and natural forests (Table 2).
Figure 3: Frequency distribution of DBH and height of planted and natural forests from the study area [Almost unimodal distribution of DBH for plantation forest (top left) and height for natural forest (bottom right); the negative skewed-distribution of height for plantation forest (bottom left) and positively skewed DBH distribution for natural forest (top right)]
The DBH-height relationship showed a wide variability but a high degree of positive correlation (r = 0.71) and was significant (p < 0.05) was observed. In plantation forest, there are larger sized trees in terms of both mean DBH and height (Fig. 3), as a result, higher biomass (or biomass carbon) was found in the plantation forest than in the naturally regenerated broadleaved forest. But the lower regeneration and density of trees, absence of under-growth, and smaller coverage diminish the future potentiality of the plantation forest for continuous carbon sequestration (Table 1, Fig. 4). The plantation Pinus forest had significantly higher biomass production and increment than naturally regenerated broadleaved forests (Table 2).
The biomass (and biomass carbon) was concentrated on medium-sized trees in case of plantation forest whereas the smaller-sized trees posed the main weight of biomass (Fig. 4). The Man-Kendall test showed that there was no significant DBH-classes distribution trend for plantation forest and the Sen's slope value was positive. On the contrary to this, the naturally-generated forest found significant (p < 0.05) negative trend of DBH-classes distribution which signifies the formation of reverse J-shaped curved and consistence size gradation of the trees (Fig. 3).
Biomass Modelling For Plantation And Natural Forests
The log10-based biomass was explained by the height response using linear regression explains only 39% (adjusted R-square: 0.3884, DF: 3426 and p-value: <0.05) and found to be significant (Fig. 5). Prediction of biomass (thus biomass carbon) is the function of DBH, height and density of the wood. The biomass (kg) was explained by the DBH linear function about 79% (adjusted R-square: 0.7863, DF: 3425 and p-value: <0.05) as presented in Fig. 6. The distinct prediction lines for biomass to DBH were observed for plantation and natural forests employing linear function (Fig. 6).
Similarly, though the biomass calculating allometric equation has not contained the polynomial response of total height (Biomass = 0.0509*pD2H), a high degree of the relation of the polynomial response of height upon log10-based function to biomass for the given data set was observed (Fig. 7). This relationship was highly significant (p < 0.05) and explained about 77% (R-adjusted: 0.77, DF: 3424) variation in biomass. The polynomial regression response of DBH to log10-based biomass explain to be quite higher than the linear function (adjusted R-square: 0.8728, DF: 3424 and p-value: <0.05) as shown in Fig. 8. The plot showed the sigmoid curve for both types of forests for biomass to DBH (Fig. 8).
Figure 5: Log10-based biomass (kg) modelling on linear response of height of forests for plantation (triangular points and dotted line) and natural forest (circular points and solid linear line) [The graphs indicate the height has a similar and significant effect on biomass production for both forests type in the study area. It is very hard to distinguish different regression lines due to overlapping one upon other]
Figure 6: Prediction of biomass (kg) based on linear relation with DBH of trees for plantation (triangular points and dotted line) and natural forest (circular points and solid line) [The graphs indicate the DBH has similar but the differential response for plantation (lower line) and higher potential for natural forest (solid-upper line in the graph) in the study area]
Figure 7: Log10 based biomass (kg) modelling against height in the response of using polynomial equations of forests for plantation (triangular points and dotted line) and natural forest (circular points and solid line) [The graphs indicate the total height has similar but the differential response for plantation (lower line) and higher potential for natural forest (solid-upper line in the graph) in the study area]