Diameter distribution

age class 1 to age class 2 and 3 but from age class 3 to age class 4 it decreases significantly. However, the ratio 3: 3: 3: 1 can also be used to roughly imitate the age composition of those stands of the three bamboo species including Indosasa angustata, Dendrocalamus latiflorus and Dendrocalamopsis sp.2 whereas the age composition of the stand of Dendrocalamus barbatus should be expressed by the ratio 4: 3: 2: 1.

In fact, except the two stands of Oligostachyum sp. and Phyllostachys edulis, the number of culms in age class 3 of all the others is significantly smaller than that of age class 1 and 2. Thus, a large amount of their culms were cut when they reach the age three. However, the smallest number of bamboo culms existing in age class 4 leads to a conclusion that the main cutting age currently applied for all the six bamboo stands is at four years.

4.2.5 Diameter distribution

4.2.5.1 Diameter distribution of stands

Diameter distribution is one of the most important stand characteristics in forestry and is also a useful tool for describing the forest structure (Loetsch et al., 1973). There are several mathematical models to describe diameter distribution of woody stands, of which Weibull function has been used for the bamboo species Phyllostachys edulis in China (under the name Phyllostachys pubescens) (Fu, 2001) and Guadua angusstifolia in Colombia (Camargo, 2005). The Weibull function is a probability density function that can be applied in a wide variety of shapes, depending on the values assigned to each of three or two constants (Pham, 2008). In this study, the Weibull function with both two and three constants is used to express the form of diameter distribution of the six bamboo stands as well.

The Chi-square test is normally used to indicate whether the difference between the distribution of the observed data and the predicted distribution is significant. If the

p-value is less than 0.05 (5% significance level), the two distributions are significantly different at 95% confidence level. In this case the observed data do not match the predicted distribution. In contrast, if the p-value is bigger than 0.05, the two distributions are relatively similar, so that the distribution of observed data fits with predicted distribution at 95% confidence level.

The data in table 11 show that the diameter distributions of the six bamboo stands do not fit with the Weibull (2) distributions at 95% confidence level while the p-values of those stands are all 0.01. With Weibull (3), only the Phyllostachys edulis stand has the p-value higher than 0.05, therefore, it is possible to conclude here that the Weibull (3) distribution can be used to describe the diameter distribution of only the Phyllostachys edulis stand at the 95% confidence level.

Table 11: p-value of Chi-square test for the diameter distribution of bamboo stands (Weibull distributions with 5% significance level) in Northern Vietnam

p-value Species n

Weibull (2)* Weibull (3)**

Oligostachyum sp. 2695 0.01 0.01

Phyllostachys edulis 1623 0.01 0.21

Indosasa angustata 870 0.01 0.03

Dendrocalamus latiflorus 625 0.01 0.01

Dendrocalamopsis sp. 2 640 0.01 0.01

Dendrocalamus barbatus 623 0.01 0.01

Note: (*) Weibull function with two constants;

(**) Weibull function with three constants

Although Weibull is a flexible function widely used to describe the size distribution of trees (Fu, 2001), the diameter distribution of five bamboo stands (excepting Phyllostachys edulis) can not be described by using this function at the 95% confidence

level in this study. In fact, it can be explained that the exploitation activities applied for those stands are uncontrolled. Surely, the size and the number of cut culms depend on the market demand, so that the human interventions significantly disorder the close-to-nature diameter distribution of those stands.

4.2.5.2 Diameter distribution following age classes

Usually each year the edge buds of mature rhizomes differentiate into shoot buds which will then develop into new culms (Fu, 1996; Fu, 2001; Nguyen, 2006). Therefore, for bamboo stands, a new class of culms generates each year. In other words, each bamboo stand consists of consecutive culm classes and the age gap between these culm classes is one year.

1 3 5 7 9

1 2 3 4

Age

DBH (cm)

Phyllostachys edulis Dendrocalamus latiflorus Dendrocalamus barbatus

Fig. 11a: Diameter distribution based age classes (n = 9) of the Phyllostachys edulis, Dendrocalamus latiflorus and Dendrocalamus barbatus stands in Northern Vietnam

As mentioned above, bamboo has no secondary growth, so that after a culm fully develops, its diameter does not change in dimension anymore (McClure, 1957; Fu, 2001; Nguyen, 2006; Xiaobing, 2007). As a result the diameter parameter of bamboo culm remains stable and is fixed at the end of the culm-development stage. Therefore, the mean diameter of bamboo culms within each age class is constant.

1 3 5 7 9

1 2 3 4

Age

DBH (cm)

Oligostachyum sp.

Indosasa angustata Dendrocalamopsis sp2

Fig. 11b: Diameter distribution based age classes (n = 9) of the Oligostachyum sp., Indosasa angustata, and Dendrocalamopsis sp. 2 stands in Northern Vietnam

The quality of shoots and young culms within a bamboo stand not only depends on the climate and management but also the situation of the stand (Fu, 1996) like the possibility of nutrient accessibility and reproduction capability. This follows the fact that the site conditions affect the plant, and in-turn the plant reflects the site condition.

Hence in a certain period of time, the variation of the quality of bamboo culms which is

here illustrated by the mean diameter of different age classes is used to obtain the situation of the stand (Fig. 11a and b).

The observed data in Fig. 11a show that the variation of diameter values of the stand of Phyllostachys edulis, Dendrocalamus latiflorus and Dendrocalamus barbatus has the same tendency. The mean diameter values of those stands all increase from age lass 1 to age class 2, afterward, decrease to age class 3, and finally negligible increase to age 4.

Although the mean diameter values of those stands are different in each age class, the variation of their diameter is not noticeable in the four age classes showing the relatively stable status of those bamboo stands.

From age class 1 to age class 3, the variations of diameters in four age classes of the stands of Oligostachyum sp., Dendrocalamopsis sp. 2 and Indosasa angustata are the same, and they all increase form age class 1 to age class 3. From age class 3 to age class 4, the diameter value of the Dendrocalamopsis sp. 2 stand changes very little whereas the diameter values of the Oligostachyum sp. and Indosasa angustata stands continuously increase (Fig. 11b). In conclusion, the three bamboo stands here have the same growing tendency that the diameter of the bamboo culms belonging to youngest age class is smaller than that of the bamboo culms of older age classes. This leads to the primary conclusion that the current situation of those bamboo stands is not stable and they may be at over mature stage and/or in the lack of nutrition status.