1. Introduction
Bamboo resources possess many advantages, including rapid growth, high productivity, a short yield period, and bamboo plants with multiple utilization [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18]. Therefore, bamboo resources are essential for people, especially in Asia. Because the weather and environmental factors are suitable for bamboo growth, over 150,000 ha of bamboo forests cover terrestrial areas, and most such forests are plantations, providing multiple ecological services in Taiwan [
7,
8,
18,
19,
20,
21,
22,
23,
24,
25]. Traditionally, culms and bamboo shoots are two main products that provide jobs and increase income for local people in villages [
7,
8,
25,
26,
27,
28,
30,
31].
However, selective cutting or thinning is necessary to improve productivity in managed bamboo plantations, regardless of harvesting culms or bamboo shoots [
21,
30,
32,
33,
34,
35]. Such an approach only removes old culms, and most culms are still maintained in the lands of bamboo plantations after harvesting, indicating the lands are always covered by most bamboo plants in bamboo forests. Because the process of harvesting bamboo forests is friendly to the environment, the products of bamboo culms are also regarded as non-timber forest products (NTFPs) [
15,
25]. In recent years, bamboo forests with high potential for carbon storage have been discovered worldwide for various species because their rapid growth results in fast accumulation of dry mass [
9,
21,
23,
25,
28,
29,
33,
36,
37,
38,
39,
40].
Makino bamboo (
Phyllostachys makinoi) is a native species with a particular ecological meaning in Taiwan. Plantations of this species are widely distributed in northern and central Taiwan [
8,
21,
24,
41]. Meanwhile, this bamboo is a crucial bamboo species because both culms and bamboo shoots have high economic value. Usually, they are planted by monoculture for financial purposes [
8,
19,
20,
21,
24,
41,
42]. Since Makino bamboo possesses high ecological and economic values, numerous studies have addressed this bamboo species in various aspects. Those studies included cost analysis for managed plantations [
43], assessing growth and biomass accumulation for the stand level [
19], analyzing stand structure of plantations [
21,
42], the impact of thinning on the growth and biomass accumulation [
20], quantifying culm height growth by growth functions [
41], and assessing the ability of carbon storage for the stand level [
9,
21,
25].
However, rare studies addressed the carbon yield model and explored the factors affecting carbon yield for the Makino bamboo plantations. Developing a carbon yield model helps assess the contribution of carbon storage for this bamboo species. Therefore, the objectives of this study were to (1) collect data from stands of various stand densities resulting from thinning treatments, (2) predict aboveground carbon storage (AGCS) based on aboveground biomass (AGB), (3) develop a carbon yield model, and (4) analyze the main factors to affect AGCS, for Makino bamboo.
4. Discussion
Bamboo plantations consist of individuals of various age classes, which display an uneven-aged structure [
8,
18,
19,
20,
21,
33,
34]. This structure results from their development pattern and management approach [
8,
18,
34]. The development of bamboo plants is based on asexual reproduction by rhizome, and new individuals sprout out year by year. Therefore, harvesting or thinning old culms (usually over 4-5 years old) is necessary because it helps increase growth space for new culms and maintain vitality for the entire stands [
21,
30,
32,
33,
34]. Because thinning is strongly related to stand development, it is crucial for stocking bamboo plantations. It indicates that thinning old culms in bamboo plantations management is necessary, regardless of whether their management purpose is on harvesting culms or bamboo shoots.
On the other hand, fertilization is another approach to improve productivity for bamboo plantations. Intensive management (IM) and extensive management (EM) are two critical strategies widely used in bamboo plantations [
30,
34,
46]. IM involves thinning and fertilization, while EM only includes thinning. Numerous studies have confirmed higher productivity in bamboo plantations performing IM [
8,
30,
46,
47]. However, the productivity may only partially reflect the stocking of bamboo plantations. If farmers harvest a significant number of bamboo shoots and only keep a few for further developing culms, bamboo plantations with IE might show lower stocking. It indicates that productivity does not fully reflect stocking because it is affected by the factor of harvesting bamboo shoots. As a result, biomass or carbon stocking does not directly correlate with bamboo plantations performing IE or ME but depends on farmers' decision to harvest culms or bamboo shoots [
30,
46]. Usually, bamboo plantations for culm harvest have higher stocking while for shoot harvest have lower stocking, regardless of IM or EM.
Despite most bamboo plants possessing wooden structures, they differ from timber trees in stems. Bamboo plants have hollow structures in culms, while boles of timber trees are solid [
33,
48]. Therefore, weight or biomass is better than volume when measuring bamboo plants to evaluate stocking or productivity [
33,
48]. Numerous studies have proposed using the allometric model for predicting bamboo plants because this approach has many advantages, such as the model being easy to use, the parameters with special meanings to explain biomass accumulation and a high predictive ability between DBH (or DBH and culm height) and biomass [
18,
49,
50]. As a result, the allometric model plays a critical role in biomass prediction in bamboo studies [
9,
18,
21,
23,
28,
29,
33,
34,
36,
37,
38,
47,
50,
51,
52,
53].
The target of the allometric model usually addresses a certain bamboo species or a combination of various bamboo species. The development of an allometric model should consider DBH distribution and age class for sampling because bamboo plantations display uneven-aged structure [
18,
21]. Suppose a high allometric relationship exists between DBH and biomass in samples. In that case, the model can effectively scale out for the whole stands and extend to the same species, obtaining the biomass of the entire stands from the summation of individuals. Consequently, researchers could easily predict carbon yield because carbon storage is approximately half of the biomass [
9,
11,
15,
16,
18,
21,
23,
29,
33,
34,
36,
37,
38,
50,
52]. Numerous researchers evaluated carbon yield based on the above processes, and the present study also followed it to predict AGCS. Since Makino bamboo is a critical bamboo in Taiwan, the allometric model and PCC have been developed and determined in a previous study by Yen et al. [
21]. Therefore, this study adopted this allometric to predict AGB, cited PCC to determine AGCS for Makino bamboo, and obtained a range from 7.16 to 28.37 Mg ha
−1. Moreover, Liu and Yen [
25] reviewed the published papers and obtained 12 records of AGCS with 22.22 ± 24.66 Mg ha
−1 for Makino bamboo plantations. We found a significant standard deviation in the AGCS. As mentioned above, the variations in the stocking resulting from farmers’ management purposes might lead to a substantial variation in Makino bamboo plantations, which covered the range of the AGCS predicted in our study.
Even with the current stocking of bamboo plantations determined by farmers’ attitudes, researchers can obtain biomass or carbon yield through the allometric model. At the stand levels, N, BA, and MDBH are critical factors that affect AGCS [
21,
25]. Liu and Yen [
25] used these three factors to develop a carbon yield model for bamboo plantations of various species and found that the factors effectively predicted AGCS. The present study referred to the same factors proposed by Liu and Yen [
25] to develop carbon yield models for a single species, obtaining a satisfactory result for Makino bamboo. In
Table 3, we found a high
Radj2 over 0.93 in the 27 models, based on the three factors with various types. The results confirmed that N, BA, and MDBH were crucial factors in predicting AGCS. Even using only two variables (ln(DBH) and ln(BA) or ln(N) and ln(BA)) can obtain the highest
Radj2. From these two best predictive models, stand density played an essential role in predicting ABCS. Usually, N and BA are crucial variables representing stand density for forests, regardless of timber tree forests and bamboo plantations [
21,
25,
54]. Our results showed that combining N and BA or using a single BA and MDBH effectively predicted AGCS. It indicated that BA played a critical role in AGCS prediction because this variable was simultaneously shown in the two best predictive models, and even only using this variable had a significant predictive ability (Figure 3).
The study used various thinning intensities to create different stand characteristics for a Makino bamboo plantation, especially in a wide range of stand densities and AGCS (
Table 1). We used the data to develop the AGCS model and found a good fit for the models, indicating that these models can effectively be used to predict AGCS in this study area. However, if researchers would like to extend the model to other regions, adding more data from such areas to develop is necessary.
Author Contributions
Conceptualization, T.-M.Y. and Y.-H.L.; methodology, T.-M.Y. and Y.-H.L.; software, Y.-H.L.; validation, T.-M.Y. and Y.-H.L.; formal analysis, T.-M.Y. and Y.-H.L.; investigation, Y.-H.L.; resources, T.-M.Y. and Y.-H.L.; data curation, T.-M.Y. and Y.-H.L.; writing—original draft preparation, T.-M.Y. and Y.-H.L.; writing—review and editing, T.-M.Y.; visualization, T.-M.Y. and Y.-H.L.; supervision, T.-M.Y.; project administration, T.-M.Y. and Y.-H.L. All authors have read and agreed to the published version of the manuscript.