Application of Forest Integrity Assessment to Determine Community Diversity in Plantation Forests Managed Under Carbon Sequestration Projects in the Western Qinba Mountains, China

Abstract

The development of carbon sequestration projects in plantation forests has the potential to offer win–win outcomes for the environment and economy. The Climate, Community, and Biodiversity (CCB) Standards ensure that a particular forest project will deliver tangible climate, community, and biodiversity benefits. According to the CCB Standards, it is necessary to assess community diversity in plantation forests. Our study provides indicators of community diversity based on Forest Integrity Assessment (FIA) according to the CCB Standards for carbon sequestration projects in Tianshui City, Gansu Province, China, which is located in the western Qinba Mountains. Herein, we estimated plantation forest conditions based on a forest condition assessment. Linear regression models were used to explore the relationships between FIA scores and community diversity (such as species richness, Shannon–Wiener index, inverse Simpson’s index, and Pielou’s evenness index quantified by species abundance) in plantation forests managed under carbon sequestration projects. The high community diversity reaches the CCB Standards. FIA scores were closely associated with Pielou’s evenness index of plant communities in plantation forests managed under carbon sequestration projects (R² = 0.104; mean square error = 0.014; standard error = 0.104; p = 0.012). A complex topography had positive effects on species richness, while a rich standing condition had negative effects on the Shannon–Wiener index. Forest conditions have been used as indicators of community diversity in plantation forests managed under carbon sequestration projects. The occurrence of climber and animal species should be used as indicators for enhancing community diversity to meet the CCB Standards. Furthermore, plant species richness benefits from a complex topography. However, our study had the limitation that the FIA could not cover the full range of environmental conditions. Our study provides a practical reference for applying the CCB Standards to plantation forests managed under carbon sequestration projects.

1. Introduction

Increasing greenhouse gas emissions can lead to rapid climate change and, thus, seriously threaten economic and social sustainability [1,2,3]. Effective nature-based solutions have been studied to protect humans from the negative impacts of rapid climatic change, support biodiversity, and secure ecosystem services [4,5,6]. As an important carbon sink, forests have received considerable attention worldwide due to climate change [7]. The development of forest sinks could play a major role in the effort to slow the accumulation of atmospheric carbon dioxide, as well as to facilitate emission reduction and energy conservation of industrial production [8,9]. However, the long-term stressful utilization of forests and grasslands has led to ecosystem degradation and carbon loss [10,11]. In this context, a series of forest projects have been developed, including the plantation of marginal agricultural land, conservation reserve programs, afforestation of agricultural land, and forest restoration projects for carbon sequestration [12,13,14]. Developing carbon sequestration projects in plantation forests can benefit the environment and regional economy [6,13,15].

These projects have contributed significantly to addressing climate change, supporting local communities and smallholders, and conserving biodiversity [4,16]. The Climate, Community, and Biodiversity (CCB) Standards ensures that a given forest project will deliver tangible climate, community, and biodiversity benefits (https://www.climate-standards.org/ccb-standards/; assessed on 3 May 2023). These standards can be applied to any land management projects, including plantation forests [17,18,19]. The CCB Standards were developed by the CCBA and have been managed by the Verra since November of 2014. The CCB Standards foster the integration of best-practice and multiple-benefit approaches into the design and implementation of relevant projects (https://www.climate-standards.org/ccb-standards/; assessed on 3 May 2023). Plantation forests have been developed as carbon sequestration projects to support the carbon economy [20,21]. The CCB Standards require the use of effective biodiversity indicators to develop carbon sequestration projects for plantation forests worldwide (https://www.climate-standards.org/ccb-standards/; assessed on 3 May 2023). However, plantation tree programs may result in bio-perversity, leading to negative outcomes for biodiversity [22,23,24]. For example, the risks of conversion of natural vegetation to plantations might be particularly pronounced in relatively low-carbon environments such as native grasslands and shrublands in the Tibet Plateau, China [9,12,13,14,15]. The CCB Standards can provide an effective approach for establishing indicators of community diversity in plantation forests managed under carbon sequestration projects to avoid bio-perversity.

Bio-perversity refers to the negative outcomes for biodiversity under carbon sequestration projects due to the reduction in community diversity, including clearing native vegetation to establish tree plantations and planting trees that become invasive taxa [22,23,24]. Under afforestation, invasive species have a stronger ability to enter forest communities than native species [23]. Plant invasions may drive bio-perversity in plantation forests under carbon sequestration conditions [23,25]. Furthermore, climatic and forest conditions determine the suitability of native and invasive plant species in forests [25]. To avoid bio-perversity, we explored whether forest conditions can be used as indicators of plant community diversity in plantation forests managed under carbon sequestration projects.

Herein, we investigated two main questions: (1) whether forest conditions have positive effects on community diversity, and (2) how forest condition scores can be applied to meet the requirements of community diversity under the CCB Standards. We used the Forest Integrity Assessment (FIA) tool to estimate the forest conditions of carbon sequestration projects; the survey was conducted by PhD and MS candidates in forestry and ecology [26]. Previously, the effectiveness of the FIA tool in estimating the condition of a tropical forest in Southeast Asia was tested [26]. The FIA tool performed well, based on the assessment of biodiversity, vegetation structure, and aboveground carbon stocks [26]. Furthermore, numerous carbon sequestration projects have been developed for ecological restoration and social development (https://verra.org/programs/verified-carbon-standard/; assessed on 3 May 2023). The assessment of biodiversity performance is necessary for such projects. Hence, FIA is a potential tool to assess plantation forests managed under carbon sequestration projects in China based on plant community diversity. Our study provides a practical reference for the application of FIA to determine community diversity in plantation forests managed under carbon sequestration projects.

To address the two research questions, we examined 60 sites belonging to 12 study areas to assess the community diversity in plantation forests under carbon sequestration projects in Tianshui City, Gansu Province, China, which is located in the western Qinba Mountains. Then, we used linear regression models to explore the relationships between FIA scores and community diversity (based on species richness, Shannon–Wiener index, inverse Simpson’s index, and Pielou’s evenness index quantified by species abundance). Finally, we provide suggestions for the effective management of plantation forests under carbon sequestration projects.

2. Materials and Methods

2.1. Fieldwork

Fieldwork was conducted in plantation forests managed under carbon sequestration projects in Tianshui City, Gansu Province, China. Tianshui is located in the western Qinba Mountains between 34°36′ and 35°42′ N and 105°32′–106°56′ E, with a total area of approximately 14,000 square kilometers [27,28]. Tianshui is situated in a typical arid inland climate zone [27]. Its regions can be divided into temperate continental arid and temperate semi-humid regions [27,28]. The most-planted species belong to the Prunus and Pinus genera. Plantation forests belong to Southeast Tibet shrublands and meadows and Central China Loess Plateau mixed forests in Qinba Mountains [29].

Using the CCB Standards, we investigated 12 study areas, each defined by a circle with a 10 km radius, to assess community diversity in plantation forests managed under carbon sequestration projects [30,31]. These study areas have similar vegetation conditions and cover the full altitude range of Tianshui. Five sites were investigated in each study area, and the total number of sites was 60, as shown in Figure 1. The selected habitat types were intended to be relatively frequent in the study areas (i.e., very rare and extreme habitats were avoided) [31]. The vegetation coverage of the study sites should be high.

One to three typical target habitats were sampled as pairs of specific sites (10 × 10 m²) within each study area, with each being under different anthropogenic disturbance conditions (relatively natural vs. disturbed) [31]. From June to July of 2022, the terrestrial plant community was established in these plantation forests managed under carbon sequestration projects along an altitudinal gradient. Following the methods described by Fang et al. (2009) [30], we recorded the following data: (1) geographic coordinates (latitude and longitude) of the center of the study sites, (2) altitudes of the center of the study sites, (3) abundance of plant species within the study sites, and (4) vascular plant species following the Plant List (http://www.theplantlist.org; assessed on 3 May 2023).

2.2. Diversity Measurements

Species richness (S), Shannon–Wiener index (H), inverse Simpson’s index (D), and Pielou’s evenness (J) were used to quantify community diversity as follows [32]:

$$S=n$$

(1)

where n represents the total number of species observed in each investigation site.

$$H=-{\displaystyle \sum _{i=1}^s{p}_i}\ln p_i$$

(2)

$$D={\displaystyle \sum _{i=1}^s{p}_i^2}$$

(3)

$$J={\displaystyle \raisebox{1ex}{$H$}\!\left/ \!\raisebox{-1ex}{$\ln S$}\right.}$$

(4)

where p_i represents the proportion of the total plant abundance belonging to species i, and s represents the number of plant species at each investigation site. H and S represent the Shannon–Wiener index and species richness, respectively, at each investigation site.

2.3. FIA for Assessing Community Diversity

The FIA tool was developed by the High Conservation Value Resource Network (HCVRN) in partnership with the SE Asia Rainforest Research Partnership (SEARRP) in Malaysia (https://www.hcvnetwork.org/; accessed on 3 May 2025) as a rapid (<1 h to complete) tool to conduct broad assessments of forest conditions in a cost-effective and efficient approach that does not require expert knowledge or extensive resources [26]. This tool has been widely used to assess forest conditions. Through the application of FIA, we assessed vegetation conditions and selected appropriate project areas for carbon sequestration [26].

The FIA tool was designed to enable forest managers with no prior experience in forestry or conservation to assess and monitor the conditions of tropical forest conservation areas [26]. The FIA tool requires no taxonomic knowledge, time-consuming measurements, expensive equipment, or inaccessible satellite technologies; it only has simple yes/no questions that can be answered based on observations during a short walk along a forest trail [26]. In this study, we used the FIA tool to estimate the conditions of plantation forests managed under carbon sequestration projects in Qinba Mountains. We recruited PhD and MS candidates in forestry and ecology to test whether the survey results were affected by prior knowledge, experience, education, or other characteristics, with a number of assessors. In the FIA questionnaire, the assessors responded to a series of 50 questions with “yes” or “no” response options regarding the site they were surveying, and the final score was calculated by determining the total number of responses, as shown in Table S1 [26].

The survey targeted seven criteria known to be associated with forest conditions: landscape, topography, water, trees, flora, fauna, and disturbances [26]. Based on the study by Suggitt et al. (2021) [26], our study regrouped the 50 questions (Table S1) into 8 overall questions because some questions were not applicable to plantation forests managed under carbon sequestration projects, as follows: (1) area of plantation forests managed under carbon sequestration projects; (2) topographical variation based on questions 5–9; (3) water body based on questions 10–14; (4) tree standing condition based on questions 16–25; (5) climber species based on questions 26–37; (6) animals based on questions 38–40; (7) earthworms based on questions 41–43; and (8) opening area based on questions 45–47. We also used “yes” or “no” response options for the eight questions as follows: (1) Q1: large area of plantation forests; (2) Q2: complex topographical variation; (3) Q3: large water body; (4) Q4: rich standing condition; (5) Q5: climber species; (6) Q6: animal species; (7) Q7: earthworm species; and (8) Q8: large opening area. We summed the total number of “yes” responses to calculate the corrected score in assessing the conditions of the forest conservation areas. Five assessors with prior forest knowledge were recruited to complete the survey, and the results among them were consistent after sufficient discussion with two professors. The courses followed the study of Suggitt et al. (2021) [26]. The final scores were determined by the two professors. Based on the study by Suggitt et al. (2021) [26], most questions could contribute to the assessment of community diversity based on the features of plantation forest management [33].

2.4. Analyses

We used linear regression models to determine the relationships between the FIA scores and community diversity (based on S, H, D, J, and P). The residuals of the relationships between the FIA scores and community diversity were assessed and mapped for analyzing spatial variations in community diversity. The ANOVA test was used to explore differences in the conditions of the forest conservation areas based on “yes” or “no” responses to the eight overall questions in the FIA. All data analyses were conducted using JMP 14 Software (https://www.jmp.com/en_gb/software.html; accessed on 3 May 2025).

3. Results

Table 1. Summary of community diversity and FIA scores in Tianshui City, Gansu Province, China.

Diversity	Mean	SD	Min.	Max.
Shannon–Wiener	2.424	0.422	1.506	3.392
Species richness	30.083	8.172	10.000	51.000
Inverse Simpson	7.845	3.792	2.097	20.228
Pielou	0.410	0.125	0.141	0.729
FIA score	4.100	0.877	2.000	6.000

shows the community diversity and FIA scores in the western Qinba Mountains, China. The ranges were 10–51 for species richness, 1.506–3.392 for the Shannon–Wiener index, 2.097–20.228 for the inverse Simpson’s index, and 0.141–0.729 for Pielou’s evenness index in plantation forests under carbon sequestration projects in Tianshui City, Gansu Province, China (Table 1). These results indicate that high community diversity was detected in these projects. The FIA scores ranged from 2 to 6, and the average value was 4.1 for the plantation forests based on our correction (Table 1). Biodiversity was detected for these carbon sequestration projects. Based on the FIA, we found climbers in 34 investigation sites, animals in 42 investigation sites, and earthworms in 22 investigation sites.

Table 2. Results of linear regression models of the relationships between community diversity and FIA scores in Tianshui City, Gansu Province, China. SE: standard error; MSE: mean square error.

Diversity	SE	MSE	R2	p-Value	Slope
Shannon–Wiener	10.504	0.181	0.001	0.854	−0.012
Species richness	3731.195	64.331	0.053	0.076	2.148
Inverse Simpson	836.807	14.428	0.014	0.373	−0.506
Pielou	0.824	0.014	0.104	0.012	−0.046

shows that linear regression models of the relationships between community diversity and FIA scores in the western Qinba Mountains, China. We found that FIA scores were negatively correlated with Pielou’s evenness index (R² = 0.104; mean square error = 0.014; standard error = 0.104; p = 0.012; Table 2 and Figure 2). The residual values of the relationship between FIA scores and Pielou’s evenness index ranged from −0319 to 0.268 (Figure 3). However, there were no significant relationships between FIA scores and the Shannon–Wiener index, species richness, and inverse Simpson’s index (p > 0.05; Table 2). The scores related to animals, climber species, and stands were significantly associated with community diversity (p < 0.05; Table 2).

Figure 4, coupled with

Table 3. Results of ANOVA test for differences in community diversity, including species richness, Shannon–Wiener index, inverse Simpson’s index, and Pielou’s evenness index, concerning the conditions of forest conservation areas according to “yes” or “no” responses to eight overall questions.

Condition	Pielou		Inverse Simpson		Species Richness		Shannon–Wiener
	F Ratio	p	F Ratio	p	F Ratio	p	F Ratio	p
Animal	0.837	0.364	4.118	0.047	6.053	0.017	6.554	0.013
Area	2.624	0.111	1.340	0.252	0.176	0.677	0.479	0.491
Climber	12.056	0.001	10.728	0.002	0.290	0.592	8.638	0.005
Earthworm	0.792	0.377	0.000	0.983	0.465	0.498	0.038	0.846
Openning	3.010	0.088	1.658	0.203	0.132	0.717	2.201	0.143
Stand	2.525	0.118	3.607	0.063	3.936	0.052	5.634	0.021
Topography	3.149	0.081	0.181	0.672	4.239	0.044	0.064	0.801
Water	0.204	0.653	0.280	0.599	0.100	0.753	0.503	0.481

, shows that differences in community diversity, including species richness, Shannon–Wiener index, inverse Simpson’s index, and Pielou’s evenness index in the western Qinba Mountains, China. Based on the ANOVA results, the presence of animals and a complex topography favored high levels of community diversity (Figure 4; Table 3). Specifically, the presence of animals had positive effects on the Shannon–Wiener index, species richness, and inverse Simpson’s index, and the presence of climber species was negatively correlated with the Shannon–Wiener index, inverse Simpson’s index, and Pielou’s evenness index (p < 0.05; Figure 4; Table 3). A complex topography had positive effects on species richness, while a rich standing condition had negative effects on the Shannon–Wiener index (p < 0.05; Figure 4; Table 3).

4. Discussion

4.1. Relationships Between Forest Conditions and Community Diversity

We used the improved FIA to assess forest conditions managed under carbon sequestration projects. We found that the FIA scores were negatively associated with Pielou’s evenness index and the inverse Simpson index, indicating that the FIA scores were not effective for achieving high levels of community diversity in plantation forests under carbon sequestration projects. Suggitt et al. (2021) [26] reported a positive relationship between FIA scores and the alpha diversity of tropical forests, which contrasted with our results. Hence, the FIA tool could be incorporated into management practices for tropical forests in southeastern Asia. However, it should be adjusted to assess the conservation value of forests under carbon sequestration projects.

Community diversity was high in plantation forests managed under carbon sequestration projects in Tianshui based on different diversity indices, indicating that the projects had a good performance for biodiversity conservation. Our study, along with previous studies, suggested that the application of FIA to the assessment of forest conditions depends on the forest type. FIA is usually used to estimate the conditions of tropical forests, and evidence indicates significant relationships between FIA scores and independent measures of forest conditions, including biodiversity, vegetation structure, aboveground carbon stocks, and other key metrics of ecosystem function, indicating that the tool performs well [26,34].

Based on the FIA, trees have a strong regeneration ability, allowing forests to maintain or revert to their natural state after disturbances [35,36]. Plantation forests create optimal conditions for biodiversity, particularly in secondary forests [35,37]. However, we found that such favorable conditions may lead to low community diversity in plantation forests managed under carbon sequestration projects. Plant invasion is the main factor affecting community diversity [38]. In plantation forests, invasive plant species may have a stronger ability to regenerate than native plant species under conditions of limited nutrient availability [25,38]. Plantation forests with high FIA scores may provide sufficient natural elements and characteristics that are retained, mimicked, or restored by invasive plant species [39,40,41]. If an alien species is more competitive than its native counterparts, it can co-opt more available resources, leading to a decline in the abundance or richness of the native species [23,40,41,42]. Therefore, better plantation forest conditions may lead to low community diversity. Plant invasion dynamics should be monitored for effective forest management in carbon sequestration projects.

4.2. Application of FIA to Assess Community Diversity in Carbon Sequestration Projects

In this study, we found that the presence of animals and a complex topography favored high levels of community diversity. The presence of climber species is negatively correlated with community diversity. Based on our results, we could improve the FIA framework of Suggitt et al. (2021) [26] to assess the conditions of plantation forests managed under carbon sequestration projects. The main objective of this study was to apply the FIA to determine community diversity in plantation forests managed under carbon sequestration projects. Based on the requirements of biodiversity under the CCB Standards, the net-positive biodiversity impacts should be assessed for carbon sequestration projects [43]. Hence, we achieved the objective of this study by assessing community diversity. Sites with high scores should incorporate terrestrial plant communities with high diversity to meet the CCB Standards.

According to the CCB Standards, forest conditions and quality should be monitored [17,18,19]. In the framework of Suggitt et al. (2021) [26], FIA is a simple and user-friendly tool for assessing and monitoring biodiversity in tropical forests. However, the conditions differ between natural tropical forests and plantation forests [43]. Some questions could not be answered regarding plantation forests. Based on our assessment, the FIA questions developed by Suggitt et al. (2021) [26] were not suitable for determining community diversity in plantation forests managed under carbon sequestration projects [26]. Hence, we corrected the framework of Suggitt et al. (2021) in our study [26]; four questions were found to be useful for FIA, namely topographical variation, tree standing condition, climber species, and animals.

Our study suggested that similar to natural forests, animals and climber species are the drivers of community diversity in plantation forests under carbon sequestration projects. Forest conditions should be considered when regulating the community diversity of plantation forests managed under carbon sequestration projects [26]. Forest conditions were assessed based on the FIA scores in our study, but the condition assessment results were different for plantation forests compared with the results for natural tropical forests reported in the study by Suggitt et al. (2021) [26].

Our study had the following limitations: (1) We should develop effective indicators of community diversity in plantation forests to meet the CCB Standards. The observation of animals and climber species should be conducted using a variety of approaches, such as remote sensing. (2) The number and the expertise of assessors need to be improved for further investigation of FIA. (3) Different vegetation types, along with aboveground carbon stocks, can lead to different results regarding community diversity. Hence, more tests should be conducted in future studies.

5. Conclusions

In this study, we determined forest conditions as indicators of plant community diversity in plantation forests managed under carbon sequestration projects in the Qinba Mountains. The biodiversity performance of carbon sequestration projects reaches the CCB Standards with high levels of community diversity. We found that FIA scores were negatively correlated with Pielou’s evenness index, indicating that forest conditions negatively affected the community diversity of plantation forests. The occurrence of climber and animal species should be used as indicators for enhancing community diversity to meet the CCB Standards. Our study provides a practical reference for the application of the CCB Standards to plantation forests managed under carbon sequestration projects based on ecological indicators. However, more detailed indicators based on forest conditions using the FIA framework should be developed. Finally, we need to improve the questions used in the FIA framework, along with involving more experts, in order to enhance the accuracy of determining the forest conditions of plantation forests under carbon sequestration projects.