Carbon sequestration and foliar dust retention by woody plants in the greenbelts along two major Taiwan highways
Abstract
Anthropogenic emissions of greenhouse gases and particulate matter have caused continued environmental concerns at both local and global scales. Greenbelts along highways have been implemented to aid in the uptake of emissions along transport sectors. The present study evaluated the capabilities of carbon sequestration and foliar dust retention in 88 woody tree species, and 1520 individuals in the greenbelts along Taiwan National Highways no. 1 and no. 3. More than 2.2 and 1.7 million average annual vehicle passages were respectively recorded for the two highways. Among species, Bischofia javanica, Acacia confusa, Swietenia macrophylla and Alstonia scholaris exhibited optimal carbon sequestration capabilities in trunks and branches, with respective carbon storage levels of 175, 105, 23.8 and 15 kg per plant. Results showed a respective estimated 19.9 and 12.3 thousand tons of carbon sequestrated by trunks and branches in greenbelts of Highways no. 1 and no. 3, respectively. The foliar dust retention capabilities of Ac. confusa and Casuarina equisetifolia were the highest in the two greenbelts, with respective foliar dust retentions of 564.9 and 60.3 g per plant. The leaves in the two greenbelts retained an estimated 47.9 and 17.3 t of foliar dust for Highways no. 1 and no. 3, respectively. The present study demonstrated that woody plant species in greenbelts exhibit a substantial contribution to carbon sequestration and foliar dust retention for two heavily used highways in Taiwan.
Introduction
The role of reforestation and afforestation via carbon sequestration on climate change attenuation is well recognised at global (Bonan, 2008) and regional scales (Xu, 1995; Yemshanov et al., 2005). In addition, the atmospheric environmental functions of urban trees have been estimated in Australia (Brack, 2002) and the USA (Nowak & Crane, 2000; Nowak et al., 2006). Pataki et al. (2006) reports that with ever-increasing urbanisation, trees will serve a more important role in the global carbon cycle. Studies have demonstrated ecological benefits to urban trees, including CO2 sequestration (McPherson, 1998; Nowak, 1994b) and air pollution abatement (Nowak, 1994a; McPherson & Simpson, 1998; Nowak et al., 2000). Total carbon storage and sequestration within cities generally increase with increased urban tree cover and increased proportion of large and/or healthy trees in the population. Large trees store approximately 1000 times more carbon than small trees (Nowak, 1994b). Urban trees in the USA store 600–900 million tonnes of carbon with $12 200–18 300 million values (Nowak, 1994b).
Among the air pollutants resulting from human activities, dusts [particulate matter (PM) of varying types and sizes] are generated from coal combustion, industrial and construction activities and mass transportation. Larger particles with diameters greater than 10 µm settle by gravity, which has led to public anxiety as potential health hazards. Smaller particles less than 10 µm have been well recognised as public health threats (Miguel et al., 1999; Kleeman et al., 2007). Wind speed and direction critically influence the particle deposition velocity generated by road traffic. Zhu et al. (2011) found that PM concentrations adjacent to roads increased during the morning hours, consistent with increased traffic volumes. Particulate matter identified in street/road dusts is composed of heavy metals, including Pb, Cu, Mn, Zn, Cd and Ni, which have been classified as significant environmental pollutants in several studies (Sezgin et al., 2003; Chiang & Huang, 2009; Faiz et al., 2009; Lu et al., 2009). Qiu et al. (2009) demonstrated the efficacy of heavy metal removal by foliar dust retention in an urban setting. However, the control of street/road dust remains a significant challenge in urban areas.
The present study addressed the most important transportation routes and emission sources stretching more than 300 km in Taiwan. The freeways are two north–south routes named National Freeway no. 1 and no. 3. Freeway traffic flow produces large amounts of PM, including SOx, NOx, CO, non-methane hydrocarbon (NMHC) and Pb. According to the Yearbook of Environmental Protection Statistics from the Taiwan EPA (2010), the annual emission of PM in Taiwan was 244 000 t, which was an estimated 9% per vehicle. These statistics demonstrated the substantial role played by mobile sources in PM emission. Vegetation greenbelts have been shown to effectively block dust, and filter suspended particles in urban areas (Bernatzky, 1982; Tong, 1991; Xueli et al., 1993). Dust deposition/capture capacity of different plants species has been investigated in urban areas and the vicinity of large industrial plants in India (Indian Central Pollution Control Board, 2007). However, the synergistic benefits of woody plants in greenbelts along heavily used highways have not been systematically quantified.
In this study, the role of woody plant species on carbon sequestration and foliar dust retention in greenbelts was elucidated by a greenbelt field vegetation survey along National Highways no. 1 and no. 3. Carbon sequestration capacity was estimated by trunk and branch biomass values tallied under the field vegetation survey. Foliar dust retention capacity was investigated by gravity methods, leaf area image analysis and leaf biomass estimates. One objective of this study was to select trees with superior carbon sequestration and foliar dust retention capabilities by analysis of their performance data in this survey.
Materials and Methods
Plot design and woody plant species survey
Twenty-five rectangular (100 m × 5 m) plots were established in the greenbelts along National Highways no. 1 and no. 3. Plot locations were selected to uniformly represent wooded areas based on aerial photographs. The plots were separated by at least 10 km (Fig. 1). From December 2009 to March 2010, each different species was identified, and diameter at breast height (DBH), tree height, and the number of each tree species identified were recorded. Two taxonomic sources were referenced to definitively identify woody species from each survey plot (Liu & Liao, 1980; Editorial Committee of the Flora of Taiwan, 1996). Tree species importance value indexes (IVIs) were calculated by the sum of species relative frequency, density and dominance (Mueller-Dombois & Ellenberg, 1974). The values of IVI were equal to the percentage of relative density plus relative dominance divided by 2. Relative density was equal to the percentage values of one species number divided by the total number of species. Relative dominance was equal to the percentage values of the area occupied by a single species divided by the total area for all species. Higher IVI values indicated increased dominance for a species.
Vegetation survey sampling plots along National Highways no. 1 and no. 3.
Plant biomass and carbon sequestration
Dust retention evaluation
The vegetation survey results were used to calculate the IVI for woody plant species. The 10 dominant woody plant species were selected for foliar dust retention evaluation. For each species, 20 leaves (or leaf fragments) were randomly collected in the middle canopy of three trees. Sampling was conducted 10 days after rain to avoid precipitation interference. The leaves were subsequently placed in 100-mL beakers filled with 50 mL deionised water, and shaken by a sonicator for 30 s. The procedure was repeated twice. The resulting solution was vacuum-filtered through preweighted filter paper with a pore size of 0.45 µm. The loaded filter paper was dried at 105°C in an oven to a constant weight. Dust retention was calculated based on weight differences between oven-dried filter paper before and after filtrations. A digital scanner was used to measure leaf area. The graph scanned was defined as the dot numbers under fixed resolution. Leaf area measurements were performed by horizontal resolution characteristics (dot per inch, dpi), and vertical resolution using ImageJ software (National Institutes of Health, Washington, DC, USA). Dust retention (mg cm−2) values were defined as dust weight divided by leaf area (cm2). Total dried mass values of each tree species were estimated by the averaged weight of 20 leaves. The average leaf biomass ratio to the sum of trunks and branches was 5.33% based on previous work covering all tree species in this study (Taiwan EPA, 2007). Dust retention capacity of tree species, exclusive of the 10 dominant taxa, was inferred as median values of each species. Dust retention capabilities of each woody plant species and greenbelt plant communities were subsequently estimated from leaf mass and area derived from the biomass survey described in the previous section. Average observed atmospheric PM concentration was 96.85 µg m−3 during the sampling period (Taiwan EPA, 2010).
Results
Woody plant surveys
A total area of 1.26 ha was sampled within 25 vegetation plots in 2 greenbelts along National Highways no.1 and no. 3, with the major information summarised in Table 1. Small numbers of species were not intentionally planted. These species were likely naturally seeded from outside sources. The biomass and carbon sequestered by the trunks and branches of trees in the 25 plots of the National Highway no. 1 greenbelt are listed in Table 2. All 10 dominant tree species were planted at the commencement of freeway operations for both highways. Therefore, these trees are roughly the same age and comparable for all variables. Natural seedings may have generated other tree species at different points in time. In the order of numbers of plants, the dominant species in the National Highway no. 1 greenbelt were Swietenia macrophylla (83), Terminalia boivinii (48), Pterocarpus indicus (38), Broussonetia papyrifera (37), Acacia confusa (35), Millettia pinnata (32), Bischofia javanica (32), Cassia fitula (30), Chorisia speciosa (20) and Tabebuia chrysantha (20). The biomass and carbon sequestered by the trunks and branches of trees in the 25 plots of the National Highway no. 3 greenbelt are listed in Table 3. The dominant woody plants in the National Highway no. 3 greenbelt were Ac. confusa (95), Br. papyrifera (67), Cerbera manghas (52), Pittosporum tobira (51), Alstonia scholaris (40) Ilex bioritsensis (34) Michellia compressa (33) Mil. pinnata (33) Cinnamomum camphora (32) and Casuarina equisetifolia (31).
| Characteristics | No. 1 | No. 3 |
|---|---|---|
| Years since initiating operations | 1978 | 2004 |
| Total highway length (km) | 372.8 | 431.5 |
| Average daily traffic volume | 906 430 | 525 353 |
| Total area of the greenbelt (ha) | 1226.4 | 1419.6 |
| Number of sampling plots in each greenbelt | 25 | 25 |
| Number of species in each of 25 sampling plots | 53 | 62 |
| Total number of trees in each of 25 sampling plots | 638 | 882 |
| Average tree height (m) in each of 25 sampling plots | 5.84 | 5.45 |
| Average DBH (cm) in each of 25 sampling plots | 14.25 | 9.2 |
| Species | Number of trees | IVI (%) | V trunk (m3) | Trunk biomass (t) | Branch biomass (t) | Plant biomassa (t) | Carbon storage (t) | Carbon storage per plant (kg) |
|---|---|---|---|---|---|---|---|---|
| Swietenia macrophylla | 83 | 10.52 | 5.79 | 3.19 | 0.76 | 4.93 | 2.46 | 23.80 |
| Terminalia boivinii | 48 | 5.55 | 1.93 | 0.96 | 0.23 | 1.49 | 0.75 | 15.52 |
| Pterocarpus indicus | 39 | 3.52 | 0.34 | 0.13 | 0.03 | 0.21 | 0.10 | 2.69 |
| Broussonetia papyrifera | 37 | 4.04 | 0.99 | 0.35 | 0.08 | 0.54 | 0.27 | 7.30 |
| Acacia confusa | 35 | 7.40 | 7.90 | 4.74 | 1.13 | 7.34 | 3.67 | 104.86 |
| Millettia pinnata | 32 | 4.64 | 2.18 | 0.87 | 0.21 | 1.35 | 0.67 | 21.09 |
| Bischofia javanica | 32 | 12.38 | 13.14 | 7.23 | 1.72 | 11.18 | 5.59 | 174.69 |
| Cassia fitula | 30 | 5.15 | 3.39 | 1.69 | 0.40 | 2.62 | 1.31 | 43.67 |
| Chorisia speciosa | 20 | 3.98 | 3.50 | 1.75 | 0.42 | 2.71 | 1.35 | 67.75 |
| Tabebuia chrysantha | 20 | 1.66 | 0.06 | 0.03 | 0.01 | 0.04 | 0.02 | 1.00 |
| Others | 262 | 41.16 | 26.24 | 11.90 | 2.83 | 18.41 | 9.20 | 35.13 |
| Total | 638 | 100 | 65.46 | 32.85 | 7.80 | 40.81 | 20.41 |
- IVI, importance value index.
- aWith leaves.
| Species | Number of trees | IVI (%) | V trunk (m3) | Trunk biomass (t) | Branch biomass (t) | Plant biomassa (t) | Carbon storage (t) | Carbon storage per plant (kg) |
|---|---|---|---|---|---|---|---|---|
| Acacia confusa | 95 | 13.66 | 3.73 | 2.17 | 0.49 | 2.80 | 1.40 | 14.00 |
| Broussonetia papyrifera | 67 | 5.68 | 0.81 | 0.29 | 0.11 | 0.42 | 0.21 | 2.99 |
| Cerbera manghas | 52 | 5.47 | 1.10 | 0.69 | 0.17 | 0.91 | 0.45 | 8.27 |
| Pittosporum tobira | 51 | 4.56 | 0.23 | 0.12 | 0.03 | 0.16 | 0.08 | 1.47 |
| Alstonia scholaris | 40 | 5.81 | 4.05 | 1.62 | 0.38 | 2.11 | 1.05 | 25.00 |
| Ilex bioritsensis | 34 | 2.39 | 0.13 | 0.06 | 0.01 | 0.07 | 0.04 | 1.03 |
| Michellia compressa | 33 | 3.32 | 1.13 | 0.69 | 0.11 | 0.84 | 0.42 | 12.12 |
| Millettia pinnata | 33 | 4.70 | 1.92 | 0.80 | 0.27 | 1.13 | 0.56 | 16.21 |
| Cinnamomum camphora | 32 | 4.33 | 1.43 | 0.60 | 0.14 | 0.78 | 0.39 | 11.56 |
| Casuarina equisetifolia | 31 | 3.43 | 0.93 | 0.41 | 0.13 | 0.57 | 0.28 | 8.71 |
| Others | 414 | 46.64 | 20.23 | 10.11 | 2.34 | 13.11 | 6.56 | 15.04 |
| Total | 882 | 100.00 | 35.68 | 17.57 | 4.19 | 22.90 | 11.45 |
- IVI, importance value index.
- aWith leaves.
Biomass and carbon sequestration
Importance value index, Vtrunk, trunk, branch, and plant biomass, and carbon storage results for the 25 plots in each greenbelt are provided in Tables 2 and 3 (Highways no. 1 and 3, respectively). S. macrophylla was the most numerous species; however, its IVI was second in the greenbelt of Highway no. 1. Bi. javanica IVI, Vtrunk, trunk, branch, and plant biomass, and carbon storage were the highest at 12.38, 13.14 m3, 7.23 t, 1.72 t, 11.18 t and 5.59 t, respectively. Importance value index >5.0% in decreasing order is as follows: Bi. javanica, S. macrophylla, Ac. confusa, Te. boivinii and Cass. fitula. Carbon sequestration results showed that Bi. javanica and Ac. confusa exhibited much greater capabilities than the summation of the rest of the species (i.e. 32 and 35 plants, respectively). The decreasing order of carbon storage is as follows: Bi. javanica, Ac. confusa, S. macrophylla, Ch. speciosa and Cass. fitula (>1.0 t). However, the carbon storage in decreasing order per tree is Bi. javanica, Ac. confusa, Ch. speciosa, Cass. fitula, S. macrophylla and Mil. pinnata (>20 kg). This result demonstrated that a majority of carbon sequestration was achieved by six well-established species, and it does not necessarily require many species or many individual trees of varied developmental stages.
The woody species sequestered 20.4 t of carbon from 40.81 t of biomass in 1.25 ha (25 sampling plots) in the greenbelt of Highway no. 1. The plots represented an even sampling of woody plants in the greenbelt; therefore, an estimated 19.9 thousand tons of carbon was sequestrated by trunks and branches. The trees in Highway no. 1 greenbelt sequestered approximately 20.3 t of C ha−1 after more than 30 years of highway operation. Average aboveground plant biomass per tree was 86.17 kg for the 10 dominant tree species and 70.26 kg for subordinate species. The above values were converted to 2.61 kg and 2.13 kg per tree, respectively, annually for Highway no. 1.
Among 62 tree species identified in the National Highway no. 3 plots, Ac. confusa was determined the dominant tree for every variable (Table 3). Ac. confusa IVI, trunk, branch, and plant biomass, and carbon storage exhibited the highest values: 13.66%, 2.17 t, 0.49 t, 2.66 t and 1.33 t, respectively. However, Vtrunk of Al. scholaris was the highest. Importance value index in decreasing order was as follows (>5.0%): Ac. confusa, Al. scholaris, Br. papyrifera and Ce. manghas. Ac. confusa and Al. scholaris carbon storage were, respectively, 1.33 and 1.0 t in the 25 plots. However, only Al. scholaris could sequester more than 20 kg of carbon per tree. It was also estimated that the trunks and branches of woody plants sequestered 12.3 thousand tons of carbon in the greenbelt along Highway no. 3. The trees in the Highway no. 3 greenbelt have sequestered approximately 8.6 t of C ha−1 for more than 7 years of plantation after the commencement of highway operation. These lower overall values for trees along Highway no. 3 can be attributed to the younger age of its woody plant community compared to the older one of Highway no. 1. The average aboveground plant biomass per tree was 24.87 kg for the 10 dominant tree species and 37.58 kg for the subordinate species. The above values were converted to 3.55 and 3.56 kg per tree annually for Highway no. 3. The overall standing carbon was greater for Highway 1, but the annual values were greater for Highway 3.
Dust retention
Foliar dust retention results for National Highway no. 1 are provided in Table 4. The average range of foliar dust retention on leaves was 1.45 × 10−2–1.56 × 10−1 mg cm−2, with the highest values exhibited by Ac. confusa (1.56 × 10−1mg cm−2) and Te. boivinii (1.51 × 10−1mg cm−2). Superior foliar dust retention per tree was exhibited by Ac. confusa (564.9 g), Ch. speciosa (129.6 g) and S. macrophylla (63.6 g). The physiological response of tree species under highly polluted environmental conditions might be different. Growth of some species might be deterred more than that of other species. Alternatively, Ac. confusa grew well in the roadside environment, showing superior growth and foliar dust retention. Larger leaf biomass could directly contribute to a proportionally larger total leaf area and total foliar dust retention.
| Species | Dust accumulation of 20 leaves (mg) | Area of 20 leaves (cm2) | Average foliar dust retention (mg cm−2) | Estimated total biomass of leaves from 25 plots (g) | Estimated total foliar dust accumulation on leaves from 25 plots (kg) | Estimated foliar dust accumulation on leaves from each tree (g) |
|---|---|---|---|---|---|---|
| Swietenia macrophylla | 833.3 | 7374.5 | 1.13 × 10−1 | 5.98 × 105 | 5.28 | 63.6 |
| Terminalia boivinii | 25.0 | 165.3 | 1.51 × 10−1 | 1.91 × 105 | 1.27 | 26.5 |
| Pterocarpus indicus | 21.6 | 422.9 | 5.11 × 10−2 | 3.47 × 104 | 0.34 | 8.8 |
| Broussonetia papyrifera | 14.2 | 851.5 | 1.67 × 10−2 | 1.35 × 105 | 0.17 | 4.5 |
| Acacia confuse | 26.9 | 172.7 | 1.56 × 10−1 | 8.38 × 105 | 19.77 | 564.9 |
| Millettia pinnata | 59.4 | 4091.8 | 1.45 × 10−2 | 2.13 × 105 | 0.50 | 15.7 |
| Bischofia javanica | 99.4 | 2254.3 | 4.41 × 10−2 | 1.30 × 106 | 2.81 | 87.9 |
| Cassia fitula | 408.8 | 9473.8 | 4.32 × 10−2 | 3.43 × 105 | 1.25 | 41.6 |
| Chorisia speciosa | 112.3 | 3710.4 | 3.03 × 10−2 | 3.33 × 105 | 2.59 | 129.6 |
| Tabebuia chrysantha | 202.1 | 2310.3 | 8.76 × 10−2 | 2.61 × 104 | 0.60 | 30.2 |
Results of foliar dust retention from National Highway no. 3 are listed in Table 5. Leaf foliar dust retention ranged from 2.15 × 10−2 to 1.71 × 10−1 mg cm−2, with the highest values exhibited in Casu. equisetifolia (1.71 × 10−1 mg cm−2) and Pi. tobira (1.27 × 10−1 mg cm−2). Superior foliar dust retention capability per tree basis was shown by Casu. equisetifolia (60.3 g) and Ac. confusa (20.9 g), respectively.
| Species | Dust accumulation of 20 leaves (mg) | Area of 20 leaves (cm2) | Average foliar dust retention (mg cm−2) | Estimated total biomass of leaves from 25 plots (g) | Estimated total foliar dust accumulation on leaves from 25 plots (kg) | Estimated foliar dust accumulation on leaves from each tree (g) |
|---|---|---|---|---|---|---|
| Acacia confusa | 12.8 | 141.7 | 9.03 × 10−2 | 1.8 × 105 | 1.99 | 20.9 |
| Broussonetia papyrifera | 227.2 | 3980.7 | 5.71 × 10−2 | 2.6 × 104 | 0.52 | 7.7 |
| Cerbera manghas | 69.1 | 1754.2 | 3.94 × 10−2 | 5.8 × 104 | 0.44 | 8.5 |
| Pittosporum tobira | 44.3 | 349.0 | 1.27 × 10−1 | 9.7 × 103 | 0.12 | 2.3 |
| Alstonia scholaris | 63.3 | 550.4 | 1.15 × 10−1 | 1.3 × 105 | 0.53 | 13.2 |
| Ilex bioritsensis | 622.6 | 13909.0 | 4.48 × 10−2 | 4.6 × 103 | 0.04 | 1.2 |
| Grevillea robusta | 217.4 | 2116.1 | 1.03 × 10−1 | 5.3 × 104 | 0.43 | 13.0 |
| Millettia pinnata | 71.1 | 3301.3 | 2.15 × 10−2 | 7.2 × 104 | 0.20 | 6.1 |
| Cinnamomum camphora | 15.3 | 317.8 | 4.81 × 10−2 | 4.9 × 104 | 0.38 | 11.8 |
| Casuarina equisetifolia | 27 | 158.0 | 1.71 × 10−1 | 3.6 × 104 | 1.87 | 60.3 |
Discussion
Although multiple benefits of afforestation have been often estimated in larger scales (Xu, 1995; Yemshanov et al., 2005), on-site survey is still required to characterise the above benefits in a detailed scale. Ac. confusa showed adaptive characters, exhibiting excellent growth and foliar dust retention among all surveyed species. Net aboveground primary productivities were estimated at 61 and 154 g C m−2 yr−1 for plants in greenbelts of Highways no. 1 and no. 3, exclusive of initial biomass. Higher productivities (annual values 3.55 and 3.56 kg per tree) were demonstrated by the woody plant community in the greenbelt of Highway no. 3, compared to annual values 2.61 and 2.13 kg per tree in the greenbelt of Highway no. 1. These results indicated that the woody plant community in the greenbelt of Highway no. 3 was still in its growing stage. On the other hand, the lower productivity exhibited by one of the greenbelts of Highway no. 1 suggested that its woody plant community was more matured. Nevertheless, the greenbelts of Highway 3 also appeared to have ∼30% higher tree density than that of Highway 1. This greenbelt had been subjected to the less environmental stress caused by fewer vehicle passages along Highway no. 3, which might also contribute to higher productivity. Calculating carbon dioxide equivalence in 15 US dollars per ton, the monetary values for 19.9 and 12.3 thousand tons of carbon sequestrated by the trunks and branches of woody plants in the greenbelts along Highways no. 1 and no. 3 were respectively 1155 000 and 714 000 US dollars. McPherson (1998) also estimated monetary values of urban forests in the USA.
Aboveground plant biomass per tree was 86.17 and 70.26 kg for the 10 dominant and subordinate species along Highways no. 1 and no. 3 (as indicated in Tables 2 and 3). The above values were equated to annual values 2.61 and 2.13 kg per tree for Highway no. 1. Average aboveground plant biomass per tree was 24.87 kg for the 10 dominant species and 37.58 kg for all other species. The above values were converted to annual values 3.55 and 5.36 kg per tree for Highway no. 3. These values were higher than those for trees used for transportation land use areas in Oakland, California (i.e. 36.8 kg per tree) (Nowak, 1993). In addition, 85% of the trees exhibited DBH values less than 30 cm for the Oakland transportation areas.
Woody plant population densities in greenbelts of Highways no. 1 and no. 3 were, respectively, 510 and 705 trees ha−1, as listed by Table 1. Mean annual increments of aboveground biomass were 2.41 and 4.40 kg per tree per year for all greenbelt samples for Highways no. 1 and no. 3, respectively. To identify the benefits of afforestation onto highway greenbelts and plantation forests, these results were compared with other homogeneous forest stands. The values of the present study were less than the reported values for common hardwoods in plantation forests in Taiwan. Lin et al. (2009) found 7.24 kg per tree per year for a 30-year-old Ac. confusa plantation of 530 trees ha−1 in eastern Taiwan and 5.69 kg per tree per year for a 26-year old Liquidamber formosana plantation of 11 090 trees ha−1 in eastern Taiwan. In addition, Lin et al. (2007) showed 14.92 kg per tree per year for a 45-year-old Ac. confusa plantation of 310 trees ha−1 in northern Taiwan. Lin et al. (2010) further determined 7.8 kg per tree per year for a 30-year-old Fraxinus griffithii plantation of 550 trees ha−1 in southern Taiwan and 5 kg per tree per year for 30-year-old F. griffithii plantation of 630 trees ha−1 in eastern Taiwan. Although there are many environmental differences between the highway greenbelts and the aforementioned sites, the above comparison could still assist quantifying the relative ratio of the benefits of trees in the highway greenbelts to the ones of plantation forests.
Mean annual aboveground biomass increments for woody plant species in the two highway greenbelts were lower than values of other common hardwoods in Taiwan forest plantations. Although each greenbelt exhibited its own soil and climate characteristics, the results of this study demonstrated the loss in tree productivity in heavily used roadside environments. However, the highway greenbelt vegetation remained useful for carbon sequestration. Terakunpisut et al. (2007) reported productivity values comparable to 13.98 per tree per year for non-dominant trees, with DBH values ranging from 4.5 to 20 cm for mixed natural hardwood forests in tropical Thailand. The DBH values reported above are similar to the two highway greenbelts in the present study (Table 1).
A partial relationship between leaf morphology and dust retention was inferred in this study. Ta. chrysantha and Br. papyrifera leaves were pubescent among the trees sampled. Casu. equisetifolia leaves were characterised as scurfy in The Flora of Taiwan, which was verified in the field. Leaf surfaces of other species were glabrous. Foliar dust retention values of Casu. equisetifolia were expectedly the highest among species. Tsai (2004) noted that the scurfy Casu. equisetifolia leaves and scale-shaped Juniperus chinensis leaves retained increased dust levels relative to trees with smooth leaf surfaces. Li & Liu (2008) found that differences in leaf surface cell structure also affected the dust detected from one locality. In the present study, random foliar dust retention was observed for tree species with glabrous leaves. Deposition and resuspension of foliar dusts from smooth leaf surfaces were easily impacted by wind patterns. Because the 25 plots were separated along more than 200 km for each highway, the range of metrological factors might affect foliar dust retention among the tree species evaluated in the study. Furthermore, total leaf area impacted overall foliar dust retention more than leaf morphology.
The ratio of leaf area/mass converted by total biomass resulted in an estimated total weight of retained dust by the leaves of sampled tree species of 47.94 and 17.37 t for National Highways no. 1 and no. 3, respectively. Compared with the 33 years of plant growth and community development for the Highway no. 1 greenbelt, the woody plant community on the Highway no. 3 greenbelt was established 7 years ago. Consequently, the canopies and leaf biomass on the greenbelt of Highway no. 3 were less developed than for Highway no. 1. In addition, fewer vehicle passages and even fewer vehicle passages per kilometer for Highway no. 3 decreased emissions and exhibited lower foliar dust retention compared with Highway no. 1. Nowak & Crane (2000) estimated that the removal of 1 kg of PM, less than 10 µm, was worth 4.51 US dollars, which equalled 6.22 US dollars per kg in 2010 following conversion by the Consumer Price Index issued by the US Bureau of Labor Statistics. Therefore, the trees in the greenbelts along National Highways no. 1 and no. 3 provided a respective 298.1 and 108 thousand US dollars.
Conclusions
Carbon sequestration and foliar dust retention by woody plant species in the greenbelts along two major highways in Taiwan were quantified in this study. Results indicated that plantation age and traffic density markedly affected carbon sequestration and foliar dust retention. Ac. confusa exhibited adaptive traits in the roadside environment of both highways. Excellent growth and foliar dust retention were demonstrated by Ac. confusa, compared to other tree species surveyed. The impact of total leaf area on overall foliar dust retention showed increased effects relative to leaf morphology.
Acknowledgements
The present study was sponsored by the joint project NSC98-EPA-M-009-001 from the National Science Council and Environmental Protection Administration, Taiwan, ROC.
