Exotic Tree Species Outnumbering Native Tree Species: an Alarming Situation for the Sustainable Urban Plantation Development

This present study mainly focuses on the tree distribution and diversity of some selected roadsides in Faisalabad, a major city in Punjab, Pakistan. A total census of the tree population on selected roads was carried out and then studied for various parameters. Shannon–Wiener index, Reciprocal of Simpson’s diversity index, and Shannon Evenness were among the different species diversity indices which were studied. Observational results showed that the selected roads in the city hold low tree diversity, with nearly 2858 trees of 34 species, 30 genera, and 15 families. Mainly exotic tree species dominated the plantation of selected roadsides. The topmost three species contributing approximately 60% of the tree population are Conocarpus erectus, Eucalyptus crebra, and Terminalia mantaly. Despite the dominance of exotic tree species, many native tree species are also growing along selected roadsides. Since, Faisalabad has been reported as one of the most polluted cities in the world due to high suspended particulate matter, appropriate efforts are needed to improve the plantation pattern and also to enhance the canopy of the trees in the city to minimize the problem of air pollution. native tree species. The results depicted that tree diversity and diversity indices of selected roadsides are extremely less than required. By keeping in view, the ndings of the present study and the literature of past studies, the government sector as well as private sectors involved in the plantation along the roads of the city of Faisalabad should emphasize the plantation of native trees in proper proportion and balance, which will ultimately help to improve the sustainable ecological functioning of the city.

functions of green spaces. To provide this vast variety of ecosystem services, urban trees must withstand a stressful urban environment in order to mature. It is critical to select adaptable trees to ensure the durability of urban tree populations. (Sjöman et al. 2016). Nonetheless, selecting species suited to urban environments is seen as a di cult task. (Bassuk et al., 2002). According to certain research, native tree species have greater health than exotic ones since they are better adapted to local circumstances. A diverse range of species and genera has been recommended as a crucial approach for maintaining a sustainable and healthy urban tree population. (Bassuk, 1988). The most imperious and most common dispute over the elevated level of diversity is the frequent occurrence of invasive diseases and pests in frequently used tree species (Tello et al., 2005;Raupp et al., 2006). Barker (1975) rstly proposed the usage of a diverse variety of species. According to him, no single species accounts for more than 5% of the entire population of trees. Smiley et al. (1986) and Miller and Miller (1991) suggested that a single species should not have more than 10% of the tree population. Grey and Deneke (1986) also suggested the same viewpoint, arguing that no single species should have contributed more than 10-15 percent of the total tree population. In another study, Moll (1989) advises that a single specie contributes approximately 5% of the total tree population and no genus should exceed 10%. Santamour (1990) broadens the rule as he aims at maximum usage of species and genera from the same family; a single species should not exceed more than 10% of the total population; in case of genera, 20% is the limit; and no family should exceed 30%. Such species choice strategies are essential guides for a more diversi ed usage of tree species in the urban context.
Trees have a vital role in creating landscapes by performing crucial ecological, social, and economic services as well as providing beauty to the human environment. (Nowak et al., 2006). Despite covering less than 3% of the worldwide terrestrial surface, cities account for over 3/4 of carbon emissions and industrial wood demand, as well as roughly 60% of household water usage. (Singh et al., 2010).
Nonetheless, despite ample evidence of trees' important function in urban settings, urban planners and managers have frequently underestimated this role. When developmental projects, especially the widening of roads take place, it is customary practice to sacri ce roadside trees rst, especially in rapidly developing cities. There is a need for data on tree census and several other elements for sustainable tree plantation along roadsides. (Maco and McPherson, 2003). The primary goals of this study were to collect information on tree distribution, species composition, diversity index, and tree density along various roadsides of Faisalabad which will help us to analyze the existing situation of the city roadside plantation. The annual rainfall is 346 mm (13.6 inches). Faisalabad City has changed tremendously in the past two decades, with various infrastructural development including the construction of o ce buildings, schools, residences, and roadways to meet the requirements of the ever-rising city's population.

Data collection and analysis:
The research study focuses on some of the most notable roadside sites in Faisalabad City. Six main roadsides were chosen for this purpose, totaling 27.36 km and varying in length from 1.81 km to 6.55 km. The entire study was based on a direct tree census, which was conducted in the survey area in 2020. Trees along roadsides were evaluated and counted up to a radius of 15 meters from the center of the road. Trees with DBHs of more than 10 cm (CBH greater than 30 cm) were evaluated. where BA stands for basal area (m 2 ), represents diameter at breast height (m), and valued as (3.142). By summing the BA of all selected roadside trees, the total BA for the selected roadsides was determined.
Species Relative Density (RD). It is an indicator used for analyzing relative distribution of species (Brashears et al., 2004). Species Relative Density was calculated using the following equation: where RD (%) represents species relative density, n i counts the individuals of species I, and N represents the count of total individual trees of all tree species along selected roadsides.
Species Relative Dominance (RD ). Species Relative Dominance (RDo), which is used to measure a tree's relative space occupancy, was calculated using (Aidar et al., 2001).
where Bai represents the total basal area of all trees of a speci c species I, and Ba n represents the total basal area of all trees on selected roadsides. Importance Value Index (IVI). The Importance Value Index (IVI) of each species was calculated using the equation below. (Brashears et al., 2004):

Species diversity indices and statistical computation
A diversity index, in terms of numerical value, indicates the structure of a biological community. It typically provides more information regarding community diversity than merely species abundance. Furthermore, the diversity index can provide information about the commonness and rarity of species in an ecosystem, making it a signi cant tool for scientists in their understanding of ecosystems.
Simpson (Simpson, 1949), Shannon Evenness and Shannon-Wiener index (Lloyd et al.,1968) were selected as species diversity indicators. The Simpson index determines the probability that a second individual chosen from a population (vegetation community) will be of the same species as the rst. The Shannon Evenness (H E ) refers to the pattern of individual distribution among species. The Shannon-Wiener index measures a vegetative community's species heterogeneity.
Simpson's Index. Simpson's index of diversity is computed as: Where, N represents the total number of tree species in an area and 'n' represents the total number of a speci c tree species. The reciprocal of Simpson's index was employed in this study to determine the tree diversity of a selected roadside. Shannon Evenness. Shannon evenness was computed using the following formula:: Where H represents the Shannon index and S represents the species richness Shannon Index of Diversity. Shannon index of diversity (H / ) is used for quantifying biodiversity in a speci c area, and it is computed as where N represents the total number of tree species in an area and pi, represents the relative abundance of the ith species. The measure of this diversity index spans from 0 to in nity in principle, combines elements of species richness and evenness, and increases as the number of tree species increases. (Stohlgren, 2007).
Biomass Index. The biomass index is used to estimate the contribution of individual tree species to the treescape and the provision of services in an ecosystem. It is calculated as follows: Where, trees of (5 -9m) height are small, (9-18 m) are medium, and (>18 m) are large tree species and scored as 1, 2, and 3 accordingly (Jim and Chen, 2009a).
Tree density. Tree density was estimated by using following equation:

Species Richness and Diversity
The ndings of the current study showed the existence of 2858 trees belonging to 34 species, 30 genera, and 15 families along chosen roads in the city. The dominant species on the basis of numbers are Conocarpus erectus (1332), followed by Euclyptus crebra (205) and Terminalia mantaly (196). (Table 1). According to the ndings, the species relative density (RD) of individual trees along chosen roadsides ranged from 0.03 to 46.60 percent (Table 1). Tree species with a high relative density (RD) were Conocarpus erectus, Eucalyptus crebra, and Terminalia mantaly, accounting for (46.60%), (7.17%), and (6.85%) respectively, Species Relative Dominance (RDo) varied from 0.00005 to 15.10% (Table 1). Tree species with high relative dominance (RDo) were Conocarpus erectus, Eucalyptus crebra, and Azadirachta indica accounting for (15.10%), (1.50%), and (0.40%) respectively. Tree species with the highest Importance Value Index were observed in Conocarpus erectus (30.8), followed by Eucalyptus crebra (4.33), and Terminalia mantaly (3.53%) on the selected roadsides of Faisalabad. Maximum biomass index percentage was shown by C. erectus (50.60%), followed by E. crebra (11.68%), and T. mantaly (7.44%). Out of all, these species account for (69.72%) of the total biomass ( Table 1). Out of a total of 15 families to which tree species belong, both Fabaceae and Moraceae rank rst with 6 species each, followed by Areaceae (4) (Table 1). Among all the roadsides, maximum species richness was recorded at Narwala Road (24) while minimum species richness was recorded at Sargodha Road (12). The maximum number of trees was recorded at Narwala Road (570) while the minimum number of trees was recorded at Jhang Road (318). Maximum tree density was recorded at Qaim Sain Road (225.96 trees/km) while minimum tree density was recorded at Narwala Road (91.48 trees/km). (Table 2).

Origin of the Species
Out of the total tree species existing at chosen roadsides, 22 are native, and 12 are exotic species (Table 3). Native tree species contribute 26% of the total tree count, with 25.75% of attainable biomass. Exotic tree species contribute 74% of the total tree count, with 74.25% of attainable biomass. The results of the biomass index percentage reveal that the maximum biomass index percentage of native trees was recorded at Narwala Road (40.49%) while the minimum biomass index of native trees was recorded at Sargodha Road (6.39%). The maximum biomass index percentage of exotic trees was recorded at Sargodha Road (93.61 %) while the minimum biomass index percentage of exotic trees was recorded at Jhang Road (59.51%). Maximum species richness of native trees was recorded at Narwala Road (16), while minimum species richness of native trees was recoded at Sargodha Road (9). Maximum species richness of exotic trees was recorded at Jail Road (9), while minimum species richness of exotic trees was recorded at Daewoo Road and Sargodha Road (4). A maximum number of native trees was recorded at Jail Road (210), while a minimum number of native trees was recorded at Sargodha Road (50). A maximum number of exotic trees was recorded at Sargodha Road (508) and a minimum number of exotic trees was recorded at Jhang Road (203). The maximum density of native trees was recorded at Qaim Sain Road (79 trees/km), while the minimum tree density of native trees was recorded at Sargodha Road (7.63 trees/km). The maximum tree density of exotic trees was recorded at Qaim Sain Road (146.96 trees/km), while the minimum tree density of exotic trees was recorded at Jhang Road (40.76 trees/km) ( Table 2). those with lower species diversity, since a large number of species have a variety of characteristics that can contribute to different functions. The roadsides of Faisalabad city have lower tree diversity, as only the rst four species, including Conocarpus erectus, Euclyptus crebra, Terminalia mantaly, and Thevetia peruviana contributed more than half of the total tree proportion of the area (65.3%). C. erectus individually contributed 46.6% of the total tree proportion and ranked rst in abundance. This shows that C. erectus is growing almost in a monoculture plantation pattern on the majority of selected roads. Researchers have criticized the single-species monocultural plantation pattern for allegedly having a number of harmful social and environmental consequences despite the documented bene ts at economic level. (Erskine et al., 2006;Alem et al., 2015). Loss of soil fertility and productivity, disturbance of the water cycle, hazards related to practices of forestry plantation (e.g., exotic species introduction), dangers of helping diseases and pests, increased danger of extremely adverse storm and re effects, and bad effects on biodiversity are just a few of the environmental consequences. (Baltodano, 2000;Evans, 2000;Bowyer, 2006). Thomson et al. (2016) in his study of street trees in 16 municipalities in Denmark, reported that genus Tilia individually contributed 26% of the total tree proportion in the study area.
On the roadsides of Faisalabad city, exotic species are present in greater adundance (78.4%) than native ones (21.5%). Our results agreed with those of Nowak (1993) and Nagendra and Gopal (2011). These researchers found that exotic species predominated in the urban woods of Oakland (USA) and Bangalore (India) by 69 and 77 percent, respectively. Further, Thomsen et al. (2016) recently reported the dominance of exotic species (71%) in some 16 large Danish municipalities. However, Sanders (1981), Hobbs (1988) and Zipperer (2002) reported contrary results, 85%, 64%, and 73% dominance of native species respectively for the Syracuse, St. Paul and Minneapolis areas of Minnesota, and Miami-Dade County of Florida, USA. Exotic tree species can also play a signi cant role in the ecosystem of the urban forest. The majority of existing species are urban exploiters in the urban environment, which may be found in urban locations all over the world. Furthermore, the value of some ecosystem services offered by exotic species is similar to, if not higher than, that offered by indigenous species. (Drayton and Primack, 1996). However, owing to continual competition between natives and exotics, urban woods are losing a substantial amount of their native species. The presence of a higher number of exotic tree species along the roadsides of Faisalabad can result in an ecological imbalance of ecosystem services. One of the most signi cant challenges to natural ecosystems is invasive or exotic tree species. (Brundu and Richardson, 2016;Hejda et al., 2017). Due to species-speci c effects on light availability (Niinemets,  respectively. These diversity indices evidently shows that the selected roadsides of Faisalabad City possess low tree species evenness and diversity. Our results are not in line with the study conduct in Gwalior city by (Bhat et al., 2016) who reported moderate value of diversity indices for roadside trees. Our results also con ict with the ndings of Jim and Zhang (2013), who calculated the tree diversity of the roadsides of Hong Kong and reported a higher value of the diversity indices. The results of the present study are also contrary to those of Nagendra and Gopal (2010), who stated a moderate value of the Shannon-Weiner index in Bangalore for roadside trees. The current study of selected roadsides depicted that the rst four tree species make a signi cant contribution (65.3 percent) to the total tree count, which results in reduced species evenness. All these four species, which make up about 65.3% of the tree community on different roadsides of Faisalabad, are exotic in nature. The dominance of exotic trees along roadsides is a serious threat to the sustainability of ecosystem functioning. Planting more native tree species could help to create biologically healthy urban forests along the roadsides, which will help not only humans but also improve the sustainability of urban ecosystems. A rich composition of species prevents the attack of pests on such species (Jim and Chen, 2009b). Wild life in the area may be attracted more when there is more species richness in the area.

Conclusion
During the study period, a total of 2858 trees of 34 species were noted at chosen roadside locations in Faisalabad City. The major tree species on the selected roads was C. erectus (46.6%). The Combretacea family dominated the selected roadsides, accounting for (53.54%) of the total tree population. All the selected roadsides of the city are mainly occupied by exotic tree species (73.03%) like C. erectus, E. crebra ,T. mantaly and T. peruviana. Native species have only (26.07%) occupancy along the selected roadsides. C. erectus, being a hardy and easy to grow specie, is spreading very quickly in the city of Faisalabad. Many different exotic tree species in higher numbers are growing and needed to be carefully substituted by bene cial native tree species. The results depicted that tree diversity and diversity indices of selected roadsides are extremely less than required. By keeping in view, the ndings of the present study and the literature of past studies, the government sector as well as private sectors involved in the plantation along the roads of the city of Faisalabad should emphasize the plantation of native trees in proper proportion and balance, which will ultimately help to improve the sustainable ecological functioning of the city.

Declarations
Funding Not applicable

Declaration of competing interest
The authors declare that they have no known competing nancial interest or personal relationships that could have appeared to in uence the work reported in this paper.  Figure 1 Map of Study Area Figure 2 Diversity of different families along the selected roads of Faisalabad