Tree species richness of SGS was found to high (34 ± 12) than PJS (9 ± 6). In SGS Ziziphus jujuba (32 ± 16 /ha) was found to be most abundant followed by Vachellia leucophloea (25 ± 15 /ha), Borassus flabellifer (27 ± 16 /ha), Morinda coreia (26 ± 16 /ha), Acacia nilotica (27 ± 11 /ha), Ficus religiosa (24 ± 14 /ha), Diospyros ebenum (21 ± 19 /ha), Albizia amara (21 ± 17 /ha), etc. Least abundant species was Mimusops elengi (4 ± 2 /ha), Chloroxylon swietenia (4 ± 1 /ha) (Fig. 4a). In PJS Prosopis juliflora (104 ± 36 /ha) was the most dominant tree followed by Borassus flabellifer (63 ± 38 /ha) and Albizia lebbeck (62 ± 21 /ha) etc (Fig. 4b). Moreover, family richness of the SGS also recorded high (range: 9–26), in contrast to PJS (range: 5–8). However, in both the vegetation types, Fabaceae family accounted to be the most abundant (Fig. 4c, d).
3.3. Morphological, Biochemical and Eco-Physiological assessment in selected tree species
The mean adaxial and abaxial stomatal density among the tree species (selected for the experiment) showed significant variation (p < 0.01*** in Kruskal-Wallis rank sum test) (Fig. 6a). Azadirachta indica exhibited the highest density at 1300 ± 198 mm2, while Prosopis juliflora displayed the lowest density at 287 ± 148 mm2 (Fig. S 1).
The mean total chlorophyll levels were found to be high in Ficus religiosa (3.3 ± 1.1 mg/g), while the species with the lowest chlorophyll content was Acacia nilotica (1.3 ± 0.87 mg/g). The higher proportion of chlorophyll in Ficus religiosa can be attributed to its greater demand for chlorophyll due to its larger mass and proportions (Fig. 6b). Prosopis juliflora exhibited highest mean plant Total Organic Carbon (leaf, stem, bark) followed by Wrightia tinctoria, Acacia nilotica, Prosopis juliflora, Albizia lebbeck, Azardiracta indica, Ficus religiosa, Ficus benghalensis, and Terminalia arjuna, The highest storage of organic carbon in Wrightia tinctoria can be attributed to its ability to fix larger proportions of organic carbon in its plant body (Fig. 6c).
The photosynthesis rates of selected tree species showed significant interspecific variation (p < 0.01***) in the Kruskal-Wallis rank sum test. Among the trees, Azadirachta indica and Prosopis juliflora exhibited higher photosynthesis rates, measuring 8.1 ± 2.0 µmol CO2/m2/sec and 8.0 ± 1.86 µmol CO2/m2/sec, respectively. The remaining species were ranked as follows: Azadirachta indicaa > Prosopis juliflorab> Terminalia arjunac > Wrightia tinctoriad > Acacia niloticae > Ficus benghalensisf > Albizia lebbeckh > Ficus religiosag. Kruskal-Wallis test and Pairwise comparisons using the Wilcoxon rank-sum test with continuity correction were conducted to assess the difference in mean photosynthesis rates of each species, results revealed species sharing different letter (in superscript) exhibited significant differences at p < 0.05. These findings underscore the variations in photosynthesis rates among the studied tree species, providing valuable insights into their relative photosynthetic capacities (Fig. 6d).
In the case of Stomatal Conductance, Azadirachta indicaa showed higher values (0.42 ± 0.15 milli mol/m2 /sec), followed by Ficus religiosab, Ficus benghalensisd, Prosopis juliflorac, Albizia lebbecke, Wrightia tinctoriaf, Acacia niloticag, and Terminalia arjunah. Kruskal-Wallis test and Pairwise comparisons using the Wilcoxon rank-sum test with continuity correction conducted to examine the difference in mean stomatal conductance rate of each species; species sharing different letter indicate significant differences at p < 0.05 (Fig. 6e).
The mean transpiration rate of tree species was found to be highest in Ficus religiosaa, followed by Azadirachta indicag, Ficus benghalensisb, Wrightia tinctoriac, Prosopis juliflorad, Acacia niloticae, Terminalia arjunaf, and Albizia lebbeckg. Kruskal Wallis test and Pairwise comparisons using the Wilcoxon rank-sum test with continuity correction were employed to analyze the difference in mean transpiration rate of each species; results revealed species sharing the different letter (in superscript)exhibit significant differences at p < 0.05 (Fig. 6f).
Leaf intracellular CO2 was observed to be higher in Ficus religiosaa and Ficus benghalensisb, followed by Azadirachta indicac, Wrightia tinctoriaf, Albizia lebbeckd, Terminalia arjunag, Prosopis juliflorae, Acacia niloticah,. The higher intracellular CO2 concentration in Ficus religiosa and Ficus benghalensis can be attributed to their greater leaf thickness compared to the other selected tree species. Kruskal Wallis test and Pairwise comparisons using the Wilcoxon rank-sum test with continuity correction were employed to analyze the difference in mean leaf intracellular CO2 concentrations of each species; results revealed species sharing the different letter (in superscript)exhibit significant differences at p < 0.05 (Fig. 6g).
<Insert Fig. 6.>
Mean RuBisCO content within the selected 8 tree species was found to be very high in Prosopis juliflora (10.8 ± 1.08 micromole/m2) compared to other trees implying its higher efficiency in primary productivity. Within SGS, RuBisCO content varied in order: Acacia nilotica > Albizia lebbeck > Azardiracta indica > Wrightia tinctoria > Ficus religiosa > Terminalia arjuna > Ficus benghalensis (Fig. 7a). However, total RuBisCO activity among the selected species was found higher in Wrightia tinctoriaa> Prosopis juliflorab> Albizia lebbeckc> Acacia niloticad> Azadirachta indicae> Ficus benghalensisf> Ficus religiosag> Terminalia arjunah (Fig. 7b).
<Insert Fig. 7.>
3.4 Assessment carbon fixing Morphological, physiological functional traits of selected tree species (8) using Structural Equation Model
Based on the initial screening from Pearson’s correlation, a best model fit was attained for the Structural Equation Model (SEM) testing the effects of environmental conditions, morphological, biochemical and physiological functional trait of selected trees on photosynthesis of the tree species (Fig. 8A, B). Exogenous variables such as stomatal density, ambient CO2, ambient temperature, available soil nitrogen and soil moisture had significantly impacted (positive/negative) the endogenous variable photosynthesis.
Effects of exogenous and endogenous variables on photosynthetic rate of selected trees
SEM derived outputs on the effects of stomatal density on photosynthesis rate of tree species found a predominant negative relationship among the species with higher stomatal density. Higher stomatal density (Stomatal density ≥ 1000 mm2) showed a direct negative effect on photosynthesis rate of the following test species - Azadirachta indica (Estimated coefficient: -0.12), Albizia lebbeck (-0.09). However, in Terminalia arjuna (0.09), Prosopis juliflora (0.05), Acacia nilotica (0.10), Wrightia tinctoria (0.16), Ficus benghalensis (0.15), and Ficus religiosa (0.16), Stomatal density < 1000 mm2 observed to have positive effect on photosynthetic rate (Fig. 8A, B).
Ambient CO2 levels had an indirect and negative effect on the optimal photosynthetic rate in the case of Azadirachta indica (Estimated coefficient: -0.03) and Albizia lebbeck (Estimated coefficient: -0.01). However, other species exhibited indirect, neutral and positive relationships to ambient CO2 levels (Fig. 8A, B). Ambient temperature between the morning hours (32.5 ± 2.02°C) had overall direct positive effect on photosynthesis rate of all the selected tree species, but photosynthesis rate of Prosopis juliflora (Estimated coefficient: -0.09) and Acacia nilotica (-0.24) were indirectly affected by temperature through RuBisCO (Fig. 8A, B).
A positive direct effect was observed between soil nitrogen, chlorophyll and RuBisCO. Subsequently, a direct positive relation was witnessed between chlorophyll, RuBisCO and photosynthesis in all the eight species. In all the test species soil moisture observed to have a very positive effect on photosynthesis. Our findings advocate that the high carbon fixing efficiency of Albizia lebbeck, Prosopis juliflora, Wrightia tinctoria, and Acacia nilotica can be attributed to the trait of Fabaceae family, which enables them to fix nitrogen to the soil, and soil nitrogen is explained to have an indirect positive relation with photosynthesis. Furthermore, it is understood that the nitrogen availability plays a crucial role in supporting essential biosynthetic metabolic processes. SEM elucidated the carbon sequestering potential of selected tree species depicted following hierarchy: Wrightia tinctoria (estimated coefficient: 1.28) > Prosopis juliflora (1.22) > Acacia nilotica (1.21) > Albizia lebbeck (0.97) > Azadirachta indica (0.74) > Ficus religiosa (0.56) > Ficus benghalensis (0.37) > Terminalia arjuna (0.26) (Fig. 8A, B.).
<Insert Fig. 8A & B.>
Effects of endogenous physiological variables such as RuBisCO, Chlorophyll, and intracellular Co2 on photosynthesis rate of eight selected tree species didn't exhibit uniformity. All the three endogenous variables exhibited a positive direct effect on photosynthesis in the following tree species Prosopis juliflora (Estimated coefficients: 0.39, 0.24, 0.07), Acacia nilotica (0.39, 0.24, 0.07), Albizia lebbeck, Wrightia tinctoria, Terminalia arjuna (0.04, 0.12, 0.11). But in case of Azadirachta indica (-0.05), Ficus benghalensis (-0.01) and Ficus religiosa (-0.09) leaf intracellular Co2 tend to have a direct negative effect on optimal photosynthesis, also RuBisCO (Azadirachta indica: 0.46; Ficus benghalensis: 0.26; Ficus religiosa: 0.59) and chlorophyll (Azadirachta indica: 0.68; Ficus benghalensis: 0.02; Ficus religiosa: 0.40) observed to have direct positive relation (Fig. 8A, B.).
A negative relationship between temperature and RuBisCO was observed in Prosopis juliflora and Acacia nilotica. However, in Ficus religiosa, Ficus benghalensis, Azardiracta indica, Albizia lebbeck, Wrightia tinctoria, and Terminalia arjuna, temperature had no notable effects on RuBisCO. This trait could be could be attributed to the leaf thickness, as the leaf thickness lessens enzymes are susceptible to high temperature (Fig. 8A, B.).
SEM elucidated the carbon sequestering potential of selected tree species in the following hierarchy Wrightia tinctoria (estimated coefficient: 1.28) > Prosopis juliflora (1.22) > Acacia nilotica (1.21) > Albizia lebbeck (0.97) > Azadirachta indica (0.74) > Ficus religiosa (0.56) > Ficus benghalensis (0.37) > Terminalia arjuna (0.26) (Fig. 8A, B.).
3.5 Tree age prediction model using random forest algorithm and maximum life expectancy of selected tree species
Pearson's correlation analysis demonstrated a positive relationship between the circumference of trees and the number of their growth rings, based on the relation a decision tree model was constructed (Fig. S 8) for all the selected eight species. Among the trees selected for study, Ficus religiosa and Ficus benghalensis exhibited longer life spans, with recorded mean ages of 55 ± 45 years and 58 ± 42 years, respectively. On the other hand, Prosopis juliflora and Terminalia arjuna displayed shorter life spans, falling within the mean age of 25 ± 23 years and 25 ± 22 years, respectively (Fig. 9). While these trees thrive amidst the polluted urban environment, which significantly affects the lifespan of trees and increases mortality rate, these findings indicate that selected trees might possess the capability to acclimate to local polluted conditions and thrive despite environmental pressures. It's worth noting that trees with extended life spans can offer various ecosystem services to the surrounding urban environment (Fig. 9).
<Insert Fig. 9.>
3.6 Carbon stock estimation and carbon dynamics in Sacred Groves Stands (SGS) and Prosopis juliflora Stands (PJS)
Above and Below Ground Biomass proportions of Sacred Groves Stands (SGS) and Prosopis juliflora Stands (PJS)
Tree stand density in PJS stood at 1056 ± 474/ha, while in SGS it is comparatively low (584 ± 331 /ha). Similarly in SGS, the basal area (25 ± 11 m2/ha) is also lower than PJS (38 ± 18 m2/ha). Leaf area index of PJS was higher (2.69 ± 0.54 m2/m2) compared to SGS (1.34 ± 0.66 m2/m2). Concomitantly, Above Ground Biomass (AGB), Below Ground Biomass (BGB) and Carbon Stock (CS) was observed to be high in PJS (AGB = 56 ± 35 tons/ha; BGB = 16.2 ± 10.3 tons/ha; CS = 32 ± 20.6 tons/ha) than SGS (AGB = 45 ± 31 tons / ha; BGB = 9.6 ± 8.8tons/ha ; CS = 27.3 ± 22.4 tons/ha); Kruskal Wallis test determined the significance with p < 0.05*for AGB; p < 0.05* for BGB; and p < 0.05* for Carbon Stock between PJS and SGS (Fig. 10).
<Insert Fig. 10.>
Carbon dynamics in Sacred Groves Stands (SGS) and Prosopis juliflora Stands (PJS)
Within the PJS, the presence of young plants in young stands showed comparatively higher values of Net Ecosystem Productivity (NEP) and could possibly act as carbon sink (0.0012 ± 0.0004 g C/m2/day). However, the matured stands within PJS, showed a shift towards carbon source, which is evident in the NEP value of -0.0006 ± 0.0003 g C/m2/day; the old growth PJS records an NEP value of -0.0034 ± 0.0012 g C/m2/day, clearly indicates the transition towards carbon source. In SGS among all the age groups Net Ecosystem Productivity values (Young: 0.07 ± 0.02 g C/m2/day, Mature: 0.13 ± 0.02 g C/m2/day, Old: 0.06 ± 0.01g C/m2/day) identified the stands to be carbon sinks for a very prolonged period when compared to PJS (Fig. 11; Table.1).
<Insert Fig. 11; Table. 1.>