The increasing concentration of CO₂ in the atmosphere has caused significant environmental changes, particularly to the lower plants such as terrestrial algae and lichens that alter species composition, and therefore can contribute to changes in community landscape. A study to understand how increased CO₂ in the atmosphere will affect algal density with minimal adjustment on its natural ecosystem, and the suitability of the algae to be considered as a biomarker, has been conducted. The current work was conducted in the Free-Air-Carbon Dioxide-Enrichment (FACE) system located in Universiti Kebangsaan Malaysia, Bangi, Malaysia. CO₂ was injected through special valves located along the ring surrounding specimen trees where 10 × 10 cm quadrats were placed. A total of 16 quadrats were randomly placed on the bark of 16 trees located inside the FACE system. This system will allow data collection on the effect of increased CO₂ without interfering or changing other parameters of the surrounding environment such as the wind speed, wind direction, humidity, and temperature. The initial density Trebouxia sp. was pre-determined on 1 March 2015, and the final density was taken slightly over a year later, on 15 March 2016. The exposure period of 380 days shed some light in understanding the effect of CO₂ on these non-complex, short life cycle lower plants. The results from this research work showed that the density of algae is significantly higher after 380 days exposure to the CO₂-enriched environment, at 408.5 ± 38.5 × 10⁴ cells/cm², compared to the control site at 176.5 ± 6.9 × 10⁴ cells/cm² (independent t-test, p < 0.001). The distance between the trees and the injector valves is negatively correlated. Quadrats located in the center of the circular ring recorded lower algal density compared to the ones closer to the CO₂ injector. Quadrat 16, which was nearing the end of the CO₂ valve injector, showed an exceptionally high algal density-2-fold higher than the average density at 796 ± 38.5 × 10⁴ cells/cm². In contrast, Quadrat 9, which was located in the center of the ring (lower CO₂ concentration), recorded only 277 ± 38.5 × 10⁴ cells/cm², which further supports the previous claim. Based on the data obtained, this study provides useful data in understanding the positive effect of CO₂ on algal density, in a natural environment, and suggests the use of epiphytic terrestrial algae as a biomarker.