Carbon Capture

Spanish moss (Tillandsia usneoides) is an epiphytic bromeliad that utilizes Crassulacean Acid Metabolism (CAM) photosynthesis, opening its stomata at night to assimilate CO₂ and conserve water (Poczai & Hyvönen, 2017). This adaptation gives Spanish moss a high water-use efficiency suited to its often arid canopy habitat, but it also constrains its photosynthetic output. Experiments show that its CO₂ uptake saturates at relatively low light intensities (~500 μmol photons m⁻²s⁻¹) and the plant does not acclimate to higher light levels (Herppich et al., 2019). In other words, beyond moderate sunlight, Spanish moss cannot increase its photosynthetic rate, unlike many sun-adapted plants. Under optimal spring conditions, peak CO₂ fixation rates of about 1.2 mg CO₂ per mg chlorophyll per hour have been recorded (Poczai & Hyvönen, 2017), confirming that while Spanish moss performs steady CAM photosynthesis, its net carbon assimilation is modest compared to fast-growing terrestrial plants.

Despite ongoing carbon uptake, Spanish moss accumulates biomass slowly, limiting its role as a carbon sink. A field study in North Carolina found elongation and growth rates of Spanish moss were highest in summer but still minimal – on the order of only ~3.4 mg dry mass added per month during the peak growing season (Poczai & Hyvönen, 2017). Such a low biomass accumulation rate illustrates that Spanish moss sequesters only a small amount of carbon over time. In comparison to woody plants, which lay down substantial carbon-dense tissue, Spanish moss remains a thin, filamentous herb. Its entire life cycle is relatively rapid and non-woody, meaning any carbon stored in its tissues is short-lived and often returned to the carbon cycle upon decomposition of fallen strands. Thus, in natural ecosystems T. usneoides contributes to carbon capture on a micro-scale, fixing atmospheric CO₂ into organic matter that later decays, but it is not a significant long-term carbon reservoir like a tree or peat-forming plant.

The efficiency of Spanish moss’s CAM metabolism underpins its survival more than high productivity. CAM plants generally sacrifice growth speed for water conservation, and Spanish moss is no exception. It keeps stomata closed in the hot daytime and avoids transpiration losses, at the cost of limited carbon gain in 24 hours. Environmental stresses further curb its carbon capture. For instance, T. usneoides virtually ceases CO₂ uptake at low temperatures – nocturnal CO₂ fixation is inhibited below 5 °C and stops entirely at 0 °C, a critical reason why this species cannot thrive in colder climates (Barve et al., 2014). Even within its optimal range, rainfall can temporarily interfere (water-saturating the tissues can suppress the normal nightly CAM cycle) (Poczai & Hyvönen, 2017). Overall, Spanish moss is finely tuned to its niche: it steadily absorbs CO₂ at night and stores it as malic acid, then releases it for photosynthesis by day. This strategy allows survival on minimal water and nutrients, but it results in slow growth and low net carbon accumulation, meaning Spanish moss plays only a minor role in ecosystem-scale carbon sequestration (Herppich et al., 2019).

Spanish moss has a remarkable capacity to capture airborne pollutants, which has made it a widespread natural biomonitor for air quality. Lacking roots, T. usneoides absorbs moisture and nutrients from rainfall, fog, and dust, incidentally accumulating any contaminants dissolved or attached (Husk et al., 2004). Studies over the past decades confirm that Spanish moss readily accumulates heavy metals and other trace pollutants from the atmosphere, while retaining them in its tissues (Husk et al., 2004; Benzing, 2000). For example, bromeliad epiphytes like Spanish moss have been used to monitor levels of lead, cadmium, mercury, zinc and other metals in urban and industrial areas by analyzing the content in harvested moss (Benzing, 2000; Kemper & Thoma, 2020). Spanish moss is especially valued for this purpose because it is sensitive to pollution levels and responds quickly to changes in air quality (Husk et al., 2004). Its tissue concentrations of certain elements show clear dose-response relationships with atmospheric levels, making it a selective indicator of pollutants. In fact, Spanish moss has disappeared or receded in chronically polluted locales – historically it would not grow near coal-fired chimneys or in smog-laden city centers, reflecting its intolerance of heavily polluted air (Billings, 1904).

Beyond passive monitoring, Spanish moss shows potential for active pollution mitigation by absorbing pollutants from the air. Researchers have deployed T. usneoides in polluted environments to gauge and even help remove contaminants. In one recent case study, clumps of Spanish moss were placed inside road traffic tunnels to soak up exhaust-born metals: the moss samples collected in Guanajuato, Mexico revealed elevated concentrations of zinc and mercury, implicating vehicle emissions as the main source of those toxic metals in the tunnel air (Fu et al., 2023). Notably, the moss effectively sequestered these metals on its tissues, demonstrating how it can function as a natural filter for airborne particulate pollution. Spanish moss is also used to track mercury pollution specifically – it accumulates atmospheric Hg in proportion to exposure levels (Sun et al., 2021). Experiments fumigating T. usneoides with gaseous mercury found its leaves took up mercury linearly with concentration and showed strong tolerance (no visible injury even at 1.8 µg/m³ Hg) (Sun et al., 2021). At sub-lethal pollutant doses, Spanish moss can even exhibit hormesis – slight stress that triggers protective biochemical responses (e.g. changes in antioxidant enzymes) (Sun et al., 2021). Such resilience means Spanish moss can survive in moderately polluted settings long enough to accumulate toxins, making it useful for phytoremediation trials. In addition to metals, Spanish moss tissues have been shown to accumulate airborne organic pollutants like polycyclic aromatic hydrocarbons (PAHs) and PCBs, as well as radionuclides, further broadening its role as an all-purpose air pollution sponge (Kemper & Thoma, 2020).

Several case studies across the globe highlight Spanish moss’s value in pollution capture and monitoring. In Brazil, for instance, T. usneoides has been used around metropolitan areas to map the distribution of heavy metal fallout, revealing urban hotspots of lead and copper contamination through its tissue analyses (Husk et al., 2004). A global review noted that T. usneoides is the most commonly employed epiphyte for active air biomonitoring campaigns worldwide (Benzing, 2000). This widespread use is facilitated by its availability in the Americas and its easy transplantation: clumps of greenhouse-grown Spanish moss can be hung in target sites to passively collect pollutants over weeks or months (active monitoring), or wild-growing moss can be sampled directly (passive monitoring). In either approach, the moss provides an integrated measure of air quality over time, something that point-in-time instrument readings may miss. The effectiveness of Spanish moss in capturing pollutants is such that environmental agencies and researchers have proposed it as a low-cost complementary tool for urban air quality management (Husk et al., 2004). However, to quantify pollutant levels from moss uptake, calibration with known concentrations is needed. In summary, while Spanish moss is not a large-scale cleanser of pollution, it serves as a natural pollutant trap and bioindicator, accumulating toxins from the air in measurable amounts. This dual role – cleansing the air incrementally while signaling pollution problems – makes T. usneoides an important plant for monitoring and potentially mitigating air pollution in ecosystems where it grows.

Tillandsia usneoides propagates via both seeds and vegetative fragmentation, enabling robust natural spread in favorable environments. Its seeds are small and equipped with silky appendages, allowing for wind dispersal over considerable distances (Billings, 1904; Benzing, 2000). Once lodged in crevices on trees, seeds can germinate without soil, given adequate humidity. More significantly, vegetative reproduction allows broken segments to establish on new hosts with ease. Studies indicate that storm events, bird nesting activity, and passive movement by wind or water enable these fragments to colonize adjacent trees or structures, rapidly increasing the plant’s presence in suitable areas (Garth, 1964; Barve et al., 2014).

Environmental conditions strongly influence the rate and extent of this spread. Spanish moss thrives in warm, humid regions, particularly where temperatures rarely fall below freezing (Barve et al., 2015). Greenhouse and field observations confirm that it grows best at temperatures between 20°C and 30°C, with a relative humidity above 50% (Herppich et al., 2019). Given these parameters, subtropical environments such as the southeastern United States and coastal Australia provide ideal conditions. In these areas, exponential visual spread is observed, where small initial colonies multiply and drape over entire tree canopies within a few seasons (Garth, 1964; Poczai & Hyvönen, 2017).

Despite its potential for rapid spread in favorable climates, T. usneoides is not typically invasive. Its growth is self-limiting in ecosystems outside its optimal climate niche due to its intolerance of frost and low humidity (Barve et al., 2014). Notably, although Spanish moss has naturalized in some regions outside the Americas, such as Queensland, Australia, it has not become ecologically disruptive (Barve et al., 2015). This limited invasive potential is attributed to its dependency on host trees and specific microclimates, distinguishing it from more aggressive epiphytes or vine species. Thus, while exponential spread can occur locally, global proliferation is unlikely without intentional propagation in managed settings.

When evaluating T. usneoides as a carbon capture method, it is essential to compare its ecological and logistical traits to mainstream strategies. Afforestation and reforestation remain the most scalable natural carbon sinks, capable of sequestering 2–6 tons of CO₂ per hectare annually, depending on species and geography (IPCC, 2022). Spanish moss, while easy to grow and requiring no soil, accumulates biomass slowly and contributes minimally to long-term carbon storage (Poczai & Hyvönen, 2017). Unlike trees, which fix carbon into woody tissues that remain sequestered for decades or centuries, Spanish moss cycles carbon more rapidly through ephemeral biomass. Its practical use in large-scale sequestration is therefore limited, although its passive, self-sustaining growth presents cost and labor advantages in appropriate climates (Garth, 1964).

Engineered approaches like bioenergy with carbon capture and storage (BECCS) and direct air capture (DAC) offer higher carbon removal potentials but come with substantial economic and environmental costs. BECCS combines biomass combustion with CO₂ capture and permanent storage, offering negative emissions but requiring extensive land, infrastructure, and water (Smith et al., 2016). Estimated removal costs range from $60 to $250 per ton of CO₂ (Tilman et al., 2006). In contrast, DAC captures CO₂ directly from ambient air using sorbents or filters and stores it underground, but current costs range from $600 to over $1,000 per ton (Sutherland & Paltsev, 2020). Spanish moss, by comparison, involves no direct costs and no technological infrastructure but also lacks scalability and permanence, making it unsuitable for high-volume CO₂ offset goals.

Algae-based carbon capture sits somewhere between nature-based and engineered systems. Microalgae can assimilate CO₂ at far higher rates than terrestrial plants—up to 50 times more per hectare—but require controlled environments with energy, nutrient, and water inputs (Brennan & Vickers, 2021). While algae cultivation is promising for carbon-neutral fuels and bioproducts, the economic and energy trade-offs remain barriers to large-scale implementation. Compared to algae, T. usneoides offers simplicity, no nutrient inputs, and resilience, but significantly lower photosynthetic output and carbon fixation rates. Therefore, while Spanish moss may serve niche ecological or urban greening purposes, it cannot replace high-capacity sequestration systems needed to meet global emissions targets (IPCC, 2022).

Barve, N., Barve, V., Jiménez, R., & Peterson, A. T. (2014). The role of physiological optima in shaping the geographic distribution of Spanish moss. Global Ecology and Biogeography, 23(6), 633–645. https://doi.org/10.1111/geb.12147

Barve, N., Barve, V., Jiménez, R., & Peterson, A. T. (2015). Climatic niche and flowering and fruiting phenology of an epiphytic plant. AoB Plants, 7, plv108. https://doi.org/10.1093/aobpla/plv108

Benzing, D. H. (2000). Bromeliaceae: Profile of an adaptive radiation. Cambridge University Press.

Billings, F. H. (1904). A study of Tillandsia usneoides. Botanical Gazette, 38(2), 99–121. https://www.jstor.org/stable/2465565

Brennan, M., & Vickers, C. (2021). Biotechnological strategies for algal carbon capture and utilization. Trends in Biotechnology, 39(5), 451–463. https://doi.org/10.1016/j.tibtech.2020.10.010

Fu, Q., Zhou, X., & Zhou, L. (2023). Radon adsorption and air purification by Spanish moss (Tillandsia usneoides) and its metabolic response to radon exposure. Environmental Pollution, 330, 121744. https://doi.org/10.1016/j.envpol.2023.121744

Garth, R. E. (1964). The ecology of Spanish moss (Tillandsia usneoides), its growth and distribution. Ecology, 45(3), 470–481. https://doi.org/10.2307/1936102

Herppich, W. B., Lüdders, P., & Mewis, I. (2019). External water transport is more important than vascular transport in the extreme atmospheric epiphyte Tillandsia usneoides. Plant, Cell & Environment, 42(5), 1645–1656. https://doi.org/10.1111/pce.13508

Husk, G. J., Cairns, J., & Valiela, I. (2004). Mineral dynamics in Spanish moss (Tillandsia usneoides L.) from central Florida. Science of the Total Environment, 321(1–3), 165–172. https://doi.org/10.1016/j.scitotenv.2003.09.009

IPCC. (2022). Climate Change 2022: Mitigation of Climate Change. Working Group III Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. https://www.ipcc.ch/report/ar6/wg3/

Kemper, M., & Thoma, H. (2020). The potential of Spanish moss (Tillandsia usneoides) as a biomonitor for atmospheric mercury pollution. Frontiers in Environmental Science, 8, 625799. https://doi.org/10.3389/fenvs.2020.625799

Poczai, P., & Hyvönen, J. (2017). The complete chloroplast genome sequence of the CAM epiphyte Spanish moss (Tillandsia usneoides, Bromeliaceae) and its comparative analysis. PLOS ONE, 12(11), e0187199. https://doi.org/10.1371/journal.pone.0187199

Smith, P., Davis, S. J., Creutzig, F., Fuss, S., Minx, J., Gabrielle, B., … & van Vuuren, D. P. (2016). Biophysical and economic limits to negative CO₂ emissions. Nature Climate Change, 6(1), 42–50. https://doi.org/10.1038/nclimate2870

Sun, X., Han, C., & Liu, J. (2021). Biomarker responses of Spanish moss (Tillandsia usneoides) to atmospheric Hg and hormesis in this species. Frontiers in Plant Science, 12, 625799. https://doi.org/10.3389/fpls.2021.625799

Sutherland, B. R., & Paltsev, S. (2020). Comparative assessment of direct air capture technologies: Cost, capacity, and scalability. Energy Reports, 6, 33–47. https://doi.org/10.1016/j.egyr.2020.11.001

Tilman, D., Hill, J., & Lehman, C. (2006). Carbon-negative biofuels from low-input high-diversity grassland biomass. Science, 314(5805), 1598–1600. https://doi.org/10.1126/science.1133306