How Does Canopy Cover in Deserts Impact Soil, Biodiversity, and Urban Climates?

• Vegetation in deserts is patchy and acts as islands of resources.
• Plant canopy can increase soil fertility, moisture, and microbial diversity.
• The tree canopy has several benefits for urban settlements in deserts, such as reducing temperatures, air pollutants, and carbon emissions.

Deserts make up 33% of terrestrial ecosystems and are present on nearly every continent. They contribute biodiversity and ecosystem services from microsites to global scales. Canopy cover is one of the chief means of understanding vegetation’s impact and ecosystem services in deserts, some of which are discussed in this article.

Typical Desert Vegetation

Deserts characteristically have vegetation growing in patches with vast empty spaces in between. The vegetation patches act as barriers, collecting dust, organic matter, and water during soil and rain erosion, gradually becoming resource islands. Climate change is predicted to make deserts more arid, which can result in vegetation loss and an increase in the gaps, affecting the ecosystem services they offer. However, the effects of aridity-driven desert vegetation loss are not well understood. To do this, it is essential to know the role of desert vegetation in deserts.

Soil Quality

1a)	1b)

Figure 1: “1a) Organic matter concentration in soil under the canopy, at 150 and 300 cm away from canopy of Capparis decidua. 1b) Nitrogen concentration in soil under the canopy, at 150 and 300 cm away from canopy of Capparis decidua,” Yasin et al. (2016). (Image credits: DOI: https://doi.org/10.35691/JBM.6102.0050).

Soil fertility depends on inorganic and organic materials. The soil organic matter (SOM) is derived from plant and animal materials returned to the soil. Add SOM to the roots of plants and their exudates, as well as leaf and woody litter. The organic matter from leaf and shoot litter is usually concentrated in the upper horizons of the soil and only under the canopy. As the distance from the canopy increases, the SOM content decreases. For example, in the Thal desert in Pakistan, organic matter and nitrogen content in soils are highest under the canopy, where the contribution from shoot detritus exists. They are less far away, where only root organic matter exists, see Figure 1.

The SOM provides several advantages:

• The litter acts as a mulch, reducing soil water loss.
• SOM in the upper soil improves the soil’s water-holding capacity.
• SOM improves and is an essential indicator of soil fertility.
• SOM can be the primary source of nitrogen in deserts.

A study found that canopy areas in deserts can have 3.1 to 3.7 times more total organic carbon than gap areas, creating fertility islands and crucial to supporting soil microbial diversity. Canopy soils have more nutrients, including minerals like nitrogen, potassium, phosphorus, sodium, calcium, magnesium, chlorides, and sulfates. Organic nitrogen is vital for maintaining the net primary productivity of deserts, as it is the main limiting factor in the ecosystem.

The types of plants and the size of their canopy will directly influence soil quality. Several places, for example, Pakistan, have only shrubs and grasses in deserts and are experimenting with growing tree plantations to improve the soil quality and use the land for agriculture.

Soil Biodiversity

Higher microbial diversity and abundance are associated with the microsite with vegetation canopies in Sonoran deserts. Bacteria and archaea are higher under the canopy than in empty spaces. However, the canopy effect was more substantial in the case of fungi. In a study, 20-70% of the fungal species were unique to communities under plant canopies.

However, in Tunisia, bacteria are less prevalent under palms than in empty areas. In the Namib desert, bacterial diversity was similar in empty and canopy areas, but fungal species were more abundant under grasses. More microbes, mainly fungi, are found under plant canopies.

The higher microbial population could be explained by more canopy litter inputs into the soils that increase nutrient availability. Another factor is the buffer that canopies offer against extreme heat, UV rays, and desiccation of soil moisture. Higher microbial populations improve the mineralization processes of nutrients, increasing soil fertility.

Global-level studies show that increasing aridity starts with vegetation decline, followed by soil disruption and systemic breakdown of saprotrophic and ectomycorrhizal fungal populations.

Temperature Moderation

Besides temperature moderation in natural ecosystems, one of the most crucial benefits of vegetation canopy in deserts is cooling in urban settlements, which improves public health.

2a: Temperature in ELV and UNLV	2b: Different vegetation in ELV and UNLV

Figure 2: “2a) Recorded temperatures in the University of Nevada Las Vegas (UNLV) are cooler than those recorded in East Las Vegas (ELV), which correlates to higher canopy percentage.2b) There is a much more concentrated area of trees and grass in the UNLV area than in East Las Vegas,” Sanchez and Sloat. (2023). (Image credits: https://digitalscholarship.unlv.edu/durep_posters/207)

Concrete and asphalt roads and buildings make cities warmer than surrounding natural ecosystems or rural areas, and this is called the Urban Heat Island Effect (UHI). The higher urban temperatures affect the quality of life and require more energy use. For example, Las Vegas is significantly warmer than the surrounding desert due to the loss of vegetation and replacement by concrete structures. As Figure 2 shows, East Las Vegas has more asphalt and less tree and grass coverage, and it is warmer than the University of Nevada,  Las Vegas area, which is cooler due to higher vegetation cover. This study found that tree canopy cover was more effective than grass ground cover in reducing air temperatures. Trees with higher canopy volume and cover have a greater cooling effect than grass.

Air Pollution

The cooling effect saves energy consumption and lowers carbon emissions. Besides these cooling effects, trees also reduce carbon dioxide levels by using gas in photosynthesis in cities, which are responsible for 70% of global emissions due to energy consumption. In Phoenix, Arizona, in 2023, trees sequestered 72.03 kt of carbon (264.11 CO2 Equiv.) and stored 1,563.68 kt of carbon (5,733.50 kt CO2 Equiv.), providing the city an economic benefit of $293,972,358.

Besides carbon dioxide, trees also remove major air pollutants such as carbon monoxide, nitrogen dioxide, ozone, sulfur dioxide, and particulate matter (PM_2.5 and PM₁₀). In Phoenix, among all vegetation types, the pollutant that was most reduced was ozone, followed by particulate matter. The reduction in pollutants adds economic value to the city, as shown in Table 1.

Table 1: “The amount and economic value of air pollution removed by trees annually,” Olgun and Karakus (2024). (Credits: https://dergipark.org.tr/en/pub/mbud)

Measurement of Tree Canopy

The studies on the effects of plant canopy in deserts use various systems of canopy measurements. Large-scale studies rely on software like the one originally designed by the USDA Forest Service called “i-Tree Canopy 7.1 software.” The tool uses aerial imagery to evaluate land cover types and extent. Small-scale tree canopy studies can use handheld tools like the CI-110 Plant Canopy Imager. The device uses a 150° fisheye lens to capture images of the canopy, and an accompanying software calculates various canopy parameters, including leaf area index (LAI), to provide GPS-tagged data for canopy research.

Find out more about the scientific device offered by CID Bio-Science Inc. for your plant canopy studies.

Sources

Karim, B., Mukhtar, A., Mukhtar, H., & Athar, M.  Effect of the canopy cover on the organic and Inorganic content of soil in cholistan desert. Pak. J. Bot., 41(5): 2387-2395, 2009.

Kushwaha, P., Neilson, J. W., Barberán, A., Chen, Y., Fontana, C. G., Butterfield, B. J., & Maier, R. M. (2021). Arid Ecosystem Vegetation Canopy-Gap Dichotomy: Influence on Soil Microbial Composition and Nutrient Cycling Functional Potential. Applied and environmental microbiology, 87(5), e02780-20. https://doi.org/10.1128/AEM.02780-20

Luqman, M., Rayner, P.J. & Gurney, K.R. (2023). On the impact of urbanisation on CO2 emissions. npj Urban Sustain 3, 6. https://doi.org/10.1038/s42949-023-00084-2

Olgun, R. & Karakuş, N. (2024). Impact of change in tree canopy cover on ecosystem services in desert cities: A case in Phoenix, USA. Journal of Architectural Sciences and Applications, 9 (2), 1031-1043. DOI: https://doi.org/10.30785/mbud.1457421

Ralls, C., Polyakov, A. Y., & Shandas, V. (2024). Scale-Dependent Effects of Urban Canopy Cover, Canopy Volume, and Impervious Surfaces on Near-Surface Air Temperature in a Mid-Sized City. Land, 13(11), 1741. https://doi.org/10.3390/land13111741

Sanchez, M., and Sloat, A. (2023). “Different Canopy Covers Effect Microclimate: How the Urban Heat Island Effect Can Be Reduced in Las Vegas.” Undergraduate Research Symposium Posters. 207. https://digitalscholarship.unlv.edu/durep_posters/207

Yasin, G., Nawaz, M. F., Rasool, F., Yousuf, M. T., Nazir, M. Q., & Javed, A. (2016). Effect of Canopy Cover of Capparis decidua Forsk. on Soil Conditions in Thal Desert,

Journal of Bioresource Management, 3 (2). DOI: https://doi.org/10.35691/JBM.6102.0050