Joshua Trees: Surviving Pollution's Slow Stranglehold

Joshua trees are an iconic part of the California landscape, but they are under threat from air pollution. Most of the pollution affecting the Joshua Tree National Park is generated in the Los Angeles basin and moves inland with the predominant westerly winds. As well as harming natural resources such as soils and surface waters, air pollution is causing an increase in invasive plant species, which are out-competing the native plants. The health of visitors to the park is also at risk, with high levels of ground-level ozone causing respiratory issues.

Nitrogen and sulfur compounds

Nitrogen and sulfur deposition change ecosystems through eutrophication (increased nutrients in soil and water, causing certain species to grow more quickly and altering community composition) and acidification (which leaches important cations from soils, lakes, ponds, and streams, decreasing habitat quality). Research in Joshua Tree National Park has found elevated soil nitrogen levels, leading to the increased growth of invasive nitrogen-loving grasses. These grasses outcompete native plant species, leading to a decline in native plant species and reduced biodiversity. Additionally, the spread of these grasses increases the risk of fires.

Atmospheric nitrogen deposition in the park tends to peak in the summer, predominantly as dry deposition, due to increased sunlight and more air mass movement from the Los Angeles air basin. Mobile emission sources in Southern California release a mix of nitric oxides and ammonia gases that travel on ocean breezes from the Los Angeles basin eastward into the desert. As these molecules are carried by the wind, they may be deposited directly on the soil surface (dry deposition) or dissolved in moisture and enter the soil with precipitation (wet deposition). As nitrogen accumulates in the soil, it can increase vegetation growth, alter microbial processes such as denitrification, and reduce the number of mycorrhizae (microscopic fungal communities that grow symbiotically with plant roots).

The introduction of invasive annual plant species, enhanced by anthropogenic sources of nitrogen from air pollution deposits, threatens the native annual plant diversity in Joshua Tree National Park. As nitrogen deposition increases, the spaces between native shrubs increase in nutrient concentrations, promoting the growth of invasive species. These changes in soil processes can influence overall desert plant communities by decreasing the richness of native plants and potentially leading to increased fire frequency and intensity when invasive species become too dense.

Eutrophication and acidification

Acidification, on the other hand, refers to the process of a solution becoming more acidic over time. In natural environments, acidification can be caused by the deposition of nitrogen and sulfur compounds from the air. Acidification leaches important cations from soils, lakes, ponds, and streams, decreasing habitat quality. Both processes, eutrophication and acidification, can have significant impacts on ecosystems and the plants and animals that depend on them.

Invasive species

Invasive grasses, such as Mediterranean split grass, red brome, and stork's bill, have become more productive and widespread in the last two decades. To test the hypothesis that elevated nitrogen levels are related to the success of these invasive species, nitrogen fertilizer experiments were conducted at four sites in Joshua Tree National Park. The results showed that non-native grass biomass increased significantly with higher levels of nitrogen fertilizer, and the species richness of native forbs declined with fertilization at sites with high non-native grass cover.

The introduction and spread of these invasive species threaten the native annual plant diversity in the park. As invasive species get too dense, they can increase the risk of fires, which further endangers the Joshua trees and the overall desert landscape. The higher elevations, where Joshua trees might survive increasing temperatures and drying conditions, are particularly at great risk of fire due to invasive non-native grasses.

The California Department of Fish and Wildlife has recognized the threat posed by invasive species and has included measures to control invasives in its conservation plan for Joshua trees. The plan aims to identify and protect lands with low risks of threats such as fires, invasive species, and development, and it seeks to permanently protect 70% of these lands by 2033.

Ozone levels

Joshua Tree National Park in California is affected by air pollution, with most of it being generated in the Los Angeles basin and carried inland by the predominant westerly winds. The pollutants are harmful to natural and scenic resources such as soils, surface waters, plants, and visibility.

Ozone is a respiratory irritant and can cause coughing, sinus inflammation, chest pains, a scratchy throat, lung damage, and reduced immune system functions. It is especially harmful to children, the elderly, people with existing health problems, and active adults. When ozone levels are high, health advisories are posted to caution visitors to Joshua Tree National Park about the potential health risks associated with exposure to elevated levels.

In addition to the human health impacts, ozone can also damage plant tissues, making it harder for plants to produce and store food. Over the course of a growing season, high ozone levels can slow growth and accelerate the ageing of sensitive plants, causing leaf injuries like bleaching or dark spots on some plant species. Studies conducted in the 1980s tested the effects of elevated ozone on herbaceous annual and perennial plants common in the Mojave Desert, which includes Joshua Tree National Park. These studies found that high levels of ozone fumigation caused leaf injury and necrosis in a variety of plant species, including members of the evening primrose and borage families, as well as sensitive wildflowers such as the desert poppy and whitestem blazingstar.

While the dry conditions in the park may limit ozone uptake by plants, a wet year or strong summer monsoon season increases the risk of ozone leaf injury. Native plants such as skunkbush sumac (Rhus trilobata) are particularly sensitive to high ozone levels. The impact of ozone on plant life can have cascading effects on the ecosystem, potentially disrupting the food chain and affecting other animal species.

Persistent pollutants

Microplastics, primarily clothing fibres like cotton, polyester, and nylon, have also been detected in park precipitation samples. Joshua Tree NP is estimated to receive an annual deposition of 3.4-3.7 metric tons of plastic. Additionally, pesticides like Metam-Sodium, Chlorpyrifos, and Diuron, which are toxic to fish and bees, are likely to drift into the park due to their application in surrounding counties.

Ozone is another critical pollutant affecting Joshua Tree. Ground-level ozone, formed from the reaction of nitrogen oxides and volatile organic compounds in the presence of sunlight, can irritate the respiratory system and damage plant tissues. Joshua Tree experiences high ozone concentrations, often exceeding the National Ambient Air Quality Standards, posing risks to both human health and plant life.

The impact of these persistent pollutants on Joshua Tree National Park highlights the vulnerability of natural ecosystems to human activities and the importance of effective pollution control and monitoring measures to protect the environment and human well-being.

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