Juglans major , Arizona walnut

This review summarizes the information that was available in the scientific literature as of 2021 on the biology, ecology, and effects of fire on Arizona walnut in North America. Arizona walnut is native to the American Southwest and Mexico. It grows primarily in riparian areas, near springs, and other areas with shallow groundwater, in mixed-deciduous, coniferous pine-oak, and oak riparian stringers, gallery forests, woodlands, and forests. Arizona walnut reproduces sexually and regenerates from seed. It also sprouts from the root crown, or higher on the trunk, after partial to complete top-kill by low- to moderate-severity fire, flood, or other disturbances. It occurs in all stages of succession, pioneering on floodplains but also growing in mid- to late-successional communities. Flooding was historically the primary disturbance in mixed-deciduous riparian communities of the Southwest; however, fire is becoming the primary disturbance in some riparian ecosystems due to changes in hydrology, climate, and plant community composition and structure. Limited research shows that Arizona walnut has relatively low postfire mortality rates, and it is most likely to survive fire as mature trees or to be top-killed and survive as sprouts. Mortality rates may increase after successive wildfires. Information on historical fire regimes in riparian areas of the Southwest is limited. Fires probably varied in severity and extent across southwestern riparian ecosystems. Most fires were likely patchy and of low severity due to the patchiness of vegetation in desert landscapes; however, mixed- and high-severity fires likely occurred as well. In many areas, stand structure and function of mixed-deciduous riparian woodlands have been significantly altered since European-American exploration and settlement began. The greatest ecological change in riparian areas since large-scale livestock ranching was introduced is probably a shift in fire regimes from mostly patchy surface fires to larger crown fires. Some research suggests that fire frequency and severity are increasing in some southwestern riparian ecosystems. More severe, intense, and/or frequent fire can interact with altered hydrology to accelerate the replacement of native plants with nonnative plants. In many riparian areas in the Southwest, there have been increases in tamarisk, Russian-olive, Mediterranean grass, and other nonnative plants that have different fuel characteristics, fire tolerances, and postfire fire responses than native species. Riparian areas are among the most diverse and productive ecosystems in desert bioregions, supporting floras and faunas that are biologically richer than adjacent uplands. Many wildlife species use riparian habits with Arizona walnut. It provides breeding, wintering, and migratory corridors for a many bird species. Riparian areas with Arizona walnut provide many ecosystem services including recreational opportunities, harboring pollinators, providing livestock forage, and enhancing water quality. Yet, many riparian communities of the Southwest are highly imperiled due to anthropogenic disturbances. Few naturally functioning riparian areas remain, and those that do are threatened with climate change. Prolonged drought may increase fire risk by decreasing water availability and reducing fuel moisture. Climate change is expected to alter water flow and lower water tables, and more frequent and severe extreme weather will likely result in more frequent and severe floods as well as more intense droughts.