The Santa Catalina Mountains are located in Pima and Pinal counties in southeastern Arizona. The study area is defined as approximately 259,000 acres (104,813 hectares) or 405 mi2 (1,048 km2), spanning an elevational gradient of 6,457 ft (1,968 m). Located on the northwestern edge of the Madrean-influenced sky island complex of southeastern Arizona and northern Sonora, highly diverse plant communities range from Sonoran Desert to subalpine forest. A total of 380 days of field work were conducted between 2007–2017, including extensive exploration of the remote east side of the mountains. The vascular flora includes 1,360 taxa in 127 families and is currently the largest of any range in southern Arizona. Non-native plants are represented by 167 taxa and comprise 12.3% of the total flora. The three largest plant families are Asteraceae, Poaceae and Fabaceae, with 213, 187 and 107 taxa respectively. Euphor-bia, Muhlenbergia and Dalea are the largest genera with 24, 22 and 16 species. A total of 375 taxa are found on lime-stone or dolomitic substrates. There are 69 historically collected taxa that have not been seen or collected in 55 years, which are excluded from this checklist. New additions to the vascular flora are vouchered at the University of Arizona Herbarium. A checklist of 169 non-vascular plants from 36 families, based on over 1,150 collections from 18 national herbaria, is included. The floristic diversity of this sky island represents nearly a third of the entire state flora, while occupying less than half a percent of the state’s area. Geographic location, elevational gradient, geological diversity, and a high percentage of species found at the edge of their ranges contribute to the rich diversity of this unique mountain range.Monsoon storms cover the west side of the range.

A checklist is provided for the vascular plants of the Guaymas region of western Sonora. This region encompasses 532,000 hectares (1,314,600 acres) where the southern Sonoran Desert transitions from subtropical thornscrub. This flora includes 820 native and non-native taxa in 113 families and 471 genera. There are 97 non-natives established in the flora area, 27 of which are grasses. Nineteen taxa are endemic to the flora area.

A large population of the fully mycoheterotrophic orchid, Corallorhiza striata var. vreelandii, was monitored for nine years, 2009—2017, in the Santa Catalina Mountains of southeastern Arizona. High elevation slopes were chosen for an unusually high density of plants. Orchid stems were counted annually, and the number of flowering stems steadily decreased by 78% during the first seven years (2009–2015) in drought conditions. Following a partial return to average rainfall on the seventh through ninth years, the number of stems dramatically rebounded on the eighth and ninth years. Overall the total number of flowering stems decreased by 35% during the study. Precipitation from the previous winter and summer strongly correlated with the number of flowering stems observed. Years of extreme drought, with less than half of annual averages, resulted in a decline of flowering stems for two consecutive years, even when the following year had average rainfall. Two years of near average rainfall resulted in an increase on the second year. Orchid numbers were observed to fluctuate as its endophyte was dynamically affected by changes in annual precipitation. This study highlights the need for research on the impact of drought to ectomycorrhizal fungi and affiliated plant species.

Portions of the eastern Mojave Desert region of southeastern California, southern Nevada, and west-central Arizona that receive significant inputs of warm-season precipitation contain large areas dominated by various C4 perennial grasses including Pleuraphis rigida, P. jamesii, Bouteloua eriopoda, and B. gracilis. The lower elevation at which the two Bouteloua species occur rises from east to west in response to diminished precipitation, especially that received during the warm season. Unpredictability of warm-season precipitation also increases from east to west, but these grasses occasionally make use of cool-season precipitation stored in the soil, once temperatures required for the C4 photosynthetic pathway are achieved in late spring, but before the onset of summer monsoonal precipitation. Species distributions vary with elevation, with P. rigida occurring at lower elevations, B. eriopoda and P. jamesii at intermediate elevations, and B. gracilis at higher elevations. Composition of communities containing the latter three species is similar to grassland formations of the cool-temperate grasslands (grama-galleta steppe) of the Colorado Plateau region. Small, less predictable amounts of warm-season precipitation probably impose the greatest limitation to the diversity of C4 grasses in the eastern Mojave Desert region. However, due to warmer minimum winter temperatures, the woody plant and succulent floras associated with perennial grasses in the eastern Mojave region bear greater resemblance to those of the warm-temperate, semi-desert grasslands of west-central Arizona, southeastern Arizona, and the Sonoran and Mojave Deserts. The presence of these woody plant and succulents in perennial grass-dominated communities in the eastern Mojave Desert imparts a structural character similar to that of the warm-temperate semi-arid grasslands of southern Arizona. Although climate (particularly warm-season precipitation) is a first-order determinant of the occurrence of perennial C4 grasses in the eastern Mojave Desert region, geological characteristics that control soil formation and soil hydrological behavior strongly influence composition of communities. The common denominator of sites dominated by grasses is a soil with relatively thick, fine-grained soil horizons that are conducive to exploitation by relatively shallow, diffuse, fibrous root systems of those grasses. Such soils occur in diverse settings, ranging from relatively steep hillslopes underlain by bedrock to gently inclined alluvial fans. In rocky hillslope environments, these kinds of soils are associated with late Pleistocene colluvium deposits in which eolian dust accumulation is principally responsible for forming the thick, fine-grained horizons. Erosion of these soils on hillslopes contributes to hydrological conditions more conducive to taproot systems of woody plants that occupy deeper fractures and joints in bedrock. Similarly, erosional truncation of well-developed soils of alluvial fans and exposure of cemented, relatively impenetrable calcic horizons produce a shift in dominance by perennial grasses to woody plants. In many settings, the presence of relatively dense perennial grass cover plays an essential role in moderating surface flows and inhibiting erosion. Prior to Anglo-American settlement of the region in the late 1800s, occasional wildfires may have fostered dominance of perennial grasses in some of these areas. Since the 1890s, livestock ranching has significantly impacted perennial grass-dominated vegetation. Removal of livestock from portions of the region around 2000, coupled with years of abundant warm-season precipitation, in some cases combined with wildfire, has led to a resurgence of perennial grasses in some areas. Effective management and conservation of these areas require a comprehensive understanding of the composition, occurrence, and ecological functioning of these communities.