Janice E. Bowers

Longevity, recruitment and mortality of desert plants in Grand Canyon, Arizona, USA

The demography of woody desert plants along the Colorado River in Grand Canyon, Arizona, USA, was analyzed using 355 pairs of replicated photographs taken as long ago as 1872. Longevity, recruitment, and mortality were determined for 38 species characteristic of ungrazed desert scrub. Indi- vidual plants that survived 100 yr or more included Acacia greggii, Ambrosia dumosa, Atriplex canescens, A. confertifolia, Echinocactus polycephalus, Ephedra spp., Fouquieria splendens, Larrea tridentata, Lycium andersonii, Opuntia acanthocarpa, 0. basilaris, 0. erinacea, Pleuraphis rigida, and Yucca angustissima. This is the first evidence of long lifespan for most of these species, particularly the succulents. Most of the long-lived species registered overall increases in population during the past century. Only four species with lifespans > 100 yr had a net loss of individuals between 1889 and the present, and only two decreased between 1923 and the present. It seems likely that climatic fluctuations over the past century are largely responsible for these recruitment and mor- tality patterns; however, nurse plants, predation refuges and other biotic factors may also play a role.

Demographic patterns of Ferocactus cylindraceus in relation to substrate age and grazing history

Three subpopulations of Ferocactus cylindraceus, a short-columnar cactus of the Sonoran and Mojave deserts, were sampled in Grand Canyon, Arizona, USA, at sites representing a range of substrate ages and different grazing histories. Age-height relations were determined from annual growth, then used to estimate probable year of establishment for each cohort. Eight years between 1944 and 1992 were especially favorable for establishment. Six of these 8 years coincided with El Niño-Southern Oscillation conditions, indicating that as for many woody plants in arid regions, somewhat unusual climatic conditions are necessary if populations are to replace themselves.Comparison of age structures showed that established and developing populations have somewhat different dynamics in that the rate of population increase was slowest on the youngest terrace. On the ancient terraces, about half the plants were less than 25 years old. Plants older than 40 years were few; however the oldest plants in the study (about 49 years) grew on the ancient terraces. On the recent terrace, 76% of the subpopulation was 25 years or younger, and the oldest living plant was about 36 years of age. The age structures of subpopulations on grazed and ungrazed sites also differed markedly. On ungrazed sites, subpopulations were more or less at equilibrium, with enough young plants to replace old ones as they died. In contrast, the subpopulation on the grazed site was in a state of marked disequilibrium. Grazing before 1981 largely extirpated a palatable subshrub that was probably an important nurse plant. Until the shrub population at Indian Canyon recovers from decades of burro grazing, a rebound in F. cylindraceus establishment is not to be expected.Nomenclature: Hickman, J. C. (ed.) 1993. The Jepson manual: Higher plants of California. University of California Press, Berkeley.

El Nino and displays of spring-flowering annuals in the Mojave and Sonoran deserts

Abstract Bowers, J. E. (U.S. Geological Survey, 1675 W. Anklam Rd., Tucson, AZ 85745). El Niño and displays of spring-flowering annuals in the Mojave and Sonoran deserts. J. Torrey Bot. Soc. 132: 38–49. 2005.—Although popular and scientific literature frequently assumes a strong connection between El Niño-Southern Oscillation (ENSO) and good displays of spring-flowering annuals in the southwestern United States, such assumptions are based on anecdotal, short-term evidence. The goals of this study were to identify good wildflower years as objectively as possible, to assess the correlation between El Niño and good displays of spring-flowering annuals, and to examine the influence of rainfall amounts on good wildflower years. The terms “good displays” and “good wildflower years” refer to times or places when populations of showy spring-flowering annuals (often called winter annuals) are abundant, robust, and diverse. In the deserts of southeastern California and southern Arizona, good wildflower years occurred about once every 5 to 7 years in the 20th century. The connection between good wildflower years and traditionally defined El Niño episodes was weak, but when El Niño was redefined in a phenologically meaningful way as any calendar year in which the average Southern Oscillation Index (SOI) between July and December was negative, 21 of 27 good wildflower years in the combined deserts were associated with El Niño. Good wildflower years were 3.6 times more likely after redefined El Niño years than after other years. Rain in the months before good wildflower years was at least 30% greater than the long-term average in the Mojave Desert and at least 50% greater in the Sonoran Desert. A diverse flora of spring-flowering annuals occurred in the region during the late Wisconsin and early Holocene, which was a period of wetter, milder winters and cooler summers. Perhaps some species of spring-flowering annuals persist today in the arid southwestern United States only because frequent El Niño conditions recreate the cool, moist conditions of the late Pleistocene.

Effects of drought on shrub survival and longevity in the northern Sonoran Desert1

Abstract Bowers, J. E. (U.S. Geological Survey, 1675 West Anklam Road, Tucson, AZ 85745). Effects of drought on shrub survival and longevity in the northern Sonoran Desert. J. Torrey Bot. Soc. 132: 421–431. 2005.—Permanent vegetation plots in the northern Sonoran Desert, USA, provided an opportunity to assess the effects of recent drought on desert shrubs and to examine survival in relation to rainfall variability during the past 76 years. Survival and maximum longevity of six species were determined for eight intercensus periods between 1928 and 2004. Average annual survival was Ambrosia deltoidea, 0.9167 ± 0.0415; Encelia farinosa, 0.7952 ± 0.0926; Janusia gracilis, 0.9334 ± 0.0247; Krameria grayi, 0.9702 ± 0.0270; Larrea tridentata, 0.9861 ± 0.0174; and Lycium berlandieri, 0.9910 ± 0.0077. The longest-lived species were Larrea, Lycium, and Krameria, with average maximum life spans of 330, 211, and 184 years. Janusia, Ambrosia, and Encelia were much shorter lived, with average maximum longevity of 53, 40, and 16 years. Winter rain equalled or exceeded 90% of the long-term average accumulation except during 1948 to 1959 (65% of average) and from 2001 to 2003 (49% of average). Summer rain did not drop below 90% of the average accumulation in any period. The 1950s drought caused modest declines in survival of Ambrosia, Encelia, Janusia, Krameria, and Lycium. The effects of the recent drought were much more pronounced, resulting in sharp declines in survival and maximum longevity of Ambrosia, Encelia, Krameria, and Larrea, and modest declines for Lycium. Despite heightened mortality during the recent severe drought, 72% of the deaths observed between 1928 and 2004 occurred during periods of average or better-than-average rain, providing support for the idea that demography of shrubs in arid regions is influenced by continuous as well as episodic processes.

Temporal and spatial patterns in emergence and early survival of perennial plants in the Sonoran Desert

Seedling emergence and survival of 15 perennial species were studied for six years in a 557-m2 permanent plot at Tumamoc Hill, Arizona, USA, an ungrazed site in the northern Sonoran Desert. The minimum rain required for germination and emergence ranged from 17.5 to 35.6 mm. Few species emerged in every year of the study. First-year survival averaged across all 15 species was 3.7%; only 0.1% of seedlings lived as long as four years. The odds of survival in the first year improved with increased rain. About three times as many seedlings died from predation as desiccation. In 2-m2 subplots, mortality of three woody species in the first 30 days after emergence appeared to be independent of seedling density. Short-, moderate-, and long-lived species displayed distinct survival strategies. Long-lived species compensated for generally poor seedling survival by frequent germination and emergence. Moderate-lived species exhibited highly episodic germination and emergence, a potentially risky behavior that might have been offset to some extent by relatively good long-term survival. Short-lived species had the highest seedling survival. Because these species can bloom in their first year, good early survival meant that some individuals were able to reproduce before they died.

Dieback and episodic mortality of Cercidium microphyllum (foothill paloverde), a dominant Sonoran Desert tree

BOWERS, J. E. AND R. M. TURNER. (U. S. Geological Survey, Tucson, AZ 85745). Dieback and episodic mortality of Cercidium microphyllum (foothill paloverde), a dominant Sonoran Desert tree. J. Torrey Bot. Soc. 128:000-000. 2001.-Past and current dieback of Cercidium microphyllum, a dominant, drought-deciduous tree in the Sonoran Desert, was investigated at Tumamoc Hill, Tucson, Arizona, USA. Logistic regression predicted that the odds of a Cercidium plant being alive should decrease with increasing circumference, association with the columnar cactus Carnegiea gigantea, and occurrence on steep slopes. Slope azimuth, parasitization by Phoradendron californicum, and distance to nearest Cercidium within 5 m did not significantly affect the odds of survival. Carnegiea was a source of background mortality rather than a primary cause of dieback. Of the >1,000 living and dead plants sampled, 7.7% had died within the past 5 to 7 years. An additional 12.8% died in the more distant past. Diebacks tended to occur during severe deficits in annual, especially summer, rain. More than half of the dead plants in the sample were 250 cm in girth. In current and past diebacks on Tumamoc Hill, it seems likely that severe drought interacted with natural senescence of an aging population, weakening large, old trees and hastening their deaths.

TWO DECADES OF CHANGE IN DISTRIBUTION OF EXOTIC PLANTS AT THE DESERT LABORATORY, TUCSON, ARIZONA

ABSTRACT A gridline survey for exotic plant species was undertaken in 1983 and repeated in 2005 at the Desert Laboratory, a 352-ha natural area just west of downtown Tucson, Arizona, USA. Coordinate data gathered during the surveys were used to plot distributions, determine frequencies (number of coordinate locations), and assess percent change. Fifty-two exotic species were encountered in all, 34 in the first survey, 44 in the second. The proportion of ornamental exotics doubled over time, reaching 50% in 2005. Casual, naturalized, and invasive exotics comprised 44%, 40%, and 15% of species found during the surveys. Minimum residence time increased significantly from casual to naturalized to invasive species, suggesting that the longer a species was present, the more likely it was to surmount barriers to naturalization and invasion. In both 1983 and 2005, casual, naturalized, and invasive species differed in mean frequency by an order of magnitude, with casual species having the lowest frequencies and invasive species the highest. Between surveys, frequency of 18 species decreased; most other species increased in frequency. The recent surge in ornamental exotics, combined with temporal trends in invasiveness, indicates that the proportion of invasive species in the flora will increase with time.

Effects of exotic grasses on soil seed banks in Southeastern Arizona grasslands

Abstract At the Appleton-Whittell Research Ranch, an ungrazed grassland preserve in southeastern Arizona, soil seed banks were sampled in June, August, and October 2002 and June 2003. Wildfire had previously burned 90% of the research ranch in May 2002. Seed density and species richness in burned native grassland (2 plots) were compared to those in burned exotic grassland (2 plots). Averaged over 4 sample dates, seed densities were as follows: burned native grassland, 591 ±243.1 seeds ·m−2 and 784 ±334.9 seeds ·m−2; burned exotic grassland, 501 ±198.9 seeds ·m−2 and 196 ±123.8 seeds ·m−2. Species richness in the seed bank, also averaged over 4 sample dates, was as follows: burned native grassland, 16.3 ±1.7 species ·m−2 and 19.5 ±1.0 species ·m−2; burned exotic grassland, 12.0 ±3.4 species ·m−2 and 11.06 ±2.5 species ·m−2. The seed bank of burned exotic grassland contained significantly fewer seeds and species than that of burned native grassland. In addition, the seed bank in burned exotic grassland comprised mainly exotic grasses, whereas annual and perennial herbs, most of them native, dominated the seed bank of burned native grassland. Of the 50 species detected in soil samples, only 20 had a persistent seed bank, and only 1 of these was a native perennial bunchgrass. The preponderance of transient species means that eradication of exotic grasses must be followed by reseeding of native grasses and herbs, perhaps repeatedly, if native grassland is to replace exotic grassland.

Branch length mediates flower production and inflorescence architecture of Fouquieria splendens (ocotillo)

The capacity of individual branches to store water and fix carbon can have profound effects on inflorescence size and architecture, thus on floral display, pollination, and fecundity. Mixed regression was used to investigate the relation between branch length, a proxy for plant resources, and floral display of Fouquieria splendens (ocotillo), a woody, candelabraform shrub of wide distribution in arid North America. Long branches produced three times as many flowers as short branches, regardless of overall plant size. Long branches also had more complex panicles with more cymes and cyme types than short branches; thus, branch length also influenced inflorescence architecture. Within panicles, increasing the number of cymes by one unit added about two flowers, whereas increasing the number of cyme types by one unit added about 21 flowers. Because flower production is mediated by branch length, and because most plants have branches of various lengths, the floral display of individual plants necessarily encompasses a wide range of inflorescence size and structure.

Plant species richness at different scales in native and exotic grasslands in southeastern Arizona

Abstract Species richness in Madrean mixed-grass prairies dominated by native or exotic species in southeastern Arizona was characterized at the community and point scales using ten 1-m2 quadrats nested within each of eight 1000-m2 plots. In the 1000-m2 plots average richness was significantly higher in oak savanna (OS, 121.0 species) than in exotic grassland on mesa tops (EMT, 52.0 species), whereas native grassland on mesa slopes (NMS, 92.5 species) and native grassland on mesa tops (NMT, 77.0 species) did not differ significantly in richness from OS or EMT. When richness was partitioned by life form, EMT was notably poorer than other community types in species of perennial grasses, perennial herbs, and summer annuals. In the 1-m2 quadrats, OS (21.2 species), NMS (20.9 species), and NMT (20.7 species) were significantly richer than EMT (5.9 species). Cover in 1-m2 plots was significantly higher in EMT than in NMT, NMS, or OS. Species richness at the point scale showed a unimodal relation to canopy cover, with cover accounting for 30% of the variation in number of species in 1-m2 quadrats. Competitive exclusion and allelopathy have perhaps limited species richness at the point scale in exotic grassland. There was no evidence of a species-pool effect between point and community scales, but such an effect between community and landscape scales was supported. Madrean mixed-grass prairies are landscapes with high species richness in comparison to other grassland types in North America, providing a large pool of potential colonizing species at the community scale. Beta-diversity (between communities) within the landscape of the Appleton-Whittell Research Ranch was consequently high despite a relative lack of habitat diversity.