Saguaro (Carnegiea gigantea) Mortality and Population Regeneration in the Cactus Forest of Saguaro National Park: Seventy-Five Years and Counting.

Annual census data spanning seventy-five years document mortality and regeneration in a population of saguaro cactus (Carnegiea gigantea) in the Cactus Forest of the Rincon Mountain District of Saguaro National Park near Tucson, AZ. On 6 four-hectare plots, each saguaro was censused and a methodical search for new saguaros was conducted annually each year from 1942 through 2016, with the exception of 1955. Regeneration has been episodic with 828 plants established from 1959 through 1993 compared with 34 plants established between 1942 and 1958 and only three plants established after 1993. The years preceding 1959 and following 1993, include some of the driest decades in centuries in southern Arizona. While woodcutting and cattle grazing are believed to be among the causes of decades of failed regeneration prior to 1958, neither of these factors contributed to the failed regeneration following 1993. The height structure of the population from 1942 to 2016 shifted dramatically from a population dominated by large saguaros (> 5.4 m tall) in the first three decades of the study to a population dominated by small saguaros (< 1.8 m tall) in the most recent two decades. Mortality is shown to be strongly age dependent. In the year following the 2011 catastrophic freeze, 21 of 59 plants older than 80 years died compared with zero deaths in 270 plants between the ages of 29 and 80 years. Saguaros under 40 years old, growing under small shrubs or in the open, have a lower probability of survival than better protected saguaros. Long-term population monitoring is essential to understanding the complex impacts of human and environmental factors on the population dynamics of long-lived species.

Spatial and Temporal Patterns of Aspergillus flavus Strain Composition and Propagule Density in Yuma County, Arizona, Soils.

Aspergillus flavus isolates from Arizona can be divided into S and L strains on the basis of sclerotial morphology. These genetically distinct strains differ in aflatoxin production. To help understand factors influencing the aflatoxin producing potential of A. flavus communities, spatial and temporal patterns of strain incidence were compared with patterns of A. flavus propagule density in Yuma County soils. Strain S isolates were found in all sampled fields, but the percentage of strain S isolates ranged from 4 to 93%. A nested analysis of variance was used to determine the spatial scale at which most variability in strain composition and propagule density occurred. For both variables, the largest component of variance occurred among fields within areas at a spatial scale of 1 to 5 km. There was also spatial structure (12 to 21% of the variance) at the subregional level (> 20 km) in strain composition, but not in propagule density. Temporal patterns for both variables were similar. The sampling periods with the highest incidence of strain S isolates, August 1994 (60%) and July 1995 (62%), occurred during cotton boll formation. The regional average for A. flavus propagule density was near 1000 propagules/g in the summer, but less than 100 propagules/g in the spring. The results suggest that insights into factors influencing the toxigenicity and propagule density of A. flavus communities might be achieved most readily by contrasting fields in close spatial proximity.

Localized migration and dispersal by the sweet potato whitefly, Bemisia tabaci.

Laboratory populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration was examined under field conditions. Insects were marked in a field of melons using fluorescent dust during two consecutive growing seasons. During the first growing season, passive traps used to collect living whiteflies, were placed along 16 equally spaced transects radiating from the field to a distance of up to 1.0 km. Wind out of the north-east consistently carried migrating whiteflies to traps placed along transects in the south-western quadrant because cold air drainages dictate wind direction during early morning hours in the desert South-west. For this reason, during the second season traps were laid out over fallow ground in a rectangular grid extending 2.7 km to the south-west of the marked field. If dispersal was entirely passive, patterns could be described using a diffusion model. Statistical examination of the data, howèver, demonstrated that the distribution on all days was patchy. Geostatistical techniques were used to describe the observed patchiness. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from near the periphery of the grid. White-flies were far less prevalent in the grid's center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field, and a portion initially respond to cues associated with skylight, ignoring cues provided by the ground, and fly for a period of time before landing in distant traps. During both years movement out of the field had an exaggerated directional component on 13 of 14 days.