July/August 2004
Temporal and Spatial Patterns in Emergence and Early Survival of Perennial Plants in the Sonoran Desert
Janice E. Bowers, USGS, Desert Laboratory, Tucson AZ
Raymond M. Turner and Tony L. Burgess,
formerly with USGS, Desert Laboratory, Tucson AZ
The episodic nature of plant establishment in the arid southwestern United States has been demonstrated repeatedly, yet large gaps remain in our knowledge of seedling behavior. How much rain is required for seedlings to emerge? What is the season of emergence? How many emergence events occur annually? How many seedlings emerge, and what proportion survive? Answers to these questions are essential for predicting the response of plant populations to environmental variability.
We undertook a 6-year study (1987-1993) of seedlings of perennial plants at the Desert Laboratory on Tumamoc Hill, Tucson, Arizona. Our study site, Area A, was a 557-m2 plot originally established by Forrest Shreve in 1910 for studying seedling survival. We visited the plot after substantial rains to look for recently emerged seedlings, which we marked with numbered aluminum tags. At irregular intervals, we recorded which seedlings had died and removed their tags. Altogether, we marked and monitored more than 4300 seedlings; our data analysis concentrated on the 15 most abundant species (N = 3728 seedlings).
Our long-term census provided unique information about the amount of rain required for emergence, season and frequency of emergence, seedling abundance, and survival in the years after emergence. Two distinct germination seasons could be discerned: early spring (February and March) and summer (July to October). The minimum rain required for germination varied among species and ranged overall from 17.5 mm to 35.6 mm. Few species emerged in every year.
Life history and germination characteristics for 15 perennial species, 1987-1993. Minimum rain = smallest amount of rain in this study that brought about germination. Ratio = emergence events : years of observation.
| Habit | Lifespan (yrs) | Minimum rain (mm) | Ratio | Month of emergence | |
|---|---|---|---|---|---|
| Species | |||||
| Acacia constricta | Shrub | > 100 | 23.6 | 4:6 | Jul, Aug |
| Ambrosia deltoidea | Shrub | 50 | 23.9 | 4:6 | Sep, Oct, Feb |
| Argythamnia neomexicana | Herb | 5 | 17.5 | 4:2 | Aug, Sep, Oct |
| Brickellia coulteri | Shrub | 40 | 35.6 | 2:6 | Aug |
| Cercidium microphyllum | Tree | > 100 | 17.5 | 12:6 | Jul, Aug, Sep, Oct |
| Encelia farinos | Shrub | 30 | 35.6 | 3:6 | Oct |
| Fouquieria splendens | Shrub | 75 | 18.0 | 5:6 | Mar, Jul, Aug, Oct |
| Janusia gracilis | Shrub | 75 | 17.5 | 2:6 | Sep, Oct |
| Jatropha cardiophylla | Shrub | 75 | ----- | 1:6 | Aug |
| Larrea tridentata | Shrub | > 100 | 17.5 | 9:6 | Jul, Aug, Sep, Oct |
| Marina parryi | Herb | 5 | 17.5 | 4:2 | Aug, Sep, Oct |
| Psilostrophe cooperi | Herb | 10 | 18.0 | 2:6 | Mar, Aug |
| Senna covesii | Herb | 10 | 18.8 | 3:1 | Aug, Sep |
| Sphaeralcea laxa | Herb | 5 | 17.5 | 4:2 | Sep, Oct |
| Trixis californica | Shrub | 40 | 18.0 | 2:6 | Mar, Aug |
As expected, seedling survival was low. Only 2 of 2008 seedlings tagged between 1987 and 1989 survived as long as 4 years (0.1%). First-year survival averaged across all species was 3.7%. Although the odds of survival in the first year improved with increased rain, about three times as many seedlings died from predation as from desiccation. There was no evidence that survival varied with seedling density.
Survival probability for seedlings of perennial plants. Short-lived,
N = 5 species; moderate-lived, N = 4 species; long-lived, N = 6 species.
Short-, moderate-, and long-lived species displayed distinct survival strategies. Long-lived species (around 75 years) compensated for generally poor seedling survival by frequent germination and emergence. Moderate-lived species (30-50 years) 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.
Bowers, J. E., Turner, R. M., Burgess, T. L. 2004. Temporal and spatial patterns in emergence and early survival of perennial plants in the Sonoran Desert. Plant Ecology 172:107-119.
For more information contact Janice E. Bowers at jebowers@usgs.gov.