Epidermal browning of saguaro cacti (Carnegiea gigantea): is it new or related to direction?
ABSTRACT Browning of epidermal surfaces of saguaro cacti (Carnegiea gigantea) is associated with premature death of the species in the northern Sonoran Desert. Analyses of authentic historical photographs of mature saguaros from 1903 to 1987 demonstrated a significant (1) loss of spines, (2) decrease in apparently healthy crests, and (3) increased trough barking over the period. All three of these changes occurred to saguaros of over 115 years of age at a projected rate of 4–5% per decade. Analyses of epidermal characteristics of saguaros at the Saguaro National Monument were performed to determine if epidermal browning was related to compass direction. Results show that both scaling and barking of crests and troughs and loss of spines, all symptoms of epidermal browning, were 10 times higher on southern exposures (between 140 and 220°) compared with northern exposures (between 320 and 40°). The two troughs of a crest were evaluated separately. The results demonstrated that injury to one trough of a crest could be as much as four times greater than that of the second trough.
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ABSTRACT: Rates that stem surfaces of saguaro cacti (Carnegiea gigantea (Engelm.) Britt & Rose) accumulate scale and bark injuries and the mortality rates of cacti were determined on a population of 1149 saguaro cacti in 50 field plots over the 9-year period of study (from 19931994 until 2002). Twenty-three percent of the saguaro population had few surface injuries throughout the 9-year period while 27% showed a marked increase in stem area with scale and bark injuries. Thirty percent of all cacti had more than 80% stem areas with combined scale and bark injuries on south-facing stem surfaces throughout the study period. Finally, 20.3% of the saguaro population died over the 9-year period, a rate of 2.3% per year. Thirty-three percent of all cacti that died by 2002 exhibited few surface injuries in 19931994 while 54% of the cacti that died over the period had more than 98% stem areas with combined scale and bark on south-facing stem surfaces in 19931994. In this manner, stem scale and bark injuries on south-facing surfaces were usually associated with the death of saguaros. The annual mortality rate of 2.3% appears high considering that this species may live for more than 200 years.Key words: saguaros, Carnegiea gigantea, Cactaceae, stem areas with scale and bark injuries, mortality rates.Canadian Journal of Botany 02/2011; 83(3):311-319. DOI:10.1139/b05-005 · 1.40 Impact Factor
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ABSTRACT: Saguaro cacti (Carnegiea gigantea) are long-lived and exhibit great variability in growth that makes age estimation problematic. A few single-site studies have focused on those locales where long-term data (e.g. 85 years) are available. Using a newly developed technique, 733 saguaros were sampled in three locales (Silverbell, Harcuvar, Kofa) across Arizona and their age structure reconstructed for the last 150 years based on a mathematical model of the heights of individuals. This is the first study to compare regeneration at multiple locations across the species' range. Regression analysis for each site (years and frequency of indivi-duals established during that year) was run and residuals extracted to determine peaks and troughs in regeneration over time. Correlation was run on the residuals between sites, and chi-square analysis was employed to compare frequency of good and bad regeneration years between Kofa and Silverbell. Peaks and troughs represent regeneration as well as sur-vivorship and mortality. Several large cohorts established at Kofa and Harcuvar since 1850, while at Silverbell well over 80% of sampled saguaros established after the late 1930s. This more recent recruitment at Silverbell may be related to the major freezing event of 1937 whose impact was likely greater at the cooler Silverbell site. Despite the widely different population structures at Silverbell and Kofa, recruitment trends in both populations were statistically linked as both locales often benefited from the same favorable periods for re-generation (P < 0.001). The Harcuvar population shares some common peaks and troughs in regeneration over time with Kofa and Silverbell, but its relationship to them is not statis-tically significant. Some trends overlap in some locales, such as the favorable regeneration period in the late 1800s and early 1900s, particularly at Silverbell and Kofa as well as at other known sites. However, each population has its own signature. Silverbell is a youthful population likely shaped by severe freezing events, while Kofa has many individuals repre-senting regeneration at several different periods. Regeneration, mortality and subsequent population structure is shaped by both regional-scale influences as well as more localized conditions over the long and short terms.
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ABSTRACT: The purpose of this study was to examine physiological correlates to the phenomenon of epidermal browning and saguaro decline in Saguaro National Monument. Gas exchange characteristics, surface temperatures, and the extent of epidermal browning in tissues of both healthy and declining saguaro cacti (Carnegiea gigantea) at different solar orientations were examined in eight long-term monitoring plots in the Rincon Mountain District of the Saguaro National Monument near Tucson, Arizona during both wet and dry seasons and years.Daily maximum surface temperatures were greatest on west-oriented tissues, as predicted by a simple model. However, south- and south-west-oriented tissues showed the highest browning indices, suggesting that browning may be more related to cumulative thermal loading than to extreme late afternoon temperature events. During the wet season, maximum nocturnal CO2uptake rates and total nocturnal CO2uptake were negatively correlated with browning index. Uptake rates during dry seasons and dry years was minimal, and not related to browning indices. However, healthy tissues of saguaros that also had tissues with high browning indices had wet-season maximum CO2uptake rates as high as saguaros without damaged tissues, suggesting that the browning is not systemic but rather is tissue- and angle-specific. Instantaneous measures of water-use efficiency did not vary significantly or predictably by angle, but rather were greatest during night-time and in tissues when CO2uptake was greatest. We suggest that browning symptoms on cacti are most likely due to natural abiotic stress, and are unrelated to systemic pollution stress or increased UV-B radiation.Journal of Arid Environments 08/1997; 36(4-36):579-590. DOI:10.1006/jare.1996.0240 · 1.82 Impact Factor