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Survival of planted star cactus, Astrophytum asterias, in Southern Texas.

A number of rare cacti occur in the SW United States and northern Mexico. Many of these cacti have very restricted ranges and low reproductive rates. While protecting existing populations is crucial to their persistence (Hernandez and Barcenas, 1996), creating new populations may be necessary for these species to survive.

The star cactus, Astrophytum asterias, is an endangered cactus with a distribution limited to extreme southern Texas and northern Mexico (Poole et al., 2007). The species is threatened by conversion of habitat, collection, and small populations (United States Fish and Wildlife Service, 2003; Terry et al., 2007). Star cactus is an obligate outcrosser, and the most common pollinators are relatively ineffective at dispersing pollen (Blair, 2007; Strong and Williamson, 2007). Fruit set is apparently limited by the number of effective pollinators (Blair, 2007; Blair and Williamson, 2008).

A pilot study on Las Estrellas Preserve, Starr County, Texas, showed that star cactus could be reintroduced to a site by planting but not by seeding (Birnbaum et al., 2011). We planted ca. 5-year-old individual star cacti on three private ranches in Starr County, Texas. Natural populations of the star cactus on these ranches were destroyed or reduced by seismic exploration.

Seeds were collected from Las Estrellas Preserve in 2004 during a study of breeding systems (Strong and Williamson, 2007) and germinated in growth-chambers in 2005. Seedlings were grown in soil mix for cactus (provided by Lady Bird Johnson Wildflower Center, Austin, Texas) until October 2009, when they were transplanted into Monarch Plant Bands[R] in preparation for planting. That same month, plants were moved from the Lady Bird Johnson Wildflower Center to the Kika de la Garza Plant Materials Center (Kingsville, Texas). Plants were initially housed in a greenhouse but were moved into a shade-house 45 days before planting to acclimate them to conditions outdoors.

We selected four sites on the three ranches. Planting site A12 was on Montell soil, while the other sites were on Catarina soil. Both soils are fine, smectic, hyperthermic Sodic Hapluserts. Within each site, we selected evenly spaced, level microsites. In October 2010, we planted four cacti at each microsite, one in each corner of a 15-x-15-cm square. Immediately before planting, we removed cacti from the plant bands and washed growth-medium from the roots. We placed plants in deep, narrow holes which were then filled with native soil; we watered plants thoroughly immediately after planting. The day after planting, we brushed any soil on the plants away with a paint brush and replaced rocks within the microsite to mimic conditions of typical habitat.

Before planting, we measured the size of each cactus: diameter along the widest axis of the plant and diameter perpendicular to the widest axis. We monitored planting sites intermittently for 18 months following planting. At each visit, we classified plants as alive, dead, or missing. When possible, we noted the cause of mortality: herbivory (root-mass still in place but aerial portion removed); excavation (hole in ground, root-mass nearby), desiccation (aerial portion of plant shriveled; no signs of damage by insects and rodents); damage by insects. We described plants as missing if the roots and the aerial portion were absent. We also noted evidence of flowers or fruit. In October 2011 and April 2012, diameters of surviving cacti were remeasured.

We watered cacti during visits in July, August (site A12 only), September, and October 2011. We watered cacti when soil moisture was <50% of field capacity, when the cumulative average daily high temperature during the previous month exceeded 37[degrees]C, or when the plants had pulled away from the soil. Due to high rates of herbivory, we placed rodent-screens over all surviving plants at site A12 in June 2011 and site A3 in October 2011. The screens (squares of hardware cloth with 0.64-cm mesh) were large enough to extend past the microsite by ca. 10 cm and were nailed directly into the ground.

[FIGURE 1 OMITTED]

For all analyses, we averaged the two measurements of diameter. We tested whether diameter and growth (change in diameter/initial diameter) differed among planting sites with a mixed model analysis (MIXED) using site as a fixed variable and microsite as a random variable, and we used the Tukey adjustment for multiple compar isons. We used logistic regression (GLIMMIX) to test whether site and initial diameter could predict mortality for all plants and for a subset of plants that excluded plants killed by herbivory. Microsite (nested within site) was again included as a random variable. We assumed that missing plants were dead but did not assign a cause of mortality. All analyses were conducted in SAS 9.3 (SAS Institute, Inc., Cary, North Carolina).

Initial diameter of the planted cacti varied somewhat among sites, with site A12 having smaller individuals and site D1 having larger individuals (Table 1). Growth of surviving individuals did not differ among sites after 1 year; only in site E1 was the average increase in size >0. From October 2011-April 2012 (months 12-18 after planting), cacti in sites A3 and E1 increased in average size.

We observed five cacti with flowers or buds in June or July 2011 or in both months at sites A3 and E1. Diameter (at planting) of these cacti ranged from 29.1-43.0 mm; average diameter was 38.4 [+ or -] 6.0 mm (mean [+ or -] 1 SD). Three of these cacti were killed by herbivory; only one was later found with fruit.

Between May 2011 and April 2012, we observed 17 cacti with fruit at sites A2 (n = 1), A3 (n = 4), and E1 (n = 12; three plants produced fruit twice). The smallest cactus to produce fruit was 32.0 mm in diameter; average diameter was 41.0 [+ or -] 5.0 mm. The cacti that produced fruit twice were all [greater than or equal to] 43.4 mm in diameter. Only one of the cacti that produced fruit died; it was killed by herbivory.

Overall mortality was 48% with herbivory accounting for 34% of the mortality (50 of 148). Mortality varied among sites (F = 11.13, df = 3, P < 0.001) but was not influenced by initial size (F = 0.00, df = 1, P = 0.95), even when plants killed by herbivory were excluded (F = 0.06, df = 1, P = 0.80). Almost 90% of the planted cacti (42 of 48) at site A12 died between January and May 2011 (Fig. 1). Eleven individuals (26%) were missing. At site A3, 68% (63 of 92) of the cacti died, mostly between July and September 2011. Herbivory accounted for 72% (45) of the deaths. Mortality was lower at sites D1 (44%, one due to herbivory) and E1 (24%, four due to herbivory, four due to damage by insects).

Observed mortality of the planted cacti (48%) was lower than reported mortality (66%) for natural populations in Tamaulipan thornscrub in Mexico (Martinez-Avalos et al., 2007). There, herbivory by the Mexican ground squirrel (Spermophilus mexicanus) was the primary cause of mortality (42%), closely followed by an unidentified cerambicid beetle (40%) and a plant pathogen (Phytophtora infestans, 19%); deaths due to desiccation were not included. Another population of planted star cacti in Starr County, Texas, had lower mortality after 1 year (27%), but mortality after ca. 3.5 years was 68% (Birnbaum et al., 2011). Causes of mortality included herbivory, uprooting (some of which was attributed to Mexican ground squirrels), infestation of weevils, and desiccation.

Mortality of planted cacti was highest during periods with little or no rainfall and high temperatures. Desiccation is clearly a problem during dry periods (region was in a drought during the study), but herbivory also appeared to increase, especially at site A3. Rodents could be using the cacti as a source of water during dry periods or the supplemental watering could have attracted them. Furthermore, a neighboring site had been root-plowed during the monitoring period, and rodents could have moved from the disturbed site into site A3, increasing herbivory. No additional plant was eaten after the rodent-exclosures were installed.

In Mexico, larger plants (>6 cm in diameter) were less vulnerable to herbivory and disease (Martinez-Avalos et al., 2007). We found no relationship between mortality and initial size, but our planted cacti were all <6 cm in diameter. Mortality was higher on Montell soils, but, because we only had one planting site on that soil type, it is unclear whether the mortality was caused by the soil or another unmeasured factor. On Las Estrellas Preserve, star cacti are primarily found on Catarina soils and, to a lesser extent, on Maverick soils (fine, smectic hyperthermic Aridic Haplusepts). No cacti have been found on the Montell soils at the site.

Exploration and extraction of oil and gas in southern Texas has the potential to impact additional populations of star cacti. While avoidance should be a high priority, mitigation is often necessary. Our results confirm that reintroduction of the endangered star cactus through planting is a viable strategy to restore affected populations. Future reintroductions should investigate the interaction between type of soil and mortality and test the efficacy of supplemental water. Furthermore, planting of nursery-grown individuals also should be explored for additional species of rare cacti.

We thank the landowners who participated in this study, the volunteers who helped to plant the cacti, and the Lady Bird Johnson Wildflower Center for providing space to grow the cacti. Funding was provided by the United States Fish and Wildlife Service.

LITERATURE CITED

BIRNBAUM, S. J., J. M. POOLE, AND P. S. WILLIAMSON. 2011. Reintroduction of star cactus Astrophytum asterias by seed sowing and seedling transplanting, Las Estrellas Preserve, Texas, USA. Conservation Evidence 8:43-52.

BLAIR, A. W. 2007. Pollinator effectiveness, pollinator importance, and pollen dispersal in star cactus (Astrophytum asterias). M.S. thesis, Texas State University-San Marcos, San Marcos.

BLAIR, A. W., AND P. S. WILLIAMSON. 2008. Effectiveness and importance of pollinators to the star cactus (Astrophytum asterias). Southwestern Naturalist 53:423-430.

HERNANDEZ, H. M., AND R. T. BARCENAS. 1996. Endangered cacti in the Chihuahuan Desert: II. Biogeography and conservation. Conservation Biology 10:1200-1209.

MARTINEZ-AVALOS, J. G., J. GOLUBOV, M. C. MANDUJANO, AND E. JURADO. 2007. Causes of individual mortality in the endangered star cactus Astrophytum asterias (Cactaceae): the effect of herbivores and disease in Mexican populations. Journal of Arid Environments 71:250-258.

POOLE, J. M., W. R. CARR, D. M. PRICE, AND J. R. SINGHURST. 2007. Rare plants of Texas. Texas A&M University Press, College Station.

STRONG, A. W., AND P. S. WILLIAMSON. 2007. Breeding system of Astrophytum asterias: an endangered cactus. Southwestern Naturalist 52:341-346.

Submitted 7 September 2012. Acceptance recommended by Associate Editor Janis K. Bush 29 March 2013.

CHARLOTTE M. REEMTS, * PATRICK CONNER, GENA K. JANSSEN, AND KIMBERLY WAHL

The Nature Conservancy, Austin, TX 78710 (CMR, PC) Janssen Biological, Austin, TX 78749 (GKJ) United States Fish and Wildlife Service, Lower Rio Grande National Wildlife Refuge, Alamo, TX 78516 (KW)

Present address of PC: 811 Washington Street, Castroville, TX 78009

* Correspondent: creemts@tnc.org
TABLE 1--Characteristics of star cacti (Astrophytum
asterias) planted at Las Estrellas Preserve, Starr
County, Texas. Initial size (average diameter) and
growth (change in diameter/initial diameter) are
least squares means [+ or -] 1 SE; values within a
column followed by the same letter are not
significantly different (P < 0.05). Growth for year
1 is from the date of planting to October 2011.
Growth for year 1.5 is from October 2011-April
2012. Herbivory is presented as a proportion of
overall mortality.

Planting    Initial size (mm)            Growth
site (n)

                                         Year 1

A12 (48)   26.8 [+ or -] 1.8 a    0.08 [+ or -] 0.11 a
A3 (92)    29.5 [+ or -] 1.4 ab   0.03 [+ or -] 0.06 a
D1 (16)    34.9 [+ or -] 2.4 b    0.04 [+ or -] 0.11 a
E1 (152)   30.4 [+ or -] 1.3 ab   0.13 [+ or -] 0.03 a

Planting           Growth             Mortality,
site (n)                             herbivory (%)

                  Year 1.5

A12 (48)   -0.001 [+ or -] 0.02 ab       88, 0
A3 (92)     0.05 [+ or -] 0.01 a        68, 73
D1 (16)     0.02 [+ or -] 0.02 ab       44, 14
E1 (152)    0.01 [+ or -] 0.01 b        24, 11
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Title Annotation:NOTES
Author:Reemts, Charlotte M.; Conner, Patrick; Janssen, Gena K.; Wahl, Kimberly
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1U7TX
Date:Mar 1, 2014
Words:2017
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