Invasive exotic plants represent one of the most significant threats to natural resources in U.S. national parks today.
Patrick J. Alexander @ USDA-NRCS Plants Database
In the national parks, exotic invasives are plants that were deliberately or accidentally introduced to park lands by humans. These plants quickly reproduce and spread. They displace native species and alter ecosystem processes at local and landscape scales.
This project had two goals:
(1) to map the location, abundance, and distribution of 74 invasive plant species on the Arizona Wildlands Invasive Plant Working Group's species list (plus five additional species of interest/concern to Tumacácori National Historical Park (NHP) staff; and
(2) to rank the recorded species based on significance of impact, ability to become a pest, and difficulty of control using the Alien Plants Ranking System (APRS) (APRSIT 2001; AZ-WIPWG 2005).
Tumacácori NHP is located in the upper Santa Cruz River Valley of southern Arizona. The park comprises three units, each housing abandoned ruins of Spanish colonial missions. San José de Tumacácori and Los Santos Ángeles de Guevavi, established in 1691, are the two oldest missions in Arizona. The third unit, San Cayetano de Calabazas, was established in 1756. The park is approximately 146 ha (360 acres). Tumacácori, with 131.5 ha (325 acres), is the largest unit. Calabazas is 11 ha (28 acres) and Guevavi 2.83 ha (7 acres). The Calabazas and Guevavi units are located 15 km and 23 km SSE of Tumacácori, respectively.
The Santa Cruz River runs through the the Tumacácori unit. It supports a rich riparian habitat of cottonwoods and willows. The Juan Bautista de Anza National Historic Trail winds through the Tumacácori unit.
Methods
An invasive non-native plant inventory was conducted at Tumacácori NHP during January and February 2006. The survey was a cooperative effort between the National Park Service’s Sonoran Desert Network and the Sonoran Institute.
Roaming surveys were performed. The goal was to comprehensively cover the entirety of all three park units. A field crew walked through the unit in a systematic way, using handheld mapping units with GIS layers showing current spatial position and the areas already surveyed and mapped.
All non-native species were mapped either as a point or polygon feature. Points were used for single individuals or patches less than 5 m across. Polygons were used for infestations larger than 5 m across. Supplemental circular-plot surveys were also performed, in which vegetation abundance and diversity were measured via visual estimates of percent cover, by species, within three quadrats of one square meter each. These surveys were done to assess any observer bias, as well as accuracy, in the roaming surveys.
Area affected was calculated as a measure of plant distribution calculated directly from mapped polygon boundaries. This measure does not reflect the intensity of invasion. For example, a 1.5-ha polygon estimated at 25–50% cover would represent 1.5 ha affected.
Area infested was calculated as a measure of the actual land area infested by each target species. Infested area was calculated using polygon data by multiplying the cover-class midpoint by the polygon’s area. For example, a 1.5-ha polygon estimated at 25–50% cover would represent 0.5625 ha (1.5 × 0.375 = 0.5625) infested. In both calculations, points were estimated to represent 0.5 squre meters.
Results
| Table 1. Non-native invasive species observed during roaming surveys, Tumacacori National Historical Park. | |
| Scientific name | Common name |
| Ailanthus altissima | tree of heaven |
| Amaranthus palmeri | Palmer’s amaranth |
| Arundo donax | giant reed |
| Conium maculatum | poison hemlock |
| Cynodon dactylon | Bermuda grass |
| Eragrostis cilianensis | stink grass |
| Eragrostis lehmanniana | Lehmann lovegrass |
| Malva parviflora | cheese weed |
| Marrubium vulgare | horehound |
| Salsola tragus | Russian thistle |
| Sisymbrium irio | London rocket |
| Sonchus spp. | sowthistles |
| Sorghum halepense | Johnson grass |
| Tamarix chinensis | tamarisk |
Species detected
Of the 79 target species, 14 were observed during roaming surveys at Tumacácori NHP units in 2006 (Table 1). All of these species had been previously recorded at these units. Of these 14 species, only five were recorded at the Calabazas and Guevavi units (these two sites were affected by the same suite of five species; see Tables 3–4). Of the 14 non-native species mapped, five were grasses (Poaceae).
Total area affected and infested
Affected-area value varied greatly by species. Several species were widespread throughout the unit (Tables 2–4). Amaranth was mapped across 76% of the unit, Bermuda grass over 72%, Russian thistle over 61%, tamarisk across 42%, and Lehmann lovegrass and poison hemlock at 36 and 34%, respectively. All other species were mapped to be affecting 1–10% of the unit.
| Table 2. Total area affected and total area infested by target species within Tumacácori NHP’s main unit. | |||||
| Area affected | Area infested | ||||
| Hectares | Percent of unit | Hectares | Percent of unit | ||
| Ailanthus altissima | tree of heaven | T | T | T | T |
| Amaranthus palmeri | Palmer’s amaranth | 101.25 | 76% | 18.20 | 14% |
| Arundo donax | giant reed | T | T | T | T |
| Conium maculatum | poison hemlock | 45.95 | 34% | 3.83 | 3% |
| Cynodon dactylon | Bermuda grass | 96.38 | 72% | 30.92 | 23% |
| Eragrostis cilianensis | stink grass | 18.06 | 13% | 0.48 | T |
| Eragrostis lehmanniana | Lehmann lovegrass | 48.67 | 36% | 0.64 | T |
| Malva parviflora | cheese weed | 1.69 | 1% | T | T |
| Marrubium vulgare | horehound | 13.58 | 10% | T | T |
| Salsola tragus | Russian thistle | 81.96 | 61% | 5.18 | 4% |
| Sisymbrium irio | London rocket | 16.14 | 12% | 1.32 | T |
| Sonchus spp. | sowthistles | 2.52 | 2% | T | T |
| Sorghum halepense | Johnson grass | 13.28 | 10% | 1.26 | T |
| Tamarix chinensis | tamarisk | 56.39 | 42% | 2.27 | 2% |
| T denotes area <0.1 ha or <0.1% | |||||
Levels of area infested also varied by species, with nine species infesting less than 1% of the total park area, three species infesting 1–4% of the total area, and two species, Bermuda grass and amaranth, infesting 23% and 14% of the total area, respectively (Tables 2–4).
| Table 3. Total area affected and total area infested by target species within the Calabazas unit. | |||||
| Area affected | Area infested | ||||
| Hectares | Percent of unit | Hectares | Percent of unit | ||
| Amaranthus palmeri | Palmer’s amaranth | 7.62 | 86% | 0.23 | 2% |
| Cynodon dactylon | Bermuda grass | 2.39 | 27% | T | T |
| Eragrostis cilianensis | stink grass | 3.24 | 37% | T | T |
| Eragrostis lehmanniana | Lehmann lovegrass | 4.92 | 55% | 0.19 | 2% |
| Salsola tragus | Russian thistle | 6.52 | 73% | 0.19 | 2% |
| T denotes area <0.1 ha or <0.1% | |||||
| Table 4. Total area affected and total area infested by target species within the Guevavi unit. | |||||
| Area affected | Area infested | ||||
| Hectares | Percent of unit | Hectares | Percent of unit | ||
| Amaranthus palmeri | Palmer’s amaranth | 3.48 | 76% | 1.05 | 37% |
| Cynodon dactylon | Bermuda grass | 0.78 | 72% | T | T |
| Eragrostis cilianensis | stink grass | 1.74 | 13% | T | 1% |
| Eragrostis lehmanniana | Lehmann lovegrass | 0.86 | 36% | T | T |
| Salsola tragus | Russian thistle | 3.07 | 61% | T | 2% |
| T denotes area <0.1 ha or <0.1% | |||||
Discussion
| Table 5. Target species occurrences and relative abundances within circular plots. | ||
| Species | Circular plot occurrences | Circular plot relative abundance (foliar cover) |
| Amaranthus palmeri | 24 | 1.6% |
| Conium maculatum | 1 | 0.02% |
| Cynodon dactylon | 17 | 13.4% |
| Eragrostis cilianensis | 8 | 0.24% |
| Eragrostis lehmanniana | 3 | 0.24% |
| Marrubium vulgare | 2 | 0.59% |
| Salsola tragus | 5 | 0.51% |
| Sisymbrium irio | 1 | 0.16% |
| Sorghum halepense | 3 | 1.76% |
| T denotes area <0.1 ha or <0.1% | ||
Roaming surveys vs. circular plots
Of the 79 target species, nine were recorded within the circular plots (Table 5). Stohlgren and others (2003) suggested randomly locating these plots and comparing the foliar cover of target species within the circular plots to that collected during the roaming surveys. When foliar cover of target species collected via circular plots was plotted against that collected via roaming surveys, the relative abundance of target species was rather consistent between the two methods (R2 = 0.62), indicating that roaming surveys performed well at estimating abundance of target species. Both datasets were transformed using the cube-root transformation.
Alien Plants Ranking System
Most species ranked under the APRS fell into the medium-tolow impact category. On a 100-point scale, plants ranged from 24 to 76 for significance of impact, from 4 to 47 for innate ability to become a pest, and from 21 to 69 for difficulty of control (Table 6). Tamarisk was ranked as having the greatest impact.
| Table 6. Ranks of target species for significance of impact, innate ability to become a pest, and difficulty of control. | ||||
| Scientific name | Common name | Impact | Ability to become a pest | Difficulty of control |
| Tamarix chinensis | tamarisk | 76 | 47 | 69 |
| Eragrostis lehmanniana | Lehmann lovegrass | 67 | 38 | 60 |
| Marrubium vulgare | horehound | 48 | 44 | 45 |
| Salsola tragus | Russian thistle | 43 | 33 | 56 |
| Cynodon dactylon | Bermuda grass | 30 | 31 | 68 |
| Sorghum halepense | Johnsongrass | 56 | 11 | 44 |
| Sisymbrium irio | London rocket | 38 | 13 | 55 |
| Arundo donax | giant reed | 60 | 20 | 25 |
| Ailanthus altissima | tree of heaven | 51 | 9 | 41 |
| Conium maculatum | poison hemlock | 37 | 22 | 32 |
| Eragrostis cilianensis | stinkgrass | 29 | 13 | 44 |
| Sonchus spp. | sowthistles | 43 | 4 | 37 |
| Amaranthus palmeri | Palmer’s amaranth | 27 | 33 | 21 |
| Malva parviflora | cheeseweed | 24 | 5 | 28 |
Tamarisk was ranked hardest-to-control, with the greatest risk of becoming a major pest. Giant reed could become more of a pest should it spread, but currently should be quite easy to control. Annuals tended to rate scores reflecting relative ease of control and minor pest status.
The less widespread annual species fell mostly within the lesser-impact and easy-to-control categories, with only tamarisk approaching both the very-hard-to-control/serious-impact category.
Conclusions
As a result of this survey, several distinctly infested areas have been identified. In general, the old agricultural areas in the southern portion of Tumacácori NHP still show signs of disturbance and are heavily infested with non-native invasive species. Areas to the north of the fiesta grounds (also old agricultural fields) also have many invasive plants, but these occur in open areas and the understory of the many trees and shrubs.
The riparian corridor has several invasive species, most notably tamarisk, Bermuda grass, and poison hemlock. Some species mapped were only found sporadically or as individual plants, and do not pose a significant threat. The two smaller units, Calabazas and Guevavi, were both observed to have the same five species. These units are in relatively good shape but management should be considered in some of the worse-affected areas.
Literature Cited
Alien Plants Ranking System Implementation Team (APRSIT). 2001. Alien plants ranking system, version 7.1. Flagstaff, AZ: Southwest Exotic Plant Information Clearinghouse. https://sbsc.wr.usgs.gov/research/projects/swepic/aprs/downloads.asp. Last accessed January 23, 2008.
Arizona Wildlands Invasive Plant Working Group (AZWIPWG). 2005. List of non-native invasive plants that threaten wildlands in Arizona. https://sbsc.wr.usgs.gov/research/projects/swepic/SWVMA/sbscmain.asp. Last accessed January 16, 2008.
Stohlgren, T. J., T. D. Barnett, and S. S. Simonson. 2003. Beyond North American weed management standards.
Prepared by Sarah Studd, Sonoran Desert Network Inventory and Monitoring Program, and Mark Zepp, Sonoran Institute, 2009.
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Last updated: November 28, 2016