Evaluation of hanging-garden endemic-plant monitoring at Southeast Utah Group national parks, 2013–2020

Abstract

Hanging gardens are the most common type of spring at Arches National Park (NP) and Natural Bridges National Monument (NM). They are also present at Canyonlands National Park, but hanging gardens are rare off the Colorado Plateau. Their cliffside setting provides stable access to water without flood disturbance. This combination provides unique habitat that is rich in endemic plant species. The diffuse, seeping emergence of water makes measuring springflow impossible at most sites. Park managers have an interest in monitoring hanging gardens—especially as the climate warms and aridity and water demand both increase. The Northern Colorado Plateau Net-work (NCPN) proposed methods for monitoring seven perennial endemic-plant species at hanging gardens as indicators of spring health and proxies for water availability. Because hanging gardens occur on bedrock outcrops, systematic or random sampling was not possible due to safety concerns and potential resource damage on steep, wet slopes. Examining eight years (2013–2020) of data, this report evaluates the suitability of endemic-plant count data at hanging gardens as a monitoring indicator. It also provides our first evaluation of status and trends at NCPN hanging gardens. The seven species included in monitoring were Rydberg’s thistle (Cirsium rydbergii), Kachina daisy (Erigeron kachinensis), alcove death camas (Zigadenus vaginatus), alcove bog orchid (Habenaria zothecina), cave primrose (Primula specuicola), alcove columbine (Aquilegia micrantha), and Eastwood’s monkeyflower (Mimulus eastwoodiae). Six of the seven species were found at each park. Up to 500 individuals of each species were counted at 42 hanging gardens in Arches NP, 14 hanging gardens in Natural Bridges NM, and 3 hanging gardens in Canyonlands NP. Larger populations were divided into count classes of 501–1,000, 1,001–10,000, and more than 10,000 individuals. Counts from two independent observers and from back-to-back years of sampling were compared for repeatability. Repeatability in count classes was less than 50% for Kachina daisy and Eastwood’s monkeyflower, which both propagate vegetatively via ramets and/or stolons. Repeatability was greater than 90% for only one species, Rydberg’s thistle. The remaining species were categorized in different classes between 15–40% of the time. Independent-observer comparisons were only available for 6.6% of the dataset, but these observations suggested that (1) observer bias was present and (2) the observer with more experience working in hanging gardens generally had higher counts than the observer with less experience in this system. Although repeatability was variable, it was within the range reported by other studies for most species. The NCPN, in discussion with park staff, has elected to make some modifications to the protocol but will continue using endemic plant counts as an indicator of hanging-garden health to maintain a biological variable as a complement to our physical-response data. This is due to their high value to park biodiversity and the difficulty of developing a more robust approach to monitoring in these sites. Endemic-plant monitoring will continue for the five species with the highest repeatability during pilot monitoring and will focus on detecting changes in smaller populations. Most hanging gardens have more than one endemic species present, so several populations can be tracked at each site. Our period of record is relatively brief, and the distribution of endemic-plant populations in different count classes at these sites has not yet shown any statistical trends over time. Be-cause of the large count classes, our methods are more sensitive to showing change in smaller populations (fewer than 500 individuals). Small populations are also of greatest concern to park managers because of their vulnerability to declines or extirpation due to drought. Over-all, more sites had endemic-plant populations of fewer than 100 individuals at the end...