Hanging Gardens of Southeast Utah's Parks: Monitoring Rare Plants, 2013–2020

A hanging garden in Arches National Park. Image from the National Park Service's page.

What hanging gardens are

On the Colorado Plateau, springs support rich plant life, including endemic species found only in small, local areas. Often the only water for miles, springs are busy with people and wildlife, yet they cover a tiny fraction of the land. A smaller share still is taken by a kind of spring found only on the Plateau: the hanging garden, which forms where water seeping through porous sandstone meets a less permeable layer. Some are small, seasonal patches with little or no vegetation; others are lush alcoves full of plants. At Arches National Park and Natural Bridges National Monument, 8–9% of known endemic plant species are hanging-garden specialists.

The Northern Colorado Plateau Network (NCPN) monitors the long-term health of springs at four park units; hanging gardens occur in three of them, Arches and Canyonlands national parks and Natural Bridges National Monument.

Key points

• More sites had endemic populations of fewer than 100 plants at the end of the record than at the start.
• Three of seven monitored species declined significantly when small-population counts were pooled across parks.
• Possible die-offs, probably from worsening drought, appeared at some sites.
• More drought stress and drying is expected in southeast Utah's springs.
• Monitoring methods will be adjusted to sample more efficiently.

How to monitor without harm

There were almost no published data on monitoring spring vegetation in the arid Southwest, and most methods describe a spring at one moment rather than over years, so NCPN ecologists and partners had to design their own. They chose the count of endemic plants as an indicator: these perennials don't swing with each year's weather the way annual grasses do, and they are good indicators of how wet a site is. Seven were chosen: 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).

Hanging gardens are fragile and often perched atop steep slopes of slick rock or loose debris. A 2011 pilot that surveyed whole gardens in quadrats raised worries about crew safety and damage, and was dropped. From 2013 to 2020 crews instead worked on the accessible parts, where walking would not disturb roots or risk a serious fall: they measured each garden roughly with a laser rangefinder, mapped its surfaces (back wall, alcove floor and slope) on graph paper, and photographed the mapped area. They counted each species on each surface up to 500 plants, and put larger populations in classes of 501–1,000, 1,001–10,000 and more than 10,000.

How repeatable the counts were

The methods were meant to be tested once used. Comparing counts from year to year, and between two observers on the same visit:

Only Rydberg's thistle was highly repeatable both ways. Kachina daisy and Eastwood's monkeyflower, which spread vegetatively, were hardest to count consistently. Two-observer data covered only 6.6% of the dataset but suggested observer bias, with more experienced observers counting more plants. Repeatability was mostly within the range other studies report. Because the upper count classes are broad, the method best detects change in small populations (under 500), which are also the ones managers worry about most, being the most vulnerable to drought. Monitoring will continue for the five most repeatable species, dropping cave primrose and kachina daisy, with a focus on small populations.

Alcove columbine (Aquilegia micrantha). Image from the National Park Service's page.

Status and trends

The record is still too short for statistical trends in the count classes. But more sites had populations under 100 at the end than at the start. Rydberg's thistle and alcove bog orchid had the largest share of small populations, and they and alcove columbine declined significantly when small-population data (under 500 in at least one count) were pooled across parks.

From 2020, crews saw possible die-offs, especially of alcove bog orchid at Natural Bridges: at one site none were found in 2020, after earlier counts of 48 to 183 plants. The likely cause is deepening drought as the region warms. Drought showed on other spring plants in 2020 and 2021 too: hackberry and chokecherry had browning canopies and dropped limbs, some woody plants in photo points died, and spring plants at the edges of hanging gardens died. More drought stress and drying is expected, likely hurting endemic plants, and continued monitoring can warn managers early even before statistics catch up.

Sources

• National Park Service, Northern Colorado Plateau Network, "Monitoring Hanging Gardens in National Parks of Southeast Utah, 2013–2020." https://www.nps.gov/articles/000/ncpn_hanging-garden-trends-21.htm
• Summarised from R. Weissinger, Evaluation of Hanging-Garden Endemic-Plant Monitoring at Southeast Utah Group National Parks, 2013–2020.