2024

Mojave

Research

Dr Lynn Sweet

(760) 834-0594

University of CA, Riverside
75-080 Frank Sinatra Drive
Palm Desert, California 92211
United States

 

lynn.sweet@ucr.edu

Post-wildfire resprouting and survival of Joshua trees and banana yucca after the 2020 Dome Fire, Mojave National
Preserve

MOJA-00378

MOJA-2024-SCI-0032

Apr 29, 2024

Dec 31, 2024

Mar 01, 2022

Dec 31, 2025

Continuing

___ A final report has been provided to the park or will be provided to the park within the next two years

___ Copies of field notes, data files, photos, or other study records, as agreed, have been provided to the park

___ All collected and retained specimens and retained material originating from such specimens have been cataloged into the NPS catalog system and NPS has processed loan agreements as needed.

Research

Vascular Plants

The Mojave National Preserve (Preserve) protects a rich diversity of plant species including two yuccas, the Eastern Joshua tree (Yucca jaegeriana) and banana yucca (Yucca baccata), that are key components of the Joshua tree woodland found in the Preserve. Since this vegetation does not readily recover from large and intense wildfires, fires such as the 2020 Dome Fire at Cima Dome provide opportunities to better understand and communicate, what are realistic expectations for ecosystem recovery following fire, and the future trajectory of populations of Joshua trees? Building on an extensive site-history of 23 study sites on Cima Dome established by Dr. McAuliffe, each with georeferenced pre-firephotos and vegetation data, Sweet et al intend to take advantage of a critical window of opportunity to investigate the impacts of the Dome Fire on Joshua trees and banana yuccas, both prominent components of the vegetation. Banana yucca is found together with Joshua trees at Cima Dome and is included to provide a comparison of post-fire responses of the two yucca species, complementing other published comparisons of post-fire responses of Joshua trees and Mojave yucca in Joshua Tree National Park. This study will contribute to ongoing efforts at Mojave National Preserve to understand and protect Joshua tree woodlands, provide opportunities for community science volunteers, and provide valuable deliverables that can be used for visitor interpretation for public enjoyment and understanding.

In 2022 we began to conduct this study of post-fire survivorship census surveys of Yucca jaegeriana (Eastern Joshua tree, YUJA) and Yucca baccata (banana yucca, YUBA) within the Mojave Preserve during two separate field campaigns; May 2022 and October 2022. During these campaigns we established and surveyed a total of 19 plots (nine in May and ten in October). 14 of these plots were established at the locations of Dr. J. McAuliffe’s long-term photo points (McAuliffe 2021). We established the remaining five plots where pre-fire data was taken during a vegetation mapping effort in 2016 (Cogan 2021). We added these plots to provide sites where Coleogyne ramosissima and Yucca species were the primary pre-fire vegetation types. We performed repeat visits May 2023 and October 2023.
During the reporting period 2024, we designed a set of unburned comparison plots (statistical control data) to gather information approximating pre-fire conditions at our burned plots. We selected plots by matching the pre-burn vegetation classification and cover, topography, slope, and aspect of our existing plots using existing GIS datasets and randomly generated plots within target spatial strata (with an oversample). In May we established the 11 new plots within unburned areas. At each of the new plots we field-verified the vegetation and topography and recorded local site data including GPS coordinates, photos, living shrub species and perennial herb species and their percent cover. We replicated the protocols used on burned plots to collect census data of YUJA and YUBA. Census data consisted of measurements of yucca individuals such as height, coordinates, number of basal sprouts and life condition. Plot sizes were 40x20 m, originating from the photo point and running along the central line of the photo. We documented both yucca species within 10 m on either side of the central line. In the case of all 11 plots we had to extend the area in increments of 100m² to reach a minimum of 20 YUJA per plot. YUBA was only counted and measured within the 40x20m plot.
We measured a total of 256 YUJA within the unburned plots, 53 of which were dead. We estimated the time since the tree had died using a repeatable protocol (developed for use with the western Joshua tree in JOTR). Trees were divided into categories based on the conditions of leaves, and wood, and an estimated time-since-mortality was assigned, generally within 1-2 years dead, < five years dead, > five years dead; and indeterminate.
Preliminary findings using these estimated mortality-timing categories indicate that 26% of the dead trees in unburned plots may have died within the last five years. We may refine this estimate based on further analysis or information regarding longevity of fine and coarse woody debris onsite locally. Dead trees (including all detectable dead trees) averaged 4.8 per 0.2 hectare across all unburned control plots, where live trees averaged 22.1 per 0.2 ha. We conclude that we sampled stands that were similar in nature and contained similar densities of trees to our burn study plots (26.9 vs. 27.8 per 0.2 ha, respectively), and so contain important contextual information for inferences about the impacts of fire. Based on our field data, these plot data should allow valuable comparisons and important context about demographics and condition in unburned stands, such as confirming the rate of background mortality in Joshua trees locally. In all, preliminary results indicate that the density of live Eastern Joshua trees may decrease from 22.1 per 0.2 ha to 5.0 per 0.2 ha post-fire.
We did not make any collections of plant material and we abided by the conditions outlined in our permit. We presented our results to the National Park Service Mojave Desert Network Science Symposium on October 16, 2024.

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