Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.6.5.6 Case Study: Conserving Climate Change Refugia for the Joshua Tree in Joshua Tree National Park, CA, USA ====

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Scale: Local

Issue: Possible extirpation of a plant species from a national park

Joshua Tree National Park conserves 3200 km 2 of the Mojave and Sonoran Desert ecosystems. The climate of the national park is arid, with an average summer temperature of 27.3°C ± 0.7°C and average annual precipitation of 170 ± 80 mm yr -1 in the period 1971–2000 ( [[#Gonzalez--2018|Gonzalez et al., 2018]] ). From 1895 to 2017, the average annual temperature increased at a significant (P &lt; 0.0001) rate of 1.5°C ± 0.1°C per century and the average annual precipitation decreased at a significant (P = 0.0174) rate of -32 ± 12% per century ( [[#Gonzalez--2018|Gonzalez et al., 2018]] ). Anthropogenic climate change accounts for half the magnitude of a 2000–2020 drought in the southwestern USA, the most severe since the 1500s ( [[#Williams--2020|Williams et al., 2020]] ).

The national park was established to protect ecosystems and cultural features unique to the region, particularly the Joshua tree ( ''Yucca brevifolia'' ), a tall, tree-like yucca that provides habitat for birds and other small animals and holds cultural significance. The national park protects the southernmost populations of the Joshua tree. Palaeo-biological data from packrat ( ''Neotoma'' spp.) middens and fossilised dung of the extinct Shasta ground sloth ( ''Nothrotheriops shastensis'' ) show that Joshua trees grew 13,000–22,000 years ago across a wider range, extending as far as 300 km south into what is now México ( [[#Holmgren--2010|Holmgren et al., 2010]] ; [[#Cole--2011|Cole et al., 2011]] ). A major retraction of this range began ~11,700 years ago, coinciding with warming of approximately 4°C, caused by Milankovitch cycles, which marked the end of the Pleistocene and the beginning of the Holocene ( [[#Cole--2011|Cole et al., 2011]] ), suggesting a sensitivity of Joshua trees of 300 km of latitude per 4°C.

Under an emissions scenario that could increase park temperatures by &gt;4°C by 2100, the suitable climate for the Joshua tree could shift northwards and the species become extirpated from the park ( [[#Sweet--2019|Sweet et al., 2019]] ). Plant mortality would increase from drought stress and wildfires, which have been rare or absent in the Mojave, but which invasive grasses have fuelled and may continue to fuel ( [[#Brooks--2006|Brooks and Matchett, 2006]] ; [[#DeFalco--2010|DeFalco et al., 2010]] ; [[#Abatzoglou--2011|Abatzoglou and Kolden, 2011]] ; [[#Hegeman--2014|Hegeman et al., 2014]] ).

The national park had been trying to conserve the species wherever in the park it was found. The future risk of extirpation prompted adaptation of conservation efforts to focus on protecting potential refugia, where suitable conditions may persist for the species into the future ( [[#Barrows--2020|Barrows et al., 2020]] ). The national park used spatial analyses of suitable climate to identify potential refugia under all emissions scenarios, except for the highest ( [[#Barrows--2012|Barrows and Murphy-Mariscal, 2012]] ; [[#Sweet--2019|Sweet et al., 2019]] ). The park prioritises the refugia for removal of invasive grasses and fire control ( [[#Barrows--2020|Barrows et al., 2020]] ) and works to restore refugia that have burned in fires, using native plants, including nursery-grown Joshua tree seedlings. The park and its partners are monitoring plant species composition and abundance in the refugia for early warnings of any changes ( [[#Barrows--2014|Barrows et al., 2014]] ).

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