A long‐term vegetation history of the Mojave–Colorado desert ecotone at Joshua Tree National Park

Abstract

Thirty-eight dated packrat middens were collected from upper desert (930-1357 m) elevations within Joshua Tree National Park near the ecotone between the Mojave Desert and Colorado Desert, providing a 30 ka record of vegetation change with remarkably even coverage for the last 15 ka. This record indicates that vegetation was relatively stable, which may reflect the lack of invasion by extralocal species during the late glacial and the early establishment and persistence of many desert scrub elements. Many of the species found in the modern vegetation assemblages were present by the early Holocene, as indicated by increasing Sørenson's Similarity Index values. C 4 grasses and summer-flowering annuals arrived later at Joshua Tree National Park in the early Holocene, suggesting a delayed onset of warm-season monsoonal precipitation compared to other Sonoran Desert and Chihuahuan Desert localities to the east, where summer rains and C 4 grasses persisted through the last glacial-interglacial cycle. This would suggest that contemporary flow of monsoonal moisture into eastern California is secondary to the core processes of the North American Monsoon, which remained intact throughout the late Quaternary. In the Holocene, northward displacement of the jet stream, in both summer and winter, allowed migration of the subtropical ridge as far north as southern Idaho and the advection of monsoonal moisture both westward into eastern California and northward into the southern Great Basin and Colorado Plateau.

Evidence against a Pleistocene desert refugium in the Lower Colorado River Basin

“…yr bp . Although this suite of species is smaller than those from other regional midden series (Van Devender, ; Holmgren et al ., , ), these mostly summer‐flowering species nevertheless provide evidence for warm season precipitation during the late glacial and early Holocene.…”

Congruence between future distribution models and empirical data for an iconic species at Joshua Tree National Park

“…Evidence from paleo-biological records indicates that Joshua trees, among many species, have shifted their distribution since the Pleistocene, when they were more broadly distributed in the southwestern United States (Smith et al 2011). While the ecotone between these deserts has shifted during glacial and inter-glacial cycles, as a whole, it is believed to have been quite stable since the end of the Pleistocene (Holmgren et al 2010). While the ecotone between these deserts has shifted during glacial and inter-glacial cycles, as a whole, it is believed to have been quite stable since the end of the Pleistocene (Holmgren et al 2010).…”

“…Today, Joshua trees occur in a jagged continuous band across the western Mojave Desert and in fragmented populations to the north and east (Cole et al 2011); the occurrence of Joshua trees within JTNP defines the current southern extent of the Mojave Desert before it transitions into the Colorado Desert within the park. While the ecotone between these deserts has shifted during glacial and inter-glacial cycles, as a whole, it is believed to have been quite stable since the end of the Pleistocene (Holmgren et al 2010). Perhaps because of their iconic and keystone status, Joshua trees have received considerable attention with respect to their sensitivity to climate change.…”

Insights into the desert living skin microbiome: geography, soil depth, and crust type affect biocrust microbial communities and networks in Mojave Desert, USA

“…Although external morphology was not visibly different, our results demonstrated that microbial communities and indicator microorganisms showed greater resolution to identify geographical pattern/separation at the Mojave-Colorado desert ecotone than morphological identification. Similar procedures previously identified a boundary of Mojave desert and Colorado desert using vascular plant taxon composition and key species [53]. Employing a similar procedure, we identified major fungal and bacterial components that contributed significantly to biocrust microbial geographical patterning (ANOVA, p < 0.05).…”