Can Desert Mosses Hide from Climate Change?: The Ecophysiological Importance ofHabitat Buffering and Water Relations to a Keystone Biocrust Moss in the Mojave Desert.

다국어 초록 (Multilingual Abstract)

Empirical and observational studies suggest a keystone biocrust moss, Syntrichia caninervis, may be sensitive to future climate change in the American Southwest due to its uniquely sensitive water relations that appear particularly challenged during summer hydration-desiccation cycles. However, the potential mitigating roles of habitat buffering, acclimatization, and winter recovery on the vulnerability of this species remain largely unexplored. I investigated potential abiotic and biotic resiliency factors driving summer stress resistance and recovery in S. caninervis along present-day aridity gradients in the Mojave Desert to strengthen the climate change vulnerability assessment for this species common to biocrusts of North America, northern Africa, and Asia.I sampled fall and spring shoots from three Mojave Desert life zone sites stratifying selection of 94 microsites across three nested aridity gradients: site elevation, mesotopographical exposure, and microhabitat exposure. I quantified microsite annual shade time with a novel photographic approach accurate for the 20 x 20 cm microsites and used on-site climate stations and microscale light sensors to estimate winter recovery conditions. This climate data informed lab-simulated recovery and stress experiments that I combined with photosynthetic performance assays to explore 1) the physiological significance of habitat buffering in summer along three scales of aridity, 2) winter-recovery potential under optimal cloudy-day conditions, 3) potential life zone legacy effects in productivity, morphology, and physiology of common-garden shoots, 4) desiccation tolerance following an extreme rapid-dry challenge, 5) in situ natural winter recovery from summer stress, 6) potential seasonal legacy effects on simulated recovery, and 7) the quality and frequency of natural winter recovery conditions during precipitation events in each life zone.I uncovered ten ecophysiological signatures of resiliency to present-day climate stress in three life zone populations of S. caninervis along a 1200-m elevation gradient in the Desert National Wildlife Refuge, Nevada. The summer stress signal (chlorophyll fluorescence ratio, Fv/Fm) and photosynthetic performance (ΦPSII) of 94 S. caninervis microsites was highly variable and not related to life zone but strongly related to topographical aspect and microhabitat shade. During simulated recovery under optimal winter conditions, all samples recovered to healthy levels within 8 h, at which point a significant positive relationship to elevation life zone emerged in photosynthetic physiology. Common-garden mosses demonstrated a strong potential for long-term recovery via apical shoot extension, while life zone patterns in rhizoidal growth and plant size were conserved in new growth. This life zone signal in physiology began to fade in the garden, but protective, 6-week legacy effects in physiology and morphology (plant size and leaf bistratosity) were linked to increased rapid-dry resiliency in the low-elevation population. These life zone patterns were largely absent after 12 weeks of dehardening, suggesting life zone plastic tradeoffs for increased desiccation tolerance may come at the cost of reduced productivity with long-term legacy effects on photosynthetic physiology and rapid-dry resistance.Mean stress and performance following natural winter recovery were 57% and 64% of that achieved during the 24-h simulated recovery, but spring-sampled shoots reached or exceeded healthy summer recovery levels within a 4- to 24-h simulated hydroperiod while demonstrating strong winter light acclimatization effects by life zone. The spring stress photosynthetic signal was greatest at the low-elevation and lowest at the mid-elevation. This inverse, nonlinear relationship to elevation meant that spring stress was not related to total winter precipitation, which increased with elevation, and was best explained by mean annual topographical exposure and higher absolute humidity and frost-free temperatures at the low-elevation site during putative periods of moss hydration.These findings implicate a strong potential for future climate resiliency in populations of S. caninervis living in one of the most extreme desert climates of the species’ global range. Contrary to predictions from global change experiments, my findings suggest (1) a combination of strong desiccation tolerance paired with summer habitat buffering may increase summer stress resistance, (2) short- and long-recovery during winter precipitation hydroperiods is plausible, (3) recovery may be strengthened by seasonal legacy effects that optimize productivity tradeoffs for increased resiliency to desiccation, and (4) the quality of winter precipitation conditions may be more important to recovery than the quantity. Collectively, these findings suggest this keystone biocrust moss and its ecosystem services may be less vulnerable to a changing Mojave Desert than previously thought.