Growth rates and sexual reproduction in <i>Dolichousnea longissima</i> transplanted along a trans-Himalayan elevational gradient

Abstract Dolichousnea longissima is famous as the world’s longest lichen, which typically reproduces asexually. Factors impacting its length and reproductive strategies are of theoretical interest and important for conservation. We discovered a Dolichousnea population with apotheciate thalli in Western China. Phylogenetic analyses using ITS sequence data confirmed specimens were D. longissima. Multiple phylogenetic branches included both apotheciate and non-apotheciate thalli, thus genetic differences within the ITS region were not related to apothecia production. There was no indication that apotheciate Dolichousnea specimens from this region belong to a separate, morphologically and genetically distinct population. In a field experiment, we transplanted non-apotheciate thallus strands of 10, 20, 40 or 80 cm length from higher to lower elevations: c. 4000 to 3400 m. Elevational gradients in temperature, relative humidity and vapour pressure deficit simulated future climatic change. We tested hypotheses regarding the impact of climatic stressors on D. longissima growth and reproduction. After three years of growth, thallus mortality was highest at the lowest sites. Of surviving thalli, 23% had fragmented, with fragmentation rates increasing with length. Other thalli thrived, reaching a maximum main stem length of 321 cm, with a maximum total length of 764 cm. Intact thalli showed negative length-dependent growth. Lower elevations were associated with initiation of sexual reproduction, reduced relative thallus length growth rates (RTLGR) and lower photobiont abundance, with apparent shifts of photobionts from the main stem to fibrils and reallocation of resources from length to central cord and fibril mass. Probability of switching to sexual reproduction increased with thallus length, but shorter thalli made greater relative investment in apothecia production. Detrimental impacts of climate change could reduce D. longissima range and biomass production, thereby also decreasing food availability for the endangered lichenivorous monkey, Rhinopithecus bieti. Conservation efforts should be prioritized at sites currently hosting abundant, fast-growing thalli, and high-elevation mature forests where D. longissima is most likely to persist under future climatic change. Apotheciate populations need protection because sexual reproduction theoretically generates genetic variability, increasing the likelihood of generating mycobiont genotypes adapted to altered climatic conditions.