Canopy openings created by partial cutting increase growth rates and maintain the cover of three Cladonia species in the Canadian boreal forest

In boreal forests partial cutting is often proposed as an alternative to the widespread use of clearcutting to better conciliate forest management with species conservation. A partial cutting experiment was implemented in black spruce boreal forests of western Québec, and the responses of three terricolous lichen species (Cladonia stellaris, C. mitis, and C. rangiferina) to three different treatments of different removal intensities (old unharvested forest stands and old forests treated by partial cutting and clearcutting) were studied. We used transplants of lichens to record growth rate over a period of 15 months, and we also recorded microclimatic variables (thallus temperature, thallus humidity, and photosynthetically active radiation [PAR]) directly at the sampling sites. We observed reduced growth rates in C. stellaris and C. rangiferina transplants located in untreated control plots compared with transplants located either in stands treated with partial cutting and or stands treated by clearcutting. We also found that lichen growth differed significantly across season, with a higher growth rate during the first summer compared to the other seasons. Lichen cover and frequency (presence or absence) of the three Cladonia species did not differ significantly between partial cut plots and control plots. PAR and thallus temperature were significantly lower in control plots, while thallus humidity was significantly higher in control plots. Thallus humidity recorded in the morning (between 6 am and 9 am) showed a reversed pattern, with lower values in control plots compared with partial cutting and clearcutting plots. Our results show that canopy openings in the partial cuttings had a positive influence on the growth of Cladonia species, probably through an increase in the quantity of light and humidity reaching the thalli early in the morning. Furthermore, canopy openings resulting from partial cuttings can maintain pre-established lichen cover at levels comparable to what is observed in our unharvested control stands.