Relationships between cyanolichen communities and nutrient cycling in sub-boreal spruce forests

Cyanolichens (lichens with cyanobacterial symbionts) form a distinct assemblage of epiphytes strongly associated with humid microclimatic conditions in inland British Columbia. Disparate abundance patterns in sub-boreal forests are examined in relation to the influence of overstorey tree species. A comparison of lichens on conifer saplings beneath five overstorey tree species revealed that saplings beneath Populus support a disproportionately abundant and species-rich community of cyanolichens. Cyanolichens also grew more rapidly and had lower rates of mortality beneath Populus than beneath conifer overstorey trees. That cyanolichens were observed beneath Populus in stands that were otherwise climatically unsuitable suggests that Populus facilitates cyanolichen communities by providing a factor that compensates for sub-optimal conditions. Chemical analyses of throughfall precipitation from beneath Populus, Picea, Abies, Pseudotsuga and Betula failed to explain the variation in lichen community structure. However, glucose-rich nectar, exuded from extrafloral nectaries on Populus leaves, may instead be supporting cyanolichen communities. The nectar accumulates during dry periods, is washed off during subsequent rain events, and may be intercepted and metabolized by cyanolichens on conifer saplings beneath mature Populus canopies. C-flux measurements and phospholipid fatty-acid analyses with experimental applications of ¹³C₆-labelled glucose revealed a strong physiological response to glucose and a rapid incorporation of exogenous-¹³C into cyanolichen fatty-acid tissues. Field evidence further supports this hypothesis with higher rates of cyanolichen establishment observed on Picea branches under treatment of 2% glucose solution compared to water. The exogenous C may enable cyanolichens to become established in regions that are otherwise too dry to support them by providing a source of C despite drought-induced inactivity of the cyanobacterial partner. The abundant communities of nitrogen-fixing cyanolichens in wet, mature forests and beneath Populus are important to ecosystem function. The contribution of cyanolichens to N-cycling is calculated at sites with varying lichen abundances from measured rates of lichen litter deposition, decomposition and nutrient release. Cyanolichen litter biomass represents up to 11.5% of the total N-input from aboveground litterfall and is estimated to release 2.1 kg N ha⁻¹ yr⁻¹ of newly-fixed N that would otherwise be unavailable in these mature sub-boreal forests.Forestry, Faculty ofGraduat