Permafrost conditions influence the abundance, distribution, and leaf traits of two closely related dominant shrub species (Rhododendron subsect. Ledum) in interior Alaska

Permafrost considerably influences boreal forest ecosystems by constraining the niche space of woody plants. The influence of permafrost on ecosystems could dramatically change with permafrost thawing due to recent rapid climate warming. However, the influence of permafrost on shrub species in the understory of boreal forests is not completely understood. We investigated two closely related common shrubs, Labrador teas (Ericaceae, Rhododendron subsect. Ledum): R. groenlandicum and R. tomentosum, which exist sympatrically in the discontinuous permafrost zone of interior Alaska. We employed field surveys and trait measurements across permafrost gradients under the same climatic conditions to examine the associations among permafrost environments, abundances, and leaf traits of the two species. Contrasting habitat attributes were found between the two species: R. groenlandicum is abundant under shaded, drier, thicker active layer conditions, whereas R. tomentosum is common under more open, wetter, and thinner active layer conditions. This suggests that habitat segregation between these species occurs in the discontinuous permafrost zone. Compared with R. groenlandicum, R. tomentosum, which dominated permafrost conditions, had smaller leaves with smaller specific leaf areas (SLA) and low nitrogen concentrations (i.e., conservative leaves). Moreover, both species presented more conservative leaves under permafrost conditions. These intraspecific variations were mainly directly associated with canopy openness in R. groenlandicum but with active layer thickness in R. tomentosum. In summary, our study suggests that large environmental variations driven by heterogeneous permafrost conditions can lead to the sympatric distributions of closely related shrub species in discontinuous permafrost zones and that conservative leaves can contribute to their adaptation under permafrost conditions.