Interactive effects of tree and herb cover on survivorship, physiology, and microclimate of conifer seedlings at the alpine tree-line ecotone

Factors affecting the establishment of trees in subalpine meadows are important to population dynamics of trees in the alpine tree-line ecotone (ATE). Interactive effects of tree and herb cover on conifer seedlings were investigated in the ATE of the Snowy Range, Wyoming, USA. Microclimate, physiology, and survivorship of first-year conifer seedlings of Pinus albicaulis Engelm., Picea engelmannii Parry, and Abies lasiocarpa Hook. were measured in response to manipulations of surrounding herb and tree cover, as well as water availability. Tree and herb cover had nearly additive effects on survivorship and photosynthesis of conifer seedlings, except under alleviated water stress. In P. albicaulis, photosynthesis was greater near compared with away from trees and herbs, and photosynthetic efficiency (Fv/Fm) increased under herb cover. Tree cover led to greater nighttime temperatures, soil water contents, and, like herb cover, shade from solar radiation for seedlings. We did not detect any negative responses of conifer seedlings to surrounding vegetation. Furthermore, the effect of surrounding vegetation on conifer establishment appeared dependent on the type of surrounding vegetation, the species of conifer, and microsite stress level. These factors may lead to variation in the way conifer seedlings interact with surrounding vegetation and could explain changes in the relative abundances of tree species during forest succession in ATEs.