Ectomycorrhizal (EM) networks link trees and seedlings belowground and are hypothesized to facilitate regeneration along an abiotic stress-gradient. This can occur through either EM colonization or resource transfer from trees to seedlings along source-sink gradients. We tested the role of networks in interactions between mature Pseudotsuga menziesii (Mirb.) Franco var. glauca (Mayr) trees and conspecific seedlings in a field experiment controlling for network formation and root competition between trees and seedlings along a climatic moisture gradient, as well as between conspecific seedlings in a growth chamber experiment, to: (1) determine the effects of climatic factors on network facilitation and root competition on seedling establishment, (2) infer the changing importance of mature trees in conspecific regeneration with climate, and (3) parse the competitive from facilitative effects of P. menziesii var. glauca trees on seedlings.
Results/Conclusions
When drought conditions were greatest, seedling growth increased when seedlings could form a network with trees in the absence of root competition. Survival was maximized when seedlings were able to form a network in the absence of root competition regardless of climate, but that this benefit was contingent upon the establishment of an EM network prior to the onset of summer drought. Regeneration facilitation appeared to occur via hydraulic redistribution from trees to seedlings based on deuterium transfer in an associated growth chamber experiment. We conclude that regeneration of P. menziesii var. glauca seedlings benefits from the ability to form networks with mature trees under climatic drought, consistent with the stress-gradient hypothesis. The results suggest that leaving mature trees on harvested or naturally disturbed forest sites will be increasingly important to forest regeneration where drought conditions increase with climate change.