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Facilitation of tree establishment by ectomycorrhizal (EM) networks (MNs) may become increasingly important as drought stress increases with climate change in some forested regions of North America. The objective of this study was to determine (1) whether temperature, CO2 concentration ([CO2]), soil moisture, and MNs interact to affect plant establishment success, such that MNs facilitate establishment when plants are the most water stressed, and (2) whether transfer of C and water between plants through MNs plays a role in this. We established interior Douglas-fir (Pseudotsuga menziesiivar.glauca) seedlings in root boxes with and without the potential to form MNs with nearby conspecific seedlings that had consistent access to water via their taproots. We varied temperature, [CO2], and soil moisture in growth chambers. Douglas-fir seedling survival increased when the potential existed to form an MN. Growth increased with MN potential under the driest soil conditions, but decreased with temperature at 800 ppm [CO2]. Transfer of 13C to receiver seedlings was unaffected by potential to form an MN with donor seedlings, but deuterated water (D2O) transfer increased with MN potential under ambient [CO2]. Chlorophyll fluorescence was reduced when seedlings had the potential to form an MN under high [CO2] and cool temperatures. We conclude that Douglas-fir seedling establishment in laboratory conditions is facilitated by MN potential where Douglas-fir seedlings have consistent access to water. Moreover, this facilitation appears to increase as water stress potential increases and water transfer via networks may play a role in this. These results suggest that conservation of MN potential may be important to forest regeneration where drought stress increases with climate change.

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Ectomycorrhizal (EM) networks are hypothesized to facilitate regeneration under abiotic stress. We tested the role of networks in interactions between P. menziesii var. glauca trees and conspecific seedlings along a climatic moisture gradient to: (1) determine the effects of climatic factors on network facilitation of Pseudotsuga menziesii (Mirb.) Franco var. glauca (Mayr) seedling establishment, (2) infer the changing importance of P. menziesii var. glauca parent trees in conspecific regeneration with climate, and (3) parse the competitive from facilitative effects of P. menziesii var. glauca trees on seedlings. When drought conditions were greatest, seedling growth increased when seedlings could form a network with trees in the absence of root competition, but was reduced when unable to form a network. Survival was maximized when seedlings were able to form a network in the absence of root competition. Seedling stem natural abundance d 13 C increased with drought due to increasing water use efficiency, but was unaffected by distance from tree or network potential. We conclude that P. menziesii seedlings may benefit from the presence of established P. menziesii trees when growing under climatic drought, but that this benefit is contingent upon the establishment of an EM network prior to the onset of summer drought. These results suggest that networks are an important mechanism for EM plants establishing in a pattern consistent with the stress-gradient hypothesis, and therefore the importance of EM networks to facilitation in regeneration of EM trees is expected to increase with drought.

Ectomycorrhizal (EM) networks (MN) are thought to be an important mode of EM fungal colonization of coniferous seedlings. How MNs affect EM communities on seedlings, and how this varies with biotic and abiotic factors, is integral to understanding their importance in seedling establishment. We examined EM fungal community similarity between mature trees and conspecific interior Douglas-fir (Pseudotsuga menziesii var. glauca) seedlings in two experiments where seed and nursery-grown seedlings originating from different locations were planted at various distances from trees along a climatic gradient. At harvest, trees shared 60% of their fungal taxa in common with outplanted seedlings and 77% with germinants, indicating potential for seedlings to join the network of residual trees. In both experiments, community similarity between trees and seedlings increased with drought. However, community similarity was lower among nursery seedlings growing at 2.5 m from trees when they were able to form an MN, suggesting MNs reduced seedling EM fungal richness. For field germinants, MNs resulted in lower community similarity in the driest climates. Distance from trees affected community similarity of nursery seedlings to trees, but there was no interaction of provenance with MNs in their effect on similarity in either nursery seedlings or field germinants as hypothesized. We conclude that MNs of trees influence EM colonization patterns of seedlings, and the strength of these effects increases with climatic drought.