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Mycorrhizal networks and distance from mature trees alter patterns of competition and facilitation in dry Douglas-fir forests


The distribution of dry Douglas-fir forests in western North America is expected to shift northward with climate change and disappear from the grassland interface in the southern interior of British Columbia. This shift may be accentuated by clearcutting, a common harvesting practice that aims to reduce the competitive effects of residual mature trees on new regeneration, but in so doing, ignores their facilitative effects. In this study, we investigated the net effects of competition from and facilitation by mature trees retained on harvested sites on seedling establishment in the dry interface Douglas-fir forests. We demonstrate that access to a mycorrhizal network (MN) and proximity to trees have important influences on seedling performance. On six sites, we established trenched plots around 24 mature Pseudotsuga menziesii var. glauca (Douglas-fir) trees, then planted Douglas-fir seedlings into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, or 250-μm pores, or without mesh) or into impermeable bags (grown in isolation) at four distances (0.5, 1.0, 2.5, or 5.0 m). Seedling survival tended to be greater and water stress lower where seedlings had full access to the MN. Seedling height, shoot biomass, needle biomass, and nutrient uptake peaked at 2.5–5.0 m from mature trees. Seedlings 0.5 m from mature trees had lower CO2 assimilation rates and wood δ13C compared to seedlings 5.0 m away. Competition for soil resources was highest near mature trees but facilitation was relatively greater at further distances, resulting in a zone of net benefit for seedlings. These results show that intraspecific tree-seedling interactions are both competitive and facilitative in dry Douglas-fir forests, and that they are spatially dependent. After disturbance, maintaining residual mature trees may be important for their beneficial regeneration zones.

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We are very grateful to Amanda Schoonmaker for her help during the field work. We are indebted to Gordon Leschyson for renting his CanDig (Kamloops, BC, Canada) mini-excavator and Rocky Hudson at High Country Cold Storage for the nursery-grown seedlings. We thank Brent Olsen and Don Brimacombe for assistance during the site selection process. We are grateful to Dr Robert Guy for his valuable advice on the physiological measurements and C isotope analyses, and to Drs Daniel Durall, Melanie Jones, and Shannon Berch for earlier comments on the study design. Tony Kozak and Val LeMay provided advice on data analysis. Funding was provided by a FSP Forest Investment Innovation of British Columbia grant, a Canadian Foundation for Innovation grant, a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to S. W. S., and an NSERC PGS scholarship to F. P. T. We declare that the experiments comply with the current laws of the country in which they were performed.

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Correspondence to François P. Teste.

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Communicated by Todd Dawson.

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Teste, F.P., Simard, S.W. Mycorrhizal networks and distance from mature trees alter patterns of competition and facilitation in dry Douglas-fir forests. Oecologia 158, 193–203 (2008). https://doi.org/10.1007/s00442-008-1136-5

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  • Tree-seedling interactions
  • Competition
  • Facilitation
  • Mycorrhizal network
  • Pseudotsuga menziesii var. glauca