Defoliation and mycorrhizal symbiosis: a functional balance between carbonsources and below-ground sinks

Herbivory is generally assumed to negatively influence mycorrhizal fungi be cause of reduced photosynthate to support mycorrhizae following defoliation . We examined effects of 60% and 100% defoliation (excluding current year n eedles) on tree growth and ectomycorrhizal associations of 10-15 year old S cots pines (Pinus sylvestris). Over 98% of short roots were colonized by my corrhizal fungi, and contrary to expectation, defoliation did not decrease the proportion of living fungi in fine roots. Furthermore, defoliation did not alter the ratios of produced needle biomass to the biomass of fine root s or living fungi in fine roots. The composition of mycorrhizal morphotypes was changed, however, which suggests competition among different mycorrhiz al growth forms owing to their carbon demands. We propose that these outcom es are a consequence of a functional balance between carbon sources in plan t foliage and below-ground sinks, i.e, growing roots and mycorrhizal associ ates.