The influence of mycorrhizas on the increased nitrogen uptake by Sitka spruce in mixed species stands
On poor peats and heathlands, satisfactory growth of Sitka spruce (Picea sitchensis), planted pure, can only be achieved with repeated applications of nitrogen fertilizer. However, when spruce is planted in mixture with Scots pine (Pinus sylvestris), Lodgepole pine (P.contorta), or Japanese larch (Larix kaempferi) it shows improved nitrogen nutrition and better growth than pure spruce. The mechanism by which the 'extra' nitrogen required to sustain improved spruce growth has not yet been elucidated. A mechanism is proposed in this thesis. The mycorrhizal community is examined in pure and mixed stands at Culloden Scotland, by recording potential mycorrhizal fruiting bodies, carrying out a seedling bioassay and characterising the mycorrhizal communities on both pine and spruce roots. Approximately 30% of the mycorrhizas on pine mixtures were formed by Suillus spp. but there were no Suillus mycorrhizas on spruce, even in mixture. In pure stands spruce mycorrhizas were mostly Tylospora fibrillosa (55%) or Russulaceae type (16%). In mixed stands, beneath pine canopy, the production of T.fibrillosa on spruce roots was lower (26%) and that of Russulaceae was higher (48%). The Sitka spruce mycorrhizal populations on younger trees in experiments at Speymouth and Shin were also examined. The results suggested that a change in mycorrhizal associations are a part of the 'mixtures effect' and that it starts before closed canopy is established. The ability of various ectomycorrhizal fungi were screened for their ability to break down and utilize protein. Suillus variegatus had the greatest proteinase activity whilst Lactarius rufus, a member of Russulaceae had little ability. The ability of L.rufus and T.fibrillosa to utilise protein hydrolysed by S.variegatus and a Calluna endophyte was examined, both isolates could utilise the filtrates as the only nitrogen source. It is suggested that in mixtures the proteolytic activity of ectomycorrhizal fungi of pine results in an extracellular pool of nitrogen for which the mycorrhizal flora of spruce compete.