SUM MARYTo test the effect of ectomycorrhizal fungi (1"'MF) on interactions between host plants, Pseiidotsuga mcnzicsii (Mirb.) Franco and Piinis potidcrosa Dougl. ex. Laws., seedlings were grown in replacement series in pasteurized soil with (a) no EMI"' added, (/;) two EMF species added -Rhizopogon vinicotor Smith (specific to Douglas-Hr) and R. octiraceortibens Smith (specific to pine), and (c) four EMF species added -the two Rhizopogon species plus two host generalists, Laccaria toccata (Scop, ex Fr.) Bk. & Br. atid Hetyctoma crusttitiniforme (Bull.) Quel. A replacement series in unpasteurized forest soil also was included. Seedlings without added EMF' were colonized by tbe greenhouse contaminant, Thctcphora terrestris. Without added EMF' (but with T. tcrrestris), the tree species mutually inhibited one another, producing Relative Yield Totals significantly < 1; with EMF added, mutual inhibition disappeared. With four EMF species added, Pseudotsuga tnenziesit seedlings were significatitly larger in mixture than in monoculture, with no corresponding decrease in the size of Piitus ponderosa seedlings; this was due solely to seedlings with L. toccata, -which apparently et-ihanced nitrogen (N) and phosphorus (P) uptake by Pseudotsuga tncnziesii at tbe expetise of luxury consumption by Pimis ponderoso. Graphical analysis suggested that better growth of Pseudotostiga menziesii in mixture with EMF added was t-elated to improved P nitrogen. Both t\ and P nutrition of Pituis ponderoso was better iti mixture with two than no EAIF species added; there was no clear nutrient effect with four EMF species added. Results indicate that EMF' can reduce cotnpetition between plant species and perhaps increase overall community P uptake. However, patterns were specific to both EMF and tree species and were quite different in unpasteurized soils. Hence generalizations about the effects of EMF" on plant-plant interactions must be made cautiously.
Nitrogen and carbon in the surface 12 cm of mineral soil, N in leaf litterfall, anaerobic N mineralization rates in the soil and forest floor, and root and N accretion to sand traps placed in surface soil layers were compared in forests with hardwoods either completely or partially removed during a conifer thinning 3 years before. An adjacent unthinned conifer–hardwood stand was also included. Conifer stocking did not differ between thinned stands with and without hardwoods. Stands without hardwoods averaged 520 kg/ha more N in mineral soil (p < 0.001), 20% more N mineralized from soil during 7-day incubations (p < 0.001), and lower soil C:N ratio (p = 0.02) than stands with hardwoods. These variables did not differ between thinned and unthinned mixed stands. Soil N did not correlate with the number of hardwoods removed. Weight of forest floor and rate of N mineralization from the forest floor did not differ between mixed and pure stands. However, stands with hardwoods returned about 10 kg•ha−1•year−1 more N in leaf litter (due to higher N concentration in conifer litter as well as the presence of high-N hardwood litter); stands without hardwoods accreted about 10 kg•ha−1•year−1 more N in sand traps. Soil N mineralization in mixed stands correlated positively with N mineralization in the forest floor but not with N accretion to sand traps, while the opposite was true in pure conifer stands. Although pretreatment variability among stands cannot be ruled out, the replicated treatments within a relatively uniform area make it appear likely that differences were related to the presence or absence of hardwoods. This was not a simple additive effect, however, but a community-level phenomenon, that is, conifers cycled N differently when mixed with hardwoods than when in pure stands.
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