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High-elevation ecosystems are expected to be particularly sensitive to climate warming because cold temperatures constrain biological processes. Deeper understanding of the consequences of climate change will come from studies that consider not only the direct effects of temperature on individual species, but also the indirect effects of altered species interactions. Here we show that 20 years of experimental warming has changed the species composition of graminoid (grass and sedge) assemblages in a subalpine meadow of the Rocky Mountains, USA, by increasing the frequency of sedges and reducing the frequency of grasses. Because sedges typically have weak interactions with mycorrhizal fungi relative to grasses, lowered abundances of arbuscular mycorrhizal (AM) fungi or other root-inhabiting fungi could underlie warming-induced shifts in plant species composition. However, warming increased root colonization by AM fungi for two grass species, possibly because AM fungi can enhance plant water uptake when soils are dried by experimental warming. Warming had no effect on AM fungal colonization of three other graminoids. Increased AM fungal colonization of the dominant shrub Artemisia tridentata provided further grounds for rejecting the hypothesis that reduced AM fungi caused the shift from grasses to sedges. Non-AM fungi (including dark septate endophytes) also showed general increases with warming. Our results demonstrate that lumping grasses and sedges when characterizing plant community responses can mask significant shifts in the responses of primary producers, and their symbiotic fungi, to climate change.

High-elevation ecosystems are expected to be particularly sensitive to climate warming because cold temperatures constrain biological processes. Deeper understanding of the consequences of climate change will come from studies that consider not only the direct effects of temperature on individual species, but also the indirect effects of altered species interactions. Here we show that 20 years of experimental warming has changed the species composition of graminoid (grass and sedge) assemblages in a subalpine meadow of the Rocky Mountains, USA, by increasing the frequency of sedges and reducing the frequency of grasses. Because sedges typically have weak interactions with mycorrhizal fungi relative to grasses, lowered abundances of arbuscular mycorrhizal (AM) fungi or other root-inhabiting fungi could underlie warming-induced shifts in plant species composition. However, warming increased root colonization by AM fungi for two grass species, possibly because AM fungi can enhance plant water uptake when soils are dried by experimental warming. Warming had no effect on AM fungal colonization of three other graminoids. Increased AM fungal colonization of the dominant shrub Artemisia tridentata provided further grounds for rejecting the hypothesis that reduced AM fungi caused the shift from grasses to sedges. Non-AM fungi (including dark septate endophytes) also showed general increases with warming. Our results demonstrate that lumping grasses and sedges when characterizing plant community responses can mask significant shifts in the responses of primary producers, and their symbiotic fungi, to climate change.

We investigated the colonization status of mycorrhizal and endophytic fungi in roots of plants established on an acidic barren at volcano Esan, Hokkaido, Japan. In total, 87 individuals of 21 plant taxa were investigated in four different vegetation sites and the surrounding area. Ericaceous plants such as Empetrum nigrum var. japonicum, Ledum palustre var. diversipilosum, Vaccinium vitis-idaea var. minus and Loiseleuria procumbens were the main colonizers at the study sites and dominated under even the most severe conditions of low soil moisture and nutrient content. They were associated with ericoid mycorrhizal (ERM) fungi at all vegetation sites and erratically with dark septate endophytic (DSE) fungi. They commonly associated together with arbuscular mycorrhizal (AM) fungi only at sites where Sasa senanensis dominated. Among the ericaceous plants, only Enkianthus campanulatus associated with AM and DSE fungi and had no associations with ERM fungi. Other herbaceous and woody plants associated commonly with AM or erratically with DSE fungi, except for Carex sp. and Polygonum spp. which erratically associated with AM and DSE fungi. Mycorrhizal associations were common in representative vegetation on the crater basin of Mt. Esan. In particular, ERM associations might play a significant role in invasion and establishment of ericaceous plants, and also in the development of plant communities in the harsh environment at Mt. Esan.

Responses of the mycorrhizal fungal community in terrestrial ecosystems to global change factors are not well understood. However, virtually all land plants form symbiotic associations with mycorrhizal fungi, with approximately 20% of the plants' net primary production transported down to the fungal symbionts. In this study, we investigated how ericoid mycorrhiza (ErM), fine endophytes (FE) and dark septate endophytes (DSE) in roots responded to elevated atmospheric CO2 concentrations and warming in the dwarf shrub understory of a birch forest in the subarctic region of northern Sweden. To place the belowground results into an ecosystem context we also investigated how plant cover and nutrient concentrations in leaves responded to elevated atmospheric CO2 concentrations and warming. The ErM colonization in ericaceous dwarf shrubs increased under elevated atmospheric CO2 concentrations, but did not respond to warming following 6 years of treatment. This suggests that the higher ErM colonization under elevated CO2 might be due to increased transport of carbon belowground to acquire limiting resources such as N, which was diluted in leaves of ericaceous plants under enhanced CO2. The elevated CO2 did not affect total plant cover but the plant cover was increased under warming, which might be due to increased N availability in soil. FE colonization in grass roots decreased under enhanced CO2 and under warming, which might be due to increased root growth, to which the FE fungi could not keep up, resulting in proportionally lower colonization. However, no responses in aboveground cover of Deschampsia flexuosa were seen. DSE hyphal colonization in grass roots significantly increased under warmer conditions, but did not respond to elevated CO2. This complex set of responses by mycorrhizal and other root-associated fungi to global change factors of all the fungal types studied could have broad implications for plant community structure and biogeochemistry of subarctic ecosys