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Tropical montane cloud forests contain a large abundance and diversity of canopy epiphytes, which depend on canopy soil to retain water and nutrients. We lack an in depth understanding of how these soils contribute to ecosystem processes and soil diversity and how sensitive they may be to projected climate change. We compared canopy and ground soils in Monteverde, Costa Rica, to determine how these two soil types differ in their extracellular enzyme activity (EEA) and fungal communities. Samples were also collected along two elevation gradients to reveal if canopy soils differed in how EEA and fungal communities responded to elevation compared to ground soils. We found that canopy soils had higher EEA than ground soils. Fungal communities were less diverse and differed significantly between the two soil types. These differences were associated with higher relative abundances of yeasts and endophytes in canopy soils. The relative abundances of free-living filamentous fungi and yeasts shifted more dramatically with elevation in canopy soils compared to ground soils. Our study suggests that canopy soils may be a reservoir for endophytes. Epiphytes may invest in symbionts that promote stress tolerance over mycorrhizal fungi whose high resource demands are costly and less beneficial. Overall, soils harbor distinct fungal communities that may be altered under projected climate change.

Wildfires are a pervasive disturbance in boreal forests, and the frequency and intensity of boreal wildfires is expected to increase with climate warming. Boreal forests store a large fraction of global soil organic carbon (C), but relatively few studies have documented how wildfires affect soil microbial communities and soil C dynamics. We used a fire chronosequence in upland boreal forests of interior Alaska with sites that were 1, 7, 12, 24, 55, ~90, and ~100 years post-fire to examine the short- and long-term responses of fungal community composition, fungal abundance, extracellular enzyme activity, and litter decomposition to wildfires. We hypothesized that post-fire changes in fungal abundance and community composition would constrain decomposition following fires. We found that wildfires altered the composition of soil fungal communities. The relative abundance of ascomycetes significantly increased following fire whereas basidiomycetes decreased. Post-fire decreases in basidiomycete fungi were likely attributable to declines in ectomycorrhizal fungi. Fungal hyphal lengths in the organic horizon significantly declined in response to wildfire, and they required at least 24 years to return to pre-fire levels. Post-fire reductions in fungal hyphal length were associated with decreased activities of hydrolytic extracellular enzymes. In support of our hypothesis, the decomposition rate of aspen and black spruce litter significantly increased as forests recovered from fire. Our results indicate that post-fire reductions in soil fungal abundance and activity likely inhibit litter decomposition following boreal wildfires. Slower rates of litter decay may lead to decreased heterotrophic respiration from soil following fires and contribute to a negative feedback to climate warming.

Soil microbes are among the most abundant and diverse organisms on Earth. Although microbial decomposers, particularly fungi, are important mediators of global carbon and nutrient cycling, the functional roles of specific taxa within natural environments remain unclear. We used a nucleotide-analog technique in soils from the Harvard Forest to characterize the fungal taxa that responded to the addition of five different carbon substrates—glycine, sucrose, cellulose, lignin, and tannin-protein. We show that fungal community structure and richness shift in response to different carbon sources, and we demonstrate that particular fungal taxa target different organic compounds within soil microcosms. Specifically, we identified eleven taxa that exhibited changes in relative abundances across substrate treatments. These results imply that niche partitioning through specialized resource use may be an important mechanism by which soil microbial diversity is maintained in ecosystems. Consequently, high microbial diversity may be necessary to sustain ecosystem processes and stability under global change.

Plant biomass accumulation and productivity are important determinants of ecosystem carbon (C) balance during post-fire succession. In boreal black spruce (Picea mariana) forests near Delta Junction, Alaska, we quantified aboveground plant biomass and net primary productivity (ANPP) for 4 years after a 1999 wildfire in a well-drained (dry) site, and also across a dry and a moderately well-drained (mesic) chronosequence of sites that varied in time since fire (2 to ∼116 years). Four years after fire, total biomass at the 1999 burn site had increased exponentially to 160 ± 21 g m-2(mean ± 1SE) and vascular ANPP had recovered to 138 ± 32 g m-2y-1, which was not different than that of a nearby unburned stand (160 ± 48 g m-2y-1) that had similar pre-fire stand structure and understory composition. Production in the young site was dominated by re-sprouting graminoids, whereas production in the unburned site was dominated by black spruce. On the dry and mesic chronosequences, total biomass pools, including overstory and understory vascular and non-vascular plants, and lichens, increased logarithmically (dry) or linearly (mesic) with increasing site age, reaching a maximum of 2469 ± 180 (dry) and 4008 ± 233 g m-2(mesic) in mature stands. Biomass differences were primarily due to higher tree density in the mesic sites because mass per tree was similar between sites. ANPP of vascular and non-vascular plants increased linearly over time in the mesic chronosequence to 335 ± 68 g m-2y-1in the mature site, but in the dry chronosequence it peaked at 410 ± 43 g m-2y-1in a 15-year-old stand dominated by deciduous trees and shrubs. Key factors regulating biomass accumulation and production in these ecosystems appear to be the abundance and composition of re-sprouting species early in succession, the abundance of deciduous trees and shrubs in intermediate aged stands, and the density of black spruce across all stand ages. A better understanding of the controls over these factors will help predict how changes in climate and fire regime will affect the carbon balance of Interior Alaska. © 2008 Springer Science+Business Media, LLC.

We examined potential large-scale controls over the distribution of arbuscular mycorrhizal (AM) fungi and their host plants. Specifically, we tested the hypothesis that AM fungi should be more prevalent in biomes where nutrients are primarily present in mineral, and not organic, forms. Values of percentage root length colonized (%RLC) by AM fungi, AM abundance, and host plant availability were compiled or calculated from published studies to determine biome-level means. Altogether, 151 geographic locations and nine biomes were represented. Percent RLC differed marginally significantly among biomes and was greatest in savannas. AM abundance (defined as total standing root length colonized by AM fungi) varied 63-fold, with lowest values in boreal forests and highest values in temperate grasslands. Biomes did not differ significantly in the percentage of plant species that host AM fungi, averaging 75%. Contrary to the hypothesis, %RLC, AM abundance, and host plant availability were not related to the size, influx, or turnover rate of soil organic matter pools. Instead, AM abundance was positively correlated with standing stocks of fine roots. The global pool of AM biomass within roots might approach 1.4 Pg dry weight. We note that regions harboring the largest stocks of AM fungi are also particularly vulnerable to anthropogenic nitrogen deposition, which could potentially alter global distributions of AM fungi in the near future.

We examined the effects of atmospheric carbon dioxide (CO2) enrichment on belowground carbon (C) pools and arbuscular mycorrhizal (AM) fungi in a chaparral community in southern California. Chambers enclosing intact mesocosms dominated by Adenostoma fasciculatum were exposed for 3.5 years to CO2 levels ranging from 250 to 750 ppm. Pools of total C in bulk soil and in water-stable aggregates (WSA) increased 1.5- and threefold, respectively, between the 250- and 650-ppm treatments. In addition, the abundance of live AM hyphae and spores rose markedly over the same range of CO2, and the community composition shifted toward dominance by the AM genera Scutellospora and Acaulospora. Net ecosystem exchange of C with the atmosphere declined with CO2 treatment. It appears that under CO2 enrichment, extra C was added to the soil via AM fungi. Moreover, AM fungi were predominant in WSA and may shunt C into these aggregates versus bulk soil. Alternatively, C may be retained longer within WSA than within bulk soil. We note that differences between the soil fractions may act as a potential feedback on C cycling between the soil and atmosphere.