Results Acknowledgments J.S.P. was supported by the University of Montana ECOS program with funding from a NSF GK-12 grant to C.A. Brewer. Special thanks to Mary Harner, Andrew Hoye, Daniel Warnock, Ben Wolfe for help in soil collection. Introduction Arbuscular mycorrhizae (AM) and ectomycorrhizae (EM) are the most common and widespread mycorrhizal associations, forming association with over 80% of all terrestrial plant. These mutualistic root relationships aid their plant hosts in nutrient acquisition, but differ in their abilities to access certain key nutrients. AM are considered most adept at phosphorus, whereas EM may best scavenge nitrogen (Read 1991). Because of their different functions, AM and ECM may occupy different niches. Thusly, during plant community succession the dominant mycorrhizal association often changes from AM to EM (Johnson et al 1991, Treseder et al 2004). Read’s (1991) hypothesis offers an explanation for the succession of mycorrhizal groups in temperate and boreal forests. Following disturbance, the initial association is AM as phosphorus is most limiting, but as an organic layer develops and N becomes more limiting, so do more EM associations. While this succession of mycorrhizal groups has been frequently observed, so too have contradictions. If an early successional species could host both mycorrhizal types would AM always be the preferred association during early succession? Herein we describe the use of a novel, floodplain chronosequence to test of Read’s hypothesis. Given this hypothesis and our system we predict: AM abundance will be greatest in early succession and EM colonization more abundant in late succession EM colonization will remain low until soils gain organic matter and surface litter As AM abundance decreases so will AM associated plants in the understory. Abstract During primary succession in temperate and boreal system the dominant soil mycorrhizal type may change from arbuscular mycorrhizal (AM) to ectomycorrhizal (EM). Read 1991 hypothesized that AM primarily aid in plant phosphorus acquisition and EM assist nitrogen acquisition, thusly P limited systems are dominated by AM and N limited systems by EM; however, exceptions have been observed. To test the strength of Read’s hypothesis we employed a 70 year cottonwood (Populus trichocarpa) dominated floodplain chronosequence. As cottonwoods (Salicaceae) may be both AM and EM, these systems are useful in studying the shift between mycorrhizal groups. We hypothesized that AM will be the preferred mycorrhizae early during succession and EM will be more prevalent during late succession. We measured soil and biotic variables to understand the shift between these mycorrhizal groups. AM biomass and inoculum potential was greatest during the first 10 years post disturbance, and then declined. In contrast to our predictions, EM colonization of cottonwood roots increased rapidly during early succession despite the absence of a litter layer or soil organic matter. While AM abundance predictably followed Read’s hypothesis, EM colonization did not. These observations suggest more important functions beyond organic nitrogen acquisition by EM in early succession. Finally, we must reconsider some of the assertions of the original Read (1991) hypothesis. Methods AMF biomass was measured by measuring soil AM hyphal lengths (Rillig et al 1999) AMF mycorrhizal inoculum potential was measured by a plant bioassay using Sorghum sudanese EMF % colonization was measured by randomly counting 50 cottonwood root tips (% EM colonization= (#EM tips/total tips)*100) Surface litter and standing understory biomass were collecting from three replicate plots (10 x 10 cm) within each age Soil pH, Olsen phosphorus, organic matter and organic carbon were measure by the South Dakota Soil testing laboratory Conclusions Our results support our prediction that the period of AM activity is a relatively short window during succession of montane floodplains. However, our second prediction was not supported. Ectomycorrhizal colonization of cottonwood trees was greater than expected in early successional soils despite low organic matter and alkaline pH, which is thought to favor AM association. Interestingly the AMF hyphal lengths decrease at each site despite the increasing grasses understory. The mycorrhizal status of the cottonwoods may affect the development of the understory’s AMF extraradical proliferation in addition to pH and P changes. Changes in the AM and EM community may not always follow Read’s hypothesis, and could be equally affected by host as well as nutrient avaliblility Discussion While Read’s hypothesis may explain the succession between AM and EM in many systems, some considerations must be made. EM may be the preferred mycorrhizal association in early succession when plants that can host EM are available (Salicaceae) EM fungi’s benefits may be greater than organic nitrogen acquisition, but their prevalence host limited Observed dominance of AMF in early succession may be a result of the prevalence of rapidly growing AM hosts and limited EM competition. Dynamics of Mycorrhizal Types during Primary Floodplain Succession Literature Cited Johnson et al Dynamics of vesicular-arbuscular mycorrhizae during old field succession. Oecologia 86: Read DJ Mycorrhizas in ecosystems. Experientia 47: Rillig et al Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands. Oecologia 119: Treseder et al Relationships among fires, fungi, and soil dynamics in Alaskan boreal forests. Ecol. App. 14: Jeff S. Piotrowski, Ylva Lekberg, and Matthias C. Rillig Division of Biological Sciences, University of Montana, Missoula, MT Figure 1. AMF biomass increased rapidly during the first 10 years of succession, and then slowly declined. The highest hyphal lengths are recorded at nine years post disturbance (Mean ± S.E.) Figure 2. Ectomycorrhizal colonization of cottonwood root tips increased quickly, reaching >50% in the first 10 years, after which maintained a slow increase to ~70% at the oldest site (Mean ± S.E.). Figure 3. Soil organic matter increased slowly during the earliest years then quickly accumulated to a peak at 30 years post disturbance, after which it declined slightly (Mean ± S.E.). Figure 4. Herbaceous understory biomass (primarily AMF hosting grasses) slowly increased along the entire chronosequence with a slight decline after 50 years (Mean ± S.E.). Figure 5. Soil pH was alkaline at all sites along the chronosequence, with a range of Initial soil pH was highest, decreasing with successional age (Mean ± S.E.).. Figure. Differences in soil phosphorus (ppm) from aged sites along the Nyack floodplain (mean ± S.E.). The Nyack floodplain near Glacier Natl. Park, MT. The Gaining and losing reaches of the floodplain are circled. The terrestrial chronosequence is on the losing reach.