Spore-Expulsion Rates and Extents of Blade Occupation by Ascomycetes of the Smooth-Cordgrass Standing-Decay System

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Abstract
Species of ascomycetes are the principal microbial secondary producers in standing-decaying shoots of smooth cordgrass (Spartina alterniflora), the foremost and highly productive grass of saltmarshes of the eastern USA. Rates of expulsion of ascospores from wet, standing-decaying leaves of S. alterniflora were measured in spore-capture chambers seasonally, over a 3-year period. Extents of occupation of decaying leaves were determined by measuring sizes of characteristic patches associated with different species or species groups. Patches of leaf blades not occupied by at least one of the ascomycetes of the smooth-cordgrass ascomycete community occupied less than 0.2% of leaf abaxial area. Patches of the ‘black-peppered’ appearance (containing ascomata of one or more of the following: Phaeosphaeria spartinicola, Mycosphaerella sp. 2, Phaeosphaeria halima) occupied 91 to 98% of abaxial leaf area for mid-shoot blades at subsites containing tall and intermediate-height shoots. At short-shoot sites, ‘peppered’ patches occupied 53% of leaf area, due to higher levels of periwinkle grazing at short-shoot sites; size of ‘peppered’ patches was negatively correlated with extent of severe invertebrate grazing of leaves (r = −0.87). Blackened patches (containing ascomata of Buergenerula spartinae) had an overall average occupation percentage of 17% of leaf area, and the percentage was not significantly affected by periwinkle grazing (r = −0.13). Attached decaying leaf blades low on shoots and leaf sheaths expelled few spores (< 30% of the average for mid-shoot and upper-shoot blades), and for sheaths, B. spartinae was apparently the principal ascomycete releasing spores. For mid-shoot and uppershoot decaying blades, mean total ascospore-expulsion rate ranged from 14 to 131 spores mm−2 leaf abaxial surface (72 h)−1, depending on marsh watershed and season. Higher percentages of expelled ascospores for Phaeosphaeria spartinicola were found for autumn and winter (52 to 62% of spores), but for Mycosphaerella sp. 2, peak seasons were summer and spring (42 to 54% of spores). Percentages of ascospores for these two species were uniform over years, marsh watersheds, and tall-to short-shoot marsh subsites, whereas Phaeosphaeria halima exhibited a high degree of spatiotemporal patchiness. Because the ascospore-capture method used does not measure total ascospore release, only minimum values could be calculated for ascospore output m−2 marsh. Calculated minimum rates of total ascospore expulsion for mid-shoot plus upper-shoot blades were 1.3 million spores m−2 marsh h−1 of blade wetness, 5.9 billion spores m−2 marsh yr−1, and 7.5 g organic mass m−2 yr−1, amounting to 4.5% of total annual fungal production within these blades.