Introduction to the Special Issue on Sierran Mixed-Conifer Research

LIKE MUCH OF THE WESTERN UNITED STATES, California’s forest has been severely altered by a century of fire suppression. The Sierra Nevada’s largest forest type, mixed conifer, which is primary habitat for more vertebrate species than any other Californian forest community, historically burned every 12–17 years. In 1894, John Muir wrote “The inviting openness of the Sierra woods is one of their most distinguishing characteristics. The trees of all the species stand more or less apart in groves, or in small, irregular groups, enabling one to find a way nearly everywhere, along sunny colonnades and through openings that have a smooth, parklike surface . . .”; (Muir 1894). Now most mixed-conifer forest is characterized by dense thickets of small shade-tolerant fir and cedar, which could quickly transfer a ground fire into the overstory crowns. Estimates based on the area burned each year project the current fire return interval at 600 years. Regional and national plans such as the Sierra Nevada Forest Framework and the Healthy Forest Initiative have made restoration a priority, yet these plans are highly controversial, in part because there is so little ecological information for guiding management prescriptions. Any effort to restore these forests to “health” or an active fire regime will only be improved with an understanding of how the ecosystem functions. This collection of articles investigates the connections between forest structure and composition, and the ecological processes that define Sierran mixed conifer. A common theme in this collection of articles is the importance of patch structure and water availability in influencing mixed-conifer’s ecological processes. All of these studies were conducted in an old-growth forest at the Teakettle Experimental Forest, which even after 140 years without a fire has a distinct patch pattern. Fire suppression has increased the stem density in tree clusters, but the forest is still characterized by three distinct patch types: tree groups, gaps, and shrubs. Several articles in this collection stratified their sampling by these patch types and found significant differences among patches in tree regeneration (Gray et al.), soil nitrogen pools and dynamics (Erickson et al.), invertebrates (Marra and Edmonds), and respiration (Ma et al.). These patches, all dynamically linked through forest seral development, appear to provide important functional and habitat heterogeneity in mixed conifer. Another important influence on forest processes was the duration and availability of soil moisture. Mixed conifer receives 90% of its annual precipitation as snow, with soils at field capacity in May, drying down to an average of 4% soil moisture by early July, and rarely receiving any precipitation until Oct. Because of this strong seasonal drought, regional climatic events such as El Niño can have a significant pulse effect on flora and fauna, influencing tree establishment (North et al.), nutrient cycling (Erickson et al.), canopy invertebrates (Schowalter and Zhang), and pest mortality (Smith et al.). Mediating the climate’s top-down effects on ecosystem function are within-stand differences in site conditions that have a bottom-up influence on ecosystem structure, composition, and function. These conditions include litter depth, soil depth, canopy-moderated surface temperatures, and stem density, all of which can significantly increase the duration and abundance of soil moisture. The important role of vegetation patch type, climate, and soil moisture may help future research understand the dynamic ecological processes of mixed-conifer forest and managers more effectively tailor restoration practices to ecosystem functions. Marra and Edmonds found a highly diverse soil invertebrate community that differed between vegetation patch types. The community, dominated by mites, was significantly influenced by annual differences in soil moisture and spatial differences in bulk density due to litter from vegetation patches. They suggest invertebrate community response to restoration treatments will likely hinge on recovery of understory vegetation, which moderates soil temperatures and provides organic inputs. Izzo et al. examined the diversity of mixed-conifer ectomycorrhizae and which species were regularly consumed by small mammals that are known fungivores. Ectomycorrhizal fungi are essential for conifer nutrient and water uptake, and their above(“mushrooms”) and below(“truffles”) ground fruiting bodies are an important food source for many forest mammals such as the flying squirrel, the main prey of the California spotted owl. Izzo and colleagues found that the ectomycorrhizal community has a high percentage of hypogeous (“truffle”) fungi, and they suggest this prevalence may be due to the Sierra’s extended seasonal drought and deep litter layers due to fire suppression. Fungi producing truffle fruiting bodies can only inoculate new areas when they’re consumed and their spores are excreted. With a greater dominance of truffle-producing fungi, the connection between trees, truffles, and small mammals may be particularly tight in these drier forests where belowground fungal fruiting produces a longer lasting food source. Canopy invertebrates, which can influence tree growth, vigor, and litter inputs into the soil, were examined by Schowalter and Zhang. They found invertebrates differed by tree species host and that there were significant differences between the communities in overstory conifers and understory angiosperms. They also suggest that significant annual variations in invertebrate abundance and diversity