Balancing biofuel production and biodiversity: Harvesting frequency effects on production and community composition in planted tallgrass prairie

Abstract

Native perennial grasslands have been proposed as a source of feedstocks for the production of second-generation lignocellulosic biofuels in the Midwestern USA. Although the consequences of some management decisions for biomass production and plant community composition are well understood (e.g. fertilization), less is known about the effects of harvesting frequency. We compared a once-and twice-annual harvesting regime at two restored prairies in southwestern Michigan established with identical seed mixtures as part of a large-scale bioenergy experiment. We determined biomass production and species composition in experimental plots and also measured the availability of light, inorganic nitrogen and soil moisture. The plant communities that established at the two sites differed markedly in composition and there was little evidence of convergence after five years. At the site dominated by warm-season C 4 grasses, single harvests generally produced more biomass than double harvests. By contrast, biomass production was unaffected by harvesting at the more diverse site. Contrary to our prediction that a summer harvest would increase diversity, we found small and subtle effects on plant community composition. This may be due in part to the timing of our harvest treatment. Our results suggest that a single, end-of-season harvest is the best practice for maximizing biomass production in prairies, especially at sites where warm-season grasses dominate. However, at more diverse sites, two harvests can produce the same total biomass and may support other beneficial ecosystem services. This study indicates that in the short term, double harvests are unlikely to affect plant species diversity or community composition in prairie plantings.

Evolution in novel environments: Do restored prairie populations experience strong selection?

“…These results differ from a study of restored populations of Elymus elymoides, where directional selection consistently favored a similar suite of traits in populations located 60 km apart (Kulpa and Leger 2013). The differences we find between Chamaecrista populations could be a direct result of environmental differences between the two sites: despite their close proximity, plant community composition and some abiotic conditions (such as soil nitrogen and water holding capacity) differ (Stahlheber et al 2016), which could generate different selection pressures at each site. Perhaps the original source population was preadapted to site conditions at Marshall but not at Lux.…”

“…In 2010, two former agricultural fields (one 13 ha and one 11 ha) near Kellogg Biological Station in southwest Michigan, Lux Arbor (42°28 0 23″ N, 85°26 0 50″ W) and Marshall (42°26 0 37″ N, 85°18 0 34″ W), were prepared for restoration using identical protocols and were planted with identical prairie seed mixes (containing 19 grass and forb species, including Chamaecrista, seeded at 0.28 kg/ha, roughly 26,700 seeds/ha) as part of a large bioenergy experiment being conducted by the Great Lakes Bioenergy Research Center. Despite being located in close proximity, being prepared in the same manner, and being seeded with the same seed mix, plant community composition and some abiotic conditions (such as soil nitrogen, phosphorus, and water holding capacity) differ between sites (Stahlheber et al 2016). Biomass from each prairie is harvested every year using identical protocols (see Stahlheber et al [2016] for full site details).…”

Forage or Biofuel: Assessing Native Warm-Season Grass Production among Seed Mixes and Harvest Frequencies within a Wildlife Conservation Framework

“…Therefore, the primary advantages of NWSG polycultures over Switchgrass monocultures for biofuel production may be limited to wildlife conservation. Previous studies have observed plant communities among harvest frequencies in conservation grasslands similar to our NWSG plantings (Jungers et al 2015, Stahlheber et al 2016. However, increased cutting frequency can increase light availability, promoting photophilic plant species otherwise deterred by tall vegetation and dense grass canopies (Hautier et al 2009, Wilson andTilman 1991).…”

“…data). Therefore, sitepreparation techniques should be explored for marginal land opportunities that can produce grasslands with significant proportions of C 4 grasses for effective ethanol yield (Adler et al 2009, Stahlheber et al 2016.…”

Evolution in novel environments: do restored prairie populations experience strong selection?

“…While we find 371 evidence of selection and/or evolutionary responses on all traits measured in the Lux population, 372 there is less evidence of selection on the Marshall population. These differences could be a direct 373 result of environmental differences between the two sites: despite their close proximity, plant 374 community composition and some abiotic conditions (such as soil nitrogen and water holding 375 capacity) differ (Stahlheber et al 2016), which could generate different selection pressures at 376 each site. Alternatively, we may find evidence of past selection in one population but not the 377 other not because of differences in selection pressures, but because of differences in the 378 populations' capacities to respond to selection or the degree to which the populations were 379 preadapted to local site conditions.…”