Observations of Spring Hard Freeze Injury to C4 Perennial Grasses Native to the Great Plains, USA

The native prairies of the Great Plains USA are dominated by perennial C4 grasses like switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii). Spring hard freeze injury to C4 perennial grasses is rare but information is lacking in the literature. Our objective was to document effects of spring hard freeze damage to C4 perennial grasses native to the Great Plains USA. On 2 May at 24:00 pm, air temperature near Mead, Nebraska was below freezing and remained below freezing until 8:00 am on 3 May, with a minimum air temperature of −2.8 ̊C. Based on 50-year of weather data for this site, a minimum threshold temperature of 0 ̊C on or after 3 May occurred 16 times, but a minimum threshold temperature of −2.8 ̊C on or after 3 May occurred only twice. Grass tillers were visually evaluated to determine extent of freeze damage. The terminal 3to 5-cm of the leaf lamina was blackened 4-d after freezing and had complete browning, rolling, and desiccation 14-d after freezing. Tiller survival was not negatively affected by the freezing temperatures in May 2004. As the growing season progressed, all agronomic and livestock responses were within normal ranges for these C4 grasses. This is the first field report of multiple C4 prairie grass species responses to a spring hard freeze following significant spring growth. Plant recovery to this late-spring hard freeze demonstrates the ecological resilience of these C4 prairie grasses. These rare spring hard freezes had short-term impacts on C4 grasses but did not negatively impact agronomic performance for forage or bioenergy later in the growing season.


Introduction
The Great Plains of the central USA are characterized by a continental climate with cold dry winters, hot humid summers, and highly variable weather patterns.
The Great Plains were historically temperate grassland ecosystems dominated by the Prairie Parkland and Great Plains Steppe Provinces, each dominated by specifically adapted species [1]. The Prairie Parkland Province of the eastern Great Plains USA is dominated by perennial C 4 grasses, including big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), switchgrass (Panicum virgatum), and little bluestem (Schizachyrium scoparium) [1] [2]. The region is characterized by temperature extremes, especially in spring and autumn [1].
Extreme temperatures during spring and autumn periodically expose living tissues of perennial grasses to a range of high and low temperatures, often within a 24-h period. High temperatures exceeding 37˚C are seldom detrimental to living tissues of C 4 grasses. However, living tissues of C 4 grasses can be injured by frost and freezing temperatures and grasses can freeze at temperatures as low as −1.5˚C to −2.5˚C [3]. Ambient air temperatures below 0˚C are used frequently to define periods of frost, whereas a hard freeze is identified by ambient air temperatures of −2.2˚C for sufficient time to kill plant tissue [4] (https://www.weather.gov/iwx/fallfrostinfo). Typically, a hard freeze is common in autumn and indicates the conclusion of the growing season [4]. For example, our research experience has demonstrated that a 5-h period of −3.9˚C or below induces autumn dormancy in C 4 prairie grasses. However, hard freeze injury to C 4 perennial grasses in spring is seldom recognized as a concern. Spring frost damage to C 4 turfgrasses such as bermudagrass (Cynodon dactylon) and Zoysia grass (Zoysia spp.) is mentioned in the extension literature [5], but the literature includes minimal field research information on the C 4 perennial prairie grasses native to the Prairie Parkland Province of the Great Plains USA. In a greenhouse study, prairie cordgrass (Spartina pectinata), a C 4 perennial prairie grass, was exposed to temperatures as low as −7˚C for up to 3-h and exhibited tolerance to freezing stress [6]. Our objective was to document the damage to field-grown C 4 perennial grasses native to the Great Plains USA associated with a naturally-occurring spring hard freeze. Weather data were accessed from the High Plains Regional Climate Center website from Weather Station Mead 6S [7]

Observations and Discussion
Beginning Measuring plant response to infrequent, extreme environmental events cannot be quantified since there is no way to design a replicated field experiment to test a hypothesis. However, relative indicators can provide insight into plant response to these environmental anomalies. We used the morphology and tiller density data from 2003 and 2004 to evaluate this anomaly (Table 1). Based on the developmental morphology data, no clear differences in plant phenology occurred between 2003 and 2004. Between 2003 and 2004, the MSC for little bluestem and sand bluestem declined slightly in July but increased slightly in August. For big bluestem, the MSC did not vary across years. However, tiller density increased in July and August for all species from 2003 to 2004, except for little bluestem in August (Table 1). This increase in tiller density in 2004 clearly indicates that tiller survival was not negatively affected by the freezing temperatures in May 2004.
During the 2004 growing season, field notes were kept on the general plant appearance and growth and development of the C 4 grasses.
June 16: "Cattle began grazing indiangrass pastures." July 13: "The indiangrass pastures look great." July 22: "The farm cut 16 round bales from the remaining 1-ha of big bluestem." July 28: "Pawnee big bluestem is just entering anthesis, with 47% of the tillers in reproductive stages. Little bluestem has 60% of the tillers in the boot stage." August 2: "Moved cattle off the indiangrass pastures." August 20: "The indiangrass pastures are re-growing and advancing nicely. Pawnee big bluestem is past peak anthesis." These field notes indicate nothing out of the ordinary for the C 4 grasses. A long-term bioenergy evaluation of "Cave-in-Rock" switchgrass biomass across multiple harvest and fertilizer treatments had average annual yields of 8.0, 5.8, 7.2, and 7.6 Mg ha −1 in 2001, 2002, 2003, and 2004, respectively. All the important agronomic and livestock responses, including grazing readiness, plant morphology, tiller demographics, livestock performance, and biomass yield were all within normal ranges for environmental variation for these C 4 grasses.

Conclusions
This is the first field documentation of the response of multiple species of C 4 prairie grasses to a spring hard freeze following considerable spring growth. events. We conclude that these rare spring hard freezes have short-term impacts on C 4 grasses, but do not negatively affect agronomic performance for forage or bioenergy later in the growing season or long-term survival. Consequently, breeding for early spring emergence and growth in perennial C 4 grasses native to the Great Plains USA should not increase the risk of stand failure due to exposure to early spring freezing stress.
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