N uptake and growth responses to sub-lethal freezing in the grass Poa pratensis L.

Climate warming has the potential to increase both the exposure and vulnerability of grass roots to frost in temperate regions by reducing snow cover and altering the timing of cold acclimation. Despite a strong research focus on the direct effects of freezing on grass mortality, the direct sub-lethal effects of freezing on grass performance have not been well-characterized. We examined sub-lethal responses of the grass Poa pratensis to variation in the timing, severity, rate and length of freezing. We assessed short term root functional responses (N-15 uptake) and longer term plant growth responses to freezing administered both under controlled conditions in a refrigerated incubator, and in the field by manipulating snow and litter cover. In fall and spring, N-15 uptake declined in response to 1 day of freezing down to -10 A degrees C or to 3 days of freezing at -5 A degrees C, whereas in winter, N-15 uptake was insensitive to freezing. Long term growth responses were similar, with reduced growth only occurring for grasses frozen for 3 days at -5 A degrees C in spring, but not for grasses frozen in fall or winter. Snow and litter removal intensified soil freezing over winter, but did not significantly affect plant growth. Our results demonstrate that while P. pratensis is relatively tolerant to frost damage over winter, it may be vulnerable to sub-lethal frost effects in fall, and particularly in spring. These sub-lethal effects occur at temperatures approximately 15-20 A degrees C warmer than the published LT50 values for this species.