Differential effects of freezing on the drought tolerance of herbaceous plants at the species, community and ecosystem scales
Abstract
Both freezing and drought cause cellular dehydration and elicit similar physiological responses in plants, where similar protective compounds are upregulated, including soluble sugars and proteins that help reduce cellular damage. Drought-freeze cross acclimation occurs when drought exposure enhances the freezing tolerance of plants. However, few studies had investigated the reciprocal effects of freezing on drought tolerance, and it was unknown if these interactions could impact plant productivity. Therefore, I examined the effects of freezing on the drought tolerance of individual species and assessed the implications of freeze-drought interactions in an old field community. Poa pratensis was exposed to fall or spring freezing in chambers and a 3-week drought in a greenhouse. Spring-frozen plants had lower reductions in biomass after a summer drought compared to drought-only plants, but this response did not appear to be correlated with the retention of soluble sugars after freezing. Subsequently, I examined freeze-drought responses for a range of herbaceous species: 6 graminoids (Arrhenatherum elatius, Agrostis stolonifera, Bromus inermis, Lolium perenne, Festuca rubra, Poa compressa) and 2 forbs (Securigera varia and Plantago lanceolata). Exposure to a freezing event enhanced the drought tolerance of Agrostis stolonifera, Bromus inermis, Lolium perenne, Poa compressa and Plantago lanceolata, where declines in biomass were lower for freeze-drought plants compared to drought-only plants. These interactions were not correlated to changes in either leaf soluble sugar or protein content before or during the drought. I then collected intact plant-soil mesocosms from an old field and subjected them to spring freezing in chambers and summer drought in the greenhouse. At the functional group level, cross acclimation was observed for legumes; however, for total biomass and nitrogen content in aboveground tissue, cross acclimation was not observed because interactions and additive effects of freezing and drought varied amongst functional groups. Overall, my results highlight the importance of considering plant stress history in experiments, because stressors in different seasons may interact with one another. Nevertheless, cross acclimation responses at the species level may not always scale up to the community or ecosystem scales.