Our results demonstrate that multiple stresses that occur over different seasons can interact; this interaction is highly relevant to herbaceous species in northern temperate regions that are experiencing more intense and frequent stress as a result of changes in snow cover and extreme climatic events.

Despite evidence that prior exposure to drought can increase subsequent plant freezing tolerance, few studies have explored such interactions over ecologically relevant time spans. We examined the combined effects of drought and subsequent freezing on tiller growth and leaf sugar concentrations in the grass, Poa pratensis. We exposed tillers to no drought (-0.04 MPa), moderate drought (-0.19 MPa) or severe drought (-0.42 MPa) for 3 weeks in summer. Tillers were then frozen in autumn or spring at -5 °C (frost damage) or at 0 °C (control) for 3 days and harvested after a re-growth period. For shoot growth, there was a significant interaction between drought and autumn freezing, whereby the relative effect of freezing on growth was least for the plants previously exposed to severe drought; however, there was no significant interaction between drought and spring freezing. For root growth, there were no significant interactions between drought and freezing in either season. Leaf sugar concentrations increased significantly with drought intensity, but these effects dissipated within a month, prior to the onset of the autumn freezing treatment. Overall, our results suggest that interactions between prior drought and subsequent freezing in P. pratensis may be most relevant in the context of autumn freezing, and despite the important role of soluble sugars in increasing both drought and freezing tolerance in this species, the retention of these compounds after drought stress does not appear to explain the occurrence of drought-frost interactions at ecologically relevant time scales.

Premise of the Study Freezing and drought both result in cellular dehydration, and similar physiological responses to these stressors may result in cross acclimation, whereby prior freezing exposure increases subsequent drought tolerance. We examined how spring freezing influences summer drought tolerance for a range of herbaceous old field species: 6 graminoids (Agrostis stolonifera, Arrhenatherum elatius, Bromus inermis, Festuca rubra, Lolium perenne, Poa compressa) and 2 forbs (Plantago lanceolata, Securigera varia), with the goal of examining the generality of cross acclimation responses. Methods We exposed the plants to –5°C in the spring and to a 3‐week summer drought, and harvested the plants after a 3‐week watering/recovery period. We also measured leaf soluble proteins and sugars to explore the potential mechanisms before and during drought stress. Key Results For Agrostis stolonifera, Bromus inermis, Lolium perenne, Plantago lanceolata, and Poa compressa there was evidence of cross acclimation based on aboveground or belowground biomass, with a reduction in the severity of the drought effect for the plants previously exposed to freezing. Freezing and drought effects were additive for Arrhenatherum elatius, and for the remaining two species the test of the freezing‐drought interaction was inconclusive, because significant drought and freezing effects did not co‐occur. When present, freezing‐drought interactions were not correlated with changes in leaf soluble protein or sugars. Conclusions Our results reveal that the phenomenon of freezing‐drought cross acclimation appears to be common in herbaceous species, and variation among species in cross acclimation indicates that multiple stresses could alter relative species abundances in plant communities.

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Low-temperature thermal acclimation may require adjustments to N and water use to sustain photosynthesis because of slow enzyme functioning and high water viscosity. However, understanding of photosynthetic acclimation to temperatures below 11 °C is limited.• We acclimated Populus balsamifera to 6 °C and 10 °C (6A and 10A, respectively) and provided the trees with either high or low N fertilizer. We measured net CO 2 assimilation (A net ), stomatal conductance (g s ), maximum rates of Rubisco carboxylation (V cmax ), electron transport (J max ) and dark respiration (R d ) at leaf temperatures of 2, 6, 10, 14 and 18 °C, along with leaf N concentrations.• The 10A trees had higher A net than the 6A trees at warmer leaf temperatures, which was correlated with higher g s in the 10A trees. The instantaneous temperature responses of V cmax , J max and R d were similar for trees from both acclimation temperatures. While soil N availability increased leaf N concentrations, this had no effect on acclimation of photosynthesis or respiration.• Our results indicate that acclimation below 11 °C occurred primarily through changes in stomatal conductance, not photosynthetic biochemistry, and was unaffected by short-term N supply. Thermal acclimation of stomatal conductance should therefore be a priority for future carbon cycle model development.