The role of perennation traits in plant community soil frost stress responses

The roles of root-nodulating bacterial associations and cyanogenesis in the freezing sensitivities of herbaceous legumes

PREMISE

Reduced snow cover and increasing temperature variability can increase freezing stress for herbaceous plants in northern temperate regions. Legumes have emerged as a plant functional group that is highly sensitive to these changes relative to other herbaceous species in these regions. We explored root-nodulating bacterial associations and cyanogenesis as potential mechanisms explaining this relatively low freezing tolerance of legumes.

METHODS

To examine the influence of bacterial associations, we grew four legume species with or without crushed-nodule inoculum at three severities of freezing, and three concentrations of nitrogen to disambiguate the direct benefits of increased nitrogen from the total bacterial effect. We quantified cyanogenesis via hydrogen cyanide production in both true leaves and cotyledons for nine legume species.

RESULTS

Root nodulation generally only affected legume survival under low nitrogen, when freezing severity was moderate or low. However, for the frost-surviving plants, the growth advantage provided by nodulation decreased (it was often no longer significant with increasing freezing severity), and greater freezing severity reduced total nodule mass. In contrast, cyanogenesis was only detected in two of the nine species.

CONCLUSIONS

The diminished performance of nodulated plants in response to freezing could place legumes at a competitive disadvantage and potentially explain their high sensitivity to freezing relative to other herbaceous species in northern temperate regions. Overall, this result has important implications for changes in soil fertility, community composition, and plant productivity in these ecosystems in the context of a changing winter climate.

High freezing sensitivity of legumes relative to other herbaceous species in northern temperate plant communities

BACKGROUND AND AIMS

Reduced snow cover and increased air temperature variability are predicted to expose overwintering herbaceous plants to more severe freezing in some northern temperate regions. Legumes are a key functional group that may exhibit lower freezing tolerance than other species in these regions, but this trend has been observed only for non-native legumes. Our aim was to confirm if this trend is restricted to non-native legumes or whether native legumes in these regions also exhibit low freezing tolerance.

METHODS

First, we transplanted legumes (five non-native species and four native species) into either an old field (non-native) or a prairie (native) and used snow removal to expose the plots to increased soil freezing. Second, we grew plants in mesocosms (old field) and pots (prairie species) and exposed them in controlled environment chambers to a range of freezing treatments (control, 0, -5 or -10 °C) in winter or spring. We assessed freezing responses by comparing differences in biomass, cover and nodulation between freezing (or snow removal) treatments and controls.

KEY RESULTS

Among legume species, lower freezing tolerance was positively correlated with a lower proportion of nodulated plants and active nodules, and under controlled conditions, freezing-induced reductions in above-ground biomass were lower on average in native legumes than in non-native legumes. Nevertheless, both non-native and native legumes (except Desmodium canadense) exhibited greater reductions in biomass in response to increased freezing than their non-leguminous neighbours, both in controlled environments and in the field.

CONCLUSIONS

These results demonstrate that both native and non-native legumes exhibit low freezing tolerance relative to other herbaceous species in northern temperate plant communities. By reducing legume biomass and nodulation, increased soil freezing could reduce nitrogen inputs into these systems.

Is a seasonally reduced growth potential a convergent strategy to survive drought and frost in plants?

BACKGROUND

Plants have adapted to survive seasonal life-threatening frost and drought. However, the timing and frequency of such events are impacted by climate change, jeopardizing plant survival. Understanding better the strategies of survival to dehydration stress is therefore timely and can be enhanced by the cross-fertilization of research between disciplines (ecology, physiology), models (woody, herbaceous species) and types of stress (drought, frost).

SCOPE

We build upon the 'growth-stress survival' trade-off, which underpins the identification of global plant strategies across environments along a 'fast-slow' economics spectrum. Although phenological adaptations such as dormancy are crucial to survive stress, plant global strategies along the fast-slow economic spectrum rarely integrate growth variations across seasons. We argue that the growth-stress survival trade-off can be a useful framework to identify convergent plant ecophysiological strategies to survive both frost and drought. We review evidence that reduced physiological activity, embolism resistance and dehydration tolerance of meristematic tissues are interdependent strategies that determine thresholds of mortality among plants under severe frost and drought. We show that complete dormancy, i.e. programmed growth cessation, before stress occurrence, minimizes water flows and maximizes dehydration tolerance during seasonal life-threatening stresses. We propose that incomplete dormancy, i.e. the programmed reduction of growth potential during the harshest seasons, could be an overlooked but major adaptation across plants. Quantifying stress survival in a range of non-dormant versus winter- or summer-dormant plants, should reveal to what extent incomplete to complete dormancy could represent a proxy for dehydration tolerance and stress survival.

CONCLUSIONS

Our review of the strategies involved in dehydration stress survival suggests that winter and summer dormancy are insufficiently acknowledged as plant ecological strategies. Incorporating a seasonal fast-slow economics spectrum into global plant strategies improves our understanding of plant resilience to seasonal stress and refines our prevision of plant adaptation to extreme climatic events.

Effects of snow on reproduction of perennial Thalictrum dioicum: Plants survive but seedlings fail to recruit with reduced snow cover

PREMISE

Future reductions in snow cover are expected in temperate climates, likely leading to more soil-freezing events and damage to plant tissues. However, whether and how plants can compensate for this damage may depend on the timing of damage and on plant allocations to seed size and number. We need more information about how seed production, germination, and seedling recruitment might respond to changes in snow cover.

METHODS

We manipulated snow cover over three seasons in a common garden experiment with four treatments: (1) "control," where snowpack was left unmanipulated throughout the winter season; (2) "late addition," where snowpack was experimentally increased at the end of the winter season in order to delay the onset of spring; (3) "late removal," where snowpack was experimentally reduced at the end of the winter season in order to advance the onset of spring; and (4) "freeze," a consistent removal treatment, where snowpack was experimentally reduced following every substantial snowfall in order to induce freeze-thaw events in the soil. In all treatments, we measured survival, growth, reproduction, and recruitment of a native perennial herb, Thalictrum dioicum.

RESULTS

Reduced snow cover minimally influenced adult survival. Instead, individuals that experienced reduced snow cover throughout the winter produced more massive seeds, whereas individuals that experienced a single snow removal at the end of the season produced less massive seeds. Seedling recruitment was lower in the removal treatments than in the control, as a result of failure to germinate in the freeze treatment and seedling mortality in the late removal treatment.

CONCLUSIONS

Both reduced snow cover throughout the winter and a single late snow removal in the spring reduced seedling recruitment, but for different reasons, suggesting that a holistic approach to the life cycle is needed to understand responses to shifting climates.

A perennial invader’s seed and rhizome differ in cold tolerance and apparent local adaptation

Extreme cold plays a key role in the range boundaries of plants. Winter survival is central to their persistence, but not all structures are equally susceptible to frost kill and, therefore, limiting to distributions. Furthermore, we expect intraspecific variation in cold tolerance both within and among tissue types. In a laboratory setting, we determined freezing tolerances of two overwintering propagule types – seeds and rhizomes – of the globally invasive Johnsongrass (Sorghum halepense), testing apparent emergence and electrolyte leakage as a proxy for cell death. We used 18 genotypes from agricultural and non-agricultural habitats spanning the climatic extremes occupied by Johnsongrass in the US. Single node rhizome fragments had an average LT90 of -5.1 °C with no significant variation based on home climate or ecotype. Seeds frozen at -85 °C suffered a decline in germinability to 10% from 25% at 22 °C. Population origin did not affect seed response to any temperature. However, non-agricultural seeds germinated more and faster than agricultural seeds from the coldest climates, with a reversed relationship among warmest origin seeds. Regardless of ecotype, seeds from the cold/dry and wet/warm sectors of Johnsongrass’s range germinated more and faster. Drastic differences in cold tolerance between seeds and rhizome and evidence for seeds’ local adaptation to land use and climate suggest that its spread is likely limited by winter rhizome survival, as well as adaptability of germination behavior to longer winters. These findings shed light on Johnsongrass’ dispersal dynamics and help identify future avenues for mechanistically understanding its range limitation.

Alpine plant growth and reproduction dynamics in a warmer world

Climate warming may stimulate growth and reproduction in cold-adapted plants, but also reduce their performance due to warming-induced drought limitation. We tested this theory using a unique experiment with the alpine forb Rumex alpinus. We examined how climate warming over the past four decades affected its annual rhizome growth, leaf production and flowering, and whether responses varied between alpine, subalpine and montane populations. Before the period of accelerated warming in the 1970s and 1980s, the primary limitation on growth had been cold temperatures and short growing seasons. Increased summer temperatures in the 1990s and 2000s enhanced rhizome growth and leaf production, but not flowering. Alpine and subalpine plants profit more than montane plants, currently producing three times longer annual rhizome increments and twice as many leaves as 40 yr ago, and achieving nearly the same values as montane plants. During the warmest 2005-2015 period, growth became contingent on summer precipitation and began to decrease across all populations, likely due to an increasing water shortage in dense monospecific stands. Warming releases plants from cold limitations but induces water shortage. Rumex alpinus exceeds its thermal optimum and becomes water-limited as the climate warms. Our results suggest that warming-induced responses in alpine plants will not be one-sided shifts to higher growth and reproduction, but rather multidimensional and spatiotemporally variable.