Introduction

Biotic homogenization in plant communities is the process in which the composition of an ecological community is altered to favor one or a few organisms (Tilman et al. 1997; McKinney and Lockwood 1999). Homogenization within a habitat may occur genetically, taxonomically, and functionally and results in lower genetic, species, and functional diversity within the community (Olden et al. 2004; Olden 2006). Drivers of biological homogenization include both natural and anthropogenic factors including climate change, disturbance events, habitat fragmentation, and invasive species introduction (Baruah et al. 2017; Bilyaminu et al. 2020). These processes can overlap and compound over time (Olden and Rooney, 2004). For example, Abadie et al. (2011) found that generalist species replaced specialist species in urban plant communities with increasing habitat fragmentation. Lougheed et al. (2008) found that anthropogenic nutrient enrichment led to lowered plant taxonomic richness in degraded wetlands. Additionally, a recent meta-analysis of 200,000 plant species showed global widespread decreases in species diversity throughout the Anthropocene (Daru et al. 2021). Disturbances not only contribute to homogenization, but the consequences of species convergence may alter the natural functions of an ecosystem thus reducing the system’s resilience to future disturbance events (Tilman et al. 1997; Hooper et al. 2005; Olden 2006).

Habitat homogenization is a prevalent phenomenon in bottomland forests and freshwater forested wetlands throughout the southeastern United States, partially due to longstanding history of disturbance to these systems (Brinson, 1990). Examples of reduced species diversity can be found throughout the range of forested wetlands dominated by bald cypress (Taxodium distinchum) and water tupelo (Nyssa aquatica). Near the northernmost historic range of bald cypress, within Virginia’s Great Dismal Swamp, specialist obligate wetland tree species (e.g., Taxodium distichum and Nyssa aquatica, and Quercus spp.) have been replaced by generalist facultative wetland communities dominated by red maple (Acer rubrum) and sweet gum (Liquidambar styraciflua), termed maple-gum (Levy 1991). The Dismal Swamp has a long history of disturbance and today ~ 60% of the Dismal Swamp is characterized as maple-gum habitat (Ludwig et al., 2021; Speiran and Wurster 2021). Toward the southernmost range of bald cypress (i.e., Texas, Louisiana, Alabama), mixed bottomland forests subjected to frequent anthropogenic and climatic disturbances are now characterized by reduced species diversity and increased dominance of invasive generalist species such as sweet gum (Hart and Holmes 2013), red maple (Abrams 1998), and invasive Chinese tallow (Triadica sebifera) (Henkel 2012).

Homogenization, such as that observed in the freshwater forested swamps and bottomland forests through the southeast U.S., is often the result of changing environmental conditions that favor the traits of competitive generalist species compared to specialist species (Naaf and Wulf 2010). Generalist and invasive plant species often possess functional traits (e.g., morphological, phenological, or physiological characteristics) favored by disturbance (Holl et al. 2022) and a wider range of abiotic tolerances compared to specialist species (Qian and Guo 2010). Plant functional traits provide information about species’ life strategies (Condit et al., 2006) and responsiveness to changing environmental cues; thus traits have been used to understand a variety of dynamics related to habitat homogenization, including differences in ecological niches (Silvertown, 2004), environmental partitioning (Drake and Franks, 2003), and relationships between traits and ecosystem function (Carlucci et al., 2020). The most commonly measured plant functional traits across plant taxa include a mix of early- and mature-life stage traits (e.g., seed mass, plant height, specific leaf area, and wood density) (Moles 2017). Early life-stage traits (e.g., seed rain, seed dormancy, germination percentage and rate, and seedling growth rate) are important first indicators of the breadth of environmental conditions under which a species may establish and spread. Additionally, the processes that drive community assembly of the soil seed bank following a disturbance are poorly understood compared to drivers of aboveground plant community assembly (Helsen et al. 2015).

Seed dormancy, or the innate delay of germination when environmental stimuli would not support plant survival (Carta et al. 2014), is one early-life stage trait that differs greatly between species and influences germination dynamics and other early-life traits. In temperate climates with cyclical temperature and precipitation regimes, seed dormancy and germination dynamics of many species are synchronized with environmental cues (Baskin and Baskin 2014), with some species requiring a period of cold exposure, or cold stratification, to overcome dormancy (Angevine and Chabot 1979). As a result, changes to the environmental stimuli that influence germination (light, temperature, moisture, etc.) due to disturbances or climate change will likely play an important role in differentially limiting species depending on their species-specific germination traits and dormancy requirements (Walck et al. 2010). As global temperatures increase, plant community structure is likely to shift if species specific germination requirements are not met, potentially resulting in increased homogenization by generalist species (Qian and Guo 2010). Daily minimum winter temperatures throughout the geographic range of swamp specialist species T. distichum and N. aquatica show a warming trend over the past century, though the rate of warming varies slightly by location (Fig. 1, NOAA, 2023a).

Fig. 1
figure 1

Native range limits of A. rubrum (red), L. styraciflua (violet), N. aquatica (yellow), and T. distichum (blue) in the United States and century trends in winter minimum temperatures (November–March) at two latitudinal range limits at which all species co-occur (Norfolk, VA, and Shreveport, LA). Temperature data were sourced from NOAA (2023), and species range limits were provided by USDA (2023). Right panel: mean length and mass of seeds from each species (n = 25 per species) derived additional seeds not used in germination trials

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Here, we explore the extent to which early life traits, specifically seed germination and germination rate, may explain observed current patterns of species homogenization in freshwater forested swamp and bottomland hardwood forests in the United States. Although the relationship between germination niche and ecological niche has been explored in other systems, to our knowledge, few studies exist that explore this in forested wetlands which are uniquely subject to strong ecological drivers (e.g., hydrology) that limit species establishment at the seedling life stage. This study also investigates the influence of changing climate on the establishment dynamics, viability, and permanence of key species in these forested wetland habitats using pre-germination cold stratification treatments of variable durations (0 days to 90 days). Species addressed in this study include two forested wetland specialist species (Taxodium distichum and Nyssa aquatica) and two generalist species (Acer rubrum and Liquidambar styraciflua) characteristic of the increasingly prevalent maple-gum habitat observed throughout freshwater forested wetlands and bottomland forests of the U.S. The objective of this work is two-fold: 1) to inform our understanding of the role of early life traits on the permanence of specialist and generalist species in freshwater forested wetlands, and 2) understand whether warming winters characteristic of climate change may further exacerbate plant community homogenization by differentially limiting germination of specialist and generalist species. The ultimate goal of this work is to increase our understanding of species-specific physiological demands at early life-stages and inform habitat management actions that may aid in preserving biodiversity and reducing species conversion in forested wetlands.

Methods

Experimental design

Freshwater forested wetlands within the United States include specialist species Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo), which are targets for restoration and conservation, as well as generalist species including Acer rubrum (red maple) and Liquidambar styraciflua (sweet gum). The seeds of four tree species used in this experiment (L. styraciflua, A. rubrum, N. aquatica, T. distichum) were obtained from Sheffield’s Seed Company, Inc. in Fall 2021. Seeds were kept in dry cool (22° C) conditions prior to pre-germination treatments. Previous research on Taxodium distichum seed germination showed that the species exhibits better germination when seeds were soaked in 1% NaOH for five minutes and then in water for the next 24 h; this process neutralizes the seed’s resin coating ex-situ that would be neutralized under natural swamp conditions (Liu et al. 2009). We conducted a fully factorial experiment, consisting of five species groups (L. styraciflua, A. rubrum, N. aquatica, non-treated T. distichum, and NaOH-treated T. distichum), seven pre-germination cold stratification durations (0, 15, 30, 45, 60, 70 and 90 days), and five replicates of each combination. Each replicate consisted of ten seeds, for a total of 350 seeds.

Pre-germination cold stratification treatments, with the exception of the 0-day duration treatment, consisted of placing seeds in air-tight plastic tub containing moist builders’ sand and exposing them to cool (4° C), dark conditions for their respective cold duration treatments in a refrigerator. Following each replicate’s cold stratification treatment, seeds were soaked in deionized water in dark conditions at room temperature for 24 h. The NaOH-treated T. distichum seeds were placed in a 1% NaOH solution for five minutes directly after refrigeration, followed by soaking in deionized water for 24 h (after Lui et. al., 2009). After soaking, all seeds were placed on Whatman 3 filter paper in a square clear plastic Petri dish (10 seeds per replicate Petri dish) with 5 mL of deionized water. Petri dishes were kept closed under a clear lid and the filter paper was kept moist with deionized water throughout the duration of observation. All Petri dishes were placed in an environmental chamber (CONVIRON Gen1000, Controlled Environments Ltd., Winnipeg, Manitoba, CAN) with alternating photoperiod and thermal conditions that mimic springtime conditions within these species' range (e.g., 12 h light at 25℃, 12 h darkness at 20℃). Seeds were observed daily for germination (indicated by the emergence of radicle) for 30 days. Germinating seeds were recorded each day and removed from the Petri dish. Fungus was observed during both cold stratification and germination. Fungal growth was mitigated by washing affected seeds with deionized water. Whatman filter paper was replaced as necessary. Sterilizing chemicals were not used to avoid possible influences on seed germination.

Statistical analysis

Each replicate Petri dish for all treatment combinations was measured for total percent germination at the end of 30-days as well as germination rate. The effect of species, cold stratification duration, and their interaction on percent germination was tested using a two-way analysis of variance (ANOVA). A one-way ANOVA with Tukey post-hoc test of pairwise comparisons was performed for each individual species to determine the effect of cold stratification within species. A two-way ANOVA was used to test the effect of NaOH treatment, and cold stratification duration, on T. distichum only.

Germination rate was analyzed as a time-to-event, in which seed germination was the event. Non-parametric and semi-parametric time-to-event analyses are recommended as a statistical technique for investigating incidence and timing of germination (Peréz and Kettner 2013). Seeds that germinated during the 30-day experiment were coded as “1” and seeds that did not germinate by day 30 were right-censored and assigned as “0”. No seeds were lost due to random censoring. Kaplan–Meier estimates of survivor functions (i.e., germination event) were generated using the survival and survminer packages in R (version 4.2.3). Survivor functions were stratified by cold exposure duration and generated for each species separately. Kaplan–Meier curves were generated using ggsurvplot with “event” function. Few species and cold stratification combinations reached 25, 50, or 75% germination, therefore the Kaplan–Meier estimator for these data never reached a failure probability > 0.25, 0.50, or 0.75; resulting in inestimable quartile values. A Cox proportional hazards model was generated using the survival (v.3.5–5, Therneau, 2023) and survminer (v.0.4.9, Kassambara, 2021) packages in R to determine the effect of species and cold stratification treatment on germination rate.

Results

Percent germination

A two-way ANOVA showed a significant interaction between cold stratification duration and species on total percent germination of seeds (F24,140 = 6.019, p < 0.0001) (Fig. 2, Supplementary Table 1). ANOVAs conducted within-species showed an effect of cold stratification on germination in A. rubrum, N. aquatica, and non-treated T. distichum seeds (Fig. 2, Supplementary Table 2). Overall, L. styraciflua exhibited superior percent germination (> 75%) compared to other species tested and showed no impact of cold stratification duration on total percent germination. Acer rubrum seed germination was ~ 25% for most cold stratification treatments with significantly greater germination (> 50%) occurring in seeds exposed to 75 days of cold. No A. rubrum seeds exposed to 90-day cold stratification germinated, however we acknowledge some of this loss may have been due to fungal growth that occurred. Nyssa aquatica seeds exposed to 15 days or fewer of cold exposure showed low percent germination (< 10%), although percent germination increased significantly (> 50%) with increasing cold stratification duration up to 75 days. Germination of T. distichum was consistently low (< 20%) compared to other species. However, germination in non-treated T. distichum seeds was greatest when seeds were exposed to 60 days of cold stratification. Contrarily, cold stratification duration had no impact on NaOH-treated T. distichum seeds. A separate one-way ANOVA comparing non-treated T. distichum seeds with NaOH-treated T. distichum seeds showed that overall germination was greater with pre-germination NaOH treatment (F1,56 = 4.840, p = 0.0320).

Fig. 2
figure 2

Percent germination (mean ± SE) of seeds from freshwater forested wetland tree species (L. styraciflua, A. rubrum, N. aquatica, T. distichum, and NaOH-treated T. distichum) exposed to cold stratification durations ranging from 0 to 90 days. Symbols above bars indicate significant differences within species revealed by species-specific Tukey posthoc analyses. Different letters indicate significant (p < 0.05) differences between groups. Vector images were sourced from open access image sources: Vectorstock, Alamy Stock Photo, and Vecteezy

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Germination rate

Liquidambar styraciflua seeds exposed to cold stratification durations of 15 days, or 45 + days resulted in more rapid germination compared to the control (0 days cold) (Fig. 3, Table 1). Although all L. styraciflua cold treatments resulted in similar germination percentages, the rate at which that germination occurred was more rapid with increasing cold duration (Fig. 3, Table 1). Acer rubrum seeds exposed to 75-days of cold exposure showed a significant increase in rate of germination compared to the control, though other cold treatment durations had no effect. Nyssa aquatica seed germination was faster than the control treatment when cold exposure treatments lasted 30 + days. Germination of N. aquatica generally began after 10 days regardless of cold treatment. Rate of germination for T. distichum was not impacted by cold stratification duration in either the treated or non-treated seeds (Fig. 3, Table 1).

Fig. 3
figure 3

Kaplan–Meier estimates of event functions (i.e., germination) for L. styraciflua, A. rubrum, N. aquatica, T. distichum, and treated T. distichum seeds exposed to varying durations of pre-germination cold stratification over a 30-day observation period. Shading represents the 95% confidence interval

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Table 1 Results of cox proportional hazards test for each species compared to 0 days cold stratification
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Discussion

The results of this study show important differences both within and between forested wetland and bottomland forest species regarding optimal conditions for seed germination. Most notably, generalist species associated with rapid colonization of bottomland forests throughout the southeast U.S. (A. rubrum and L. styraciflua) showed markedly greater germination success compared to specialist species (T. distichum and N. aquatica) across a wide range of seed cold exposure treatments. The impact of longer pre-germination cold exposure was most important for N. aquatica seeds as longer cold stratification (30 + days) significantly increased overall germination success. Although there was limited effect of cold stratification on germination for other species, the rate of germination showed a positive correlation with duration of cold exposure for both N. aquatica and L. styraciflua. Our results provide important information about each species’ germination niche and development under a changing climate. Additionally, our results have implications for explaining and predicting shifts in community composition where these species co-occur as well as informing their management.

Variability in early life-history traits is important to understanding shifts in plant community structure and functional diversity in response to changing environmental conditions. Several studies have investigated the relationship between breadth of seed germination dynamics and ecological niche and found that generalist germination strategies, such as not requiring specific cues to germinate, may have evolved in response to higher spatial variation (Marques et al. 2014; Barga et al. 2017). Wetlands are characterized by specialist species that are uniquely adapted to the variable physiological stressors inherent to inundated and or saline soils; in fact, in the U.S., plant species are assigned a wetland indicator status which indicates the species’ likelihood for occurrence in wetlands (i.e., wetland specialist vs. generalist) (USACE, 2020). In forested wetlands and bottomland forests, existing research regarding species’ niche and physiological thresholds, specifically as it pertains to habitat suitability, predominantly focuses on post-germination life history stages. Seed morphological traits, such as seed size and dispersal mode, have been used to explain functional diversity of a variety of forest types (Swenson et al. 2012; Lamanna et al. 2014; Andrew et al. 2022). Generally, seeds with smaller mass are associated with having dormancy, greater seed production per unit canopy area, and lower seedling survival than seeds with greater mass (Moles 2017). However, inclusion of seed germination dynamics as a predictor of functional diversity or community complexity is less common. Our findings counter common trends in predictive seed traits, as the smaller-seeded generalist species (Fig. 1) did not show a strong requirement for dormancy.

Previous studies support our findings that A. rubrum and L. styraciflua have high rates of fecundity. Abrams (1998) documented that requirements for A. rubrum seed germination are broad and seeds may germinate immediately following dispersal or in the second year after they are produced. Moore and Lacey (2009) found that Liquidambar styraciflua show rapid germination rates (first germination in less than 15 days) and high overall percent germination (60–83%). Species-specific forestry guides (USDA, 1965) provide broad recommendations for cold stratification requirements for these species (e.g., 30 to 90 days for A. rubrum, 30 to 120 days for N. aquatica, and 30 to 90 days for T. distichum). Our findings, coupled with these broad recommendations, demonstrate there may be significant variability within species regarding germination requirements, potentially depending on population, phenotype, or climate/geographic range. Additionally, it is important to note that the majority of information on the basic ecology for these species is sourced from studies originating more than 50 years ago (USDA, 1965) and regional variability in germination dynamics due to genotype or provenance may impact germination. For example, Krauss et al. (1998) found that genetic lineage influenced T. disctichum seed germination in response to increasing salinity. Tremblay et al. (1996) identified local variability in germination response to cold stratification across A. rubrum populations at its northernmost range in Canada. And Millan & Winstead (1976) found adaptive variation within seedling growth traits across populations of L. styraciflua from Mexico to U.S. Although some provenance studies have been performed to understand regional physiological differences and responses to climate change in freshwater forest and bottomland species (Carter 1996; Mohan et al. 2004; Arnold et al. 2012), the influence of provenance on temperature-dependent seed germination has not been thoroughly explored in these species. Although many aspects of individual species’ traits may not change over, the large gap in time from when these species’ reproductive ecologies were last analyzed provides impetus to revisit basic phenology and reproductive dynamics given the impact of a rapidly changing climate. Additionally, there remains a strong need for provenance and common garden studies to understand how individual populations may or may not have adapted to changing climate.

Implications of climate change on community structure

Following disturbance, species establishment (i.e., seed germination) is one of the first ecological filters on community composition. Seed germination requires complex interactions between both internal and external cues (e.g., red light exposure, soil water potential, Abscisic acid concentration, temperature) (Taiz et al. 2023). In forested wetlands throughout the southeastern United States, historical shifts in climate and anthropogenic activities have altered soil chemistry, hydrology, and forest structure (Mulholland et al. 1997). Consequently, tree seeds germinating today are doing so under conditions that are potentially vastly different from the conditions in which their mature neighbors germinated. Whereas many changing environmental variables can impact species establishment, here we focus on isolating the influence of duration of winter temperatures on germination as a primer to decoupling more complex influences on community composition.

Shortened winters, as projected under anthropogenic climate change, may have differential impacts on germination and seedling establishment depending on plant species and life stage. Seeds and seedlings are especially sensitive to changes in temperature and water availability and they often rely on these factors to signal the shift from dormancy to germination (Walck et al. 2010). A change in these cues may cause change in community composition. Liquidambar styraciflua’s consistently superior germination percentages paired with its extended habitat range suggest that this species will exhibit similar germination success under shorter, warmer winters. However, germination of N. aquatica will be negatively impacted by projected climate changes of shorter winters as peak germination was recorded after 30 or more days of cold exposure. Although A. rubrum showed maximum germination when exposed to 75 days of cold stratification, this species was able to germinate successfully (~ 25%) regardless of cold duration suggesting germination of this species will most likely be unaffected by climate change effects on temperature. T. distichum showed maximum percent germination when exposed to 60 days of cold stratification. Our findings support field observations by Lei and Middleton (2021) who found that germinating T. distichum was more prevalent in seedbanks collected from the northern range of the species compared to south. But even maximum germination of T. distichum in our study was < 25%, suggesting germination and recovery of bald cypress may be slow regardless of future climate condition.

In L. styraciflua and N. aquatica the rate of germination increased with increasing cold exposure. Speed of germination may have conflicting effects on establishment and survival. Strong competition dynamics observed between new seedlings may favor the first to germinate, as has been observed in both grasslands (Louda et al. 1990) and tree species (Jones et al. 1997). However, slower and asynchronous germination may increase seedling survival under unpredictable environmental conditions (Sales et al. 2013). In the case of warmer and shorter winters due to climate change, variable springtime temperatures where snap freezes may occur may favor the survival of slower germination mechanisms that allow the plant to remain in a dormant state until later in the spring avoiding tissue damage from freeze.

Our results show that replacement of freshwater forested specialist species, such as Taxodium distichum and Nyssa aquatica, by generalists with broad germination requirements may be exacerbated by warming winter temperatures, further accelerating ongoing habitat homogenization. Though many factors (e.g., habitat fragmentation, abiotic stressors, pests and pathogens) contribute to shifts in species abundance, we predict rising winter temperatures throughout the range of these species will particularly limit N. aquatica’s presence. This should be of concern for habitat managers as changes in freshwater forested wetland and bottomland forest community composition can challenge long standing ecosystem functions and services, including biodiversity, carbon sequestration and storage, nutrient cycling, and flood mitigation (Hooper et al. 2005; Lougheed et al. 2008). Further, the continued plant community shift toward redundant functional diversity will reduce the ecosystem’s plasticity, leaving it more vulnerable to stress events (Olden et al. 2004).

Opportunities for future inquiry

From this study we have identified several areas of opportunity for future inquiry. First, we recognize that although germination percentage and rates are an important part of a species’ reproductive success; to best predict community composition dynamics we also need information about total seed rain. Similar to other aspects of a species’ reproductive ecology, seed production per tree may vary across geographic ranges, environmental conditions, or even within the individual’s life-span. Available information on average seed production for the species addressed in this study is limited, variable, and often sourced from older field studies with geographic limitations. For example, Abbott (1974) found that A. rubrum can yield annual seed crops between 12,000 and 1,000,000 seeds depending on tree size. For L. styraciflua, seed balls may contain between 8 and 56 sound seeds per ball, though there is little data available on the number of seed balls produced per tree per year or factors that predict seed fecundity (USDA, 1965; USDA, 1974). Taxodium distichum trees average 16 viable seeds per cone (Faulkner and Toliver 1983), and while some seeds/cones are produced every year, good seed crops only occur at 3- to 5-year intervals (USDA 1974). Data on the actual quantity of seed/cone production for these species is obfuscated by forestry conventions which report number of seeds/cones per bushel rather than annual production by tree. We believe there is value in revisiting some basic descriptive ecology for species under current climate conditions with consideration of local- to landscape-scale phenological variability.

Next, cold stratification studies performed on many species use constant pre-germination cold exposure, albeit of variable durations. Climate data from throughout temperate regions show that not only will winters be of shorter lengths but may be interrupted (NOAA, 2023a). Weather station data sourced from the Great Dismal Swamp in Suffolk, VA, one of the northernmost range limits for all four species, showed that the maximum number of consecutive days where temperatures did not exceed 4 °C was 18 days over a 30  year period (NOAA, 2023b). Consistent and consecutive cold winter days may not be a reality in many of the regions in which these species co-occur, thus research on the effects of interrupted cold exposure may be equally informative. It’s important to recognize that the effects of cold stratification may depend on incubation temperatures during germination. Weber and Sorensen (1990) found that the effectiveness of stratification duration on Pinus ponderosa seed germination was correlated with incubation temperature (e.g., higher incubation temperatures promoted germination when paired with shorter stratification periods, whereas the effects of stratification were greater when paired with lower incubation temperatures). Germination responses to climate change may be generational (Fernández-Pascual, 2018). Chen et al. (2011) showed that in Arabidopsis spp. the climate conditions experienced by the mother plant is integrated into the fruit bearing its seeds (thermotolerance), which controls seed dormancy. The presence of this phenomena may aid species in adjusting to changing winter conditions, but deserves further study across taxa.

Finally, our results have implications for forest management. The range of species’ hydrothermal germination niches has been used to explain overall species range limits and inform the conservation of threatened species and habitats (Rajapkshe et al., 2022). To operationalize this information and promote biodiversity, forest managers are encouraged to explore controlled seed preparation (i.e., cold stratification) and manual seed dispersal in habitats where seed rain for desired species may be limited. Although using seed for forest enhancement is not a common practice in freshwater forested wetlands compared to seedling transplantation, which ideally has higher rates of survival, it may be a cost effective and labor-efficient option. Of course, successful seed dispersal must also consider other environmental factors that may limit seed germination and survival (e.g., light, hydrology, soil pH, herbivory, etc.). Direct seeding has been employed as a successful restoration technique in neotropical savanna (Sampaio et al. 2019) and tropical forests (Palma and Laurance 2015). This type of forest restoration or enhancement technique may be particularly useful in bottomland forests that have been severely disturbed or selectively logged.

Conclusion

In this cold stratification study, we found that shorter durations of cold exposure strongly decreased the overall germination success of specialist swamp species N. aquatica, but had little impact on generalist species other than slowing the rate of germination. Whereas many factors contribute to biotic homogenization, including propagule pressure, dispersal abilities, life history traits, and tolerance of species to the biophysical attributes of their surrounding habitat (Qian and Guo 2010), factors that control seed germination are one of the first filters on community composition. Climate change brings about more variable temperature regimes, and with that, conditions that will favor species with broader germination requirements. Worldwide, across multiple taxa, disturbances to habitat and climate have contributed to the replacement of specialist species by generalists (Clavel et al. 2010). Among the species tested in this study, we predict that warming shorter winter temperatures will contribute to the decline in presence of N. aquatica and convergence toward generalist species in freshwater forested wetlands. Understanding the differential effects of changing winter temperatures on early life stage traits of temperate forest species is important to understanding the multiple interacting factors driving observed patterns of habitat homogenization, particularly in systems like wetlands characterized by species with broad tolerances to other abiotic stressors.