CLIMATE CHANGE IMPACTS TO PLANT AND NATURAL COMMUNITIES
The information in this appendix is based on the theoretical understanding of community, behavioral and restoration ecology, as well as data published in the scientific literature on how climate is affecting plant and natural communities around the globe. The goal of presenting this information is to help us think about how it specifically applies to the Chicago Wilderness region. Ultimately we need to garner the knowledge of Chicago Wilderness’s natural resource managers to capture the patterns of change happening on the ground, and use it to inform adaptive management in a rapidly changing environment. This section is intended as a platform for managers, stewards and volunteers to contribute their qualitative and quantitative knowledge of how climate change is affecting the natural communities they work at in the decades to come.
1. PHENOLOGICAL and RELATED CHANGES
1.1 Pollination: Potential impact to Prairie Grasslands, Savannas/Wooded Communities
Most animal pollinated species in the Chicago Wilderness region have relatively open pollination systems, with a large number of potential pollinators. This means that if phenology mismatches occur between a plant’s flowering period and its current primary pollinators, pollination services may be largely provided by other species and not have a particularly strong effect. On the other hand, different suites of pollinators may have different degrees of effectiveness, especially if the shift results in a significant change in the taxonomic make-up of the pollinator community on a particular species of plant. This situation could result in decreased pollination efficiency (Schemske and Horvitz, 1984) and reduced seed set. Common plant species that also rely on common pollinators may not necessarily be impacted. For example, the likelihood of a phenology mismatch between hummingbirds in the Chicago Wilderness region (ruby-throated hummingbird; Archilochus colubris) and their dominant food resource (Jewelweed; Impatiens capensis, I. pallida) is low because both species are very common and ruby-throated hummingbirds are present throughout most of the jewelweed flowering season (May-September).
At the other end of this spectrum, the de-coupling of historically synchronized phenological events could be most damaging to conservative insect species with specific, narrow habitat requirements, host-plant specificity, and limited dispersal capabilities. However, even as evidence that species are shifting their phenologies in response to recent climate changes continues to mount (reviewed in Parmesan, 2006; Amano et al., 2010), and insect phenology in particular is showing a steeper advance than plant phenology in certain regions (Gordo and Sanz, 2005), there is little empirical data to show which species’ traits might make them more sensitive to future climate change. Researchers have recently begun to address this question for species that have long-term life history data available. For example, research by Diamond and colleagues (2011) on butterflies in the U.K. indicates that species with narrower larval dietary breadth, smaller range sizes and more advanced overwintering stages experienced relatively greater advances in their date of first appearance over the last 30 years. The finding that species with a narrower larval dietary breadth would be indicative of relatively greater spring advancement was unexpected, mainly because phenological advancement may be limited by the availability of host plants (Memmott et al., 2007) and generalist host use has been observed to facilitate the climate-driven range expansion in U.K. butterfly species (Braschler and Hill, 2007). Diamond et al. (2011) suggest the advancement of specialized butterflies may in fact be enabled by the phenological advancement of their host plant, while species with a greater number of potential host plants may be buffered from such shifts in plant phenology. If correct, and if applicable to species in the Chicago Wilderness region, this could mean conservative species are not necessarily more vulnerable to phenological mismatches.
This question would indeed be a research topic of great interest because while a large percentage of the plants in our region’s prairies, savannas, forests, and wetlands are wind-pollinated or pollinated by generalists, some plants have developed specialized relationships with specific pollinators. In fact, there are at least 453 conservative insect species (those species largely restricted in distribution to intact natural community remnants) in prairies and savannas throughout the Chicago Wilderness region (Panzer, 2010) that could be affected by a climate change induced phenology mismatch. A well-known example is the specialized relationship between sphinx moths (Sphingidae family) and the eastern prairie white-fringed orchid (Platanthera leucophaea). The specialized structure of the flowers of this federally threatened orchid limits pollination to only a few species of large sphinx moths. Further, this species exists in small isolated populations, making it difficult for pollinators to locate them. Because of this, a phenology mismatch that disrupted the interaction between the sphinx moth and orchid would compound the pollination challenges this species already faces and could result in the extinction of the eastern prairie fringed orchid (WICCI Plants and Natural Communities Working Group Report, 2011).
Phenological mismatches between pollinator and plant could occur in several ways. For example, while many species respond to changes in temperature as a cue for when to emerge, the specific trigger (e.g., the total number of days above a certain temperature—or accumulated warmth—versus crossing a particular temperature threshold) may differ. Alternatively, daylight length may be the driving force behind emergence for some pollinators, which could lead to a timing mismatch if their host plants are instead more closely cued into temperature change. Even if pollinators are capable of continuing to synchronize their phenological events with host plants that are advancing in their timing of emergence, the increased variation in precipitation (wetter winter and springs; longer dry periods in the summer interspersed with more frequent, extreme storms) and temperature fluctuations (early winter or spring warm-ups followed by cold snaps) this region is likely to experience may hinder one, or both, species from sustaining healthy populations in the long-term.
1.2 Seed production and dispersal: Potential impact to Prairie Grasslands, Savannas/ Wooded Communities and Wetlands
In addition to shifts in pollination opportunities, as discussed in the previous section, there are a variety of ways that changes in climate can affect seed production. For example, temperature, carbon dioxide (CO2) levels and the timing of rainfall all influence fruit development. Warmer temperatures, along with higher CO2 levels, have the potential to increase the number of flowers and fruits, and the number and size of seeds produced by a plants (Jablonski et al., 2002). However, wild plants (as opposed to agricultural crops) are constrained by what they can do with increased CO2 and may use it for survival and defense rather than boost reproduction. Some research suggests that even though seed quantity and size may increase in wild plants under higher CO2 levels, the trade-off is that they contain less nitrogen (Jablonski et al., 2002). On the other hand, more frequent and longer periods of reduced rainfall could reduce fruit size and/or the number of fruits produced both of which can impact species’ dispersal ability. Changes in the number of flowers, fruits and seeds and their nutritional quality could have far-reaching consequences, since changes in the amount of nutrients in seeds could affect reproductive success and seedling survival. Such changes could also have long-term effects on ecosystem functioning.
Another consideration is that germination and seedling establishment are the highest-risk phases in the life cycle of plants (Harper, 1977) and thus mechanisms that reduce the risk, such as germination patterns that increase the probability of successful seedling establishment, are under strong selection pressure (Meyer et al., 1997). For many species, germination cues are temperature-mediated and therefore changes in temperature might be expected to have significant effects on plant distribution and survival because the timing of this key life-history trait may no longer be optimal (Gworek et al., 2006; Cochrane et al., 2010).
Ultimately, as temperatures rise and rainfall patterns change, it is likely that some plant species may increase seed production while others decrease. These types of trade-offs will influence how species abundance and composition will shift. Some species are likely to have more plasticity in their responses, or ability to adapt via natural selection, than others. There will also be species that will not be able to either respond or adapt, and consequently will become uncommon, rare or even disappear from our regional flora. This community-level trade-off in species composition and abundance will likely result in unpredictable and novel ecosystems (Cochrane et al., 2010).
1.3 Dormancy: Potential impact to Prairie Grasslands, Savannas/Wooded Communities, and Wetlands
Although many prairie grasses do not need cold dormancy to germinate, some terrestrial native species do require a period of cold (10–90 days) dormancy—and sometimes an interaction of cold and wet conditions—in order to break germination. Many trees, shrubs, and forbs require cold stratification prior to dormancy. For example, prairie restorationists have learned that white false indigo (Baptisia leucantha) requires 90 days of cold in order to foster optimal germinations. Warmer winters, along with increased seasonal temperature variability, may affect the ability of certain prairie species to fulfill their dormancy requirements. This could end up favoring species without strict dormancy requirements and alter species composition of prairies, savannas, forested communities, and wetlands.
Currently, plants in the Chicago region likely meet their dormancy and cold stratification requirements rather early in the winter season. This would mean that plants could respond to unusually early warm-ups, making them susceptible to damage during subsequent cold snaps that can occur later in the season. Plants that have a later completion of the required dormancy, however, might be less affected by these early warm-ups.
1.4 Early bud burst: Potential impact to Savannas/Wooded Communities and Wetlands
Trees and shrubs may break dormancy earlier in the spring in response to earlier, warmer springs. Early bud burst can impact trees and shrubs because the new growth may be killed by subsequent frosts. Early bud burst may also lead to early flowering and phenological mismatch.
For example, if plants leaf out significantly earlier with climate change, the synchrony among herbivores (mostly insects) and predators (especially birds) could deteriorate. The large majority of forest insectivorous birds are migratory, and winter in the Neotropics. Most of those migrants arrive in the Chicago area in May. Already in years when spring has been warmer than usual, the oaks were largely leafed out before most of the insectivorous migrants arrived. This is the period when the leaves are most susceptible to insect herbivory. Presumably the lack of avian predators could mean a greater degree of insect damage to these trees is possible than what historically has been the case.
(Amano et al., 2010). As the warm-up continues to advance, and organisms such as trees and insects responding to temperature cues begin to emerge earlier, a mismatch between food availability and that of avian migratory timing could occur.
While there is some evidence that bird migration does appear to be advancing to some degree along with bud burst (Root and Hughes, 2005; Bradley et al., 1999), the timing of these migrants is mostly set by photoperiod, especially for Neotropical migrants that have no weather cues on their wintering grounds. Because of this, it seems likely that the mismatch between leaf out and the arrival of migrating birds will become larger and more consistent, and will likely negatively impact populations of migratory birds, and in turn decrease their ability to provide natural pest control for the elms and other trees in the region’s woodlands.
2. BIOTIC/ABIOTIC FACTORS
2.1 Range shift: Potential impact to all communities
As the region becomes hotter, drier and more extreme, plants will be faced with three options—move, adapt, or cease to exist. For those species that are able to shift, their range alterations will be influenced by competitive regimes as well as by physiological tolerances. This means that although plant species for which Chicago Wilderness represents the southern end of their range could certainly be expected to shift northward and out of the region completely, and species either at the middle or northern end of their range may expand their distribution and/or abundance, the actual resulting patterns of range shifts are likely be more complex than simple north-south shifts. It is also possible that even as species shift their range in order to track their “climate envelope”, some remnant populations will persist near the southern limit of their distribution (e.g., similar to hemlock or tamarack currently). Wind pollinated species and generalist species will likely fare better than those with a restricted pollination or habitat requirement. This is especially true of species that occur in the dramatically altered landscapes of urban areas, where they are virtually suspended in small, isolated fragments.
2.2 Community disaggregation: Potential impact to all communities
Since individual species are likely to respond differently to climate change, it is possible that some level of disaggregation in all communities will occur. Some species, especially the less conservative ones, will be able to easily move between isolated and changing areas. Others will not be able to easily move if dispersal distances are too great. Even those conservative species that have highly dispersive seeds could be impacted as assemblages shift. The expectation is that new species will move into the Chicago Region while some current species will shift out, resulting in novel species assemblages and interactions, as well as opportunities for invasives to get a foothold in many communities. One of the biggest challenges community disaggregation presents is figuring out how to manage new types of communities composed of novel inter-taxa interactions (plant-animal, plant-microbe, etc.) we have never seen before (WICCI Plants and Natural Communities Working Group Report, 2011).
Changes to climate and community assemblages could likely have a ripple effect, causing changes to aspects such as 1) the decay rates of leaves and their biochemical composition, 2) rhizosphere dynamics, 3) fungal recruitment and associated nutrient acquisition capacity and 4) element cycling (e.g., K, Na, Si). These changes could in turn act upon soil fauna, microbes and geochemistry in unpredictable ways that influence cation and dissolved oxygen content (DOC)/N balances in watersheds, dynamics of soil organic matter, as well as the proportion of active and stabilized soil C pools (Filley, 2008).
2.3 Invasives/diseases/pests: Potential impact to all communities(Leatherman, 2007). An example of a pest that already occurs in the Chicago Wildnerss region that is likely to be affected by climate change is the gypsy moth (Lymantria dispar), which could experience population expansions in response to a longer growing season. However, the most potentially harmful interaction between climate change and invasive pathogens and pests would arguably be the establishment of species that do not currently survive here, but could thrive under warmer conditions (e.g., Sudden Oak Death [Phytophthora ramorum], Kudzu [Pueraria montana], Hydrilla [Hydrilla verticillata]). And because climate change will generally favor species that are very adaptable, mobile and aggressive, which are characteristic traits of many invasive and weedy species, it is a reasonable to expect many invasives will have opportunities to further increase in range and abundance.
2.4 Fragmentation/isolation: Potential impact to all communities
Many of the natural communities in the Chicago Wilderness region are already highly fragmented and while climate change may exacerbate this threat, it does not cause further fragmentation and isolation. The exception, however, may be in wetland complexes. Chicago Wilderness summers are predicted to be hotter and drier, which may alter the hydrology of wetlands and cause some wetland areas to dry up and others to become more isolated. Coastal wetlands, such as those at Illinois Beach State Park and Indiana Dunes National Lakeshore, are at risk if lake levels drop significantly; but wetlands that are no longer connected to Lake Michigan (e.g., Hegewisch Marsh) are perhaps most vulnerable because they are entirely reliant upon precipitation patterns for inundation. Wetlands that are spring-fed (e.g., fens) will not be as greatly impacted in the short-term, although sustaining groundwater aquifers in the long-term may prove challenging. Wetlands that have been disconnected from their original water source and are now regulated by pumps (e.g., Hennepin Lake is completely disconnected from the Illinois River and its water level is regulated via a pump) also may not be immediately or greatly impacted. In contrast, bogs are not linked to groundwater and receive their water through precipitation and runoff only. Thus, drier summers could affect bogs more quickly and severely than other wetlands. Isolated ephemeral wetlands in prairies and agricultural systems also may have shorter periods when they have water, or be smaller in extent. Through time, this may allow agriculture or other vegetation types to replace the wetland community currently supported by these sites. Consequently, frogs, turtles and migratory waterbirds (ducks and shorebirds primarily) that rely on these ephemeral habitats may have to look farther afield to find appropriate habitat.
Although climate change will not directly cause more fragmentation or isolation of other habitats, it will compound the effects of existing fragmentation. Most obviously, if species are tracking appropriate climatic conditions, they will need to shift geographically and fragmentation will make such movement much more difficult. Certain species will be more heavily impacted than others in this regard. For example, birds can easily move from place to place but species with limited dispersal abilities are likely to be affected more strongly by fragmentation, as will species that are more specialized in their habitat use. These features will also interact; specialized species with poor dispersal abilities will be most at risk from fragmentation under climate change.
There are a variety of indirect effects that could also occur. Any tendency of climate change to interfere with breeding success would increase the probability of local extinctions within a fragmented system. For example, phenological mismatches that result in species having a reduced seed set, or disaggregation occurring due to plants and pollinators differentially responding to climate change, would greatly increase the probability of extirpations in fragmented habitats.
Additional indirect effects might include increasing microclimatic edge effects in fragments as conditions become warmer and drier, reducing the available area appropriate for forest interior species, not just due to light levels, but also humidity and temperature, and more frequent severe storms causing an increase in wind damage to trees, which is typically most severe at edges of patches.
2.5 Herbivory: Potential impact to Prairie Grasslands, Savannas/Wooded Communities
Climate change could impact herbivory by insects and mammals. For insects, milder winters will enable insects to survive the winter better, and potentially become active earlier and increase in abundance, increasing the level of herbivory on trees, shrubs, and forbs. Adding to this is the fact that as the greening of local vegetation advances with milder temperatures and earlier bud burst, migrant insectivorous birds will likely be behind the leaf out of oaks. This will increase the herbivory on the young leaves (Marquis and Whelan, 1994). In addition to leaf damage, root hebivory might if larvae of native and invasive beetles, invasive worms, etc…were to cause early stress to fine root production and potentially nutrient and water uptake.
3. PRESCRIBED FIRE
3.1 Change in fire management: Potential impact to Prairie Grasslands, Savannas/Wooded Communities(WICCI Plant and Natural Communities Working Group, 2011).
Potential impacts are drought conditions and/or higher temperatures making fires burn hotter and more thoroughly. Changes in wind patterns could also play a role in changing the dynamics of fire. In addition to wind currents themselves undergoing changes, more energy in the atmosphere could increase wind strength, making fires hotter and more thorough. This change in conditions might be helpful in making it possible to bring fire to marginal low-fuel situations, such as unhealthy areas with little grass matrix or other natural fuel. The effects of hotter and more thorough fires are likely to be variable across taxa. For example, hotter fires might be beneficial in managing forest systems for oak dominance and may help knock back shrubs and woody growth in prairies, as long as managers were still able to get fire onto the landscape under these more challenging conditions. However, this may be less desirable from a bird or insect perspective. Both of these groups can show increased diversity with increased variation in the vegetation structure that is maintained by incomplete burning.
The combination of more frequent droughts in the summer and fall, which increases the difficulty of controlling a prescribed burn, and wetter springs could significantly reduce the number of potential days that fire can be used, especially during the traditional spring and fall burn windows. As a result, the timing of burn windows could change, with prescribed burns needing to occur earlier in spring and/or later in fall or throughout the winter, both for strictly climatic conditions, but also because of vegetation stages or activities of fire susceptible fauna. Already many Chicago Wilderness land managers are thinking of one burn season that extends from fall through spring and have been able to successfully conduct prescribed burns in late December and January due to warmer winter temperatures and snow free days.
Recent trends in earlier spring green-up and warmer temperatures have indeed shortened the spring burn window in several recent years and made spring burns more complex. Managers need to be more aware of faunal responses to warmer springs, for example snakes may be emerging from their hibernacula earlier and insects may be active earlier in the season. In the past few years the spring burn season has also been complicated by the cool, wet springs this region has experienced. This could be a short-term event, or it could be the beginning of a long-term trend related to changing climate that would require more fall burning than currently takes place. However, if more intense fires result from either 1) warmer summers and fall or 2) more dry fuel from droughts then the number of possible burn days in the fall would ultimately be reduced. Fewer appropriate prescribed burn days could mean fewer prescribed fires, which would negatively impact fire dependent forbs and grasses. Many shrubs are set back by fire so they may increase with decreased prescribed burning. What this latter point could mean for the associated impacts to soil experiencing shrub or woody encroachment is unclear; however less light reaching the ground layer due to encroachment would negatively impact the existing plant community. However, because suppression of burning in grasslands and savannas is a significant cause for the expansion of woody plants into grasslands and consequently changes the rates of the biogeochemical cycling of nitrogen and carbon in those ecosystems (Filley et al., 2008). It seems likely that changes to the long-term pools of soil carbon and feedback to local ecosystem services and to atmospheric CO2 are already underway and could accelerate.
4. WEATHER IMPACTS & EXTREME EVENTS
4.1 Change in frost dates: Potential impact to Savannas/Wooded Communities, Lakes and Lakeshores
During the last several decades the Midwest has experienced a shift toward shorter winters and increased air temperatures (Sinha and Cherkauer, 2010). The length of the frost-free season has increased by over a week in this region, mainly due to earlier dates for the last spring frost (USGCRP, 2009). Longer growing seasons should provide favorable conditions during spring for planting, however, the projected rise in soil temperatures during the cold season is likely to increase the risk of pest infestation (Sinha and Cherkauer, 2010). Overall trends in extreme and mean seasonal soil temperature from 1967–2006 indicated a warming of soil temperatures at a depth of 10 cm in northwest Indiana, north-central Illinois, and southeast Minnesota, leading to a reduction in the number of soil frost days (Sinha et al., 2010). While continued future warming is likely to enhance rising soil temperatures, it’s also possible that reduced snow cover during winter could work in the opposite direction to cool soil temperatures (Sinha and Cherkauer, 2010). Both changes will cumulatively affect cold season processes in the region (e.g., soil frost days, snow accumulation, and soil temperatures).
If the date for the last spring frost (historically late April) continues to advance and/or the date for first fall frost continues to delay (historically mid-late October), this would result in an even longer growing season and a shorter winter season. Fewer days with soil frost implies higher infiltration, specifically during early spring which may result in decreased soil moisture retention and drier soils in spring owing to enhanced evapotranspiration losses as compared to the present (1977–2006) conditions (Sinha and Cherkauer, 2010). This may also reduce the risk of soil frost enhancing winter and spring flood events; however, increased precipitation in the winter months is likely to keep river levels higher throughout the cold season (Sinha and Cherkauer, 2010).
There are a variety of additional ways that changes in frost patterns could indirectly effect natural communities. For example, researchers have found that, in arctic environments, extended frost-free time causes soils to have longer active periods when microbes can mineralize nutrients (Altrichter et al., 2010). While the soils remain frost-free longer, plants continue their normal cycle dictated by the length and intensity of daylight, which has not changed. Microbes may continue to create nutrients, but the plants no longer use them, so that when rain or snow comes the nutrients leach into the rivers and streams. For example, in the arctic concentrations of nutrients such as nitrate and ammonium in the water are substantially increasing due to extended frost-free durations (Ball et al., 2011).
Whether this would be true for mid-latitudes is uncertain. Higher temperatures and longer frost-free time would be expected to increase rates of organic matter decomposition in soils. Yet more than temperature controls soil decomposition rates, and it is unclear how factors such as soil structure, chemical make-up of soil, oxygen levels, water availability, and frost-free time will interact to influence decomposition rates in our region (Davidson and Janssens, 2006).
Aquatic plants may also have an extended growing season as ice melts earlier on lakes, and lakes and streams warm up earlier in the spring. Somewhat related to changes in frost dates, and milder winters overall, is the potential for a decrease and a delay in the amount of ice cover on local lakes and wetlands, leading to increased evaporation in the winter. This could also affect lakeshore communities as more sand and beach is exposed with lower water levels. It’s possible that winter survivorship of aquatic invasives will change as well. For example, climate change will influence the likelihood of new species becoming established by eliminating cold temperatures or winter hypoxia that currently prevent survival and by increasing the construction of reservoirs that serve as hotspots for invasive species (Rahel and Olden, 2008). Climate change could also modify the ecological impacts of invasive species by enhancing their competitive and predatory effects on native species and by increasing the virulence of some diseases (Rahel and Olden, 2008).
4.2 Change in freeze-thaw cycles affect: Potential impact to all communities(Wuebbles et al. 2010), combined with higher seasonal variability in temperature could cause a shift in the freeze-thaw cycle. The pattern has typically been characterized by one or several longer periods where land and water are completely frozen during the winter months, but this could shift to a pattern of multiple, shorter periods of freeze/thaw cycles during the next several decades (Takle and Hofstrand, 2008). This type of shift in the freeze-thaw cycle would expose plants and seeds to periods of thawing throughout the winter, and might enable some species to get a head start on spring emergence. Changes in freeze-thaw cycles can alter soil physical properties and microbial activity (e.g., a higher number of cycles would increase the risk of soil erosion by weakening soil structure; Sinha and Cherkauer, 2010). However, the overall impact of these changes on soil functioning remains unclear (Henry, 2007). In the longer-term (late 21st century), the number of freeze-thaw cycles may actually be reduced due to rising soil temperatures that provide fewer opportunities for freeze-thaw cycles to occur (Sinha and Cherkauer, 2010).
In aquatic systems, freshwater ice-covers control most major interactions between the atmosphere and the underlying systems including solar radiation, thermal regimes and oxygen levels, and therefore biological productivity (Prowse et al., 2007). In general, ice is likely to break-up earlier and freeze later and result in reduced and/or delayed ice cover in aquatic communities. It is expected that reductions in lake-ice covers will produce changes in temperature and light levels, water circulation patterns, nutrient availability, aquatic UV radiation exposure and layering of warm and cold water during the ice-off period (Prowse et al., 2007).
In smaller aquatic systems, such as rivers and streams, the risk of experiencing freeze-thaw cycle interruptions is greater due to water flow and could lead to increased evaporation in the winter and lower water levels in rivers and streams. In contrast, although Lake Michigan is already experiencing delayed and reduced ice cover, once it does freeze over it is unlikely to experience interruptions in the freeze-thaw cycle. All of the Great Lakes have experienced reduced ice cover during the last several decades. Wang (2010) has estimated total ice cover on the Great Lakes has shown an overall decline of ~15% over the period 1973–2009. Over the middle to deepest parts of the Great Lakes, the occurrence of very densely packed ice declined by more than 30% over the period 1973–2002. Near the lakeshores, the occurrence of very densely packed ice declined by ~20% over the period 1973–2002. Lake Michigan in particular has experienced the second highest rate of ice loss among the Great Lakes, with a negative trend of -2.05 yr¹ between 1973–2010 (Wang et al., 2011).
How these changes may impact lake levels has been an active field of research with multiple approaches being used in modeling this function. Recent models for Lake Michigan, derived from using an energy budget-based approach to adjusting the potential evapotranspiration (PET) instead of using air temperature as a proxy to compute PET, suggest either a smaller decrease in net basin supply and smaller drop in lake levels than using the temperature proxy, or a reversal to increased net basin supply and slightly higher lake levels (Lofgren et al., 2011). In other words, lake levels are highly variable, making it imperative that we manage and restore our systems to handle that fact.
4.3 Milder winters: Potential to impact all communities
As discussed in several previous sections, milder winters will interact with a many of the phenological, abiotic, biotic, and weather variables. Generally, milder winters are likely to decrease and delay the amount of ice cover on lakes and wetlands—a phenomenon already occurring on Lake Michigan (Wang, 2010; Wang et al., 2011)—and lead to increased evaporation. Milder winters may also affect terrestrial communities if they impact species dormancy requirements and extend the growing season. In addition, this will almost certainly enable many pests and diseases to increase in range and abundance.
4.4 Increased evapotranspiration: Potential to impact Prairie Grasslands, Savannas/Wooded Communities, Wetlands, Streams/Rivers, Lakes
Evapotranspiration, driven by temperatures and solar radiation, is the amount of water evaporated from land, water, plants, and soils. It is expected that, as temperatures increase with climate change, evapotranspiration rates will also increase. Climate modelers in Wisconsin evaluated potential evapotranspiration rates using downscaled climate models considering that evapotranspiration rates will increase with increased temperature, while increased moisture or cloud cover will decrease solar radiation and evapotranspiration rates. Overall the models predict that under various emission scenarios, annual rates of potential evapotranspiration will increase with the highest rate of increase in the spring and fall when increased temperatures are less offset by increased moisture or cloud cover (WICCI Plants and Natural Communities Working Group Report [2011)].
On an ecosystem-level, increased evapotranspiration is expected to lead to lower lake levels and lower stream flows in the Great Lakes Region (Magnuson, 1997). Increased evapotranspiration may also have a drying effect on all soils, especially wetland soils. On a plant-level, the stomata in plants become smaller in response to higher levels of CO2, thus reducing the amount of water they transpire into the atmosphere. However, rising temperatures would likely make plants transpire more, so how evapotranspiration would play out is unclear. Studies from the agricultural field have shown that overall rates of evapotranspiration decrease under elevated CO2 due to the stomatal response (Bernacchi et al., 2007). Currently, it is not well understood how the decrease in plant transpiration from increased CO2 levels could affect overall ecosystem level evapotranspiration rates.
4.5 Ice storms: Potential to impact Savannas/Wooded Communities
4.6 Droughts (hydrology): Potential to impact all communities
An increase in the number and duration of droughts could stress vegetation in all terrestrial communities due to lack of water availability. Longer and more frequent droughts could shift the species composition of our region’s natural communities, and favor communities adapted to drier conditions. Prairies may be more resilient to droughts as many prairie grasses and forbs have deep root systems and are adapted to withstand drier summers. In prairies where topography and hydrology combine to provide a gradient in moisture, we might expect to see a shift toward dry prairie species and away from wet prairie species. This shift could reduce the overall diversity of a prairie, potentially favoring more opportunistic species over more conservative species.
Droughts could potentially improve the chances of species that currently occur south and west of the Chicago Wilderness region to survive well here. Ultimately, however, there are multiple requirements necessary for a species to do well in a new region, and it may be that only the most opportunistic species will be capable of successfully shifting their range.
The impact of more frequent droughts on plants would inherently have a cascading effect on the wildlife that depend on them. In the Chicago Wilderness region, for example, the pipevine swallowtail (Battus philenor) exclusively uses pipevines (Aristolochia) to lay eggs. A study looking at the effect extreme weather change had on oviposition rates of pipevine swallowtails found rates to be significantly lower during the dry period (Papaj et al. 2007). This was mainly due to a lower density and quality of pipevines during extreme dry periods. By extension, more severe droughts may be associated with more markedly reduced rates of oviposition of this species. If chronically low rates of oviposition translate to chronically low population levels, then they may be at a higher risk of population extinction, a documented consequence of climate change in butterflies and other organisms (Papaj et al. 2007).
4.7 Floods: Potential to impact Prairie Grasslands, Savannas/Wooded Communities, Wetlands, Streams/Rivers, Lakes
Precipitation patterns for this region are expected to shift toward an increased frequency of extreme rainfall in a short period of time (i.e. deluge rains) combined with greater incidences of consecutive rainfall of events (1–2 inches) that results in heavy saturation of soils. Increased flooding would result in greater deposition of sediments into regularly flooded areas and, because places subject to regular severe flooding tend to have poor diversity especially in the herbaceous layer, it’s likely that few species will be able to tolerate those conditions, and those that are likely be opportunistic weedy species.
Increased floods would also result in more areas of standing water in prairies, savannas, forests, and wetlands near stream channels and potentially favor species that can withstand wide fluctuations in water availability. More flooding resulting from an increased frequency of extreme rain events, and associated increase in runoff, would also contribute to greater nutrient loading of streams, rivers, and lakes. It is important to define the control that changing hydrology will have on organic carbon, nutrients, and soil exported from the land in order to understand how streams rivers and lakes will respond to this aspect of climate change. In mixed land use watersheds, short duration, high discharge events are responsible for most of the introduction of soil derived materials to our waterways (Dalzell et al., 2007).
It is important to bear in mind that this region will experience changes in both precipitation and temperature, which together can result in a non-intuitive climate shift. First, more of the region’s precipitation is likely to come down in fewer, but more extreme, rain events that will increase the incidence of flooding and stormwater runoff. This will in turn cause a greater percentage of water to leave the region and reduce the amount available to recharge groundwater levels. Secondly, temperatures will continue to increase, leading to higher rates of evapotranspiration. The combined effect, however, is likely to be less water available in the system annually. So, even though flooding may increase, the overall effect of anticipated precipitation and temperature changes could actually be a drier and warmer environment.
4.8 Scouring/erosion (water, ice): Potential impacts to Streams/Rivers, Lakes and Lakeshores
More severe storm events may lead to more scouring, possibly creating more incised stream channels in rivers and streams. Scouring can also affect the herbaceous communities in the floodplains of rivers and streams. Removing existing vegetation can cause the herbaceous layer to be dominated by a small number of early successional species either tolerant of regular disturbance, or able to quickly re-colonize. In lakes, more severe storm events can increase sand and gravel loss along lakeshores, re-shape shorelines, and prevent vegetation establishment.
4.9 High winds: Potential impacts to Savanna/Wooded Communities and Lakeshore
Along with increases in air and water temperatures, wind speed has also increased in Lake Michigan over the last several decades (Table 1 in Austin and Coleman, 2007). If wind patterns were to continue to increase, resulting in more extensive and stronger straight-line winds, then storm damage to large, mature trees could increase. The loss of large canopy trees is particularly concerning in many of our region’s degraded forests, woodlands, and savannas, where the subcanopy and sapling trees are mostly weedy species and few native species exist to fill the gaps created when these mature trees are damaged in storms. The most susceptible species would be those with brittle wood, such as maples (although large stout trees like oaks would not be immune), and those in wetter areas that have shallow rooting and more windthrow.
High winds can result in increased windthrow in forested communities. The effects of high straight-line winds should be most severe at edges of wooded patches on the windward side. It could be that the relatively isolated trees in savannas could be more susceptible to wind damage. Interior trees in woodlands should see less damage, and less effects of increasing storm intensity. Beaches tend to accumulate less sand during windy spring and summer periods and lose sand through erosion during strong fall and winter storms.
Whether tornados or microbursts will increase in abundance and severity as a result of climate change is unclear (Trapp et al., 2009). However, if they do, it does not seem likely that there would be changes in habitat or species patterns associated with storm damage from tornados. Tornados and microbursts are a mechanism for disturbance, and this means in the short-term opportunistic/weedy species will be able to thrive after a tornado hits. This disturbance in the middle of a forest would also create more edge habitat, which then may be affected more by straight-line winds. In the long-term, stronger windstorms could possibly increase dispersal distances in wind-dispersed species and help with species migration, but because most seeds fall very close to the parent plant, even in small-seeded species, the mean dispersal distance would not change dramatically in the short-term. The species most likely to show increased dispersal distances due to increased windstorms are likely to be easily-dispersed weedy species rather than more conservative species.
(Barry et al., 2007). As temperatures increase, we could see increases in freeze-thaw cycles that would otherwise be moderated by a more typical snow cover. Thawing changes the structure of snow and reduces its insulating properties, increasing the potential for frost penetration into the soil and root damage to certain plant species (Barry et al., 2007). Spring deluge, which is key for exporting C and N-laden nutrients to rivers and streams, is also likely to occur earlier and as a smaller event due to earlier spring warm-up and less persistent snow cover.
4.10 Persistence of snow cover: Potential impacts to Prairie Grasslands, Wetlands, and Savanna/Wooded Communities
Less snow cover and lack of frozen ground can limit management and ecological restoration activities that are best carried out under frozen ground and snow cover conditions to avoid soil damage. Overall the decrease in duration and depth of snow cover can be expected to exacerbate other stresses to terrestrial ecosystems associated with climate change.
In the lake effect region of Chicago Wilderness (mainly Indiana lakeshore counties—Lake, Porter and La Porte, and Berrien county in Michigan), more limited ice cover on Lake Michigan could result in increased lake effect snow in the next few decades. This could result in deeper, more persistent snow cover in these parts of the region. The specific predictions will depend on the details of winter weather patterns. Important parameters include temperature and wind patterns. One study examined the correlation between air and waters temperatures, ice cover, and lake effect snow in Cleveland, OH and Buffalo, NY since 1950. The analysis showed that in years with above normal summer and fall temperatures, lake waters remained warmer into the winter. This effect combined with decreased ice cover will likely increase lake effect snowfall as long as mean winter temperatures remain below freezing (Ferian, 2009).
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