Range margins and refugia by Johan Dahlberg

Plants & Ecology

The Department of Ecology, 2013/1 Environment and Plant Sciences Stockholm University

Range margins and refugia by Johan Dahlberg

Supervisors: Kristoffer Hylander & Johan Ehrlén

Plants & Ecology

The Department of Ecology, 2013/1 Environment and Plant Sciences Stockholm University

Plants & Ecology

The Department of Ecology, Environment and Plant Sciences Stockholm University S-106 91 Stockholm Sweden

© The Department of Ecology, Environment and Plant Sciences ISSN 1651-9248 Printed by FMV Printcenter Cover: Lathyrus vernus at a south facing slope, a refugium in the middle of Sweden (the county of Ångermanland), north of its main distribution range. Photo: Johan Dahlberg, 2012.

Sammanfattning Arters utbredningsområden är begränsade av fysiska barriärer, arters spridningsförmåga, abiotiska faktorer såsom klimat samt interaktioner mellan arter. Hur en art reagerar på miljöfaktorer och biotiska faktorer bestämmer dess abundans, utbredning och avgränsning av utbredning. Det finns fyra olika typer av respons på globala klimatförändringar. En population kan acklimatiseras, anpassas, migrera eller dö ut. Under ogynnsamma förhållanden kan populationer migrera och överleva i refugier med gynnsamma miljöförhållanden. Därefter kan de återvända till deras förra utbredningsområden när förhållandena återigen har blivit gynnsamma. Begreppet refugium har använts för storskaliga refugier såsom interglaciala och glaciala refugier (makrorefugier) samt för småskaliga refugier såsom mikrorefugier. Mikrorefugiers existens gynnas av att det lokala klimatet frikopplas från det regionala klimatet eftersom detta buffrar mot klimatförändringar. Sådan frikoppling sker mest sannolikt i heterogena landskap. När en art flyttar sitt utbredningsområde så kan nya refugier bildas vid den retirerande utbredningsgränsen. Makrorefugier kan bildas från artens huvudutbredningsområde om det minskar.

Summary Species ranges are restricted in distribution by physical barriers, dispersal ability, abiotic factors such as climate and interspecific interactions. The responses of a species to environmental conditions and biotic factors determine its abundance, distribution and range limits. There are four possible responses for populations facing global climate change. They can acclimate, adapt, shift their ranges or go extinct. During unfavorable conditions populations may shift their ranges and survive in refugia with favorable environmental features. Thereafter they might be able to return to their former distribution when the conditions get favorable again. The term refugium has been used for large scale refugia such as interglacial and glacial refugia (macrorefugia), and for small scale refugia such as microrefugia. The existence of microrefugia is promoted by decoupling of the local climate from the regional climate because this buffers against climate change. Such decoupling is most likely to occur in heterogeneous landscapes. At a range shift, new microrefugia may arise at the eroding edge, while macrorefugia may form from a contracting main (continuous) range.

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Introduction

An evolutionary perspective In earth’s history there has been several periods with large species turnover, old species has died out and new species has replaced the former (Willis & McElwain 2002). In fact, five mass extinction events for the marine faunal record over the last 600 million years are clearly defined (Raup & Sepkoski 1982), accompanied by high extinction numbers in the terrestrial faunal record (Willis & McElwain 2002). Gould (1985) even proposes that mass extinction is the punctuating events leading to speciation and so drives macroevolution in the animal kingdom. Among plants, Willis and McElwain (2002) recognizes five major periods of evolution and species turnover where the fossil records indicates more originations over extinctions. They further suggest that increased atmospheric CO2 associated with each of five pulses of global plate spreading might be the underlying cause behind these main events of plant evolution.

The periods of great species turnover may have extraterrestrial causes or earth-bound causes. The former includes large impacts from meteorites, comet storms, radiation from supernovae and large solar flares, while the latter includes geomagnetical reversals, continental drift, massive volcanism, sea-level change, global cooling and glaciation, and decreasing levels of oxygen and/or salinity in the oceans. In excess of change in climate due to positional changes of the land masses, continental drift may lead to increased CO2 levels. This is due to increased mantle outgassing caused by increased volcanism and faster seafloor spreading (Willis & McElwain 2002).

These extreme events caused large numbers of extinctions through large scale disturbance, and subsequent, high rates of speciation has been observed. Extinctions may occur when populations cannot migrate or disperse due to the rate or magnitude of the disturbance (Dimichele et al. 1987). In contrast, populations exposed to smaller scale disturbance often can avoid extinction through migration, survival in refugia, or adaption (Willis & McElwain 2002). During such exposure to smaller scale disturbances species may shift their ranges during unfavorable conditions and return to their former distribution when the condition gets favorable again (Willis & McElwain 2002).

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Species ranges and refugia The causes and consequences of species exposure to disturbances, in particular the one that Willis and McElwain (2002) regard as small scale, will be further dealt with in this essay. It covers the characteristics of species ranges and its special case, refugia, where populations can survive during range contraction. To understand the characteristics of refugia it is necessary to have knowledge of the wider concept of species ranges. In the context of climate change and refugia it is especially interesting to further investigate the properties of range shifts. How can species cope with climate change by shifting their ranges, adapt or acclimate? How and where may refugia arise through range shifts?

Geographic ranges

Climate and other limiting factors Species ranges are restricted in distribution by physical barriers, dispersal ability, abiotic factors such as climate and interspecific interactions (Gaston 2003). Physical barriers may be rivers, particularly large and fast-floating rivers, mountain chains (Gaston 2003), oceans, human development (Mott 2010), topographic features in the sea (McClain & Hardy 2010), transitions between freshwater and saltwater (Mott 2010), ocean currents (Won et al. 2003), land masses (Vinogradova 1997) and large distances (Wright 1943). Physical barriers may also be of periodical character, for example volcanic eruptions, flooding or fires (Mott 2010).

Physical barriers may influence the limitation of species ranges either directly or, more commonly, indirectly through the spatial, climate gradients that accompany them (Mott 2010). These gradients may limit the ranges due to constraints in the environmental tolerances or dispersal abilities of the species. The either direct or indirect effect of physical barriers on dispersal can be range limiting for species with different dispersal capacities. For species with poor dispersal ability, both small and large barriers may be limiting while species with an effective dispersal may be restricted by large barriers (Gaston 2003; Mott 2010).

According to Gaston (2003) evidence for climate limiting of distribution can be divided in two major parts. First, species ranges often coincide with specific climate conditions and second, temporal range shifts often track changes in environmental conditions.

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There have been many observations of how distributions are limited by climatic factors (Gaston 2003). The first work that elucidated the role of climatic factors in restricting species ranges was written by Grinnell in 1917. Often several climatic variables are significantly related to the distribution pattern of a species. There are hundreds of possible combinations of climatic variables; for example minimum, maximum and mean temperature (Gaston 2003). For example the distribution of Ilex aquifolium is limited by the combination of mean temperature in the coldest and warmest months. Additionally, this species is only present if mean temperature of the coldest month exceeds -1 degrees Celsius (Iversen 1944). Other evidence for climate as a range limiting factor are the observations of species that are restricted to refugia with locally favorable microclimate, outside their continuous distribution range, and species that decreases with depth or elevation towards higher latitudes. Also, there are species that increasingly favors north or south facing slopes with changing latitude and thereby maintains the experienced climatic regime (Gaston 2003).

There has been reliance in the climate as an important factor for species distribution. Several studies have tried to reconstruct past patterns in climate by use of historical plant distributions (Gaston 2003). Plant distributions have also been used to correct climate maps of recent years (Boyko 1947).

The reliance of climate as a factor determining species ranges is coupled with several problems. First, many climate variables covary. Second, the microclimate experienced by a species may not be the same as the climate measured by weather stations. Third, there may be indirect effects of climate on the distribution of species. This is explained by the fact that climate factors may influence the distribution of resources, competitors, predators or parasites which in turn affect the distribution of the studied species (Gaston 2003).

To assess the climate as a range limiting factor and exclude the possibility of a merely coincidence between the climate and range limits of species, it can be appropriate to demonstrate that the climate is unsuitable beyond the range limits of the species (Gaston 2003). Hutchins (1947) described two ways in which the climate may be unsuitable. Due to environmental tolerance, it may directly kill individuals outside the range limit. Even though many species tolerates larger range of climate conditions then encompassed by their normal ranges, range limited by lethal climate factors has been observed (Gaston 2003). Additionally, the climate could be unsuitable for the reproduction of the species or for stages in their life

6 cycle. Range limiting by climate unsuitable for reproduction or stages in the life cycle are more common than limiting by mortality of adults due to climate (Gaston 2003). For example, minimum temperature during the growing season is an important determinant of seed mortality of forest trees in Sweden (Blennow & Lindkvist 2000). Plant species that at their range limits fails to reproduce sexually, may survive by vegetative reproduction or since they are long-lived (Gaston 2003). Therefore, the geographical difference between the reproductive and distributional limit is likely to be bigger for long-lived than for short-lived species. This can be demonstrated by the long-lived, broadleaved tree Tilia cordata, for which the reproductive limit in UK is about 200 km south of its distributional limit. In contrast, in an annual species with a short-lived seed bank, it is probable that the distributional and reproductive limits are more evenly matched. For the long-lived species, the current northern distributional limit probably reflects a previously warmer climatic period (Woodward 1997). Following this, the probability that past events reflect the current range of a species increase with longevity (Gaston 2003).

Hutchins (1947) especially considered temperature as the limiting climate factor. This is generally seen as the most important climatic factor limiting species ranges (Gaston 2003). For example, frost damage may have both short- and longterm effects on plants, as well as evolutionary effects (Inouye 2000). Even though climate is an important range limiting factor it is not obvious whether it is a primary or secondary cause of the range limits. It may be that another factor than the climate has been crucial for determining the range limits of a species. By evolution the species may have lost the physiological requirements to exist in the climate outside its range limits, especially if the maintenance of these requirements is costly. This leads to a tendency of coincidence between tolerated and experienced conditions (Gaston 2003).

Beside temperature there may be a range of other abiotic factors that could determine the distribution of a species. For example, fire and other disturbances, trace metals, pH, salinity, water availability, soils, geology, content of carbon dioxide and light (Gaston 2003). Water balance is seen as critical for the distribution of vegetation, both at larger and smaller scales (Gaston 2003). Furthermore, limitation by interspecific interactions may occur.

First, there is limitation of consumers by resources. For example a specialist consumer that is restricted in distribution to the range of its host. Mostly, the range of the consumer is smaller

7 than the range of its host. This may be due to limitation by environmental factors (Gaston 2003) or low abundance and small size of host populations close to range margins (Kawecki 2008).

Second, already Darwin (1859) noticed that competitors may limit the range of species. Connor and Bowers (1987) stated that interspecific competition plays an important role in species distribution across different spatial scales. In fruitfly experiments it has been shown that interspecific competition alters the distribution and abundance of the species (Davis et al. 1998). Bullock et al. (2000) showed that co-occurrence of two species of Ulex disappeared when studied at finer scales. This shows that the scale of the analysis is important when determining the cause of range limitation (Gaston 2003). Additionally, MacArthur (1972) argued that many northern species are restricted to spread towards the south because of competition. Hampe and Jump (2011) summarizes that interspecific competition often is stressed to be the most important biotic factor influencing retraction of species low-latitudinal range margins. However Gaston (2003) doubts that interactions (competitors or consumers) in general are sufficient for determining range limits. Studies of palaearctic mammals reveals that temperature range and habitat size (e.g. biomes) may be the most important range limiting factors (Letcher & Harvey 1994). The combination of climatic factors and competition might be a common cause of range limits. The role of competition cannot be excluded even if the range limits coincide with specific climate conditions (Gaston 2003).

Third, limiting of the consumed by predation or parasitism is often unlikely because the existence of the predator or parasite relies on their prey or host. It is unlikely that the host or prey goes extinct before the predator or parasite goes extinct or switches resource (Gaston 2003). However, Hochberg and Ives (1999) showed with a mathematical model that a natural enemy can enforce a range limit. They also observed that a predator may limit its prey by strong influence on its reproductive rate. Further on, the density of one of two prey species can depend on the rate of predation. During a high rate of predation, species a can be a stronger competitor for resources and therefore outcompete species b. Conversely, during a low rate of predation, species b may be dominant over species a (Holt & Barfield 2009). Additionally, the struggle to avoid the predator can be regarded as interspecific competition (Williamson 1957). Because predators/parasites may be lacking at range edges (Hodkinson 1999) it is more likely that the prey or host can avoid being consumed in these areas (Gaston 2003). Total exclusion by predator/parasites is more likely if the prey or host has limited

8 dispersal abilities (Gaston 2003). Lastly herbivores may influence the distribution of their resources (Gaston 2003).

There has been much debate over the relative importance of abiotic factors and biotic factors for range limits (Gaston 2003). On one hand, abiotic factors may lead to stress at range edges, reduced competitive ability and thereby increased vulnerability to biotic factors such as competition or pathogens. On the other hand, individuals often are scarce at range edges which render small effects from biotic factors. Additionally, facilitation, i.e. positive interactions between organisms, may enlarge the realized niche of a species, sometimes even to a potentially larger size than the fundamental niche (Bruno et al. 2003).

According to Salisbury (1926) many factors limit species ranges in contrast to the ideas of Twomey (1936) that only very few factors limit species ranges. Gaston (2003) expand upon Salisburys statement. First, range limits may be determined by interactions of many factors. Second, there may be a temporal variation of factors limiting a range. Third, it is almost certainly a norm that the range limiting factors may be different from one area to another. Fourth, the species characteristics may vary over space because the range size may be larger than the dispersal distances. In conclusion, both abiotic and biotic factors are important for setting the range borders of species.

Austin et al. (1984) describes three kind of environmental gradients: a. indirect or complex gradients (e.g. altitude) – other factors that covary with these have the direct influence on growth. b. direct gradients (e.g. pH) – factors with direct effect on growth. c. resource gradients – factors that are used as resources. The response of species abundance to environmental gradients is known as response curves. Abundance of species tends to peak at intermediate, favorable levels of these gradients (Gaston 2003). In other words, they have hump-shaped response curves. More realistic, the response curve consists of several abundance peaks which declines towards the range margins.

The spatial distribution of abundance is determined by the species optimum response surfaces and their local population dynamics. The optimum response surface can be regarded as the abundance structure of a species range that reflects the environmental gradients in all directions from the point of highest abundance (Hengeveld 1990). Where the environment is favorable the abundance is high and vice versa. The abundance structure will change with

9 spatial and temporal variations in the environmental conditions. Therefore, many different shapes of the abundance structure may occur due to the sensitivity of species to specific environmental conditions. The optimum response surface model has several assumptions which may be violated. For example they assume that the population density is coupled with favorable habitat conditions. In fact, there may be a decoupling between some habitat conditions and abundance, especially if the environment is strongly seasonal, temporally unpredictable or spatially patchy, and for generalists, long-lived species or species with high reproductive capacity (Gaston 2003). Also, the source-sink dynamics can lead to high abundance in the low quality sinks (Pulliam 1988). Another assumption is that species response to environmental factors is constant in space. It has been shown that there may be adaption and selection as a consequence of abiotic and biotic conditions, especially during range expansion (Hewitt 2000). In addition, the assumption that ecological conditions become less favorable towards range edges may not be true. According to Soulé (1973) not all peripheral populations experience less favorable conditions and not all populations with less favorable conditions are located at range margins.

In conclusion, the responses of a species to environmental conditions and biotic factors determine its realized niche. They also set the species abundance, distribution and range limits. Additionally, these structures are influenced by source-sink dynamics and spatial variation in the influence of these conditions on individuals with different genetics (Gaston 2003).

Edge processes At range edges species often resides in specific habitats or microhabitats. This might be due to a declining number of individuals towards the range edges and therefore by chance in a smaller number of habitats, frequently in the most widely distributed habitats where the species can exist. Sometimes, these habitats are atypical for the species compared to the habitats occupied elsewhere. For, example Picea rubens occurs in bogs and adjacent upland slopes at its lower elevational range margin, although bogs is an unusual habitat in its main range (Webb et al. 1993). This might reflect changing conditions at the range margin, either in the typical or in the atypical habitats of a species, or it might reflect the absence of typical habitats at the range margin. Further on, there are many observations that habitat shifts at range edges are systematic. For example, on the northern hemisphere there can be a shift in distribution to south facing slopes with favorable microhabitats at the northern range limit of a

10 species. Microhabitats are maybe most apparent in transitions zones, areas where several range margins meet. Therefore they possess many microhabitats and high species richness (Gaston 2003).

However, local abundance of species may not decrease towards the range edge, instead they might be more patchily distributed. Also, it is unlikely for the abundance to decline towards range edge for expanding populations, for populations where individuals move unidirectional and for populations disrupted by physical barriers (Gaston 2003).

The limiting factors at range edges affect the vital rates of the populations such that for most of the time the criterion below isn’t met, i.e. the result of b – d + i – e is smaller than zero instead of larger than or equivalent to zero. b is the number of births in the population, d is the number of deaths, i is the number of immigrants and e is the number of emigrants. Small changes in these variables may determine range edges (Gaston 2003). b – d + i – e ≥ 0

According to several papers, the population dynamics towards range edges are dependent on abiotic factors and less density-dependent (Gaston 2003). Gaston (2003) concludes that less density-dependent populations at range margins might possess greater temporal variation in population size. This in turn could lead to greater extinction risk for margin populations, a circumstance that has been commonly observed (Gaston 2003).

Multiple populations or metapopulation structure are more likely at range edges because of more fragmented abundance structures. Further on, the number of establishments minus the number of extinctions is less than zero at and beyond the range edge (Gaston 2003). This relationship has been further analyzed by Carter and Prince (1981) as follows. For the species to spread, the rate of colonizable sites that becomes colonized must be positive (for the species to survive, the rate must be equal to or greater than zero). This can be written as do / dt > 0 where o is the number of colonizable sites that are occupied.

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Further on, this requires that a ≥ δ / β if do / dt = βao – δo where a is the number of unoccupied colonizable sites available, β is the rate at which colonizable sites become occupied and δ is the rate at which colonized sites cease to be occupied.

Building on Levins metapopulation model (Levins 1969), Holt & Keitt (2000) shows that a species will be absent from all points along a smooth environmental gradient x if k(x) < e(x)/c(x) where k is the fraction of potentially colonizable sites, e is the extinction rate (per colonized site) and c is the colonization rate of empty, colonizable sites (per colonized site). Even if the conditions are held constant a range limit could be reached if the number of colonizable sites declines (Holt & Keitt 2000). Furthermore, species with relatively lower extinction rate may survive closer to unfavorable climate where there are relatively fewer potentially colonizable sites (Gaston 2003). A range limit will arise if the colonization rate declines while k and e are held constant (Holt & Keitt 2000). Declining colonization rate may be due to reduced population size with fewer potential colonists and reduced successful dispersal by less favorable environmental conditions (Holt & Keitt 2000). Gaston (2003) concludes that changes in whichever of these three variables (k, e and c) may result in a range limit and that all three variables need to be studied to reveal the cause of range limitation.

Evolutionary adaptions that allow for range expansion without changes in the environmental conditions seem to be rare (Eldredge 1997). There may be several reasons why a species doesn’t expand its range beyond the limits set by abiotic and biotic factors (Hoffmann & Blows 1994; Hoffmann & Parsons 1997). First, there may be low heritabilities of the traits that determine range boundaries. In marginal populations this might be explained by low

12 genetic variation due to small population size, directional selection or large environmental variability. Second, this may be explained by genetic interactions as follows. Evolution of multiple traits is required for range expansion due to covariance in limiting environmental factors. Also, there can be genetic trade-offs between fitness in stressful and favorable environments. The possession of genes that favors species in stressful conditions can be costly in the more frequent favorable conditions. Third, the effect of deleterious mutation may be enhanced under stressful conditions (Gordo & Campos 2008), although the opposite has been showed by Kishony & Leibler (2003). In addition, genotypes favored at range margins may be replaced by gene flow from central range populations (Kirkpatrick & Barton 1997). However, Gaston (2003) claims that this gene flow seems too small for such a replacement of genes.

Climate change and range shifts There are four possible responses for any species facing global climate change. They can acclimate by phenotypic plasticity (Pfennig et al. 2010), adapt through genetic evolution (Davis & Shaw 2001), shift their ranges (migrate) to follow the shift in favorable climate conditions (Parmesan 2006) or go extinct. The term range shift has been used for expansions, contractions and movement of species ranges (Parmesan et al. 2005; Sorte et al. 2010).

The earth has faced frequent climate change during its history. During the Pleistocene there has been a time of constant change in climate with glacials and interglacials shaping the landscape and species ranges (Beatty & Provan 2010). According to fossil pollen data and macrofossil records there has been major range shifts during the glacials and interglacials of the Quaternary period (Petit et al. 2008). Climate change between glacials and interglacials has resulted in both latitudinal and altitudinal range shifts (Hewitt 2004). Many species have migrated during glacials, survived in refugia, and later recolonized previous occupied areas during subsequent interglacials. In fact, migration has been the usual response to climate change during the Quaternary (Mott 2010).

Also, the recent anthropogenic climate change has leaded to observed range shifts in both animal and plants (Parmesan 2006). Many species preferring cooler climate have shifted their distributions towards higher latidudes or altitudes (Parmesan 2006). Already in the 1940s, Ford (1945) observed northwards range shifts in butterflies in England due to warmer summer

13 temperatures. During anthropogenic climate change the range shifts are expected to occur towards the poles or towards higher altitudes (Parmesan 2006).

Models of future scenarios predict distribution shifts during the decades to come (Parmesan & Yohe 2003; Thuiller et al. 2008; Pereira et al. 2010). However, caution is needed because there is an individualistic response to climate change among species (Gaston 2003). Because of climate change, but maybe not only for this reason, species assemblages vary over time (Gaston 2003). Also, range shifts with other causes than temperature has been observed. For example, tracking of water balance may be the reason for range shifts among Californian mountain plants (Crimmins et al. 2011).

The colonization process during climate change can be described by the leading edge model of colonization. In this model the expansion of the species range is mostly controlled by long- distance dispersal from the range front. The subsequent population growth is exponential (Hampe & Petit 2005).

Refugia

Different types of refugia, what do we mean? The concept of refugia was originally used for glacial refugia, locations where species survived the glacial periods during the Quaternary (Blytt 1876; Warming 1888; Sernander 1896; Lindroth 1931; Dahl 1946; Heusser 1955; Rundgren & Ingólfsson 1999; Brochmann et al. 2003; Marko 2004; Schönswetter et al. 2004; Bennett & Provan 2008). Heusser (1955) was first in using the latinized form of the word refugia (Bennett & Provan 2008) when he searched pollen profiles in peat for plants surviving the last glacial on the Queen Charlotte islands in Kanada. Later on the usage of the term refugia has been widened. Billings (1974) and Mac-Vean and Schuster (1981) used the term interglacial refugia. At around the same time the term was introduced for areas that escape fire and to which species later can re- colonize (Zackrisson 1977; DeLong & Kessler 2000; Clarke 2002; Hylander & Johnson 2010) and for habitats in streams protected from disturbances such as fast flowing water, dry periods and ice cores (Williams & Hynes 1974; Sedell et al. 1990; DoleOlivier et al. 1997; Matthaei et al. 2000; Taylor et al. 2006). In later years, the term has also been used for refugia from current climate change, areas where conservation should be prioritized to limit the effects of

14 climate change (Ashcroft 2010), and for refugia from other forms of stress and disturbances (Schmalholz 2007). Further on, different locations of refugia can be addressed as in situ or ex situ refugia (Fig. 1). An in situ refugium constitute a part of the previous, wider distribution range of a population. An ex situ refugium on the other hand, is located outside the previous, wider distribution of a population, i.e. a place where the species didn’t occur before. Both of these expressions has been used for glacial refugia and for potential, future refugia (Holderegger and Thiel-Egenter 2009; Ashcroft 2010).

Figure 1. An in situ refugium is located in the former distribution range of the species, while an ex situ refugium is located outside the former distribution range.

This broadening of the term refugia and the inconsistency in its definition has potentially decreased its usefulness (Bennett & Provan 2008). Therefore, this essay describes different contexts in which the term refugia have been used. This information can be valuable when using the term in specific studies.

It is clear that refugia may arise when species ranges contracts. Often this is associated with the expansions and contractions of species ranges during Quaternary glacial and interglacial periods (Stewart et al. 2010). Refugia may also arise during even longer time periods of climate change caused by for example plate tectonics (Ledru et al. 2007), or as a consequence of range contraction during the ongoing climate change (Ashcroft 2010). The reason for the contraction is most likely to be some sort of disturbance at varying scales (Schmalholz 2007) which makes the environment, except for the refugia, unsuitable for occupancy.

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In other words, the definition of refugia/refugium mostly aims at the area where the species remains after contraction (Sedell et al. 1990; Camp et al. 1997; Schmalholz 2007; Rull 2009). In addition, the definitions of Sedell (1990) and Schmalholz (2007) includes that refugia house populations that are important for re-colonization of the earlier occupied habitats. Lancaster and Belyea (1997) have a broader view by stating that a refugium is a place or time where the negative effects of disturbance are lower than in the surrounding area or time.. In contrast, Bennett and Provan (2008) suggests that the term refugia should be replaced by the term bottleneck when aiming at the abundance of species. The authors think that this term better describes the continuing process that species goes through when contracting to a refugium before later re-colonization of the previous occupied area. Stewart et. al. (2010) excludes bottlenecks from their definition. They mean that bottlenecks don’t entail range contraction but may result from it. Stewart et. al. (2010) points out Quaternary refugia and defines these as “the geographical region or regions that a species inhabits during the period of a glacial/interglacial cycle that represents the species’ maximum contraction in geographical range”. In this definition it is added that refugia should be the area under maximum contraction. Hampe and Jump (2011) defines refugium as an area with a climate relict population, a population that persists in an area with suitable climate space surrounded by unsuitable climatic conditions. He points out microrefugia as a small area with local favorable environmental features, in which small populations can survive unfavorable periods outside their main distribution range, protected from the unfavorable regional environmental conditions.

It seems for me that a suitable definition for the term refugia could be: refugium is an area with favorable environmental features, in which a population can survive after contraction of its range due to unfavorable environmental conditions in the surroundings. This definition resembles Rulls (2009) definition of microrefugia which is presented below (see Microrefugia). The differences are that all sorts of refugia is included in the presented definition. Also, it is clarified that a refugium should originate from a contracting phase. This follows both Rull (2010) and Stewart et al. (2010). In contrast to them, I suggest that refugia also can be expanding by migration or dispersal of the populations. ‘Population’ is used in the definition to exclude individual shelters from for example predation which instead might be called refuges (Ashcroft 2010). It is also clarified that the unfavorable, environmental conditions are the cause of the isolation. This excludes populations that have been isolated due to other factors than unfavorable conditions, for example due to recent dispersal.

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Due to the various meanings of the term refugia in different studies, Bennet & Provan (2008) thinks that we need to use more descriptive terms and clearly defined contexts. For example terms such as interglacial microrefugia (Rull 2009) or microclimatic refugia (Trivedi et al. 2008) might be used. I agree with clarifying the context but it could be contra productive to use more terms. A solution proposed by Rull (2010) is to, instead of terms such as microrefugia and relicts, always use the term refugia (either glacial or interglacial) to avoid confusion.

In short, the term refugia and its different forms have been used at many different spatial scales. Descriptions of some large and small scale refugia follow. Also the climatic prerequisites of microrefugia and their role at range shifts are discussed. Only terrestrial refugia are considered.

Large scale refugia

The Milankovitch cycles: For at least 200 million years (Jurassic) and probably throughout geological time there have been regular intervals with changes in the amount and distribution of incoming solar radiation (Willis & McElwain 2002). These cyclic intervals are called Milankovitch cycles (Milankovitch 1930) and occurs at intervals of around 413, 100, 41, and 24-19 thousand years (Hays et al. 1976; Berger 1977; Fischer 1986; Imbrie et al. 1993). The reduction in solar radiation at these intervals leads to global climatic change, at some intervals with cooling and formation of ice caps by the poles (Bradley 1999). There are different climatic/geological phenomena that regulate the impact of the Milankovitch cycles, and during some periods there were no buildup of ice caps (Frakes et al. 1992; Willis & McElwain 2002). These different impacts may be related to much larger intervals of approximately 300 million years characterized by ‘icehouse’ or ‘greenhouse’ mode (Fischer 1981, 1982; Frakes et al. 1992) within which the Milankovitch cycles are nested (Willis & McElwain 2002). During the last 2 million years (the Quaternary) the Milankovitch cycles have been expressed as glacial/interglacial cycles (Bennett 1990).

Glacial refugia Glacial refugia are places where species survived the ice ages. Mostly the term has been used for the most recent glaciation (Holderegger & Thiel-Egenter 2009). Originally, glacial refugia

17 were identified with palaeoecological evidence (Huntley & Birks 1983; Bennett et al. 1991), but since around 1990 also phylogeographic studies has been used in the identification (Hewitt 1996, 1999; Hewitt 2000; Bennett & Provan 2008). Together with the fossil record they have showed that many extant populations of primarily temperate taxa are derived from southern refugia (Hewitt 1996; Taberlet et al. 1998; Hewitt 1999; Hewitt 2000; Jackson et al. 2000; Lacourse et al. 2005; Soltis et al. 2006). Often these glacial refugia comprise the southern part of the interglacial distribution of the species, in Europe mainly mountainous regions within the Balkan, Iberian and Italian peninsulas (Bennet et al. 1991; Stewart et al. 2010) and in southeastern U.S. (Jackson et al. 2000). Compared with Iberia and Balkan, the role of the Italian peninsula as a source for recolonization is limited due to the barrier exerted by the Alps (Taberlet et al. 1998). Glacial refugia in European, southern regions followed by interglacial re-colonization of northern areas are a general pattern for several species groups, for example plants, insects and vertebrates (Hewitt 1999, 2004). The forests of the current interglacial period became established in northern Europe due to expanding populations from glacial refugia (Anderson et al. 2006; Cheddadi et al. 2006). In recent years, analogous northern refugia have been recognized at the southern hemisphere (Byrne 2008).

The combining of genetic data with fossil records has also revealed glacial refugia closer to the ice cap. Probably trees survived in glacial refugia only tens of kilometres south of the ice sheet in North America (Cheddadi et al. 2006; Shepherd et al. 2007), while more cold-adapted species even survived in refugia north of the ice (Stewart et al. 2001; Pruett & Winkler 2005; Anderson et al. 2006). These higher latitude, isolated, glacial refugia are distinguished by Stewart et al. (2010) as cryptic northern refugia (see also Microrefugia).

Holderegger and Thiel-Egenter (Holderegger & Thiel-Egenter 2009) suggests three main types of glacial refugia for species currently living above the timber line in mountains.. These terms are Nunatak glacial refugia, Peripheral glacial refugia and Lowland glacial refugia. Nunataks are glacial refugia on mountain peaks above glaciers in the core of mountain ranges (Blytt 1876; Brochmann et al. 2003; Crawford 2008; Holderegger & Thiel-Egenter 2009). Peripheral glacial refugia are used for areas, located at the edge of mountain ranges, in which mountain species survived (Tribsch & Schonswetter 2003; Schonswetter et al. 2005; Holderegger & Thiel-Egenter 2009). Lowland glacial refugia were placed outside both mountain ranges and the ice sheet (Holderegger & Thiel-Egenter 2009).

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Interglacial refugia Studies have showed increase in population sizes of cold-adapted species during the beginning of the last glaciation (Stewart et al. 2010). Furtheron, several tundra and steppe species were more widespread during the last glaciation (Bennett & Provan 2008). This is the case for Dryas octopetala in Europe. This species was widespread throughout the lowlands of central and parts of northern Europe towards the end of the last glaciation. Now it has a disjunct European distribution restricted to the northernmost and alpine regions (Godwin 1975). Another example is the arctic fox Alopex lagopus that was distributed over large parts of central and northeastern Eurasia during the last glacial maximum. Now it is confined to the tundra regions of the northern hemisphere (Dalén et al. 2007; Bennett & Provan 2008). This supports the hypothesis that a high proportion of cold-adapted species contracts to refugia during interglacials. Today, many arctic species are restricted to interglacial refugia in the high-latitude, Polar Regions, for example muskox and arctic fox (Stewart et al. 2010). These species that are restricted to interglacial refugia during the current interglacial will be under additional stress when temperature increases due to climate change (Ashcroft 2010). Stewart (2010) divides interglacial refugia in polar refugia and cryptic southern refugia (see Microrefugia)

Small scale refugia

Microrefugia Rull (2009) distinguish the terms macrorefugia and microrefugia. He points out microrefugia as a small area with local favorable environmental features, in which small populations can survive unfavorable periods outside their main distribution range (the macrorefugium), protected from the unfavorable regional environmental conditions. In this definition macrorefugium (synonymous to refugia) is linked to glacial–interglacial alternation while microrefugium includes several different terms such as for example ‘microrefuges’, ‘cryptic refugia’, ‘northern refugia’, ‘isolated microhabitats’, ‘relict isolates’, ‘sparse stands’, ‘humid microsites’ and ‘interglacial refugia’.

Cryptic refugia can be viewed as small areas with sheltered topography and stable microclimate, mainly in montane regions, earlier considered as inhospitable (Bennett & Provan 2008; Stewart et al. 2010). They are scattered outside their macrorefugia during glacials or interglacials (Provan & Bennett 2008; Stewart et al. 2010). Cryptic northern

19 refugia for mainly temperate taxa are situated in glacial refugia north of their southern macrorefugia during glacials. Analogous, southern cryptic refugia for cold-adapted species are located in interglacial refugia at lower latitudes, presumably mountain areas, south of their northern macrorefugia (Stewart et al. 2010). Rull (2010) summarizes that cryptic refugia refers to our incapability of knowing the correct location of a given refugia.

Cryptic northern refugia were introduced as a possible solution to Reid’s paradox, the discrepancy between interglacial migration rates and the dispersal capacity of trees, because they might allow for local dispersal (Stewart & Lister 2001; McLachlan et al. 2005; Pearson 2006; Birks & Willis 2008; Provan & Bennett 2008; Dobrowski 2011). The detection of cryptic refugia with palaeoecological methods, especially with pollen data, might be difficult because of their local and small character (Willis et al. 2000; Birks & Willis 2008). Anyway, cryptic refugia have been indicated through palaeoecological methods, mainly with the help of macrofossils (Willis et al. 2000; Willis & van Andel 2004; Birks & Willis 2008), and their existence have been further supported through the use of phylogeography (Cruzan & Templeton 2000; Provan & Bennett 2008; Stewart et al. 2010). The rising evidence for cryptic refugia means that the migration capacity of deciduous trees might be smaller than previous thought. (Petit et al. 2008).

Climatic prerequisites for microrefugia Climatic factors may limit refugia either by directly limiting the refugial population, or indirectly by restricting antagonistic species or a biotic factor that is required for the survival of the refugial population (Hampe & Jump 2011). In this chapter I will focus on the climatic prerequisites for microrefugia.

Air temperature is the most important factor in mountain climate (Barry 2008; Dobrowski et al. 2009) regarding its influence on ecosystem processes (Dobrowski et al. 2009). Temperature is influenced by regional and local controls. Regional controls may be circulation patterns, distance to the sea and latitude while local patterns include slope and aspect, elevation, solar insolation, topographic convergence and others (Dobrowski et al. 2009). Often, elevation has been regarded as the most important factor controlling temperature in areas with complex terrain. However, Vanwalleghem & Meentemeyer (2009) showed that both topographical and ecological factors such as forest cover regulate the temperature.

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The finescale variation in solar heat transfer influence the temperature regimes in areas with complex terrain (Fridley 2009). The term topoclimate, derived by Thornthwaite (1953), is the in situ experienced climate by an organism, a sum of the regional and the local climates (Dobrowski 2011). Topoclimate and edaphic conditions sets the physical environment experienced by species (Dobrowski 2011), for example in microrefugia. The climatic prerequisites of microrefugia require knowledge of present and future regional climate, how regional climate limits species ranges, and how the terrain can modify the regional climate so microrefugia can persist (Dobrowski 2011).

According to Dobrowski (2011) there are several factors that promote decoupling of the local climate from the regional climate and by that also promote the existence of microrefugia. These are cold air drainage, elevation and slope and aspect. In addition, the decoupling may arise due to microtopography within a slope (Scherrer & Körner 2010), edaphic conditions, water table or vegetation characters (Spitzer & Danks 2006; Suggitt et al. 2011). A discussion of these decoupling factors follows.

Cold air drainage Convergent environments have lower minimum temperatures than the surroundings due to cold-air drainage and temperature inversions. Therefore, the local climate is decoupled from the regional climate at these sites. Due to the decoupling from the regional climate with low minimum temperatures, convergent environments are the most potential areas for microrefugia. This is also true for mesophilous species due to the accumulation of water and soil at the bottom of terrain depressions (Dobrowski 2011).

When the temperature cools down after sunset cold air drains into convergent environments, especially in calm weather (Geiger 1950; Whiteman 1980; Lindkvist et al. 2000; Dobrowski 2011). The temperature then increases with altitude, a phenomenon called inversion. Decoupling from the free atmosphere arises when the vertical mixing of air is absent (Daly et al. 2010; Dobrowski 2011). Cold air pooling is common in sheltered positions such as valleys, basins, sinks and other depressions in mountain areas (Dobrowski 2011). The magnitude and persistence of an inversion is determined by the amount of incoming solar radiation and to which extent the local climate mixes with the regional climate due to wind effects. These factors depend on topographical characters (Whiteman 1982; Muller & Whiteman 1988; Colette et al. 2003; Whiteman et al. 2004; Dobrowski 2011). Inversion mostly occurs in

21 nighttime, but could also last 2 to 5 hours after sunrise (Whiteman 1982; Muller & Whiteman 1988; Whiteman et al. 2004). The modeling of cold-air drainage and estimates of minimum temperature is improved by taking surface water accumulation factors into account (Dobrowski 2011).

Elevation Elevation in itself is not a good predictor for minimum temperature at sites where a varying topography affects the local climate in different ways (Lookingbill & Urban 2003; Dobrowski et al. 2009; Dobrowski 2011). Mountain peaks are exposed to wind and drained which leads to a climate that is more often coupled with the regional climate then at protected, wetter sites further down the slopes (Pepin & Seidel 2005; Dobrowski 2011). Therefore, without combining it with other topographic factors, elevation is a poor predictor of the degree of decoupling from the regional climate and thus of the location of microrefugia (Pepin & Seidel 2005; Dobrowski 2011).

Slope and aspect Slope and aspect exhibits varying exposure to incoming solar radiation and wind which are factors that influence air temperature and water availability (Dobrowski 2011). For example, south-facing slopes have often been addressed as location for microrefugia. The incoming solar radiation in the northern hemisphere is highest on southwest-facing slopes and lowest on northeast-facing slopes, which leads to gradients of air and soil temperatures, humidity and soil moisture (Hutchins et al. 1976; Xu et al. 1997; Hutchinson et al. 1999). Additionally, microtopography within a slope has very strong effects on local temperature due to varying microexposure (incoming solar radiation), cold air flows and snow cover (Löffler et al. 2006; Scherrer & Körner 2010).

It seems that the daily maximum temperatures are more controlled by incoming solar radiation than the minimum temperatures (Dobrowski 2011). This is due to that the daily minimum temperatures are foremost controlled by moving, cold air masses, and only indirectly by radiation (Bergen 1968; Manins & Sawford 1979; Fridley 2009). Therefore, populations in microrefugia on south-facing slopes are more limited by snow, ice and season- length then by minimum temperature (Dobrowski 2011). Furthermore, the cloud cover has a dampening effect on solar radiation, and also the solar radiation changes from short-wave to latent at wet sites due to high soil moisture and canopy cover (Dobrowski 2011). Therefore,

22 the slope and aspect has a bigger effect on local temperature in arid sites than in wetter areas (Fridley 2009).

The effect of slope and aspect on water availability is higher than on local temperature. The evapotranspiration (and the water balance) is affected by slope and aspect through varying shortwave radiation and wind exposure (Hupet & Vanclooster 2001; Gong et al. 2006; McVicar et al. 2007; Dobrowski 2011). Also, soil moisture and groundwater level is affected by slope and aspect (Hutchinson et al. 1999; Zinko 2006). Soil moisture and near-ground humidity influences transpiration and evaporation and thus the temperature. The daytime maximum temperature is lowered by soil moisture (Dai et al. 1999) because wet surfaces are dried out before warming (Fridley 2009). This limits the heat carrying capacity of the soil (Geiger et al. 2003). The dampening effect of soil moisture on nighttime minimum temperature is less pronounced (Dai et al. 1999), but soil moisture may buffer against cold extremes (Geiger et al. 2003). Mainly because of the large effect of slope and aspect on water availability and the decrease of short-wave radiation in mesic regions (Dobrowski 2011), slope and aspect are more important for the location of microrefugia in arid regions than in temperature limited regions. For example, the use of depressions as refugia during the glacial suggests water availability as a limiting factor instead if minimum temperature (Dobrowski 2011).

Also, the duration of the snow cover is influenced by aspect. After canopy cover, differences in site aspect is the most important factor for duration of the snow cover. The effects of these factors on snow cover are mainly driven by their influence on incoming solar radiadion (Coughlan & Running 1997). The effect of aspect on snowpack is most pronounced at lower elevations where the solar angle is lower early in the season (Coughlan & Running 1997). Duration of the snow cover is one of the factors controlling local temperature (Löffler et al. 2006).

Vegetation cover, water table and edaphic conditions The vegetation cover and the water content in plants can have a significant effect on temperature regimes (Fridley 2009). A forest canopy dampens the diurnal and seasonal temperature variation as compared to the variation in the free atmosphere (Fridley 2009). Additionally, it has been showed that higher vegetation height in heathlands reduces temperatures close to the ground (Thomas et al. 1999).

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Sites with extreme water surplus (such as high elevations and protected streamsides) may have a delayed warming in spring and dampened diurnal temperature variations (Shanks 1954; Fridley 2009), which indicates that they are less coupled to the regional temperature than drier sites (Fridley 2009). Also, small scale water bodies, e.g. springs, ravines, lakes or mires act as thermoregulators (Caissie 2006; Hampe & Jump 2011). On bogs, the Sphagnum and peat layer insulates the ground habitats. Along with the dampening effect of the water table this leads to a cooler microclimate than in the surroundings (Spitzer & Danks 2006).

The role of refugia at range shifts The picture of a species range in figure 2 shows the continuous range (main population) of a species distributed along an environmental gradient. When the environmental gradient changes, the species will shift its range towards more favorable environmental conditions, in this case towards the top of the paper which can represent either south or north. During this shift, the leading edge refers to the expanding edge, while the eroding edge refers to the rear edge. Both a leading and a rear edge is characterized by small, more or less isolated populations. At the leading edge these are the result of expansion (Hampe & Jump 2005), or

Figure 2. Example of a distribution range of a species across an environmental gradient. When this gradient change, the range of the species will shift its main population (continuous range) range towards more favorable environmental conditions (towards the top of the paper, either to the south or to the north). Both the leading and the rear edge are characterized by small isolated populations. At the leading edge these are the result of expansion, or previous refugial populations that expand, and at the rear edge these are refugial populations. The rear edge can be stable or trailing (modified from Hampe & Petit 2005).

24 they may be microrefugia from an earlier contracting phase that now expand. At a rear edge, these populations are located in microrefugia. The rear edge can be stable with small or altitudinal range shifts owing to a heterogenous landscape. Such landscapes might locally buffer the climatic change due to decoupling of the local climate from the regional climate (Dobrowski 2010). The rear edge can also be trailing with larger latitudinal range shifts (Hampe & Jump 2005).

The picture of a shifting species range in figure 2 is simplified in several ways. A continuous range is also limited to the west and to the east by one or several factors. This means that there may be refugia also in these directions. Furthermore, a range shift could take place towards other directions than north and south if there are changes in environmental gradients in these directions. In addition to shifts of both the rear and the leading edge towards one direction, there might be situations when there is only one edge of the shifting continuous range. In such cases, it is more suitable to use the terms expanding or eroding edge for the shifting range margin (see Fig. 3 for ranges with eroding edges only). Finally, if the environmental gradient inverse, the leading edge will become the rear edge and the other way around.

Let’s focus on the temperature as the main environmental gradient. Consider a species adapted to specific temperature conditions and consider it has both a leading and a rear edge. This species could be adapted to cold, warm or intermediate temperature conditions. When the temperature changes it may shift its range towards the more favorable temperature conditions, preferably towards one of the poles if it gets warmer or towards the equator if it gets colder. Notably, the leading range may also shift towards higher altitudes during climate warming or towards lower altitudes during climate cooling. Continuously, new isolated populations will form ahead of the leading range through colonization, to a largely extent controlled by long-distance dispersal (Hampe & Jump 2005). Whether these isolated populations are newly formed or earlier formed microrefugia, isolated populations at the leading range will expand into the surrounding areas, depending on where the conditions gets favorable, until they merge with the shifting continuous range. At this moment, populations will cease to be refugial. At the eroding rear edge, microrefugia with locally favorable temperature conditions may arise, both at a stable and at a trailing rear edge.

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Figure 3. Contracting species ranges with eroding edges but no expanding edges. a) This pattern might arise during a warming scenario for a cold-adapted species distributed around one of the poles, around a mountain-top or on a plateau. b) A similar pattern might also arise if the species is distributed along the equator.

This range shift might lead to the contraction of the continuous range. If so, the range forms one or several macrorefugium in addition to the microrefugia at the range edges. It is not obvious whether a shift towards more favorable temperature conditions leads to an expansion or a contraction of the continuous range (main range), or if its size is more or less stable. First, this depends on the characteristics of the leading edge, which includes the dispersal capacity of the species and to which degree the range expansion is hindered for different reasons. One kind of expansion limiter can be physical barriers such as mountain ranges, ice sheets and large water bodies. For example, the Mediterranean and the Sahara may hinder southward range shifts by temperate species during climate cooling. Another option is that the range expansion is limited due to abiotic or biotic factors (Gaston 2003; Mott 2010). Secondly, this also depends on the properties of the rear edge, i.e. if it is stable or trailing.

At the extremes, if there is only an eroding edge or only an expanding edge, the range will contract or expand respectively. These circumstances might prevail if the species is distributed on a plateau, around a mountain-top, around one of the poles, or along the equator. Two examples that comprise ranges with only eroding edges belong maybe to the most

26 obvious range shift scenarios. These are the range contraction and the subsequent arise of refugia for cold-adapted species when the climate gets warmer and for warm-adapted species when the climate gets colder (Fig. 3). The main reason for the former case might be that cold habitats around the poles or at high altitudes are likely to decrease during climate warming (Fig. 3a). Similarly, the latter case is explained by that warm habitats close to the equator are likely to decrease during climate cooling (Fig. 3b). One example is the rainforest cover in Africa that was reduced during the last glacial maximum due to cooler and drier climate (Leal 2004). Also bear in mind the higher species richness in the tropics compared to temperate regions (Wallace 1878). If species migrates towards the equator during climate cooling, there might be more biotic interactions with influence on the species ranges.

Furthermore, the rear edge characteristics of a cold-adapted species when a glacial is approaching will largely depend on the ice cap that builds up. If it is an arctic species, an advancing ice cap may force the continuous range of the species to shift towards the equator. For this reason, no or few rear edge microrefugia might exist at the rear edge. It is possible though, that there might instead be refugia beyond the ice sheet limit, at nunataks, along the coasts or on islands in the sea (Stewart et al. 2001; Pruett & Winkler 2005; Anderson et al. 2006; Holderegger & Thiel-Egenter 2009). Other, less cold-adapted species will not be as close to the advancing ice sheet, so the chance is bigger that microrefugia arise at their rear edges.

Figure 4. Example of climatic oscillations during the glacial-interglacial oscillations. a) The mean temperature decrease from the interglacial toward the glacial period. b) The mean temperature increase from the glacial toward the interglacial period. c) The mean temperature decrease during an interglacial period. d) The mean temperature increase during an interglacial. All these temperature oscillations may result in range shifts and the arise of refugia.

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A temperature change scenario, leading to range shifts, can take place at the scale of the Milankovitch cycles, resulting in an interglacial or a glacial (Fig. 4a-b; Bennett 1990). It can also take place as regional climatic fluctuations at a smaller temporal and spatial scale during an interglacial or glacial (Fig. 4c-d; IPCC 2001). The persistance of climatic refugia depends on the scale during which they were formed. If they were formed during climate cooling, they can last until the next warming period. At stable edges they might even persist over several glacials and interglacials (Hampe & Petit 2005). At trailing edges they are likely to be more short-lived due to the latitudinal movements of the ranges. The persistance of climatic refugia requires that the local climate is decoupled from the regional climate; otherwise the refugial population will be at large risk for extinction.

One conclusion drawn from the range shift process described above is as follows. Even if there are more or less obvious range shift scenarios, the described process infers that a species may form refugia both at warming and cooling scenarios. This is especially true for species adapted to intermediate temperature conditions, but it may as well be true for species adapted to cold or warm temperature conditions. The temperatures resulting from climate change may be too extreme even for the most cold- or warm-adapted species, possibly leading to range shifts.

In summary, the formation of refugia from climate change depends on many factors such as terrain and land mass characters as well as on abiotic and biotic factors. It also depends on the species dispersal capacity, the degree of temperature change and the temperature niche of the species.

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