4.2 Drivers of charophyte species distribution
It was not possible to model the species with less than eleven occurrences because of the paucity of observations. In the literature, very few predictive models have been applied to rare and endangered species, mainly for the same reason. Modelling those species is nevertheless still a challenge because of its relevance for conservation (for example, Lyet et al. 2013). The species with a higher number of occurrences (≥12) were successfully modeled.
The waterbody size represented the main environmental gradient explaining the distribution of most charophyte species. Looking at the species responses curves (Fig. 3 and Supplementary material), the likelihood of finding C. tomentosa, C. strigosa and N.
obtusa increased with the waterbody size. In contrast, the probability of recording C.
vulgaris was maximized in small waterbodies (100 m2) and in medium sized ecosystems (1000 m2) for C. intermedia. The waterbody size itself has no direct effects on the presence of charophyte species but is a proxy for many properties. For example it allows the distinction between the functioning of lakes and ponds. By providing a wider diversity of micro-habitats, larger areas can support more plant species and more individuals. Lakes and small ecosystems also differ in origin and in age; many lakes originate from the last glacial period whereas the majority of small ecosystems are more recent. Because they are older and of larger size, lakes have more stable environmental conditions that are favorable for aquatic plants, including many charophytes if oligo or mesotrophic conditions are met. Aquatic plant stands can last for decades in lakes, whereas they commonly disappear after a few years in permanent small waterbodies because of the rapid ecological succession, further accelerated by eutrophication. By contrast, in temporary small waterbodies, disturbance such as drought events can be favorable to pioneer species by eradicating desiccation-sensitive competitive species.
Other proximal conditions such as the permanence of water, the water temperature and the light regime associated with water depth also diverge between lake shores of large deep lakes and smaller sized or shallower waterbodies. The life cycle of many species also differs with the size of the waterbody: because of its plasticity a species can be perennial in large deep and permanent waters and annual in smaller one.
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Additionally to waterbody size, the distribution of most charophyte species can be explained by their thermal preferences. A few species exhibited a bell shaped curve along the temperature gradient, with an optimum range (Supplementary material). This was the case for N. opaca whose distribution was optimal at sites where the mean July temperature reached between 11 and 16°C (reference 1960-1990), as well as for C.
tomentosa (11 and 18 °C), and C. strigosa (around 11-12°C). Amongst all species, C.
strigosa had the narrowest temperature range and can be identified as a cold water specialist (see also Rey-Boissezon and Auderset Joye 2014, this issue). C. hispida, N.
obtusa, C. globularis, C. contraria and C. aspera all showed a wider temperature optimum.
The probability of finding them increased up to around 20°C and decreased beyond this value. However, none of our localities had a mean July temperature exceeding 22°C (Table 3). Temperature was not included in the distribution model of C. vulgaris, indicating that other factors are more important in explaining the distribution of this species. In fact, ubiquitous species like C. vulgaris and C. globularis are more difficult to model because of their low degree of specialization.
In some models the July precipitation also played an important role. For example, rainfall was the second main variable explaining the distribution of C. strigosa and a major factor accounting for that of C. hispida, C. tomentosa, C. aspera and C. vulgaris. The distribution of the last two species decreased quite linearly as the rainfall increased, indicating the preference of those species for waterbodies which may dry after the growing season. C. strigosa is more likely in localities with a weak to moderate July rainfall, localities that probably do not, or only partly dry out after the summer, offering both conditions in the same waterbody. Apart from the direct effect on the fluctuations of water level, rainfall can also have an impact on aquatic ecosystems through the transport of nutrients by the flow from land to water. In that sense, climate, even if indirectly, can be related to land-use.
The proportion of forest in the catchment area was included in three quarters of our species models. However, the effect of this land-use variable on charophytes was weaker than temperature and precipitation. Land used for agriculture in the zone adjacent to the waterbody was more frequently incorporated in the models than that of the whole catchment, indicating that nutrients transport through soil leaching and direct anthropogenic effects from the surrounding areas influence the distribution of some
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charophyte species. Similar results have been found in a study on boreal lakes (Alahuhta et al., 2012).
Finally, the calcium carbonate content of the soil in the catchment area also explains a part of the species distribution. The role of calcium carbonate in discriminating charophyte species has already been reported by several authors (Olsen, 1944; Corillion, 1957). Extremes of the gradient generally separated calcifuge species (Nitella) from calciphile species (Chara). In Switzerland calcium carbonate is abundant in the environment and can probably explain why Charae species are much more common than the Nitellae (Auderset Joye et al., 2002). Among species of the genus Chara, C.
globularis and C. hispida will be found on the two extremes of the calcium gradient, with C. hispida on the richer part.
The habitat of charophyte species was established on the relationship between species records and several large environmental gradients. As an alpine country, Switzerland provides an elevation gradient that offers an excellent opportunity to investigate the potential impact of climate variations on organisms.
Our species occurrence models can be used for many geographic regions where the range of explanatory variables is comparable (waterbody size from 2 to 106 m2, land-use from 0 to 100 %, mean July temperature from 5 to 22°C, July precipitation from 400 to 2500 mm) and which include the alpine countries and large parts of Europe. Indeed, a 1°C decrease in the mean July temperature can be experienced either by moving 231 km northwards along the latitudinal gradient or by climbing 200 m along the elevation gradient (De Frenne et al. 2013).
4.2. Potential losers and winners of future changes
The model used allowed us to calculate a probability of occurrence for each species in each locality from the whole country. We also computed the probabilities, taking into account our scenario, of climate changes (+ 2°C in July temperature and -15 % July rainfalls). We defined species as winners if the sum of probabilities in the 21,092 sites increased and as losers if this sum decreased. We could have predicted also the number of sites where a species can be found by using a threshold: if the probability is greater than a certain percentage, then the site is counted as a potential locality. However, such an operation is threshold sensitive and we chose to use a more global procedure.
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Concerning the changes in probabilities calculated for the 11 species that have been modeled, it must be underscored that such a prediction takes into account the “real geography” of the country. The potential losers were found amongst species colonizing permanent waterbodies, almost exclusively the large deep lakes of the lowland. These species are predicted to decrease in occurrence from 5 % (Nitella opaca) to 49 % (C.
contraria). These foreseen losses in species occurrence are the consequence of the aquatic ecosystems distribution in Switzerland: as large deep lakes do not exist in other part of the country, fewer localities with new climatically suitable conditions will be available for the species. The rate of decline of loser species depends on their degree of
“fidelity” for a type of ecosystem. For example, the large decrease in occurrence predicted for C. contraria results from its distribution, mainly in the mesotrophic deep lakes located on the Swiss Plateau, which have no equivalent elsewhere in the country.
The small decrease in occurrence predicted for N. opaca is caused by its wide range of distribution which involves lakes of the lowland as well as lakes with colder climates, thus at higher elevation.
By contrast, species predicted to be winners will increase their occurrence from 5 % (N. obtusa) to 90 % (C. hispida). Most of these were recorded principally in moderate sized and small ecosystems (C. vulgaris, C. hispida, C. intermedia, C. globularis). The consequences of warming will not deprive these species from potential favorable ecosystems because waterbodies of this range of size are distributed within a wide extent of mean July temperatures, a parameter closely related to altitude. The risk to become a loser or a winner depends mostly on the distribution of the different types of waterbodies along the temperature gradient, i.e. altitude. The results are thus subordinate to the geography (climate, landuse, geology) of the country from which the data were issued. Therefore, the predictions are valid only for Switzerland and probably for alpine countries with similar habitat and species.
The results take into account the species relation to the environment (ecological traits) but do not include the biological traits (morphology, physiology, reproduction, etc.) which play also a crucial role in species current occurrence and abundance. In the future, the risk to be a loser or a winner will depend not only on the availability of suitable ecosystems but also on the aptitude of a species to migrate to new localities and/or to acclimate by adapting its life cycle.
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4.3. Biological traits and life strategies
The species biological traits can explain a part of their present-day distribution and their potential ability to be resilient to future climate changes. For example, the success of the most common species can be explained by their fertility; they generally produce many oospores which are long lasting propagules (C. aspera, C. contraria, C. globularis and C. vulgaris). Some species are tolerant to desiccation to a certain extent and occur in the drier regions of Switzerland (C. aspera, C. hispida and C. vulgaris). This is consistent with the literature which considers C. vulgaris as a species resilient to drying (van Geest et al., 2005), associated with waterbodies frequently disturbed (Bornette and Arens, 2002). C. aspera and N. obtusa produces bulbils, i.e. starch reserves designed for vegetative multiplication that can be seen as an adaptation to resist to disturbed conditions such as short droughts, floods and waves. Van den Berg et al. (2001) showed that all the bulbils of C. aspera tested emerged during a germination experiment. Small fragments of these organs are easily transported by birds and increase the chance of survival of these species. According to Bociag and Rekowska (2012), C. aspera, C.
globularis, C. tomentosa are able to adopt a clonal regeneration strategy. This type of growth has been observed in C. intermedia, N. obtusa and some Nitella (personal observations). C. hispida shows physiological and morphological light acclimation (Schneider et al., 2006), an advantageous trait to resist to lowering water levels when July precipitation is low and irradiance high. C. contraria does not adapt morphologically to high light (Schneider et al., this issue) but like many other charophytes, produces photo-protective pigments (Schlagerl and Pichler, 2000). This light adaptation also appears in N.obtusa, N. opaca and C. tomentosa growing at shallower depths (personal observations). This is also a beneficial trait for plants exposed to lower water level due to warmer and drier conditions. The longevity of charophyte oospores buried in sediment, which are able to germinate after several decades (Rodrigo et al., 2010, 2013), is also a favorable attribute to ensure species survival.
Among the potential losers, species living currently in oligo-mesotrophic deep Swiss lakes are commonly recorded in deep zones at low light and temperature. Under these harsh living conditions, the species remains sterile and reproduces vegetatively by sprouting, whereas sexual reproduction has been observed at shallower water conditions (N. obtusa and N.opaca). In contrast to other species, we have rarely observed
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C. tomentosa producing ripe oospores. This under-performing sexual reproduction and its sensitivity to temperature make C. tomentosa a particularly vulnerable species.
In the future, thanks to their multiple traits, most species appointed as potential losers could be resilient and able to adapt their life strategy to certain limits. For example, some species might be able to survive at shallower depth by producing photo-protective pigments and /or by reproductive adaptations but they might also disappear due to the concurrence of more competitive species at these new environmental conditions. The shifts toward deeper parts of lakes as a means to escape warming, is unlikely because lake-species are already growing at suboptimal light conditions. To survive, lake-species should be able to migrate and colonize lakes and ponds located at higher altitudes where they will find temperatures similar to their present-day conditions. The water quality in high-altitude ecosystems is potentially favorable for the establishment of charophytes, as many remain in pristine environments. As oospores and propagules are dispersed mainly by birds, the chance for charophytes to colonize the higher elevation waterbodies will depend also on the probability that birds cross the Alps during their migration.