11.5.1 Interspecific Variation
In mountain and alpine environments, life histories are often characterised by long-lived iteroparous perennial life cycles. A trade-off between allocation to vegetative growth and sexual reproduction is expected as a consequence of nutrient limitation. Thus, the increased allocation to vegetative growth should reduce the availability of resources for reproduction (Obeso
2002). In general terms, alpine and arctic plants invest more in maintenance and less in reproduction (Jónsdóttir
2011).
In the harsh climatic environment of high altitudes, new plant establishment is a particularly risky (unsuccessful) mode of reproduction because of the high nutrient demand of seed production (Watson
1984), infrequent germination and low seedling survival (Bliss
1971; Scherff et al.
1994). Accordingly, there may be a reduction in seed rain and seed bank size as elevation increases (Molau and Larsson
2000). The demography of alpine plant populations is often characterised by low seedling recruitment and high seedling mortality at early developmental stages compared with lower-elevation populations (Bliss
1971; Hautier et al.
2009; Milla et al.
2009). In general terms, this implies that the successful establishment ex-novo of new genets (independent physiological units, or clonal colonies, sensu Watson and Casper
1984) is infrequent. However, these paradigms regarding alpine plants are currently changing and seedling establishment may be more common and successful than previously thought (Jolls and Bock
1983; Chambers et al.
1990; Forbis
2003, Forbis and Doak
2004; Giménez-Benavides et al.
2007; Venn and Morgan
2009; Kim and Donohue
2011).
The main evidence of the rarity of seedling establishment in alpine plants is the fact that the size-class distributions within the populations are often characterised by the absence of the smaller size-classes (Philipp
1997; Jónsdóttir
2011). These distributions may also be a consequence of longer intervals between ‘windows’ of regeneration by seeds in the extremely variable alpine conditions (Eriksson
1997). Additionally, taking into account that long-lived alpine plants may reach ages of one thousand years or more, the selective pressures that conditioned the establishment of the parent plant were likely not the same that seeds and seedlings currently face.
Persistence of established genets, through somatic maintenance, clonal growth and vegetative reproduction is thought to be one of the most remarkable adaptations to the conditions of high mountain habitats and its importance tends to increase with altitude. Survival of adult plants has been suggested to be a key demographic parameter for maintaining alpine plant populations, and their demography is often characterised by high adult survival compared with lower-elevation populations (Bliss
1971; Hautier et al.
2009; Milla et al.
2009; García and Zamora
2003; Kim and Donouhe
2011). As a consequence, the decline of annual species with increasing altitude is remarkable, as it is the number of long-lived species that relies on clonal reproduction for population maintenance (Stöcklin
1992; Klimes et al.
1997).
Despite the above generalisations, life histories of alpine plants are highly diverse due to a great variety of growth and multiplication models. This diversity may be associated with different growth forms and varying degrees of physiological integration within genets. Most alpine plants are clonal perennials, the lifespan of which is one order of magnitude longer than that of non-clonal perennials (de Witte and Stöcklin
2010). Clonal perennials range from ‘splitters’ to ‘extensive integrators’. In the former case, the new clonal individuals (ramets) split from the parental genet shortly after their development (seeds produced by agamospermy, bulbils, plantlets, and some bulbs and tubers) and in the latter, the offspring ramets (normally rhizomes) remain physiologically integrated with the parent genet throughout their lifetime. There is an intermediate situation (‘intermediate integrators’) in which the offspring ramets remain connected to the parental plant for a time, as is the case of stolons, rosettes, rhizomes and root shoots (Jónsdóttir
2011).
Arctic and alpine non-clonal perennial lifespans from several decades to more than one hundred years are common (Callaghan and Emanuelsson
1985) and genet age of ‘extensive integrator’ clonal perennials may reach over one thousand years or more. As an expected consequence of a trade-off between longevity and sexual reproduction, the allocation to sexual reproduction is generally lower in clonal than in non-clonal plants (Jónsdóttir
1995; Stenström
1999; Stenström and Jónsdóttir
2006).
Taking into account the reduction in reproductive allocation at high elevations, we can expect that plants have developed some adaptations in their life histories to reduce the risk of costly reproductive investment. In this sense, we can expect alpine plants to increase offspring survival throughout life-history variables related to parent care: larger seed size to produce larger seedlings, pseudovivipary (Lee and Harmer
1980) and nursing of seedlings to increase their survival. Established cushion plants (such as
Silene acaulis) can act as nurses of seedlings increasing their survival (Bliss
1971). This nursing effect has been mostly observed in an interspecific context. However, we can predict that this may be an important phenomenon from an intraspecific perspective, as has been proposed in environmental contexts others than alpine ones (Fajardo and McIntire
2011).
Pseudoviviparity consists of the formation of vegetative diaspores in inflorescences, with the already developed flower parts undergoing proliferation and transformation into leaf-like structures (Pijl
1972). Species with pseudovivipary are mostly found in arctic, alpine and arid environments. In the local high-altitude floras, the proportion of pseudoviviparous species reaches 10% and, in exceptional cases, even up to 25% (Sarapul’tsev
2001). These habitats may favour pseudovivipary because they are extraordinarily coarse-grained for seedling establishment and the probability of an offspring being dispersed to a suitable patch is very low. The success of pseudovivipary may also be related to the problems of establishment and growth in the short, cold growing seasons of these regions (Lee and Harmer
1980; Elmqvist and Cox
1996). Furthermore, parental care is not restricted to seedling establishment, as the survival of daughter ramets may be greatly enhanced by translocation of resources from the parental plant through the vascular connections. This extended parental care depends on the degree of physiological integration or independence and is prolonged in the case of ‘extensive integrators’ (Callaghan
1984; Jónsdóttir
2011). As a rather general trend, parental care to seeds is substituted by parental care to daughter ramets, which are much more costly to produce but exhibit much higher survival. Seedling survival is probably the most critical stage in the life histories of long-lived perennial alpine plants, determining species’ distribution and range shifts (Kitajima and Fenner
2000).
Seed weight should be affected by altitude because heavier seeds are more likely to produce larger seedlings that successfully establish in harsh conditions (Westoby et al.
1992), which is in accordance with the ‘stress-tolerance’ hypothesis (survival depends on plant stress resistance). However, despite the fact that elevation gradients in seed mass have repeatedly been reported (Baker
1972; Blionis and Vokou
2005), findings were often conflicting and had not revealed any consistent pattern thus far. Although an increase in seed mass with elevation was reported by Pluess et al. (
2005), there is also evidence of negative relationships between seed mass and elevation supporting the ‘energy constraints’ hypothesis, which states that lower temperatures and shorter growing seasons at higher elevations may reduce resource acquisition and the energy available for seed development and seed provisioning (Baker
1972; Körner
2003; Bu et al.
2007). Additionally, seed size is subjected to allometric constraints and thus determined by plant size variation with altitude.
In detail, Pluess et al. (
2005) tested the hypotheses that between related species-pairs and among populations of single species a similar trend for increasing seed weight with increasing altitude should be present. These authors determined seed weights from 29 species-pairs, with each pair consisting of one species occurring in a lowland area and a congeneric species from a high altitude area. Compared to the related lowland species, 55% of the alpine species had heavier seeds, 3% (one species) had lighter seeds and 41% had seeds of approximately equal weight. However, Wu and Du (
2009), who examined the hypothesis of a positive effect of altitude on both interspecific and intraspecific variation in seed mass, found that in 50% of the 44 species that occurred in both low and high altitudes, seed mass increased with altitude, but decreased in the other 50%. Moreover, Wang et al. (
2014) examined seed mass variation in 42 species of
Rhododendron along an altitudinal gradient from a few hundred metres to 5500 m above sea level on the Tibetan Plateau. They found that seed length, width, surface area and wing length were negatively correlated with altitude, and positively with plant height. Conversely, Qi et al. (
2014), using a large database involving 1355 species from the Tibetan Plateau, found a non-significant seed mass-elevation relationship across all species after controlling for phylogeny and plant height. These authors also found a mass-dependent response to the elevation gradient: smaller seeds tended to increase in mass with elevation but large seeds tended to decrease.
11.5.2 Intraspecific Variation
When the same plant species occurs along a mountainside, within-species variation in life histories is expected since a suitability gradient is found within each mountain range (Körner
2003). Depending on the biogeographic origin of the species, plants occurring at the highest or lowest altitudinal limits should face especially harsh constraints on reproduction and establishment via seeds (Hampe and Petit
2005; Arrieta and Suárez
2006; Giménez-Benavides et al.
2007). In this sense, the ‘centre–periphery’ hypothesis proposes that conditions for the regeneration of plant populations are less suitable in the boundaries than in the centre of the distribution area, and at the same time, life cycles should slow down at high altitudes (Lawton
1993; Vucetich and Waite
2003; Angert and Schemske
2005).
Arx et al. (
2006) used the width of annual rings in roots to study plant demography along an altitude gradient after determining plant age and lifetime growth in three perennial forbs. For all three species, the plants from the highest altitudes tended to be considerably older and produced more flowering shoots than lowland plants. Highland plant growth, estimated by ring width, was approximately half that of lowland plants. However, ring width of the high-altitude plants increased during the first years and later decreased. These results highlight the importance of investing resources in plant growth during the first years to ensure plant establishment. This initial investment in growth is a characteristic behaviour of life cycles in which mortality decreases considerably with the age of the individual.
When comparing demography and life-history traits of populations of
Erysimun capitatum from alpine and low-elevation populations, Kim and Donohue (
2011) found that mortality of all life stages was higher at lower elevations than at an alpine site. At the same time, they found that low-elevation plants reproduced more quickly and were more frequently semelparous than alpine plants. Thus, low-elevation semelparous populations depended primarily on seedling recruitment and precocious reproduction, whereas alpine plants tended to be iteroparous and to produce more vegetative rosettes. These results showed an altitudinal variation in parity (number of reproductive events), and its demographic consequences, indicating that plastic or evolutionary changes in this trait have a clear influence on population performance along altitudinal gradients.
As the allocation of resources to reproduction results in a reduction of allocation to vegetative growth and, therefore, an impact on future reproductive success, the trade-off between allocation to reproduction and vegetative growth is also a determinant of iteroparous perennial cycles within species. Hautier et al. (
2009) conducted a transplant experiment to assess the influence of both the altitudinal origin of populations and the altitude of the growing site on vegetative growth and reproductive investment in
Poa alpina. According to the general trend in plants, the variation in reproductive investment was mainly explained by plant size. However, the vegetative growth and the relative reproductive allocation decreased in populations originating from higher altitudes compared to populations originating from lower altitudes. They also found that the importance of plasticity was scarce in relation to genetic effects and interpreted these results as a consequence of local adaptations.
Gao-Lin et al. (
2011) tested the hypothesis that seed mass was positively correlated with altitude within species in four congeneric Saussurea (Asteraceae) that occur in the Tibetan Plateau. They found a general trend of a significant increase in seed mass with altitude. Contrarily, Meng et al. (
2014) showed that along an altitudinal gradient in the Hengduan Mountains, mean seed weight of
Sinopodophyllum hexandrum decreased significantly. Pluess et al. (
2005) compared seed weights among populations of four species from different habitats and with different life histories along an altitude gradient (
Scabiosa lucida, Saxifraga oppositifolia, Epilobium fleischeri and Carex flacca). In all the four species, they found no indication for heavier seeds at higher altitudes. Similarly, in the cactus
Gymnocalycium monvillei seedling height increased with altitude, whereas seed mass was not related to this variable (Bauk et al.
2015).
Assessing adaptive differentiation of plant populations along altitude gradients is useful for predicting how they may respond to climatic change. Local adaptation along altitudinal gradients has been demonstrated in several alpine plant species after reciprocal transplant experiments (Byars et al.
2007; Kim and Donohue
2013; Toräng et al.
2015) or transplants to a common garden (Stenström et al.
2002). However, information about local adaptation in traits related directly to life history is still scarce. Leimu and Fischer (
2008) reviewed the information about local adaptations and found that although local plants performed better than foreign plants in 71% of the studies, local adaptation, sensu stricto, was demonstrated in approximately 40% of the case studies.
Surprisingly, genetic diversity of alpine plant populations is not as depleted as predicted from small population sizes and repeated vegetative multiplication, a fact that suggests that gene flow and repeated seedling recruitment during succession might be more frequent than commonly thought (Diggle et al.
1998; Pluess and Stöcklin
2004; Reisch et al.
2007).