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
11 Life-History Responses to the Altitudinal Gradient
267
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
11 Life-History Responses to the Altitudinal Gradient
267