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.
11 Life-History Responses to the Altitudinal Gradient
269
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.
11 Life-History Responses to the Altitudinal Gradient
269