elevations and dictates, together with seasonal changes in temperature and photoperiod, the duration of the growing and/or breeding seasons of alpine organisms,
apart from providing thermally protective snow blankets (Hodkinson 2005). Since
evapotranspiration declines with elevation, the balance for plant water supplies is
rarely critical. However, in periods of high evaporative demand or prolonged soil
moisture depletion, the need to reduce transpiration may affect the rates of CO 2
photosynthetic uptake and nutrient assimilation (Schulze and Chapin 1987). This
trade-off has direct consequences for alpine plant recruitment, survival and growth
patterns, and ultimately shapes their phenology and morphology (Körner 2003), as
well those of the fauna they host (Hodkinson 2005).
11.2.4 Primary Productivity
Mountain environments display sharp gradients in soil fertility, largely dependent
on climate, bedrock, soil structure and age, micro-topography and soil fertilisation
by primary consumers. At high elevations, low temperatures (or a short duration of
mild temperatures) negatively affect soil enzymatic activities, the rate of nutrient
mineralisation and turnover, thus reducing nutrient availability for primary producers with consequences for upper trophic levels (Laiolo et al. 2015a). In some
mountain areas, shorter grazing seasons and reduced nutrient inputs from herbivores also contribute to lower productivity, and the permanence of nutrients
shortens in steep and shallow soils (Mariotti et al. 1980; Huber et al. 2007). Alpine
plants may respond by reducing their size and enhancing mineral nutrient concentration (Körner 2003), although there are exceptions (Laiolo et al. 2015a). It is
also worth mentioning that soil nutrient concentration does not always reflect
availability for organisms, as nutrients may be in a form that cannot be absorbed.
Nutrient availability unquestionably affects the overall performance of plants up to
higher trophic levels and plays a key role in life-history evolution (Stearns 1992).
11.2.5 Biotic Interactions
The above factors, together with land area and history, are evoked as the primary
drivers of the decline in species richness observed in many taxa on mountains, as
well as of the decrease of biotic interactions (McCain and Grytnes 2010). Under the
Stress Gradient Hypothesis, negative interactions should decline with environmental harshness while positive ones should increase (Brooker 2006). Negative
trends of competition, predation and parasitism with elevation have indeed been
documented in a large number of studies (Hodkinson 2005; Boyle 2008; Meléndez et al.
2014), but some positive relationships have as well (e.g. Abbate and Antonovics 2014).
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P. Laiolo and J.R. Obeso
apart from providing thermally protective snow blankets (Hodkinson 2005). Since
evapotranspiration declines with elevation, the balance for plant water supplies is
rarely critical. However, in periods of high evaporative demand or prolonged soil
moisture depletion, the need to reduce transpiration may affect the rates of CO 2
photosynthetic uptake and nutrient assimilation (Schulze and Chapin 1987). This
trade-off has direct consequences for alpine plant recruitment, survival and growth
patterns, and ultimately shapes their phenology and morphology (Körner 2003), as
well those of the fauna they host (Hodkinson 2005).
11.2.4 Primary Productivity
Mountain environments display sharp gradients in soil fertility, largely dependent
on climate, bedrock, soil structure and age, micro-topography and soil fertilisation
by primary consumers. At high elevations, low temperatures (or a short duration of
mild temperatures) negatively affect soil enzymatic activities, the rate of nutrient
mineralisation and turnover, thus reducing nutrient availability for primary producers with consequences for upper trophic levels (Laiolo et al. 2015a). In some
mountain areas, shorter grazing seasons and reduced nutrient inputs from herbivores also contribute to lower productivity, and the permanence of nutrients
shortens in steep and shallow soils (Mariotti et al. 1980; Huber et al. 2007). Alpine
plants may respond by reducing their size and enhancing mineral nutrient concentration (Körner 2003), although there are exceptions (Laiolo et al. 2015a). It is
also worth mentioning that soil nutrient concentration does not always reflect
availability for organisms, as nutrients may be in a form that cannot be absorbed.
Nutrient availability unquestionably affects the overall performance of plants up to
higher trophic levels and plays a key role in life-history evolution (Stearns 1992).
11.2.5 Biotic Interactions
The above factors, together with land area and history, are evoked as the primary
drivers of the decline in species richness observed in many taxa on mountains, as
well as of the decrease of biotic interactions (McCain and Grytnes 2010). Under the
Stress Gradient Hypothesis, negative interactions should decline with environmental harshness while positive ones should increase (Brooker 2006). Negative
trends of competition, predation and parasitism with elevation have indeed been
documented in a large number of studies (Hodkinson 2005; Boyle 2008; Meléndez et al.
2014), but some positive relationships have as well (e.g. Abbate and Antonovics 2014).
256
P. Laiolo and J.R. Obeso