Altitude as an Ecological Tool

This month I’ve chosen to look at an article that is more relevant to my ongoing research this year by discussing a paper titled: “The use of ‘altitude’ in ecological research” by Christian Kömer, 2007 (TREE: 22 (11): 569-574).

This article is about the confusion in the scientific literature regarding altitude phenomena – namely ecological and evolutionary responses of biota to geophysical influences.  About ¼ of the Earth’s land surface is covered by mountains, where about 1/3 of terrestrial plant species diversity is endemic and supply half of the human population with water.  Furthermore, mountains offer steep environmental gradients, creating a great opportunity for biological research – particularly for exploring evolutionary adaptation over short spatial distances.

The author of this opinion article stresses to point out that many researchers have made the mistake of interpreting results that often include unique local conditions, such as fire, land use or drought that are not generally associated with altitude above sea level.  In reality, many findings that report supposed mechanisms that influence biota from different altitudinal gradients are in fact confusion between other environmental drivers that have been put under the umbrella term ‘altitude’.

Science strives to make over-arching blanket theories to explain observations or phenomena in order to simplify our understanding of the natural world.  Attempts have been made to correlate altitude with reduction of species, productivity, body or organ size, physiological and morphological trait trends or gene-ecological and life-history characteristics.  Seemingly altitude phenomena are particularly difficult to distil into a general, global altitude-related theory of biological phenomena because of many confounding variables, such as variable moisture gradients that occur along altitude gradients.  This article attempts to clearly distinguish those variables that change with altitude (e.g. atmospheric pressure and temperature) and those that do not (e.g. moisture and wind).

I find this article particularly interesting, as my current research project is attempting to ascertain the underlying causes of the high mortality rates of the Clanwilliam cedar (Widdringtonia cedarbergensis) in the Cederberg Mountains.  I am expecting that there will be a link between higher mortalities and higher temperatures and lower moisture availability across its range.  This article stresses that I will need to be careful when analysing the relationship between altitude and moisture availability as there is no global trend.  However, temperature is a variable that is physically tied to altitude giving me confidence in correlating these factors.

The primary global geophysical change with altitude is the decrease in land area with increasing altitude.  This seems rather obvious, but it is important for science to define such relationships.  This is a particularly interesting trend, because available land area has a major influence on biotic diversity and evolution.  Above the altitudinal tree limit (which varies around the world), on average, every increase of altitude by 167m results in a halved land area.  This average varies between mountain ranges.

Figure 1. The pattern in the European Alps of land area per 100m steps above sea level (a.s.l) starting from 1500m a.s.l.  

This figure clearly illustrates how land area consistently decreases with increasing altitude.  In the Alps an average of 150m of altitude results in half the amount of land area.

Along with the altitudinal loss of land area, there are 4 primary atmospheric changes associated with altitude:

1.       Atmospheric pressure declines for every kilometre gained in altitude with some regional trends although the general pattern is global.  This has an impact on respiration in animals and gas exchange in plants due to the reduced partial pressure of gases such as CO₂ and O₂.  This has allowed for interesting adaptations, such as more efficient CO₂ fixation in plants and more porous egg shells in birds to facilitate higher rates of O₂ diffusion to the embryo.

2.       Air temperature drops by 5.5 degrees Kelvin per kilometre of altitude on average.  Many plants have learnt to adapt to this by decoupling themselves from free convection and the ambient temperatures, allowing them to warm significantly under solar radiation, creating stable conditions for photosynthesis.  However, there is no global trend for all plant’s departures from ambient conditions and local and/or life form-specific patterns are likely to govern the degree of departure.  Instead, there is a trend seen in low-stature vegetation (e.g. cushion plants) of all mountains, where a thermal contrast is apparent in high altitude vegetation.  Furthermore, tall plants (trees) generally exhibit similar altitudinal reductions in temperature as the atmosphere does, which helps to explain the uniform position of tree lines that are restricted by a common isotherm of the mean temperature during the growing season (between 6˚C and 7˚C)(see Figure 2).  Due to these differences among organisms, the author points out that in order for altitudinal thermal gradient analyses to be biologically meaningful, only the temperatures that are physiologically effective need to be considered.

      Figure 2.  A photograph of the mountain tree-line in at Banf National Park, USA. 

3.       & 4. For both solar and UV-B radiation, the actual amount and effect has been misrepresented.  In both cases, the effects of clouds and fog mitigating radiation have not been considered.  For solar radiation, under a clear-sky profile only, solar radiation increases with altitude, but this is never realised due to effect of clouds and fog.  UV-B radiation contributes a higher fraction at any given solar radiation with increasing altitude.  Again, however, clouds and fog mitigate this effect.  At peak solar radiation, there is a consistent increasing global trend of increasing UV-B radiation with altitude, but this is not seen in mean daily or seasonal trends.

  

Perhaps the most important part to this article is the climatic trends that are not generally related to altitude.  The most important thing to note for all of these variables is that there are no clear global altitudinal phenomena for precipitation, wind velocity or seasonality.  The importance of this cannot be understated, as it is a far worse mistake to attribute a false causational variable to a pattern than to mistakenly not attribute a (true) causational variable to a pattern.

In conclusion, the importance of this article can be seen in the potential consequences of misusing altitude and its associated variables as an ecological tool.  In the case of my research project, if I was to falsely assume that moisture availability was to decrease with increasing altitude, this information could potentially be used in replanting efforts.  These efforts would be likely to fail, as it far more likely that the cedar trees rely on micro-climates between and on rocky outcrops than on particular altitudinal-moisture trends, even if this the general trend.  As Kömer (2007) notes, because there is no ‘standard mountain’ any data collected along altitudinal gradients will reflect a combination of the regional peculiarities and general altitude phenomena.