Biomass production of alpine vegetation

Global comparisons of biomass production of vegetation types are commonly done on a yearly basis. This may be valid if one wishes to account for the total contribution of certain biota to the globe's productivity or if one considers annual yield of certain products. A comparison that aims at the "ecological well being" and "physiological capacity" of certain biota will, however, not use the year, but the duration of the growing period for such a comparison. When biomass production is restricted to a 10 week snowfree period, the remaining 42 weeks under snow pack are of little relevance for the judgement of the biological performance.

1 - Snow pack controls the length of the growing period in extra-tropical mountains at very small spatial scales: a key driver of total productivity per unit of land area.
1 - Snow pack controls the length of the growing period in extra-tropical mountains.
As long as soil moisture is high, biomass production in natural vegetation is a matter of time, not climate.
Monthly net primary production
(kg m-2month-1)
Length of growing season
(months)
Annual net primary production, mean (range)
(kg m-2a-1)
Humid tropical forest 12 2.5 (1.8 - 3.0)
Temperate deciduous forest 5 1.2 (1.0 - 1.5)
Boreal forest 5 1.1 (0.3 - 2.0)
Tropical grassland 10 2.5 (0.2 - 4.0)
Temperate grassland 6 1.0 (0.2 - 1.5)
Alpine vegetation
(northern temperate zone)
2 0.4 (0.2 - 0.6)

Dashed bars illustrate the variance around the mean calculated from various data sets for humid regions.

When based on the length of the growing season, the biomass production of closed alpine grassland is not different from the mean for other not-water-limited grasslands of the globe. In fact, the productivity does not differ from that of humid tropical forests.

This is a masterpiece of evolution. The adaptation of plant physiology and development is so perfect that the last remaining factor limiting biomass production is the length of the growing period.

We do not know enough about the productivity of tropical alpine vegetation, such as the tussock grassland found on all tropical and subtropical mountains. Does their 12 month productivity still fit the pattern in Fig. 2? It is likely that the monthly alpine productivity in the tropics is less than in temperate mountains, because the productivity in a short growing season will profit from winter time mineralisation under snow and the accumulation of N in the snow pack (see Alpine plant nutrition). In seasonal alpine climates nutrients become available for a short pulse of growth in summer. Furthermore, the tropical alpine day lasts only c. 12 h, compared to 16 h in the temperate zone. New data for the semi-arid Andean Altiplano in the Sajama region of Bolivia suggest an annual above-ground biomass production of >400 <700g m-2 a-1 at a ground cover by tussock grasses of 15 %. This productivity represents the average one month productivity of the ecosystems listed above.

tropical tussocks
2 - Tropical tussock grass: Festuca orthophylla gets more than 1 m tall (Bolivia, 4200 m).

Is their monthly productivity comparable to temperate zone alpine grassland? Possibly it is lower. Do you recall why? Could you design a study protocol for tropical grassland productivity? Remember that the key problem is that there is no beginning or end of the season. Just take the above tussock: How much biomass does it produce in a month? A hint: the technique you will have to employ does not require any high tech equipment. In fact, you can do it with simple equipment you already have ...

Answer: The most practical procedure is a study of tiller or leaf turnover (the birth and length growth of tillers or leaves). If you have a conversion of tiller size to tiller weight you can estimate biomass production (we call this a demographic approach). What you need are tags, a folding meter, a field book, a string with a few pegs to define the unit area, and time. This is for above ground productivity. There is no simple solution for below ground productivity because root demography is tricky.

2 - The dominant tall tussock grass in the Bolivian altiplano (3800-4300 m): Festuca orthophylla.