Principles of terrestrial ecosystem ecology.pdf

138 6. Terrestrial Production Processes
Shoot Root Root Total
Biome (g m -2 ) (gm -2 ) (% of total) (g m -2 )
Tropical forests 30,400 8,400 0.22 38,800
Temperate forests 21,000 5,700 0.21 26,700
Boreal forests 6,100 2,200 0.27 8,300
Mediterranean shrublands 6,000 6,000 0.5 12,000
Tropical savannas and grasslands 4,000 1,700 0.3 5,700
Temperate grasslands 250 500 0.67 750
Deserts 350 350 0.5 700
Arctic tundra 250 400 0.62 650
Crops 530 80 0.13 610
a Biomass is expressed in units of dry mass.
Data from Saugier et al. (2001).
terrestrial biomes (Table 6.4). Forests have the
most biomass. Among forests, average biomass
declines 5-fold from the tropics to the lowstature
boreal forest, where NPP is low and
stand-replacing fires frequently remove biomass.
Deserts and tundra have only 1% as
much aboveground biomass as do tropical
forests. In any biome, disturbance frequently
reduces plant biomass below levels that the
climate and soil resources could support. Crops,
for example, have a biomass similar to that
of tundra or desert, despite more favorable
growing conditions; regular removal of crop
biomass by harvest prevents it from accumulating
to levels that climate and soil resources
could potentially support. When disturbance
frequency declines, for example through fire
prevention in grasslands and savannas, biomass
often increases through invasion of shrubs and
trees.
Patterns of biomass allocation reflect the
factors that most strongly limit plant growth in
ecosystems (Table 6.4). Between 70 and 80% of
the biomass in forests is aboveground because
forests characterize sites with relatively abundant
supplies of water and nutrients, so light
often limits the growth of individual plants. In
shrublands, grasslands, and tundra, however,
water or nutrients more severely limit production,
and most biomass occurs belowground.
Because of favorable water and nutrient
regimes, crops maintain a smaller proportion of
biomass as roots than do most unmanaged
ecosystems.
Tropical forests account for about half of
Earth’s total plant biomass, although they occur
on only 13% of the ice-free land area; other
forests contribute an additional 30% of global
biomass (Table 6.5). Nonforest biomes therefore
account for less than 20% of total plant
biomass, although they occupy 70% of the icefree
land surface. Crops for example, account
for only 1% of terrestrial biomass, although
they occupy more than 10% of the ice-free land
area. Thus most of the terrestrial surface has
relatively low biomass (see Fig. 5.20). This
observation alone raises concerns about tropical
deforestation, independent of the associated
species losses.
Biome Differences in NPP
Table 6.4. Biomass distribution
of the major terrestrial
biomes a .
The length of the growing season is the major
factor explaining biome differences in NPP.
Most ecosystems experience times that are too
cold or too dry for significant photosynthesis or
for plant growth to occur. When NPP of each
biome is adjusted for the length of the growing
season, all forested ecosystems have similar
NPP (about 5gm -2 d -1 ), and there is only about
a threefold difference in NPP between deserts
and tropical forests (Table 6.6). These
calculations suggest that the length of the
growing season accounts for much of the biome
differences in NPP (Gower et al. 1999, Körner
1999).
Leaf area accounts for much of the biome
differences in carbon gain during the growing
season. Average total LAI varies about sixfold
among biomes; the most productive ecosystems
generally have the highest LAI (Table 6.6;
Fig. 6.4). When NPP is adjusted for differences