Environmental Engineering Reference
In-Depth Information
computations of global NPP showed annual fluctuations
up to 3.8% (NTSG 2006). A longer comparison, using
satellite-based models of global NPP, indicated that
climate change weakened several constraints on plant
productivity, resulting in a 6% (3.4 Gt C) rise of NPP be-
tween 1982 and 1999 (Nemani et al. 2003). Enhanced
photosynthesis was also found as a result of lingering vol-
canic aerosols from the 1991 Mount Pinatubo eruption
(because plant canopies use diffuse radiation more effi-
ciently than the direct beam) and stronger monsoon
winds over the western Arabian Sea. They increase nutri-
ent upwelling and boost average summertime phyto-
plankton mass by more than 350% (Gu et al. 2003;
Goes et al. 2005).
On the other hand, the European heat wave in 2003
reduced the continent's GPP by about 30% and was re-
sponsible for a strong (500 Mt C/a) anomalous net
source of CO
2
(Ciais et al. 2005). Long-term averages
of NPP and standing phytomass are bound to change
with progressively more pronounced global warming,
but the extent and pace of these changes remain highly
uncertain, often even as to the direction of the sign (will
boreal or tropical forests be a long-term sink or source of
carbon?). A model by Cao and Woodward (1998) indi-
cated much enhanced global NPP (by 25%) and substan-
tially higher phytomass stocks (up by 20%) with doubled
CO
2
, but other studies predict no gains or minimal gains
in some key biomes. Similarly, it is unclear to what extent
the continuing deforestation of sub-Saharan Africa, parts
of Latin America, and Asia will be balanced by reforesta-
tion and enhanced NPP in other regions.
to the productivity and structure of the tree-dominated
biomes (Reichle 1981; Landsberg 1986; Perry 1994;
Waring and Running 1998; Barnes et al. 1998; Roy, Sau-
gier and Mooney 2001), and it has produced extensive
databases that reveal universal patterns as well as numer-
ous peculiarities. Tropical rain forests produce most of the
biosphere's new phytomass. Field et al. (1998) ascribed
about 48% of terrestrial NPP to forests and 32% to tropi-
cal rain forests alone. Reliable moisture, steady high tem-
peratures, and very high leaf area index (LAI, the upper
area of foliage per unit area of ground) explain the
unmatched rates. While the LAI of grasslands and cereal
crops is often no higher than 2-3, tropical rain forests
have an LAI of 4-7.5 (Myneni, Nemani, and Running
1997; Scurlock, Asner, and Gower 2001).
Available data show NPP averaging around 2.5 kg/m
2
(about 1.3 W/m
2
), almost equally divided between
above- and below-ground phytomass, and the reported
maxima reach about 1.7 W/m
2
. As already noted, pho-
tosynthesis in the wet tropics is limited by insolation and
nutrient-poor soils. Unlike the temperate rain forests
(rooted in relatively nutrient-rich soils covered with thick
litter mats), most tropical trees are shallowly rooted in
highly weathered and leached nutrient-poor soils overlaid
with rather thin litter layers. Nutrients necessary for pho-
tosynthesis reside in the phytomass itself, and the fertility
of the forest depends on their constant rapid recycling.
Nutrient-conserving adaptations include rapid direct ab-
sorption of scarce nutrients, extensive mycorrhizal symbi-
oses, absence of denitrifying bacteria, leaves scavenging
nutrients but resistant to rainfall leaching and heterotro-
phic attack, and quick regrowth in clearings (Jordan and
Herrera 1981; Primack and Corlett 2005).
The efficient use of nutrients, moderately high produc-
tivity, high efficiency of stemwood production (as a share
3.3 Productivities of Ecosystems and Plants
Division of the global NPP shows the expected domina-
tion of forests. A great deal of research has been devoted
