Environmental Engineering Reference
In-Depth Information
growing in cooler settings (King et al. 2006). Forest response remains especially
unclear because their R A :GPP ratio actually declines when the mean temperature
is below about 11°C (Piao et al. 2010b). In any case, available data indicate that
increases in forest GPP do not lead to preferential allocation to any particulate plant
tissues (Litton, Raich, and Ryan 2007).
The combination of higher NPP, improved water-use efi ciency, and unchanged
(or only mildly elevated) R A may thus bring noticeable improvements in primary
productivity of annual crops and increase carbon storage on long-lived tissues. Some
early studies coni rmed such a response. For example, a model by Cao and Wood-
ward (1998) indicated a much enhanced global NPP (by 25%) and substantially
higher phytomass stocks (up by 20%) with doubled CO 2 , and Nemani et al. (2003)
concluded that between 1982 and 1999, changing climate had eased some key
climatic constraints to plant growth (optimum temperature, precipitation), and as
a result, global NPP increased by about 6% (3.4 Gt C) over those 18 years.
The effects of CO 2 “fertilization” have been found on scales raging from local
to national. Higher annual productivity became evident throughout most of the
United States during the latter half of the twentieth century (Nemani et al. 2002),
and Tao et al. (2007) found that even in such a poorly forested country as China,
the net ecosystem productivity (NEP) rose between 1981 and 2000, as did the
overall carbon storage in plants. But greater carbon sinks may not be the norm, as
photosynthesis may remain limited by shortages of key macronutrients (particularly
nitrogen) or as the new photosynthate may produce mostly short-lived litter (foliage,
i ne roots) rather than long-lived stems and taproots. In particular, mature temperate
trees may not respond to CO 2 enrichment in the same way smaller trees would. For
example, a four-year experiment by Körner et al. (2005) found a sustained enhance-
ment of carbon l ux through the trees but no overall stimulation of stem growth or
leaf litter production.
Northern forests are also particularly sensitive to changes in spring and fall tem-
peratures, and the past increase of about 1°C may already have led to autumnal
losses high enough to offset 90% of increased spring CO 2 uptake (Piao et al. 2008).
Greater warming in the fall could thus substantially reduce the future capacity
of northern ecosystems to store more carbon; additional losses may result from a
higher frequency of regional wildi res and longer duration of droughts (Westerling
et al. 2006). Further, a study by the International Union of Forest Research Orga-
nizations maintains that the capacity of forests to store carbon could be lost entirely
once the average global temperature rises by 2.5°C over the preindustrial level as
the forests become a massive source of CO 2 (Seppälä, Buck, and Katila 2009). In
Search WWH ::




Custom Search