Here we focus on forest productivity and its contributing factors (see Section 1.3.5 for phenological aspects). Rising atmospheric CO2 concentration, lengthening of the growing season due to warming, nitrogen deposition and changed management have resulted in a steady increase in annual forest CO2 storage capacity in the past few decades, which has led to a more significant net carbon uptake (Nabuurs et al., 2002). Satellite-derived estimates of global net primary production from satellite data of vegetation indexes indicate a 6% increase from 1982 to 1999, with large increases in tropical ecosystems (Nemani et al., 2003) (Figure 1.5). The study by Zhou et al. (2003), also using satellite data, confirm that the Northern Hemisphere vegetation activity has increased in magnitude by 12% in Eurasia and by 8% in North America from 1981 to 1999. Thus, the overall trend towards longer growing seasons is consistent with an increase in the ‘greenness’ of vegetation, for broadly continuous regions in Eurasia and in a more fragmented way in North America, reflecting changes in biological activity. Analyses in China attribute increases in net primary productivity, in part, to a country-wide lengthening of the growing season (Fang and Dingbo, 2003). Similarly, other studies find a decrease of 10 days in the frost period in northern China (Schwartz and Chen, 2002) and advances in spring phenology (Zheng et al., 2002).
Figure 1.5. Estimated changes in net primary productivity (NPP) between 1982 and 1999 derived from independent NDVI data sets from the Global Inventory Modeling and Mapping Studies (GIMMS) and Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Land (PAL). An overall increase in NPP is observed, which is consistent with rising atmospheric CO2 and warming. From Nemani et al., 2003. Reprinted with permission from AAAS.
However, in the humid evergreen tropical forest in Costa Rica, annual growth from 1984 to 2000 was shown to vary inversely with the annual mean of daily minimum temperature, because of increased respiration at night (Clark et al., 2003). For southern Europe, a trend towards a reduction in biomass production has been detected in relation to rainfall decrease (Maselli, 2004), especially after the severe drought of 2003 (Gobron et al., 2005; Lobo and Maisongrande, 2006). A recent study in the mountains of north-east Spain (Jump et al., 2006) shows significantly lower growth of mature beech trees at the lower limit of this species compared with those at higher altitudes. Growth at the lower Fagus limit was characterised by a rapid recent decline starting in approximately 1975. By 2003, the growth of mature trees had fallen by 49% when compared with pre-decline levels. Analysis of climate–growth relationships suggests that the observed decline in growth is a result of warming temperatures. For North America, recent observations from satellite imagery (for the period 1982 to 2003) document a decline for a substantial portion of northern forest, possibly related to warmer and longer summers, whereas tundra productivity is continuing to increase (Goetz et al., 2005). They also confirm other results about the effects of droughts (Lotsch et al., 2005), as well those made by ground measurements (D’Arrigo et al., 2004; Wilmking et al., 2004).
Climate warming can also change the disturbance regime of forests by extending the range of some damaging insects, as observed during the last 20 years for bark beetles in the USA (Williams and Liebhold, 2002) or pine processionary moth in Europe (Battisti et al., 2005). The latter has displayed a northward shift of 27 km/decade near Paris, a 70 m/decade upward shift in altitude for southern slopes, and 30 m/decade for northern slopes in Italian mountains.
Trends in disturbance resulting from forest fires are still a subject of controversy. In spite of current management practices that tend to reduce fuel load in forests, climate variability is often the dominant factor affecting large wildfires, given the presence of ignition sources (McKenzie et al., 2004). This is confirmed by an analysis of forest fires in Siberia between 1989 and 1999 (Conard et al., 2002), which detected the significant impacts of two large fires in 1996 and 1998, resulting in 13 million ha burned and 14 to 20% of the annual global carbon emissions from forest fires. The increase in outdoor fires in England and Wales between 1965 and 1998 may be attributable to a trend towards warmer and drier summer conditions (Cannell et al., 1999). Repeated large forest fires during the warm season in recent years in the Mediterranean region and North Africa, as well as in California, have also been linked to drought episodes. One study of forest fires in Canada (Gillett et al., 2004) found that about half of the observed increase in burnt area during the last 40 years, in spite of improved fire-fighting techniques, is in agreement with simulated warming from a general circulation model (GCM). This finding is not fully supported by another study, which found that fire frequency in Canada has recently decreased in response to better fire protection and that the effects of climate change on fire activity are complex (Bergeron et al., 2004). However, it seems to be confirmed by another recent study (Westerling et al., 2006), which established a dramatic and sudden increase in large wildfire activity in the western USA in the mid-1980s closely associated with increased spring and summer temperatures and an earlier spring snow melt.