The role land-use change has played in increasing terrestrial productivity over the last 200 years, relative to climate and atmospheric changes, is controversial. If changing atmospheric chemical composition is increasing productivity, increasing Ca is without question regarded as the key driver, stimulating photosynthesis and widely documented as being able to provide varying degrees of 'protection' against many plant stresses and the deleterious effects of other important atmospheric changes, such as the increase in tropospheric ozone concentrations (Drake et al., 1997; Hymus et al., 2001; Ashmore, 2005). However, the role that increasing Ca has played in recent global productivity remains the subject of much debate. Recently Casperson et al. (2000) concluded that increases in Ca over the last century were not a factor that could explain increases in US forest productivity over the same time period. This finding has been challenged by Joos et al. (2002) who, using the same data-set, concluded that the same increases in productivity could indeed have been the result of CO2 fertilization. Some studies have made more specific conclusions. In a modelling study, Osborne et al. (2000) reconciled a 25% increase in NPP in the Mediterranean in the last 100 years primarily with a 70 ppm increase in Ca and consequent increases in water use efficiency. Thornton et al. (2002) modelled carbon accumulation after disturbance for coniferous forests and showed that increases in Ca reduced the period in which the forest was a source of CO2 to the atmosphere following disturbance, the total carbon lost during this period, the time to reach peak sink strength and increased total carbon storage. Nitrogen deposition was an important additive factor in this assessment, with the effects described earlier increasing with nitrogen deposition.
What is less contentious than attributing increases in terrestrial productivity with increases in Ca specifically is that a combination of changes in climate and atmospheric composition has had significant effects on terrestrial productivity over the last few decades. The last two decades have seen rapid changes in key atmospheric and climate variables: Ca increased by 9%, they were two of the warmest decades in the instrumental record, they experienced three persistent El Niño events and a human population increase of 37% (Nemani et al., 2003). These changes combined with nitrogen deposition and forest regrowth are well documented to have contributed to regional increases in NPP, particularly in the mid to high latitudes (Myneni et al., 1997; Houghton, 1999). However, Nemani et al. (2003) reconciled 18 years of satellite data with climate records collected between 1982 and 1999 to conclude that a broad suite of climatic changes over that time period had eased climatic restrictions on NPP and increased NPP by 6% or 3.4 Pg C. Importantly, this study highlighted much regional variability, with the greatest increase in NPP, of 42%, observed in the tropics primarily due to decreases in cloud cover. This observation of an increase in productivity in the tropics is in keeping with the recent findings of Lewis et al. (2004) who concluded that there had been significant increases in productivity over the last three decades in South American tropical forests which would have resulted in the region being an abundant carbon sink. This was also explained in terms of a combination of increases in key resources, including Ca and radiation.
In addition to easing of limitations to productivity across the globe, global change has also caused specific changes in vegetation phenology that have implications for productivity. We highlight the example of advancing spring bud break and the greening up of forest canopies in mid- and high-latitude forests at a rate of several days per decade over the last 50 years (Badeck et al., 2004). This effect lengthens the growing season, which is in turn well correlated with increase in NEP (Baldocchi and Wilson, 2001).
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