There was a notable difference in the slopes of the graphs of NPP against precipitation for the mountainous and lowland sites in Figure 17.10. The slope was much lower in the mountainous case and it seems likely that the steeper terrain in this region resulted in a higher rate of water runoff from the land and, thus, a lower efficiency in the use of precipitation (Jobbagy et al., 2002).
A related phenomenon has been soil texture can observed when forest production on influence productivity sandy, well-drained soils is compared with soils consisting of finer particle sizes. Data are available for the accumulation through time of forest biomass at a number of sites where all the trees had been removed by a natural disturbance or human clearance. For forests around the world, Johnson et al. (2000) have reported the relationship between above-ground biomass accumulation (a rough index of ANPP) and accumulated growing season degree-days (stand age in years X growing season temperature X growing season as a proportion of the year). In effect, 'growing season degree-days' combine the time for which the stand has been accumulating biomass with the average temperature at the site in question. Figure 17.11 shows that productivity of broadleaf forests is generally much lower, for a given value for growing season degree-days, when the forest is on sandy soil. Such soils have less favorable soil-moisture-holding capacities and this accounts in some measure for their poorer productivity. In addition, however, nutrient retention may be lower in coarse soils, further reducing productivity compared to soils with finer texture. This was confirmed by Reich et al. (1997) who, in their compilation of data for 50 North American forests, found that soil nitrogen availability (estimated as annual net nitrogen mineralization rate) was indeed lower in sandier soils and, moreover, that ANPP was lower per unit of available nitrogen in sandy situations.
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