Figure 12.2. (a) Above-ground biomass change (dry weight) of trees greater than 10-cm diameter in 59 Amazon plots, based on initial and final stand-biomass estimates calculated using an allometric equation relating individual tree diameter to biomass, and incorporating a correction factor to account for variation in wood density among species (from Baker et al., 2004a). As would be expected in a random sample of small plots measured for a finite period, some sites show a decline in biomass during that period indicating that at that particular point in space and time tree mortality has exceeded tree growth. However, the mean and median are shifted significantly to the right (P < 0.01). (b) Stem number change in 91 plots from across South American tropical forests. Stems were counted during the first and final censuses of each plot (plots are the same as those used by Phillips et al., 2004). The mean and median are shifted significantly to the right (P < 0.05).

convention (Phillips and Gentry, 1994) we estimate stem turnover between any two censuses as the mean of annual mortality and recruitment rates for the population of trees > 10-cm diameter. Second, we examine changes in biomass fluxes of the forest— in terms of growth of trees and the biomass lost with mortality events. These stand-

Figure 12.3. Annualized rates of stand-level basal-area growth, basal-area mortality, stem recruitment, and stem mortality from plots with two consecutive census intervals (i.e., the subset of RAINFOR sites that have been inventoried on at least three successive occasions), each giving the mean from 50 plots with 95% confidence intervals. Paired t-tests show that all of the increases are significant. The average mid-year of the first and second censuses was 1989 and 1996, respectively (from Lewis et al., 2004b).

Figure 12.3. Annualized rates of stand-level basal-area growth, basal-area mortality, stem recruitment, and stem mortality from plots with two consecutive census intervals (i.e., the subset of RAINFOR sites that have been inventoried on at least three successive occasions), each giving the mean from 50 plots with 95% confidence intervals. Paired t-tests show that all of the increases are significant. The average mid-year of the first and second censuses was 1989 and 1996, respectively (from Lewis et al., 2004b).

level rates of "biomass growth'' and "biomass loss'' should be approximately proportional to the rate at which surviving and recruiting trees gain basal area and the rate at which basal area is lost from the stand through tree death (Phillips et al., 1994).

Among 50 old-growth plots across tropical South America with at least three censuses (and therefore at least two consecutive monitoring periods that can be compared), we find that all of these key ecosystem processes—stem recruitment, mortality, and turnover, and biomass growth, loss, and turnover—are increasing significantly (Figure 12.3), between the first and second halves of the monitoring period (Lewis et al., 2004b). Thus, over the past two decades, these forests have become, on average, faster growing and more dynamic. Notably, the increases in the rate of dynamic fluxes (growth, recruitment, and mortality) are about an order of magnitude larger than are the increases in the structural pools (above-ground biomass and stem density; Lewis et al., 2004b).

These and similar results can be demonstrated graphically in a number of ways. In Figure 12.4, we plot the across-site mean values for stem recruitment and mortality as a function of calendar year. This shows that the increase has not been short-term (e.g., the result of a spike around a year with unusual weather), that recruitment rates have on average consistently exceeded mortality rates, and that mortality appears to lag recruitment (Phillips et al., 2004).

Using data for the 50 plots with two consecutive census intervals, we can also separate them into two groups: one faster growing and more dynamic (mostly western Amazonian), and one slower growing and less dynamic (mostly eastern and central Amazonian). Both groups showed increased stem recruitment, stem mortality, stand basal-area growth, and stand basal-area mortality, with larger absolute increases in rates in the faster growing and more dynamic sites than in the slower-growing and less dynamic sites (Figure 12.5; Lewis et al., 2004b). However, the proportional increases

Year

Figure 12.4. Mean and 95% confidence intervals for stem recruitment and mortality rates against calendar year, for plots arrayed across Amazonia. Rates for each plot were corrected for the effects of differing census-interval lengths, for "site-switching", and for "majestic-forest bias". A detailed justification methodology for these corrections is given in Phillips et al. (2004); all trends are robust and hold equally if these corrections are not applied. Black indicates recruitment, grey indicates mortality, solid lines are means, and dots are 95% confidence intervals (from Phillips et al., 2004).

Year

Figure 12.4. Mean and 95% confidence intervals for stem recruitment and mortality rates against calendar year, for plots arrayed across Amazonia. Rates for each plot were corrected for the effects of differing census-interval lengths, for "site-switching", and for "majestic-forest bias". A detailed justification methodology for these corrections is given in Phillips et al. (2004); all trends are robust and hold equally if these corrections are not applied. Black indicates recruitment, grey indicates mortality, solid lines are means, and dots are 95% confidence intervals (from Phillips et al., 2004).

in rates were similar, and statistically indistinguishable, across both forest types (Lewis et al., 2004b). This shows that increasing growth, recruitment, and mortality rates are occurring proportionately similarly across different forest types and geographically widespread areas.

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