study site and data
The study was conducted in the moist lowland tropical forest on Barro Colorado Island (BCI), a research reserve of the Smithsonian Tropical Research Institute. Rainfall averaged 2637 ± 462 mm year−1 for the period 1929–2001, with a 4-month dry season between January and April (Condit et al. 2001). Mean humidity is 77.9%, and daily maximum and minimum temperatures are 30.81 °C and 23.42 °C (1971–2001 average). Further meteorological information about BCI is available at http://www.stri.org/tesp. The forest is partly deciduous, with about 10% of canopy leaves dropped at the peak of the dry season (Condit et al. 2001). The 50-ha plot was established in 1981 (Hubbell & Foster 1983, 1986; Condit 1995, 1998; Condit et al. 1995; Leigh 1999). All stems ≥ 1 cm in trunk diameter were mapped, measured and identified to species between 1981 and 1983 (taxonomy follows Croat 1978; D’Arcy 1987, or Condit et al. 1996, unless specified otherwise). The census was repeated in 1985 and every 5 years thereafter.
More than 200 000 individual trees of over 300 species have been recorded in each census. From 1985 forward, trunk diameter (henceforth referred to as diameter) was measured to the nearest millimetre at 1.30 m above ground or above buttresses. If a trunk was swollen at 1.30 m, the measurement was taken 20 mm lower (Condit 1995, p. 50). Points of measure (POM) were painted on buttressed stems during 1990–2000. In 1982, measurements were not always taken above buttresses, so we excluded the 1982 census from our analysis. Multi-stemmed, broken and re-sprouting trees were recorded and handled separately in growth analyses. Measurement problems were handled during each census by checking field sheets for consistency and by re-measuring problematic trees. A subsample of 1715 trees was re-measured to assess independently the quality of diameter data. About 96% of the data had a relative error of less than 10%. For a more detailed description of the field methodology, the reader is referred to Condit (1998).
Above-ground biomass change, denoted ΔAGB, is due to growth G, plus recruitment R, minus loss M. More precisely, G can be defined as the annual increment of AGB due to the growth of trees that were alive during two successive censuses, M as the annual loss of AGB due to the mortality of trees that died by the second interval, and R as the annual ingrowth of AGB due to recruitment into the minimal diameter class between the first and second censuses. Clark et al. (2001a) refer to AGB increment as the sum of AGB growth plus recruitment.
Denoting the above ground biomass of tree i at time t by AGBi(t) and defining its increment between census time t and census time t + Δt as
- ΔAGBi(t) = AGBi(t + Δt) − AGBi(t)(eqn 3)
The total biomass is , and the total increment is, in Mg ha−1 y−1
- (eqn 4)
where the sum is taken over all the surviving trees in the census plot. The precise interval of measurement between two censuses, Δt, may vary among trees.
By convention, AGB loss is defined by
- (eqn 5)
where EM is the subset of trees alive at time ti and dead at the next census, and T is the mean census period (5 years). Clark et al. (2001a) define AGB recruitment R as given by equations 3 and 4, where AGB at time t is the AGB of a minimum-sized tree, and AGB at time t + Δt is that of the recruited tree. The alternative hypothesis is to assume that the AGB of the tree before it recruited was zero (Y. Malhi et al., unpublished results). When the minimal tree diameter is 1 cm, we tested that both assumptions yield almost undistinguishable results.
Using discrete time steps leads to a slight underestimate of AGB increment and loss because trees that die between two censuses grow some biomass before they die (Delaney et al. 1998). Trees that die between censuses survive, on average, half of the census period Δt. During this period, they grow a fraction (G + R)/AGB of biomass above ground, thus the additional growth term is roughly (G + R)/AGB × M × (T/2). If G+ R= M = 5 Mg ha−1 year−1, T= 5 year, and AGB = 300 Mg ha−1 this additional term would be about 0.2 Mg−1 ha−1 year. However, AGB losses should also be inflated by a term of the same magnitude. Thus, the estimates of AGB change are unaffected.
To reduce measurement error in calculating fluxes, problematic records were checked individually. There were 788 trees (0.3% of the total sample, 2.7 m2 ha−1) with anomalous diameter increases (> 35 mm year−1) or decreases (< 5 mm year−1) between two censuses. The diameter measurement in each of four censuses, 1985–2000, were checked in all 788 cases, and most had one obviously outlying diameter record. We used the three other measurements to replace the egregious one with an interpolated value. After these changes, we were left with five anomalous measurements, large changes where we could not easily detect the error. We excluded these five from estimates of AGB growth and loss (this is equivalent to setting the growth of these trees to zero). In addition, for 5071 cases (out of 591 099) where the point-of-measure of stem diameter changed between censuses, we assumed a zero change: these trees (c. 10% of the stand basal area) were excluded from AGB flux estimates. We also tested the importance of these excluded trees in the estimate of AGB change by assuming that these trees had had an average growth in diameter.
Major trunk or crown loss above the POM was noted in the field in 1985 for all trees, and in 1990–2000 for trees ≥ 10 cm diameter. In 2002, we checked a random sample of 191 of the trees marked as damaged in 2000. For each, we estimated the height at which the trunk broke and the amount of crown lost, compared the broken height to the predicted height from regression, and assumed that trunk biomass was reduced by the fraction of height lost. We further assumed that crown biomass was reduced by the amount of crown that we estimated to be missing. If crown AGB is 25% of total AGB (Malhi et al. 1999; J. Chave, unpublished results), we can convert these figures to a crude estimate of the percentage AGB lost to the break. The mean percentage lost per tree was multiplied by the number of trees recorded with a new break above the POM in each census period. For trees < 10 cm diameter, we had to assume that the percentage AGB loss per tree was the same as it was in the sample of trees > 10 cm diameter we checked in 2002, as no damaged smaller trees were checked.
All trees recorded as broken below the POM, but which survived and sprouted a new stem, were noted in each census (Condit 1998). Here, we treat these cases as mortality and then recruitment in the AGB flux estimates.