Tree Growth and Ecosystem Respiration in Central Amazon Forest

1. Tree Growth and Ecosystem Respiration in Central Amazon Forest Jeffrey Q. Chambers, Edgard S. Tribuzy, Roseana P. da Silva, Ligia C. Toledo, Joaquim dos Santos, Niro Higuchi, and Susan E. Trumbore

2. Calculating Forest Net Primary Productivity When estimating production from permanent inventory plots both old and new losses must be considered. Typically, for above-ground NPP, the biomass increment and fine litterfall is considered. Branchfall is usually not considered in NPP estimates. Trees are not simple mass accumulators.

3. Allometric Models for Predicting Tree Mass Tree mass allometry should be based on a representative sample including damaged and senescent trees. Accelerating losses for the largest trees shows up as curvature in the relationship. We have developed a generic moist forest allometric model from five tropical forest sites. An unbiased relationship followed a log-log-cubic curve.

4. Why does branchfall need to be included in NPP estimates? Envision a hypothetical forest where trees are losing branches as fast as new wood is being produced so that tree mass over time does not increase (Mt1 = Mt2). The production not being accounted for here is equal to branchfall (Lbranch). Because trees both gain and lose mass as they increase in size, both old and new losses must be included.

5. Estimates are based on numerous field studies over large temporal and spatial scales. Below-ground productivity can be estimated using a modified Raich and Nadelhoffer method ... 0.4 4.0 ± 0.3 (?) 2.1 ± 0.2 Units: Mg C ha-1 yr-1 Mass losses from damaged trees is a relatively small but significant flux.

6. Central Amazon Forest Autotrophic Respiration Total= 29.3 BG Assumptions: Changes in SOM relatively small; BG production and mortality are about equal; and carbon use efficiencies (CUEs) AG and BG about equal. 17.5 Rleaf 6.8 5.0 RBG Rstem CUE = 0.23 Units: Mg C ha-1 yr-1 AG respiration is considerably higher than BG respiration.

7. Values presented for NPP and respiration are long-term average fluxes. We are also finding considerable temporal and spatial variability in these fluxes. Total = 8.6 0.4 4.0 ± 0.3 2.1 2.1 ± 0.2 Units: Mg C ha-1 yr-1 AG-NPP is considerably greater than BG-NPP.

8. Partitioning of NPP in Central Amazon Forest To quantify how carbon storage responds to an increase in productivity both the allocation of NPP ... Woody tissues represent 49% of above-ground NPP

9. Carbon Pools in Central Amazon Forest and the size of the carbon reservoir determines the residence time and the capacity for sequestering carbon. Only large wood and SOM have a high capacity to sequester carbon for long periods of time.

10. 20 m We have developed a stochastic-empirical individual-based model to explore the carbon cycling dynamics of large wood in Central Amazon forest. Tree Stand Plot

11. CO2 CO2 respiration growth mortality live wood coarse litter fragmentation recruitment large wood Carbon cycling structure of the model We can use this model to explore how changes affecting individual trees and canopy gaps influences ecosystem scale carbon cycling and storage.

12. Carbon balance with only background mortality Total large wood (TLW) includes both dead and live material. Points above the line indicate TLW is a carbon source, and below the line TLW is a carbon sink. TLW can act as a carbon sink by and increase in production or a decrease in coarse litter respiration. 100 ha model run Considerable temporal variability but long-term carbon balance.

13. Carbon balance w/ background and catastrophic mortality This simulation shows how large wood carbon balance responds to a single 50% mortality event at year 1000. There is a quick and large loss of ecosystem carbon right after the event. Afterwards large wood acts as a small sink for many years. 50% mortality event Also in long-term carbon balance but this forest is usually a carbon sink.

14. Carbon balance w/ background and catastrophic mortality Catastrophic mortality can play an important role in temporal variability in ecosystem carbon balance. It is important to quantify the frequency and intensity of these events. Large mortality events referred to as “blowdowns” are known to occur throughout the Amazon. 20% mortality events There is a similar response to 20% mortality events every 200 years.

15. Ikonos image of 10x10 km area in the Central Amazon High resolution satellite imagery is useful for quantifying the frequency and extent of catastrophic mortality events such as this blowdown which can exceed 2,000 ha in size. 200 m Severe downburst type winds are associated with late dry season storms.

16. The carbon sequestration potential of large wood We can also use the model to explore how large wood carbon balance responds to an increase in NPP. NPP was increased 0.5% per year for fifty years (the gray area). Large wood sequestered carbon for many years but at a low annual rate. The annual sink (0.5 Mg C ha-1 yr-1) is much lower than estimate from eddy covariance tower studies.

17. Precipitation Variability in Manaus from 1910-85 monthly precipitation (mm) The Central Amazon experiences considerable intra- and inter-annual variability in precipitation. Changes in moisture availability affects a number of physiological processes such as respiration and photosynthesis. El Niño often results in a dry early wet season.

18. Seasonal Variability in Forest Wood Production Linear gauge data There is a steep increase in wood production at the onset of the rainy season. Using linear gauge tensiometers and TDR we are comparing stem volume changes associated with moisture with changes from actual growth. Daily shrinking and swelling associated with transpiration is small compared with growth. Production changes seasonally

19. Precipitation Variables and Growth using Regression Analysis Changes in wood production is not only dependent on the amount of precipitation, but also on how precipitation is distributed. When the same amount of precipitation is distributed over many days, growth rates decline. This is probably because light availability is reduced on cloudy days limiting photosynthesis.

20. A Synthesis of Global NPP Studies A workshop at NCEAS compiled NPP data from numerous globally distributed studies. Some empirical relationships were evident that may help extrapolate NPP estimates from sites to regions.

21. Conclusions • NPP estimates based on tree mass allometry must consider both old and new litter losses. • BG-NPP is about 20% of total NPP. • Only large wood and SOM can sequester large amounts of carbon in response to an increase in NPP. • The frequency and extent of catastrophic mortality is important for understanding net carbon balance. • Central Amazon forests can sequester a large amount of carbon with increasing NPP, but in any given year sequestration is limited to about 0.5-1.0 Mg C ha-1 yr-1. • NPP is not a constant and exhibits both intra- and inter-annual variability with environmental changes. For further information and reprints contact Jeff Chambers at chambersjq@yahoo.com