Hawaii

 

Why does forest growth decline with stand age? An experimental test of carbon acquisition and allocation over stand development. Funded by the National Science Foundation, with Dan Binkley, Colorado State University and Jim Fownes, University of Massachusetts at Amherst.

 

The decline in wood production after canopy closure helps set the rotation age in production forestry and the potential for carbon sequestration in older forests. Decline in production appears to be universal, but the mechanism is unknown. We planted fast-growing Eucalyptus saligna in Hawaii to test basic hypotheses about the mechanism that causes decline. Our hypotheses dealt with resource supply, resource use, and carbon allocation: 1) wood production declines as a result of increasing C allocation to woody respiration as trees become larger; 2) wood production declines because C allocation to belowground production increases as trees become larger; and 3) wood production declines because photosynthesis declines as a result of declining soil nutrient supply or increasing hydraulic limitation as trees growth larger. Our plantations used two tree spacings (1x1 m and 3x3 m) to vary the display of canopy leaves and the balance between leaves and woody tissues, and two fertilization regimes (only at time of planting, or 4 applications/yr with all elements). We measured the complete C budget over 6 years of plantation development. We also planted a second set of plots to allow comparisons of plots of different age under the same weather conditions (primarily to test the hydraulic limitation hypothesis). Current results show: 1) wood production and canopy photosynthesis declined even with very high resource availability; 2) woody respiration, belowground allocation, and hydraulic limitation do not explain the decline in production; and 3) the decline in wood production was more pronounced and belowground allocation was greater in the less fertile stands.


Wood production and respiration both declined substantially after plots reached full leaf area, following the decline in gross primary production (GPP). The decline in production occurred despite sustained leaf area (with fertilization), light interception, canopy N, and water use. 1x1 or 3x3 m spacing; C = control, F=heavily fertilized; error bars = standard error of treatment means (Ryan, Binkley, Fownes, Giardina, Senock, unpublished)


     Aerial view of the plantation.

 


     Dan Binkley, Jose Luiz Stape, Christian Giardina
    and Jim Fownes in a 3x3 plot.


     Eucalyptus 6 months after planting
     18 months after planting   
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