Testing causes of age-related productivity decline in forests

Collaborators: Dan Binkley, Colorado State University; Christian Giardina, USDS FS, Pacific Southwest Research Station; Jim Fownes, University of Massechusetts; Randy Senock, Chico State University

Productivity of Tropical Plantations Meeting Porto Seguro, Brazil, Nov 10-14, 2008 Understanding why tree productivity declines with forest age is a universal phenomenon, important to forest managers, and a fundamental problem in forest biology. Working with fast growing trees allows quicker answers and easier manipulations to understand the phenomenon. Results from a 7-year study in Hawaii showed that productivity declines in fast-growing trees were similar to those in subalpine forests, but happened in 5 years, not 150. Results also showed that reduced canopy photosynthesis and changes in carbon allocation were important causes of the decline, and that the decline was less under more fertile conditions. Major findings: Ryan et al. (2004): We followed all components of the carbon budget in an age sequence of fast growing Eucalyptus saligna in Hawaii with 3 fertility (minimal, high, and restore fertility after 3 years) and 2 tree density treatments (1111/ha and 10000/ha). Wood growth and aboveground NPP declined in all treatments, with greater declines in the high density and minimal fertility treatments. Lower canopy photosynthesis was largely responsible for the decline in wood growth and aboveground NPP with a smaller contribution from increased below ground carbon allocation and foliar respiration. Giardina et al. (2004): ~90% of soil respiration is derived from carbon that was produced within the past year. Fertilization lowers relative carbon allocation belowground and lowers the carbon flow to soil carbon. Hubbard et al. (2004): Sapflow per unit sapwood area quickly increased in response to fertilizer application, but then decreased to control levels after a few months. Wood production per unit of water use increased with fertilization, but not significantly. Giardina et al. (2003): Fertilization increased canopy photosynthesis and decreased belowground carbon allocation as a fraction of photosynthesis. Barnard and Ryan (2003): Hydraulic limitation was not responsible for lower photosynthesis in older taller trees. The older trees adjusted by loweing leaf water potential mid-day. Giardina and Ryan (2002): Total belowground carbon alocation can be measured even in rapidly growing ecosystems using a carbon balance approach (soil respiration minus litterfall), as long as changes in coarse root biomass is counted.		January 1996 Same location,. January 2002
Publications and Presentations Control of Production and Carbon Allocation in Eucalyptus – Seminar October 2007 Ryan MG, D Binkley, JH Fownes, CP Giardina and RS Senock. 2004. An experimental test of the causes of forest growth decline with stand age. Ecological Monographs 74: 393-414. Causes of Age-Related Growth Decline in Forests and Limits to Tree Height – Seminar, 2003 Giardina CP, D Binkley, MG Ryan, JH Fownes and RS Senock. 2004. Belowground carbon cycling in a humid tropical forest decreases with fertilization. Oecologia 139: 545-550. Hubbard RM, MG Ryan, CP Giardina and HR Barnard. 2004. The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest. Global Change Biology 10:427-436. Giardina CP, MG Ryan, D Binkley and JH Fownes. 2003. Primary production and carbon allocation in relation to nutrient supply in an experimental tropical forest. Global Change Biology 9:1438-1450. Barnard HR and MG Ryan. 2003. A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna. Plant, Cell and Environment 26:1235-1245. Giardina CP and MG Ryan. 2002. Total belowground carbon allocation in a fast growing Eucalyptus plantation estimated using a carbon balance approach. Ecosystems 5: 487-499. Binkley D, JL Stape, MG Ryan, HR Barnard and JH Fownes. 2002. Age-related decline in forest ecosystem growth: an individual-tree, stand-structure hypothesis. Ecosystems 5:58-67. Related Ryan MG, D Binkley and JH Fownes. 1997. Age-related decline in forest productivity: pattern and process. Advances in Ecological Research 27: 213-262. Ryan MG and BJ Yoder 1997. Hydraulic limits to tree height and tree growth. Bioscience 47: 235-242.		Jim Fownes, Tom, Elda Rae, Randy Senock, Dan Binkley in 1996.
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