Nutrient Limitation of Young Douglas-fir
Collaborators:	Doug Maguire, Rick Fletcher, Doug Mainwaring (OSU)
Funding:	Private and Oregon Department of Forestry

Fertilization is one of the primary tools used in intensive plantation forestry in the Pacific Northwest. Most fertilization research in Douglas-fir stands has focused on growth response to nitrogen (N), and shows N-limitation of approximately 2/3 of forest stands. However, fully 1/3 of forests do not respond to N additions, and decreased growth in response to added N has also been observed. Nutrient limitations by phosphorus (P) or calcium (Ca) deficiency are prime candidates on N-rich sites, as well as deleterious effects of high aluminum (Al), and co-limitation by multiple nutrients is also possible.

This project examines overall nutritional limits to Douglas-fir productivity based on application of site-specific, multi-nutrient blends and also assesses more specifically which soil nutritional factors (i.e., P, Ca, low pH) may contribute to growth limitations when N is abundant. Currently 12 study sites are distributed across a range of soil types throughout the Oregon and Washington Coast Range and Cascades. At each installation, target stands are 20 yrs old (± 5 ys), at densities common to managed stands (about 300 ± 100 tpa), without prior fertilization or pre-commercial thinning in the last eight years.

Replicated fertilization treatments were initiated in Fall 2006 on an individual tree basis to examine growth response to (1) Unfertilized control, (2) Nitrogen (224 kg/ha N as urea), (3) Calcium as lime (1000 kg Ca/ha), (4) Calcium as calcium chloride (100 kg Ca/ha, tests calcium deficiency without pH adjustment), (5) Phosphorus (420 kg/ha P as di-sodium phosphate), (6) site-specific Fenn prescriptions, and (7) site-specific Kinsey prescriptions. Period re-measurement will indicate growth response to nutrient applications. Results of this work will inform forest fertilization management prescriptions, particularly on N-rich sites, and will help sustain the historically high productivity of Pacific Northwest forest lands.

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