1. N Fertilization of PNW Forests: Things we know and some things we ought to
Jeff DeRoss
4/26/07

2. The Growth Story
Growth response studied for many years, starting circa late 1950’s - early 1960’s
Early studies looked at N in various dosages, confounded with irregular additions of other nutrients (P, K, S)
Promising results spurred on larger, carefully designed studies – Shawnigan Lake, RFNRP

3. RFNRP tests fertilization effects across a variety of stand types
Three rates of N fertilization applied as urea (0, 200, 400 lb./ac.) – other rates/sources/elements examined to a lesser extent
Natural and planted stands of varying ages in Douglas-fir and hemlock
Different silvicultural treatments (commercial thinning, PCT) considered
Installations across western Washington and Oregon

4. Results from RFNRP and other regional studies tell the same story:
Approximately 2/3 of stands respond positively
Direction/magnitude of response is highly site dependent
Gains from first 200 lb./ac. of N are greater than gains from the second 200 lb./ac.

5. More results…
Fertilization helps stands recover from “thinning shock” quickly
Young and “old” stands have capacity to respond
Urea treatments are generally economically efficient, with net returns greater than 6%

6. Miller et al. summarized several regional studies (three here):

8. More results from Miller et al
More results from Miller et al. – responsiveness and contribution by size class in thinned and unthinned stands
Volume response by small and large trees at Shawnigan Lake, BC

9. Response by tree size at Rocky Brook

10. Basal area growth response
Opalach et al summarized growth response to single and multiple fertilizations (RFNRP Report #8, 1987)
Basal area growth response
refert
rethin
Total gross basal area growth response by two–year growth period

11. Volume growth response
refert
rethin
Total gross volume (CVTS) growth response by two–year growth period

12. Aboveground biomass increment
from Brix (1983)
Direct + indirect effect

13. Growth rate corrected for initial size at start of each growing season (analogous to relative growth rate)
from Brix (1983)
Direct effect

14. Direct effect + functional insight
Aboveground biomass increment per unit initial foliage mass
Growth efficiency
from Brix (1983)
Direct effect functional insight

15. These figures highlight a few things have learned:
Growth response appears to be long-lived (one or more decades) as fert. plots continue to outgrow controls
But when corrected for initial volume, growth response lasts ~5 years (direct effect)
Volume growth increases are due mainly to increases in basal area growth, not height growth

16. Growth Response and Site Quality50
Heath (1988) predicted gross volume growth response to fertilization by initial basal area and site index.
Prediction contours for unthinned Douglas-fir stands at right:
Site Index (feet) 90
Initial Basal Area (ft2/ac.)

17. Prediction contours for thinned Douglas-fir stands at right:
Unthinned stands have greater absolute growth response potential (more growing stock)
Thinned stands have greater percentage growth response potential
Also consider that after thinning, growth will be concentrated on higher value stems
Site Index (feet) 40 70
Initial Basal Area (ft2/ac.)

18. Notice from previous slides that increasing site quality is associated with decreasing growth response to N fertilization
Percent Difference in Volume Increment vs. Total Nitrogen Fertilizer Applied
% Difference in Volume Increment (Fertilized - Control)
Total N Fertilizer Applied (lb./ac.)
Sidell Thesis

19. Relationship between site quality and response to fertilization leads to more questions
Why do stands with equal site index respond differently?
What site variables can be used to explain response to fertilization?
Why is western hemlock so weird?

20. Start With The Obvious
Soils are a good starting point for explaining fertilization response
Fertilizer is a soil amendment
Soils are easy to sample
Large variety of tests for C, N, S, P, Cations available
Many options for predicting response

21. Measures of soil N are a good place to start
Total N
C/N ratio also considered
Mineralizable N a good measure
Shumway and Atkinson found mineralizable N in top 15 cm of soil correlates positively with site index, negatively with growth response to urea
46 ppm mineralizable nitrogen considered the point where N fertilization will give to positive response

22. Soils change in response to fertilization
Long term increases in soil N concentrations, mineralizable N possible (Binkley and Reid 1985)
Soil N, mineralizable N may not change though (Chappell et al. 1999)
Only partially explains the duration of response to N fertilization
Other elements can be implicated
P for hemlock (Radwan)
Ca of recent interest (Perakis et al. 2006)

23. Foliage can also help explain responses to fertilizer
Figures from Hinckley et. al
Leaf Nitrogen
Light
Harvesting
Chlorophyll
Other
Metabolic
Functions
CO2
Harvesting
Enzyme
Other
Leaf
Properties
Defense
Chemicals
Maximize (Leaf Longevity*Productivity)

26. Increasing leaf Nitrogen levels tends to increase productivity, however…
Only to a certain point, after which other nutrients or water may become limiting factors
Foliar N levels of about 1.5% or more can typically be considered fully adequate

27. Foliar considerations continued
Adequate foliar S and B are necessary for a positive response, tend to be the most limiting to N response
P can become an issue in cases of high N soils, P-sorbing volcanic soils
Considering these factors can give a reasonable prediction of response to N fertilization

28. A few things to look into…
Understanding the mechanisms of response to N fertilization – there are a number of ways to go about this
Understanding the interaction between fertilization and thinning, and how this interaction changes (or does not change) the mechanisms of response

29. Mechanisms of response to fertilization
Comprehensive study of nutrient levels in tree tissues and seasonal nutrient fluxes in the tissues
Does fertilization change the seasonal flux or amount stored through the seasons?
What does this indicate about growth response, nutrient budgets?
Do patterns indicate induced deficiencies (S, P, Ca, etc.)?

30. Other tissues worth a look
Foliage a starting point for studying seasonal nutrient flux in response to fertilization
Most relevant to growth
Beyond N Study
Other tissues worth a look
Inner and outer bark
Xylem – by radial and vertical position
Branch tissues

31. Check a few of the assumed relationships regarding response to N fertilization
Changes in leaf area as a result of fertilization in unthinned stands
Building leaf area in thinned stands
Does leaf area:sapwood area ratio remain constant after fertilization – studies (e.g. Vose and Allen 1988) indicate yes

32. Investigate linkages between fertilization response and environmental variables
Water stress
On older installations via Δ13C ratios
On new installations using irrigation x fertilization factorials (e.g. Albaugh et. al 2004)
Soil Parent Material
Accessible from NRCS maps, other sources – cheap & easy
Applicable to broad areas

33. Many More Possibilities
Thanks
Time for Questions & Discussion