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

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

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

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.

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%

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

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

Response by tree size at Rocky Brook

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

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

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

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

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

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

Growth Response and Site Quality 50 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.)

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.)

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

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?

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

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

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)

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)

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

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

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

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.)?

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

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

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

Many More Possibilities Thanks Time for Questions & Discussion