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College of Forest Resources

 

ARTICLES OF INTEREST

Effects of Organic Matter Retention and Management on Long-term Productivity of
Pacific Northwest Coastal Douglas-fir Plantations

Robert B. Harrison, Associate Professor of Forest Soils
Robert L. Edmonds, Professor of Soil Microbiology
Dale W. Cole, Professor of Forest Soils
Amy Sidell and Barry Flaming, Graduate Students
College of Forest Resources, University of Washington
Thomas A. Terry, William Scott, & Ron Heninger, Senior Scientists Weyerhaeuser Corporation
Alex Dobkowski, Senior Research Forester Weyerhaeuser Corporation
Richard Miller, Emeritus Soil Scientist USDA Forest Service, PNW Station.

Introduction:

Increased demands for wood fiber coupled with reductions in the acreage of production forests have created the need to produce more material from a given area; i.e., increased biomass utilization and more intensive harvesting and management practices. The higher intensities of management can result in increased pressure on the basic soil factors that contribute to site productivity potential, with possible negative impacts as a result of compaction of the soil from mechanized harvesting, direct removal of nutrients in biomass, and weed control.

Understory vegetation was collected from 5 subplots per treatment plot		Forest floor and organic soil horizons (Oi, Oe, Oa) were collected from 2 randomly selected 0.25 m2 subplots per treatment plot

 

 

 

 

This study was initiated to investigate the long-term effects of common industry practices on forest productivity and should begin to fill a critical data gap that exists in the Pacific Northwest Region for Douglas-fir management. The specific objectives of this study are:

1) To develop nutrient budgets for various levels of organic matter removal from typical highly productive Site II Douglas-fir stands.

2) To evaluate the effects of increased harvest utilization (biomass removal) and soil compaction on organic matter, nutrients, soil porosity, and productivity of the subsequent stand.

3) To evaluate the effectiveness of ameliorative fertilization and tillage on organic matter, nutrients, soil porosity, and subsequent stand growth, and to assess the effect of weed control on nutrient redistribution, soil properties, and stand development and growth.

 

 

The two photos show coarse woody debris being quantified by measuring diameter, length and decomposition class along 3 randomly located 15m transects per treatment plot. Samples of each decomposition class were collected for nutrient analysis
Soil A horizon and B horizon (0-10cm, 10-30cm, 30-50cm, and 50+cm) samples were collected and measured from 2 randomly-selected subplots per treatment plot using a 3.5cm diameter soil probe

Work completed to date:

We are now nearing the final phases of fieldwork in order to characterize the initial stand conditions before harvesting and installation of the treatments. Presently, we have completed sampling the understory vegetation, forest floor, and soils.

To quantify the amount of understory vegetation, 5 randomly located subplots were established in each of the 48 treatment plots. For each subplot, all oxalis was collected from a 0.25 m2 area. All other understory species present within a 2-meter radius were collected, oven-dried to constant weight, and weighed. Presently, we are composting vegetation samples for nutrient analysis.

Forest floor was removed from the same 0.25 m2 area in only two of the five randomly located subplots. Samples have been oven-dried to constant weight, and are currently being prepared for nutrient analysis.

Soil samples were taken with a soil probe to about 80 cm deep, and separated as A horizon, B 0-10 cm, B 10-30 cm, B 30-50 cm, and B 50+ cm. The length of each depth was recorded to allow for bulk density volume of the soil probe. Additional samples were taken with a traditional bulk density sampler and revealed a high correlation (r2=0.70) with the soil probe estimation of bulk density. Samples have been air-dried and are being weighed and prepared for nutrient analysis.

 

In addition to the above sampling, coarse woody debris was also estimated. We ran three randomly located transects 15 m long by 30 cm wide in each plot. Large woody material not sampled as forest floor was collected, decomposition class noted, and weighed. We rated decomposing logs into five continuous classes based on observable characteristics, such as the presence or absence of bark and twigs. Class 1 logs are recent additions to the forest floor and still have intact bark and twigs present. Class 3 logs have only trace signs of bark and no twigs present. A Class 5 log is the most highly decomposed with a soft and powdery texture and is almost completely incorporated into the forest floor. Material that was too large to cut with a chainsaw was “mapped” by recording its dimensions and decomposition class. Samples of each decomposition class were randomly collected for nutrient analysis.

Future Work:

The above samples will be analyzed for the parameters listed in Table 1.

 

Table 1: Analysis Parameters
Vegetation and Forest Floor	Soil
Dry Weight pH	Dry Weight pH	Mineralizable N Bray-2 Extractable P
Total Elemental Concentration	Total Elemental Concentration	Cation Exchange Capacity
(C,N,P,K,Ca,Mg,S, B)	(C,N,P,K,Ca,Mg,S, B)	Excangeable cations
		Bulk density

 

 

 

 

 

 

 

 

Tree Biomass Estimation:
Our next step to complete the initial stand characterization will be to estimate standing tree biomass. Diameters at breast height have been recorded for all trees in the study. We will select at random thirty Douglas-fir and thirty western hemlock trees based on their diameter distributions. These trees will be felled and we will then be able to quantify the biomass components of live branches, dead branches, and bole wood and bark.

This site has been scheduled to be logged this winter, as soon as all fieldwork is completed. Treatments will then be randomly selected and installed. The study design consists of four blocks of twelve treatment plots which are 0.63 acres in size and internal measurement plots which are 0.33 acres in size to ensure that valid long-term productivity trends (i.e., with fertilization or possibility for thinning) can be monitored. The twelve treatments of interest are listed in Table 2.

 

Table 2: Study Design and Treatments
			OM & Nutrient Treatments				Compaction Treatments
Plot	Treatment Designation		Harvest Level	Nitrogen Fertilization*	Weed Control		Soil Compaction	Soil Tillage
1	O1FoWo	CoTo	Bole-only	____	____		____	____
2	O1F1Wo	CoTo	Bole-only	200 lb. N	____		____	____
3	O1FoW1	CoTo	Bole-only	____	Weed Control		____	____
4	O1F1W1	CoTo	Bole-only	200 lb. N	Weed Control		____	____
5	O2FoWo	CoTo	Total-tree	____	____		____	____
6	O2F1Wo	CoTo	Total-tree	200 lb. N	____		____	____
7	O2FoW1	CoTo	Total-tree	____	Weed Control		____	____
8	O2F1W1	CoTo	Total-tree	200 lb. N	Weed Control		____	____
9	O2F1Wo	C1To	Total-tree	200 lb. N	____		Compaction	____
10	O2F1Wo	C1T1	Total-tree	200 lb. N	____		Compaction	Tillage
11	O2F1W1	C1To	Total-tree	200 lb. N	Weed Control		Compaction	____
12	O2F1W1	C1T1	Total-tree	200 lb. N	Weed Control		Compaction	Tillage

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

---" indicates this treatment will not be made for this plot

*200 lb N fertilizer will be applied at age 10 and 6 year intervals; adjustments to fertilizer applications

(amounts and elements) may be made based on diagnostic results

 

The study site will be planted with genetically improved family-mix stock of 680 trees/acre after harvesting and treatment installation. Stand and soil measurements, including temperature and moisture will continue to be recorded. Atmospheric inputs and CO2 flux will also be monitored for a nutrient cycling study.

Questions can be addressed to:

Dr. Rob Harrison at: robh@u.washington.edu

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