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Growth of Immature, Spaced and Fertilized Douglas-Fir Stands:
Second Measurement Results

Eric Turnblom, Assistant Professor, M&E, Forest Biometry, College of Forest Resources, University of Washington and Rob Harrison, Associate Professor, ES, Forest Soils, College of Forest Resources, University of Washington

This report summarizes four-year (first growth period) results of auxiliary-fertilized treatment regimes set up in the Stand Management Co-op (SMC) Silviculture Project Type I installations. Type I installations are well-established juvenile stands that have not experienced substantial inter-tree competition.

OBJECTIVES

One of the secondary study objectives for Type I installations is to assess the effects of pruning, fertilization, and tree selection during pre-commercial thinning on growth, yield, and commercial value of stands that have received early and continued density control over the coastal Douglas-fir region of western British Columbia, Washington, and Oregon.

STUDY SITES

The SMC has been establishing a variable number of Type I installations per year since 1986. Presently, there are 30 Douglas-fir Type I installations, nine of which were augmented with fertilized plots. Fertilized plots were established in stands ranging in density from 304 to 762 stems per acre (average: 535 stems per acre). The stand Site Index (according to King) ranged from 90 to 135 ft. at 50 years breast height age (average: 120 ft.), and initial stand ages ranged from seven to fifteen years (average: 10 years). At the time of fertilization, average DBH’s ranged from 1.9 to 4.6 in. (average: 3.0 in.) and average total heights from 15 to 33 ft. (average: 22 ft.).

Treatments were assigned randomly at each installation. Urea was hand delivered to each treated plot at a rate of 200 lbs/acre. Fertilizer was applied in the establishment year, and will be applied every four years thereafter. Three fertilized and three unfertilized plots were examined at each installation. Pre-commercial thinning was systematically employed in the establishment year to yield a range of density levels. Two plots at each installation (one fertilized and one not) were not thinned, two plots were pre-commercially thinned to half their original density, and two plots were thinned to one-fourth their original density. Not all density levels were represented at each installations. Density was therefore classified into four levels (51 to 150 Trees Per Acre (TPA): midpoint 100, 151 to 250 TPA: midpoint 200, 251 to 450 TPA: midpoint 350, and 451 and over: labeled 550+). Site index was classified into three levels (low: SI <= 105, medium: 105 < SI <= 125, high: SI > 125).

EXPERIMENTAL DESIGN

The data were analyzed as a three-factor fixed-effect model design. The design was unbalanced because the 54 plots available for analysis did not equally represent all levels of every factor. Tests for fixed effects: Site class (high, medium, low), density (very low, moderate, high), and fertilization (yes, no) were conducted as well as all two-way interactions. Multiple comparisons of least square means were also made.

RESULTS

Growth observed over the first measurement interval (four years) as a result of the various intensities of early spacing and fertilization can be summarized as follows.

NET BASAL AREA GROWTH

Total stand response can be summarized by stand basal area. It is derived from directly measured variables (diameter and number), and accounts in some sense for both numbers of trees and the average size of those trees. Growth figures reported are adjusted for any initial (pre-treatment) differences in trees per acre (TPA) on the plots.

Average net periodic annual basal area increment of all the treatment regimes was found to be 8.4 sq.ft/ac/yr

Net periodic annual basal area increment was significantly different among the different treatment regimes (p=0.0001).

• The impact that thinning is generally expected to have on basal area growth was observed. Main effects for density were statistically significant (p=0.0001), though density did not interact significantly with any other factor.

Combining all sites and both fertilization levels, very low density stands (100 TPA) grew 3.4 sq.ft/ac/yr.

• Combining all sites and both fertilization levels, the low density class (200 TPA) exhibited mean growth of 6.5 sq.ft/ac/yr. This was significantly different from the very low density class (p=0.0001).

Combining all sites and both fertilization levels, the moderately dense plots (350 TPA) exhibited mean growth of 10.6 sq.ft/ac/yr. This is significantly different from the 200 TPA class (p=0.0001).
The high density class (550+ TPA), when averaged over all sites and fertilization levels exhibited mean growth of 12.3 sq.ft/ac/yr. Growth at this density level was significantly different from the 350 TPA class (p=0.0135).

Site quality and fertilization both impacted growth in the expected fashion. Greater net periodic annual increment was observed on higher sites (p=0.0001), and fertilization also had a positive effect on growth (p=0.0057). However, effects on growth of these two factors cannot be interpreted independently since their interaction was significant (p=0.0051).

• Fertilization had the greatest impact on low sites (SI <= 105). On average, low site, fertilized plots grew 4 sq.ft/ac/yr. more than unfertilized plots (p=0.0001).

Fertilization did increase growth on medium sites (105 < SI <= 125), with fertilized plots growing on average 0.9 sq.ft/ac/yr. more than unfertilized plots. However, this increase was not significant (p=0.0842).

• Fertilization did not affect net basal area growth on high sites (SI > 125, p=0.8300).

Net Growth in Diameter (QMD)

Examining net change in Quadratic Mean Diameter breast height (QMD) is a way to summarize change in DBH for the average individual. It characterizes change in DBH considering all processes, mortality, ingrowth, and survivor growth. The growth figures reported have been adjusted for any initial (pre-treatment) differences in basal area per acre found among the plots.

Average net periodic annual diameter increment of all the treatment regimes in this study was found to be 0.71 in./yr.

Periodic annual diameter increment was significantly different among the different treatments (p=0.0001).

• Main effects for site (p=0.0001), density (p=0.0001), and fertilization (p=0.0007) were all significant. Here again, the impacts one would expect or hope to achieve using these treatments were generally observed with none of the factors interacting significantly.

Very low density stands (100 TPA class midpoint) averaged growth of 0.75 in./yr, regardless of site or fertilization level. Low density stands (200 TPA) averaged about 0.72 in./yr, which was not significantly different from the very low density plots (p=0.2681). Moderately dense stands (350 TPA) averaged 0.69 in./yr, which was not significantly different from low density stands (p=0.4871), nor was it significantly different from very low density stands (p=0.0928). High density stands (550+ TPA) grew 0.53 in./yr on average, which was significantly lower than all other densities (p=0.0001).

The growth exhibited by low site stands (regardless of density or fertilization) was 0.58 in./yr. There was no difference between growth on medium and high site plots (p=0.4271) which averaged about 0.72 in./yr. The exhibited growth on these plots was detectably different from the growth on the low sites (p=0.008).

The fertilized plots (regardless of site or density) grew on average 0.08 in./yr more than unfertilized plots (p=0.0001).

CHANGES IN 40 LARGEST DIAMETER TREES (D40)

Using average growth of a fixed number of largest diameter trees (usually 40, which will represent the exact same trees as those in the top height component) as a measure focuses attention on what will likely be the future crop tree component. It also removes the effects of ingrowth. Mortality is partially compensated for by the next largest diameter tree being included in the sample at time 2, should a top 40 tree die during the measurement interval. The growth figures reported here have been adjusted for any initial (pre-treatment) differences existing among the plots in both D40 and TPA.

• Average periodic annual D40 increment of all the treatment regimes in this study was found to be 0.83 in./yr.

Periodic annual diameter increment was significantly different among the different treatments (p=0.0001).

• Main effects for site (p=0.0003), density (p=0.0021), and fertilization (p=0.0451) were all significant. Here again, the impacts one would expect or hope to achieve using these treatments were generally observed with none of the factors interacting significantly.

Very low density stands (100 TPA) averaged growth of 0.81 in./yr, regardless of site or fertilization level. Low density stands (200 TPA) averaged about 0.82 in./yr, which was not significantly different from the very low density plots (p=0.8303). Moderately dense stands (350 TPA) averaged 0.82 in./yr, which was not significantly different from low density stands (p=0.9376), nor was it significantly different from very low density stands (p=0.7872). High density stands (550+ TPA) grew 0.67 in./yr on average, which was significantly lower than all other densities (p=0.0010).

• The growth exhibited by low site stands (regardless of density or fertilization) was 0.70 in./yr. There was no difference between growth on medium and high site plots (p=0.0771) which averaged about 0.82 in./yr. The growth found on these plots was detectably different from the growth on the low sites (p=0.008).

The fertilized plots (regardless of site or density) grew on average 0.06 in./yr more than unfertilized plots (p=0.0001).

 

DISCUSSION

The impacts that thinning and fertilization is expected and designed to produce on tree growth were observed. Increased intensity of thinning resulted in increased average diameter growth (both in QMD and D40), while total stand basal area growth declined. Applying 200 lbs/acre of N as urea caused increased growth in QMD, D40, and basal area per acre for this first measurement interval. Generally speaking, though detectable differences in growth were not found on all sites, growth increased with increasing site index as well.

Interestingly, site was found to interact with fertilization level only when total basal area response was considered. Basal area growth response due to fertilization in this case tapered off (became less) as site index increased. Changes in QMD and D40 in response to fertilization remained about the same, regardless of site.