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Articles of Interest
Carryover Effects of Nitrogen Fertilization on Douglas-Fir Stands
Rob Harrison, Associate Professor, Forest
Soils, College of Forest Resources, University of Washington
Bob Gonyea, SMC Program Manager, College of Forest Resources, University
of Washington
BACKGROUND
Powers et al. (1990) recognized soil organic matter as being one of the most important soil variables in forest management because of its fundamental influence over physical and chemical processes affecting water, nutrient, and energy balances. Studies of direct removal of nutrients in biomass during harvesting have been seen to reduce Pacific Northwest Douglas-fir productivity by up to 40% in growth of the subsequent stand (Compton and Cole, 1991; Grier et al., 1989). However, these growth reductions were reversed with N-fertilization (Compton and Cole, 1991).
Although most studies indicate that N-fertilization generally increases productivity of Douglas-fir forests and future forest productivity may be impacted by the removal of organic matter and associated nutrients, little is known about the long-term effects of additional organic matter produced on a forest site following N-fertilization. This additional organic matter will almost certainly have some effect on the growth of the subsequent forests; however, the nature of that effect is unknown.
OBJECTIVES
The specific objectives of this study are to determine:
- Whether N-fertilization increases the growth of subsequent stands, and after harvesting
| Table 1: Hypotheses to be Tested | |
| Null Hypotheses | Reject Null Hypothesis If: |
| N-fertilization will not increase site organic matter following harvest | The quantity of C in the stand with N-fertilization is greater than that in the stand with no fertilization |
| N-fertilization will not increase site N in the long-term following harvest | The quantity of N in the stand with N-fertilization is greater than that in the stand with no fertilization |
| N-fertilization will not decrease C/N ratio of soil in the long-term following harvest | The C/N ratio of the forest floor and soil in the stand with N-fertilization is less than that in the stand with no fertilization |
| N-fertilization will not increase long-term site productivity | Growth of a subsequent stand on a previously fertilized plot is greater than growth in the stand with no fertilization |
METHODS
Currently, four of the proposed ten carryover effects study sites have been installed and four others are scheduled to be installed: The sites listed in Table 2 are each former RFNRP plots and thus, the sites are already monumented and the growth of previous stands documented.
| Table 2: Former RFNRP Plots | |||
WORK TO DATE
Tree growth data and soil, forest floor and vegetation samples were collected from Installation 134 at Pack Forest in September and October 1998, respectively. To quantify the amount of understory vegetation, 3 randomly located subplots were established in the previously fertilized and control plots. Understory species present within a 0.25 m2 area were collected by species (Figure 1), oven-dried to constant weight, and weighed. We are presently compositing the understory vegetation samples from each subplot for nutrient analysis. The percent groundcover of each species within a 1m2 grid at each subplot was also recorded.
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Forest floor was removed from three 0.25 m2 areas located just outside of each of the previously fertilized and control plots (Figure 1). Samples have been oven-dried to constant weight, and are currently being prepared for nutrient analysis. |
Due to the rocky nature of the soil, we were not able to collect soil samples using a soil probe, and thus, samples were collected from a pit located just outside the plot boundaries (Figures 2 (control) and 3 (fertilized)). Samples were collected from the A horizon, B 0-10 cm, B 10-30 cm, B 30-50 cm, and B 50+ cm. Samples have been air-dried and are being weighed and prepared for nutrient analysis.
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Foliage samples were collected from Installation 156, Coyle which was logged in December, 1998. These are currently being prepared for elemental analysis.
RESULTS TO DATE
The height and DBH of each of the trees in the new stand at Installation 134 at Pack Forest were measured at the ends of the 1997 and 1998 growing seasons. The average heights from both years are greater in the former fertilized plot than in the control plot. The average growth rate of trees in the former fertilized plot also appears to be greater than that of the control plot. Furthermore, in 1998, the average quadratic mean diameter of the Douglas-fir seedlings on the control and fertilized plots were 0.3 inches and 0.43 inches, respectively.
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Visual comparison of the plots revealed that the understory vegetation at the site varied between the fertilized and control plots (Figures 4 and 5 respectively). The primary species present in the control plot were salal, sword fern, Oregon grape, and pearly everlasting; these species were also present on the fertilized plot but at a lesser frequency. Other species observed on the fertilized plot included bracken fern, blackberry, foxglove, and thistle. Overall, the vegetative cover on the fertilized plot was significantly denser than that on the control plot. There was question as to whether the slight differences in the topography of the plots creates different soil moisture regimes, thus causing the observed differences in vegetation. Soil moisture content was determined for soil samples collected at each depth for both plots. The soil moisture did not vary significantly between plots, and thus, based on the available data, the differences in vegetation can not be attributed to different soil moisture regimes.
FUTURE WORK
Installations 17 and 168, which have already been harvested and replanted, will be characterized during 1999. Installations 53, 167, 177 and 179 are scheduled to be harvested during 1999; foliage samples will be collected at this time. Understory and soil samples will also be collected from each installation during 1999.
We will characterize the previous stands by using SMC data as well as taking samples before and during harvesting. Vegetation and forest floor samples will be analyzed for dry weight, pH, and total C, N, P, K, Ca, Mg, S, B, and other elements. Soil samples will be analyzed for the above properties as well as mineralizable N via in-situ incubation, Bray 2-extractable P, cation exchange capacity and exchangeable cations. The bulk density of each mineral soil horizon or depth will be determined on <2mm material using either a core or excavation method (Canary, 1994).
New stands will be measured for growth rates, initially at yearly intervals. Foliage samples will be taken at the same interval to monitor differences in nutrient concentrations and content among stands.
REFERENCES
Canary, J.D. 1994. Carbon and Nitrogen Storage Following Repeated Urea Fertilization of a Second Growth Douglas-fir Stand in Western Washington. M.S. Thesis. University of Washington, Seattle WA.
Compton, J.E. and D.W. Cole. 1991. Impact of Harvest Intensity on Growth and Nutrition of Successive Rotations of Douglas-fir . p. 151-161 in W.J. Dyck and C.A. Mess, Eds. Long-term Field Trials to Assess Environmental Impacts of Harvesting. IEA/BE T6/A6 Workshop, Florida, February, 1990.
Grier, C.C. , K.M. Lee, N.M. Nadkarni, G.O. Klock, and P.J. Edgerton. 1989. Productivity of forests of the United States and its relation to soil and site factors and management practices: a review. USDA Forest Serv. Gen. Tech. Rep. PNW-GTR-222. Pacific NW Res. Stn., Portland, OR. 51 p.
Powers, R.F., D. H. Alban, R. E. Miller, A. E. Tiarks, C.G. Wells, P.E. Avers, R.G. Cline, R.O. Fitzgerald, N. S. Loftus, Jr. 1990. Sustaining site productivity in North American Forests: Problems and Prospects. In: S.P. Gessel, D.S. Lacate, G.F. Weetman, and R.F. Powers. Sustained Productivity of Forest Soils. Proceedings of the 7th North American Forest Soils Conference, University of British Columbia, Faculty of Forestry Publication, Vancouver, B.C.
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