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Forest floors under second growth Douglas-fir stands : their chemical variability and some relationships to productivity Carter, Reid Ewart


The variability of 23 chemical properties of LF and H horizons was established under six, sixty to one hundred and thirty year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands representing three hygrotopes and two geographic locations within the Coastal Western Hemlock Biogeoclimatic Zone. pH (CaCl₂) total P, S, N, C and Zn; and C:N, N:P and N:S ratios were found to have the lowest within-plot variability—all requiring less than 15 samples per plot to predict a mean value at the 5% level of significance with an allowable error of 10%. Total Fe, Al, Mn and Ca were found to have the greatest within-plot variability. Variability within plots was shown to be significantly less than variability between plots at the 1% level for all properties tested. Composite samples prepared from 15 individual samples within each plot were shown to be significantly correlated with the depth-weighted arithmetic means of individual samples at the 1% level for most forest floor properties tested. In a related study, the productivity of second growth Douglas-fir stands in the Coastal Western Hemlock Zone was examined in relation to 28 chemical properties of the forest floor. Fifty-three plots were sampled from a wide range of site types over a broad geographic distribution. LF, H, and Ah horizons of each plot were characterized by a single depth-weighted composite sample. Seventeen different forest floor properties were found to be significantly correlated with site index. Total Mn was able to account for more than 44% of the variability in site index. A jackknifed stepwise discriminant analysis procedure was able to classify site classes correctly 59.5% of the time using a combination of total lipids, Mn, Ca and KCl-extractable N in the LF horizon of the unstratified sample population expressed on a kg/ha basis. From the results of all analyses, total Mn, lipids, ash-free lipids, KCl-N, Mg, K, Ca and S and N:S, C:N and Ca:Al ratios were thought to have the strongest relationship to Douglas-fir productivity of all forest floor properties examined. Studies that examined relationships between individual site parameters and forest productivity did not reveal any methods of direct productivity assessment. Such studies appear to be most useful for clarifying many of the nutrient interactions within forest stands.

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