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Growth and nutrient relations in black cottonwood in South-Coastal British Columbia McLennan, Donald S.

Abstract

Initially, the study examined within and among site temporal and spatial variation of foliar nutrients, and spatial variation of soil nutrients to assess the sampling methods employed, and to provide background for the interpretation of nutrient-site index interactions. The study then examined relationships between the growth of black cottonwood, expressed as site index, and site units, under story vegetation, soil nutrient contents, and foliar nutrient concentrations in29 black cottonwood stands in south-coastal British Columbia. The final phase of the study was a fertilizer trial in three juvenile black cottonwood stands, with treatments based on used DRIS diagnosis of limiting nutrients. Significant levels of variability in foliar nutrient concentrations were identified within tree canopies, and from tree-to-tree within stands. A protocol was suggested to standardize sampling procedures to reduce spatial variability. Sample size requirements for different levels of accuracy and precision were presented. Important variation in foliar nutrient concentrations was also recorded seasonally, and from year to year, in foliage samples collected according to the same protocol. It was shown that the temporal variability was sufficient to alter the interpretation of foliar nutrient concentrations for the stands. Spatial variation in soil nutrient concentrations was high and was attributed to order-of-magnitude concentration differences between soil strata in each pedon. Spatial variation of soil nutrient contents (expressed in kg/ha over a 1 m sampling depth) was generally higher than soil nutrient concentrations, because of factors such as bulk density and percent coarse fragments that were used to calculate soil contents, and that are themselves subject to variation. It was shown that the compositing procedure used to reduce costs approximately doubled the variability seen in the intensively sampled sites, and alterations to the compositing procedure were suggested. It was also argued that sampling over a depth of 1 m, and not over the main rooting depth, provided the most biologically meaningful estimates of soil nutrients available to black cottonwood. The ANOVA comparing black cottonwood growth within site units was highly significant (p < .001), and explained 87% of the variance in site index within the 29 study sites. This general result suggests that, relative to the ecological requirements of black cottonwood, the site classification provided an ecologically-meaningful differentiation of the edatopic gradients sampled. For operational purposes, this result predicts that black cottonwood site index can be estimated with considerable accuracy by identifying the site unit on which a stand is located. Growth was best on the high bench of alluvial floodplains (Ss-Salmonberry site association), and on moist upland sites with seepage (Cw-Foamflower site association). Growth was poorest on the low bench of alluvial floodplains (Ac-Willow site association), and on gleyed, marine site units (Cw-Salmonberry and Cw-Black twinberry site associations). About 50% of the variation in site index could be accounted for using understory vegetation from within the stands as predictors. This relatively low explanatory power was attributed to the fact that black cottonwood site index changed significantly across the indicative range of many of the understory plants. All methods of analysis revealed consistent relationships between measures of site nutrient status and site index. Sample stands with high pH, high levels of exchangeable Ca and Mg, and low levels of soil N, P, and K, had foliar concentrations of N, P, and K diagnosed as limiting to black cottonwood growth, and had the lowest site index. High site index was recorded in stands with more or less opposite soil and foliar properties. Site index was seen to decrease in site units with increasing flooding frequency and duration on alluvial floodplains. The decrease was attributed to the negative impact of flooding on the rate of organic matter mineralization, on nutrient uptake, and on the negative effect of high levels of soil Ca and high soil pH on the availability of soil P. On upland sites, soil gleying and prolonged rooting zone flooding during the growing season was correlated with low site index. Using DRIS analysis based on foliar norms from the 25 fastest-growing, fertilized trees at the Squamish 23 site, it was concluded that black cottonwood stands in the high site index class were limited by K, and then P. In three juvenile black cottonwood stands, the application of fertilizer based on diagnosis of foliar nutrient concentration using DRIS norms had the following 3 year responses - basal area increment increased by 65%, and height growth increment by 15% at the Squamish 23 site; basal area increment increased by 65% and height growth increment increased by 30% at the Strawberry site; and basal area increment increased by 27% without a significant height growth response at the Soowahlie site. At the Squamish 23 and Soowahlie trials, response was attributed to fertilization with K and P, as suggested from the foliar nutrient diagnosis of the fast-growing group. Given that relatively low dosages (ca. 100 kg/ha) of P were required to achieve a significant growth response, and acknowledging that, in many forest fertilization programs response to P fertilization occurs for a considerable period of time, the results suggest that the fertilization of fast-growing, juvenile black cottonwood stands in coastal British Columbia may be economically justified. Significant correlations between measures of foliar response and wood production were not seen in the study, and this finding limits the usefulness of the graphical procedure for interpretation of the experimental results. Foliar concentrations from the 25 fastest-growing black cottonwoods at the Squamish 23 site are presented as DRIS standards that will be useful in the diagnosis of the nutrient status of black cottonwood stands in coastal British Columbia.

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