UBC Faculty Research and Publications

Does coastal western hemlock respond to fertilization? 2008

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Does Coastal Western Hemlock Respond to Fertilization? Scientia  Silvica Extension Series, Number  44, 2001 Introduction Response to fertilization is a function of the degree to which nutrients are limiting growth, the capacity of individual trees to respond to nutrient inputs, the degree to which other factors limit growth, and the possible extrinsic effects of treatment (e.g., root mortality due to fertilizer-induced soil pH effects). Recognition and examination of these factors is essential if response to fertilization is to be predictable. Over the past 25 years numerous western hemlock fertilizer trials have shown responses ranging from negative to positive with no clear trends. Theories for this erratic response include: (a) different nutritional requirements during different stages of stand development; (b) high native N availability or low supplies of other nutrients (P and S, in particular); (c) differential adverse effects of N fertilizers on surface roots, mycorrhizal populations and P nutrition; (d) a requirement for slow release N; and (e) induced water stress. The objective of this study was to develop site-specific guidelines for western hemlock fertilization decision-making for industrial use. This study reports on the first and third growing-season response to two different fertilizer treatments, and identification of possible relationships between fertilizer response and site and stand conditions. Methods Forty-four sites were chosen to represent the full range of growing conditions observed for western hemlock in six CWH subzones. All sites supported young hemlock stands with >80% basal area, breast height age ranging from 11 - 39 years, and site index ranging from 29 to 38 m @ 50 yr bh. All stands had been spaced 2 - 6 years prior to this trial to between 550 and  900 stems.ha-1, and had considerable opportunities for crown expansion. Site quality was assessed by regional climatic, soil moisture, and soil nutrient conditions using the methods of biogeoclimatic ecosystem classification. Foliar characteristics are summarized in Table 1. Property  Minimum Maximum Mean Standard Error Average weight (mg/100 needles) 147 302 195 5 N (%) 0.91 1.63 1.27 0.03 P (%) 0.10 0.27 0.16 0.01 K (%) 0.42 0.91 0.67 0.02 Ca (%) 0.22 0.39 0.31 0.01 Mg (%) 0.09 0.17 0.13 0.00 S (%) 0.08 0.21 0.12 0.00 SO4-S (ppm) 32 873 199 20 Mn (ppm) 572 2338 1312 57 Al (ppm) 286 1157 499 24 B (ppm) 12 33 18 0.6 Cu (ppm) 3 23 5 0.4 Fe (ppm) 42 1037 97 22 Zn (ppm) 6 15 9 0.3  Table 1. Minimum, maximum, mean and standard error of observed foliage chemical and physical properties over the 44 sites. Fertilizer treatments applied in the spring of 1990 were control (no fertilizer), N alone (225 kg.ha-1 urea N) and blend (225 kg.ha-1 urea N, 100 kg.ha-1 P as triple-super-phosphate, 60 kg.ha-1 K as K2SO4, 100 kg.ha-1 S as SO4, 40 kg.ha-1 Mg, 10 kg.ha-1 Cu as CuSO4, 20 kg.ha-1 zinc as ZnSO4 and 2.5 kg.ha-1 B). The design for the experiment was a randomized complete block with subsampling. At each of 44 sites (blocks) there were two replications (0.04 ha plots) of each of the 3 treatments. Within each plot, 6 dominant or codominant trees (subsamples) were chosen and tagged for measurement. Tree measurements recorded at the time of trial establishment included diameter at 1.3m, age, height, and length and width of live crown. Three growing seasons after treatment the same sample trees were measured for dbh and radial growth. Based on quadratic means of the two radial increment measures, individual tree inside bark basal area and basal area growth for the three years prior and the three years post treatment were calculated. In each plot, the current year's foliage was sampled during late fall 1990. Needles were selectively sampled from the proximal portion of leading shoots. Foliage samples were oven-dried and 100 needles from each sample tree were weighed. A single composite sample per plot was then ground, and the samples were analyzed for macro and micro-nutrients. The forest floor and top 30 cm of the mineral soil were also sampled and analyzed for pH, total C, total N, total and extractable P, and extractable Ca, Mg, and K. First growing-season response was measured in terms of changes in needle weight and foliar nutrient concentrations. Analysis of variance (ANOVA) was conducted on needle weight and foliar nutrient concentrations.  Orthogonal contrasts of the control treatment versus N alone, and the control treatment versus blend were carried out separately for each installation. Third growing-season response was measured in terms of total individual tree basal area growth over the three growing seasons following treatment. A simple linear model of three growing-seasons’ post-treatment basal area growth as a function of three growing-seasons’ pre-treatment basal area growth was fitted to the control tree data for each installation. These equations were then used to predict how the treated sample trees would have grown if they had not been fertilized. Absolute basal area response was calculated by subtracting the predicted from the observed for each sample tree. To avoid the potential large influences of small trees, relative basal area response was calculated on a per plot basis. Relative basal area response was calculated as the average observed growth in the plot divided by the average predicted unfertilized growth for the plot. ANOVAs on absolute and relative basal area response followed by contrasts of control versus N alone and control versus blend were then carried out for each installation. Results First Growing-Season Response The N-alone treatment generally resulted in little or no change in needle weight with 2 sites showing a significant increase while one site showed a significant decrease in needle weight. In contrast, 7 sites exhibited a significant positive needle weight response to the blend treatment (Table 2). Unit needle weight and foliar nutrient concentrations in unfertilized western hemlock showed no relationship with site index. Uptake of applied nutrients and increases in needle weight appear to be independent of pretreatment foliar nutrient levels. Treatment Needle weight (mg/100) N % P % K % Mg % S % SO4-S ppm Cu ppm Zn ppm B ppm Control 195 1.27 0.16 0.67 0.13 0.12 198 5.3 8.6 18 Urea 197 1.60 0.15 0.65 0.12 0.11 43 5.2 9.4 17 Blend 216 1.72 0.17 0.75 0.11 0.14 173 5.5 10.2 37  Table 2. Average unit needle weight and foliar concentrations of applied nutrient elements following treatment. The mean foliar N concentration across all test sites was 1.27% (Tables 2 and 3). Foliar N levels in this range are considered to be "slightly deficient" to "adequate" for western hemlock and are relatively high compared to other coniferous species growing on comparable sites in the Pacific Northwest. Foliar P was low across all sites with a mean value of  0.16% considered to represent a moderate to severe deficiency.  Significant increases in needle weight in response to fertilization occurred most often on sites with low extractable P in the forest floor and low foliar P concentrations. Sites showing a significant increase in needle weight, a common predictor of first growing-season fertilizer response, had forest floor-extractable P levels of <50 ppm and pre-treatment foliar P concentrations of <0.21 %. Site index (m @ 50 years) n Needle weight (mg/100) N (%) P (%) K (%) ≤26.0  10 197 (6.9) 1.275 (0.045) 0.165 (0.015) 0.752 (0.022) 26.1-28.0 7 203 (13.5) 1.241 (0.069) 0.171 (0.018) 0.728 (0.043) 28.1-30.0 6 177 (7.6) 1.240 (0.125) 0.131 (0.007) 0.657 (0.059) 30.1-32.0 8 190 (11.0) 1.248 (0.053) 0.147 (0.010) 0.655 (0.035) 32.1-34.0 6 202 (18.0) 1.267 (0.077) 0.160 (0.026) 0.572 (0.075) ≥34.1 7 198 (19.5) 1.317 (0.031) 0.172 (0.010) 0.597 (0.044)  Table 3. Average weight per 100 needles, and foliar N, P and K concentrations by site index class. Standard errors in brackets. Third Growing-Season Response Seventeen of the 44 study sites showed a significant absolute basal area response to fertilization (p ≤0.05). Six sites showed a significant response to the N-alone treatment while 11 sites showed a significant response to the blend treatment (Tables 4 and 5). Significant absolute basal area responses were found in stands growing across a wide range of site indices and sites. Site no. Absolute Response (cm2) Relative Response Site Index (m/50yr) Control Needle Wt. (mg/100) Change in Needle Wt. (mg/100) Control Foliar N (%) Change in Foliar N Control Foliar P (%) Change in Foliar P 3 34.5 1.37 33 250.4 (-7.1) 1.39 (0.34) 0.125 (0.021) 5 36.8 1.51 27 226.7 55.0 0.91 (0.75) 0.180 (-0.021) 6 27.4 1.34 28 192.9 (13.8) 1.49 0.59 0.104 0.069 15 31.9 1.60 30 221.7 (-2.5) 1.38 0.63 0.121 (0.028) 18 28.2 (1.54) 32 163.3 27.9 1.26 (0.41) 0.158 (0.014) 19 40.8 1.90 35 186.7 (13.3) 1.03 (0.47) 0.107 0.028 20 23.8 1.41 34 190.4 (28.3) 1.01 0.53 0.098 0.037 21 21.8 1.54 26 174.6 (10.0) 1.25 (0.16) 0.111 (0.000) 25 28.3 1.37 30 206.7 (22.9) 1.49 (0.19) 0.189 (0.028) 34 39.9 1.46 27 190.8 (35.8) 1.45 0.60 0.202 (0.013) 40 32.8 1.68 30 172.1 (-4.6) 1.05 (0.46) 0.150 (0.018)  Site no. Absolute response (cm2) Relative Response1 Site index (m/50yr) Control needle wt. (mg/100) Change in needle wt. (mg/100) Control foliar N (%) Change in foliar N2 Control foliar P (%) Change in foliar P2 1 29.3 1.38 35 177.5 35.8 1.24 (0.25) 0.139 (-0.007) 5 24.7 1.34 27 226.7 (7.5) 0.91 (0.60) 0.180 (-0.035) 15 34.0 1.58 30 221.7 (-17.9) 1.38 0.50 0.121 (-0.010) 18 21.0 (1.40) 32 163.3 15.0 1.26 (0.32) 0.158 (-0.005) 25 21.3 1.23 30 206.7 (25.8) 1.49 (0.08) 0.189 (0.028) 34 18.1 (1.22) 27 190.8 (9.6) 1.45 0.54 0.202 (-0.025)  Table 4. Characteristics of sites showing a significant absolute basal area response to urea. Changes in brackets are not significant. 1Basal area response relative to control where control growth equals 1.0. 2Change in absolute foliar N and P concentrations expressed on a dry mass basis. Table 5. Characteristics of sites showing a significant absolute basal area response to the blend treatment. Changes in brackets are not significant. 1Basal area response relative to control where control growth equals 1.0. 2Change in absolute foliar N and P concentrations expressed on a dry mass basis. The small number of responsive sites precludes most opportunities for identifying of factors determining fertilizer response. Few stand or site variables were found to offer consistent, significant utility as predictors of absolute basal area response. Among individual tree and stand characteristics examined only (i) height-diameter ratio showed a weak (0.23≥ r2 ≤0.67) but significant (p ≤0.05) negative correlation with basal area response in 4 of the 12 sites that responded to the blend treatment; and (ii) length of live crown showed a weak, but significant (p ≤0.05) relationship to basal area response on 2 sites. Understory vegetation characteristics (occurrence and cover of salal), and soil chemical properties did not show a significant relationship with basal area response. Pretreatment foliar P and post-treatment foliar SO4-S concentrations were the only variables that showed a significant relationship to relative basal area response among sites. Pretreatment foliar P concentration was also significantly related to reponse to the blend treatment (p ≤0.05). Sites with a significant absolute basal area response had an average foliar P concentration of 0.140% while non-responding sites had an average concentration of 0.165%. Post-treatment foliar SO4-S concentration accounted for a small but significant portion of the explained variation in relative basal area response for the blend. Comparison of first and third growing season response measures First growing-season response variables did not show a strong relationship to third growing season response variables (Table 5).  Six sites showed an increase in first growing season unit needle weight in response to the blend treatment with only two of these sites showing a significant increase in third growing season basal area. Scientia Silvica  is published by the Forest Sciences Department, The University of British Columbia, ISSN 1209-952X Editor: Karel Klinka (klinka@interchange.ubc.ca) Research: Reid E. Carter (Reid.Carter@NBFinancial.com), E.R. McWilliams (eleanor@istar.ca), and  K. Klinka Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca) Financial support: Science Council of British Columbia For more information contact: R. E. Carter Copies available from: www.forestry.ubc.ca/klinka or K. Klinka, Forest Sciences Department, UBC, 3036-2424 Main Mall, Vancouver, BC, V6T 1Z4 Discussion When there is adequate moisture  there is an expectation that tree growth in general will respond positively and consistently to an increase in the nutrient status of the site. Other species such as Douglas-fir meet this expectation of a positive growth response to fertilization. Douglas-fir site index increases linearly with increasing soil mineralizable N levels, with foliar N concentrations highly correlated with soil mineralizable N levels. Additionally, the effectiveness of first growing-season fertilizer response measures such as changes in foliar nutrient levels and needle weight has been supported by studies examining many different determinate species. However, in this study, the response of western hemlock to fertilizer additions did not result increased growth, and first-year growing season fertilizer response measures did not show any useful relationship to third growing season basal area response. These results suggest that western hemlock productivity may not be consistently linked to nutrition in the absence of a moisture deficit or surplus. One of the explanations may be that hemlock growth is not nutrient-limited on most sites.  Mean foliar N and P concentrations across all sites were 1.27 and 0.16 % respectively, with pretreatment levels of both elements apparantly independent of  hemlock site index (Table 3). While fertilizer responses can be quite high, average responses are generally 5 - 10%. This combination of low response level and high uncertainty has been a principal constraint to operational fertilization of western hemlock in the Pacific Northwest. Conclusions Successful fertilization of western hemlock will require application of multi-nutrient fertilizers - likely containing N, P, and S as a minimum - severely constraining opportunities for economic applications of fertilizer. Application of N and S as sulphur- coated urea or urea-ammonium sulphate blends and P as triple super phosphate at the rates used in this study increases both the fertilizer purchase price and application payload, increasing costs by up to 65% and making fertilization of western hemlock a very expensive proposition. Unless we can improve both the magnitude and the certainty of gaining a fertilizer response, fertilization of western hemlock cannot be recommended. Reference Carter, R.E., E.R.G. McWilliams, and K. Klinka. 2001. Three year growth response of western hemlock to fertilization in coastal British Columbia. (Submitted to Forest Ecology and Management, 01/01/20)


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