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Relationship of site index to estimates of soil moisture and nutrients for western redcedar in south… Klinka, Karel; Kayahara, Gordon J.; Chourmouzis, Christine 1997

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Home  Scientia Silvica  Extension Series, Number 5, 1997  Relationship of Site Index to Estimates of Soil Moisture and Nutrients for Western Redcedar in South Coastal British Columbia Introduction  Study Site and Procedure  Where timber production is the primary management objective, knowledge of the relationship between the potential productivity of candidate tree species and levels of light, heat, nutrient, moisture and aeration is necessary for species- and site-specific decision making. For example, foresters need to decide which tree species to regenerate on a particular harvested area to obtain maximum sustainable productivity. Similarily, when considering the application of silvicultural treatments such as spacing or fertilizing, foresters need to determine whether the potential productivity of a particular site warrants the cost of the treatment.  We sampled 105 plots from stands with western redcedar as the major tree species. The plots were distributed within two climates, a drier and a wetter cool mesothermal climate in south coastal British Columbia, the CWHdm and CWHvm subzones respectively (latitude 49º20’N, longitude 122º35' to 123º05’W, elevation 160 to 295 m). The stands in both subzones originated from logging during the early 1900’s followed by fires. The natural regeneration that followed resulted in relatively extensive areas of even-aged western redcedar stands with a minor component of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Natural regeneration of a large component of western redcedar after fire is unusual for coastal British Columbia where post-fire regeneration is typically Douglas-fir and/or western hemlock.  We used the site index (height of dominant trees at breast height age) of western redcedar (Thuja plicata Donn. ex D. Don.) as a measure of productivity, and described the pattern of mean site index in relation to field identified soil moisure and soil nutrient regimes.  Table 1. Site index for western redcedar in (A) the dry maritime and (B) the very wet maritime Coastal Western Hemlock (CWHdm and CWHvm, respectively) subzones. Mean site index (m at 50 years breast height), 95% confidence interval (m) in parentheses and sample size are given for each SNR and SMR combination sampled. (A) CWHdm Actual soil moisture regime  very dry  (B) CWHvm Soil nutrient regime  very poor  poor  medium  rich  very rich  13.4 (5.4) n=2  Actual soil moisture regime  Soil nutrient regime very poor  poor  medium  rich  very rich  20.9 n=1  22.0 (6.3) n=2  24.8 (0.5) n=3  23.0 (0.6) n=5  24.7 (0.8) n=6  26.6 (0.7) n=7  26.3 (0.6) n=5  28.3 (0.5) n = 16  21.8  25.2  n=1  n=1  27.9 (1.6) n=7  27.5 (0.6) n=5  17.8 (1.9) n=7  22.5 (0.5) n=3  26.2  very dry  moderately dry  22.2 (7.2) n=2  slightly dry  24.4 (0.6) n=5  moderately dry  26.7 (1.4) n=4  slightly dry  fresh  29.7 (0.8) n=9  fresh  moist  30.3 (2.7) n=3  very moist  16.5  32.2 (3.5) n=3  29.0  29.1 (1.5) n =3  32.0  moist  n =1  very moist  n=1  25.4 wet  wet n=1  n=1  n=1  The highest western redcedar site index occurred on sites identified as having no moisture deficit or excess and high nitrogen availability. Site index decreased on sites identified by field procedures as being outside this optimum moisture and nutrient combination. The mean site index was significantly greater in the drier cool mesothermal climate than the wetter cool mesothermal climate on sites identified as having similar soil moisture and nutrient conditions. Due to a small sample size, the results reported here should be considered as preliminary until a larger data base is collected. However, until such a data base is generated, operational needs must be addressed. Thus for operational use we present estimates of mean and 95% confidence intervals of measured site index for various combinations of SMR and SNR for two subzones (Tables 1a and 1b). For preliminary use in areas south and north of Vancouver, users should consider the influence of even a small difference in climate on the site index of western redcedar growing on the same soil conditions.  We also compared the site index of western redcedar to that of western hemlock (Figure 1). Sixty-three plots had both western redcedar and western hemlock available for comparisons. On sites where both species were sampled, the site index of western redcedar was consistently lower. For forest management scenarios where timber production is the primary objective, practitioners may take into account that the site index of western redcedar was consistently less than western hemlock. These values can be used in a growth and yield model to compare productivity between the two species.  Reference Kayahara, G.J., K. Klinka, and A.C. Schroff. 1997. The relationship of site index to synoptic estimates of soil moisture and nutrients for western redcedar (Thuja plicata) in southern coastal British Columbia. Northwest Science 71: 167-173.  12  Results and Discussion  Western hemlock site index (m) 16 20 24 28 32 36  Heights and ages were measured for five dominant trees in a 0.04 ha plot, and the height at a base age of 50 years was calculated to give site index. Sites were identified to one of seven soil moisture regimes (SMR) and one of five soil nutrient regimes (SNR). Identification of SMR and SNR was based on a heuristic synthesis of topographic and soil morphological properties, augmented by indicator plant analysis.  40  Home  12  16  20  24  28  32  36  40  Western redcedar site index (m) Figure 1. Site Index comparisons between western redcedar and western hemlock (n = 63). The dashed line represents a line of equal site index.  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: Gordon Kayahara gordon.kayahara@mnr.gov.on.ca) and K. Klinka Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca) Financial support: Imajo Cedar Management Fund For more information contact: G. Kayahara  Copies available from: www.forestry.ubc.ca/klinka or K.Klinka, Forest Sciences Department, UBC, 3036-2424 Main Mall, Vancouver, BC, V6T 1Z4  


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