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Relationships between coastal Douglas-fir site index and synoptic categorical measures of site quality Klinka, Karel; Carter, R. E. (Reid E.); Chourmouzis, Christine 2001-12-31

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Scientia Silvica Extension Series, Number  37, 2001Relationships Between Coastal Douglas-fir Site Index and SynopticCategorical Measures of Site QualityIntroductionKnowledge of ecological characteristics of trees, sites and tree growth on different sites is fundamental for silviculturaldecision-making and planning. With the biogeoclimatic ecosystem classification in place, silvicultural management inBritish Columbia has been given an ecological foundation; however, relationships between growth and site have not yetbeen fully investigated. The purpose of this study was to determine how height growth of Douglas-fir within the drierportion of the CWH zone varies with site.We adopted site index (m @ 50 yr bh) as a species-specific measure of forest productivity, recognizing that it indicatesheight growth performance at a selected point in time. If forest productivity is correlated with ecological measures of sitequality, what site factors should be used to quantify the relationships? Because of compensating effects, the numerous sitefactors can be reduced to four primary (synoptic) factors that directly affect plant establishment and growth: climate (lightand temperature), soil moisture, soil nutrients, and soil aeration (not used in this study).Materials and MethodsOne-hundred and thirty-three plots were located in even-aged immature Douglas-fir stands with a relatively wide agerange  (18 to  69 yrs) and stocking (400  to  900 stems ha-1),  and without a history of damage. All stands had similarmanagement history: slashburning, planting to Douglas-fir, and pre-commercial thinning. The stands were located in thevery dry and dry maritime subzones of the CWH zone on Eastern Vancouver Island and the adjacent mainland across awide range of elevation, aspect, and soil conditions. In each stand, a 20 x 20 m plot was located to represent an individualecosystem with relatively uniform vegetation and soil. The relative soil moisture regime (SMR) and soil nutrient regime(SNR) of each plot were estimated in the field, and then converted to actual SMRs.In each plot, the five largest diameter trees of the study species were measured for breast-height age, using an incrementborer, and top height, using a clinometer. Site index was taken from height growth tables. On each plot the current year'sfoliage from the upper crown of 15 dominant trees was sampled and analyzed for total nitrogen. Samples of forest floorand the  top  30 cm of mineral soil were taken  at each plot, air-dried, and analyzed for  a number of nitrogen-relatedproperties. Biogeoclimatic subzone (identified from maps and representing climate), actual SMR, and SNR were used asindependent categorical variables in stratification and data analysis. Analysis of variance (ANOVA) and multiple regressionanalyses were used to examine relationships between site index and climate, actual SMR, and SNR.ResultsRelationship between Site index and ClimateStratification of study stands by subzones did not indicate any meaningful relationships. By using a soil moisture factorseparately from a temperature factor, biogeoclimatic ecosystem classification reduces the role of climate as categoricalvariable to a temperature factor. Although temperature characteristics of the two study subzones are not the same, theirvariation is probably too small to exert a significant influence on plant activity via potential evapotranspiration.Relationship between Site index and Soil MoistureA strong productivity gradient coinciding with the assumed soil moisture gradient was detected when all study stands (n =133) were stratified according field-estimated SMRs (Table 1). Douglas-fir site index increased from very dry throughmoist sites, but the difference between fresh and moist  was not significant (alpha = 0.05).  The variation of direct (continuous)measures of soil moisture and soil nutrients paralleled this trend but none of them separated the five SMRs from eachother.Relationship between Site index and Soil NutrientsStratification of all study stands according to field-estimated SNRs produced the similar results as for SMRs  - a strongproductivity gradient coincided with the assumed soil nutrient gradient (Table 2). Douglas-fir site index increased consistentlyfrom very poor through very rich sites. The variation in values of direct measures of stand and soil nutrients paralleled thistrend, but again none of them separated five SNRs from each otherRelationship between Site index, Climate, Soil Moisture, and Soil NutrientsRegression models were developed to quantify relationships between site index and categorical measures of site quality(Table 3) as well as analytical measures of site quality, using actual evapotranspiration or water deficit and mineralizable-N (data not presented). The SMR and SNR models each had strong relationships with site index but the combined SMR& SNR model accounted for the largest proportion of the variation in site index of all models tested. The rate of increasein site index from very dry through moist SMRs was nearly the same as that from very poor through very rich SNRs;however, SMR and SNR were found to exhibit high collinearity. As a categorical variable, biogeoclimatic subzone wasfound to be significant (alpha  = 0.05) but did not improve the performance of any categorical model tested. The models usinganalytical variables also showed strong relationships with site index. Testing of the models using an independent data setshowed that the SMR, SNR, and SMR & SNR models provided the best results compared to analytical models.Table 1. Means and standard deviations (in parentheses) of selected stand and soil characteristics of the study standsstratified according to actual soil moisture regimes. Values in the same row with same superscripts are not significantlydifferent (p <0.05; Tukey?s test).Table 2. Means and standard deviations (in parentheses) of selected stand and soil characteristics of the study stands stratifiedaccording to actual soil nutrient regimes. Values in the same row with same superscripts are not significantly different (p <0.05;Tukey?s test).Characteristic   Very dry n = 18  Moderately dry n = 56  Slightly dry n = 33  Fresh n = 21  Moist n = 5 Douglas-fir site index  21d (2.5) 27c (2.9) 33b (2.8) 35a (2.3) 36a (1.9) Actual growing season precipitation (mm)   206b (26) 219a,b (32) 227a,b (32) 243a,b (22) 254a (26) Water deficit (mm/yr)  97a (48) 68a,b (50) 39a,b (42) 8b,c (13) 0c (11) Mineral soil (0-30 cm) mineralizable -N (ppm)  6.8c (2.5) 9.8c (4.9) 22.6b (14.0) 27.1b (13.0) 48.0a (30.1) Foliar N (%)  1.09b (0.13) 1.14b (0.09) 1.20a (0.09) 1.20a (0.09) 1.20a (0.07) Mass of 100 needles (mg)  434b (44) 480b (67) 501b (64) 497b (82) 636a (102) G3Characteristic   Very poor n = 11 Poor n = 60 Medium n = 39 Rich n = 19 Very rich n = 4 Douglas-fir site index  22d (2.4) 26c (3.7) 32b (2.6) 36a (2.1) 37a (1.4) Mineral soil (0-30 cm) mineralizable-N (ppm)  7.0c (2.8) 9.5c (4.5) 17.5b (13.5) 31.1b (11.4) 49.6a (37.9) Foliar N (%)  1.14a (0.06) 1.13a (0.13) 1.19a (0.10) 1.19a (0.15) 1.24a (0.26) Mass of 100 needles (mg)  470b (61) 469b (69) 479b (64) 552a (87) 610a (28) G3DiscussionDespite a limited representation of certain combinations of SMRs and SNRs (edatopes) in the study, the large amount ofvariation explained by the models using either categorical or continuous variables suggests the presence of strong relationshipsbetween direct as well as indirect measures of ecological measures of site quality. Increases in Douglas-fir site index withdecreasing growing-season water deficit and increasing soil nutrient levels were described in other studies, but have notbeen quantified, particularly over a large area. This study has also given evidence that the mineralizable-N in the mineralsoil is a good single measure of a soil nutrient gradient related to potential forest productivity. Insignificant differences in siteindex between subzones indicated that the two study subzones are climatically equivalent. In consequence, site index - sitequality relationships can be examined within the study area without considering a climatic factor. Thus indirect measures(i.e., SMR and SNR) are good estimates of direct measures (water deficit, soil mineralizable-N). In view of difficultiesinvolved in measurements of actual evapotranspiration and nutrient levels in forest soils, it was not surprising to find theperformance of the analytical models to be inferior to the categorical models.The categorical models require only identification of SMR and SNR for each site in a forest without the need for extensivedata collection and laboratory analysis. Examination of a few selected topographic and soil properties aided by keys andindicator plant analysis have proved to be an efficient means for identifying ecological quality of forest sites using system ofbiogeoclimatic ecosystem classification.ConclusionsThere were significant differences in site index and direct measures of soil moisture and nutrients between Douglas-firstands when stratified according to field-estimated soil moisture and nutrient regimes. Regression models developed showedstrong relationships between Douglas-fir site index and field-estimated SMR and SNR. The similarity in the relationshipsobtained when employing direct measures of soil moisture and nutrients justifies the use of indirect qualitative measures -soil moisture and nutrient regimes - in estimating potential forest productivity within drier cool mesothermal climates ofsouthern coastal BC.Table 3.  Models for the regression of Douglas-fir site index (m @ 50 years bh) on soil moisture regimes(SMRs) and soil nutrient regimes (SNRs). All models are significant at p <0.01. SEE - standard error ofestimates; abbreviations for SMRs: VD -  very dry, MD - moderately dry, SD - slightly dry, F - fresh, M -moist; abbreviations for SNRs: VP - very poor, P - poor, M - medium, R - rich, VR - very rich.[1] SI = 34.6 - 14.0(VD) - 7.7(MD) - 3.0(SD) - 0.01(F) + 0.0(M)   Adjusted R2 = 0.74, SEE = 2.6 m [2] SI = 35.5 - 13.2(VP) - 9.7(P) - 4.0(M) -0.6(R) + 0.0(VR)   Adjusted R2 = 0.66, SEE = 3.0 m [3] SI = 35.5 - 7.0(VD) - 1.4(MD) + 0.1(SD) + 3.1(F) + 0.0(M) - 9.4(VP) - 7.4(P) - 4.9(M) - 1.5(R) + 0.0(VR)    Adjusted R2 = 0.85, SEE = 2.0 m G3ReferencesKlinka, K. and R.E. Carter. 1990. Relationshipsbetween site index and synoptic environmentalfactors in immature coastal Douglas-fir stands.For. Sci. 36: 815-830.Scientia Silvica is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: R.E. Carter (Reid Carter@NBFinacial.com) and K. KlinkaProduction and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Natural Science and Engineering Council of Canada and CanadianForestry Service and BC Ministry of Forests under the Canada-British ColumbiaForest Resource Development Agreement, Extension, Demonstration, and Research,and Development Sub-program (1985-1990).For more information contact: R.E. CarterCopies available from: www.forestry.ubc.ca/klinka orK. Klinka, Forest Sciences Department, 3036-2424 Main Mall, UBC, Vancouver, BC  V6T 1Z4

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