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"Trembling Aspen Site Index in Relation to EnvironmentalMeasures of Site QualityScientia Silvica Extension Series, Number  43, 2001IntroductionTrembling aspen (Populus tremuloides Michx.) is one of the most common tree species in the boreal and temperate forestsof North America. It grows on many different sites and associates with a variety of tree species. In BC, aspen is frequentthroughout all submontane and montane continental forested zones. Relationships between environmental factors and forestproductivity have been the subjects of many studies. Most of these studies, using various topographic, soil, physical andchemical properties as independent variables, had limited success in accounting for the variation in SI over a large geographicarea.  The objectives of this study were (1) to quantify relationships between aspen SI and environmental factors at twospatial scales, and (2) to develop predictive SI models from easily measurable environmental factors.Study Stands and MethodsStudy stands were deliberately selected to capture the widest range of climatic, soil moisture, and soil nutrient conditionssupporting aspen growth throughout BC. 142 plots were situated in naturally established, unmanaged, fully stocked, even-aged aspen stands older than 50 years @ 1.3 m on a variety of sites across BC in the BWBS, SBS, ICH, MS, IDF, and ICHzones. Each plot, 20 x 20 m in size, was uniform in stand and environmental characteristics.Biogeoclimatic maps were used to identify the subzone in which each plot was located. Topographic maps were used toidentify the latitude and longitude of each plot; elevation was measured with an altimeter, and aspect with a compass. Soiland vegetation were described at each study site. Site index (SI) for each plot is calculated as the average top height of thesampled trees at breast-height age 50, with top height calculated from stem analysis data.Study stands were stratified according to selected variables of site quality, and one-way analysis of variance and Tukey?s testwere used to detect differences in SI among the strata. To predict SI from environmental measures, multiple regressionmodels were developed. SI was individually regressed on climatic (spatial) variables (latitude, longitude, elevation, andzone), topographic variables (aspect, slope gradient), and edaphic variables (soil moisture and nutrient regimes). Thentopographic and edaphic measures were combined with climatic variables in an all-factor model to test whether climaticvariables improve the model?s predictive power.Results and DiscussionEffect of spatial (climatic), topographic, and edaphic gradients on site indexAspen SI varied from a minimum of 5.5 m, on a moderately dry, nutrient-poor site on a ridge crest in the BWBS zone, to amaximum of 30.7 m, on a moist, very rich site, on a flat in the ICH zone. On zonal sites, mean SI was the highest in the ICHand MS zones (21 and 23 m, respectively), and the lowest in the BWBS and SBS zones (17 and 16.5 m); SI in the IDF zonewas intermediate (19 m) (p &lt;0.0001) (Figure 1).As the pattern of the influence of environmental factors on SI varied with spatial scale, we examined productivity - siterelationships at two spatial scales: (1) provincial, including the data from all 6 zones and (2) zonal (regional), including dataspecific to each of the 6 zones. Relationships between SI and climate surrogates (longitude, latitude, and elevation) differedwith spatial scale. On zonal sites, at the  provincial scale, SI decreased with increasing  longitude  and latitude,  but norelationship between SI and elevation was found (Figure 2). At the zonal scale, SI decreased with increasing longitude in theBWBS and IDF zones, but not in the MS, SBS, and ICH zones. It also decreased with increasing latitude in the BWBS, IDFand MS zones, but not in the SBS and ICH zones (Figure 3).  With increasing elevation, SI decreased in the BWBS and MSzones, but increased in the IDF and ICH zones, while no change was detected in the SBS zone (Figure 3).Figure 1.  Mean site index of aspen on zonal sites in relation tobiogeoclimatic zones. Error bar represents one standard errorof the mean; numbers in the bases of bars are the numbers ofplots; different letters in bars indicate significant difference insite index between zones (p &lt;0.05).Figure 2. Scattergrams and fitted regression lines of aspen site index on zonal sites in relation to (A) longitude (SI = 93.468 ? 0.610 (LON),R2 = 0.41, SEE = 3.50, p &lt;0.0001), (B) latitude (SI = 54.012 ? 0.660 (LAT), R2 = 0.24, SEE = 3.95, p &lt;0.0001), and (C) elevation (SI =16.699 + 0.002 (ELE), R2 = 0.0, SEE = 4.56, P = 0.425).Figure 3. Scattergrams and fitted regression lines of aspen site index on zonal sites in relation to (A) longitude, (B) latitude, and (C)elevation. Regression lines were developed for each biogeoclimatic zone.117 119 121 123 125 127 129 131 133Longitude (W)5101520253035Site index (m @ 50 yrs bh)49 50 51 52 53 54 55 56 57 58 59Latitude (N)200 400 600 800 1000 1200Elevation (m)C117 119 121 123 125 127 129 131 133Longitude (W)5101520253035Site index (m @ 50 yrs bh)49 50 51 52 53 54 55 56 57 58 59Latitude (N)200 400 600 800 10 00 12 00Elevatio n (m)BWBSICHIDFMSSBSZONEC051015202530BWBS SBS IDF ICH MS22b13b10ab15a4aBiogeoclimatic zoneSite index (m @ 50 yrs bh)  VP P M R VR VD 14.8  n = 1   13.1  n = 1    MD 12.6  n = 1 16.3a (4.97) n = 8 13.8b (4.81) n = 16 13.4b (5.68) n = 6   SD 7.8c (0.94) n = 6 13.6b (5.68) n = 14 16.9ab (5.09) n = 13 17.3ab (4.84) n = 19 19.4a (1.13) n = 2 F  15.1b (3.12) n = 6 18.7a (2.80) n = 13 19.9a (4.17) n = 14 17.8 ab (6.72) n = 3 M  10.2  n = 1 25.1a (2.09) n = 4 23.3a (4.55) n = 9 26.2 a (3.93) n = 3 VM   12.2  n = 1 20.0  n = 1   G3The effect of aspect on SI varied among zones. In the BWBS zone SI was lower on north-aspect slopes than on south-aspect slopes; the opposite was found in the MS and ICH zones, while aspect had no effect on SI in the SBS and IDF zones.A similar relationship was observed in relation to drier and wetter portions (subzones) of study zones. On north-aspectslopes, SI in drier climates were significantly higher than in wetter climates, while the opposite trend was observed on thesouth-aspect slopes. This implies that SI is affected by complex interactions, and compensatory effects among latitude,elevation, and slope-aspect determine differences in the length of growing season and soil moisture conditions on a particularsite.SI varied greatly with edatope at the provincial scale (Table 1). On slightly dry and fresh sites, SI increased from 7.8 m onvery poor sites to 18.5 m on very rich sites. On medium and rich SNR sites, SI increased from 13.7 m on moderately drysites, to 23.9 m on moist sites, and then decreased to 16.1 m on very moist sites. At the zonal scale, the relationshipsbetween SI and SMR and SNR were consistent with those at the provincial level.Table 1. Mean aspen site index, standard error (inparentheses), and sample size for each soil moisture regime(SMR) - soil nutrient regime (SNR) combination. Differentsuperscript letters indicate significant differences in siteindex values in rows (Tukey's test; p &lt;0.05).Site index predictionsAt the provincial scale, although significant (alpha = 0.05), multiple linear regression models based on individual environmentalfactors explained little of the variation in aspen SI (12%&lt; R2 &lt;37%). When spatial (latitude, longitude, elevation) gradientswere combined with slope-aspect, predictive power was increased (R2 = 52%); however, the spatial gradients combinedwith edatope explained still more variation (R2 = 61%).Since the zone gives an ecologically meaningful pattern of regional climates across the province and spatial gradients reflectgeneral climatic trends, further regression models were developed by nesting measured environmental factors within zones.The nesting of spatial gradients, slope-aspect, and edatope within zones resulted in models with high accountability of SIvariation (Table 2). From a practical point of view, Eqs. [1] and [2] are especially useful because they allow relativelyprecise SI predictions based on information that can be easily obtained from maps (i.e., latitude, longitude, elevation, zone,and slope-aspect).The model combining all measured variables nested within zones (Eq. [3], Table 2) accounted for 81% of SI variation,markedly exceeding that of any other model; however, predictors varied with zones. In the BWBS zone, SI in the BWBSzone decreased with increasing elevation, decreasing soil nutrients and moisture, and on the northern and western slopes. Inthe SBS zone a weak, positive relationship between SI and elevation was observed, likely due to the drier climate of the areawhere sampling was conducted. In the IDF zone, decreases in aspen SI were associated with increasing latitude and with thesouthern slope-aspect, and the most productive growth occurred on slightly dry (the ?wettest? sites supporting aspen growthin this zone) and very rich sites. In the MS zone SI was the lowest on water-deficient sites and south-aspect slopes, and thehighest on nutrient very rich sites. SI on north-aspect slopes in the MS zone were higher than that on south-aspect slopes,due to the reduced angle of solar incidence and decreased summer temperatures and soil water deficit.  Flat topography inthe ICH zone is associated with increased SI, likely due to the enhanced capacity of these sites to retain water relative toslopes. The all-variable zone model (Eq. [3]) is recommended when all environmental factors within zone are inexpensivelyobtained, or when the value of increased model precision is greater than the cost of obtaining appropriate predictor data.ConclusionsAspen SI varied (1) along spatial gradients (latitude, longitude, and elevation), used as surrogates for climate, and with (2)zone, each delineating a regional climate, (3) slope-aspect, (4) actual soil moisture regime, and (5) soil nutrient regime. Thepattern of variation and the strength of relationships between SI and environmental factors, however, varied with spatialscale, specifically with climate. The variation in SI followed unique trends depending on the climatic conditions of eachzone. At the provincial scale, these relationships were weaker than on the zonal (regional) scale. High-elevation, north-aspect slopes compensate for relatively warm climates at low latitudes, while low-elevation, south-aspect slopes compensatefor relatively cool climates at high latitudes. Aspen SI is predictable across a large area from a combination climatic (spatial),slope-aspect, edatope  variables.ReferenceKrestov, P.V., H.Y.H. Chen, K. Klinka, and B. Collins. 2001. Trembling aspen site index in relation to environmentalmeasures of site quality at two spatial scales. (Submitted to Canadian Journal of Forest Research)No. Predictor  Model [1] Zone; spatial gradients SI = 16.9511 + BWBS ? [2.3011 (LAT) ? 1.0725 (LON)] + IDF ? [-7.4875 (LAT) + 3.1224 (LON)] + MS ? [-14.7737 (LAT) + 7.0683 (LON) ? 0.0858 (ELE)] + ICH ? [-4.6229 (LAT) + 2.0057 (LON)]    Adjusted R2 = 0.58, SEE = 3.70, p &lt;0.0001 [2] Zone; slope-aspect SI = 16.5897 + (BWBS) ? [-3.7219 (N) - 8.7921 (RG) - 3.5586 (W) ? 4.3651 (STR)] + (IDF) ? [3.9371 (E) - 6.6050 (RG) - 7.0756 (W)] + (MS) ? [9.4117 (E) + 10.4150 (F) + 8.7259 (N)] + (ICH) ? [10.9098 (F) + 9.6138 (N) + 4.2418 (S)]    Adjusted R2 = 0.58, SEE = 3.69, p &lt;0.0001 [3] Zone; all-predictors SI = [29.4764 ? 0.0191 (ELE) - 7.3582 (VP) - 2.1196 (P) ? 3.5947 (N) ? 2.6742 (W) + 3.1232 (F - BWBS ? SMR) + 6.2063 (M - SMR)] + SBS ?[-10.9565 + 0.0311 (ELE)] + IDF ?[21.7516 ? 4.0038 (LAT) + 2.1517 (SD - SMR) + 5.6432 (VR) ? 6.4011 (S)] + MS ?[24.2986 ? 4.3168 (MD - SMR) + 3.9359 (R) + 2.7436 (N) ? 5.2927 (S)] + ICH ? [12.5709 + 0.0109 (ELE) + 4.6104 (M - SMR) + 5.3712 (F)]    Adjusted R2 = 0.81, SEE = 2.50, p &lt; 0.0001  Scientia Silvica is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: P.V. Krestov (farrex@vtc.ru), H.Y.H. Chen (han.chen@mnr.gov.on.ca), K. Klinka and B. Collins (brad@locin.com)Produced and designed by: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Site Productivity Working Group, BC Ministry of ForestsFor more information contact: K. KlinkaCopies available from: www.forestry.ubc.ca/klinka orK. Klinka, Forest Sciences Department, UBC,3036-2424 Main Mall, Vancouver, BC, V6T 1Z4Tabled23dZoneUspecificdpredictiondmodelsdfordaspendsitedindexdatdthedzonaldscale3dELEdUdelevationDdLATdUdlatitudeDdLONdUdlongit ude;dNUdnorthDdEdUdeastDdSdUdsouthDdWdUdwestDdRdUdr idgeDdFdUdflat;dSEEdUdstandardderrordofdthedestimate3"@en
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