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The association between western hemlock fine roots and woody versus non-woody forest floor substrates.. Klinka, Karel 2001-04-09

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The Association Between Western Hemlock Fine Roots andWoody versus Non-Woody Forest Floor Substrates in CoastalBritish ColumbiaScientia Silvica Extension Series, Number 46, 2001IntroductionIn the wetter climates associated with the coastal forests of the Pacific Northwest, coarse woody debris (CWD) accumulationsin the form of snags, downed boles, and large branches can be large in natural forest ecosystems.  Although maintainingorganic matter for sustainable site productivity is not in dispute, the importance of CWD as a source of soil organic matteris questionable.  Forest managers attempting to optimize timber production need to know how CWD affects short-termforest tree growth and productivity.  This study addresses the question of the immediate value of CWD for growth ofmature (90 year old) western hemlock (Hw). Because of practical difficulty with mature trees growing in different substrates,we utilized fine root distribution or proliferation, as an indicator of important substrates.Roots in higher plants tend to proliferate in locally rich patches of water or nutrients.  Root proliferation can thus be used totest the relative importance of CWD compared to non-woody forest floor substrates and to mineral soil for tree survival andgrowth.  In this study we test the importance of CWD for mature trees in the summer-wet cool mesothermal climate ofcoastal BC.  The objective was to investigate whether the fine roots (<2 mm diameter) of mature Hw proliferate to adifferent degree in different rooting substrates.  Hw was chosen since it is most strongly associated with CWD.  If rootgrowth is  greater in  CWD compared to  other  substrates,  this  implies  that  CWD provides an immediate  benefit as amoisture and/or nutrient source.Study Stands and MethodsWe investigated how two humus forms (Mors and Moders) and the adjacent CWD substrates affected root proliferation.The soil moisture regime of sites with Mors were identified as fresh; sites with Moders were fresh to moist.  Three areasin the Submontane Very Wet Maritime Coastal Western Hemlock (CWHvm1) variant were sampled: the Capilano watershednorth of Vancouver, the Malcolm Knapp Research Forest east of Vancouver, and southeastern Vancouver Island.  In orderto isolate only Hw roots, the study sites were second-growth, naturally regenerated stands approximately 90 years old, with> 70% Hw cover.  The closed canopy shaded out most herbs and all shrubs.Six sites were sampled within each area: three sites each with Mor and Moder humus forms, for a total of 18 sample sites.Within each stand a sampling site of approximately 100 m? was selected containing only Hw, and having a Hw growingupon a class II downed log (the log is elevated on support points with intact to partly soft wood) in the centre.  Samplinglocations were selected as follows (Figure 1):i  A Hw tree or sapling growing on class II log was plot centre.  Two locations approximately 2-4 m from each side of thetree along the log were sampled;ii  From the  centre tree,  as close as possible to  a 90? angle from the  log,  another two  locations  (one on each side)containing a decay class IV or V log (these classes of decayed wood form a mound on the ground with small softblocky pieces or a soft powdery texture) were sampled.  These sampling locations were 2-4 m away from a hemlocktree; andiii The prevalent non-woody humus form was sampled from two locations as close as possible to location ii.An exact distance and angle was not possible but the closest point criterion prevented observer influence.  The substratessampled were (a) non-woody humus form over mineral soil; (b) the mineral soil under the non-woody humus form; (c)non-woody humus form over the decay class II log; (d) the decay class II log; (e) non-woody humus form over the decayclass IV or V wood; and (f) the decay class IV or V wood (Figure 1).Root density was determined by excavating from 750 to 1500 cm? of each respective substrate, and collecting the substrateand associated fine roots from the hole.  The volume of the hole was determined using 2 mm diameter glass beads.  Foreach substrate, two holes were excavated at each sample point (i.e., two holes for each of the six sampling points).  Rootswere washed and separated according to size class and then oven dried at 65?C to constant weight.  The density of the fineroots was expressed as mg?cm-? calculated from the volume of sample cores.  The mean density of the four sample coresfor each substrate was used for statistical analyses.  Chemical analyses for each substrate determined pH, total C, and N-related properties.  Statistical analyses for the density of fine roots and chemical properties were done separately for eachof the humus forms.Results Significant differences (p < 0.001) in mean values for root density were detected between the different substrates withinboth humus forms. Root densities were stratified into six classes: three classes representing the LFH layers, two classesrepresenting the two CWD decay classes and one class representing the mineral soil sampled (Table 1; Figure 2).  Rootdensity was approximately two to  three times  greater in  the non-woody LFH substrates  compared to the  CWD andmineral soil.  Significant differences (p < 0.001) in mean values for all chemical measures were detected between thedifferent substrates within both humus forms (Table 1).  Total N, C:N ratio, and mineralizable-N data were stratified into thesame six classes (Figure 3).  With some exceptions, two groups were distinguished:  a very strong group of the non-woodyLFH layers and a weaker group of the CWD and the mineral soil.Figure 1. Layout showing the sampling locations (i,ii, iii). Two substrate samples were collected per location (a to f).Rooting substrate characteristic  LFH on mineral soil  LFH on log II  LFH on log IV  log II  log IV  mineral soil Moder humus form         root density (mg cm-3) 2.86a (0.56)  3.71a (0.82)  3.87a (0.44)  1.41b (0.31)  1.27b (0.36)  1.40b (0.37) pH   3.87b (0.13)  3.39c (0.07)  3.41c (0.09)  3.40c (0.10)  3.34c (0.06)  4.43a (0.06) total-C (%)  44.18b (6.15)  58.68a (2.27)  57.15a,b (2.45)  61.14a (1.23)  59.44a (1.19)  7.78c (1.13) total-N (%)  1.614a (0.131)  1.660a (0.054)  1.599a (0.105)  0.270c (0.012)  0.442b (0.018)  0.341b,c (0.068) C:N   27.6c,d (2.3)*  35.5c (2.6)  37.3c (5.0)  247.8a (12.9)  139.2b (2.2)  23.3d (1.9) mineralizable-N (ppm)  467.64a (51.53)  447.09a (39.40)  459.92a (66.77)  40.37c (6.54)  65.19b,c (8.59)  95.96b (33.39) Mor humus form         root density (mg cm-?) 5.66a (1.05)  4.44a (0.30)  5.40a (0.82)  1.90b (0.31)  2.09b (0.56)  2.47b (0.64) pH 3.56b (0.09)  3.44b,c (0.11)  3.35b,c (0.08)  3.23c,d (0.04)  3.06d (0.08)  4.13a (0.18) total-C (%)  55.73a (0.83)  52.47a (5.07)  56.44a (0.96)  59.59a (2.22)  60.24a (1.91)  3.85b (0.89) total-N (%)  1.373a (0.067)  1.341a (0.107)  1.391a (0.011)  0.302b (0.057)  0.357b (0.080)  0.110c (0.040) C:N 41.0b (2.1)  41.2b (7.9)  40.9b (0.9)  220.6a (52.2)  189.0a (34.9)  38.9b (3.1) mineralizable-N (ppm)  445.91a (22.24)  322.49a (47.07)  380.42a (21.82)  35.01b,c (18.43)  38.72b (10.19)  19.21c (9.98) G3Table 1.  Means and standard errors (in parentheses) of root density and chemical measures within the Moder and Morhumus forms between the rooting substrates (n=3).  LFH = non-woody humus form; log II = decay class II log; log IV = decayclass IV/V wood.  Values with the same letter in the same row are not significantly different (p > 0.05).;;;;yyyyDecay class II logHemlocktreeHemlocktreeHemlocktree;;;;yyyy;;;;yyyy;;;;yyyy;;;;yyyy;;;;yyyy;;Non-woody humusMineral soilClass II CWDClass IV CWDRooting substrateAll substrate samplingpoints are 2 to 4m from treesiiiiiiiiiiiie fc da bFigure 2.  Plot of the mean and standard error of the fine root density associated with each rooting substrate.  (A)is the Moder humus form, and (B) is the Mor humus form.  For the rooting substrate:  hms = LFH over the mineralsoil, hdw2 = LFH over the decay class II log, hdw4 = LFH over the decay class IV log, dw2 = decay class II log,dw4 = decay class IV log, and ms = upper 10 cm of the mineral soil under the LFH substrate.Rooting substratehms hdw2 hdw4 dw2 dw4 ms01234567Rooting substratehms hdw2 hdw4 dw2 dw4 ms01234567Fine root density (mg.cm3)ABConsidering the three nitrogen measures (total-N, C:N ratio, and mineralizable-N), there was a general pattern for bothhumus forms.  Mean values for the non-woody LFH layer were consistently significantly different from the two decayclasses but with no significant differences detected among the three non-woody LFH layers.  For both humus forms, the twoCWD decay classes and the mineral soil showed different groupings for nitrogen measures.Figure  3. Plot of the mean and standard error of (A) pH, (B) total-N, (C) C:N ratio, and (D) mineralizable-N associatedwith each rooting substrate.  The Moder humus form is represented with solid circles and the Mor humus form with hollowcircles.  The acronyms for the rooting substrates are: hms = non-woody humus form over the mineral soil, hdw2 = non-woody humus form over the decay class II log, hdw4 = non-woody humus form over the decay class IV log, dw2 = decayclass II log, dw4 = decay class IV log, and ms = upper 10 cm of the mineral soil under the non-woody humus form.2345012Rooting substratehms hdw2 hdw4 dw2 dw4 ms050100150200250300Rooting substratehms hdw2 hdw4 dw2 dw4 ms0100200300400500600pHC:N ratiomineralizable NTotal -N (%)A BCDScientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Gordon J. Kayahara (gordon.kayahara@mnr.gov.on.ca)Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: National Science and Engineering Resaearch Council of CanadaFor more information contact:  G. KayaharaCopies available from:  www.forestry.ubc.ca/klinka, orK. Klinka, Forest Sciences Department, UBC,3036-2424 Main Mall, Vancouver, BC, V6T 1Z4DiscussionAccording to the root proliferation hypothesis, CWD appears to provide little immediate benefit to tree growth in thepresence of non-woody humus forms on sites without a water deficit.  Since the climate of the areas used in this study doesnot typically have a long dry growing season, and the study sites did not have a soil water deficit, the moisture value ofCWD can be disregarded for this study.  Root proliferation generally follows the nutrient content of the substrates.ConclusionsIf root proliferation is accepted as a good indicator of substrates that benefit tree survival and growth, then CWD has lessshort-term value to trees compared to non-woody humus substrates on sites without a water deficit in the summer-wetclimates of coastal BC.   Despite the  strong association of seedlings and saplings with CWD, there  appears to  be norelationship between CWD and tree productivity.ReferenceKayahara, G.J. 2000. The effect of coarse woody debris on site productivity of some forest sites in southwestern BritishColumbia. Ph.D. Dissertation, Department of Forest Sciences, University of British Columbia, Vancouver, BC. 123 pp.

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