UBC Faculty Research and Publications

Forest floor dynamics across a chronosequence in the coastal western hemlock zone Klinka, Karel 1997-04-07

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Forest Floor Dynamics Across a Chronosequence in the CoastalWestern Hemlock ZoneIntroductionThe forest floor  represents  the  uppermost organic and organic-enriched mineral  soil  horizons. They have been formed by thedeposition of organic material and the  subsequent  biologically-mediated decomposition.  The forest floor influences rooting-zonetemperature, aeration, moisture, and nutrient conditions, and hence,forest productivity. Considering the importance of the forest floor,and the  fact  that  it  is  exposed to  disturbance (being  the  surfacelayer), we need to assess the potential impacts our logging practicesmay have.Clearcutting, one of the contentious silvicultural practices used inBritish Columbia, is imputed to most adversely affect ecosystemsand sustainability.  We assessed the long-term impact of clearcuttingon the  forest  floor  by documenting changes in  the  thickness,chemical and biotic properties of the  humus form  across achronosequence of forest  stands.    The study  was located  in  thelargest and most representative portion of the  coastal rainforest-the  Very Wet Maritime Coastal Western Hemlock (CWHvm)subzone.The chronosequenceIn  order to  study  long-term  changes expediently,  we used achronosequence approach, i.e., we investigated three stages of standdevelopment (or succession) compared to a benchmark old-growthstage. The developmental stages examined were:(1)  old-growth,  represented  by an open-canopy,  multi-storied,uneven aged (trees  from  100 to  890 years) mixture of westernredcedar,  western hemlock, and Pacific silver fir,  with scatteredDouglas-fir, and well developed understory vegetation;(2) stand initiation, represented by a 7 year-old cutover with plantedDouglas-fir,  naturally regenerated  western hemlock and westernredcedar and early seral vegetation;(3) stem exclusion, represented by a naturally established (30-40years after cutting), unmanaged, dense, closed-canopy, even-agedmixture of western hemlock with scattered Pacific silver fir andDouglas-fir, and poorly developed understory vegetation;(4) understory reinitiation, represented by a naturally established(50-80  years after clearcutting), unmanaged, semi-open  canopy,even-aged mixture of   western hemlock, Pacific silver  fir  andDouglas-fir, with moderately well-developed understory vegetation.Twelve stands, three for each developmental stage, were locatedon zonal sites identified according to the biogeoclimatic ecosystemclassification system as having a fresh soil moisture regime (nowater deficit or excess) and medium soil nutrient regime.Humus form profiles(1) Old-growth(2) Stand initiation? Lv&n horizon of new (n) and old (v) litter? Fa (amphimorphic) horizon, moderate non-compact matted structure with fungalmycelia and fine roots common? thick Hh (humic) horizon, with a blocky togranular structure and scattered clustersof decaying woodThis profile represents a Mormoder humus form.? Lv horizon of mostly old litter? thin Fz (zoogenous) horizon of partlydecomposed plant residue which areweakly aggregated and have loose orfriable consistency? thick Hz horizon has a granular structuredue to the faunal droppings whichconstitute most of the fabricThis profile is a typical Leptomoder and although Mormodersare present too, this is the predominant humus form.(3) Stem exclusionThis profile represents a Leptomoder humus form.This profile is a Mormoder.    Leptomoders are still present inthis successional stage, accounting for 30% of the humusforms.(4) Understory reinitiation? Lv horizon? Fz as in stand initiation stage? Hz horizon is thinner than in stand initationstage and faunal droppings are lessabundant? Lv&n horizon? Fa horizon? Hh/z (humic/zoogenous) horizon, withscattered faunal droppings and predomi-nance of fine substances;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;Lv&nLvLv&nFaFaFzFzHhHzHzHz/hScientia Silvica Extension Series, Number  4, 1997LvHow has clearcutting affected the forestfloor materials?The presence of a relatively thick forest floor is needed to protectthe mineral soil from erosion (erosion risk is high in the coastalrain  forest)  and to  maintain a nutrient pool for  the  bioticcommunity.  In this study the forest floor was preserved duringharvesting operations, so its thickness in the stand initiation stageremained about the same as it was in the old-growth stage. Theforest  floor  thickness  sharply  decreased between the  standinitiation and stem exclusion stages, where it reached a minimum,and increased  from  the  stem  exclusion to  the  understoryreinitiation stage. This decrease and subsequent increase parallelsthe changes in litter inputs and decomposition rates in the forestfloor.  In  the  stand initiation  and stem exclusion stages, whereforest  growth is  vigorous, the  litter  inputs  are low  anddecomposition is high, while the reverse is true in the old-growthstage.How has clearcutting affected the forestfloor nutrient pool?We used two indices - carbon:nitrogen (C:N) ratio and mineralizableN (minN) - to infer the amount of easily available soil nutrients in theforest floor. In general, higher C:N ratios are associated with lowerorganic matter decomposition, and higher amounts of N mineralizationare associated with higher soil nutrient availability.Changes in thickness of the  combined L and F and Hhorizons across the four developmental stages.Variations in the C:N ratio and minN from the old-growth stageacross  the  chronosequence  gives  an  indication  of  thedecomposition  rate and nutrient availability.  C:N was highest andminN lowest in the old-growth stage.  Immediately after clearcutting,C:N declined and minN increased.  This trend continued betweenthe stand initiation and the stem exclusion stages for C:N and minNin the LF horizons. The increased mineralization is a result of theexposure of the original forest floor material, added logging residueand high quality litter from new vegetation to the open area climate(increased  temperature  and moisture) created by clearcutting.However, this increase was not so rapid as to dramatically reducethe forest floor before canopy closure, a period of high demand forsoil nutrients, had occurred.From the stem exclusion stage to the understory reinitiation stage,C:N  increases somewhat and minN decreases. This trend may reflectthe  changes in  microclimate through  canopy closure, and theaccumulation of recalcitrant materials, such as the large inputs of ligninfrom decaying wood.Changes in C:N ratio and mineralizable N in the LF and  Hhorizons across the four developmental stages.03691215standinitiationstemexclusionunderstoryreinitiationold-growthThickness (cm)LF horizonsH horizonsstandinitiationstemexclusionunderstoryreinitiationold-growthLF horizonsH horizonsMineralizable N (mg/kg)202530354045100200300400500C:N ratio202530354045100200300400500Mineralizable N (mg/kg)C:N ratioHas clearcutting affected the hidden life:soil fauna, fungi and bacteria?Decomposition of the  forest  floor  materials results  from  thecomplex interaction  of many different  soil  organisms, knowncollectively as the  food web.   Any substantial variation in  thenumber of organisms or composition of the soil biotic communitymay lead to a disruption (undesirable change) in decompositionand nutrient release.Bacteria and fungi form the  base of the  soil food web. In thisstudy, the bacterial biomass showed very little change throughthe entire chronosequence.  On the other hand, the fungal biomassshowed a large decrease after clearcutting, reaching a minimumat the stand initiation stage, and then showing signs of increasein subsequent stages.  The depression of the fungal communityafter clearcutting may be the result of a decrease in live tree roots(therefore,  a decrease in  the  mycorrhizal community) and thedecreased acidity of the  LF and H horizons.   This decrease inacidity,  from  pH 3.6 in  the  old growth to  pH 4.4 in  the  standinitiation stage  may be due to the mineralization of the Ca-richlitter of western redcedar.  Forest floor fungi are less tolerant ofpH above 4.5.Groups of soil fauna represent the grazers (different groups feedon the various saprophytic and mycorrhizal fungi, and living anddead plant material) and predators in the food web.  In this study, 31groups of soil fauna were identified to the taxonomic level of Familyand above.  Unfortunately, only broad taxonomic groups can beidentified, due to the difficulty of identifying soil fauna to the specieslevel, the level at which food and habitat preferences are displayed.Furthermore, there is a great deal of variation in the samples collected.Nevertheless, some broad trends were evident.Roundworms, mites, and springtails  accounted for  97% of allthe fauna.  Only the springtails were significantly higher (alpha=0.05) inthe  old-growth stage  compared to the  three  stages  of thechronosequence.  The stand initiation stage had the lowest number ofsoil faunal groups, and lacked three groups which occurred in theother stand developmental stages.  One of these groups, the spiders,was only observed in the understory reinitiation and the old-growthstages. This one group is important as it represents a top predator inthe soil food web and when present, can be seen as indicative of anincrease in the complexity of the food web.Changes in fungal and bacterial biomass in the F and  Hhorizons across the four developmental stages.Clearcutting, and the  associated changes in  microclimate anddecomposition rates, induced alterations in the forest floor bioticcommunity.  The greatest impact was on the biomass of the fungalcommunity and on the numbers of springtails, both decreasingsignificantly  after cutting.   Neither of these  two  communitiesshowed a recovery to the original old-growth amounts even atthe  understory reinitiation  stage.  The bacterial communityappeared to be unaffected by clearcutting, although differencesin taxonomic classes were not looked at. Besides springtails, othergeneral taxonomic groups in the faunal community appeared tomaintain the same levels.Numbers of individual (x103m-2) roundworms, mites andspringtails in the four developmental stages.-hdnmnyns) 5324052374332483s) 093322722612s) 731680745r 2 44l 4 6 4 (mg.g-1 dry weight)LF horizonsH horizons FungiBacteriaFungiBacteriaWhat are the impacts of clearcutting?After clearcutting, there were inevitable qualitative and quantitativealterations to the forest floor as the result of the change from amodified to an open-area climate.  Even if the forest floor materialsare preserved during harvesting, the structure and compositionof the forest floor community will be altered.Using the  old-growth stand  as a benchmark, clearcuttingbrought clear changes and some  of these  differences  weremaintained up to the understory reinitiation stage.  In the old-growth forest  a non-compact matted Mormoder was theprevailing humus form.   In the  stand  initiation  stage,  theincreased decomposition of the forest floor materials resultedin  the  development of a friable,  Leptomoder humus form,which was maintained through the stem exclusion stage. Inthe  understory reinitiation  stage a non-compact Mormoderbecomes the dominant humus form. These changes in humusforms were accompanied by increased minN, and decreasesin LFH thickness, C:N ratio, fungal biomass, and springtailpopulations.  Although some of these features appeared to berecovering at the understory reinitiation stage (LFH thickness,C:N, minN, spider populations), other features decreased afterclearcutting and were maintained at this reduced level (fungalbiomass, springtail population).Scientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Jaume Fons (jfons@filnet.es) and K. KlinkaWritten by: Christine Chourmouzis and Gordon Kayahara (gordon.kayahara@mnr.gov.on.ca)Production and design: C. Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Forest Renewal British Columbia and the Spanish Ministry of EducationFor 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 1Z4warkariekarnreba.tgrnevu((etrg.kusr(oe.llrbkencnk.ut.ju(ukoe iju )usrinukoe une @rte k elcikarie irnr.ibaFe 8toe rllrbkeune.(n )r@rt)rtkec@ t/G,yekare)rgirre le@irnris.ku te lekare ()Jgi vkael irnkel(  i.lkriebckkutg-G2yevarkarie.es.iurkoe lehubi rtsui thrtknevu((ejre@i su)r)ute cieh.t.gr)el irnkndevaubaevu((eirnc(keutekare)rsr( @hrtk le.e)usrinre@.kkrite leachcnel ihn-GVye.kevaubaenk.grenrb t)Jgi vkae.t)encjnrfcrtkel irnknevu((jrea.isrnkr)e.g.utde i.e. whether at the understory reinitiationstage or older; and(4) the size and pattern of clearcutting across the landscape.ReferenceFons J. and K. Klinka. 1990. Temporal variation of forest floorproperties in the Coastal Western Hemlock zone of southernBritish Columbia. Can. J. For. Res. 28:582-590.


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