Patterns of initial versus delayed regeneration of white spruce in boreal mixedwood succession

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<ul><li><p>Patterns of initial versus delayed regeneration ofwhite spruce in boreal mixedwood succession</p><p>Vernon S. Peters, S. Ellen Macdonald, and Mark R.T. Dale</p><p>Abstract: The timing of white spruce regeneration in aspen (Populus tremuloides Michx.) white spruce (Piceaglauca (Moench) Voss) boreal mixedwood stands is an important factor in stand development. We examined borealmixedwood stands representing a 59-year period of time since fire and determined (1) whether and when a delayed re-generation period of white spruce occurred, (2) whether the relative abundance of initial (</p></li><li><p>real mixedwoods consist of (1) if spruce regeneration occursonly immediately after fire, stands will be even aged, re-flecting an initial floristics model, (2) if there is only de-layed regeneration, stands will be multiaged, reflecting relayfloristics. These two patterns differ with respect to the rela-tive importance of the mechanisms driving stand dynamics.In both cases seed source and availability of regenerationmicrosites are undoubtedly important but competition (bothintra- and inter-specific) is likely to have more influence ondelayed regeneration; as well, the types of regenerationmicrosites that are important may change over time. Thetiming of delayed regeneration after fire in boreal mixed-woods and its importance to age structures, and thus standsuccessional development, are not well understood.</p><p>Provided a seed source exists, most white spruce estab-lishment occurs over a 3- to 5-year period after fire (mainlyon mineral soil seedbeds); this is particularly true followingmast seed years (Zasada and Gregory 1969; Purdy et al.2002; Peters et al. 2005). Abundant initial regeneration ofwhite spruce would subsequently influence the light regimein the stand (Constabel and Lieffers 1996) and thus could in-hibit later white spruce regeneration. Even-aged stands dom-inated by immediate postfire regeneration may, therefore, bemore likely to occur on sites with strong seed sources andsuitable seedbeds or where fires coincide with mast years.Stands burning in nonmast years have significantly less re-generation immediately postfire than stands burning in mastyears; this is because of initial seed limitation and the rapiddeterioration of the seedbed (Peters et al. 2005). It seemslikely that regeneration of white spruce might be limitedonce the forest floor microsites become unfavourable untilthe competitive influence of the aspen canopy declines (dueto self-thinning) and suitably decayed logs become availableas regeneration microsites (i.e., stand exclusion stage; Oliverand Larson 1996). Most snags in mixedwoods fall downwithin 20 years after fire; they must then decay sufficientlyto provide a suitable substrate for spruce to establish (Lee1998).</p><p>In this study, we address the following questions regard-ing the regeneration of white spruce in fire-origin borealmixedwood forests of Alberta: (1) Does a delayed regenera-tion pulse occur, and if so, what is the timing of delayed re-generation after a fire? (2) Is dominance of a stand by initialor delayed regeneration related to seed availability at thetime of the fire (mast versus nonmast year, proximity to and</p><p>direction from seed source)? (3) What are the important re-generation substrates for initial versus delayed regeneration?(4) Do density-dependent factors influence the relative im-portance of initial versus delayed regeneration withinstands? We also present a rules-based aging approach forwhite spruce that uses cross-dating and yet facilitates rapidaging, such that factors influencing successional patterns canbe assessed across a large number of stands.</p><p>Materials and methods</p><p>Study areaThe study took place in a 60 000 km2 area of boreal</p><p>mixedwood forest in east-central Alberta, between 5446N,11002W and 5743N, 11508W. The study area is situatedwithin the Boreal Mixedwood Ecoregion. The average tem-perature is 13.5 C from May to August and 13.2 C fromNovember to February. Sixty percent of the mean annualprecipitation occurs as rain during July. Gray Luvisols andEutric Brunisols are the predominant soils on upland sites(Strong and Leggat 1992). Intense crown fires annually burnan estimated 0.42% of the land base within the Boreal PlainsEcozone (Stocks et al. 2003).</p><p>The fire-origin stands that we sampled were dominated byan overstory of trembling aspen, balsam poplar (Populusbalsamifera L.), and (or) paper birch (Betula papyriferaMarsh.), and an understory of white spruce (Table 1). Allstands occurred on mesic sites that were typically flat togently sloped (slope </p></li><li><p>red squirrel trapping records (19411958). From the poten-tial mast and nonmast years identified, we selected fires thatwere accessible, were more than 200 ha in size, and initiatedbetween 26 April and 6 June. The procedures for identifyingmast years, fires, and stands are outlined in more detail inPeters et al. (2005).</p><p>Stands were sampled during the summers of 19982000.In each stand, sampling was conducted at two sites, 20 and100 m into the burn away from the unburned seed source.Three sampling points were randomly located (at least 8 mapart) at each distance (site). At each sampling point we col-lected the five live white spruce nearest to the plot center foraging, for a total of 15 trees at each site in each stand (360trees in total from the 12 stands). Trees were cut at groundlevel, and the belowground stumps were excavated and col-lected. Regeneration substrate was also recorded (see be-low).</p><p>We used the following procedures to determine the age ofeach tree. A ground-level disk was scored with a fine razorblade, and a dissecting microscope (18110 magnification)was used to count rings along two radii. Trees that by thissimple method dated to within 20 years of the fire were clas-sified as initial regeneration and were used to address the re-maining objectives. Previous cross-dating work verified thatmost trees for which ground-level ring counts aged the treeto within 20 years of the fire could actually be dated towithin 10 years of the fire. Our approach in this study reliesheavily on the fact that most trees initiate either within10 years of the fire or more than 20 years after fires. Evi-dence for this was based on previous cross-dating work thatshowed that in the first 20 years after fire, 89% of the regen-eration established within 5 years of the fire (31 stands, 8fires) (Peters et al. 2005). Furthermore, a delayed regenera-tion pulse did not occur within the first 20 years after fire,as evidenced by many empty age-classes between 8 and20 years (Peters et al. 2005). Thus, it was not necessary inthis study to age these initial recruits more carefully.</p><p>If the ground-level ring count indicated that the tree mayhave established as late as 21 years postfire, the tree wascross-dated above and below ground, according to the proce-dures in Peters et al. (2002). Both above- and below-groundcross-dating are necessary for accurate ages of white sprucebecause missing rings are common at ground level, and sub-stantial portions of the trunk may be buried below ground,contributing to further age underestimation. A total of 134trees were finally determined to have regenerated 21 or moreyears postfire, and the cross-dated ages of these trees wereused in analyses of the timing of delayed regeneration.</p><p>Initial versus delayed regenerationTo quantify the relative importance of initial versus de-</p><p>layed regeneration, and to examine which factors operatingat the time of the fire might influence the relative abundanceof each within a stand, we used trees collected from the twostands in each of the 58- (nonmast) and 59-year-old (mast)fires described above, plus similarly sampled trees from anadditional three stands per fire. Hence a total of 15 exca-vated stumps were typically collected at each of two sites(20 and 100 m from the seed source) in each of five standsfor four fires. One of the sites lacked spruce, and fewer than</p><p>15 trees occurred at a few sites, so in the end there were 39sites and 577 trees.</p><p>Having identified appropriate threshold ages for classify-ing trees as either initial regeneration or delayed regenera-tion, we used seven steps, as follows, to classify treeswithout the time-consuming process of cross-dating everyone. (1) Aging errors (difference between ground-level ringcount and best estimate of true age) were quantified in onestand from each of the 58- and 59-year-old fires (30 treesper stand, from 4 stands total) using above- and below-ground cross-dating (Peters et al. 2002). (2) For each of theremaining trees, ground-level disks were sanded to 400 gritand the rings were counted. (3) The mean aging error (as de-termined in step 1: 5.5 years; range: 4.86.4 years) wasadded to this ground-level ring count. (4) Trees that basedon the age after step 3 originated within 20 years of the firewere classified as initial regeneration. (5) When the age,after step 3, indicated regeneration was between 21 and31 years postfire, the tree was assigned to cross-dating (step7). (6) For the remaining trees (age at step 3 indicated regen-eration 31+ years postfire), we measured the length of theburied trunk. When the length was less than 15 cm (whichrepresents a mean error of 5.1 buried years, based on data inPeters et al. 2002), the tree was assumed to have regeneratedapproximately 26+ years postfire and was classified as de-layed regeneration. When the length of the buried trunk wasmore than 15 cm, the tree was assigned to cross-dating (step7). (7) Trees assigned to cross-dating were cross-dated be-low ground using skeleton plots, a technique that depicts rel-ative ring width for each tree ring and permits cross-datingamong samples (Peters et al. 2002). If the age after cross-dating indicated that the tree originated within 20 years ofthe fire, it was classified as initial regeneration. If the age af-ter cross-dating indicated that the tree regenerated more than20 years after fire, it was classified as delayed regeneration.Based on this process, and on the ring counts and cross-dating of the trees used for the assessment of timing ofdelayed regeneration, we were able to classify each of the577 trees as either initial (within 20 years postfire) or de-layed (20+ years postfire) regeneration.</p><p>Regeneration substratesWe recorded the rooting substrate for each of the white</p><p>spruce trees classified as initial or delayed regeneration (577trees in the 58- and 59-year-old fires, including those used toassess the timing of delayed regeneration) as well as for thesimilarly sampled trees from an additional three stands fromeach of the four fires (an additional 12 stands and 217 trees).For each site we recorded the position as either downwind ofthe seed source (relative to prevailing winds: 60150) ornot downwind of the seed source (all other directions). Wealso had rooting substrate data for 1332 spruce sampled in71 stands from fires that were 19 to 41 years old (see Peterset al. 2005) for comparison. To determine rooting substrate,we removed the leaf litter and humus at the base of each treeand recorded the establishment substrate as log, humus, ormineral soil. Trees rooted on mineral soil and humus weresubsequently pooled into a single mineral soil humus classbecause the location of the root collar could not be exactlydetermined in the field (see Peters et al. 2002). The condi-tion of logs was recorded as burned or unburned (indicating</p><p> 2006 NRC Canada</p><p>Peters et al. 1599</p></li><li><p>prefire versus postfire origin), and above or below ground,depending on whether the trunk outline was visible withoutexcavation. In fires up to 41 years old, belowground logshad existed as downed wood prior to the fire; in 58- and 59-year-old fires, belowground logs might have been downedwood that existed before the fire or a fire-killed tree thatlater fell and had decayed considerably (V.S. Peters, per-sonal observation).</p><p>To quantify the availability of logs as regeneration sub-strates and describe the general vegetation structure forstands at different times since fire, we sampled 135 stands ina total of 17 fires between 4 and 59 years old (included 46stands from the sampled 38-, 41-, 58-, and 59-year-old fires;other fires are described in Peters et al. 2005). In one plot(5 m2) at each of the two sites (20 and 100 m from seedsources) per stand, we recorded percent cover for downedlogs, bryophytes, herbs, grass (live, lying dead, standingdead), and shrubs. Canopy closure was estimated using aconvex spherical densiometer. Shrub cover was estimatedseparately for three height strata (00.2, 0.20.5, 0.52 m).A prism was used to quantify basal area of postfire regenera-tion by species. Postfire regeneration was easily distin-guished from residual trees by their much smaller diametersand heights.</p><p>Factors influencing initial versus delayed regenerationDensity and height of regeneration were quantified in the</p><p>20 stands (39 sites) from the 58- and 59-year-old fires, forwhich we had classified 577 trees as initial or delayed regen-eration (3 sample points per site, but only 115 in total be-cause we were unable to collect height data at two sites). Ateach of the three sampling points per site, we recorded thedensity and height of trees classified as initial or delayed re-generation within a 28.3 m2 circular plot and within a nestedsubplot (5 m2). We also noted each trees location with re-spect to these plots, or as being outside the 28.3 m2 plot. Wecalculated a mean height of initial regeneration within the28.3 m2 plots for use in subsequent data analyses. We calcu-lated the density of initial- and delayed-regeneration treesfrom the total regeneration density and the proportionalabundance of both initial- and delayed-regeneration trees ateach site. Densities were calculated for each sample point(based on density in the 28.3 m2 plot) and also for each site(averaging density for the three 28.3 m2 plots). As an indica-tion of seed-source strength, we counted the number ofstanding and fallen white spruce trees in the adjacent un-burned patch that were seed producing at the time of the fire(for details see Appendix D in Peters et al. 2005).</p><p>In these same stands we quantified the basal area of liveresidual deciduous and coniferous trees at each of the 39sites (using a prism) as an indication of surviving tree com-position postfire. We assumed that the proportional represen-tation of deciduous and coniferous trees in basal area wassimilar to what it would have been immediately postfire.Detailed vegetation data had been collected at each of the 39sites as part of the description of general vegetation structure(see above).</p><p>To quantify mortality (self-thinning) of postfire regenera-tion, we counted all dead spruce 1 year or older in 5 m2</p><p>plots at each of 258 sites in 135 stands from 17 fires be-tween 4 and 59 years old (the same sites we sampled for log</p><p>availability and general vegetation structure; see above; firesare described in Peters et al. 2005). This included all sam-pling points in the 39 sites from the 58- and 59-ye...</p></li></ul>