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Species diversity and floristic relationships of the understory vegetation in black spruce and trembling.. Klinka, Karel; Qian, H.; Krestov, Pavel; Chourmouzis, Christine 2001

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Scientia Silvica Extension Series, Number  32, 2001Species Diversity and Floristic Relationships of the UnderstoryVegetation in Black Spruce and Trembling Aspen Stands in theBoreal Forest of British ColumbiaIntroductionThe boreal forest is confined to the Northern Hemisphere and is the most continuous and extensive forest in the world. InNorth America boreal forest extends from the Pacific to Atlantic coast spanning over 10? latitude. White spruce (Piceaglauca (Moench) Voss), black spruce (P. mariana (Mill.) B.S.P.), and trembling aspen (Populus tremuloides Michx.)are among the dominant tree species. Black spruce and trembling aspen may form pure stands and occupy similar sites astheir edaphic amplitudes overlap; however, spruce is rare on water-deficient sites and aspen does not tolerate excesswater.Despite many studies conducted in the North American boreal forest, little is known about relationships between theboreal understory vegetation and softwood or hardwood canopy species in different climate regions. Furthermore, thevariation in species diversity and succession between the stands dominated by coniferous trees and those dominated bybroadleaved trees within the same region is unknown. The objectives of this study are to determine (1) the difference inthe species diversity and floristic composition of understory vegetation between black spruce and trembling aspen standswithin the same climatic region, and (2) how the species diversity and floristic composition of understory vegetation ineach stand type vary with climate, and soil moisture and soil nutrient conditions.Study Stands and MethodsA total of 231 sample plots, each 20 x 20 m, representing black spruce and trembling aspen stand types, were located from52? 14' to 59? 59' N latitude and from 120? 02' to 133? 17' W longitude. The stands were stratified into three climaticregions according to zonal classification of the BC Forest Service: (1) drier montane boreal (DMB), (2) wetter montaneboreal (WMB), and (3) mild montane boreal (MMB). The DMB and WMB climates are delineated by the Boreal Whiteand Black Spruce (BWBS) zone, and the MMB climate is delineated by the Sub-boreal Spruce (SBS) zone. The drierportion of the BWBS zone affected by DMB climate is west of the Rocky Mountains, the wetter portion of the BWBSzone affected by WMB climate is  east and northeast of the  Rocky Mountains, and the  SBS zone affected by MMBclimate is west of the Rocky Mountains but south of the BWBS zone.Soil moisture regime (SMR) and soil nutrient regime (SNR) of each sample plot were estimated in the field. In this study,we used three classes for both SMR and SNR:  SMR1 (moderately dry + slightly dry), SMR2 (fresh + moist), and SMR3(very moist + wet); for soil nutrients these classes were: SNR1 (very poor + poor), SNR2 (medium), and SNR3 (rich +very rich).Floristic similarity between each pair of the 231 sample plots was measured by S?rensen's index:where a and b are the numbers of species in sample plots i and j, respectively; and c is thenumber of species common to both sample plots.Non-metric multidimensional scaling (NMDS) was applied to detect to what extent different dominant canopy speciesand climatic regions in the sample plots of were segregated in an ordination space.G45G46._G36G2c+=G15ResultsSpecies richness  between aspen and spruce stands in  the  same climatic region  was not significantly different whenvascular plants and cryptogams were combined (Table 1). However, when different growth forms were compared separately,trembling aspen stands were significantly more diverse in vascular plants and less diverse in cryptogams than blackspruce stands in the DMB and WMB regions (BWBS zone). Spruce and aspen stands had equal species richness in theMMB region (SBS zone). In most comparisons, species richness stand types was similar between the DMB and WMBregions (BWBS zone) but significantly higher in the MMB region (SBS zone) (Table 1). Spruce stands Aspen  stands Drier montane boreal  Number of stands  24  28 All plants  20.4 (5.5)Aa 21.0 (4.8)Aa Vascular plants  12.2 (4.2)Ba 16.7 (2.7)Ab Woody plants   4.8 (2.0)Aa 4.8 (1.5)Ab Herbaceous plants  7.4 (3.2)Ba 11.9 (2.3)Aa Cryptogams 8.3 (3.1)Bb 4.3 (3.6)Ab Wetter montane boreal Number of stands   67  60 All plants  20.2 (8.1)Aa 21.6 (3.9)Aa Vascular plants  13.6 (7.2)Ba 20.3 (4.4)Aa Woody plants   4.9 (2.1)Ba 6.6 (1.9)Aa Herbaceous plants  8.7 (5.9)Ba 13.7 (3.7)Aa Cryptogams 6.6 (3.1)Ba 1.3 (1.7) Aa Mild montane boreal Number of stands   27  25 All plants  27.0 (6.1)Ab 27.5 (6.5)Ab Vascular plants  22.1 (6.8)Ab 25.2 (5.4)Ac Woody plants   6.5 (2.2)Ab 7.4 (2.5)Aa Herbaceous plants  15.6 (5.7)Ab 17.8 (3.8)Ab Cryptogams 4.9 (1.8)Bc 2.4 (2.4)Aa G3Table 1.  Mean and standard deviation (in parentheses) ofspecies richness per sample plot according to dominantcanopy species and climatic regions. Means with thesame uppercase superscripts within the same rowbetween different stand types are not significantlydifferent (t-test, alpha = 0.05); and means with the samelowercase superscripts within the same column betweenclimatic regions for the same group of plants are notsignificantly different (Tukey's HSD multiple comparison,alpha  = 0.05).Species richness of vascular plants increased with increasing nutrient availability from poor to rich while that of cryptogamsdecreased in both types of stands (Figure 1a). Overall species richness increased consistently with increasing soil moisturefrom dry to wet in aspen stands but increased and then decreased in black spruce stands (Figure 1b).Figure 1.  Comparisons of species richness (mean ? 1 SE) of overall plants and vascular plants between trembling aspen(At) and black spruce (Sb) stands on sites of different soil moisture (a) and nutrient (b) conditions. Means with the sameuppercase and lowercase letters on the bars for the same stand type are not significantly different in species richness ofall plants and vascular plants (alpha = 0.05), respectively.Soil nutrient regimeSNR1 SNR2 SNR3Number of species05101520253035All plantsVascular plantsSoil moisture regimeSMR1 SMR2 SMR3Number of species051015202530(a) (b)At At At At At At SbAa Aa AaBb BbBcAa AaBb AbBbAaClimatic region/ Stand type DMB/ At (28) DMB/ Sb (24) WMB/ At (60) WMB/ Sb (67) MMB/ At (25) MMB/ Sb (27) DMB/At  0.440 a DMB/Sb 0.319 0.378 b WMB/At 0.282 0.199 0.461 a WMB/Sb 0.291 0.359 0.231 0.376 b    MMB/At 0.224 0.142 0.299 0.178 0.401 a  MMB/Sb 0.328 0.286 0.301 0.310 0.303 0.394 a G3G30G30G25G10G36G25G30G30G25G10G24G37G3aG30G25G10G36G25G3aG30G25G10G24G37G27G30G25G10G36G25G27G30G25G10G24G37G2eG28G3cWithin the same climatic region, floristic similarity was on average higher among aspen stands than spruce stands (Table2). Furthermore, within-stand type floristic similarity was significantly higher (p <0.05) than between stands across allclimatic regions (Table 2). Spruce stands in different regions were more similar than aspen stands (Table 2). The NMDSordination differentiated well between stand types (Figure 2).Table 2. Mean of S?rensen's indices between sample plots according to stand type (At - tremblingaspen, Sb - black spruce) and climatic region (DMB - drier montane boreal, WMB - wetter montaneboreal, MMB - mild montane boreal). Bold values indicate the highest mean in the row and/orcolumn. Numbers in parentheses are the number of sample plots. Mean of S?rensen's indices withthe same superscripts within the same climatic region between the two stand types are notsignificantly different (alpha  = 0.05).Figure 2.  NMDS ordinations showing relationships among the 231 study stands differentiatedaccording to two stand types (AT - trembling aspen, SB - black spruce) and three climatic regions(DMB - drier montane boreal, WMB - wetter montane boreal, MMB - mild montane boreal). Ellipsesfor the stand types were constructed with a probability value of 0.7.Scientia Silvicais published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: H. Qian (hqian@interchange.ubc.ca), K. Klinka, and P.V. Krestov (farrex@vtc.ru)Production and design: 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, 3036-2424 Main Mall, UBC, Vancouver, BC  V6T 1Z4DiscussionAs black spruce and trembling aspen may occupy similar upland sites, what are the determinants of their understoryvegetation differentiation?  We suggest two principal causes:  understory light conditions and forest floor quality (humusforms).Compared to black spruce stands, trembling aspen stands have more light in the understory, indicated by the presence ofshade-intolerant and moderately shade-tolerant species (such as Epilobium angustifolium, Lupinus arcticus and Rosaacicularis), more balanced temperature and soil moisture conditions, and most importantly, higher nutrient availability, asindicated by the presence of Aster conspicuous, Epilobium angustifolium, Festuca altaica, Leymus innovatus, Lathyrusochroleucus, Lonicera involucrata,  Mertensia paniculata  and Osmorhiza berteroi.  Probably the  best indicator  ofhigh nitrogen availability is Epilobium angustifolium that typically inhabits cutovers and signifies rapid decomposition offorest floor materials. In contrast, black spruce stands are dominated by indicators of low nitrogen availability such asericaceous shrubs (e.g., Ledum groenlandicum and Vaccinium vitis-idaea) and oxylophytic mosses (e.g., Hylocomiumsplendens,  Pleurozium shreberi  and Ptilium crista-castrensis).Compared to acidic Mor humus forms (particularly Hemimors) in black spruce stands, a variety of Moder or, rarely Mull,humus forms develop in boreal trembling aspen stands. Many studies investigating relationships among humus forms,forest floor nutrient properties, and understory vegetation concluded that the regional decomposition and nutrient availabilitygradients are reflected in the development of different humus forms that, in turn, are associated with different vegetation.The nutrient availability (measured by plant-available nitrogen) in the forest floor increases from Mor < Moder < Mullhumus forms,  and vegetation changes in  the  same  order from  bryophyte and ericaceous-dominated communities toherbaceous communities. Thus, according to these general relationships, aspen stands should have richer forest floors andmore diverse herbaceous understories than spruce stands.ReferencesQian, H., K. Klinka and P.V. Krestov.  2001.  Species diversity and floristic relationships of the understory vegetation inblack spruce (Picea mariana) and trembling aspen (Populus tremuloides) stands in the boreal forest of British Columbia.Submitted to Journal of Vegetation Science 01/02/25.

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