Abstract: Differences in lichen diversity and abundance and lichen N and C pools were examined across the two dominant host tree species (Picea engelmannii Parry ex Engelm. x Picea glauca (Moench) Voss) and Abies lasiocarpa (Hook.) Nutt.)) and two soil types (fine- and coarse-textured soils) in an old-growth sub-boreal forest in central British Columbia, Canada. Forty-four epiphytic macrolichen species were identified across the study area. Hair lichen species, particularly nonsorediate Bryoria species, were more abundant in spruce on coarse-textured soils, while cyanolichens were most commonly observed in subalpine fir on fine-textured soils. Overall macrolichen biomass and C pools were greatest in subalpine fir trees on coarse-textured soils. The tripartite species Lobaria pulmonaria (L.) Hoffm. was the dominant macrolichen, particularly over fine-textured soils where its stand level biomass was greater than that of all other species combined. The N pools of L. pulmonaria in combination with the less abundant N-rich bipartite cyanolichens amounted to 7.5 1.9 kg Nx[ha.sup.1] on fine-textured soils. These results indicate that epiphytic cyanolichens may make substantial contributions to ecosystem N despite their relatively insignificant contributions to overall forest biomass and C stocks.
Key words: lichen abundance, sub-boreal forest, soil texture, carbon pool, nitrogen pool, lichen nutrition.
Resume : Les auteurs ont examine les differences dans la diversite et labondance des lichens, ainsi que dans les pools N et de C, sur deux especes darbres hotes dominants, le Picea engelmannii Parry xglauca (Moench) Voss) et lAbies lasiocarpa (Hook) Nutt.), et sur deux types de sols (sols a texture fine et grossiere), dans une foret surannee sub-boreale du centre de la Colombie Canadienne. Ils ont identifie 44 especes de macrolichens epiphytes sur lensemble de laire etudiee. Les lichens chevelus, surtout les especes de Bryoria sans soredies, sont les plus abondants chez lepinette venant sur des sols a texture grossiere, alors quon observe les cyaonolichens surtout sur les sapins sub-alpins venant sur des sols a texture fine. Dans lensemble, la biomasse des macrolichens et les pools de C sont plus importants sur les sapins sub-alpins et sur les sols a texture grossiere. Lespeces tripartite, Lobaria pulmonaria, est le lichen dominant, surtout sur les sols a texture fine, ou limportance de sa biomasse in situ est plus grande que celle de toutes les autres especes confondues. Les pools de N du Lobaria pulmonaria, (L.) Hoffm. en combinaison avec les cyanolichens bipartites riches en N, moins abondants, representent 7,51,9 kgx[ha.sup.1], sur les sols a texture fine. Ces resultats indiquent que les cyanolichens epiphytes peuvent apporter une contribution substantielle a lazote de lecosysteme, en depit de leurs contributions relativement peu significatives a lensemble de la biomasse de la foret et des pools de C.
Mots-cles : abondance des lichens, foret sub-boreale, texture du sol, pool de carbone, pool dazote, nutrition des lichens.
[Traduit par la Redaction]
Introduction
Forest canopies, critical to many important ecosystem functions, may support about 40% of the earths extant species, of which 10% are predicted to be canopy specialists (Ozanne et al. 2003). A key component of forest canopy diversity in interior, sub-boreal conifer forests is the epiphytic macrolichen community. While many of the lichens of the sub-boreal forests of central British Columbia are common to the boreal vegetation zone that stretches across the circumpolar north (Ahti 1977), the same is not true of the cyanophytic component. These species are infrequent in the interior forests of British Columbia and are largely restricted to old-growth, rainforest environments (Goward 1994). Cyanolichen species assemblages that were once thought to be endemic to coastal forests are now known to occur in inland rainforest environments (Goward 1994; Campbell and Fredeen 2004; Radies and Coxson 2004; Goward and Spribille 2005) and now in the interior hybrid spruce (Picea engelmannii Parry ex Engelm. xPicea glauca (Moench) Voss.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) forests of central British Columbia. The abundance of lichens in inland forests is thought to be a factor of stand age and appropriate moisture regime (Goward and Arsenault 2000a; Goward 2003a). Lichen growth is limited by water availability because physiological activity occurs only when the thallus moisture content is >50% and >150% for chlorolichens (Hajek et al. 2001) and cyanolichens (Lange et al. 2004), respectively. While hair lichens are known to require cyclical desiccating and hydrating conditions, cyanolichens tend to be more restricted to moist microhabitats, a pattern due in part to the 200% hydration required for nitrogenase activity (Antoine 2004). However, not all old, moist, interior forest ecosystems provide the same level of habitat even for cyanolichens, and disparate loadings are commonly observed. Possible explanations for these observed differences include soil characteristics (Loppi et al. 1998) and tree species composition (Goward and Arsenault 2001). To date, there has not been an empirical comparison of epiphytic lichen biomass between soil texture types or tree species within interior forest ecosystems. Such a comparison would provide further insight into the continued debate over the ecological factors determining spatial and temporal distributions of these species.
Lichens play an important role in forest nutrient cycling (Knops et al. 1996). This may be particularly true of N2-fixing cyanolichens, which have the potential to significantly contribute to the N budget of forest ecosystems (Wollum and Davey 1975; Cleveland et al. 1999). Conservative estimates of the rate of biological [N.sub.2] fixation by vascular plants range from 200 kg Nx[ha.sup.-1]x[year.sup.-1] for leguminous species (Stewart 1969) to 320 kg Nx[ha.sup.-1]x[year.sup.-1] for 20-year-old stands of red alder (Alnus rubra Bong.) (Newton et al. 1968). These species are often less abundant in mature sub-boreal spruce forests with atmospheric N inputs as low as 0.5 kg Nx[ha.sup.-1]x[year.sup.-1] (Rosen and Lindberg 1980; Hope 2001), and N is thought to be the limiting nutrient (Brockley 2000). Nitrogen-fixing epiphytes, on the other hand, are more abundant in older forests (Goward 1994; Sillett and McCune 1998; Campbell and Fredeen 2004) and have been estimated to fix 35 kg Nx[ha.sup.-1]x[year.sup.-1] in coastal old-growth forests (Franklin et al. 1981). While no comparable N2 fixation rates are available for northern interior forests, it may be that cyanolichens are equally important sources of N in these regions.
The relative role of substrate features such as underlying soil texture and tree species on the abundance, C stocks, and N pools of lichen functional groups is wholly unknown for interior old-growth forests. Thus, the objective of this study was to compare lichen diversity, biomass (at the species and functional group level), C, and N between soil types and tree species to query the roles of edaphic conditions and host species in epiphytic macrolichen distribution.
Methods and materials
Study area description
The study was conducted in the Aleza Lake Research Forest (ALRF) in central British Columbia, Canada, approximately 60 km east of Prince George (54[degrees]11N, 122[degrees]40W). The ALRF is in the wet cool variant of the Sub-Boreal Spruce biogeoclimatic zone (SBSwk1) (Meidinger and Pojar 1991) and is broadly transitional between drier plateau forests to the south and west and the wet trench and mountain forests of the adjacent Interior Cedar Hemlock (ICH) / Englemann Spruce--Subalpine-fir (ESSF) zones to the east. The ALRF is characterized by cold, snowy winters and cool, moist summers. It receives an average of nearly 900 mm of precipitation per year, approximately one third as snow, and has a mean summer temperature of 14.8 [degrees]C (Murphy 1996). The research forest is approximately 10 000 ha in area and is forested primarily by interior hybrid spruce and subalpine fir with smaller components of interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), paper birch (Betula papyrifera Marsh.), trembling aspen (Populus tremuloides Michx.), black spruce (Picea mariana (Mill.) BSP), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and occasional individuals of western redcedar (Thuja plicata Donn ex D. Don) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Soils on the research forest were formed from glaciolacustrine parent materials. Fine-textured soils are the more common texture type and are a mixture of Orthic Gleyed Luvisols and Orthic Luvic Gleysols. The less common coarse-textured soils are predominantly Orthic Humo-Ferric Podzols (Arocena and Sanborn 1999).
Study design
To examine the influence of substrate (soil type and host tree species) on epiphytic lichens, we established a factorial experiment with combinations of two factors: soil type (with coarse- and fine-textured soils, C-T and F-T soils, respectively) and tree species (with the two most common trees, subalpine fir and interior hybrid spruce). By examining both soil type and host tree, we hoped to determine the variation in lichen abundance patterns attributable to each. This factorial design also allowed the testing for potential interactions.
Two old-growth sites (120240+ years) were selected in each of F-T and C-T soils. Only two old-growth sites could be found on C-T soils owing to an historic harvesting preference on this soil type. Three 10 m x 10 m plots were established near what was determined to be the centre of the stand. The first plot was located 20 m from the site centre along a random compass bearing. The second and third plots were located 20 m from the centre at a random compass bearing of +120 and +240[degrees], respectively. The terrestrial vegetation, nonvascular community, and stand characteristics of each plot are detailed in Botting and Fredeen (2006). One subalpine fir (fir) and one interior hybrid spruce (spruce) were identified for arboreal lichen assessments within 5 m of each plot centre. Trees that were structurally safe and supported a lichen loading that was most representative of that occurring at the site were identified and three study trees of each species were randomly selected from this pool. Selected trees were subsequently rigged, climbed, and sampled vertically for epiphyte biomass and species diversity.
Lichen assessments
Lichen biomass was assessed on every branch of the study trees. Lichens were grouped for assessments using a modified version of the functional groups described by McCune (1993). The four functional groups were Alectoria (including Alectoria sarmentosa (Ach.) Ach. and lesser amounts of Usnea spp. and Ramalina thrausta (Ach.) Nyl.), Bryoria, foliose chlorolichens, and cyanolichens. Cyanolichen assessments included all species with a cyanobacterial partner, and all references to cyanolichens herein refer to both bipartite species and the tripartite lichen Lobaria pulmonaria (L.) Hoffm. unless specified otherwise. Hair lichens were separated into the two functional groups owing to niche partitioning between Alectoria and Bryoria as identified by Campbell and Coxson (2001). Biomass estimation techniques are described in Campbell and Coxson (2001) and Benson and Coxson (2002).
Lichen surface area, as a proxy measure for cover, was assessed on one branch selected from each of three distinct height zones in the canopy of each sampled tree. The height zones were based on vertical changes in lichen species composition with the upper canopy characterized by abundant Bryoria (Brodo & D. Hawksw.) spp. (particularly nonsorediate species) and the lower canopy by the presence …
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