Bryoid Layer Response to Soil Disturbance by Fuel Reduction Treatments in a Dry Conifer Forest

Amanda Hardman and Bruce McCune1

ABSTRACT. .Weinvestigated the response of the bryoid layer, bryophyte and lichen communities on the soil surface, three years after fuel reduction treatment (logging and burning) in the central Blue Mountains of eastern Oregon. Both treatment and control areas had been decimated by spruce budworm and drought before the fuel reduction treatments. Treatments reduced overstory and understory woody vegetation, litter, and coarse woody debris and disturbed the soil surface. In the untreated stands minor local disturbances had created bare mineral soil over about 1% of the ground area, about half of that was from burrowing rodents. Fuel reduction treatments disturbed an additional 23% of the ground area, beyond the 1% disturbed in untreated sites. Over half of the recently disturbed treatment areas had been colonized by pioneering short mosses. Bare soil from rodent disturbances covered about 10 times more area in treated sites than in untreated sites, increasing from 0.4% to 5.5% cover. The bryoid layer responded to the treatments by changes in species composition, rather than species richness. Treated areas had more cover of small acrocarpous pioneer bryophytes (i.e., Funaria hygrometrica, Ceratodon purpureus, and especially Bryum caespiticium), whereas cover of larger pleurocarps, such as Brachythecium and Rhytidiadelphus was reduced by soil disturbance. We infer that pioneer bryophytes perform a valuable ecosystem service in these dry forests by rapidly colonizing and stabilizing the soil surface, reducing its vulnerability to erosion by wind and water.

KEYWORDS. .Biotic crusts, Brachythecium, Bryum caespiticium, Ceratodon purpureus, ecosystem services, Funaria hygrometrica, lichen, logging, Pacific Northwest, Rhytidiadelphus, rodents.

In the last few decades forests in much of western crusts, the thin living skin on the surface of the soil. North America have experienced an onslaught of Biotic soil crusts can include any association of disturbances, including insects, disease, fire, and mosses, lichens, liverworts, microfungi, logging. Although researchers have studied many cyanobacteria, algae, and other microscopic ecosystem responses to these disturbances, we are organisms found at the soil surface and in the upper relatively ignorant about the responses of biotic few mm of soil in exposed habitats. Biotic crusts grade into a "bryoid layer" (Achuff & La Roi 1977), a particularly appropriate term for bryophyte- dominated surface layers. Arid and semi-arid environments often host persistent, highly-developed biotic crusts that are frequently dominated by lichens Belnap et al. 2001). Free-living, and both lichen- and

or cyanobacteria, and tend to increase in cover with moss-associated cyanobacteria fix N2 (DeLuca et al. time since disturbance. In recently deforested areas in 2002), providing a significant input, particularly in moister climates, the bryoid layer is often initially nitrogen-poor environments. Schlatterer and Tisdale dominated by short-lived mosses whose cover usually ( 1969) found a positive relationship between biotic diminishes with time since disturbance. crusts and the growth of perennial bunchgrass Many studies have demonstrated important species; this also seems possible in open, dry forests. functions of biotic crusts in arid and semi-arid Finally, moss cover can alter moisture and environments (reviewed by Belnap & Lange 2001). temperature at the soil surface, affecting seed Many of these roles should also apply to bryoid layers germination. A few studies have found a positive in other environments. In particular, the bryoid layer relationship between biotic crusts and the can colonize recently disturbed soils, potentially germination of perennial bunchgrasses (Schlatterer & stabilizing the soil surface. This may be especially Tisdale 1969; St. Clair et al. 1984), though this true when hot fires kill canopies and root systems of relationship has been questioned by others (Prasse & higher plants, opening the soil surface to erosion by Bornkamm 2000; West 1990). In some cases intact wind and water. biotic crusts may reduce establishment of invasive Soil stabilization results from various properties species (Serpe et al. 2006). of the bryoid layer. The protonemata, or juvenile The Blue Mountains of northeastern Oregon stages of a bryophyte, bind soil particles as they grow have a long history of logging, fire suppression, and over and through the soil. Mature mosses trap cattle grazing (Langston 1995). Twentieth-century airborne soil particles (Danin & Ganor 1991). Moss forest management led to dense forests that are prone colonies in the Blue Mountains appear to accumulate to insect outbreaks and catastrophic fire (Hessburg et soil; local soils are developed from volcanic ash, al. 2005; Mutch et al. 1993). Furthermore, reduced unstable where disturbed, and easily transported by species diversity may result from fire exclusion in wind. The bryoid layer reduces wind erosion by these fire-adapted ecosystems (Metlen & Fiedler lowering friction threshold velocities (Belnap & 2006; Vavra et al. 2004). Many of the forests in this Gillette 1997; Leys & Eldridge 1998; Williams et al. region incurred heavy canopy mortality and fuel 1995). Rhizoids, or the root-like structures of a moss, buildup from spruce budworm infestations and also stabilize soil by forming a fine filamentous drought in the 1990s. Research on lichen and matrix near the soil surface. Lichen rhizines and bryophyte community response to these changes, as rhizohyphae act similarly, trapping soil particles. well as to prescribed fire and timber extraction, is Algae, cyanobacteria, and fungi bind soil through the almost nonexistent in western North America. To secretion of polysaccharides (Belnap & Lange 2001; help to fill this gap, we studied the effects of fuel Mazor et al. 1996). Mosses reduce rainsplash- reduction treatments (logging and burning) on the sheetwash erosion by physically intercepting the extent, species richness, and composition of the impact of raindrops (Tchoupopnou 1989); bryoid layer in insect-decimated forests. We also presumably other cryptogram covers do the same. measured soil disturbance by the treatments and the Cryptogam cover can increase rainwater infiltration response of the bryoid layer to that disturbance. This in disturbed soils (Eldridge 1993). Although we did contributes to our basic understanding of ecosystem not measure soil stability per se in this study, the services provided by the bryoid layer in these dry substantial evidence cited above supports an forests. assumption that the conversion of bare soil to bryophyte cover will increase stability of the soil STUDY AREA surface. Starkey Experimental Forest is in the central Biotic crusts can increase above- and below- Blue Mountains of eastern Oregon about 40 km west ground development of vascular plants in arid soils of La Grande, 450 15' N 1180 38' W. Starkey is a by increasing soil fertility (Pendleton et al. 2003; primary research facility for studies of elk, Cervus canadensis, and the influence of herbivory, fire, and logging on the landscape (Rowland et. al. 1997). The region is semi-arid but Pacific maritime air masses provide an average annual precipitation of 550 mm at the higher forested elevations, over half of that as snow. Temperatures recorded on site from 1989- 1995 averaged -7.2 °C in December, and 25.4 °C in August (U.S. Forest Service, unpublished data in Clark et al. 2000). The study sites lie on broad plateaus of basalt at 1350-1470 m in elevation. Soils in forested areas are mostly Andisols, predominately well drained Olot silt loams, which are moderately susceptible to erosion by wind and water (USDA 1985). Abies grandis, Larix occidentalis, Pinus ponderosa, and Pseudotsuga menziesii dominate in the Experimental Forest (nomenclature: Hitchcock & Cronquist 1973). Stand density varies from open similar as possible to each other in elevation, grasslands to closed forests, but all six sites that we topography, soils, and vegetation. An eighth pasture studied were formerly closed forests, apart from a few at each site lay outside of the exclosure and was open grassy, thin-soil inclusions. Mountain pine beetles to free-roaming ungulates. Fuel reduction treatments (Dendroctonus ponderosae) in the early 1970s, and consisting of partial removal of standing live, severe spruce budworm (Choristoneura occidentalis) standing dead, and down wood followed by on-site infestations that began in the 1980s, combined with burning were applied in 2001-2002 to three of the six prolonged drought, caused heavy tree mortality and sites before construction of the exclosures. Burns fuel loads. were in broadcast slash rather than piles. These Our study was part of a larger study of the treatments left only scattered live trees (stand basal influence of ungulates on the development of forest area 4.2-9.6 m2/ha; McCune et al. 2008). The vegetation (M. Wisdom et al. 2005, unpublished remaining three had not been harvested or burned in study plan). A companion study at the same sites over forty years and had abundant snags and documented reductions in the forage lichen Bryoria crisscross jumbles of downed wood. fremontii after insect outbreaks and fuel reduction Sampling was carried out in summer months treatments (McCune et a1. 2008). The Pacific from 2004 to 2006, 3-4 yr after treatments were Northwest Research Station has constructed fenced completed. Nine 0.5 X 4 m plots, which were exclosures in forests classified as the Abies grandis permanently staked equidistant from each other, habitat type (Franklin & Dyrness 1973). Our research were sampled along parallel transects in each pasture, was carried out in these exclosures after fencing was for a total of 432 plots. The plot frame was made installed, but before grazing trials had begun, so from 1 cm nylon webbing strung between two pieces differential ungulate influence is not included here. of 50 cm X 1 cm aluminum channel stock. Within Our sampling, however, followed the nested these plots we estimated cover by species in the sampling design of the larger study. bryoid layer. Species growing directly on rock, intact epiphytes on bark or wood fragments, or those that MATERIALS AND METHODS had fallen into the plot were excluded. Any The infrastructure of this study consisted of six bryophytes or lichens growing on soil, humus, and grazing exclosures (sites) made of game-proof well-rotted wood or other plant litter were included. fencing, each of which was further divided into seven, Microtopographic relief, and cover of rock, litter, 1-ha pastures (Fig. 1). Sites were selected to be as wood, and recent rodent disturbance were scored for each site. Nomenclature follows Norris and Shevock (2004) and McCune and Geiser (2009). Plot data (432) were averaged within pastures (48) for the analyses. Permutation-based nonparametric multivariate analysis of variance (perMANOVA, Anderson 2001; McCune & Mefford 2006) tested for differences in community composition by treatment, using 5000 permutations of the data. A two-level nested model that nested pastures within sites was chosen along with the Sorensen distance measure (Bray & Curtis 1957). Variance due to treatment was tested against variance among sites. Because of the small number of unique permutations possible for this F-ratio, we also Taxonomic resolution for the quantitative data evaluated treatment effects with multi-response was defined by what was distinguishable in the field. permutation procedures (MRPP). MRPP is a Macroscopically similar species were lumped, so that nonparametric test of the hypothesis of no difference "species" in the community data actually represent between groups (Mielke 1984; McCune & Mefford species groups in some cases (Table 2). For example, 2006). PerMANOVA allows more complex designs "dark crust" represents a mixture of minute lichens, than MRPP but obtains a p-value by a large number cyanobacteria, and free-living algae. Discerning these of actual permutations of the data rather than species requires microscopic examination. asymptotic approximation. To simplify the design for Microscopic identification of vouchers (OSC) MRPP, we averaged pastures within sites to yield a confirmed accurate field determination for most matrix of six sites X 31 species. Sorensen distance specimens and allowed us to construct a species list measure was also used for this analysis. The chance- corrected within-group agreement statistic (A) burrowing rodents. Fuel reduction treatments measures the strength of the difference between disturbed an additional 23% of the ground area, groups. A = 0 indicates groups no more different relative to the untreated areas, based on the increases than expected by chance, whereas the maximum in bare soil and recently bare soil now occupied by value of A = 1 indicates that all sites were identical pioneer mosses. Over half of that recently disturbed within groups. area had been colonized by pioneer mosses, for an Differences in species richness at the site, additional 12.6% of the total land area, relative to the pasture, and plot level were analyzed with nested untreated areas, increasing the cover of pioneer univariate ANOVA. Plots were nested within mosses to 13.6%. Cover by bare soil from rodent pastures and pastures within sites. To elucidate disturbances was about 10 times higher in the treated differences in species composition of the bryoid layer areas than the untreated areas, increasing from 0.4% among sites and between treatments we ordinated to 5.5% of the land area (Table 3). pastures in species space using a matrix of 48 Microtopographic relief and rock cover did not differ pastures X 31 species and nonmetric between treated and untreated areas. multidimensional scaling (NMS; Kruskal 1964; Rodent burrowing was a prominent cause of McCune & Grace 2002). Sorensen distance was used disturbance to the soil surface, particularly in treated to measure dissimilarity among the pastures. PC- sites (Table 3). Areas where microtopography had ORD's slow-and-thorough "autopilot" sought the been altered by rodents (soil piled up or dug out) in solution with lowest stress (best fit) with 250 runs the current year were essentially devoid of vegetation. from random starting configurations (McCune & Slightly older rodent mounds were colonized by Mefford 2006). A randomization test with 250 runs pioneer bryophytes (see below). The increase in determined whether the fit of the ordinations to the rodent activity at treated sites vs. untreated sites had F data was stronger than expected by chance. weak statistical support at the site level (ANOVA: 1,4 = 3.8, p = 0.12); statistical power was too low to be RESULTS conclusive, because only three sites received each Soil disturbance and recolonization. At the treatment and disturbance by rodents was quite untreated sites minor local disturbances had created variable among sites within treatments. bare mineral soil over about 1% of the ground area Species richness. Differences in species richness (Table 3). Almost half of that was caused by of bryophytes and lichens between treated and untreated areas varied with the scale of the sample the entire project (24 moss, 20 lichen, and 4 liverwort unit. At the site scale, species richness of bryopbytes species). Although the difference between treatments and lichens, when analyzed as field-based species and must be tested at the scale of the treatments (site species groups (Table 2), did not differ significantly level, as given above), the average species richness at 2 between treated and untreated areas (Table 4; nested the plot level (2 m ) is actually higher in the treated ANOVA, F = 1.8, p = 0.25). The total species list, than in untreated areas (Table 4). This resulted from however, after identifying vouchers to species, relatively consistent species presence in the treated included 25 species from treated sites, and 45 species area, in contrast to higher heterogeneity and beta from untreated sites, with a total of 48 species across diversity in the control areas (see also NMS results below). Species composition. Fuel reduction treatments created compositional differences across sites (Tables 3, 5, Appendix 1). MRPP showed a strong difference in the species composition between treated and untreated sites (A = 0.25, p = 0.001). Because we had only three replicates sites for each treatment, the p-value from the perMANOVA is necessarily high, despite the large F ratio for the treatment effect. NMS revealed that patterns in species composition related to environmental variables, site differences, and treatments. A 2-dimensional solution with a final stress of 15 and final instability of 0.00174 was best. This solution represented 90% of the variation in the distance matrix, with 72% on axis 1 and 18% on axis 2, after rotation to load the treatment effect on axis 1. Treated pastures were separated from untreated pastures on axis 1. Treated pastures lie close to each other in the ordination and are therefore similar in species composition (Fig. 2). Species composition of untreated pastures was more variable then treated pastures, indicated by the larger spread of the untreated pastures in the ordination. Bryoid communities varied along a primary gradient from more vegetative cover and wood on untreated sites to more bare soil on treated sites (axis 1, Fig. 2). The treated sites had more bare soils primarily because down wood, standing trees, and understory vegetation were removed through logging and fire, secondarily through additional rodent disturbance. Species highly correlated with recently bare soils were the small acrocarpous pioneer mosses to provide habitat for pioneer species. But, because Funaria hygrometrica, Ceratodon purpureus, and untreated sites seemed less disturbed by rodents, it especially Bryum caespiticium (Fig. 3). In contrast, appears that their influence on the bryoid layer will bryophytes favoring more stable micro sites were diminish as more stable habitat conditions ensue. primarily Brachythecium albicans (Fig. 3), and less Treated areas were relatively homogeneous in so, Barbula eustegia, Cephaloziella divaricata, the bryoid layer as compared to the controls as Polytrichum Juniperinum, Rhytidiadelphus triquetrus, revealed by the degree of scatter of points in the and Rhytidiopsis robusta, along with the lichens ordinations and the scale-dependence of species Cladonia and Peltigera spp. These species associated richness. The only way that treated areas can have with the less disturbed side of the gradient (left side lower richness than controls at the site level, but on Fig. 2). Brachythecium albicans did occur in higher richness than untreated areas at the plot level, treated plots, but only in unburned areas that had is by higher beta diversity in the untreated areas. We sufficient vascular vegetation to create shady cover. attribute this community heterogeneity to more This moss was much less common in treated than heterogeneous microsites in the untreated areas, untreated plots (Fig. 3, Table 3). including patchier tall vegetation and more abundant fallen logs. Shade and humidity provided by forest

DISCUSSION remnants and coarse woody debris probably foster Soil disturbance associated with fuel reduction more pleurocarpous mosses, such as Brachythecium. treatments greatly increased the area of bare mineral Worldwide research on the response of the soil, both directly and apparently by stimulating bryoid layer to fire has revealed a recurring pattern. populations of burrowing rodents. The bryoid layer The mosses Funaria hygrometrica and Ceratodon responded quickly, covering about half of the newly purpureus frequently colonize rapidly on burned sites disturbed area within two years. Pioneer bryophytes in the northern hemisphere (de las Heras et al. 1994; thus perform a valuable ecosystem service in these Hoffman 1966; Southorn 1976; Thomas et al. 1994). dry forests by rapidly colonizing and stabilizing the These species also increased in cover after fuel soil surface after disturbance, reducing its reduction treatments (timber harvest and prescribed vulnerability to erosion by wind and water. If rodents fire) in the insect- and drought-decimated forests maintain their presence in a forest with increasing that we studied. The dominant colonizer in our case understory vegetation, their mounds would continue was, however, Bryum caespiticium, whose life form and successional role are similar to those species. redevelopment of the bryoid layer and its protective These pioneer mosses tend to be small and short- functions. Until pleurocarpous mosses and larger lived, and invest heavily in reproduction by spores plants with well-developed root systems recover from (During 1992; Soderstrom & Gunnarsson 2003). large-scale disturbances, such as fire or logging, the With time they are displaced by grasses, forbs, and early successional bryoid layer is important for soil other bryoid species. By continually moving to retention. freshly bared soils, these small species can escape from competition. In contrast, later-seral species put ACKNOWLEDGMENTS more effort into vegetative growth than into We thank Brian Dick, Corinne Duncan, Bryan Endress, Patricia reproduction (op. cit.). Muir, Bridgett Naylor, Catherine Parks, the staff at Starkey Activities that expose mineral soil promote the Experimental Forest, and anonymous reviewers for their colonization of pioneer bryoid species. Recent soil assistance. 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