Canopy-tree influences along a soil parent material gradient in Pinus ponderosa-Quercus gambelii forests, northern Arizona
Abella, Scott R
ABELLA, S. R. (Public Lands Institute and School of Life Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154-2040) AND J. D. SPRINGER (Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011-5017). Canopy-tree influences along a soil parent material gradient in Pinus ponderosa-Quercus gambelii forests, northern Arizona. J. Torrey Bot. Soc. 135: 26-36. 2008.-The distribution of canopy trees can impose within-site patterns of soil properties and understory plant composition. At ten sites spanning a soil parent material gradient in northern Arizona Pinus ponderosa-Quercus gambelii forests, we compared soils and plant composition among five canopy types: openings, Pinus ponderosa single trees, Quercus gambelii single stems, dispersed clumps, and thickets. Soil texture on average did not differ significantly among canopy types, whereas Oi horizon thickness and weight, 0-15 cm soil loss-on-ignition, and gravimetric soil moisture differed significantly among three or more canopy types. Understory plant richness per 4 m^sup 2^ ranged from five species below P. ponderosa to 12 species in openings, with richness below Q. gambelii single stems significantly greater than below Q. gambelii thickets. C^sub 4^ graminoids (e.g., Aristida purpurea) inhabited openings, while C^sub 3^ species like Poa fendleriana also occurred below trees. The forbs Thalictrum fendleri and Lathyrus laetivirens were strongly associated with Q. gambelii dispersed clumps and thickets. We also conducted an experimental planting with T. fendleri that was consistent with these correlational results, with outplanted T. fendleri seedling survival 2-7 times greater when planted below Q. gambelii compared to openings. Previous research and our results suggest that understory species associated with Q. gambelii canopies vary regionally, but there are consistently some associated species. Canopy types affected understory vegetation similarly across soil parent materials, not supporting a hypothesis that positive plant interactions changed along soil gradients. Our results suggest that forest management that manipulates both the density and the pattern of trees, together with the growth forms of Q. gambelii trees, can induce within-site spatial patterns of soil properties and understory species.
Key words: Lathyrus laetivirens, Pedicularis centranthera, positive plant interactions, single-tree influences, Thalictrum fendleri, understory.
Canopy-tree influences on soils and understory vegetation have long been studied in vegetation science (e.g., Ovington 1955, Earth and Klemmedson 1978, Crozier and Boerner 1984). Some authors have described savannas and forests as mosaics of single-tree influence circles where tree distributions constrain soil and understory compositional patterns (Zinke 1962, Wu et al. 1985, Boettcher and Kalisz 1990). In savannas of northwestern Kenya, for example, Weltzin and Coughenour (1990) found that aboveground herbaceous biomass averaged 260 g m^sup -2^ under individual trees of Acacia tortilis (Forsskal) Hayne (Fabaceae), compared to only 95 g m^sup -2^ in openings. Several plant species in that study were distributed according to distances from tree boles. Understory habitat below tree canopies differs from that of openings (Parker and Muller 1982, Leach and Givnish 1999). Habitat variables that may differ among canopy types and between openings include: light, throughfall quantity and chemistry, soil moisture, litter thickness, allelopathy, organic matter, nutrients, pH, soil structure, herbivory, and invertebrate and animal communities (Scholes and Archer 1997, 0kland et al. 1999). Single-tree influences have been detected by long-term studies tracking changes near individual trees (Quideau et al. 1996), chronosequence studies of different tree sizes and ages (Barth 1980, Everett et al. 1983), comparative studies among tree species and canopy openings (Jackson et al. 1990, Finzi et al. 1998), and resource manipulations or species additions and removals (Maranon and Bartolome 1993, Belsky 1994).
There has been increasing appreciation in vegetation science for positive, rather than only competitive, interactions between plants (Callaway 1995). Canopy-tree influences on understory plants can be negative, neutral, or positive, depending on the tree and the plant species or the environmental setting (Scholes and Archer 1997). Some authors have hypothesized that positive interactions intensify in stressful environments (e.g., Callaway 1997, Tewksbury and Lloyd 2001). Various studies have supported this hypothesis while others have not (Mordelet and Menaut 1995, Tewksbury and Lloyd 2001, Maestre et al. 2005). Outcomes can depend on several factors, including the plant community attributes examined (e.g., richness or composition; McClaran and Bartolome 1989, Maestre et al. 2005).
Using both correlational and experimental approaches, we undertook this study to evaluate canopy-tree influences of Pinus ponderosa P. & C. Lawson (Pinaceae) and Quercus gambelii Nutt. (Fagaceae) on soils and understory vegetation along a soil parent material gradient in semi-arid P. ponderosa-Q. gambelii forests in northern Arizona. Quercus gambelii is commonly the only deciduous tree in otherwise pure P. ponderosa forests (Harper et al. 1985). This clonal Quercus species has several different growth forms in P. ponderosa forests related to numbers and spacing of stems within clumps, also providing an opportunity to test whether different growth forms of the same species have different influences. Our hypotheses were that: (1) influences on soils and understory vegetation differ among P. ponderosa and Q. gambelii canopy types relative to openings, (2) Q. gambelii exhibits the most positive associations with understory plant species by containing the greatest understory species richness, and (3) positive associations between tree and understory species increase along a soil parent material gradient from moist, nutrient-rich sites to dry, nutrient-poor sites.
Materials and Methods. STUDY AREA. We performed this study at ten Pinus ponderosaQuercus gambelii sites on the Northern Arizona University Centennial Forest and on the northern half of the Coconino National Forest surrounding the city of Flagstaff. The mean distance between sites was 23 km, with an extent of 42 km (Table 1). We selected these sites, based on previous research, to span a range of soil parent materials and soil properties (Abella and Covington 2006a, 2006b). Based on three weather stations, precipitation across the study area averages 42-56 cm/yr, snowfall from 152-233 cm yr^sup -1^, and maximum daily temperatures from 15.717.5°C (Western Regional Climate Center, Reno, NV). Elevations at the study sites range from 2071-2270 m, and slope gradients are
PLOT LAYOUT. On a 1 ha (100 x 100 m) grid at each site, we randomly selected for sampling five each of the following five canopy types: openings, Pinus ponderosa single trees, Quercus gambelii single stems, dispersed clumps, and thickets (Fig. 1). We sampled a 4-m2 circular plot below each canopy because this plot area fits within typical drip lines of the tree canopies (Gill et al. 2000). There were a total of 25 plots per grid (5 canopy types x 5 canopies of each type). Tree boles were plot centers for single trees, with plot radii corrected for bole area to maintain plot sizes of 4 m^sup 2^ excluding boles. Plots were located in the centers of openings and of Q. gambelii clumps and thickets. Sampling occurred between June 8-30, 2004.
SOIL SAMPLING AND ANALYSIS. We randomly selected one plot of each canopy type on each grid for soil sampling. We collected a 0-15 cm mineral soil sample ([approximate] 1 kg field moist including coarse fragments) about 0.5 m from boles of single trees and from plot centers of other canopy types. We analyzed the
VEGETATION SAMPLING. In all 25 plots on each grid, we categorized Quercus gambelii litter (Oi horizon) cover and aerial cover of understory plant species rooted in plots using cover classes modified from Peet et al. (1998): 1 =
We recorded diameter at 1.4 m (breast height) and mapped all living trees > 1 cm diameter on six of the 1 ha grids. We divided grids into 100, 10 × 10 m cells and mapped stems to the nearest 0.1 m. Repeated measurement errors averaged
EXPERIMENTAL PLANTING. To test the hypothesis that plant survival differs among Quercus gambelii canopy types and openings, we outplanted six-month-old, greenhouse-grown seedlings of Thalictrum fendleri Engelm. ex Gray (Ranunculaceae) and Penstemon virgatus Gray (Scrophulariaceae). Seedlings were purchased from a local grower (Flagstaff Native Plant and Seed, Flagstaff, AZ). We chose these species based on our correlational field data to include a species with an affinity for Q. gambelii (T. fendleri) and for openings (Penstemon virgatus). We stored seedlings outdoors for two months before outplanting on 21 November 2005 near the Dry Lake site (Table 1). Three seedlings each of T. fendleri and P. virgatus were planted below three each of three Q. gambelii canopy types (single stems, dispersed clumps, and thickets) and in openings 12 m away from trees paired with each Q. gambelii canopy. Thus, there were a total of nine Q. gambelii canopies and nine openings, for a total of 54 seedlings planted for each of T. fendleri and P. virgatus. We planted seedlings in a 1 m^sup 2^ circular area below each canopy or in openings. We recorded plant survival on 9 September 2006, approximately 10 months after planting.
DATA ANALYSIS. We compared soil variables and species richness per 4 m^sup 2^ and diversity among canopy types with sites serving as blocks using analysis of variance (ANOVA) and Fisher’s least significant difference for mean separation. Analyses were performed with the software JMP (SAS Institute 2002). To avoid pseudoreplication (Hurlbert 1984), we averaged all variables on a site basis for ANOVA that had the following degrees of freedom: whole model = 13, error = 36, site = 9, and canopy type = 4. We calculated diversity based on relative cover class as Shannon’s Diversity Index in the software PC-ORD (McCune and Mefford 1999). We ordinated combinations of soil variables using principal components analysis (correlation matrix) to examine if canopy influences on species richness changed along multivariate soil gradients. For these analyses, we relativized richness site means of tree canopies by subtracting from open canopy site means. We analyzed categorical (alive or dead) survival data from the experimental planting using a generalized linear model with binomial error terms. The factors in this model were Quercus gambelii canopy type (three levels: single stems, dispersed clumps, or thickets), habitat (two levels: below Q. gambelii or in openings), and planted species (two levels: Thalictrum fendleri or Penstemon virgatus). To avoid pseudoreplication, we used the fraction of the individuals of each planted species that survived (out of three) from each planting for a particular Q. gambelii canopy or opening as the raw data. Due to overdispersion in the data, we used the F statistic, rather than the chi-square, to assess statistical significance at α = 0.05. We used R software (http://www.r-project.org/) to perform this analysis.
Results. Although 0-15 cm gravel concentration tended to be 8-17% lower below openings than below tree canopies, gravel and soil texture were not significantly different among canopy types (Table 2). Soil pH below Pinus ponderosa tended to be 0.2-0.4 units lower than below other canopy types, but pH of Quercus gambelii canopy types did not appreciably differ from openings. Loss-on-ignition below Q. gambelii canopies consistently exceeded that of openings, increasing in the order: openings
Understory plant richness per 4 m^sup 2^ ranged from 4.7 species below Pinus ponderosa to 12.0 species in openings (Fig. 2). Richness was intermediate below Quercus gambelii canopy types, with richness below thickets averaging 2.5 species fewer than below single Q. gambelii stems. Shannon’s diversity index ranged from 1.2 below P. ponderosa to 2.3 in openings and exhibited multiple comparisons identical to those for species richness. There was a slight trend for greater richness of annuals and biennials below open canopies and single Q. gambelii, but canopy types overall were dominated by perennials. Forbs on average composed 64% of richness in openings, 51-56% below Q. gambelii canopy types, and 42% below P. ponderosa. Less than 5.3% of mean richness below all five canopy types consisted of exotic species.
Several species occurred more frequently below two or fewer canopy types (Table 3). The C^sub 4^ grass Aristida purpurea Nutt. (Poaceae), for instance, occurred in 43% of plots in openings on the six grids this species occupied, but was absent below Pinus ponderosa and Quercus gambelii dispersed clumps and thickets. Compared to C^sub 3^ species, C^sub 4^ grasses occurred more frequently below openings and to a lesser extent single Q. gambelii than below other canopy types. In contrast, the forbs Thalictrum fendleri and Lathyrus laetivirens Greene ex Rydb. (Fabaceae) occurred most frequently below Q. gambelii dispersed clumps and thickets. Pedicularis centranthera Gray (Scrophulariaceae) also was most frequent below Q. gambelii canopies. No species was most frequent below P. ponderosa, although Poa fendleriana and Elymus elymoides maintained frequencies ≥ 50% below P. ponderosa.
For single Quercus gambelii, species richness was weakly correlated with Q. gambelii diameter (Pearson r = 0.41, n = 50), but there was large variation in richness for a given diameter (e.g., 4-16 species per 4 m^sup 2^ for 26 cm diameters). On the six 1-ha stem-mapped grids, richness in Q. gambelii dispersed clumps and thickets was negatively correlated with Q. gambelii stem density within clumps (r = -0.34, n = 60) but positively correlated with basal area (r = 0.32, n = 60). Richness was negatively correlated with Oi thickness for Q. gambelii dispersed clumps and thickets (r = -0.48, n = 100), a relationship that strengthened when single Q. gambelii were included (r = -0.62, n = 150). Across Q. gambelii canopy types, richness was not correlated with mean Q. gambelii canopy cover (r = 0.06, n = 150).
Correlations between soil variables and species richness site means of Quercus gambelii canopy types provided little support for the hypothesis that Q. gambelii influence on richness changed along soil gradients (Fig. 3). Richness of Q. gambelii dispersed clumps, for instance, was not strongly correlated with pH (r = 0.05, n = 10), sand concentration (r = 0.11), or soil moisture (r = -0.36). One exception for Pinus ponderosa, however, was that richness more closely approached that of open canopy richness with increasing soil pH. Results of principal components analysis were consistent with these bivariate findings, with little relationship between multivariate combinations of soil variables and richness of canopy types (Fig. 4).
In the experimental planting, planting habitat (below Quercus gambelii or in openings) and planted species interacted significantly (F^sub 1,26^ = 12.1, P
Discussion. SOILS. Inferences were strengthened in this study for isolating tree influences from within-site abiotic variation because soil texture did not differ significantly among canopy types (Boettcher and Kalisz 1986). While tree canopies can reduce throughfall (Anderson et al. 1969), more soil moisture below trees compared to openings could be the result of several factors. Loss-on-ignition was greater below trees, suggesting increased organic matter that probably increased moisture-holding capacity (Saxton et al. 1986). Hydraulically lifted water from tree roots also could have increased soil moisture (Horton and Hart 1998). Furthermore, thicker Oi horizons combined with shading below trees likely affected microclimates and reduced evaporation (Parker and Muller 1982). For example, Evenson et al. (1980) found that light intensity in summer was 48% lower below Pinus ponderosa canopies and 69% lower below Quercus gambelii canopies relative to openings in northern Utah. At a 7.5 cm depth in the mineral soil in June-July, Boyle et al. (2005) reported that temperatures were approximately 0.5-2°C cooler below canopies of old P. ponderosa than in openings in northern Arizona.
Consistent with Klemmedson (1987), Quercus gambelii did not affect mineral soil pH. Klemmedson (1987) found that pH of freshly fallen Q. gambelii leaves was 4.9 (less than our measured mineral soil pH values; Table 2) compared to 3.9 for Pinus ponderosa needles. Stand-level pH of only the Oi+e horizon increased in his study with increasing Q. gambelii relative to P. ponderosa. While all three Q. gambelii canopy types in our study consistently increased LOI, absolute amounts occurring under a given canopy type strongly depended on the site. Loss-on-ignition below Q. gambelii thickets, for instance, ranged from 4% on dry limestone/chert soils at the Garjon Tank site, to 16% on loamy benmoreite soils at Fisher Tank. These changes were approximately proportional to changes in open canopy LOI along the gradient, however, suggesting that Q. gambelii effects on measured soil properties did not change along the sampled soil gradient.
SPECIES RICHNESS AND DIVERSITY. While all tree canopies reduced understory species richness and diversity relative to openings, Quercus gambelii effects depended on canopy type (Fig. 2). Different growth forms of Q. gambelii differentially affected richness. While there might be a minimum diameter (and age) below which Q. gambelii does not affect species richness (Everett et al. 1983), richness for single Q. gambelii was not strongly correlated with stem diameter across our range of sampled diameters (7-77 cm). Based on Q. gambelii diameter-age regressions developed from Fulé et al. (1997) within the study area, estimated ages of our sampled single Q. gambelii ranged from 66-377 yr. Quercus gambelii clones, however, may be older than the oldest living stem (Harper et al. 1985). Differing from our results, Everett et al. (1983) found that tree size and age affected understory composition in northern Nevada Pinus monophylla Torr. & Frém. (Pinaceae) woodlands. Consistent with our results, however, Haworth and McPherson (1994) reported that diameters of Quercus emoryi Torr. (Fagaceae; ranging from 12-48 cm diameter) did not affect understory plant composition in south-eastern Arizona Quercus woodlands.
Stem density and basal area were not strongly correlated with understory richness for Quercus gambelii dispersed clumps and thickets. Differences in richness between these canopy types instead could be partly related to stem spacing, with the closely spaced stems in thickets resulting in reduced richness. Variations in herbivory or wildlife habitat also could have contributed to these differences (Scholes and Archer 1997).
SPECIES COMPOSITION. Within-site tree distribution in Pinus ponderosa-Quercus gambelii forests constrains distributions of understory plant species. All five prevalent C^sub 4^ grasses occurred more frequently in openings than under any tree canopy, while three C^sub 3^ graminoids were relatively frequent under both openings and tree canopies (Table 3). These data support the theory that C^sub 4^ species are most competitive in warm, dry environments (Sage and Monson 1999). While not exclusively occurring below Q. gambelii, the forbs Thalictrum fendleri, Lathyrus laetivirens, and to a lesser extent Pedicularis centranthera, were strongly associated with Q. gambelii. Similar to differential influences of Q. gambelii canopy types on species richness, T. fendleri and L. laetivirens were more prevalent below Q. gambelii dispersed clumps and thickets than under single Q. gambelii. Apparently these species are fairly shade tolerant, and their positive association with these canopy types could result from shading or favorable moisture or nutrient regimes (Table 2; Klemmedson 1987). Greater survival of T. fendleri below Q. gambelii than in openings in the experimental planting supported the correlational finding of T. fendleri being more prevalent below Q. gambelii (Fig. 5). However, T. fendleri survival did not differ among Q. gambelii canopy types in the experimental planting, as its distribution did in the correlational findings. Inferences could be strengthened in a future experimental planting, however, by including additional sites and attempting to more closely isolate potential reasons for differences in survival, such as controlling for animal activity which may differ between openings and below trees (Scholes and Archer 1997).
Our study supports previous investigations in other parts of Quercus gambelii’s range that have found positive associations between Q. gambelii and some understory species. In western Colorado, Brown (1958) reported that Carex geyeri Boott (Cyperaceae) biomass averaged 229 kg/ha below Q. gambelii compared to only 28 kg/ha in openings. Evenson et al. (1980) in Utah also found that C. geyeri was abundant below Q. gambelii, in addition to Pseudostellaria jamesiana (Torr.) W.A. Weber & R.L. Hartman (Caryophyllaceae).
Apparently associated species vary regionally, but a few species occur most frequently below Q. gambelii in several regions within its range.
PLANT-PLANT INTERACTIONS ALONG ENVIRONMENTAL GRADIENTS. Research has been conflicting in evaluating the hypothesis that positive plant-plant interactions (e.g., nurse plants) are more prevalent in stressful environments within landscapes (Mordelet and Menaut 1995). Our study sites span fairly wide soil texture, pH, and productivity gradients within this regional climate (Fig. 4), a conclusion supported by significant statistical differences among sites (Table 2). However, we did not find strong evidence that Quercus gambelii’s influence on soils or species richness or composition was more intense on dry sites or changed in detectable ways along our sampled gradient. For example, Thalictrum fendleri was associated with Q. gambelii on both the lowest and highest soil pH sites (5.6 at Fisher Tank and 6.8 at Campbell Mesa). However, we did not sample Q. gambelii in riparian areas or canyons in the study area, which may affect the perceived length of our sample gradient (Callaway 1997). Also, Q. gambelii is infrequent on some productive basalt and limestone soils in the study area that do support T. fendleri. Experimentally testing for positive interactions using species removals or additions across environmental gradients may contribute further insights about canopy-tree influences along environmental gradients in these forests (Moir 1966). The experimental planting that we conducted at one site, for example, could be extended to additional sites.
MANAGEMENT IMPLICATIONS. Pinus ponderosa-Quercus gambelii forests consist of mosaics of tree influences inducing within-site soil and understory plant patterns (Zinke 1962, Finzi et al. 1998). Results suggest that manipulating tree densities and spatial patterns will affect distributions of soil properties and understory species. For Q. gambelii canopy types, dispersed clumps seem to provide a compromise between maintaining fairly high species richness (Fig. 2), while providing habitat for the three plant species associated with Q. gambelii in our study (Table 3). Quercus gambelii thickets exhibited depressed species richness, probably because of their dense, closely spaced stems, many of which are small diameter (
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Scott R. Abella1,2
Public Lands Institute and School of Life Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154-2040
Judith D. Springer
Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ 86011-5017
1 From the Ecological Restoration Institute, we appreciate help from Mark Daniels with plant sampling; Brian Zimmer with soil analyses; and Kristen Pearson, Jeff Rainey, Claire Fuller, Danielle Gift, Lang Suby, and Jennifer Tsonis with stem mapping. Rudy King, statistician with the U.S. Forest Service, Rocky Mountain Research Station, provided advice on statistical analyses of community and soil data, and Cheryl Vanier, biometrician with the University of Nevada Las Vegas, analyzed experimental planting data. We also thank Sheila Sandusky of the Coconino National Forest, and J. J. Smith and Keith Pajkos of the Centennial Forest, for facilitating and supporting this study’s implementation; and Sharon Altman (University of Nevada Las Vegas) for formatting tables. Two anonymous reviewers and the Associate Editor provided helpful comments on an earlier draft of the manuscript. The U.S. Forest Service and the Ecological Restoration Institute provided financial support.
2 Author for correspondence. E-mail: scott.abella@ univ.edu
Received for publication September 17, 2007, and in revised form December 26, 2007.
Copyright Torrey Botanical Society Jan/Feb 2008
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