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2 Department of Agronomy and Range Science and Center for Population Biology, University of California, One Shields Avenue, Davis, California 95616 USA; and 3 Section of Evolution and Ecology and Center for Population Biology, University of California, One Shields Avenue, Davis, California 95616 USA
Received for publication August 31, 1999. Accepted for publication March 2, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: annual grasses • Bromus • California • oak canopy • plant competition • Poaceae • Quercus • soil nutrients • species distribution
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ebersohn and Lucas, 1965
; Parker, 1977
; Schott and Pieper, 1985
; Scholes and Walker, 1993
; Vila and Sardans, 1999
; Weltzin and McPherson, 1999
). In California oak woodlands and savannas, species compositional shifts between open grassland and understory vegetation are often well developed and have been found within both deciduous and evergreen oak communities (Holland, 1973
; Parker and Muller, 1982
; McClaran and Bartolome, 1989
; Callaway and Davis, 1998
). Studies on mechanisms causing differences in species composition under oak canopy usually have emphasized differential adaptation to canopy-caused gradients in soil nutrients and light (Holland, 1973
; Mahall, Parker, and Fonteyn, 1981
; Maranon and Bartolome, 1993
; Dahlgren, Singer, and Huang, 1997
; Davis, Wrage, and Reich, 1998
; Davis et al., 1999
). Although Maranon and Bartolome (1993)
suggest that herbaceous competition under dense evergreen oak stands is weak, the importance of competition in affecting changes in composition under deciduous oaks is unknown. In addition, despite the long-standing interest in the competitive interactions between congeners (Tansley, 1917
; Harper et al., 1961
; Marshall and Jain, 1967
; Werner and Platt, 1976
), there are surprisingly few studies that have used experimental manipulations in the field to examine how competition between congeners along an environmental gradient may contribute to observed shifts in species composition and dominance (but see Grace and Wetzel, 1981
).
Throughout much of the range of blue oak (Quercus douglasii Hook & Arn.) woodlands in California, the annual grass Bromus diandrus Roth typically dominates under the canopy of this deciduous oak, while a congener, B. hordeaceus L., is usually dominant in the adjacent open grassland (Holland, 1973
; McClaran and Bartolome, 1989
). In order to use this woodland system to examine the effect of competition between congeners on plant species distribution, we conducted a field experiment that examined how the competitive balance between B. diandrus and B. hordeaceus changed between open grassland and blue oak understory. We measured the performance of each species using a demographic index (i.e., relative reproductive rate) that (1) facilitated comparisons of performance between species and (2) allowed us to examine whether the population performance of a species correctly predicted its distribution and abundance. Because it has been suggested that increases in soil nutrients beneath blue oak canopy may be important in causing this shift in distribution (Holland, 1973
), we also manipulated soil type in order to examine edaphic effects independently of competition and other environmental factors such as shading. Finally, we repeated the entire experiment at another site in the following year. Because we used a different site in the second year, we could not partition site and year effects; however, this second year of the experiment did allow us to examine the robustness of our results across different growth environments.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The modal texture of the soils at the Hopland site is silty loam (ultic haploxeroll), while the soil texture at the Sierra site is rocky loam (mollic haploxeralf). The Hopland site is grazed by sheep, while the Sierra site has been grazed primarily by cattle.
At both field sites, we used a split-plot design with habitat type (grassland or oak canopy) as the main plot factor and species, soil type, and density of each Bromus species as factorial subplot factors. At each site we established five rectangular (30 x 10 m) replicate blocks under different blue oak trees. These blocks were fenced to prevent livestock grazing and were oriented at random with respect to compass direction. Blocks were located near each tree so that approximately one-third of the block (10 x 10 m) was under canopy, one-third in open grassland, and one-third of the block was located in a transition zone between the two habitats. The transition area of the block was not used for planting. We planted the blocks at Hopland on 9–13 September 1987 before the onset of the rainy season. Similarly in the following year, we planted before fall rains at Sierra on 5–8 September 1988. Hereafter, for convenience, we will refer to the separate experiments by their field sites, but it should be noted that differences between the field experiments represents the combined effects of both site and year. We examined the influence of soil nutrients on compositional shifts between habitats by using a reciprocal transplant approach. Thirty-two 20 cm diameter holes were augured within each replicate block to a depth of 30 cm, 16 in the oak canopy main plot and 16 in the grassland main plot. Although some roots may penetrate more deeply than 30 cm, studies on root distribution of these annual grass species indicate that the majority of roots occur in this upper layer of soil (Gordon and Rice, 1992, 1993
; Holmes and Rice, 1996
). The 16 holes within each main plot were arranged in a four by four array with a 50-cm spacing. We collected and bulked the soil from the 16 holes within each habitat type. From each bulk collection, soil was either refilled into the holes within the original habitat or transplanted to holes within the other habitat. Within each main plot, these refilled holes served as the planting locations for the various subplot treatment combinations. Removal and refilling of soil for all the planting subplots provided the necessary experimental controls for the disturbance caused by transplanting soils. When replacing the soil, we compacted the soil as much as possible, however, measurements taken from a set of test holes indicated that the bulk density of transplanted soil was less than that of undisturbed soil. Because of this reduction in bulk density, we were unable to examine the influence of bulk density variation as a factor causing compositional shifts across habitats. Although effects of bulk density variation on species shifts could not be examined experimentally, from each of the five blocks we removed cores of undisturbed soil (20 cm depth) in open grassland and under oak canopy to examine bulk density variation. To test for soil nutrient differences along this gradient, subsamples of the bulk density cores were submitted to the University of California Division of Agriculture and Natural Resources Analytical Laboratory for analysis of total Kjeldahl nitrogen, phosphorus, and exchangeable potassium, calcium, and magnesium. Differences between field sites and habitats in soil characteristics were analyzed as a randomized complete block design using the General Linear Model procedure in the SAS statistical package (Freund, Littell, and Spector, 1986
).
At each field site, differences in the light and temperature environment of each habitat were measured when the oak trees were leafless (February) and when the trees had developed full leaf canopy (April). Using a sun-fleck ceptometer (Decagon Instruments, Pullman, Washington, USA), photosynthetic photon flux density (PPFD) was measured at five points within each canopy subplot and expressed as a percentage of the average PPFD measured for the adjacent grassland subplot. Air temperatures at a height of 10 cm were measured with a thermocouple thermometer (Decagon Instruments) at five points within both canopy and open grassland subplots; any canopy effects were expressed as a difference between canopy and open grassland temperatures. Both PPFD and air temperatures were measured between 0900 and 1000, 1200 and 1300, and 1600 and 1700. A repeated-measures analysis was used to detect effects of season, time of day, and site on light and temperature differences between habitats; canopy PPFD data, expressed as a percentage of open grassland PPFD, were angular (arcsine) transformed before analysis.
In order to document species composition shifts between open grassland and oak understory, we measured species cover using a stratified random sampling design. A point sampling frame developed for sampling annual vegetation (Heady and Rader, 1958
) was located at 20 randomly selected locations within each main plot, resulting in a total of 200 points per main plot. To facilitate identification at both field sites, we made these measurements when most annual species were flowering (Hopland—10 May 1988; Sierra—5 May 1989). Changes in composition between open grassland and understory habitats were analyzed as a randomized complete block design with habitat type and field site as factors. As noted above, because a different field site was used in each year, the joint effects of site and year cannot be separated statistically. In addition to analyzing the absolute cover data of both Bromus species, we also analyzed total cover as well as cover data for the other most common grass (Avena barbata Link) and forb species (Erodium cicutarium L'Her.). Percent cover data were angular transformed before analysis by the GLM procedure in SAS.
To test the potential importance of competition among congeners in causing shifts in dominance between the two Bromus species, demographic responses to changes in the density of conspecifics and congeners were monitored for both species. Planting densities for both species were 45, 90, 135, and 270 seeds/dm2. These planting densities represent a range that is typical for these grass species in the California annual grassland (Heady, 1958
). Seeds of Bromus diandrus and B. hordeaceus used in plantings at Hopland were collected from six different populations at this site on 3–5 June 1987, while seeds for the experiment at Sierra were collected from eight populations at Sierra on 24–26 May 1988. We estimated population growth rate (relative reproductive rate) by dividing seed output per plot by seed input (i.e., planting density per plot). To reduce edge effects, we harvested plants only from the central 15 cm diameter area of each plot, resulting in a 2.5-cm buffer around the edge of the plot.
Data were analyzed as a split-plot design using the GLM procedure of the SAS statistical package (Freund, Littell, and Spector, 1986
). Estimates of seed output required for the calculation of relative reproductive rate (RRR) were obtained from regressions relating seed mass to seed number. In order to properly compare relative reproductive rates across treatment combinations, it was necessary to confirm that reproductive parameters other than seed number (i.e., individual seed mass) were not affected by the treatments (except species identity). In order to test this assumption, we examined treatment effects on individual seed mass from a random subsample of seeds from the experimental subplots. To meet parametric assumptions of the linear model, individual seed masses and relative reproductive rates were ln-transformed before analysis. For each site (Sierra 1988–1989 and Hopland 1987–1988), the potential interactive effects of the various factors were examined by testing the significance of all interaction components, beginning with the highest order (i.e., five-way) interaction. Interactions that were significant (P < 0.05) were then further studied by inspection of plots of the appropriate interacting factors.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The overall ANOVA model for relative reproductive rate (RRR) was highly significant at both sites (P < 0.001) and explained 77.2% (Hopland) and 81.2% (Sierra) of the variance in relative reproductive rate (Table 4).
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A strong two-way interaction between species and B. diandrus density indicated that, similar to the Hopland site, B. diandrus appeared to be a stronger interspecific competitor than B. hordeaceus (Table 4B, Fig. 6B). In addition, a significant three-way interaction at the Sierra site (Table 4B; P < 0.05) among habitat type, species, and density of B. diandrus indicates that interspecific competitive suppression of B. hordeaceus by B. diandrus was more strongly expressed under oak canopy than in open grassland habitat (Fig. 7).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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also found that B. hordeaceus was relatively more abundant under canopy at Sierra than at the other four sites they examined. This weaker gradient in species distribution at Sierra parallels the weaker species by habitat interactions in demographic performance measured at this site. Significant crossing interactions between habitat and species were not detected for RRR at the Sierra site. However, significant three-way interactions among habitat, species, and B. diandrus density still indicate a relative advantage for B. diandrus under canopy and a slight advantage for B. hordeaceus in open grassland. In a sense, the evidence from the Sierra site is negative; however, the results are still consistent with a population-based explanation for the shifts in distribution of the two species. The weaker gradient in species dominance between grassland and oak understory measured at Sierra would be predicted from the weaker interaction between species and habitat in demographic performance.
Competition and species distribution
An important question is whether interspecific competition between the congeners could contribute to the observed species distribution gradient. In their review of field competition experiments, Goldberg and Barton (1992)
noted that a proper test of the importance of interspecific competition in affecting distribution patterns requires the examination of the interaction of habitat, target species and competitive interaction (hereafter referred to as the competition by habitat interaction). They argue that for a conclusive demonstration of the importance of interspecific competition in affecting the distribution of a species, the competition by habitat interaction should indicate that: (1) interspecific competition reduces a species performance most in a habitat where the species does not normally occur; and, (2) in habitats where it does not normally occur, the reduction in performance is greater for this species than for another species that normally occurs in the habitat. At both the Hopland and Sierra sites, B. diandrus was a stronger interspecific competitor than B. hordeaceus. However, it is only at the Sierra site that the effect of interspecific competition from B. diandrus appears to vary with habitat (Fig. 7). With increasing B. diandrus density, B. hordeaceus population growth decreases more rapidly under oak canopy (ANCOVA, P < 0.05; regression slope = -0.0166) than in the grassland plots where it is more common (slope = -0.0096). Thus for estimates of population growth in B. hordeaceus, the first competition criterion noted by Goldberg and Barton (1992)
is satisfied because negative interspecific competitive effects on B. hordeaceus are most pronounced in the site where this species is in low abundance (i.e., under oak canopy). Next it is necessary to demonstrate that, under oak canopy where B. diandrus is dominant, interspecific competition has a stronger effect on B. hordeaceus than on B. diandrus. This condition is supported by the general results indicating that B. diandrus is the stronger interspecific competitor regardless of habitat type. Variation in B. diandrus density has a highly significant effect overall on B. hordeaceus population growth rates (P < 0.001; Table 3), while variation in B. hordeaceus density does not significantly affect population growth rates in B. diandrus (P > 0.20; Table 3). Thus, it appears that the distribution of B. hordeaceus could be significantly influenced by interspecific competition from its congener, B. diandrus. However, this does not appear to be the case for B. diandrus because, as noted above, variation in B. hordeaceus density did not have a significant effect on population growth rates in B. diandrus. The relative insensitivity of B. diandrus to density variation in B. hordeaceus suggest that interspecific competition from B. hordeaceus is not an important factor in determining distribution patterns of B. diandrus. In this reversal of species dominance from grassland to oak understory, interspecific competition between these two species may affect distribution patterns of the grassland dominant only. Larger individual seed mass in B. diandrus may contribute to its apparent competitive superiority to B. hordeaceus. Both theoretical (Geritz, 1995
; Rees and Westoby, 1997
) and empirical studies of annuals (Mack and Harper, 1977
; Law and Watkinson, 1987
; Turnbull, Rees, and Crawley, 1999
) have demonstrated repeatedly that increased seed mass is correlated with increased competitive ability.
The fact that the competition by habitat interaction was not significant at the Hopland site suggests that the effect of interspecific competition on the distribution of these annuals may be vary spatially or temporally, or both. Unfortunately, spatial and temporal effects on the competition by habitat interaction cannot be separated in this study because the experiments at each field site were also conducted during different years. We are not aware of other studies that have looked at spatial or temporal variation in the occurrence of competition by habitat interactions (Goldberg and Barton, 1992
), so it is unknown whether our results can be generalized to other systems. Whether competition occurs at all often seems to be dependent on spatial and temporal variation (Goldberg and Barton, 1992
), so it seems a reasonable extrapolation to expect that these more complex interactions involving competition might also be environment dependent.
Finally, it is possible that the nonsignificance of the competition by habitat interaction at the Hopland site may result from intraspecific genetic differences between populations of the two Bromus species at the two field sites. Previous work on B. hordeaceus in California has demonstrated that there is significant variation among populations of this species in growth and demographic response to both intra- and interspecific competition (Wu and Jain, 1979
). Because we only used locally collected seeds of both species at each site, we cannot use our results to test for this possibility. It should be noted, however, that at both Hopland and Sierra, B. diandrus was consistently insensitive to interspecific competition from its congener, while B. hordeaceus was strongly suppressed by increasing B. diandrus density.
With regard to the role of competition in affecting species distribution, the study by Maranon and Bartolome (1993)
on vegetation shifts under the evergreen oak Quercus agrifolia provides an interesting contrast to our study conducted under deciduous oak canopy. They argued convincingly that because deep shade under a dense evergreen oak canopy reduces herbaceous cover to such a low level, interference among herbaceous species should be very weak. As a result, they suggested that competitive effects of understory species such as B. diandrus are not important in suppressing populations of open grassland species such as B. hordeaceus under evergreen canopy. In contrast, our results indicate that interspecific competition from B. diandrus is likely an important factor reducing the abundance of B. hordeaceus under blue oak canopy. We suggest that our contrasting results reflect the greater light availability and resulting higher plant density under deciduous oaks compared to evergreen oaks (Holland, 1973
). For example, the plant density measured by Maranon and Bartolome (1993)
under dense evergreen canopy was much lower (10 plants/dm2) than the density gradient we used in our experimental treatments (i.e., 45–270 seeds/dm2).
The demographic results from the Sierra site provided evidence for the importance of competition from B. diandrus in determining the distribution of B. hordeaceus. In contrast, B. diandrus was found to be relatively insensitive to interspecific competitive effects from its congener either under canopy or in open grassland. This raises the question as to what factors reduce population growth of B. diandrus in open grassland (Fig. 4) and thus contribute to its lower abundance in this habitat (Table 3).
Our data on nutrient levels under canopy indicate significantly higher levels of both N and P relative to the open grassland at both Hopland and Sierra (Table 4). Because reduced soil fertility in grassland soils decreased the population growth of B. diandrus to a greater extent than in B. hordeaceus at both sites (Fig. 5), we propose that this gradient in soil fertility between the two habitats helps to explain the lower abundance of B. diandrus in less fertile open grasslands. Our results agree with previous studies that have indicated that B. diandrus responds favorably to increased nutrient availability (Jones and Evans, 1960
; Jones and Winans, 1967
; Hull and Muller, 1976
; Heady et al., 1992
). As part of their study on the distribution of B. diandrus and A. fatua beneath coast live oaks, Parker and Muller (1982)
examined the effects of soil nutrients on both grass species. Their results indicated that B. diandrus was more responsive than A. fatua to the higher nutrient status associated with soils under canopy, especially under shaded conditions. They thus concluded that soil fertility gradients contributed, at least in part, to the dominance of B. diandrus under live oak. Similarly, higher soil nutrient levels under blue oak canopy were also found by Holland (1973)
. Although Holland did not conduct any experiments, he suggested that increased cover by B. diandrus under canopy resulted from soil nutrient differences.
In sum, we suggest that factors determining the relative abundance of these two bromegrasses along the environmental gradient between open grassland and oak understory may be different for each species. The grassland dominant B. hordeaceus appears to be restricted in its distribution by competition from its congener, the oak canopy dominant B. diandrus. In contrast, B. diandrus is largely unaffected by interspecific competition from B. hordeaceus. It is possible that competition from other species that are common in the open grassland but relatively rare under canopy may act to reduce B. diandrus abundance in open grassland. For example, competition from Erodium cicutarium, a species absent from the understory but abundant in open grassland, may affect B. diandrus distribution. Similarly, Maranon and Bartolome (1993)
argued that dominant tall grasses such as Avena fatua might limit establishment of understory species such as B. diandrus in open grassland. Although competition from other species like Erodium or Avena may play a part in reducing B. diandrus abundance in open grassland, it is important to note that we measured reduced growth for open grassland populations of B. diandrus populations within an experimental design that strongly reduced potential competition from such species. As a result, we suggest that physical conditions in the open grassland might be just as important as competition from grassland dominants in reducing B. diandrus population growth. In particular, our results from the soil reciprocal transplant treatments indicate that reduced nutrient availability found in open grassland sites may limit the abundance of B. diandrus in this habitat.
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FOOTNOTES |
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4 Current address: Mountain Lake Biological Station, Department of Biology, University of Virginia, Charlottesville, Virginia 22903 USA
5 Author for reprint requests (Tel.: 530-752-8529; e-mail: kjrice{at}ucdavis.edu
).
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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