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Ecology |
Department of Biology, Box 355325, University of Washington, Seattle, Washington 98195-5325 USA
Received for publication January 30, 2006. Accepted for publication August 28, 2006.
ABSTRACT
The persistence of poor competitors within species-rich assemblages is often tied to habitat heterogeneity. Here, the persistence of foxtail pine (Pinus balfouriana) in the Klamath Mountains of northern California was addressed using a two-step approach. First, the response of foxtail pine to shading from six co-occurring conifers was examined using two morphological indices. Foxtail pine increased the height to the first branch that supported foliage, and this branch was shorter when compared with those on all other sampled conifers, suggesting that foxtail pine is a poor competitor for light. Second, three hypotheses to explain foxtail pine persistence were tested: habitat heterogeneity at large spatial scales (substrate hypothesis), habitat heterogeneity at small spatial scales (microsite hypothesis), and the long lifespan of foxtail pine (successional hypothesis). Habitat heterogeneity at multiple spatial scales favored the persistence of foxtail pine. At large spatial scales, ultramafic substrates affected the importance and competitive abilities of shade-tolerant conifers. At small spatial scales, species richness, species diversity (H'), and stand density were positively correlated with microsite availability. No support was found for the successional hypothesis. Results are subsequently linked with general hypotheses of species coexistence in species-rich assemblages.
Key Words: competition • habitat heterogeneity • Klamath Mountains • microsites • Pinaceae • Pinus balfouriana • species diversity • ultramafic substrates
The persistence of poor competitors within species-rich assemblages is enigmatic. The canonical theoretical and empirical explanation for the coexistence of unequal competitors is their separation in space and time. Poor competitors typically inhabit low quality or disturbed sites, escape competition by inhabiting microsites, and are limited in their distribution across the landscape (reviewed by Chesson, 2000
; Sarr et al., 2005
). These uncommon species are a major component of the overall patterns of diversity within the landscape.
Competition among tree species is typically for light (Canham et al., 1994
; Barnes et al., 1998
), especially in the montane and subalpine forests of temperate North America (Barbour, 1988
; Rundel et al., 1988
; DeClerck et al., 2005
). The ability to compete for light differs among species. Within the Pinaceae, firs (Abies) and hemlocks (Tsuga) are generally more shade tolerant, while pines (Pinus) and larches (Larix) are less so (Kobe and Coates, 1997
; Mason et al., 2004
). Typical morphological responses to shading include thinning of branches, loss of needles, and stunted growth (Barnes et al., 1998
). Where species from these genera co-occur, the inability to tolerate shade separates these taxa in time (e.g., successional status) or space (e.g., inhabitance of canopy gaps). In this paper, I examine hypotheses addressing the persistence of foxtail pine (Pinus balfouriana Grev. & Balf.), a supposed poor competitor for light within the conifer-rich subalpine forests of the Klamath Mountains (KM), California.
The KM are a geologically complex system of mountain ranges well noted for high levels of conifer richness and endemism (Whittaker, 1960
). Forest types are mixtures of Cascadian and Sierran species, with a suite of rare and endemic conifers that supplements local patterns of species diversity (Sawyer and Thornburgh, 1988
). Foxtail pine is one such conifer that is distributed throughout the subalpine of the KM and in the southern Sierra Nevada. In the KM, it is long-lived, inhabits high elevation sites on and off ultramafic substrates, and typically co-occurs with Shasta red fir (Abies magnifica Andr. Murray var. shastensis Lemmon), western white pine (Pinus monticola Douglas), and Jeffrey pine (P. jeffreyi Grev. & Balf.) on dry, southwest-facing slopes (Sawyer and Thornburgh, 1988
; Eckert and Sawyer, 2002
). It also occurs, however, within enriched subalpine stands containing up to nine other conifer species (Sawyer and Keeler-Wolfe, 1995
). This stand type is typically located on igneous glacial moraines that have northeast-facing slopes (Eckert and Sawyer, 2002
). The structure and composition of foxtail pine stands in the KM is markedly different from the southern Sierra Nevada where foxtail pine occurs as the dominant species in low-diversity timberline stands on metamorphic and igneous substrates (Ryerson, 1983
; Rourke, 1988
).
I thought of three hypotheses to explain the persistence of foxtail pine in the KM. These hypotheses differ in the spatial scale at which they are relevant in explaining the persistence of foxtail pine and are predicated upon the assumption that this species is a poor competitor for light relative to co-occurring conifers in the KM. All three hypotheses, therefore, make implicit conceptual use of either lack of competition or alteration of competitive hierarchies with respect to light resources (Chesson, 1985
, 2000
; Keddy, 1990
; Canham et al., 2006
).
Foxtail pine is classified as an indicator species for ultramafic substrates in the KM (Kruckeberg, 1984
; Safford et al., 2005
). The typical explanation for fidelity of forest trees to ultramafic substrates (Zobel and Hawk, 1980
; Kruckeberg, 1984
), or to varying substrates in general (Mason, 1946
; Bigelow and Canham, 2002
), is the decreased abundance of superior competitors. Many authors have concluded, therefore, that at large spatial scales the persistence of foxtail pine can be explained by a substrate hypothesis (Mastroguiseppe and Mastroguiseppe, 1980
; Ryerson, 1983
; Kruckeberg, 1984
; Sawyer and Thornburgh, 1988
). The central tenet of this hypothesis is that foxtail pine escapes competition from shade-tolerant species that are excluded or grow poorly on ultramafic substrates.
The majority of foxtail pine stands are located on ultramafic substrates. Some stands, however, are located on other types of substrates and contain many species. The persistence of foxtail pine at small spatial scales, therefore, could be a function of microsite availability (Eckert and Sawyer, 2002
). This hypothesis is referred to as the microsite hypothesis. Conifer diversity and foxtail pine occurrence would then be regulated by the availability of microsites that spatially separate competing individuals. The role of microsites or microhabitats in promoting coexistence among competing species is well documented in forest trees and has been offered as a mechanism that promotes high levels of alpha diversity (Parker, 1988
; Duncan, 1991
; Basnet, 1992
; Gray and Spies, 1997
; Beckage and Clark, 2003
).
It is also possible to explain the persistence of foxtail pine with a successional hypothesis. The long lifespan of foxtail pine could enable this species to persist within diverse stands at small spatial scales solely because of its ability to reach old ages. In other words, most mixed stands should contain predominantly old foxtail pine trees. Maximum ages of foxtail pine in the KM have been estimated at 1000 to 1500 yr (Mastroguiseppe, 1972
). Similar explanations based on life history traits have been offered for coexistence of poor competitors in many types of plant assemblages (Chesson, 2000
; Sarr et al., 2005
). A requisite condition for this hypothesis is that small- to large-scale disturbances (Loehle, 2000
) or temporal variation in environmental conditions (Chesson, 1985
) are frequent.
Subalpine forest stands in the KM are diverse because of the presence of rare conifer species, such as foxtail pine, and unusual combinations of conifers, such as Douglas fir [Pseudotsuga menziesii (Mirbel) Franco] and whitebark pine (Pinus albicaulis Engelm.). These qualities provide a unique system with which to analyze the persistence of a poor competitor within species-rich assemblages. In this paper, therefore, I address two questions: (1) Is foxtail pine a poor competitor for light? (2) Why does foxtail pine persist in the KM? Answers to the first question provide a conceptual basis for addressing the second question through testing of the aforementioned substrate, microsite, and successional hypotheses.
MATERIALS AND METHODS
Study area and characterization of stands
Study area
The extent of the KM in northern California covers approximately 30 300 km2 of geologically complex mountains (Fig. 1). Substrates are complex and derived from diverse terranes accreted to western North America during the Mesozoic (Sawyer and Thornburgh, 1988
). The oldest of these are located in the eastern portion of the KM and are dominated by ultramafic substrates primarily derived from gabbro, peridotite, and serpentinite parent materials. Summit elevations range from 1500 m to 2700 m with a maximum of 2750 m. The highest peaks in the region were glaciated during the Pleistocene, with the largest alpine glaciers being located in the Trinity Alps (Sharp, 1960
). Regional climate conforms to a modified Mediterranean type with abundant winter and marginal summer precipitation, which produces a marked west-to-east moisture gradient across the complex regional topography (Major, 1988
). Forest types range from lowland Douglas fir to enriched subalpine stands (Sawyer and Thornburgh, 1988
; Sawyer and Keeler-Wolfe, 1995
). Regional conifer richness is approximately 30 species with 13 of these occurring in the subalpine and nine of the 13 co-occurring with foxtail pine in one or more stands (Eckert and Sawyer, 2002
).
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were used as the strata. These subregions are higher order clusters of individual stands. Sampling in this fashion resulted in a set of 25 stands located throughout the region.
Stand characteristics
Within each stand, the point-centered quarter (PCQ) method was used to determine tree density and basal area from 25 randomly spaced points (N = 100 sampled trees) sampled along five 200-m transects (Bonham, 1989
). Transects followed elevation contours and were spaced approximately 100 m apart. Each sampled tree was identified to species and its diameter at breast height (DBH) was measured. Sampling was conducted separately for adult and young trees. Individuals >1.37 m tall were considered as adult trees, while young trees were defined as individuals 
1.37 m tall. The cover of boulders (diameter > 50 cm) was used to define microsite availability and estimated from 15 one-meter-squared plots proportionately distributed along the five PCQ transects. Substrate type was determined from exposed rock and verified with soil maps (Natural Resources Conservation Service, U.S. Department of Agriculture) when available. I defined ultramafic substrates as those which contained more than 50% ultramafic rock (e.g., serpentinite, gabbro, peridotite, dunite, and pyroxenite) estimated from five 1-m2 plots placed on exposed rock in each stand.
Stand and species-specific densities for adult and young trees were calculated from PCQ data using Pollard's (1971)
unbiased estimator. Data for adult trees were also used to determine basal area for each species from its average DBH and frequency (Bonham, 1989
). Relative values of density and basal area of adult trees were combined into an index of importance (Ii) following the formulation of Jackson and Petty (1971)
for each species. Importance values were subsequently used to calculate species diversity with the Shannon–Wiener index (H'). Species diversity was also calculated for young trees using density rather than importance values.
Two additional transects were placed in adjacent downslope stands 25 m and 75 m below the lowest observed foxtail pine. The PCQ method with 12 randomly spaced points was utilized to determine stand density and basal area in these stands as described for comparison to sampled foxtail pine stands. Differences in stand density, basal area, species richness, and H' among adjacent downslope and foxtail pine stands were determined with paired t tests.
Responses to shading
Two morphological measures indicative of response to shading were chosen to test the assumption that foxtail is a poor competitor: height from the ground to first branch (m) and length of the first branch that supported foliage (cm). These measures were chosen as simplistic indices of competitive ability for this resource because shade-intolerant species typically lose branches and needles when exposed to low levels of light (Barnes et al., 1998
). The expectations are that if shading has been severe there should be an increase to the height of the first branch and a decrease in the cover of foliage on this branch relative to a control tree. Total tree height and branch length were also measured to gauge the influence of these variables on the measurements of interest.
Subalpine forest stands in the KM typically do not form closed canopies and are instead collections of widely spaced groups of individuals (Sawyer and Thornburgh, 1988
). Therefore, 30 pairs (foxtail pine plus one competitor) of trees with overlapping crowns were randomly located within each stand. The height of the first branch and the length of the first branch that supported foliage were measured on each tree and compared to a control of the same species and similar DBH no more than 25 m away. Only pairs with similarly sized trees (
5 cm DBH; 10 m height) were chosen, and paired trees were less than 3 m apart so as to have overlapping crowns. Trees used as controls were isolated from other individuals by at least 10 m on each side.
Hotelling's T2 tests were used to identify statistically significant effects of shading using sampled trees and their controls (Zar, 1999
). Separate analyses were conducted for foxtail pine and all competitors grouped together. Effects of single variables were explored with paired t tests for foxtail pine and all competitors grouped together. Pairs of trees were defined as the sampled tree plus its control. Analyses were combined across all stands (N = 750 pairs).
Species-specific competitive effects were estimated as the difference between sampled foxtail pine trees and their controls. For both morphological measures, this difference was defined so that positive numbers refer to effects of the paired conifer on foxtail pine. For height to first branch and length of this branch supporting foliage, the mean value of this difference is expected to be large if shading is severe. Differences for each character among the six species observed in pairs with foxtail pine were subjected to single-factor analysis of variance (ANOVA) followed by a Bonferroni multiple comparison test (Zar, 1999
). Analyses were conducted for all pairs of trees combined across stands.
Substrate hypothesis
If foxtail pine persists in the KM because of its occurrence on ultramafic substrates, nonultramafic substrates should host stands with lower importance values for foxtail pine and higher values for species richness, H', density, and basal area. Conversely, stands on ultramafic substrates should have increased importance values for foxtail pine and have lower levels for species richness, H', density, and basal area. The substrate hypothesis was evaluated using two-sample t tests to detect differences in species richness, H', and foxtail pine importance between substrate types. All t tests were one-tailed according to the expectations listed.
A corollary of the substrate hypothesis is that competitors should decrease in their importance and competitive effects on ultramafic substrates. Each of these expectations was tested in turn for the six most commonly sampled competitors. First, differences between substrate types in the importance values of Jeffrey pine, mountain hemlock [Tsuga mertensiana (Bong) Carriè], Shasta red fir, western white pine, white fir [A. concolor (Gordon & Glend.) Lindley], and whitebark pine were evaluated with separate two-sample t tests. Distributions of importance values were skewed for each species because of the presence of zero values. Therefore, prior to performing t tests, the data were log10-transformed, accounting for the lowest nonzero importance value, using the formula
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). Second, two-sample t tests were used to examine the effects of substrate type on response by foxtail pine to shading from each of the six species of paired competitors.
Microsite hypothesis
Persistence of poor competitors such as foxtail pine would increase species richness and H' at small spatial scales. Therefore, if foxtail pine persists in the KM because of the availability of microsites, then species richness and H' should be positively correlated with microsite abundance. Microsites in previous studies of montane and subalpine forests of California were based on canopy gaps or open microhabitats in which shade-intolerant species avoided shading (Parker, 1988
; Barbour et al., 1990
; Gersonde et al., 2004
). Canopy gaps in those studies were a form of habitat patchiness that isolated competitors. Closed canopies in foxtail pine stands are virtually absent (Eckert and Sawyer, 2002
), so boulder cover (%) was chosen as an index of habitat patchiness under the assumptions that boulders partition space within stands and that high levels of boulder cover indicate high levels of microsite abundance. These assumptions imply that stand density should also be correlated with boulder cover. The microsite hypothesis was tested using simple linear regression (SLR) with boulder cover as the explanatory variable.
Successional hypothesis
If foxtail pine persists in the KM because of its ability to reach old ages, then mixed stands should contain predominantly old foxtail pine trees. This expectation was tested in two ways. First, SLR was used to examine the relationship between the densities of young and adult foxtail pine trees. This relationship should have a slope less than one under the successional hypothesis. I tested this prediction using ANOVA to gauge the fit of the linear model and a one-tailed t test to assess the null expectation of the slope being less than one (Zar, 1999
). Second, foxtail pine age distributions from diverse and nondiverse stands were compared using a Kolmogorov–Smirnov test for continuous data (Zar, 1999
). Tree ages were calculated using the log-transformed SLR relating DBH to age given by Eckert and Sawyer (2002)
. The ages used in this SLR are based on growth ring counts from cored trees (N = 44). Data were obtained from Mastroguiseppe (1972)
. Stands were classified as either diverse or nondiverse using the regional median value for H'.
Statistical software
All statistical analyses were conducted with the NCSS statistical software package (Hintze, 2001
) using a critical value of P = 0.05 unless otherwise noted.
RESULTS
Stand characteristics
Sampled foxtail pine stands differed in their species composition and structure (Table 1). Shasta red fir, western white pine, Jeffrey pine, mountain hemlock, white fir, and whitebark pine were the most common associates of foxtail pine. Three additional conifers were rarely sampled and differed in their occurrences across the region. Incense cedar [Calocedrus decurrens (Torrey) Florin] was predominantly sampled in the southern portions of the region, while lodgepole pine [Pinus contorta Loudon subsp. murrayana (Grev. & Balf.) Critchf.] and Douglas fir were predominantly sampled in the northern portions of the region. Measures of species richness and H' for adult trees varied from three to 10 (median = 4) and from 0.47 to 1.93 (median = 1.00), respectively. Values for species richness and H' for young trees were highly correlated with values for adult trees (species richness: r2 = 0.91, F1,23 = 15.05, P < 0.001; H': r2 = 0.93, F1,23 = 25.41, P < 0.001). Stand densities for adult trees ranged from 51 to 381 trees/ha, while stand basal areas varied between 4.52 and 45.68 m2/ha. Similar densities were found for young trees. Age class distributions (40-yr age classes) for foxtail pine in all 25 stands were reverse J-shaped with the first age class accounting for approximately 34% to 69% of the sampled trees.
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Responses to shading
Morphological responses to shading were statistically significant for foxtail pine (T2 = 26.11, F2,748 = 13.03, P < 0.001), but not its competitors (T2 = 2.76, F2,748 = 1.37, P = 0.255). Height to the first branch for foxtail pine was higher than control trees, while the length of this branch supporting foliage was less than control trees. These differences were significant when all sampled pairs of trees were considered and when sampled pairs of trees were grouped by species of competitor and analyzed separately for each morphological character (paired t tests: P < 0.001). The height to first branch was not correlated with total tree height (r2 = 0.04, F1,748 = 0.78, P = 0.377), nor was the length of this branch supporting foliage correlated with total branch length (r2 = 0.07, F1,748 = 1.01, P = 0.315).
Foxtail pine was sampled with western white pine in 38% (N = 285), Shasta red fir in 24% (N = 180), white fir in 16% (N = 120), mountain hemlock in 10% (N = 75), Jeffrey pine in 6% (N = 45), and whitebark pine in 6% (N = 45) of the pairs. The species with which foxtail pine was paired affected its response to shading for the difference between treatment and control trees in height to first branch (F5,745 = 86.10, P < 0.001) and length of this branch supporting foliage (F5,745 = 95.65, P < 0.001). Shasta red fir, mountain hemlock, and white fir produced the largest effects on foxtail pine for height to the first branch and the length of this branch supporting foliage (Table 2). The effects of shading by western white, Jeffrey, and whitebark pines on foxtail pine were relatively smaller. Bonferroni multiple comparison tests showed that patterns among species were the same for both characters, with shade-tolerant species producing the largest effects on foxtail pine.
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Responses to shading
Significant morphological responses to shading were documented for foxtail pine. Species of the predominantly shade-tolerant genera produced the largest negative effects. When sampled with firs or hemlocks, foxtail pine illustrated visible signs of stress that included thinning of branches, stunted growth, and high levels of dead or dying needles. These observations were verified by the statistically significant increase in height to first branch and decrease in length of this branch supporting foliage for foxtail pine when paired with other conifers (Table 2). Correlations of the height to first branch with total tree height and the length of the first branch supporting foliage with total branch length were not statistically significant. Therefore, allometric relationships did not confound these measurements.
Effects of shading were asymmetric. Foxtail pine illustrated significant responses to shading, but associated conifers did not. The responses by foxtail pine to shading from firs and hemlocks were large and ranged from increases of 1.56 to 3.09 m in height to the first branch and decreases of 48.38 to 66.30 cm in length of this branch supporting foliage relative to control trees (Table 2). Crown modifications of these magnitudes, without compensatory increases in growth rate or crown diameter, affect tree survival (Takahashi, 1996
). Moreover, foxtail pine in the southern Sierra Nevada survives longer and grows faster in high light environments where stands are less dense, species-poor, and contain widely dispersed trees (Bunn et al., 2005
).
Responses by foxtail pine to shading were interpreted as indications of its poor ability to compete for light relative to other co-occurring conifers. The observed asymmetry in shading response between foxtail pine and associated shade-tolerant conifers could be a function of plastic responses in crown architecture by firs and hemlocks. As light intensity decreases, shade-tolerant conifers produce flat-shaped crowns to efficiently capture light (Takahashi, 1996
), reduce height and diameter growth (Kobe and Coates, 1997
), and retain foliage for longer periods of time (King, 1994
). Pinus species are not as plastic in their response to shading (Kobe and Coates, 1997
). Therefore, the large morphological responses of foxtail pine to shading by firs and hemlocks are likely a function of the ability of these shade-tolerant species to experience light-dependent crown modifications that increase their effective degree of shading to foxtail pine. Reduced responses of foxtail pine to shading by co-occurring Pinus species are consistent with this argument. Furthermore, crown modifications have been shown to be important when spacing of trees becomes aggregated (Sorrensen-Cothern et al., 1993
; Canham et al., 2004
), which is likely influenced by microsite availability through the cover of boulders.
Substrate hypothesis
Stand density, stand basal area, species richness, H', and foxtail pine importance were not affected by substrate type (Table 3). When species were considered individually, however, differences between substrate types were apparent. Foxtail pine stands on ultramafic substrates had decreased importance values of shade-tolerant conifer species. Moreover, downslope stands had increased densities, basal areas, and importance values of shade-tolerant firs and hemlocks. Mountain hemlock, Shasta red fir, and white fir are the regionally dominant trees of the subalpine in the KM (Sawyer and Thornburgh, 1988
). These observations are consistent with the persistence of foxtail pine at large spatial scales being promoted by a reduction in or lack of competition on ultramafic substrates.
The response of foxtail pine to shading by firs and hemlocks on ultramafic substrates was less severe than when observed on nonultramafic substrates. Average changes in the difference in height to first branch and the difference in length of this branch supporting foliage on ultramafic relative to nonultramafic substrates were approximately 0.5 m and 15 cm, respectively (Table 4). These values represent net changes between substrate types of approximately 20–25%, consistent with alteration of competitive hierarchies on ultramafic substrates. As discussed, the magnitudes of shading responses are physiologically and ecologically important; it cannot be ruled out, however, that these substrate-specific changes represent statistical artifacts or decreased growth rates of foxtail pine on ultramafic substrates. The latter explanation is possible given the correlation of foxtail pine growth with high light and moisture conditions in the Sierra Nevada (Bunn et al., 2005
).
The substrate-specific reduction in effects of shading on foxtail pine by firs and hemlocks strengthens the conclusion that ultramafic substrates act as sites in which foxtail pine escapes competition. Nutrient availability can modify the response to shading in conifers due to amelioration of physiological responses to intense shading by abundant soil nutrients (Mitchell and Arnott, 1995
). Amelioration of shade-induced physiological effects by abundant soil nutrients implies that limitations in soil nutrients can also affect the growth and survival, and hence competitive ability, of conifers co-occurring with foxtail pine on ultramafic substrates. These substrates are typically limited in the availability of common nutrients, such as calcium, potassium, and nitrogen, and have increased levels of toxic heavy metals, such as nickel and chromium (Kruckeberg, 1984
). Attributes such as these have been shown to decrease plant cover, density, and seed production in herbaceous alpine plant assemblages (Nagy and Proctor, 1997
) and to affect leaf morphology and nutrient resorption from senescent leaves in chaparral plant assemblages (Pugnaire and Chapin, 1993
). Furthermore, substrate type has been shown to affect the competitive abilities of annual grasses, which was offered as a mechanism of species coexistence in some native California grasslands (Reynolds et al., 1997
). Data in the present paper support the hypothesis that a similar situation may be occurring within foxtail pine stands located on ultramafic substrates, but further work is need to quantitatively examine soil characteristics within sampled stands.
Microsite hypothesis
The cover of boulders was positively correlated with species richness, H', and stand density, which suggests that boulder cover facilitates the persistence of foxtail pine at small spatial scales. The correlation of stand density with the cover of boulders (Fig. 2C) supports the inference that boulders partition space within stands and this partitioning of space results in separation of competitors. This pattern is different from equilibrium-based canopy gap models previously documented within montane forests as a function of wind disturbance and tree mortality (Spies et al., 1990
; Ulanova, 2000
; Kramer et al., 2001
) or nonequilibrium disturbance models based on frequent fires (Agee, 1998
; Heyerdahl et al., 2001
). The importance of canopy gap and disturbance dynamics to the persistence of poor competitors is lessened in foxtail pine stands due to the general lack of closed canopies and infrequent fires (Sawyer and Thornburgh, 1988
). Patterns documented in this study support the role of microsites as similar to that of canopy gaps or other small-scale, disturbance-induced environmental heterogeneity, especially because sampled stands appear to be compositionally and structurally stable. These patterns emphasize the separation of and interactions among individuals, however, and not the mosaic nature of discrete patches or gaps.
Theoretical modelers have long recognized that the coexistence of poor competitors within species-rich assemblages is facilitated by spatial segregation of competing species, but the importance of habitat heterogeneity to patterns of species coexistence within plant assemblages has been questioned when each species must inhabit a unique type of microhabitat (Crawley, 1997
). Stochastic effects of dispersal limitation and neighborhood interactions can, however, promote coexistence of species with differing competitive abilities in spatially homogeneous (Pacala et al., 1996
) and spatially heterogeneous environments (McEuen and Curran, 2004
). Under the microsite hypothesis, the persistence of foxtail pine at small spatial scales is then a function of the distribution of microsites, which are defined by the cover of boulders, the likelihood of multiple colonization events within a single microsite, and patterns of tree mortality that result in empty microsites.
Patterns within this study support the idea that competition is spatially defined (i.e., in pairs of trees with overlapping crowns) and that microsite availability facilitates the persistence of foxtail pine at small spatial scales. Although these patterns are solely observational, they are indicative of stochastic dispersal and recruitment limitations operating at small spatial scales within sampled stands. Observations of multiple colonization events within individual microsites further support the putative role of dispersal and recruitment limitations to the persistence of foxtail pine in diverse stands. The estimated age of foxtail pine in each of these multiply colonized microsites was sufficient to conclude that foxtail pine colonized the microsite first. In all cases, however, younger firs and hemlocks were producing negative effects upon the older foxtail pine. Factors such as these have been shown to be important to species coexistence and patterns of diversity in empirical (Ribbens et al., 1994
; Clark et al., 2004
; McEuen and Curran, 2004
) and in theoretical studies (Hurtt and Pacala, 1995
). Further work to quantify patterns of recruitment and dispersal limitations in foxtail pine stands could provide such demographic explanations to patterns observed in this study.
Successional hypothesis
The prediction that diverse stands should contain predominantly old foxtail pine trees was not supported by the data. The density of young trees was directly proportional to the density of adult trees for foxtail pine (Fig. 3) and for mountain hemlock, Shasta red fir, western white pine, and white fir (data not shown). Species richness and H' for young trees (1.37 m tall) were highly correlated with the same measures for adult trees (>1.37 m tall). These observations agree with data from Whittaker (1960)
that show subalpine conifer assemblages in the KM to be stable across long time scales. The role of foxtail pine's longevity in promoting its persistence in the face of stochastic dispersal and recruitment limitations or climate variability, i.e., the storage effect of Chesson (1985
, 2000
) as discussed and applied to forest trees by Clark et al. (2004)
, however, cannot be verified with data presented in this paper.
Cumulative relative age distributions of foxtail pine did not differ between diverse and nondiverse stands (Fig. 4) suggesting that diverse stands containing foxtail pine are not successional artifacts. In other words, foxtail pine is actively reproducing in all sampled stands, and frequent disturbances do not appear to be important in promoting the persistence of this species in the KM at small spatial or temporal scales. If patterns of disturbance, such as high intensity fires, were important across small time scales, age class distributions of foxtail pine should have been more variable among stands as a response to localized disturbance regimes. Contrary to this expectation, all sampled stands had reverse J-shaped age class distributions of foxtail pine and showed no signs of recent disturbance (i.e., no observable fire scars on trees). Fire has been shown to be important, however, in driving long-term patterns of vegetation change (i.e., over millennia) within the subalpine of the KM (Whittaker, 1960
; Sawyer and Thornburgh, 1988
; Mohr et al., 2000
).
Implications
Understanding the persistence of foxtail pine aids in our understanding of what makes subalpine forests in the KM diverse and establishes a framework for further studies of other relict conifers in the region, such as Brewer spruce (Picea breweriana S. Watson). It also provides a comparison to work in the southern Sierra Nevada where foxtail pine stands are fundamentally different in structure, composition, and diversity (Ryerson, 1983
; Rourke, 1988
; Eckert and Sawyer, 2002
) as well as in environmental conditions (Bunn et al., 2005
).
Observed patterns also serve as a foundation for further hypothesis-based tests of diversity patterns within foxtail pine stands of the KM. Specifically, the correlations of species richness, H', and adult tree density with boulder cover indicate that stochastic effects of dispersal and recruitment limitations may affect the persistence of foxtail pine at small spatial scales. A substantial empirical and theoretical literature suggests that such factors are important over multiple spatial scales within broadleaf deciduous forests of eastern North America (Ribbens et al., 1994
; Clark et al., 2004
; McEuen and Curran, 2004
). Opposed to those studies, factors such as disturbance, environmental limitations, or climate variability are typically offered as important drivers of small- to large-scale diversity patterns within temperate forests of western North America (Agee, 1998
; Ulanova, 2000
; Urban et al., 2000
; Bunn et al., 2005
). Data from this study, however, suggest that additional factors must be included, at least as a null model, for understanding patterns of species coexistence at small spatial scales within some forest types of the KM.
FOOTNOTES
1 The author thanks M. L. Eckert, S. Hwang, and J. West for assisting with fieldwork and S. Liang for assistance with Fig. 1. Funding was provided by the Giles Graduate Student Field Research Award available from the Department of Biology, University of Washington and by research funds from B. D. Hall. Valuable suggestions for improving the manuscript were provided by M. L. Eckert, R. del Moral, J. O. Sawyer, and two anonymous reviewers. 
2 Author for correspondence (aje2{at}u.washington.edu
)
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