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0 The Nature Conservancy, Department of Botany, University of Florida, Gainesville, Florida 32611 USA; and 3 Department of Agronomy and Range Science, University of California, Davis, California 95616 USA
Received for publication August 10, 1998. Accepted for publication September 14, 1999.
ABSTRACT
Reduced recruitment of blue oak (Quercus douglasii) seedlings in California grasslands and woodlands may result from shifts in seasonal soil water availability coincident with replacement of the native perennial herbaceous community by Mediterranean annuals. We used a combination of container and field experiments to examine the interrelationships between soil water potential, herbaceous neighborhood composition, and blue oak seedling shoot emergence and growth. Neighborhoods of exotic annuals depleted soil moisture more rapidly than neighborhoods of a perennial grass or "no-neighbor" controls. Although effects of neighborhood composition on oak seedling root elongation were not statistically significant, seedling shoot emergence was significantly inhibited in the annual neighborhoods where soil water was rapidly depleted. Seedling water status directly reflected soil water potential, which also determined the extent and duration of oak seedling growth during the first year. End-of-season seedling height significantly influenced survival and growth in subsequent years. While growth and survival of blue oak seedlings may be initially constrained by competition with herbaceous species, subsequent competition with adult blue oak trees may further contribute to reduced sapling recruitment.
Key Words: blue oak woodlands • competition • Fagaceae • oak regeneration • Quercus douglasii • recruitment • seedling emergence • seedling bank • seedling growth • soil water potential
Competition with herbaceous species has been demonstrated to inhibit or prevent the growth of several woody species (da Silva and Bartolome, 1984
; Davis and Mooney, 1985
; Williams, Hobbs, and Hamburg, 1987
; Cohn, van Auken, and Bush, 1989
). In arid savanna and woodland systems, water is often implicated as the resource that is rapidly or most effectively depleted by the herbaceous species (Walter, 1979
; Knoop and Walker, 1985
; Warren, Skroch, and Hinseley, 1987
; Cunningham et al., 1989
). The root systems of woody seedlings can be initially quite shallow. As a result, herbaceous species that rapidly deplete water in the upper soil profile before roots of woody species reach lower depths can exclude woody plant establishment, especially in years without precipitation toward the end of the growing season when competition from senescing herbaceous vegetation may be less (Schultz, Launchbaugh, and Biswell, 1955
; Williams and Hobbs, 1989
).
Regeneration of several oak species in California currently appears threatened because of poor seedling survivorship into sapling size classes (Muick and Bartolome, 1987
; Bolsinger, 1988
). In particular, age structure within stands of blue oak (Quercus douglasii Hook & Arn.) is strongly bimodal, with biases toward seedling-sized individuals (
15 cm height) and trees >70–100 yr old (McClaran, 1987
). The current poor recruitment has occurred over the last 50–90 yr, before which recruitment within mature Q. douglasii stands occurred regularly, although episodically (McClaran and Bartolome, 1989
). Thus, intraspecific competition alone appears unlikely to explain the current lack of sapling recruitment. However, low rates of oak recruitment are coincident both with a decrease in fire frequency (McClaran and Bartolome, 1989
) and a shift in the oak woodland understory from a perennial to a non-native annual grassland (Biswell, 1956
; Burcham, 1957
; Bartolome, Muick, and McClaran, 1987
). Positive effects of fire on recruitment appear most likely to result from stimulation of resprouting in seedling-size individuals and increased growth rates in saplings established before the fire (McClaran and Bartolome, 1989
). Other factors controlling seedling survival and growth are likely to have been more important in creating the current deficit of saplings.
The annual grassland species are hypothesized to more rapidly deplete soil water in the upper soil layers during the spring than did the native perennial species (White, 1967
; Gordon et al., 1989
; Welker, Gordon, and Rice, 1991
). This rapid depletion may result both from the earlier phenologies of the annual species (Hull and Muller, 1975
; Jackson and Roy, 1986
) and from the greater density and cover in annual than in perennial grasslands (Saenz and Sawyer, 1986
). Earlier work (Gordon et al., 1989
; Gordon, Rice, and Welker, 1991
; Gordon and Rice, 1993
; Rice et al., 1993
) supports these hypotheses and indicates that the rate of soil water depletion depends on the species identity and density of the herbaceous "competitive neighborhood" (sensu Goldberg and Werner, 1983
). Following shoot emergence, the growth rates, water relations, and growing season of blue oak seedlings may also depend on soil water potential (Gordon et al., 1989
; Gordon, Rice, and Welker, 1991
).
The studies reported here further explore this phenomenon of seedling growth suppression in response to competition for soil moisture. We focus on the competitive effects of herbaceous neighbors on the water resource, and response of oaks to that resource (sensu Goldberg, 1990
). If the control of soil water availability on root and shoot development shown in other species (Vartanian, 1981
; Eissenstat and Caldwell, 1988
; Stuart-Hill and Tainton, 1989
) occurs in Q. douglasii, we hypothesized that seedling water status and growth should be dependent on soil water potentials and thus be suppressed under rapidly drying conditions.
A combination of container and field experiments was used to examine the relationship between competitive effects on soil water and oak shoot emergence and growth. A neighborhood experiment that examined both above- and belowground growth responses of acorns planted in perennial and annual herbaceous neighborhoods (Experiment I) was conducted in large containers. We then conducted a similar experiment in the field (Experiment II), where acorns were planted into neighborhoods of annual herbs or no competitors. Finally, we directly examined the relationship between seedling emergence and soil water by manipulating water availability without competitors present (Experiment III).
MATERIALS AND METHODS
Experiment I: perennial vs. annual neighborhood box experiment
Three herbaceous species were used to create competitive neighborhoods around germinated Q. douglasii acorns: a native perennial grass, Nassella pulchra (Hitchc.), and two non-native annuals, the grass Bromus diandrus (Roth.) and the forb Erodium botrys (Cav.). Herbaceous species will hereafter be referred to by their genera. Each of two containers was planted with low (10 seeds/dm2) or high (100 seeds/dm2) density of Bromus or Erodium, Nassella (ten clones of two tillers each), or no herbaceous neighbors. A higher density treatment is unrealistic for Nassella, which reaches a maximum of ~10% cover in the field (White, 1967
). The containers were boxes 1 m tall, 48 x 24 cm at the top and 48 x 12 cm at the base. An obliquely slanted glass pane shielded from light penetration along one wider side of the boxes allowed root growth observation (see Gordon et al., 1989
, for complete description). This incomplete factorial design resulted in a total of 12 experimental units (boxes) that were randomly arranged outdoors at the Orchard Park facility of the University of California, Davis (Yolo County), California, USA.
Boxes were filled with oak woodland soil (Mollic Haploxeralf) collected from the University of California Sierra Foothill Research and Extension Center (SFREC), Yuba County. Natural soil layers of 0–10, 10–30, and 30+ cm were maintained when transferred to the boxes. Soil was saturated as the boxes were filled with soil to increase compaction. To increase consistency with field conditions, the soil surface was then covered with 3 cm deep sods of standing dead litter brought from SFREC that were autoclaved to remove the resident seed pool. The standing litter was cut to ground level in the high-density and control treatments. Litter was left intact in the low-density neighborhood treatments so that lateral shading would inhibit grass tillering and prevent plants in those treatments from growing to the same biomass as plants in the high-density neighborhoods (see Gordon et al., 1989
).
Calibrated screen-cage thermocouple psychrometers (J. D. Merrill Specialty Equipment, Logan, Utah, USA) were placed in all boxes at 40 cm depth to monitor soil water potentials. Welded wire frames covered with 56% shade cloth were erected over each box to simulate shading from a continuous sward and reduce edge effects. Boxes were watered to field capacity until 1 March 1988 to approximate mean annual precipitation. Water potential (MPa) was monitored weekly starting in March from the soil psychrometers using a data-logger (Campbell CR-7, Logan, Utah, USA) (Brown and Bartos, 1982
). Measurements were collected within 1 h after dawn to reduce the effect of temperature gradients along the psychrometer leads.
Nassella propagules with at least two tillers each were separated from several larger plants brought back from the SFREC and planted in the boxes in early November 1987. Each propagule was cleaned of dead material, shoot growth was trimmed to 7 cm, and root growth to 2 cm. Pairs of propagules were planted at each of ten evenly spaced locations within each of two randomly determined boxes and were randomly thinned to one set of tillers once they had established.
Bromus seeds were collected at the SFREC, and Erodium seeds were collected at Jepson Prairie in Yolo County in Spring 1987. Erodium seeds were manually scarified, and seeds of both annual species were germinated on filter paper prior to sowing in the boxes in early December 1987. The low-density Erodium neighborhoods were replanted in late December because of poor establishment in the sods. Seeds were planted in a regular pattern within the sods in the low-density treatments, forming an equilateral triangle around each acorn location.
Acorns were collected from three Q. douglasii trees at the SFREC in September 1987 and divided into four 0.5-g fresh mass classes per tree (mass range across trees was 2.5–5.5 g). Eighteen randomly selected acorns with emergent radicles were planted into each of the boxes in January 1988 to correspond with the timing of acorn germination in the field. Across all boxes, equal numbers of acorns from each tree and each mass class group per tree were planted. Maternal tree identity and fresh mass class were used as statistical covariates.
We took weekly measurements of root growth along the glass sides of the boxes, shoot emergence, stem height, leaf number, leaf area (estimated from a linear regression of area on leaf length; Gordon and Rice, 1993
), and mid-day leaf conductance and transpiration rates (LI-COR null-balance porometer, Lincoln, Nebraska, USA). Pressure chamber measurements of oak seedling predawn xylem water potential were taken monthly on two destructively sampled seedlings per box as long as green seedlings were available. The experiment was terminated when all seedlings and herbaceous neighborhoods had senesced by August 1988. Intact seedlings were then harvested from the boxes for root and stem mass measurement. Root mass of the oak seedlings and herbaceous species was also determined in 0–25, 25–50, 50–75, and 75–100 cm soil depth classes.
Logistic regression (BMDP; Dixon, 1985
) was used to examine the relative effects of neighborhood species and density on oak emergence. Analysis of variance (SAS; Freund, Littell, and Spector, 1985
) was used to compare oak growth responses to neighborhood treatments or soil water potential. Oak seedling growth was analyzed using ANCOVA, with acorn mass and maternal tree as covariates. Analyses were conducted on all oak seedlings, as all were alive by the end of the experiment and produced some roots regardless of stem production or timing of senescence. Relative growth rates (RGR) were calculated as the difference in natural logs of stem height, leaf area, or root length over the growth period measured in days. Oak response variables measured over time were tested in ANOVA and ANCOVA models through an orthogonal polynomial decomposition of the time effect (Myers, 1972
; see Gordon and Rice, 1993
). Main and interactive effects and Tukey tests were thus tested against specified nested Type I error terms. We also examined correlations between soil and green seedling water status. Continuous variables were log transformed, whereas count variables were square-root transformed to increase consistency with linear model assumptions.
Experiment II: annual neighborhood field experiment
This field examination of neighborhood effects was conducted at the SFREC in an area that was fenced from cattle grazing just before the experiment began. Mature Q. douglasii trees were removed from the area so that the experiment could be established on canopy-influenced soil without excessive shading effects. Neighborhood treatments using the non-native annuals Bromus and Erodium only were analogous to the annual neighborhood treatments established in the boxes (none, 10 seeds/dm2, and 100 seeds/dm2). Each of the five species and density treatment combinations was replicated once within each of ten blocks in a randomized complete block design. Plots within blocks were 20 cm apart; blocks were 3–20 m apart.
Seeds of both annual species were collected as described for Experiment I, and Erodium seeds were scarified as described above. We removed the litter and top 2 cm of soil in 20 cm diameter circular plots to eliminate most of the resident seedbank. Annual seeds were sown in October 1987 in these plots.
Acorns, collected in September 1987 from four Q. douglasii trees at the SFREC, were planted in January 1988 when radicles were 1–10 mm in length. Acorn mass within 0.5-g intervals and maternal parent tree identity were again recorded for use as covariates in the statistical analysis. Acorns were randomly distributed across all blocks. Germinated acorns were planted into the centers of each neighborhood. Oak seedling shoot emergence, stem height, leaf number, and estimated leaf area were monitored weekly. Calibrated thermocouple psychrometers were buried at 35–40 cm depth in each of two replicates of each treatment, randomly selected across the blocks.
Aluminum screen cages were placed over each individual plot (0.6 m height x 0.25 m basal diameter) to prevent acorn predation and seedling herbivory. The screening reduced ambient light by ~50%, approximating the shading of an average oak canopy (Welker and Menke, 1987
). Plots were weeded as necessary to maintain density and species composition treatments. Because of unusually dry conditions in early 1988, we watered the plots weekly with ~7.5 L of water (drip irrigation) during January and February. Beginning in March, soil water potential was monitored every 2 wk (within 2 h after dawn) using a data-logger. Water potential, emergence, and growth data were analyzed as described in the methods for Experiment I.
In July of 1988, 1989, and 1990 we measured the end-of-season heights of the oak seedlings; in the latter two years we also measured the heights of overwintering shoots in January. These data allowed evaluation of growth in subsequent years from either existing or new shoots. The influence of emergence in the first year and neighborhood treatment on survival and growth was evaluated using logistic regression. We used chi-square analysis to examine the relationship between height after one growing season and survival to the third year. Expected probabilities were generated under the assumption that survival was independent of height.
Experiment III: soil moisture availability experiment
This second container experiment, by removing the influence of other species, was designed to more directly examine the control of Q. douglasii shoot emergence by soil water availability. We collected acorns from ten trees at the SFREC in October 1997. The acorns were stored at 3°C until they germinated in December. We then planted one randomly selected acorn from each of the ten trees into each of 12 pots that were 15 cm diameter x 20 cm deep (N = 120 acorns total). The pots were filled with compacted "U.C. Mix" (50% peat / 50% sand) and watered to field capacity. Three water treatments were then imposed: pots were watered from below for 1 wk, 1 mo, or 3 mo. Each watering treatment was replicated four times. Pots were randomly positioned in an unheated, ventilated greenhouse on the University of California, Davis campus.
We recorded shoot emergence in the pots weekly. The number of shoots per pot that emerged were analyzed using ANOVA on square-root transformed data.
RESULTS
Perennial vs. annual neighborhood box experiment (Experiment I)
Effects of neighborhoods on soil water availability
The competitive neighborhoods induced significant differences in temporal changes in soil water potential (Fig. 1). Repeated-measures ANOVA revealed that both low- and high-density Bromus and low-density Erodium neighborhoods induced drier soil conditions than did Nassella and high-density Erodium neighborhoods (Tukey test, P < 0.05). However, over time the soil in high-density Erodium neighborhoods was drier than that in the low-density Erodium neighborhood (Fig. 1), and soil water in all the herbaceous neighborhoods was significantly lower than that in the no-neighbor control treatment (df = 5, F = 5.25, P = 0.03). Rates of water depletion, evaluated through contrasts on the slopes of water potential over time, were most rapid in neighborhoods of Bromus, moderate in neighborhoods of Erodium, and slowest in neighborhoods of Nassella and the control (P < 0.05 for all). By early May, however, the rate of soil water depletion in the Nassella neighborhood exceeded that in all others, reaching water potentials comparable to those in the annual grass treatments within a month (Fig. 1). Differences in soil water depletion rates in the competitive neighborhoods may be explained by the significantly greater biomass (df = 4, F = 21.02, P = 0.002) and root length (df = 4, F = 30.71, P < 0.001) developed by the annual grass compared to either the perennial grass or the annual forb (data not shown). Soil water potential in the control treatments did not significantly change until mid-June, when oak seedling transpiration and surface evaporation rapidly depleted soil water (Fig. 1).
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Effects of soil water availability on oak seedling water status
Oak seedling water status responses exhibited similar trends to the growth responses. Predawn seedling water potential (df = 1, F = 172.90, P < 0.0001; Fig. 2), conductance (df = 1, F = 41.26, P < 0.0001) and transpiration (df = 1, F = 34.67, P = 0.0002) were directly related to variation in soil water potential over time. As this relationship suggests, seedling stomatal conductance was correlated (r = 0.94, P < 0.0001) with predawn xylem water potential. Because of the herbaceous neighborhood effects on soil water potential (see above), water status in the Q. douglasii seedlings decreased from seedlings growing alone > Nassella neighborhood > Erodium neighborhoods > Bromus neighborhoods. Timing of seedling senescence showed the opposite pattern such that seedlings with more favorable water status senesced later than seedlings under more water stress (Fig. 2).
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Seedling survival and growth in the first growing season influenced survival in subsequent years. Final heights of seedlings in 1989 (r = 0.72, P < 0.0001), but not 1990, were positively correlated with heights in 1988. Neighborhood treatment influenced the re-emergence in 1990 of those seedlings that had emerged in 1989. Larger numbers of seedlings growing in high- and low-density Erodium neighborhoods re-emerged in 1990 than did those in any of the other neighborhoods (Tukey, P < 0.05). The probability of seedling survival into the 1990 growing season was also related to seedling height after the first growing season. Seedlings that were taller than 6 cm by the summer of 1988 were significantly more likely than expected by chance alone to survive to 1990 (
2 = 12.40, P < 0.01; Fig. 3). Final height in 1988 and subsequent years was independent of initial acorn mass.
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DISCUSSION
All three experiments indicate the importance of the soil water depletion rate for Q. douglasii seedling establishment and growth. These relationships exist whether competitors (Experiments I and II) or direct manipulation of water (Experiment III) regulate soil water potentials. Seedling shoot emergence was most inhibited when high densities of annual species induced rapid rates of soil water depletion, while the probability of shoot emergence increased with the duration of watering in the container study where water was independently manipulated.
These results are consistent with earlier observations of higher Q. douglasii recruitment in wet years (Griffin, 1971
; Borchert et al., 1989
) and experimental container studies by Gordon et al. (1989) that reported an emergence rate of only 20% for Q. douglasii shoots growing in a high-density (100 seeds/dm2) annual grass neighborhood, despite vigorous early root growth. In contrast, low-density neighborhoods (10 seeds/dm2) of either the annual grass (Bromus diandrus) or annual forb (Erodium botrys) allowed 56% of the oak shoots to emerge, while 89% emerged when oaks were grown without neighbors (Gordon et al., 1989
). Additional experiments using field irrigation increased oak shoot emergence in annual grassland plots by an average of 40% over nonirrigated plots (Gordon, Rice, and Welker, 1991
). Taken together, these results suggest that during initial establishment, oak seedlings require some minimum level of water availability for stimulation of shoot elongation or that herbs directly inhibited oak emergence, or both.
Following emergence, water potentials continue to determine both the extent and the duration of growth in oak seedlings. These results are consistent with those of other studies on this species (Momen et al., 1994
; Gordon et al., 1989
; Welker and Menke, 1990
; Welker, Gordon, and Rice, 1991
; Gordon and Rice, 1993
) and on Quercus lobata, (Danielsen, 1990
) in California. Griffin (1971) found higher blue oak survival when competition from grasses was experimentally reduced. Further, rapid rates of soil moisture depletion resulted in smaller oak seedlings with less perennating tissue available for growth in subsequent seasons (Gordon, Rice, and Welker, 1991
). As in Experiment II, that study also found that seedling height at the end of the first growing season influences survival and sometimes height in subsequent seasons. Thus, variation in early growth of Q. douglasii seedlings may affect probability of recruitment into sapling age and size classes. A more gradual decline of soil water both allows oak seedlings to regulate their osmotic components to maintain positive turgor potentials at increasingly negative soil water potentials (Momen et al., 1994
) and to resprout following herbivory (Welker and Menke, 1990
). Higher growth rates that persist longer into the summer drought would produce cumulatively larger seedlings with deeper roots, better able to exploit soil resources not available to the herbaceous understory.
The same mechanisms that have been suggested in this system to explain the decrease of Q. douglasii have been linked to the increasing dominance of woody species in other arid or semi-arid systems (summarized in Archer, 1995
). Changes in climate (Archer, Schimel, and Holland, 1995
), fire frequency (Young and Evans, 1981
; Baisan and Swetnam, 1990
), grazing regime (Brown and Archer, 1987
; Bush and van Auken, 1991
) or the phenology or morphology of competing species (Eissenstat and Caldwell, 1988
; Archer, 1994
) have been suggested to explain this increased recruitment. Many of these systems have been subject to similar suites of factors like grazing, fire suppression, and vegetation changes that have been prevalent in California over the last century (Archer, 1995
; van Auken and Bush, 1997
). Why these factors should result in woody dominance in some areas and suppression in others is unclear; however, it may be that different combinations of the factors result in greater competitive effects of herbs in systems like the one studied here (non-native annuals in California rangelands), and lesser effects in others (e.g., native perennial grasses in Texan rangelands; van Auken and Bush, 1997
).
Several examples exist of other woody species whose establishment is inhibited by competition with herbaceous species in grasslands and savannas in California (Schultz, Launchbaugh, and Biswell, 1955
; Davis and Mooney, 1985
) and elsewhere (including woody species that are now increasing because of a reduction in herbaceous competition) (van Auken and Bush, 1987, 1997
; Harrington, 1991
; Archer, 1994
). The shrub Baccharis pilularis is sensitive to herbaceous competition for soil water and can establish in California annual grassland only in years with late spring rain (Williams and Hobbs, 1989
). Although establishment was not affected, annual grass density and soil water potential influenced Baccharis growth rates in a container experiment (da Silva and Bartolome, 1984
). Similarly, infiltration of water below the grass rooting zone of 0–30 cm was limited by grass uptake rates, decreasing the probability of Acacia establishment (Knoop and Walker, 1985
).
A lack of regeneration because of low seedling survival to sapling stages has been documented for many oak species worldwide (Evans, 1982
; Pallardy, Nigh, and Garrett, 1988
; Savill, 1991
; Masaki et al., 1992
). Although the causes of this lack of regeneration can be complex, reduced seedling survival and growth because of competition from herbaceous and woody understory species have been either documented or suggested for several forest and woodland systems (Abrams, 1992
; McPherson, 1993
; Lorimer, Chapman, and Lambert, 1994
). Within more mesic forest types, where light is thought to be the primary limiting factor, the primary competitors with the oak seedlings are shade-tolerant woody species such as understory shrubs or subcanopy tree species (Lorimer, 1984
; Crow, 1988
; Abrams and Downs, 1990
; Lorimer, Chapman, and Lambert, 1994
).
In California oak woodlands, research to date has suggested that competition from the herbaceous understory is more important than competition from other woody species for oak seedlings. Callaway (1992)
demonstrated that establishment and growth of Q. douglasii seedlings were actually facilitated by association with shrub species such as Salvia leucophylla and Artemisia californica. Callaway (1992)
argued that the shrubs served as "nurse plants" for the oak seedlings by providing direct benefits from shading (e.g., reduced temperature stress) and indirect benefits such as reduced competition from herbaceous species under the shrub canopy.
Although these results suggest that competition from woody species may be a relatively unimportant factor constraining regeneration in Q. douglasii, a largely unexplored possibility is that belowground competition from adult blue oak trees may sometimes suppress recruitment of oak seedlings to the sapling stage. Seedlings can survive for extended periods (>20 yr) in a suppressed state and form a type of seedling bank (Griffin, 1971
; Phillips et al., 1997
). When fire was more prevalent in these systems, such seedlings may have been released following fire, forming the basis for the episodic recruitment documented through tree ring analysis (McClaran and Bartolome, 1989
). Recent field surveys have indicated that sapling recruitment from this seedling bank is highest in canopy gaps and lowest in sites with the extreme conditions of either no tree canopy or closed canopy (Swiecki, Bernhardt, and Drake, 1997
). Adult Q. douglasii with relatively shallow root systems have been demonstrated to competitively suppress understory vegetation (Callaway, Nadkarni, and Mahall, 1991). Holmes and Rice (1996)
detected continued depletion of soil moisture through the summer to very negative water potentials (-4.0 to -6.0 MPa) under senesced stands of annuals. Their conclusion that soil moisture depletion under these stands was the result of transpiration of nearby adult oaks is further evidence suggesting potential competition between seedlings and adults. Conclusive evidence as to whether adult oaks significantly suppress sapling recruitment will require field manipulations such as root trenching experiments. Although logistically difficult, these experiments would be invaluable in providing a potential mechanism for the observed pattern of sapling recruitment in canopy gaps.
FOOTNOTES
1 The authors thank J. Welker, B. Momen, C. Walsh, J. Menke, T. Foin, L. Flowers, J. Hardison, T. Holmes, N. Willits, P. Hunter, and the staff of the Sierra Foothill Range and Extension Center for assistance during the research. Funding for this research was provided by Jastro Graduate Research Awards and Intercampus Travel Grants from the University of California, Davis and Sigma Xi (D. R. Gordon) and the Integrated Hardwood Management Program (CA-D*-ARS-4053) (K. J. Rice, J. M. Welker, and J. W. Menke). 
2 Author for correspondence (dgordon{at}botany.ufl.edu)
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