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Plant architecture and allocation in different neighborhoods: implications for competitive success.

INTRODUCTION

A plant's competitive ability is determined by its ability to acquire resources from resource pools shared with neighbors. Resource acquisition ability depends on the size and placement of plant parts, the physiological activity levels of these parts, and how these change through time. As a consequence, resource acquisition, allocation, and deployment abilities will be important determinants of competitive success.

Plant part deployment in space is controlled by the morphological traits of a particular species, and by the amount of flexibility in the expression of these traits. The study of plants as modular organisms, championed by Harper (e.g., 1984, 1985), has resulted in a resurgence of interest in incorporating the analysis of plant morphology into investigations of ecological phenomena (White 1979, 1984, Porter 1983, Bell 1984), and provides a framework for examining the dynamics of plant form. The morphology of a species is determined by rules of meristem activity and module iteration. The architecture of an individual is determined by the responses of meristem activity and module placement to environmental conditions. Thus, plant architecture helps to determine resource-gathering ability and therefore, in situations where resource availability is altered by neighboring plants, competitive success (Grime et al. 1986, Bazzaz 1991).

There is a growing literature on the relationships between plant form and competition in ecology (e.g., Kuppers 1985, Lovell and Lovell 1985, Schmid and Harper 1985, Caldwell and Richards 1986, Franco 1986, Geber 1989, Weiner et al. 1990). These studies have shown promise for understanding the importance of resource allocation and architecture to plant competitive ability. Bell (1984, 1985) has suggested ways in which models that describe plant growth patterns can be used to explore the consequences of these patterns for within- and between-plant interactions. Others (e.g., Barnes et al. 1990) have explicitly modeled multispecies competition for light and found that such competition is influenced by plant architectural traits.

In this study we examined the relationships between plant performance and biomass allocation and architecture in target plants grown with different neighbor species. In a previous paper (Tremmel and Bazzaz 1993), we showed that neighbor plants with different architectures produced canopies that differed in light penetration, but that these differences did not translate straight-forwardly into differences in the performances of target species growing in these canopies. In this paper we will examine how the architecture of, and allocation to, target plant parts is affected by different neighbor species, and consider the implications of architectural and allocational traits, and levels of plasticity in these traits, for competitive ability.

MATERIALS AND METHODS

Experimental design

Four naturally co-occurring old-field annuals - Abutilon theophrasti Medic., Datura stramonium L., Polygonum pensylvanicum L., and Setaria faberii Herm - were used in this study. Abutilon, Datura, and Polygonum were used as "targets." Each target species was grown with each of these three species, and with Setaria, as "neighbors."

Growth conditions and experimental design are described in detail in Tremmel and Bazzaz (1993). Seedlings of equal size within species were transplanted into 13.3-[cm.sup.2] pots containing a 1:1:1 mixture of sterilized local topsoil, coarse sand, and Turface (chipped montmorillonite clay). Plants that died within 13 d of transplanting were replaced with plants of equal size and age. Each pot had one central target plant surrounded by four neighbor plants, with each neighbor plant 5 cm from the target plant. Plants were grown on four replicate glass-house benches using a split-plot design. On each bench there were four subplots, one for each neighbor species. Within subplots, pots were placed next to each other so that their sides were touching, creating closed canopies with at least a 4:1 ratio of the neighbor species to other species. Each subplot was surrounded by pots containing four plants of the neighbor species to minimize edge effects.

Pots were top-watered with an automatic drip watering system, and each pot had a saucer underneath it. Nutrients, in the form of water-soluble 20-20-20 NPK fertilizer (Peter's brand), were supplied four times at 10-d intervals beginning 7 d after transplanting. Day length was 14 h, with the natural photoperiod being extended by mercury vapor lamps. The photon flux density of photosynthetically active radiation (PAR) at the top of the canopy was [approximately equal to]2000 [[micro]mol] [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1] on clear days, and was maintained at [greater than or equal to] 900 [[micro]mol] [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1] using mercury vapor lamps on cloudy days. The ratio of red: far red light measured in these glasshouses is similar to that measured outside the building. Daytime and nighttime temperatures were 28 [degrees] C and 25 [degrees] C, respectively.

Five harvests were performed at 10-d intervals starting 5 d after transplanting; detailed allocational and architectural measurements on target plants were made at the final three harvests. These measurements included plant heights, the number of nodes, internode and petiole lengths, and individual leaf areas (whole plant leaf areas at harvest 4). Petiole and leaf inclination angles were measured as displacement from 0 [degrees] (horizontal) using a clinometer. These measurements were made only during daylight hours because of the nighttime changes in leaf angles observed in Abutilon and Polygonum. Inclination angle values were increased by 90 [degrees] for analysis to remove negative numbers.

At harvests 3 and 5 the plants were separated by node (for both main stem and branches) into individual internodes, petioles, leaves, and reproductive structures for weighing. At harvest 4 plants were separated into leaves, petioles and stems, and reproductive structures for weighing. All samples were then oven-dried to a constant mass. Root biomass was not determined at these harvests.

Calculation of architectural and allocational parameters

For each plant we determined within-plant maximum internode and petiole length and leaf area. We also calculated within-plant maximum internode, petiole, leaf, and metamer (sensu White 1979) masses. Metamer mass was calculated as the mass of each repeated morphological unit within the plant shoot. A metamer consisted of a leaf, its petiole, and the stem of the internode subtending that leaf. We also calculated specific leaf mass (leaf mass per unit area), internode specific stem mass (internode mass per unit length), and specific petiole mass (petiole mass per unit length) for individual nodes and whole plants.

Because of Datura's morphology, parameters for its nodes were calculated in a slightly different manner from other species. In Datura the main axis, after producing several nodes (4-8 in our plants), begins successive bifurcations at each subsequent node. Because developmental rates of the resulting branches are similar throughout the growth of successive branch orders, a mirror-image branching pattern results. In order to facilitate comparisons of part and metamer sizes, single values of size and mass parameters for each branch order within an individual were obtained by averaging all values for that branch order.

To examine how allocation patterns and architecture changed with plant mass, we also compared allocational and architectural parameters among plants grouped according to biomass. Among other things, this facilitated calculations of within-metamer means of length, area, and mass characters, because plants in the same mass class generally had similar numbers of nodes. Individuals of each target species (regardless of neighbor identity) at harvests 3 and 5 were separated into three mass classes by their aboveground biomass percentile rank. Comparisons were then made within target species among these mass classes at each harvest. The relationships between these derived categories and neighbor identity were examined using contingency tables and chi-square tests.

Statistical analyses

Analyses of variance (ANOVAs) were performed on aboveground biomass and on a variety of architectural and allocational parameters using Data Desk 3.0 (Velleman 1989). Dependent variables were log or square-root transformed as necessary to obtain both normal distributions of residuals, and equal variances among groups [as determined by Scheffe-Box tests (Sokal and Rohlf 1981)]. Separate analyses were performed for each species using split-plot models, with block and neighbor species identity as main plot effects, and harvest date as the subplot effect. The neighbor effect was tested using the block X neighbor variance; the remaining main effects and interactions were tested using the residual (error) variance. In all cases, Type III sums of squares were used. For post-ANOVA comparisons of subclass means, least-squares (adjusted) means of target plant parameters were computed for plants grown with each neighbor species using SAS (Joyner 1985). Dunn-Sidak LSD tests (Day and Quinn 1989) were then used to assess differences among these neighbor identity "treatments."

For species where significant effects of neighbor identity on target architectural and allocational parameters were found, analyses of covariance (ANCOVAs) were performed, using plant aboveground biomass as the covariate, to identify effects on these parameters due to neighbor species independent of effects on plant size. The model was specified as for the ANOVA described above, except for the inclusion of the covariate and the use of Type I sums of squares. Tests of residual distributions and variances were performed as for ANOVA. In all cases the relationship between the dependent variable and the covariate was tested for linearity and equal slope among groups; only cases for which these ANCOVA assumptions were met are reported.

Analyses of petiole and leaf inclination angles were performed on data from the main axis of the plant only. Observations of the plants suggested that petiole and leaf angles changed in a predictable way along the stem. To test whether these patterns were consistent and thus represented possible constraints on flexibility in these characters, analyses of these parameters were performed using node position, as well as harvest date, as subplot effects. For these analyses node position relative to the top of the plant was used, so that comparisons were made among nodes of similar developmental age. Because Datura target plants had so few main axis leaves at the time of the later harvests, there were too few data to perform these analyses for this species.

To compare the amount of variability within target species among the architectural and allocational traits we measured, we calculated the coefficient of variation of each of these traits for each target species using all plants (regardless of neighbor) from harvest 5.

Biomechanical (sensu Givnish 1986) constraints could limit flexibility in size of support structures, because of limits on support structure density imposed by the amount of biomass these structures are supporting. We therefore used linear regression analysis to examine allometric relationships between the specific masses of support structures and the biomass they were supporting, and to determine if the slopes of these relationships were altered by different neighbors. For each target species, F tests (Sokal and Rohlf 1981) were used to determine whether the slopes of the relationships between specific petiole mass and leaf biomass, and internode specific stem mass and plant biomass above that internode, varied significantly among neighbor species identities ([Alpha] = 0.05). When F tests were significant, pairwise comparisons of slopes for plants grown with each neighbor species were performed using Tukey-Kramer mean significant difference tests (Sokal and Rohlf 1981).

RESULTS

Neighbor effects on part number, size, and mass

In Abutilon, neighbor identity had no effect on above-ground biomass, number of main axis nodes, maximum internode length, petiole length, area per leaf, and metamer mass, or specific masses of stems, petioles, or leaves (0.0786 [less than or equal to] P [less than or equal to] 0.3531 from ANOVA; Table 1). However, means of these characters differed markedly among neighbor-identity treatments, with the largest values being 24 to 174% larger than the smallest values (Table 1).

In Datura, neighbor identity had significant effects on aboveground biomass, number of nodes, and maximum petiole length and area per leaf (Table 2). In all cases where differences were significant, values for plants with Datura neighbors were significantly lower than those for plants with Abutilon neighbors (Table 1). For all characters for which ANCOVA could be performed, there was a highly significant relationship between that character and the covariate (aboveground biomass; see Table 2). Values of the model coefficient of determination ([r.sup.2]) were also higher for ANCOVA than for corresponding ANOVA models (Table 2). When variability due to individual target plant mass was removed from the analysis by ANCOVA, there were no longer any significant effects of neighbor identity on any character (Table 2).

In Polygonum, neighbor identity had significant effects on all characters except maximum internode length [TABULAR DATA FOR TABLE 1 OMITTED] (Table 2). Where differences were found, Polygonum plants grown with Abutilon neighbors always had significantly smaller values for these characters than did those grown with Setaria neighbors (Table 1). As with Datura targets, the covariate term from ANCOVA was always highly significant, and ANCOVAs had higher model [r.sup.2] values than analogous ANOVA models (Table [TABULAR DATA FOR TABLE 2 OMITTED] 2). Significant neighbor identity effects from ANCOVA were found for number of nodes and specific leaf mass, indicating that these characters were affected by neighbors independent of neighbor effects on plant biomass (Table 2). However, for all other characters for which ANCOVA could be performed effects of neighbor identity were no longer significant (Table 2).
TABLE 3. Mean leaf and petiole angles at each node of Abutilon and
Polygonum target plants at harvest 4. Means represent data from
plants grown with all neighbor species, and have been adjusted for
the main ploteffect of neighbor species identity. Within target
species and characters, means for different nodes followed by
different letters are significantly different at the [Alpha] = 0.05
level by Dunn-Sidak LSD tests.


Node
number
(0 = top     Leaf                   Petiole
of plant)    angle                  angle


             Abutilon targets


0            -17.90                 12.82(c)
1            -26.46                 39.31(b)
2            -33.13                 56.45(ab)
3            -39.58                 69.42(a)
4            -42.15                 52.71(ab)


             Polygonum targets


0            15.03(a)               30.23(e)
1            -7.18(ab)              38.47(de)
2             7.41(a)               42.66(cd)
3             2.32(ab)              47.41(cd)
4            -1.13(ab)              50.73(bc)
5            -8.57(ab)              59.35(ab)
6            -23.08(b)              65.75(a)


Neighbor effects on target leaf display

Petiole leaf inclination angles of both Abutilon and Polygonum followed a regular pattern with metamer age, and were generally unaffected by neighbor identity. In Abutilon targets, neighbor identity had no effect on petiole or leaf angles (P = 0.9049 and 0.6542, respectively). Developmental age (as assessed by node position relative to the top of the plant, a rough equivalent of leaf plastochron index) had a marginally significant effect on leaf angle (P = 0.0871) but a highly significant effect (P [less than] 0.0001) on petiole angle. Neighbor identity had a significant effect on Polygonum petiole angle (P = 0.0406). Comparisons of means revealed only one significant (P = 0.0438) difference: plants with Abutilon neighbors held their petioles more upright than did those with Setaria neighbors (Table 3). Neighbor identity had no effect on Polygonum leaf angles (P = 0.1726). Part age had highly significant effects on both leaf and petiole angles in Polygonum (P = 0.0005 and P [less than] 0.0001, respectively). In general petiole angles of fully expanded leaves increased, and leaf angles decreased, with relative age (Table 3).

Neighbor effects on relative allocation to support tissue

In all target species there was a positive relationship between specific petiole mass and leaf biomass within metamers. The slope of this relationship did not vary among neighbor identities in Datura ([Alpha] = 0.05), and differences in the amount of leaf biomass being supported by different petioles explained a significant amount of the variance in specific petiole mass ([r.sup.2] = 0.82; significant at [Alpha] = 0.001). In Abutilon and Polygonum targets this allometric relationship was significantly altered by different neighbors. In Abutilon, the slope was significantly higher (i.e., there was a larger increase in specific petiole mass with a given increase in leaf mass) for plants grown with Datura neighbors (slope = 0.92) than for those grown with Abutilon or Polygonum neighbors (slope = 0.64 and 0.62, respectively). In Polygonum, the slope was significantly lower when plants were grown with Setaria neighbors (slope = 0.53) than with Datura or Polygonum neighbors (slope = 0.75 and 0.77, respectively). In all cases there was a strong relationship between within-metamer leaf biomass and specific petiole mass (coefficients of determination were [r.sup.2] = 0.71-0.97 in Abutilon, [r.sup.2] = 0.75-0.92 in Polygonum).

There was a strong, positive relationship between internode specific stem mass and plant mass above that internode in both Datura or Polygonum targets ([r.sup.2] = 0.83 for both species; both [r.sup.2] significant at [Alpha] = 0.001), and slopes of this relationship did not vary among neighbor identities ([Alpha] = 0.05). In Abutilon, the slope of this relationship was significantly smaller for plants with Polygonum and Setaria neighbors (slopes = 0.435 and 0.282, respectively) than for those with Abutilon and Datura neighbors (slopes = 0.668 and 0.754, respectively), though there were no differences in maximum specific stem mass. In all cases there were strong, positive relationships between these characters (0.76 [less than or equal to] [r.sup.2] [less than or equal to] 0.90; all [r.sup.2] significant at [Alpha] = 0.001). Thus, increases in stem density per unit increase in plant mass being supported were smaller in plants with Polygonum and Setaria neighbors than in those with Abutilon and Datura neighbors.

Relative variability in size and mass characters

In all target species plant aboveground biomass, and within-plant maximum internode and metamer mass, were the most variable characters (among individual plants) measured (Table 4). Maximum internode and petiole lengths were significantly less variable than aboveground biomass, maximum internode, petiole, and metamer mass, and internode stem specific mass, and were always less variable than plant-part maximum masses and specific masses (except for specific leaf mass) in all species (Table 4). Maximum leaf mass, and leaf specific mass, were always less variable than both internode and petiole maximum masses and specific masses, respectively, and these differences were significant in 7 of 12 comparisons (Table 4).
TABLE 4. Coefficients of variation ([r.sup.2]) of plant aboveground
biomass, and of architectural and allocational characters, among all
plants of each target species at harvest 5. Within species, values
followed by different superscript letters are significantly
different as determined by confidence limits calculated for each
value ([Alpha] = 0.05).


                                                      Polygo-
          Character            Abutilon   Datura      num


Aboveground biomass            80.4(ab)   74.4(a)     83.6(a)
Maximum internode length       28.8(d)    25.2(bc)    22.7(e)
Maximum petiole length         32.2(d)    26.6(bc)    25.3(de)
Maximum area per leaf          44.7(cd)   27.2(bc)    37.0(cd)
Maximum internode mass         95.9(a)    82.1(a)     75.8(ab)
Maximum petiole mass           66.7(bc)   63.9(a)     69.6(ab)
Maximum leaf mass              56.8(bc)   34.3(b)     68.0(ab)
Maximum metamer mass           78.2(ab)   69.1(a)     72.8(ab)
Internode specific stem mass   61.2(bc)   67.9(a)     71.9(ab)
Specific petiole mass          45.6(cd)   36.2(b)     56.2(bc)
Specific leaf mass             31.0(d)    20.7(c)     40.8(c)


Relationship between performance and architecture

Because differences in part size and biomass characters were strongly related to differences in total aboveground plant biomass, we examined changes in biomass allocation and architecture among plant mass classes (see Materials and methods: Calculation of architectural and allocational parameters). Contingency tables for each target species relating plant mass class rank to neighbor species identity revealed patterns consistent with those for target relative biomass with different neighbors (see Table 1). Chi-square tests for these tables were significant for all target species ([Alpha] = 0.05 for Abutilon targets; [Alpha] = 0.01 for Datura and Polygonum targets), indicating a non-random relationship between mass class rank and neighbor identity.

At day 25 (harvest 3) there were no marked differences in patterns of within-metamer allocation among mass classes for any target species, though plants in the smallest mass class had fewer nodes than did those in the largest mass class [ILLUSTRATION FOR FIGURE 1 OMITTED]. By day 48 (harvest 5), however, there were allocational differences among mass classes in Abutilon and Datura; plants in the smallest mass class had a larger number of younger nodes at which there was little relative allocation to stem. No such pattern was seen in Polygonum. Instead, Polygonum plants in the lowest mass class had many fewer nodes than did those in the larger mass classes, and produced fewer new nodes between the two harvests on both absolute and relative scales [ILLUSTRATION FOR FIGURE 1 OMITTED]. The relative difference between the number of nodes on plants in the smallest and largest mass classes was also much greater in this species than in either Abutilon or Datura.

The consequences of these allocation patterns for architecture are illustrated in Fig. 2. The number of canopy leaves was similar within target species across mass classes at both harvest dates. At day 25, when allocation patterns were similar across mass classes (within species), the relative sizes of parts were also similar among mass classes, though part size varied with biomass. However, at day 48 the differences in allocation patterns were manifested in differences in plant shape between plants in the smallest mass class and those in the larger two mass classes. This was especially true of Abutilon, where stem allocation was severely reduced in the upper canopy in the smallest mass class. In this class upper-plant internode lengths were extremely short, and as a result the vertical distribution between successive leaves was severely reduced [ILLUSTRATION FOR FIGURE 2 OMITTED]. Similar allocation changes in Datura plants [ILLUSTRATION FOR FIGURE 1 OMITTED] also resulted in less elongated upper canopies, though Datura's bifurcating growth pattern probably resulted in less self-shading than in Abutilon. By contrast the general shape of, and number of leaves in, Polygonum canopies was similar across mass classes at day 48, but plant heights and node numbers decreased with mass class (Fig. 2).

DISCUSSION

Responses to different neighbors

It has been shown that light quality, specifically the red: far-red ratio, decreases with increasing vegetational shade and canopy density (Raynal and Bazzaz 1975, Morgan and Smith 1981, Ballare et al. 1988), and differs among canopies of different species (Thompson and Harper 1988). Both internode and petiole lengths increase with decreasing red: far-red ratios (Holmes and Smith 1977, Morgan and Smith 1979, Smith 1982, Ballare et al. 1987, 1988, Smith et al. 1990). Additionally, internode length has been shown to increase with increasing planting density in monocultures of several annuals (Verheij 1970, Muchow 1979, Nienhuis and Singh 1985, Ballare et al. 1988). Light penetration into Setaria canopies was between two and five times higher than that into canopies of Abutilon, Datura, and Polygonum (Tremmel and Bazzaz 1993). We therefore expected to find differences in how target plant internode and petiole lengths (traits which affect leaf placement within the canopy and therefore light harvesting ability) responded to different neighbor canopies. Instead we found no effects of different neighbor species on these two target characters that were not explained by variation among individuals in overall shoot biomass.

The plants in this study were growing in a continuous closed canopy environment. This should have resulted in there being a premium on height growth, and little advantage in lateral growth or extraordinary petiole extension, regardless of neighbor identity. Because no target species consistently overtopped any neighbor (Tremmel and Bazzaz 1993), it is likely that our target plants had maximal internode extension within biomechanical limits, and therefore showed no significant effects of neighbor identity independent of shoot biomass. The fact that maximum internode length was one of the least variable characters measured, while maximum internode mass was one of the most variable (see Table 4), supports this view, and suggests that allocational flexibility in these species allowed them to maximize part size at the expense of part mass to some extent.

In contrast to part lengths, relative allocation to supportive tissue was affected by neighbor identity. While strong, positive relationships between within-metamer specific petiole mass and leaf biomass, and internode specific stem mass and plant biomass above that node, follow from biomechanical necessity, the slopes of these relationships changed significantly with neighbor in some cases. The most marked neighbor effect on these relationships was found for Abutilon targets. In this species, plants growing with Polygonum and Setaria neighbors had heavier plant parts, denser support structures, and a weaker relationship between these two characters, than those growing with Abutilon and Datura neighbors. It is possible that photosynthesis and transpiration were greater in the relatively more open canopies of Setaria (Tremmel and Bazzaz 1993), and therefore that support structures had higher specific masses because they had more conductive tissue to accommodate greater transport demands for water, nutrients and photosynthate. Plants have also been found to develop thicker stem tissue in response to touch stimuli and wind (Jaffe 1973, 1980, Grace 1988). Wind velocities may have been higher in Setaria canopies. It may also be that Abutilon plants growing with Setaria, which has many culms that are easily moved by the wind, were physically struck by their neighbors more often than were plants growing with other neighbors. Similarly, Polygonum neighbors may have indirectly caused relatively higher allocation to stem tissue in Abutilon. None of the Polygonum neighbor plants in this mixture at the harvest examined were able to physically support themselves once removed from the stand, while all the Abutilon targets were. Therefore these targets may have had heavier stems because they were supporting the mass of neighbor plants.

The fact that these allometric relationships differed among neighbor treatments indicates that within-metamer allocation can be determined, at least in part, by environmental conditions. It is not possible to determine, from the data collected, whether these environmental conditions directly altered the nature of the biomechanical requirements related to support tissue, or if they elicited these responses independent of such requirements. Since the latter could result in unnecessary allocation to support structures, plants that are able to cause such responses in neighbors may gain an advantage via this non-competitive mechanism.

Implications for competitive success

It has been shown that subtle changes in canopy structure can affect the competitive balance in mixtures of species with similar architectures (Barnes et al. 1988, 1990, Ryel et al. 1990). In this study we did not find consistent architectural or allocational changes with neighbor species that could be related to target performance. Changes in allocation and part size paralleled changes in biomass, while petiole and leaf angles were constrained by developmental patterns. We therefore cannot implicate changes in specific characters as means by which plants overcame the effects of specific neighbor canopies. However, this does not mean that architectural and allocational flexibility are not important in determining competitive success in these species. For example, McConnaughay and Bazzaz (1992) found that Abutilon seedlings grown in pots surrounded by varying densities of hollow glass rods used flexibility in petiole length to enhance their potential for carbon gain. While most of the leaves became trapped by the rods and consequently were poorly displayed for light-gathering, some "escaped," and were thus able to hold their leaf blades in a more natural position. These leaves tended to have longer petioles (and smaller blades) than the leaves that remained trapped, suggesting that the relationship between specific petiole mass and leaf biomass resulted in a trade-off between petiole and leaf size. Though we found no significant differences in Abutilon petiole length with neighbor identity in our study, it is possible that the experimental conditions in the former study resulted in allocation to maximize petiole length at the expense of leaf size, while the completely closed canopy conditions of our experiment resulted in allocation to maximize leaf size, since there was no benefit to extraordinary petiole elongation. The same study (McConnaughay and Bazzaz 1992) also showed that architectural flexibility in stem habit enabled Polygonum seedlings to "escape" from neighborhoods of simulated shoot neighbors. Stems of many Polygonum plants in our study also showed flexibility in habit, often being decumbent or even procumbent at the base of the plant. Though complete escape from canopy neighbors was not possible, this growth pattern may have allowed these individuals to "forage" for small canopy gaps in which to grow.

Overall plant shape also changed with biomass in these species, though dramatic changes in plant shape and stature were confined to plants in the smallest mass class. The extent of these changes differed among species. The general architecture of Abutilon changed markedly with mass class. Suppressed Abutilon plants had a large reduction in biomass allocation to stem, but only a modest reduction in the number of nodes produced. This resulted in these plants being quite short (and therefore presumably low in the canopy) and having a monolayer canopy instead of the usual columnar multilayer canopy [ILLUSTRATION FOR FIGURE 2 OMITTED]. Horn (1971) has argued that this canopy shape would allow for better light harvesting in understory trees, and the same may be true for these plants.

Because of its rigid morphological pattern, Datura plants in different mass classes had similar overall shapes, though there was some compression of the canopy in small plants of this species also. Suppressed Polygonum plants showed no evidence of canopy compression. Instead, small plants of this species produced many fewer nodes, and lost fewer leaves low in the canopy, than did larger plants. As a result, suppressed Polygonum individuals had much of their leaf area in very low quality light environments. However, because light compensation points for this species are exceedingly low under these conditions (Wieland and Bazzaz 1975, Bazzaz and Carlson 1982), these plants may have maintained a net carbon gain.

Conclusions

Since neighboring plants are an important cause of variability in resource availability, an understanding of the allocational and architectural mechanisms by which plants can alter their resource-gathering ability is useful in determining their potential for success in different competitive situations. Despite differences in canopy structure and light penetration among the neighbor species used in this study, and appreciable amounts of variability in allocational and architectural traits of target plants, no consistent differences in these traits in response to different neighbors were found, even within target species, that were independent of differences in total biomass. It was therefore not possible to determine whether changes in architecture with different neighbors were causes or effects of performance differences. There were, however, within-species differences in allocation patterns and overall plant architecture when plants in different size classes were compared, as well as among-species differences in the way allocation and architecture varied with size class. Target plants in this study may have been responding to finer-scale environmental variability than could be defined by our "neighbor treatment" categories. Plants most likely respond to local variability in resource availability with changes in architecture to maximize their resource-gathering ability. Their responses will be determined by the rules that govern their morphological development, by their physiological capabilities, and by the amount of resources they have access to and how these resources are allocated within the plant.

ACKNOWLEDGMENTS

We thank D. Ackerly, G. Berntson, W. Bossert, A. Knoll, K. McConnaughay, S. Morse, O. Solbrig, and P. Wayne, and four anonymous reviewers for helpful discussions and for comments on earlier versions of this manuscript. K. Mc-Connaughay also provided invaluable help with data analysis and interpretation. This research was funded in pan by grant number DE-FG02-84ER60257 from the United States Department of Energy.

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Author:Tremmel, D.C.; Bazzaz, F.A.
Publication:Ecology
Date:Jan 1, 1995
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