Belowground Competition in Mirabilis:
The Effect of Bulb Size
Elizabeth A. Koniers '01

    This research project, under the direction of Dr. Rich Niesenbaum at Muhlenberg College dealt with the effect of neigbors in soil.  Tubers of differing sizes store a variable amount of nutrients and thus are unequal competitors for nutrients garnered from the soil such as nitrogen.The main goals of the study were to determine the effect of  stored resources on competition in Mirabilis jalapa between bulbs and seeds, between bulbs in equal size classes and between bulbs of different size classes.  If the R* hypothesis is valid, then bulbs should have a statistically significant competitive advantage over seeds and larger bulbs should display similarly significant advantages over smaller bulbs, despite the proposed symmetry of belowground competition.


    Studies relating to competition among plants have a long history and unlike in zoology, botanists generally accept competition as a force instrumental in structuring plant communities (Keddy, 1991).  Among plants, competition stems from limiting factors such as light, carbon dioxide, and mineral nutrients such as nitrogen (Harper, 1977, Cahill, 1999).  In general, asymmetric competition has been regarded as common, with one plant showing a significant advantage over another plant (Barbour, 1999, Weiner, 1992).  Although a majority of studies have dealt with the general topic of plant competition, few studies have separated the effects of competition into above- and belowground components (Twolan-Strutt, 1996, Cahill, pers. comm, 1999).
    Essentially, belowground competition stems from limited water and nutrients (Harper, 1977). Nitrogen is the principal limiting resource below the surface (Keddy, 1991). Furthermore, studies suggest that where soil nitrogen levels are low, root competition overshadows shoot competition in terms of importance (Wilson and Tilman, 1991). Although asymmetry in competition has been widely reported,  the effect of competition below the ground has been hypothesized to be more symmetric than competition above the surface, because both plants show similar effects as compared to plants grown individually (Weiner, 1992). Traits affected may include form, biomass, size, rigor, growth rate, reproductive output, or other characteristics.  Conversely, studies suggest that preemption of space and resources by individual plants lends distinct and significant advantages over competitors (Cahill, 1977). It has also been supposed that competition below the soil is stronger and more inclusive -- in that it affects more individual characteristics -- than that which occurs aboveground (Casper, 1997).
     One keystone hypothesis in competition, especially that which occurs below the ground, is the limiting resource model (R*). This concept has its origins in Liebig's law of the minimum and it supposes that the individual who is able to create a smaller R* value, or who can develop using lower levels of key resources, will successfully out-compete others for resources (Armstrong and McGeehee, 1980).
     Theoretically, the more stored resources a plant has available, the lower its R* value should be, at least initially.  If the limiting resource model is correct, a plant that starts with more stored resources will require a lower R* and will therefore outcompete neighboring individuals.  These stored resources may be in the form of seeds or bulbs.  Because allocation to bulbs in previous seasons may not be equal between individuals, this is a key factor in determining competitive ability.  The question then, is if  tubers have a significant effect on competitive ability, measured in terms of biomass.  Essentially, do bigger bulbs make for  a better competitor?  The effect of storage on competitive ability has not been greatly investigated (Cahill, pers. comm. 1999),  but this project aims to investigate its importance.


The plants used in this study are from the genus Mirabilis, belonging to the family Nyctaginaceae.  Their common name is the four o'clock.  They are regarded as tender perennials, but will occasionally grow annually in colder climates.  Mirabilis  prefer a sunny environment in dry to moist soil, which may or may not be sandy.  The adult height of a plant may reach 12-36 inches and inflorescence may be red, yellow, white or fuchsia.  They have a practically nation-wide distribution in the United States, with populations growing wild in Western and Southwestern states, and they may more recently be found as far as the East Coast.  In Mirabilis, as growth of the plant occurs, resources are allocated to the root system based on environmental conditions.  When possible, resources are stored in enlargements known as perenniating storage organs, or bulbs.

The goals of the study included determining the effect of stored resources on competition in Mirabilis jalapa between bulbs and seeds, between bulbs in equal size classes and between bulbs of different size classes.  If the R* hypothesis is valid for competition between storage organs, then bulbs should have a statistically significant competitive advantage over seeds and larger bulbs should display similarly significant advantages over smaller bulbs, despite the proposed symmetry of belowground competition.

Of the bulbs available to me, having been collected from previous experiments in the lab, 300 were suitable for scientific experimentation.  They were gathered together, most attached hair roots and dried soil was removed and then they were sized and arranged.  The organs were separated into groups of 10 according  to size (that is, the largest were grouped together first and so on). Classifications based on mass measurements. The group with a mass larger than the median is named Large and the smaller group, Small accordingly. The bulbs were divided as follows: Overall, 300 bulbs were planted in the Shankweiler greenhouse. 40  bulbs having a mass closest to the median were planted in competition with  seeds.  40 having a mass below the median labeled as Small were planted in competition with 40 other small bulbs (80 with each other = Small-Small) and 40 having a mass above the median, Large, were planted in competition with 40 Large bulbs (80 with each other = Large-Large).  40 small bulbs were planted in competition with 40 large bulbs = Large-Small.  Each pair in this group was planted with the Large set on the left and the Small on the right in relation to the pot label, and all pairs were distinct.

A group of 20 bulbs, 10 large and 10 small, comprised the control, and were planted in isolation in the greenhouse.
The greenhouse soil used was be nitrogen-poor, in order to enhance the effects of belowground competition.  Because the soil was reused, a sieve was implemented to prevent stray Mirabilis seeds from skewing experimental data.

Progress was observed through the course of the experiment in terms of aboveground attributes such as leaf number and height, but can only be used as approximations of belowground results.  Because biomass measures are destructive, they will only be collected at the end of the experiment in terms of total and  individual dry biomass, above and belowground.

The research was devised and conducted during the Fall 2000 semester, which provided adequate time to plan, plant, collect data, analyze data and establish relationships between groups being tested.  The facilities, materials and resources at Muhlenberg College prove a research environment that is not only adequate, but ideal.  The greenhouse and laboratory of Dr. R. Niesenbaum have been made available to me and provide a useful space for growth and data collection, as well as a place to analyze and review.

Once the study period was determined to be over, the attributes of all the plants that grew were recorded, and the plants were removed from the bulb at their base.  These were dried out in the same manner as the bulbs and their dry mass was recorded. The bulbs were extracted from their respective regimens during the first week of February 2001.  They were placed in aluminum boats and dried in ovens for 2-4 days at ~32oC.  Their mass was recorded in the same manner as before.
After completion of the weighing, all data was entered into Excel and compared with the pre-planting data. The relative losses were equalized by computing and comparing the percent change in biomass for each group, rather than by looking at the absolute mass.


From the useable results, it was determined that there was an overall net loss in biomass within all groups.  The difference in biomass loss was not significant when comparing the percent biomass loss in the two same-paired groups (i.e., Small- Small and Large-Large), and when comparing the Large-Small group.

 In looking at small bulbs that grew in the Small-Large regime, these did not have a significantly higher percent biomass loss than those grown in the Small-Small pairing.  The Large bulbs in the Small-Large regime did have a significantly lower percent biomass loss than those in the Large-Large treatment. This was substantiated using a one-way ANOVA, where P=0.002191.

Turning to the relationship between belowground resources and resulting aboveground biomass, there was evidence from the planting to suggest plant dependence on bulb size. Indeed, all bulbs within the Large-Large group that successfully grew plants were significantly larger as a group than those "Large" group members without plants. See Fig 3. P=0.000111 for this data, based on a one-way ANOVA.

In addition, the bulbs that successfully supported aboveground biomass had a significantly lower percent biomass loss than did those bulbs lacking plants.  P=0.00973 after a one-way ANOVA.   See Fig 2.

Results from the treatments with bulbs and seeds yielded no useable results.


 The occurrence of widespread biomass loss among all bulb types may be consistent with competitive theories in relation to resource competition and plant density. Bulbs of relatively equal sizes showed relatively equal losses in biomass, as in Small-Small and Large-Large.  These results were consistent with the research hypothesis. However, if success in competition is looked at in terms of least output required in exchange for survival or even for shoot growth, there is a clear advantage in larger bulb biomass.  It seems that all bulb competition results in losses in biomass, but those that were able to minimize relative stored nutrient use and those that produced aboveground biomass were those bulbs with the largest overall biomass.

 Why bulbs of smaller sizes in all treatments did not succeed in sending up plants or minimizing losses may be a function of several factors:

* there may be a cutoff value, some sort of optimal bulb size, which does not sacrifice the fitness of the plant from which it is formed, but is large enough to ensure success in the next generation.

* there may be temporal issues not taken into account by this study, where smaller bulbs may have sent up shoots at a later time.  This seems unlikely based on the status of bulbs when they were unearthed, but it remains as a possibility.

* there may be an optimal bulb morphology, which was not readily identifiable, that is favored for competition.

It should be noted also, that the bulbs available to me may have been limited in their range due to growing cycles and greenhouse propagation.  All bulbs used were collected from previous experiments and were all within a relatively narrow age range.

One issue that exists is if size is the principal determining factor for bulb success, then what became of the large bulbs planted in the Large-Small group?  They did indeed show smaller losses in biomass, but they would have also been expected to show more aboveground mass.

* one reason for this might be that they randomly had a smaller overall mass than those selected (also randomly) to be planted in the Large-Large category. That is, even though all plants were designated as Large, this group had a larger range, and perhaps there was some disparity between subgroups.  This occurrence, however unlikely, remains as a possibility.

* another more likely explanation is that large bulbs in this regime did not need to send up shoots so urgently, because the small bulbs in Large-Small presented a minimal competitive situation.  The issue that competition never even occurred when such a significant disparity between bulbs existed might be raised.  Perhaps this group more than any would have shown different results over a longer time span.

Why were bulbs with plants the lowest consumers of stored resources?  This may be because they were able to put in a large effort first, through which plant matter was formed.  Once the presence of photosynthetic material was realized, this enabled the plant to gain a competitive advantage by replacing lost resources, or decreasing the amount of resources needed for the initial plant-forming stages.  The presence of sprouts might be considered the hump over which competing plants must grow if they are to remain successful.


The presence of stored resources provides some competitive advantage, but in a scaled manner.  There was a distinct difference in the performance of bulbs of a large size and bulbs of a small size when they were in competition with other bulbs.

It appears that the ability of bulbs to outcompete their soilmates preempted the possibility for aboveground competition in almost all cases.  Depending on the amount of resources a bulb had stored and the fierceness of the competitive situation (competition with and equally matched bulb versus one that posed little competitive threat) the bulb seemed more likely to send up shoots.  The size of a bulb may be optimized to predict success of several generations, based on the R* of one of them. This theory of R* limiting competition may be applied to bulb growth, for only the group with the very largest biomass and therefore stored resources had the ability to minimize resource use for competition and to produce plants.

Whether the size of bulbs was due to the allocation of past generations to asexual methods, this would explain larger bulbs, and smaller bulbs, whose plants relied more on seed production in past now had no chance in presence of relatively larger bulbs.  If this allocation decision is cyclical, it may acccount for the disparity between bulb sizes.

It must be kept in mind that the time frame of this study, as well as that for the growth of the bulbs involved in the study  may have placed an unusual skew on the results, if they were inconsistent with what happens in Mirabilis in the wild.

Although bulbs which did the best in this time period came were a subset of the Large group, the group was large enough that the subset might be considered unique. In relation to its own and all other groups, this subset constituted a superior group of competitors.  Their competitive advantage could be linked to the stored resources of these bulbs.

Further experimentation, with more bulbs, a longer growing period and more stringent mass classifications would be needed to confirm these findings.

Klimt Gustav - Bluehender Garten


Fig 1.     This figure illustrates the relationship between the mean percent change in bulb mass for all large bulbs,
either grown in the Large-Large condition or the Large-Small.  The difference in bulb mass change
between the Large-Large groups and the group from Large-Small was significant, with P= 0.002191

Fig. 2   In this figure, the relationship of initial bulb size is compared for Large bulbs with and without
resulting plant biomass. In this group,  P=0.00973.

Fig. 3      This figure portrays the relationship between groups for the amount of biomass lost over
the experiment.  The groups are Large bulbs without plants and Large bulbs with
plants.  P=0.000111 for this data.

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Klimt Gustav - Der Kuss

I would like to thank Dr. Niesenbaum and the DANA scholars program for allowing me to dabble in botany while I figured out what I wanted to do with my life...  Thanks for your patience!

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