James F. Cahill

FERTILIZATION EFFECTS ON INTERACTIONS BETWEEN ABOVE- AND BELOWGROUND COMPETITION IN AN OLD FIELD

Recent theoretical and experimental studies have addressed whether the rel- ative importance of aboveground and belowground competition changes along gradients of biomass productivity. Results have been contradictory, with some researchers finding a decrease in the importance of belowground competition and an increase in aboveground competition with increased productivity, and others finding either no relationship, or a positive correlation between the various factors. Belowground competitive intensity (BCI), resulting from root interactions, and total competitive intensity (TCI), resulting from both root and shoot interactions, have usually been measured as the proportional growth reduction due to competition (relative to growth without competition). Instead of direct measurement, aboveground competitive intensity (ACI) has been estimated by assuming that aboveground competition and belowground competition do not interact to affect plant growth, and there- fore ACI + BCI = TCI. In this study, Abutilon theophrasti was used as a focal species to determine whether an interaction between the two competitive forms could exist. Target plants were grown with varying degrees of interaction with the roots of neighboring plants, through the use of modified root exclusion tubes, and by tying back the aboveground neighboring vegetation. In total, 16 combinations of varying intensities of aboveground and belowground interactions with neighbors were created at each of two fertilization levels. The strength of belowground competition decreased with fertilization, while neither above- ground competition nor total competition (occurring both above- and belowground simul- taneously) varied among fertilization treatments. Not only was there evidence for an in- teraction between above- and belowground competition, the form of interaction varied with productivity, switching from no interaction in the unfertilized block to a positive interaction in the fertilized block. With fertilization, belowground competition decreased a plants ability to compete in asymmetric competition for light. These results contrast with existing models of the role of competition in plant communities, and a new model is presented. In order to understand the role of aboveground and belowground competition in plant com- munities, the potential for interactions between the two competitive forms must be con- sidered in future studies.

Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science

There is a growing realization among scientists and policy makers that an increased understanding of todays environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

Plants integrate information about nutrients and neighbors.

Plant root growth is modified in the presence of within-species competition and uneven local resource distributions. Animals regularly integrate information about the location of resources and the presence of competitors, altering their foraging behavior accordingly. We studied the annual plant Abutilon theophrasti to determine whether a plant can demonstrate a similarly complex response to two conditions: presence of a competitor and heterogeneous resource distributions. Individually grown plants fully explored the pot by using a broad and uniform rooting distribution regardless of soil resource distributions. Plants with competitors and uniform soil nutrient distributions exhibited pronounced reductions in rooting breadth and spatial soil segregation among the competing individuals. In contrast, plants with competitors and heterogeneous soil nutrient distributions reduced their root growth only modestly, indicating that plants integrate information about both neighbor and resource distributions in determining their root behavior.

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

DISRUPTION OF A BELOWGROUND MUTUALISM ALTERS INTERACTIONS BETWEEN PLANTS AND THEIR FLORAL VISITORS

Plants engage in diverse and intimate interactions with unrelated taxa. For example, aboveground floral visitors provide pollination services, while belowground arbuscular mycorrhizal fungi (AMF) enhance nutrient capture. Traditionally in ecology, these processes were studied in isolation, reinforcing the prevailing assumption that these above- and belowground processes were also functionally distinct. More recently, there has been a growing realization that the soil surface is not a barrier to many ecological interactions, particularly those involving plants (who live simultaneously above and below ground). Because of the potentially large impact that mycorrhizae and floral visitors can have on plant performance and community dynamics, we designed an experiment to test whether these multi-species mutualisms were interdependent under field conditions. Using benomyl, a widely used fungicide, we suppressed AMF in a native grassland, measuring plant, fungal, and floral-visitor responses after three years of fungal suppression. AMF suppression caused a shift in the community of floral visitors from large-bodied bees to small-bodied bees and flies, and reduced the total number of floral visits per flowering stem 67% across the 23 flowering species found in the plots. Fungal suppression has species-specific effects on floral visits for the six most common flowering plants in this experiment. Exploratory analyses suggest these results were due to changes in floral-visitor behavior due to altered patch-level floral display, rather than through direct effects of AMF suppression on floral morphology. Our findings indicate that AMF are an important, and overlooked, driver of floral-visitor community structure with the potential to affect pollination services. These results support the growing body of research indicating that interactions among ecological interactions can be of meaningful effect size under natural field conditions and may influence individual performance, population dynamics, and community structure.

THE HERBIVORY UNCERTAINTY PRINCIPLE: VISITING PLANTS CAN ALTER HERBIVORY

In 1927, Werner Heisenberg proposed that there are fundamental limitations to the study of subatomic particles, as the act of measuring them affects their behavior. Here we show that experimenter-induced uncertainty also applies in plant ecology, with potentially dramatic consequences for field biologists. We tested whether the simple act of visiting marked plants once per week for eight weeks influenced the intensity of herbivory experienced by six plant species in an old field community. Half of the plants were touched once per week to simulate taking morphological measures, while the other half were left undisturbed (neither visited nor touched). After eight weeks, visitation resulted in (1) decreased leaf damage by insects on one species, (2) increased leaf damage on a second species, (3) a marginally significant increase in survival for a third species, and (4) no effect on the remaining three species. These results serve as an important reminder that seemingly benign experimental methods may themselves dramatically affect the performance of experimental subjects. Our results raise concern about studies that use repeated visitation of focal plants either to compare rates of herbivory among species or to investigate some factor that can either directly or indirectly be influenced by the rate of herbivory (e.g., seed production, competition, etc.). Since the six species in our study responded differently to visitation, visitation effects must be accounted for in the design of future field experiments.

Focusing the metaphor: plant root foraging behaviour

Many authors assert that plants exhibit complex behaviours which are analogous to animal behaviour. However, plant ecologists rarely root these studies in a conceptual foundation as fertile as that used by animal behaviourists. Here we adapt the optimality principles that facilitated numerous advances in the study of animal foraging behaviour to create one possible framework for plant foraging behaviour. Following the traditions of animal foraging ecology, we discuss issues of search and handling in relation to plant root foraging. We also develop a basic plant-centered model that incorporates modular growth and foraging currencies relevant to plant growth. We conclude by demonstrating how this new foundation could be adapted to address five fundamental questions in plant foraging ecology.

Increased competition does not lead to increased phylogenetic overdispersion in a native grassland

That competition is stronger among closely related species and leads to phylogenetic overdispersion is a common assumption in community ecology. However, tests of this assumption are rare and field-based experiments lacking. We tested the relationship between competition, the degree of relatedness, and overdispersion among plants experimentally and using a field survey in a native grassland. Relatedness did not affect competition, nor was competition associated with phylogenetic overdispersion. Further, there was only weak evidence for increased overdispersion at spatial scales where plants are likely to compete. These results challenge traditional theory, but are consistent with recent theories regarding the mechanisms of plant competition and its potential effect on phylogenetic structure. We suggest that specific conditions related to the form of competition and trait conservatism must be met for competition to cause phylogenetic overdispersion. Consequently, overdispersion as a result of competition is likely to be rare in natural communities.

Independent Evolution of Leaf and Root Traits within and among Temperate Grassland Plant Communities

In this study, we used data from temperate grassland plant communities in Alberta, Canada to test two longstanding hypotheses in ecology: 1) that there has been correlated evolution of the leaves and roots of plants due to selection for an integrated whole-plant resource uptake strategy, and 2) that trait diversity in ecological communities is generated by adaptations to the conditions in different habitats. We tested the first hypothesis using phylogenetic comparative methods to test for evidence of correlated evolution of suites of leaf and root functional traits in these grasslands. There were consistent evolutionary correlations among traits related to plant resource uptake strategies within leaf tissues, and within root tissues. In contrast, there were inconsistent correlations between the traits of leaves and the traits of roots, suggesting different evolutionary pressures on the above and belowground components of plant morphology. To test the second hypothesis, we evaluated the relative importance of two components of trait diversity: within-community variation (species trait values relative to co-occurring species; α traits) and among-community variation (the average trait value in communities where species occur; β traits). Trait diversity was mostly explained by variation among co-occurring species, not among-communities. Additionally, there was a phylogenetic signal in the within-community trait values of species relative to co-occurring taxa, but not in their habitat associations or among-community trait variation. These results suggest that sorting of pre-existing trait variation into local communities can explain the leaf and root trait diversity in these grasslands.

Water and nitrogen addition differentially impact plant competition in a native rough fescue grassland

We examined how water and nitrogen addition and water–nitrogen interactions affect root and shoot competition intensity and competition–productivity relationships in a native rough fescue grassland in central Alberta, Canada. Water and nitrogen were added in a factorial design to plots and root exclusion tubes and netting were used to isolate root and shoot competition on two focal species (Artemisia frigida and Chenopodium leptophyllum). Both water and nitrogen were limiting to plant growth, and focal plant survival rates increased with nitrogen but not water addition. Relative allocation to root biomass increased with water addition. Competition was almost entirely belowground, with focal plants larger when released from root but not shoot competition. There were no significant relationships between productivity and root, shoot, or total competition intensity, likely because in this system shoot biomass was too low to cause strong shoot competition and root biomass was above the levels at which root competition saturates. Water addition had few effects on the intensity of root competition suggesting that root competition intensity is invariant along soil moisture gradients. Contrary to general expectation, the strength of root competition increased with nitrogen addition demonstrating that the relationship between root competition intensity and nitrogen is more complex than a simple monotonic decline as nitrogen increases. Finally, there were few interactions between nitrogen and water affecting competition. Together these results indicate that the mechanisms of competition for water and nitrogen likely differ.