Plant Community Ecology-Plant Interactions

(competition/facilitation)

CompetitionReduction in fitness (how an organism apportions resources among competing demographic functions (survival, growth,

reproduction) due to shared use of a resource that is in limited supplyIntraspecific

Interspecific

Plants can tell the difference between the shade of an inanimate object and the shade of another plant. When a plant detects competition from neighboring plants, it initiates a set of responses, called collectively the shade avoidance syndrome, that alter its growth and physiology. A rapid and transient increase of newly synthesized auxin via a newly discovered auxin synthesis pathway allows plants to elongate and grow toward the sun.

- / - both parties lose0 / - or + / - asymmetric competition - largest individuals have disproportionate negative effects on their smaller neighbors

Competition

Competition

Tall plants may intercept light, but small plants may intercept water, soil nutrientsAsymmetric, but larger plants usually have much greater effects

Competition

Trade-offs and strategiesWinning competition for one resource may compromise ability to win comp. for another (light vs. nutrients)Outcome may change as resource availability changes

Competition

Immediate density most important - average density essentially irrelevantEffects of neighbors decrease sharply with distance

DEBATE between Grime and Tilman (1980s) regarding the effects of competition on species coexistence and community composition

Resource competition-tendency of neighboring plants to utilize the same quantum of light, ion of a mineral nutrient, molecule of water, or volume of space

Both believe that understanding plant traits, resource acquisition and loss by plants and the effect of disturbance on individuals holds the key to understanding patterns of diversity

GRIME CRS Theory regarding the effects of competition on species coexistence and community composition

In high productive environments, competitors that confer rapid growth rates will always dominate

Competition is unimportant in unproductive environments and success is dependent onthe ability to tolerate abiotic stress

In unproductive habitats species have characteristics that confer stress tolerance, such as long-lived leaves with high nutrient retention rates (biggest contrast with Tilman’s R theory)

In disturbed habitats plant have traits that allow them to withstand disturbance or high dispersal ability that allows them to recolonize rapidly

Grime’s Triangular Model

TILMAN R* (Resource-Ratio ) Theory regarding the effects of competition on species coexistence and community composition

Extended MacArthur (1972)—niche theory ideas on plant competitive strategies

R* is the concentration of available resource that a species requires to survive in a habitat … If all species are limited by the same nutrient, the species with the lowest R* should displace all competitors.

Tilman (1987) stated that competition in low productive sites would be for belowground resources and in productive sites for aboveground resources (light).

In unproductive habitats, superior competitive ability depends on the ability to reduce soil nutrients to a level below that at which competitors can exist and to persist at that this low level

Change in resource levels over time occurs because of incorporation of resources into the biomass of the plant population. (1) The population starts at a high resource level. Growth by the population results in the uptake of resources which are incorporated into standing biomass. This in turn results in the (2) decrease of available resources in the environment. This process continues until there is a (3) dynamic balance between resource uptake due to growth and resource release due to mortality. Essentially b=d and the population size remains fairly constant and resource levels are held at the level of R*, the minimum resource level for the maintenance of the population.

If two species are competing for the same resource, the species that can grow at the lowest resource levels will be able to drive the other species out of the system. Species B above will outcompete species A, since it can exist at lower resource levels

Craine 2005 Journal of Ecology-critiqued Grime and Tilman’s theories

General Overview of effects of competition on species coexistence and community composition

Ecologists agree that competition is intense in productive, nutrient-rich habitats when herbivory and disturbance is low

Importance in unproductive habitats remains debatable because environments can be unproductive for a variety of reasons (low water supply, cold temps, short growing season, saline soil, toxic metals)

Future Research: reducing vast diversity of terrestrial plants to conceptual categories so that plants can be grouped in ways that enable one to pose testable hypotheses and make predictions

Chemical warfare among neighboring plantsRelease toxins into soil to reduce growth or kill adjacent plantsWay to gain competitive advantageKnapweed on rangelands - little effect on Eurasian plants, strong effects on N. Amer. plants

Allelopathy

Plants influence their environments in many ways—altering balance of nutrients, acidifying the rhizosphere, secreting materials, and shedding parts. Plant can also affect soil microbes that affect other plants (indirect)

Allelopathy?

Artemisia californica

FUTURE RESEARCH-ALLELOPATHY

Need to know the concentrations and release rates of hypothesized allelochemicals, followed by experiments to manipulate concentrations of chemicals

Need to link greenhouse studies with large-scale field manipulations to bring clarity to subject

Need to know how plants change their soil environment and how these changes affect their interactions with both conspecifics and competitors?

Facilitation

Positive effects on neighbors rather than negative (opposite of competition?)May be particularly common under conditions of high abiotic stress, or high herbivory

FacilitationNeighbors’ effects differ depending on other aspects of the environment

Eucalyptus spp.

Falcataria spp.

Eucalyptus benefitted at high P to be near Falcataria but had negative competition interactions with Eucalyptus at low P

Facilitation-nurse plants

Mature “nurse” plant may facilitate germination, establishment, growth of juvenile plant of a different growth form; increase soil moisture, decrease temperature and light intensity

Facilitation-CMNs

Common mycorrhizal networks - extensive connections linking many plants of differing age, speciesMay facilitate seedling and sapling survival and growth by way of nutrient and water transfers among plantsMature helping juvenile

Mycorrhizae

Symbioses between mycelium of a fungus with roots of terrestrial plants

Mutualism or Parasitism?

Fungi get carbon, energy from plant hostPlant gets nutrients, other benefitsEither “partner” can function as parasite at times - plant sheds fungus when times are good, or fungus gives little to plant

Mycorrhizae

Arbuscular mycorrhizae - most abundant where phosphorus is limited, in warm, dry climatesImportant in tropical ecosystems, and for crop pants - woody and herbaceousFungal body grows inside root cells, with hyphae extending outward

MycorrhizaeEctomycorrhizae - woody plants, especially temperate conifersHartig net between root cells, mantle network of hyphae outside root

Mycorrhizae

Fungal hyphae increase nutrient uptake from soilTransfer nutrients to root cells of host plantsImprove metal uptakeImprove water uptakeBreak down soil proteinsProtect roots from toxinsProtect plant from fungal, bacterial diseaseshttp://www.youtube.com/watch?v=bq1bTduTzC0