Abstract
Form and function of mycorrhizas as well as tracing the presence of the mycorrhizal fungi through the geological time scale are herein first addressed. Then mycorrhizas and plant fitness, succession, mycorrhizas and ecosystem function, and mycorrhizal resiliency are introduced. From this, four hypotheses are drawn: (1) mycorrhizal diversity evolved in response to changes in Global Climate Change (GCC) environmental drivers, (2) mycorrhizal diversity will be modified by present changes in GCC environmental drivers, (3) mycorrhizal changes in response to ecological drivers of GCC will in turn modify plant, community, and ecosystem responses to the same, and (4) Mycorrhizas will continue to evolve in response to present and future changes in GCC factors. The drivers of climate change examined here are: CO2 enrichment, temperature rise, altered precipitation, increased N-deposition, habitat fragmentation, and biotic invasion increase. These impact the soil-rhizosphere, plant and fungal physiology and/or ecosystem(s) directly and indirectly. Direct effects include changes in resource availability and change in distribution of mycorrhizas. Indirect effects include changes in below ground allocation of C to roots and changes in plant species distribution. GCC ecological drivers have been partitioned into four putative time frames: (1) Immediate (1–2 years) impacts, associated with ecosystem fragmentation and habitat loss realized through loss of plant-hosts and disturbance of the soil; (2) Short-term (3–10 year) impacts, resultant of biotic invasions of exotic mycorrhizal fungi, plants and pests, diseases and other abiotic perturbations; (3) Intermediate-term (11–20 year) impacts, of cumulative and additive effects of increased N (and S) deposition, soil acidification and other pollutants; and (4) Long-term (21–50+ year) impacts, where increased temperatures and CO2 will destabilize global rainfall patterns, soil properties and plant ecosystem resilience. Due to dependence on their host for C-supply, orchid mycorrhizas and all heterotrophic mycorrhizal groups will be immediately impacted through loss of habitat and plant-hosts. Ectomycorrhizal (ECM) associations will be the principal group subject to short-term impacts, along with Ericoid mycorrhizas occurring in high altitude or high latitude ecosystems. This is due to susceptibility (low buffer capacity of soils) of many of the ECM systems and that GCC is accentuated at high latitudes and altitudes. Vulnerable mycorrhizal types subject to intermediate-term GCC changes include highly specialized ECM species associated with forest ecosystems and finally arbuscular mycorrhizas (AM) associated with grassland ecosystems. Although the soils of grasslands are generally well buffered, the soils of arid lands are highly buffered and will resist even fairly long term GCC impacts, and thus these arid, largely AM systems will be the least affect by GCC. Once there are major perturbations to the global hydrological cycle that change rainfall patterns and seasonal distributions, no aspect of the global mycorrhizal diversity will remain unaffected.
Highlights
For the purposes of this review: mycorrhizal fungi are dual soil-plant inhabitants; a mycorrhizal association is the relationship between the fungus and the plant-host; and mycorrhizal diversity encompasses the entire range of form and functions—ranging along the functional continuum from symbiotic, commensalistic, mutualistic all the way through to exploitive/parasitism [34]
Central to examination of these possibilities is addressing the following fundamental questions: (1) What are the effects of environmental drivers on mycorrhizal plant-hosts? (2) What are the effects of environmental drivers on mycorrhizal fungi? (3) How do these changes to mycorrhizal fungi affect mycorrhizal association with their plant-partner? (4) What will happen to mycorrhizal diversity as a consequence of any de-coupling of mycorrhizal associations? (5) Will changes to mycorrhizal fungi in terrestrial soil-vegetation systems influence plant, community, ecosystem, and biospheric/global processes?
Mycorrhizal changes in response to ecological drivers of Global Climate Change (GCC) will in turn modify plant, community, and ecosystem responses to the same
Summary
The biology of mycorrhizas is a function partially of the wide diversity of fungi, partially of the wide diversity of plants involved, and partially the wide diversity of soils where plant and fungus interact, and by the various morphologies these associations take. These associations between plant and fungus are symbiotic and almost entirely mutualistic in that both fungus and plant partners benefit [1]. There is evidence, that one of the largest groups of fungi, those that form the arbuscular mycorrhizas, are heavily controlled by their plant partners These plants lean towards parasitism of the fungi [2]. PHYSIOLOGICAL AND METABOLIC: PLANT-LEVEL Decomposition of organic matter; volatilization of C, H, and O Elemental release and mineralization of N, P, K, S and other ions Elemental storage: immobilization of elements Accumulation of toxic materials Synthesis of humic materials Instigation of mutualistic, commensalistic and exploitive symbioses Increased survivability of seedlings Protection from root pathogens ECOLOGICAL: PLANT-COMMUNITY-LEVEL Facilitation of energy exchange between above- and below-ground systems Promotion and alteration of niche development Regulation and successional trajectory and velocity MEDIATIVE AND INTEGRATIVE: PLANT-ECOSYSTEM/BIOME-LEVEL Facilitation and transport of essential elements and water from soil to plant roots Facilitation of plant-to-plant movement of essentials elements and carbohydrates Regulation of water and ion movement through plants Regulation of photosynthetic rate of primary producers Regulation of C allocation below ground Modification of soil permeability and promotion of aggregation Modification of soil ion exchange and water-holding capacity Detoxification of soil (degradation, volatilization or sequestration) Participation in saprotrophic food chains Production of environmental biochemicals (antibiotics, enzymes and immunosuppressants)
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