Argüello et al. 2016). Cross talk between plants through MF networks can also assist
recovery and succession of forests following disturbance events (Song et al. 2015).
The composition of mycorrhizal types is increasingly recognized as an important
factor in biogeochemical cycling and ecosystem function. For example, MF are
major determinants of soil carbon stocks, which store more carbon than the atmosphere and vegetation combined. Ecosystems composed largely of plants with EcMF
store more carbon than those with AMF, by (1) producing a larger biomass of
recalcitrant mycelia and, (2) imposing nitrogen-limitation on free-living soil
microbes that slows saprotrophic decomposition (Averill et al. 2014). Globally,
agriculture and other land use changes have reduced EcMF-associated vegetation,
reducing carbon sequestration in the soil (Soudzilovskaia et al. 2019). Climate
change may further drive declines in EcMF abundance (Parrent et al. 2006). For
example, coast live oaks, Quercus agrifolia, form tripartite symbioses with AMF
and EcMF, but drought differentially impacts fungal colonization abilities, such that
water-limited plants may become increasingly reliant on AMF (Querejeta et al.
2009). Anthropogenic changes, including GCC, nitrogen pollution, and fire suppression have induced continent-wide shifts toward AMF in US forests (Jo et al. 2019;
Averill et al. 2018). Even with climate mitigating strategies, declines in EcMF are
predicted in North American pines over the next 50 years with negative implications
for biogeochemical cycles (Steidinger et al. 2020).
Although studies are typically of limited duration and biased to the northern
hemisphere, MF show potential to improve plant resilience to warming, elevated
CO 2 levels, and rainfall variability (Mohan et al. 2014; Compant et al. 2010; Bennett
and Classen 2020) by several mechanisms. First, elevated levels of atmospheric CO 2
can increase hyphal colonization and rates of nutrient exchange. As noted above,
however, this does not necessarily correspond with increased production as plants
are more often nitrogen, rather than carbon limited. Here the type of MF is important.
In plant species with AMF, nitrogen limitation indeed inhibits CO 2 fertilization,
whereas biomass increases regardless of nitrogen availability in plants associating with EcMF (Terrer et al. 2016). Similarly, temperature often positively correlates
with MF abundance and activity, and may affect the structure of the hyphal networks, affecting storage versus growth dynamics (Mohan et al. 2014; Hawkes et al.
2008). Third, as soils dry, nutrient uptake is often reduced. Experimental studies
show that MF alleviate water stress under both drought and variable water conditions
(Bowles et al. 2018; Augé 2001). Furthermore, AMF may improve stomatal conductance (rate of CO 2 entering, H 2 O vapor exiting) and photosynthetic rates,
increase antioxidant activities, or regulate channel proteins involved in water transport that reduce water stress (Quiroga et al. 2017; Li et al. 2019; Augé et al. 2015).
While the effects of drought on MF abundance and activity are mixed, overall MF
plants are more productive compared to non-MF plants during drought (Mohan et al.
2014). Increased usage of fertilizer and fossil fuels have also increased reactive
nitrogen deposition in soils. This impacts ecosystem functioning, leads to nitrate
leaching and subsequent water pollution, as well as the release of N 2 O; a potent
greenhouse gas that also damages the ozone layer (Vitousek et al. 1997; Montzka
16 Symbiosis in a Rapidly Changing World
281
recovery and succession of forests following disturbance events (Song et al. 2015).
The composition of mycorrhizal types is increasingly recognized as an important
factor in biogeochemical cycling and ecosystem function. For example, MF are
major determinants of soil carbon stocks, which store more carbon than the atmosphere and vegetation combined. Ecosystems composed largely of plants with EcMF
store more carbon than those with AMF, by (1) producing a larger biomass of
recalcitrant mycelia and, (2) imposing nitrogen-limitation on free-living soil
microbes that slows saprotrophic decomposition (Averill et al. 2014). Globally,
agriculture and other land use changes have reduced EcMF-associated vegetation,
reducing carbon sequestration in the soil (Soudzilovskaia et al. 2019). Climate
change may further drive declines in EcMF abundance (Parrent et al. 2006). For
example, coast live oaks, Quercus agrifolia, form tripartite symbioses with AMF
and EcMF, but drought differentially impacts fungal colonization abilities, such that
water-limited plants may become increasingly reliant on AMF (Querejeta et al.
2009). Anthropogenic changes, including GCC, nitrogen pollution, and fire suppression have induced continent-wide shifts toward AMF in US forests (Jo et al. 2019;
Averill et al. 2018). Even with climate mitigating strategies, declines in EcMF are
predicted in North American pines over the next 50 years with negative implications
for biogeochemical cycles (Steidinger et al. 2020).
Although studies are typically of limited duration and biased to the northern
hemisphere, MF show potential to improve plant resilience to warming, elevated
CO 2 levels, and rainfall variability (Mohan et al. 2014; Compant et al. 2010; Bennett
and Classen 2020) by several mechanisms. First, elevated levels of atmospheric CO 2
can increase hyphal colonization and rates of nutrient exchange. As noted above,
however, this does not necessarily correspond with increased production as plants
are more often nitrogen, rather than carbon limited. Here the type of MF is important.
In plant species with AMF, nitrogen limitation indeed inhibits CO 2 fertilization,
whereas biomass increases regardless of nitrogen availability in plants associating with EcMF (Terrer et al. 2016). Similarly, temperature often positively correlates
with MF abundance and activity, and may affect the structure of the hyphal networks, affecting storage versus growth dynamics (Mohan et al. 2014; Hawkes et al.
2008). Third, as soils dry, nutrient uptake is often reduced. Experimental studies
show that MF alleviate water stress under both drought and variable water conditions
(Bowles et al. 2018; Augé 2001). Furthermore, AMF may improve stomatal conductance (rate of CO 2 entering, H 2 O vapor exiting) and photosynthetic rates,
increase antioxidant activities, or regulate channel proteins involved in water transport that reduce water stress (Quiroga et al. 2017; Li et al. 2019; Augé et al. 2015).
While the effects of drought on MF abundance and activity are mixed, overall MF
plants are more productive compared to non-MF plants during drought (Mohan et al.
2014). Increased usage of fertilizer and fossil fuels have also increased reactive
nitrogen deposition in soils. This impacts ecosystem functioning, leads to nitrate
leaching and subsequent water pollution, as well as the release of N 2 O; a potent
greenhouse gas that also damages the ozone layer (Vitousek et al. 1997; Montzka
16 Symbiosis in a Rapidly Changing World
281