Sunday, February 20, 2022

Through the Fire: How Grasses Rise Up from the Flames

Aristida beyrichiana (wiregrass), a resprouter
The fact that the Poaceae use fire to create stable ecological states like grasslands and savannas is well known, but resolving the fine details on the mechanisms underlying these processes is a work in progress.

Grass populations can recover from these fire events via two paths or so-called persistence strategies. Some species die back and resprout from underground rhizomes or undamaged crowns. These are the so-called resprouters. For example, both Imperata cylindrica (cogon grass) and Aristida beyrichiana (wiregrass) come back after a fire from surviving remnants (using tough rhizomes in the case of the former). 

Sharp rhizome of Imperata cylindrica, which is a resprouter
The other way to get a population through a fire is by reseeding. These grasses are the so-called seeders, and they include species like Taeniatherum caput-medusae (medusahead grass), Bromus tectorum (cheatgrass), and Microstegium vimineum (stiltgrass). In this case, the population develops to sexual maturity rapidly, and then sets seed, which can survive past the fire event to form a new population and generation.

Dried seedheads of Taeniatherum caput-medusae, a seeder
The two types of persistence strategies significantly affect the various morphological and physiological traits of each species, as listed in the table below.

Trait

Relationship with resprouting ability

Photosynthetic pathway
Resprouters are more likely to be C4 than C3 (Moore et al., 2019). C4 species are highly efficient in fire-prone environments and may therefore have greater stored resources to resprout (Tix & Charvat, 2005; Ratnam et al., 2011)

Bud position

Resprouters are more likely to have buds below the soil surface (rhizome resprouters) where they are protected from intense heat (Pausas & Paula, 2020)

Specific leaf area (SLA)

Resprouters will have lower SLA than seeders (Forrestel et al., 2014). High SLA will aid the rapid growth of seeder species

Leaf nitrogen (N) content

Resprouters will have lower leaf N contents than seeders. Resprouters may experience fire multiple times in their lifetime and thus low-N availability (due to N volatilisation during fire; Reich et al., 2001; Hern├índez & Hobbie, 2008). In these conditions, a high N-use efficiency (low leaf N content) may be advantageous (Wedin & Tilman, 1990; Reich et al., 2001)

Leaf C : N ratio

Resprouters will have higher leaf C : N ratios than seeders. High leaf C : N ratio, which is linked to low decomposition rates and the accumulation of a highly flammable fuel load (Aerts, 1997), may be advantageous to shade-intolerant resprouting species in maintaining an open canopy (by aiding the removal of standing dead and woody biomass; Everson et al., 1988)

Life history

Resprouters are more likely to be perennial than seeders. Perennial-grass species have buds from which to regrow, which annual species may lack

Table 1. From Simpson et al, 2021

These persistence strategies in grasses have a complex and fascinating relationship with the existing fire regime, but in order to understand it, the different characteristics of an existing fire regime have to be defined.

Fire Frequency is the time between fires in an area. More frequent fires tend to be more damaging to a plant.

Fire Intensity refers to the amount of energy released by the flames. Higher amounts of released energy increases the probability of severe damage or death to a plant.

Fire Severity is an index of the damage caused by a fire by measuring the amount of organic matter lost.

These three attributes are related in a simple manner. The less frequent fire events are, the more biomass can be accumulated, which leads to more intense fires and a corresponding increase in the severity of the event.

Grass seeders proliferate under fire regimes of high frequency and low intensity, whereas resprouters dominate when the fires are more intense after a longer period between fire events. The rapid sexual maturity of seeders allows them to seed quickly, and these grasses can persist even in fire frequencies that are as short as a year!  

However, if fires do not occurs frequently, this leads to a larger accumulation of fuel biomass, and more intense fires. In this case, reprouters have an advantage and will dominate an area because they do not need to pass through the more vulnerable seed and seedling stages. 

The golden hued seeder T. caput-medusae (medusahead) carpets an area
Knowledge about the relationship between fire regimes and persistence strategy is not only fascinating, but can also be used to predict community composition in a habitat given specific changes in the fire regime. For example, changes in fire regimes due to climate change can cause dramatic alterations in the ecology. It may also be possible to use this knowledge to dent the advance of invasive grasses that hew to a particular persistence strategy.


References

Simpson, K.J., Jardine, E.C., Archibald, S., Forrestel, E.J., Lehmann, C.E.R., Thomas, G.H. and Osborne, C.P. (2021), Resprouting grasses are associated with less frequent fire than seeders. New Phytol, 230: 832-844. https://doi.org/10.1111/nph.17069

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