Hello all! Its #EarthDay2021 and I am here to tell you about the latest savanna paper news! this is a 2021 paper by Adam Pellegrini, a superstar in savanna biogeochem. Often, when we think about savannas, we think of fire. after all, fire kills encroaching trees. but! https://twitter.com/itatiVCS/status/1385194657639026688

What happens if there is something else is also killing trees?
We know fire and disease are big disturbances in ecosystems, but we know very little about how they interact.

here we are looking at the interaction between how fire and a plant pathogen, oak wilt, in the restored savannas at Cedar Creek.
Cedar Creek is a cute LTER where savannas have been maintained using different fire frequencies--so, different times since the last disco inferno

The combo of fire treatments at Cedar Creek means there is a savanna to forest gradient across the site. We already know that fire kills fire-sensitive saplings, reducing tree recruitment. Additionally, grass the predominant fuel at Cedark Creek--so we dont need trees to party

However, the site has been invaded by an oak specific pathogen, oak wilt. Oak wilt klls red oaks preferentially, while white oaks are relatively safe. This interaction between disease and fire could have unique consequences, since they affect tree mortality in different ways

Oak wilt itself is spread via root networks. and, its increased the amount of standing dead trees at Cedar Creek.
Does fire kill off this pathogen in the savanna? survey says...no.
So, how does disease in trees affect how fire maintains the savanna?

It's actually pretty cool to see fire and disease basically tag team these trees.
What do I mean by this? Let me explain.
Foremost, we see that fire and disease jointly regulate the persistence of savannas [ie, the absence of trees]

Unburned plots had WAY more trees than burned plots, as well as bigger trees than intermediately burned or frequently burned plots. However, after 2010, because of this higher density of large trees, oak wilt barreled through these plots like a bull in a china shop

In fact, the loss of trees by oak wilt was so extreme that the losses due to wilt were more than twice as rapids as all the gains made by trees during the period of fire exclusion. The sensitive trees, Q. ellipsodalis, only occured in unbured or infrequently burned plots.

So, while they attained large size due to fire exclusion, this in turn made them susceptible to oak wilt--which primarly murks large trees.
But there's more to the story!

We know different tree species are more or less suceptible to fire, right? at Cedar Creek, we know that Q. ellipsoidalis [pin oak] and Q. macrocarpa [bur oak] occur together but grow very differently. Pin oak grows significantly faster than bur oak--but this advantage vanishes

as fire frequency goes up. In fact, pin oak saplings need to grow 13% larger to become as resistant to fire as the same age bur oak. Pin oak eventually declined in burned plots while bur oak increased in those toasty plots.

Pin oaks then are more abundant in infrequently burned or unburned plots. They crave peace.
As it turns out, the pin oaks played themselves because oak wilt chose violence. The spread of oak wilt meant that pin oak population plummeted after 2010 while bur oak increased.

So, choosing the spoiled life always comes to bite eventually.
We also know from the tree ring ages at Cedar Creek that fire occurence every decade to every 3 years effectively eliminates the recruitment of trees. Tree can only recruit really in low burn frequency or unburned.

However, the density of large adult trees in the unburned/infrequently burne plots was again their downfall. Oak wilt attacked these trees and spread easily, so much so that wilt-induced mortality was equal to the rate of sapling recruitment.

Finally, we see that disease presence changed how efficiently fire controls ecosystem structure. The outbreak of oak wilt essentially decreases the fire frequency neccessary to maintain an open canopy savanna. With so many dead trees, you don't need a disco inferno to have fun!

Disease also changes the properties of a forest that can make-or-break their fire resistence. Oak wilt driven tree mortality effectively reverses the transition from savanna to woodland, because as adult trees die, grasses supercede them and replace them. Grasses rn at CC:

the rapid death of large and in charge adult trees also means that the few trees who do replace them are fire-sensitive saplings. These saplings are MUCH more likely to perish in case of fire.
In burned plots, the low density of host trees keeps oak wilt at manageble levels.

Its almost like oaks need to hit a Goldilocks zone to survive, where fire frequency is low enough that saplings can grow up, but not so low that the density of eventual adult trees ignites and fuels an oak wilt outbreak.

Now, these unburned plots at Cedar Creek are something novel, where their size structure is that where saplings have a high relative abundance but tree cover is lower that in disease free forest. This "diseased woodland" that can be pushed back to savanna if fire were to occur.

TL:DR fire hates pin oaks and so does oak wilt. So, they work conjointly to cancel trees and maintain a grassy savanna system.
Thanks for reading, and happy #EarthDay !