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HUGE shoutout to my co-authors @EnvEcoLab @amy_trowbridge @wtp_abq @McDowell_Trees @ShealynMalone @DianaSix1 @PeltierDrew @Tree_Phys.

I’m pretty stoked about this paper because it links some pretty disconnected ideas from physiology, ecology, biochemistry, and mathematics to test an idea that has been really tough to test in the past. However, there remains lotsa work to do on this topic.

To summarize two key findings:. 1. Defenses of Pinus edulis are carbon limited during drought. 2. Stored defenses are likely more important for tree survival from bark beetles during drought.

Perhaps because we are very lucky people, bark beetles started to kill our study trees (about 40% of them died). This allowed to opportunistically test our models predictions. Altogether, our model had a correct classification rate of nearly 70%!

When I simulate this across a range of alpha values, we find that this survival threshold increases exponentially with decreasing response rate

This yielded an interesting result - trees that are slower to respond to an attack have to mount a larger defensive response to survive an attack!

Finding this cusp point depends on some tree-specific parameters, specifically how quickly a tree can mount a defense. We quantified this by measuring how fast a tree maximized its resin flow relative to the fastest tree at the site

To move forward, we want to know: at what threshold of carbon reserves does the fixed point of tree survival vanish? This is the cusp point in the figure two lines up.

We end up with three fixed points: two stable fixed points representing either beetle or tree mortality, and an unstable fixed point where both co-exist (unstable because that’s biologically impossible).

Without going into detail, these equations describe the change in defense and the change in successful beetle attacks as carbon reserves change. Using qualitative analysis, we can analyze the equations together and find their fixed points

But… this doesn’t tell us much about whether the defenses we can actually attribute to de novo synthesis is enough for trees to survive. So we came up with a set of differential equations and referenced the Bible (i.e. @stevenstrogatz book) to analyze them

This implies that *most* of the defenses that seemed to be induced were actually old, and were translocated from elsewhere in the tree to the site of inoculation!.

So, we inoculated trees with the fungus and measured pre and post inoculation defenses and carbon reserves. What we found was honestly surprising: local carbon reserves couldn’t support the defenses we measured.