SO3 in wood ash

I have seen several sources note that wood ash typically contains 3–4 % $\ce{SO3}$ by weight. My question is, why would ash have any $\ce{SO3}$? $\ce{SO3}$ boils at 45 °C, so it should have all boiled away by the time the wood turned to ash.


Some sources for wood ash composition:  

[Source 1](https://digitalfire.com/4sight/material/pine_wood_ash_1119.html)  

[Source 2](https://www.researchgate.net/publication/51190303_Sodium_and_Potassium_Released_from_Burning_Particles_of_Brown_Coal_and_Pine_Wood_in_a_Laminar_Premixed_Methane_Flame_Using_Quantitative_Laser-Induced_Breakdown_Spectroscopy)  

[Source 3](https://www.sintef.no/globalassets/project/cenbio/cenbio-final-conf/2017-03-14_bjarte_oye_cenbio_final_conference.pdf)




Generally, ashes of natural samples (e.g. plant material or soil) do not actually contain chemical compounds like $\ce{Na2O}$, $\ce{P2O5}$, $\ce{SO3}$, $\ce{K2O}$ etc. These are just the reference compounds for the calculation of results of the elemental analysis.


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