Hello,
I got the fact, that he was the arrendator of Jõõpre estate from the book:
Jääajast siia ja edasi ehk Killukesi Audrust (compiled by Kersti Põllu)
https://www.digar.ee/arhiiv/et/raamatud/108390
From the chapter: JÕÕPRE VARAJANE AJALUGU JA MÕISAAEG / JÕÕPRE: EARLY HISTORY AND MANOR PERIOD (p172–206) written by Heljo Konts
In this article there is a list of Jõõpre arrendators and only one short paragraph about him (p 182 – 183):
"23.04.1900 – 23.04.1920 anti mõis rendile Aleksander Eduardi p Kerstenile (Kärstens). Arvatakse, et ta oli lätlane. Tegelikult tegutses ta mõisas 1910. aastani. Mai Kalm, ajakirjanik Jõõprest, on kirjutanud, et Kärsten oli oma aja kohta edumeelne põllumees. Näiteks rajas ta praeguse Kaseküla maadele 1,5 km lahtise äravoolukraavi, samuti raiuti võsa. Tegelikult juba 26.11.1911 on mõisa rentnikuks viimane Jõõpre mõisnik Baron Wolter Arnold von Stackelberg."
The translation would be:
"For 23.04.1900 – 23.04.1920 Jõõpre estate was leased to Aleksander Kersten, Eduard's son (Kärstens). It is believed that he was Latvian. In reality he worked at the manor until 1910. In fact, from 26.11.1911 the arrendator of the estate was Baron Wolter Arnold von Stackelberg, the last Jõõpre landlord.
Mai Kalm, a journalist from Jõõpre, has written that Karsten was a progressive farmer for his time. For example, he built a 1.5 km open drainage ditch on the lands of present-day Kaseküla, and also cut down brush."
You can make an enquiry with the National Archives of Estonia about him. Probably they can provide some more information. The enquiry forms are here
https://www.ra.ee/vau/index.php/en/enquiry/form/index (Genealogical Research)
All the best,
Aiki Agasild
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Alexander Kersten