There's a staggering lot we don't know about plants. It's partly due to their nightmarish genome complexity (often polyploid), their slow generation time (taking often decades to mature and reproduce) and the impossibility to reproduce the complex micro-organism communities found in the soil in which they thrive.
Much of the cool stuff we know (such as e.g. transmitting responses to stress to other individuals) we've only learned a couple years ago. This is truly an exciting time to make discoveries.
from the paper's synposis, paraphrased -- consider two large groups of trees, one of which thrives in warm temperatures with moderate to high rainfall, another which thrives in strong seasonal changes including a cold and dry time of the year. Each of these two groups has co-evolved with respective microbes in the soil which do regulate the nutrition available from the roots. The physical areas in which these microbes appear, can change "relatively abruptly" over the land, and are sensitive to climate conditions.
The implication is, that if climate conditions like more rain or sustained hotter temperatures, change rapidly, then the microbes in the soil are increasingly out of synch with the weather conditions. I would guess that more stress and increased reproductive failures, to name two extinction drivers, become more pronounced with less available nutrition from the soils plus the new weather.
If anyone is interested in this topic, Joe Rogan (I know, groan from some) has a great podcast episode with the mycology expert Paul Stammet where they discuss this topic. It's the thing that first got me into mycology and mushroom cultivation. He also has two fantastic Ted Talks if memory serves me correctly. Well worth a listen.
No microbe map in the paper unfortunately, just global maps of where the samples were taken. Like many papers these days: wonderful titles, disappointing abstracts and empty conclusions.
Much of the cool stuff we know (such as e.g. transmitting responses to stress to other individuals) we've only learned a couple years ago. This is truly an exciting time to make discoveries.