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.