Why are trees in one section of the Miyawaki Miniforest taller than their cohorts in other sections?

The miniforest is divided into four sections, each with a different soil amendment.

Section 1 had no amendments.

Section 2 had compost from the Town of Ithaca DPW. 

Section 3 had compost + biochar.

Section 4 had a living inoculum from a natural forest that was cultured on wood chips and applied around the roots as they were planted in the hole. 

In fall 2026 we noticed that the trees in section 4 looked taller than those in the other sections. We decided to measure the heights of four tree species, chosen because they had been planted from pots and had been growing in White Oak nursery under similar conditions. (Most of the other trees were planted as bare root saplings that had been stored in bundles in soil over the summer and were separated for planting.)

The 4 charts below show the heights of each tree sorted by the sections. For all four species trees in section 4 were, on average, taller than trees in the other sections.  However, some individuals of both birch and hackberry in sections 2 and 3 achieved the same height as those in section 4.  Both persimmon and basswood grew taller in section 2 (compost) than in section 3, (compost + biochar). 

Data from this small sample of trees suggests that something about the conditions in section 4 favored vertical growth. It may have been that lush growth of these plants produced a shadier understory and competition for light  propelled them upward. But we didn’t measure light levels in the understory, it just seemed to be more lush than in the other sections. What other factors might be considered?