Modern attempts to remedy previous environmental injustice patterns structures urban street tree biodiversity

Background/Question/Methods

Humans promote and inhibit other species on the urban landscape, shaping biodiversity patterns. Institutional racism may underlie the distribution of urban species by creating disproportionate resources in space and time. Here, we examine whether present-day street tree occupancy, diversity, and composition in Baltimore, MD, USA neighborhoods reflect their 1937 classification into grades of loan risk—from most desirable (A= green) to least desirable (D= “red- lined”)—using racially discriminatory criteria. We ask whether neighborhoods that were redlined contain different present-day occupancy rates of tree size classes, ⍺-diversity, or communities that exhibit different patterns of species turnover or reordering. We predict that street trees in these neighborhoods (i.e., D-graded) will exhibit (1) lower occupancy of large (i.e. older) trees, (2) lower ⍺-diversity, (3) different tree species community composition compared to better-rated HOLC neighborhoods, and (4) lower among-neighborhood species turnover (i.e. homogenization). However, these predictions may not hold in smaller (i.e. younger) cohorts of street trees that have been established more recently. Here, we predict that older assemblages, indicated by trees with larger diameters, should exhibit significant divergence in assemblage structure by HOLC grade, but such differences should be less distinct with newer assemblages (smaller trees) due to similar contemporary planting practices implemented citywide.

Results/Conclusions

We find that neighborhoods that were redlined have consistently lower street tree ⍺-diversity and are 9 times less likely to have large (old) trees occupying a viable planting site. Simultaneously, redlined neighborhoods were locations of recent tree planting activities, with a high occupancy rate of small (younger) trees. However, the community composition of these young trees exhibited lower species turnover and reordering across neighborhoods compared to those in higher grades, due to heavy reliance on a single tree species. Overall, while the negative effects of redlining remain detectable in present-day street tree communities, there are clear signs of recent investment. A strategy of planting diverse tree cohorts paired with investments in site rehabilitation and maintenance may be necessary if cities wish to overcome ecological feedbacks associated with legacies of environmental injustice.