The effect of extensive green roof substrate composition on native low-bush blueberry (Vaccinium angustifolium) and crowberry (Empetrum nigrum) growth and health
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The establishment of native vegetation on green roofs may reduce maintenance requirements and increase local biodiversity and ecosystem services. In Nova Scotia, native species from coastal barrens have been selected for green roof use due to their adaptations to harsh climates; yet the persistence of dominant native species (Vaccinium angustifolium and Empetrum nigrum) on green roof systems is unexplainably low. To test whether commercial green roof substrate is limiting shrub growth, shrub health and survivorship were monitored in an experimental study using four different substrate treatments. Under greenhouse conditions, survival, growth (height and number of leaves) and a categorical ranking of health were compared between plants grown in four substrate compositions: (1) control, or 100% commercial green roof growing medium; (2) control+peat moss, (3) control+native soil inoculum; and (4) control+peat moss+native soil inoculum. After 12 weeks, the control treatment for both species had significantly higher height, number of leaves and health scores than the other treatment, with the exception of V. angustifolium in the peat moss treatment. The mixture treatment had the lowest pH and highest organic matter content of all treatments, making it the closest treatment to resemble two of the three characteristic factors of coastal barren soils: high organic matter content; low pH; and low nutrient levels. However, the mixture treatment had the lowest height, number of leaves and health scores apart from V. angustifolium’s number of leaves in the inoculum treatment. Plant growth and health was lower in mixture substrates due to increased nutrient levels, high organic matter levels in soils with a relatively basic pH to coastal barrens, or waterlogged roots due to increased decomposition of added organic matter. The results indicate that substrate composition may not be limiting dominant native shrub growth on green roofs. However, before substrate composition is omitted as a potential explanation for shrub deficiency in green roof systems, this study must be replicated: (1) with a mixture treatment that has a lower pH, as pH is a leading factor in determining nutrient levels in soil; and (2) on an outdoor green roof system due to potential effects environmental factors have on shrub growth and health.