Carbon, nitrogen, and phosphorus variation along a green roof chronosequence: Implications for green roof ecosystem development

While the developmental dynamics of ecosystems have been studied in many natural systems, it is unclear if these patterns are to be expected in engineered ecosystems where components have not co-developed over time. Green roofs often begin with a nutrient rich substrate that is paired with slow-growing plants adapted to nutrient poor conditions - perhaps resulting in different developmental dynamics than natural ecosystems. We evaluated changes over time in green roof nutrient content and vegetation communities using a chronosequence in southern Sweden of similarly designed but different aged green roofs spanning between 2 and 22 years. Substrate depth, substrate nitrogen (N), and total N pool sizes varied positively with roof age. These dynamics suggest an accumulation of 2.9 ± 1.1 g N/m2/yr with no indication of leveling off after 22 years. Plant N content (%) positively varied with roof age but plant biomass, plant nutrient pools, and plant diversity did not vary with age. These dynamics indicate a novel developmental scenario where the ecosystem begins with near-stable plant biomass but still accumulates N in the substrate at rates on par with many secondary successional systems. The apparent accumulation of N could not be accounted for by N deposition rates for the region, suggesting substantial N-fixation. The novel developmental dynamics outlined in this study point to the need for a new or expanded ecosystem developmental paradigm that better suits green roofs and perhaps other emerging engineered ecosystems. •Shallow green roofs in this study appear to follow novel developmental trajectories.•Nitrogen content positively varied with green roof age despite steady plant biomass.•Apparent nitrogen accumulations greater than deposition suggest nitrogen fixation.