Linking overstory relative density to light availability and understory plant community composition in disturbance-dependent, Pinus rigida -dominated forests of the mid-Atlantic coastal plain, USA

There is a vigorous debate around whether and how to manage forests for climate mitigation or for other goals such as biodiversity. To achieve any of these goals it is necessary to define the parameters within which they can exist; ideally, the metrics used to evaluate progress towards one goal should be able to speak to other goals, especially when there are competing objectives at play. In the pitch pine (Pinus rigida) dominated coastal plain of New Jersey, open-canopy habitats are the locus of biodiversity, so we sought to define ‘open-canopy’ using a common metric of forest occupancy, Relative Density (RD). Using forest inventory data and light intensity measurements we found non-linear relationships between overstory RD and metrics of canopy light use. We then used RD to define upper limits of forest occupancy for fairly common open-canopy understory taxa above which one is very unlikely to find these plants. Our methods found credible thresholds in overstory density for four of five taxa, placing boundaries on the envelope of survivable conditions for these open-canopy community members. Native warm-season grasses (subfamily Panicoideae), Carex, Quercus ilicifolia, and Q. marilandica were not present in pitch pine forests above RD of roughly 0.57, aligning well with the approximate stocking level for stem exclusion. Separate validation data supported our quantified limits for warm-season grasses and demonstrated the importance of considering ground-layer competition but did not support overstory density thresholds for Hudsonia spp. Managers of similar pine forests with social goals in tension may be better able to quantify or illustrate tradeoffs between objectives using these results.