Trans-Atlantic connections between North African dust flux and tree growth

James King 1, Grant Harley2, Justin Maxwell3
1Université de Montréal, Montréal, Canada, 2University of Southern Mississippi, Hattiesburg, USA, 3Indiana University, Bloomington, USA

The role of mineral dust aerosols within the context of land-atmosphere interactions is poorly understood. Recently, studies have shown novel connections between atmospheric dust and vegetation dynamics. African dust is speculated to have positive effects on plant growth in the Amazon Basin and the Caribbean region, providing seasonal inputs of nutrients. However, increased aerosol optical depth close to source regions have been shown to alter the amount of direct and indirect surface photosynthetically active radiation (PAR) available to plants. Here we use monthly cell production from a pine tree species (Pinus elliottii var. densa) during a growing season from the Florida Keys with a suite of supporting datasets uncover a link between atmospheric aerosols and tree growth. Annual growth ring formation of P. elliotti, the foundation species and sole canopy tree in endangered pine rockland ecosystems of the U.S., was found to not have a strong long-term climate signal (i.e. precipitation, temperature) within the annual growth rings of the species, but was shown to have significant correlation with solar radiation flux. In addition, some were found to form an intra-annual density fluctuation (IADF; commonly known as a ‘false ring'), which is an anomalous radial growth characteristic, concurrent with a period of reduced incoming solar radiation.
This study analyzed MODIS aerosol optical depth (AOD) and fraction of photosynthetically active radiation (fPAR) products along with the measurements of AOD and extinction Ängstrom coefficient from a proximal AERONET station to ascertain the local effects of transported mineral dust aerosols over a concurrent period of the cell production measurements. From these measurements, we suggest that dust-derived increases in AOD reduced the amount of direct PAR available at the surface triggering the reduced tree growth anomaly. However, it is not clear the mechanisms responsible for the subsequent increase in growth after the dust-derived AOD decreased. The passing of a Tropical Storm directly after the IADF formation not only could have acted to clear any aerosols in the atmosphere increasing total PAR, but it would have also enhanced nutrient deposition, both of which could have resulted in the positive growth response in P. elliottii var. densa. Further research should aim to disentangle the complex intra-annual relationship between the potentially radiation limiting atmospheric dust AOD and fertilizing deposition of dust as a P input. Future research should investigate the potential for developing growth anomaly chronologies using P. elliottii var. densa as a novel proxy with which to reconstruct annual or seasonal dust emissions over the south Florida and other sub-tropical pine habitat regions. As a long-lived conifer (300+ years), P. elliottii var. densa could represent a novel proxy with which to reconstruct annual or seasonal mineral dust aerosol fluxes over the Caribbean region.