Comparative functional mechanisms of co-occurring shrubs resulting in dominance of a thicket forming N-fixer

Background/Question/Methods

Thicket forming shrub species can quickly and dramatically alter microclimate conditions beneath thickets and also community level variables such as species richness and diversity and nutrient and hydraulic cycling. Post-thicketization consequences are well documented, but causes of species establishment and subsequent rapid expansion are less known. The objective of our study was to determine functional physiological mechanisms resulting in aggressive expansion of an invasive shrub species compared to co-occurring shrubs. We compared the invasive dominant shrub, Elaeagnus umbellata, to the abundant and co-occurring shrubs, Clethra alnifolia and Vaccinium corymbosum in an Eastern temperate system. Of the three species, all of which have a deciduous leaf habit, Elaeagnus is the only N-fixer. Our approach was to determine resource use parameters including variations in light capture, nitrogen, and water use. For each species, we measured electron transport rate (ETR), leaf angle, and pigment concentrations as well as foliar δ¹³C, δ¹⁵N, C:N, and %N during the peak of the growing season at Fort A.P. Hill, Virginia.

Results/Conclusions

Light capture parameters for sun exposed leaves showed that Elaeagnus had a 50% greater ETR compared to Clethra and Vaccinium. Elaeagnus had the highest pigment concentrations and leaves were angled for maximal light capture suggesting that Elaeagnus is well suited to high light environments. Based on our δ¹⁵N results, Elaeagnus primarily utilizes fixed N as opposed to available soil N indicating low soil N availability. Clethra and Vaccinium are non-fixers and had ~50% less foliar %N compared to Elaeagnus. Vaccinium had the highest nitrogen use efficiency even compared to Clethra as determined by C:N ratios. δ¹³C values were highest for Elaeagnus suggesting greater stomatal regulation compared to Clethra and Vaccinium. The three shrubs examined are all abundant within the community, however Elaeagnus showed the greatest potential for higher light capture, more rapid ETR, higher N acquisition and allocation to leaves, and greater stomatal control and water use compared to the other species. This study suggests that for rapidly expanding shrub species, mechanisms of thicket formation may include photosystem machinery equipped for maximal light capture and processing, root symbionts promoting nutrient uptake, and greater regulation of stomatal aperture and water uptake.