Gas exchange and growth of landscape tree species in response to drought and post establishment applied organic mulch

Landscape industry employees often advises homeowners to place organic mulches on soils surrounding trees in new and existing landscapes. However, limited research has been conducted to determine if organic mulch placed below established landscape trees enhances tree growth. Under non-irrigated conditions, our research objective was to compare gas exchange and growth of established landscape trees that either had, or did not have organic mulch placed on the soil surface surrounding each tree. English oak (Quercus robur), Chinquapin oak (Q. muehlenbergii), and Shantung maple (Acer truncatum) trees were planted 2002. In Fall 2008, six trees of each species were randomly assigned a mulch treatment: organic mulch or no organic mulch placed on the soil below the tree canopy. From Fall of 2008 through Fall of 2011 trees received weekly irrigation. Beginning 2012, trees were not irrigated. Daily weather data were collected by an onsite weather station. Throughout the 2012, 2013, and 2014 growing seasons bud break data was collected from each tree. In addition, throughout each growing season mid-day gas exchange data (stomatal conductance, photosynthetic rate, etc.) were collected. At the end of each growing season, growth data were assessed. Data indicate soil under mulch was cooler, had greater soil moisture, and less extreme temperature fluctuations extremes when compared to soil without mulch. Throughout the experiment, species and treatment differences were found in gas exchange data. Shoot growth tended to be greater for non-mulch trees. However, leaf area varied between species and mulch treatments. Results suggest benefits (greater gas exchange, growth, etc.) of organic mulch placed under non-irrigated established trees is species specific, and should be carefully considered prior to making recommendations.