Urban root-zone temperatures and their impact on tree hydrology and growth

William Richard Graves, Purdue University

Abstract

Urban trees are subjected to environmental constraints that decrease their life spans. The goal of this project was to characterize urban root-zone temperatures during summer months and assess their influence on growth and water status of tree species used in cities. Mean root-zone temperature at urban street tree planting sites was 7$\sp\circ$C higher than the mean in a rural area 4 km away. Urban temperatures between 25$\sp\circ$ and 35$\sp\circ$ were common, but others as high as 69$\sp\circ$ were documented. Redbud (Cercis canadensis L.), red maple (Acer rubrum L.), honey locust (Gleditsia triacanthos inermis L.), and tree of heaven (Ailanthus altissima Mill.) were evaluated for responses to 18$\sp\circ$ to 36$\sp\circ$ root-zone temperatures using a unique temperature-controlling apparatus. Growth and water relations data suggested honey locust was most tolerant of root-zone temperatures exceeding 30$\sp\circ$. Honey locust and tree of heaven were then evaluated together for responses to 24$\sp\circ$ and 34$\sp\circ$ root zones. Growth of tree of heaven, a species assumed tolerant of urban conditions, was reduced by 34$\sp\circ$ root-zone temperature. However, growth of honey locust was greater among plants with 34$\sp\circ$ root zones than those in the 24$\sp\circ$ regime. Species-dependent growth responses to elevated root-zone temperature were associated with different leaf water potentials and transpiration rates, but osmotic adjustment and altered cell wall elasticity did not contribute to turgor of either species under temperature stress. These data suggested that root water transport capacity was affected by temperature, so hydraulic conductivity (L$\sb{\rm p}$) of honey locust and tree of heaven roots was assessed as a temperature-sensitive mechanism influencing whole-tree water status. Rhythmic oscillations in root water flow complicated determination of root L$\sb{\rm p}$ as the slope of force-flux relations. Analysis of variance and multiple regression were used to quantify L$\sb{\rm p}$ accounting for endogenous rhythms and the effects of 24$\sp\circ$ and 34$\sp\circ$ temperatures. Roots formed at 34$\sp\circ$ had lower L$\sb{\rm p}$ than 24$\sp\circ$-grown roots, especially among tree of heaven in which altered stelar anatomy and membrane composition probably reduced water transport capacity. Exposure to 34$\sp\circ$ during resistance measurement increased flux through roots of both species grown at both temperatures. Results of this project show root-zone temperature influences the resistances and driving forces governing water balance of tree species used in cities.

Degree

Ph.D.

Advisors

Dana, Purdue University.

Subject Area

Botany

COinS