Quercus macrocarpa and Q. Prinus physiological and morphological responses to drought stress and their potential for urban forestry

Bur (Quercus macrocarpa Michx.) and chestnut (Q. prinus L.) oaks are nearly allopatric species. Both are drought resistant and tolerant of basic soils, and thus, potentially suitable for urban sites. Their morphological and physiological responses to substrate moisture stress were studied by subjecting container-grown seedlings to no, one or two substrate moisture stress cycles. There were greater differences in response to the stress cycles between species than within species. Quercus macrocarpa seedlings had four-fold higher root:shoot ratios than Q. prinus seedlings receiving no, one or two stress cycles and twice the root surface areas as Q. prinus. However, Q. prinus roots absorbed twice as much water per unit root surface area as Q. prinus. Unstressed Q. prinus seedlings had twice the leaf area ratio (LAR) and were twice as tall as stressed or unstressed Q. macrocarpa seedlings. Unstressed Q. macrocarpa seedlings had the highest relative growth rate (RGR) and twice stressed Q. prinus seedlings the lowest. Within a species, moisture stress had no effect on LAR or net assimilation rate (NAR), but Q. macrocarpa had lower LAR and higher NAR than Q. prinus seedlings. Seedlings of both species had similar leaf water potentials when unstressed but were significantly lowered following one or two stress cycles. Twice stressed Q. macrocarpa had significantly lower leaf solute potentials than similarly stressed Q. prinus seedlings. In both species, drought postponement traits were more strongly expressed than drought tolerance traits. We propose that Q. prinus maybe better adapted to urban planting sites than Q. macrocarpa because of its more efficient water absorbing root system and its apparent adaptation to shallow soils.