Growth of four conifer species during establishment and the effects of recurring short-term drought on growth and photosynthetic capacity

Abstract

The Midwest and southern Great Plains regions of the United States are known for historic and severe droughts. However, short-term recurring drought events are more common and can limit tree survival in landscape and production settings. The pressure of environmental stress combined with numerous diseases and pests are decimating existing Pinus L. spp. (pine) plantings and driving the effort to identify alternative species. Four species of conifer were grown in a pine bark substrate and subjected to recurring moderate to severe drought in a controlled environment glass greenhouse as well as field planted to observe root and shoot growth during the initial 12 months after transplant. The species utilized were Abies nordmanniana (nordmann fir), Cupressus arizonica (Arizona cypress), Picea engelmannii (engelmann spruce), and Thuja x ‘Green Giant’ (‘Green Giant’ arborvitae). Results indicate that C. arizonica exhibited extraordinary growth after establishment and was able to maintain growth and photosynthesis following several drought cycles. Thuja x ‘Green Giant’ exhibited significant increase in root and shoot growth after transplant. Under conditions of moderate and severe drought, only minimal reductions in height and shoot dry weight were observed while root growth and photosynthesis were unchanged. Abies nordmanniana experienced minimal increases in root and shoot growth throughout the growing season and was unaffected by drought. In contrast, P. engelmannii had only modest increases in root dry weight after transplant, while shoot growth was non-existent. Under conditions of severe drought, photosynthesis was reduced. Cupressus arizonica, a known drought tolerant species, with its ability to establish quickly and endure drought may have an advantage when establishing in harsh climates such as the Midwest and southern Great Plains. Thuja x ‘Green Giant’ is known to be a rapid grower, however, it established slowly during the current study and may require additional time before resuming rapid growth and maximum drought tolerance. Slow establishing species such as A. nordmanniana and P. engelmannii may require greater attention to season of planting to coincide with rapid root growth. However, P. engelmannii may not be a suitable choice for the Midwest due to the inability to maintain photosynthesis during periods of drought.