Altering Seedling Physiology to Improve Reforestation Success

Abstract

Reforestation success depends largely upon the physiological and morphological preparedness of seedlings to survive and grow after planting. Frost hardiness, mineral nutrition, and carbohydrate reserves are critical elements of seedling physiology. This paper reviews these elements to illustrate how they are affected by nursery cultural practices and, in turn, their impact on field survival and growth. Inappropriate application or timing of some nursery practices, such as irrigation and fertilization, can lengthen the growing season and delay the onset of dormancy; for some species, such as Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), extended summer growth can lead to fall frost damage in the nursery. However, well-planned schedules of irrigation and fertilization, as they relate to species phenology, can increase frost hardiness. Seedling damage that does occur may or may not be visible in the nursery, and the performance of the damaged seedlings after planting may be severely reduced under some field conditions. Field performance also will be reduced in seedlings that are not fall-hardy when lifted, processed and stored for planting, and in seedlings that, for any number of reasons related to cultural practices, lack resistance to subfreezing temperatures in the field. Mineral nutrition of seedlings can be dramatically increased by growing irrigated seedlings at low seedbed densities and moderately high fertilizer levels. Nitrogen (N) concentrations in the range of 1.7 to 2.3% generally result in seedlings that exhibit enhanced field survival and height growth. The benefits of nutrition, however, can be altered by its effect on seedling size, degree of frost hardiness, and levels of carbohydrates. Cold storage, an accepted and sometimes necessary nursery practice for holding seedlings between lifting and planting, often depletes carbohydrate reserves. Poor seedling survival after cold storage has, for a number of species, been attributed to depleted carbohydrate reserves. Because seedling root and shoot growth after field planting may also be dependent on carbohydrate reserves, depleted levels could have a marked impact on first-year performance in the field.