Getting Loaded

Spring is off to a warm and fast start here in Michigan.  March was unseasonably warm and the past week or so has seen temperature 20 degrees above average or more.  Needless to say this is pushing all of our landscape trees and shrubs.  Forsythia and saucer magnolia are in full bloom, at least two weeks ahead of schedule.  The warm weather also has us scrambling to get some research projects in the ground as well.  Today I was working with members of my lab to install a trial to look at the relationship between fertilization in the nursery and subsequent of shade trees in the landscape.  For the past two years we’ve grown Acer miyabei (‘State street’ maple) and ‘Harvest gold’ Linden trees in 25 gallon containers as part of a trial on controlled release fertilizer.  Interestingly, in the nursery we saw a significant increase in chlorophyll index and foliar nitrogen with fertilization (no surprise) but no difference in caliper or height growth (somewhat of a surprise).  This indicates that in the nursery, fertilization induced ‘luxury consumption’ or an uptake of nutrients beyond what the trees need to meet their growth requirement.  This observation provided the opportunity for our current, follow-up study.  In the forest nurseries there is a growing interest in the practice of ‘nutrient loading’ seedling trees before they are lifted.  Forest nursery managers deliberately induce luxury consumption by fertilizing late in the season.  At this time seedlings have set a hard bud and won’t grow but can take up additional nutrients.  Numerous studies, particularly by Dr, Vic Timmer and his associates at the University of Toronto have shown that nutrient loaded seedlings will outgrow standard seedlings when out-planted on reforestation sites; even though the seedlings are the same size when transplanted.  How does this apply to large-caliper shade trees?  We don’t know.  There are certainly some underlying commonalities that are intriguing.  Nutrient loaded forest seedlings have an advantage when planted on tough sites where follow-up culture is minimal – basically the seedling has to get by initially with its own energy reserves and resources.  Shade trees planted as street trees often face the same hardship; once planted they may receive little or no after-care beyond an initial watering.  Could nutrient loading provide a better internal nutrient reserve and jump start the re-establishment process for street trees like it does for the smaller forest cousins?  We should gain some insights this summer and next.