Interspecific and interannual variation in the duration of spring phenophases in a northern mixed forest

A wide range of intra- and interspecific variation occurs in spring leaf phenology as a result of biotic factors such as, life strategy, ecological niche and genetic adaptation, and abiotic factors such as environmental condition. Whereas knowing when the start of bud-burst occurs is necessary for determining the beginning of the growing season, and the subsequent start of carbon uptake, the duration of phenophases is equally important (to estimate the rate of carbon uptake, for example), but rarely reported. Here, we investigate variation in the timing and duration of 3 key phenophase categories (bud-open, leaf-out, full-leaf unfolded) from a range of 8 broadleaf and 2 conifer species in a mixed forest in northern Wisconsin, USA over a 5-year period. As expected, the start of each phenophase category varied across species and years and an earlier start to one phenophase did not necessarily result in an earlier start to subsequent phenophases nor did it mean a faster or slower progression. Ecological niche was not always a useful predictor of the timing or duration of the spring phenology season. The spring phenology season from bud-burst to full leaf open for the entire forest community took an average of 13 days ranging from 12 to 18 days across species. Bud-open and leaf-out lasted an average of 4 days whereas, full-leaf unfolding lasted 5 and again there were variations among species. Full leaf unfolded for A. incana lasted significantly (p < 0.001) longer than other species. Variation in the duration of the spring phenology season among years closely tracked local seasonal air-temperature based on growing degree hours (GDH). These results could be used to help determine the relationship between phenology and the potential for carbon storage in early spring in a mixed forest and highlight the value of direct field observation data at species level, the detail of which cannot, at present, be captured by satellite remote sensing.