Relationship between soil temperature and the onset of vegetative growth in northern New Hampshire

Sales, Tracie
Peter Palmiotto, PhD
Department of Environmental Studies
The current definition of a wetland, as determined by the U.S. Army Corps of Engineers, requires that a jurisdictional wetland contain hydric soils, hydrophytic vegetation and wetland hydrology. Wetland hydrology in turn requires that presence of saturated or inundated substrate during a portion of the growing season. Formally, growing season occurs when the soil at 50cm depth is at or above 5ºC, yet in many of the colder areas of the United States, leaves are unfurling, flowers are blooming and new shoots are showing well before this critical soil temperature has been reached. To examine the timing of bud break as an indicator of the growing season, this study, conducted as part of the Mascoma Headwaters Project located in Dorchester, New Hampshire, compares soil temperatures at 50cm depth with observed vegetative growth between 1998 and 2005. Data was collected from soil temperatures probes placed at 50 cm depth in eight plots, four wetland plots and four upland plots, between 1998 and 2005. During the same years, researchers visited the site approximately three times per week to determine vegetative growth using Bud Score method, beginning at first snow melt and continuing until leaf growth was clearly established. Results showed that 50% or more of plants in a plot showed signs of vegetative growth, defined as a Bud Score of one or higher, 13.3 ± 8.9 days before the soil temperature reached 5ºC at 50 cm depth. On average, 74% of plants showed signs of growth when the soil temperature reached 5ºC at 50 cm depth in this study. Finally, the mean soil temperature on the day that the first signs of growth were apparent was 2.11 ± 1.03 ºC at 50cm depth and was 3.11 ± 1.18 ºC on the day when 50% of more of plants were growing. These results suggest that an alternate definition of growing season should be applied for wetland delineation in northern New Hampshire, with the most accurate determination coming from direct observation of bud break and growth from early growing species such as Betula alleghaniensis Britt., Prunus pensylvanica L.f. and Acer pensylvanicum L.