Frost risk in northern Appalachian trees

A late spring frost in the White Mountains, NH resulted in swaths of autumn-like color in trees at around 2500 feet in elevation this summer. The AMC Research team monitoring tree canopy development as part of an Appalachian Trail wide ecological monitoring program (AT Seasons) also observed this unusual coloration in individual trees. Starting in late May and early June we noticed that some of the leaves looked reddish brown.

Aerial Photo taken June 2015 near Jefferson, NH by R. Hanavan

Nipped in the bud

The culprit was frost damage that stopped leaf development at the tops of the trees before they had grown to full size. We recorded below freezing temperatures at the AMC Pinkham Notch Visitor Center’s NOAA climate station during May 23rd to May 25th. According to the NH Division of Forests and Lands the frost damage was widespread throughout the White and Green Mountains. A similar event happened in 2010 when record warm spring temperatures were followed by a late frost event May 9th-11th. Researchers examined that earlier event in the context of leaf development and overall damage (Hufkens et al., 2012) in trees up to 2,700 elevation. They concluded that sugar maple leaf out responds relatively stronger to warming spring temperatures, therefore they may be more at risk from spring frosts than beech and yellow birch, putting them at a competitive disadvantage.

Fall foliage bust?

How this year’s late frost will impact the higher elevation fall foliage colors is still to be determined. Other variables from the amount of spring and summer rainfall to the number of cool September and October nights also play a role in the final fall landscape color palette. Just look at the fall of 2010 (http://naturenotes.outdoors.org/2010/10/peak-foliage-is-here.html), even with the documented spring frost event it was not a bust for autumn colors. So there is still hope for our fall fix of yellow, orange, and reds. The damage appears isolated to the upper canopy of the trees, which can be explained by the upper canopy acting as a heat retention blanket that prevented damaging below freezing temperatures in the trees’ lower canopy.

What about climate change?

Looking at the 1935-2015 AMC Pinkham Notch Visitor Center’s NOAA climate data record the 2015 late May frost was uncommon: less than 25% of all freezing events for the month happen in the latter half of the month. However, this was also a record warm May for Pinkham Notch (see top graph in box), with 2015 registering the highest ever recorded average maximum temperature. Statewide average May temperature also hit record warmth.

While warmer springs and late frosts may spell trouble for some species in the coming years, we may experience fewer late frost events as the region continues to warm. Indeed, that is the trend we are observing in AMC’s Pinkham Notch Visitor Center’s NOAA climate data (see bottom graph in box).

So even with an ever increasing variable climate the old Yankee farmers adage still applies --“the first and last frost are the worst!”