Climate warming and more frequent droughts threaten the effectiveness of circumpolar boreal forests as carbon sinks.  To the immediate south of boreal forests in eastern North American lie regions that still support natural island-like growths of boreal conifers, where warming (expressed by higher temperature maxima) has increased at similar rates but with seasonal precipitation increases rather than the decreases that are commonly seen in other warming regions.  Studying tree growth under these circumstances offers the opportunity to examine effects of heat stress in the absence of drought. 

We studied growth of mature White spruce (Picea glauca), planted as pure stands in east-central New York State, at the top of the Helderberg Plateau (600 m above sea level) during the 1920s. During 2013-2017 period we observed 1) radial growth of spruce trees, 2) sap flow, 3) soil moisture in the top 10 cm layer as well as timing of spruce budbrake and detailed survey of tree species at two 30x30 meters plots. 

Results demonstrate that the spruce responded to the early arrival of a warmer-than-average spring with a 3-4 day advance in the start of radial growth (SRG).  During the same years, the end of radial growth in late summer occurred earlier by 14-18 days indicating a cessation in growth despite moist soils and favorable solar conditions. Therefore, the observed advance in the SRG did lead to a shorter radial growth period due to early cessation dates.  Abundant precipitation and relatively high soil moisture supported relatively stable sap flow and allowed us to dismiss soil drought as a factor.  

If heat stress is leading to declines in photosynthesis, a likely explanation based on previous experimental work on this species, reduced allocation of non-structural carbon to tree stems leads to early cessation of radial growth and therefore reduced carbon storage, independent of seasonal precipitation. 

This work took place in the Edmund Niles Huyck Preserve (42.5155° N, 74.1402° W https://phenocam.sr.unh.edu), an 800 ha nature preserve and biological research station established in 1931 about 300 km south of the southern border of the boreal forest biome.  The preserve is located on the Helderberg Plateau within the Glaciated Low Allegheny Plateau Ecoregion. We surveyed and mapped two 30x30 m plots within White spruce (Picea glauca) plantations begun as seedlings in 1924-1927. The plots are separated by 1500 m and lie uphill from two different water bodies Lincoln Pond (LP) and Lake Myosotis (LM). Locations of all trees >10 cm dbh were mapped. 

Radial growth of trees during 2013-2017 was monitored with six ICT high resolution ( ± 0.001 mm) two-point dendrometers at 30 min intervals. These records were averaged over 24 h intervals to match the resolution of weather data. In addition to radial growth, every spring we recorded the date of budbreak. This final stage of bud break was estimated directly by visual inspection of the spruce canopy as the day when bud scales burst and fresh needles became visible.

Volumetric soil moisture content was measured in the top 10 cm of soil profile in 2016 at LP and LM sites and in 2017 at only LP site. These measurements were derived from 5 “wave” soil moisture sensors (MP3063, ICT International), with 3 sensors installed at the LP site and 2 sensors at LM site. 

The rate of radial growth was modeled with partial least square regression (PLSR) with a nonlinear iterative partial least squares (NIPALS) algorithm that uses the Variance Inflation Factor (VIF) to measure the impact of collinearity among the variables in a regression model.