Patterns of susceptibility to soil frost and the hydraulic properties of yellow birch (Betula alleghaniensis Brit.) stands over natural gradients in a northern hardwood forest

Background/Question/Methods

This study is important in determining if and/or how winter climate change (reduced snowpack and increasing soil frost) is currently affecting yellow birch distributions in the northern hardwood forest. We established plots over north and south-facing elevation transects as a gradient of susceptibility to winter soil frost induced root damage in yellow birch stands at the Hubbard Brook Experimental Forest (HBEF) during the growing season of 2010 and 2011 in central New Hampshire. Air temperature and snowpack data were used from the HBEF hydrometeorological database from weather stations near our plots as a proxy for determining vulnerability differences and frequency and intensity of winter thaw-freeze events for each year.  We compared average root percent embolism and sap-flux densities across plots to determine (1) if/and to what extent vulnerability to soil freezing  currently has on root embolism patterns under natural conditions, and (2) is root embolism a factor limiting sap-flux density (productivity)?

Results/Conclusions

The frequency and intensity of thaw-freeze events strictly based on air temperature for both years revealed that south-facing slopes were more susceptible to level 1 (109) and level 2 (6) thaw-freeze events compared to north-facing slopes (83 and 4 for levels 1 and 2 respectively). For both years, all north-facing plots had lower average root percent embolism than south-facing plots, with the exception of our north-facing mid plot in 2011.  All middle elevation plots in their class of aspect, had the greatest root percent embolism compared to their respective high and low elevation plots during both years. All plots with the greatest average root percent embolism exhibited the lowest sap-flux density (Js) in 2010 and 2011, with the exception of our south-facing high site in 2011 when compared to north-facing sites only for ordinal dates 200-203. The pattern of average root percent embolism over space supports the idea that susceptibility to winter thaw-freeze events greatly influences root health in yellow birch stands. Furthermore, plots with the greatest root percent embolism displayed the lowest sap-flux densities and visa-versa, supporting the idea that root embolism is disrupting the hydraulic pathway enough to reduce sap-flow. This means an increasing number of thaw/freeze events, due to warming winters, may incite crown dieback and be responsible for limiting future yellow birch distributions. This study raises interesting questions regarding future shifts in the distribution of the northern hardwood forest and interactions among ecotones in relation to future winter climate change in the northeast United States.