Potential role of Ca in the response of paper birch (Betula papyrifera) to ice storm-induced decline in Vermont, USA

Background/Question/Methods In recent years, an increasing number of mature paper birch (Betula papyrifera Marsh.) in northeastern U.S. forests have exhibited decline symptoms. Visual indicators of this decline include a lack of foliage in upper crowns, limited fine branching, and tree mortality. A severe ice storm in 1998 adversely impacted crown conditions of many tree species in the northeast including paper birch, and may have served as a catalyst in the current decline. Tree species declines in the region are not novel and have been attributed to various causes, including the effects of acidic deposition and associated depletion of calcium (Ca) from soil pools. Despite region-wide exposure to acidic inputs, depletion of Ca has not been investigated in relation to birch dieback. The cumulative effects of continued acidic inputs, coupled with acute physical damage from the 1998 ice storm, have the potential to be significant factors in current decline of paper birch. In 2006 we assessed tree health and collected increment cores from paper birch throughout the Green Mountains, VT, to identify both the onset and severity of decline. We examined the role of Ca-depletion in current birch decline by analyzing cation content of soils from the forests evaluated.

Results/Conclusions Using increment core data, we found no significant differences among the ages of trees with different degrees of crown dieback - suggesting that age (natural senescence) was not associated with decline. The dendrochronological record indicated that decline originated in 1998 – the year of the ice storm. All trees (regardless of their current decline status) experienced reduced growth starting in 1998 and trees continued to decline in growth for the two years following the ice storm. A wide-spread drought in 1999 may have contributed to this prolonged growth decline. Ultimately, trees with reduced crown vigor continued to experience reduced growth, while those with healthy crowns were able to rebound to growth levels comparable to years prior to 1998. Soil Ca status was significantly associated with crown condition and growth – trees that rebounded in growth and crown health following the 1998 ice storm grew in soils with higher extractable Ca. Although previously overlooked as a factor influencing birch dieback and decline, our data suggest that Ca-depletion may predispose paper birch to decline following an instigating event – such as the ice storm of 1998. This predisposition could prove very significant to the health of the northern forest in light of predicted climate change.