Tree seedling growth and mortality responses to manipulations of calcium and aluminum in a northern hardwood forest

To assess potential forest compositional responses to exchangeable soil calcium (Ca-exch) and aluminum (Al-exch), we characterized light-dependent growth and mortality of tree seedlings under amendments of CaCl2 and AlCl3 at Hubbard Brook Experimental Forest (HBEF), New Hampshire, U.S.A. Seedlings of Acer saccharum Marsh., Fagus grandifolia Ehrh., Betula alleghaniensis Britton, Abies balsamea (L.) Mill., and Picea rubens Sarg. were transplanted into field plots, which were randomly assigned to control, CaCl2, or AlCl3 treatments and stratified across <1 to 35% full sun. Acer saccharum and P. rubens exhibited significantly higher mortality in Al-amended than Ca-amended or control plots. Acer saccharum showed significant increases in relative diameter growth in Ca-amended plots versus controls; all other species showed nonsignificantly higher relative diameter growth under Ca amendments. We incorporated significant seedling responses into a model of forest dynamics (SORTIE) to assess potential changes in species composition under Al-exch increases and Ca-exch losses. SORTIE predicts that further increases in Al-exch would have negligible effects on canopy composition within 200 years but that the estimated Ca-exch depleted from HBEF between 1968 and 1995 and its influence on seedling dynamics could lead to substantial decreases in A. saccharum canopy dominance within a single forest generation (<125 years).