| Fir waves are spatially and temporally repeating patterns of stand development and destruction in subalpine, balsam fir-dominated forests of the northeastern United States, perpetuated by downwind migration of strips of dying trees, "dieback zones," through mature stands at 60-80 yr intervals.;I hypothesized that variations in annual carbon balance (the difference between foliar carbon assimilation and woody tissue carbon respiration) underlie the fir wave phenomenon. This balance varies in response to age-related changes in canopy structure and to mechanical foliage loss in dieback zones due to winter rime ice deposition, declining gradually with age following canopy closure, then dropping rapidly to zero within dieback zones, causing tree death.;Field observations supported in most respects the postulated structural trends. A computer model linking canopy structure and growing season climate to gas exchange responses of balsam fir predicted a rapid increase in annual carbon balance to a maximum at canopy closure, followed by a gradual decline in mature stands, with the exception of a large but transient drop at the age of maximum leaf area index caused by inadequate model treatment of light extinction through the canopy. In the dieback zone, carbon balance increased slightly.;Qualitatively, the model results supported some predictions of the hypothesis but did not support others. Quantitatively, the model was not validated because it predicted annual carbon uptake insufficient to meet the carbon demands of net primary productivity, except in the oldest dieback zone stands. Therefore, model results could not support or disprove declining carbon balance as the cause of tree mortality in fir waves.;I studied potential causes of fir wave tree death at 1220 m elevation on Mt. Moosilauke, New Hampshire. Old age, insects, fungi, nitrogen growth limitation, and summer water stress were not contributory factors to tree mortality. |
