Responses of Northern U.S. Forests
to Environmental Change
Chapter 7: Tree Health and Physiology in a Changing
Walter C. Shortle, Kevin T. Smith, Rakesh Minocha,
Subhash Minocha, Philip M. Wargo, and Kristiina Vogt
Tree rings are good measures of growth and indicators of health.
Tree ring characteristics are a composite response to intrinsic
and extrinsic factors, and although growth is integrative, it is
restricted by the essential factor that is most limiting in supply.
Trees sampled at or near the edge of their range are likely to contain
a stronger common signal, especially of climate. Given these principles,
the use of tree rings as a proxy record of climate or environmental
disturbance depends on uniformity of linkage between external conditions
and tree biology. Separating the signal of external and internal
factors in the tree-ring record can be difficult.
Based on a large sample of red spruce tree rings, enrichment of
Ca and Mg evident in stemwood formed in the 1960's is consistent
with the mobilization of base cations in the soil, which also coincides
with increases in the atmospheric deposition of nitrates and sulfates.
Root physiology and pathology are affected indirectly by acidic
deposition, through changes in soil chemistry and through changes
in carbon allocation patterns to the roots. The Ca/Al molar ratio
of the soil solution seems to be one of the prime mechanisms by
which acidic deposition affects forest growth and is an important
indicator of potential stress.
The availability of biological markers that can assess the current
status of stress in apparently healthy trees in a forest is crucial
for planning a potential treatment or management practice for mediating
or removing the stress. In conjunction with soil chemistry, putricine
and/or spermidine may potentially be used as early indicators of
Al stress before the appearance of visual symptoms in red spruce
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