Climatic Inferences from Dendroecological Reconstructions - Dendroclimatology - page 277

and with careful consideration of available evidence, annual or near-annual reso-
lution of death dates can be obtained for examination of stand- to landscape-scale
mortality events (e.g., Margolis et al. 2007 ) .
Another requirement of tree demographic studies is an adequate sample size.
However, no studies to our knowledge have addressed how many trees may be
needed to adequately characterize population age structure in varying forests, and
more work is needed in this regard. In old, uneven-aged forests, sometimes hun-
dreds of trees must be sampled to obtain adequate characterization of age structure
distributions. This requirement is mainly due to the decline in survivorship of trees
as they age (often following an inverse J-shaped curve). This decline in quantity or
quality of evidence with increasing time before the present is what has been referred
to as the 'fading-record' problem, and it is common to paleoecological studies. The
oldest trees that represent germination pulses, or 'cohorts,' in the earliest periods
may not be detected unless numerous trees are sampled, but again, few studies have
addressed this question in any type of systematic manner (but see Johnson et al.
1994 ) . Furthermore, many studies have found relatively poor relationships between
tree size and age. Therefore, some sort of systematic sampling procedure should
be used to select trees for aging rather than merely selecting the largest trees in a
stand.
Similar to tree recruitment dates, preservation of dead trees is a declining func-
tion with time before present, and obtaining estimates of past mortality events
depends both on persistence of woody material and the ability to adequately sample
the material to obtain death dates. Old forests usually contain complex recruitment
and mortality patterns, and to temporally resolve the different pulses and hiatus peri-
ods of establishment and mortality events often requires very large sample sizes and
careful sample techniques, particularly with sampling difficulties of obtaining the
bark rings on dead trees.
However, despite these and other difficulties in obtaining high-resolution tree
demographic data, concentrated efforts have resulted in very useful time series for
evaluating population dynamics related to climatic influences. One of the most
successful of these studies to date is the work of Villalba and Veblen ( 1997a )
on Austrocedrus chilensis woodlands in Argentina. They demonstrated a clear set
of linkages between favorable moisture conditions promoting regional tree cohort
establishment, and unfavorable (drought) conditions promoting reduced tree estab-
lishment and mortality (Fig. 9.7 ) . Another successful example is a recent study by
Brown and Wu ( 2005 ) that illustrated the contingent effects of climate and fire
occurrence on tree establishment in a ponderosa pine ( Pinus ponderosa) forest in
southwestern Colorado (Fig. 9.8 ) .
Examples of regional- to landscape-scale tree natality, mortality, and climate
associations found by Villalba and Veblen ( 1997a ) , Brown and Wu ( 2005 ) , and
Brown ( 2006 ) inspired us to undertake a 'meta-analysis' of regional tree establish-
ment data in ponderosa pine forests from the southwestern United States. For this
analysis, we compiled data from 12 studies that sampled hundreds of ponderosa
pine trees distributed around the Southwest (Fig. 9.9 ) . Some of these studies esti-
mated stand densities (number of stems or trees per hectare) from a sampling of
trees, while others reported total counts of trees establishing by date or period over