Prospective students could develop any of the ideas described below
using data I have from Alberta, using a variety of publicly available
GIS and paleo databases, or using data they collect from a geographic
area of specific interest.
Reconstructing histories of fire, climate and vegetation
I have made advances in four different methods of reconstructing the
ecological past.
Fire
The ability to reconstruct pre-historic fire frequency is difficult in
areas where large intense fires kill most of the vegetation in their
path and thus leave few records of the fire”Ēs occurrence. I have helped
develop and refine methods of analyzing tree-rings and lake-sediments to
address this problem. My broad background in fire reconstruction allowed
me to coauthor a review paper on this topic that will be published in
what should be a standard reference book for paleoenvironmental studies
(Whitlock and Larsen 2001).
Climate
I found that annual tree-ring widths in northern Alberta were more
strongly related with seasonal averages of fire-weather indices than
with mean monthly climate (Larsen and MacDonald 1995). The tree-ring
records I created were thus useful for relating to the life-table
reconstructions of decadal-scale variations in fire frequency with
variations in the summer average in the fire weather. In Stuart-Williams
et al. (submitted) we have shown that oxygen isotopes from the calcite
fraction of annual laminae can provide a record of mean summer
temperature with a sub-decadal resolution. This 227-year record shows
that the temporal pattern of climate change in the central boreal forest
was no different from that in the arctic. I have submitted a NSF grant
to extend this record to the past 2000 years. I have recently found that
changes in the pollen accumulation rate of all terrestrial plants
evidences centennial-scale climate cycles that have been observed in
arctic regions (Larsen and Morris in prep.). This finding refutes the
general belief that, because of inertia in vegetative responses to
climate change, pollen records are only useful for reconstructing
climate changes that occur over periods of 1000 years or more.
Sediment mixing
Since paleoenvironmental research is best performed on lake sediments
that are little mixed, and especially on ones with annual laminations, I
compiled and developed heuristics that related the surface area and
maximum depth of a lake with the degree of sediment mixing (Larsen and
MacDonald 1993). These heuristics, though not deterministic (Larsen et
al. 1998), aid the identification of lakes with minimal mixing and the
exclusion of ones with excessive mixing. Although massive
(non-laminated) sediments are mixed, I have found that if sampled at an
approximate 5-year resolution they can still exhibit a relatively
unsmoothed stratigraphy that evidences fire events and forest succession
(Larsen and MacDonald 1998b). This usefulness appears to be lost in
shallower lakes as their sediments are more mixed (Larsen 1994).
Sediment aging
Radioisotopic methods allow massive sediments to be absolutely dated
with a decadal-scale error over the last 150 years, but with only a
centennial scale error during less recent periods. One method that has
been suggested to allow sub-centennial scale dating of massive sediments
is the slotting of a record of geomagnetic secular variation contained
in its sediments with an absolutely dated record of them from annually
laminated sediments. This method has not been applied to recent lake
sediments because it has not been possible to recover undisturbed recent
sediments. I applied two methods for obtaining undisturbed sediments and
have shown that sub-centennial scale changes in the geomagnetic
declination are recorded in lake sediments, and that the changes from
laminated sediments could potentially be used to absolutely age massive
(non-laminated) sediments (Larsen et al. 2000).