Transcript Shifts in treeline* at upper and lower forest borders

Geog696M: Geography and Dendrochronology
Fall 2008
Dendrochronology and Forest Demography
• Variability in spatial patterns of tree demography due to climate and disturbance as documented
with dendrochronology.
• Shifts in treeline* at upper and lower forest borders
• Changes in forest composition and density due to fire suppressing, grazing and other
disturbances,
• Patterns and process related to episodes of recruitment and mortality over past centuries to
millennia.
When and where to meet? This time/place OK?
Focus of class: upper, lower treeline?
Idea of publication of literature review? If so, what aspects should the review cover and can we
make an outline of the sections of the review, and select an order of readings?
Organization: generate outline that identifies topic areas of interest to be covered (for a review or
not); students take one or more topics, identify relevant papers to read, and lead weekly discussions.
*by treeline, I mean the limit of tree growth of any form; treeline sometimes means the limit of upright trees,
while timberline means the limit of tree growth
Large scale, region-wide treeline
related to climate
At a smaller scale, more spatial
variability obvious; tree forms,
disturbance agents, topographic
limits
Topographic and substrate constraints on
treeline
Variability in species, tree
growth form, changes over
time?
Disturbance at treeline or a climate
influence?
Changes at treeline; recent recruitment
above current treeline
Changes in treeline: Evidence of a higher
treeline in the past
Some General Treeline Topics (many of these overlap with each
other):
Causes and controls of treeline, physiologic, topographic, climate (including large
scale circulation)
Treeline ecotone dynamics, patterns, processes, feedbacks
Subarctic treeline, changes over recent times, role of climate
High elevation treeline, evidence for change, role of disturbance
Lower forest border treeline changes, esp. drought, changes in species, invasion into
grasslands
Paleo treelines and climate, evidence of past treeline
Other topics to consider:
Treeline and the forms it takes
 Upright trees
 Krummholz
 Tree islands
 Lower treeline characteristics
Influences on treeline – constant vs variable
 Topography (aspect, slope, top vs basin)
Possible to define and/or narrow
focus:
• Upper vs lower treeline?
• “Upper” as in elevation or
latitude?
• Types of influence – all or
narrow?
• Time scales of change?
• Locational focus?
 Substrate (soils, geology, frozen ground)
 Climate (for both recruitment and mortality); precip, snowpack and deposition, temperature, wind….
 Disturbance (fire, avalanche, drought, insects and disease, humans; inter-relationship between
these)
 Biological factors (seed crops, dispersal, recruitment and survival, competition)
Time scales and time characteristics of treeline change
 Short term (decadal), long term (millennial)
 Holocene; late, early, entire period, recent changes, human-influenced climate change impacts
 Trends vs extreme events
 Threshold responses vs gradual response
Spatial scales to consider: Micro-environments, Regional, U.S. west-wide,or broader,
circumpolor
Strategy for Structuring the Tree
Demography/Treeline Seminar/Literature Review
• 3 general background papers to get started
• Develop an outline of topics and/or a lit. review
• Students select topics of interest (several of your per topic area is fine)
and come up with papers for discussion
• Each week, we’ll discuss 2 papers
• If we go the literature review route, these papers can be the basis for the
review for this topic area, which will be written by discussion leaders .
* We have 14 weeks after today (I’ll be gone Oct. 20th)
Three paper to get us started:
Brubaker, L.B. 1986. Responses of tree populations to climate change.
Vegetatio 67, 119-130.
Overview of biological mechanisms controlling tree-population response to climate; general (not
specific to treeline) background on factors that influence populations and timing of changes, as
well a focus on treeline (high elevations and high latitude)
Smith, W.K., M.J. Germino, T.E. Hancock, and D.M. Johnson, 2003.
Another perspective on altitudinal limits of alpine treelines. Tree Physiology
23, 1101-1112.
Review of hypotheses for timberline causation from a physiological perspective, and a new
hypothesis regarding the role of tree establishment based on microsite factors.
LaMarche, V.C., 1973. Holocene climatic variations inferred from treeline
fluctuations in the White Mountains, CA. Quaternary Research 3, 632-660.
Classic paleoclimatic study of treeline changes over the Holocene, including a global literature
review, and a discussion of the role of climate as a control on treeline. Long paper, but lots of
detail on site, methods.
Today:
• Time and place to meet each week
• 3 papers for next meeting on web site, password protected (geog696m)
• 3 volunteers to do brief overviews of these papers (20 minute max);
everyone else come with questions
• Decide on literature review (yes/no?)
• Decide on focus for seminar (how can we limit it?)
• Outline/order for seminar topics – at least the first topic, and someone
(or more than one person) to select a couple of papers for discussion