Geog. 4371
Take-home test, Sept. 29, 2005: 30 points
This take-home test is open book and open notes, but you cannot consult with anyone
about the test.
The total length of your test cannot exceed 5 pages of double-spaced text (i.e. not longer
than about 1250 words). Use the points allocated to each question as a guide to the
length of your answers.
The test is due at the beginning of class on Tuesday, Oct. 4.
1. In temperate rain forests, seedlings of some tree species often establish on logs. For
example, some western hemlock in the Pacific Northwest establish on logs and grow to
maturity on these sites. Similarly, some Nothofagus species in New Zealand and in
Chilean temperate rain forests establish as seedlings on logs or stumps from windsnaps
and grow to maturity. Apparently, either the rate of seedling establishment is greater or
the rate of seedling survival is greater on these log and stump sites than on the forest floor
(i.e. on the litter covered soil).
Use the concept of the operational environment to discuss the environmental
differences between the two types of sites (on logs vs. on the forest floor) which might
account for the differences in seedling abundances. Which factors of the operational
environment are likely to be critical? How do they differ between the two types of sites?
Do not indiscriminately list all the factors of the operational environment. Instead,
explain why you think certain factors may be the most important in affecting seedling
establishment or survival. Remember to refer to the direct factors of the operational
environment. 10 pts.
The most likely factors include:
a- Elevation to a zone of higher solar radiation (above understory competitors). This
difference affects the amount of NPP (via photosynthesis) and subsequent metabolic
processes (such as the shoot/root ratio) and weaknesses against pathogens.
b- Reduced fungal pathogens on logs compared to the litter-covered forest floor (In
general, there is smaller temperature oscillations and higher soil moisture on the
forest floor).
c- Soils may be waterlogged which reduces soil gases (oxygen).
d- Logs could be richer in nutrients (soil chemicals); litter accumulation may
negatively affect nutrient availability (depends on type of litter).
e- In cool climates, minimum air or soil temperatures may be lower on the forest
floor.
f- Reduced seed predators (herbivores) on logs compared to forest floor.
g- Animal seed vectors (rodents, birds) may perch or next on logs and disperse seeds
to log sites.
h- Even in a rainforest environment, occasional drought may cause forest floor litter
to dry out whereas logs retain moisture better.
i- Thick layer of slow decaying litter on the forest floor (wet and cool = slow decay)
can prevent the seedlings from rapidly developing root contact with mineral soil with
adequate soil moisture and nutrients.
There are other possible but less likely factors such as:
i- Fewer allelochemicals (soil chemicals) on the logs than in the litter (not too likely
in wet climates).
j- Surface fires could kill seedlings on the forest floor (not likely in rain forests).
k- Etc.
2. After logging of Douglas-fir forests in the Pacific Northwest (PNW), foresters often
burn slash as a treatment to make the site more suitable for tree seedling establishment
and survival. Use the concept of the operational environment to identify the changes in
the environment caused by burning that are most critical to enhanced seedling
establishment and survival. 5 pts.
The key factors are:
1. removal of slash decreases shade, which increases solar radiation. This removal
also increases the temperature oscillations (warmer during the day and cooler during
the night).
2. burning of litter decreases fungal pathogens (damping off fungi).
3. removal of litter exposes bare mineral soil which usually facilitates access to soil
moisture by seedlings that otherwise would have to grow longer roots to penetrate
thick litter that can easily dry out.
4. burning sometimes controls weeds (competitors) in the context of the Operational
Environment.
5. nutrients released by decay of ash.
6. burning can lower the risk of future low severity fires that could kill the seedlings.
3. Some ecologists try to predict future vegetation patterns of North America expected as
a result of global warming associated with elevated carbon dioxide in the atmosphere
based on existing relationships of plant species distributions to climate. In particular,
monthly and seasonal temperature and precipitation are analyzed for the distribution of a
particular plant species or community type. Then, a model of future climate is used to
create a map of future vegetation patterns based on the relationships of the current
vegetation to the current climate. Critically evaluate this procedure from an ecological
perspective. In other words, assume that climate can be accurately predicted and just
focus on the prediction of future vegetation patterns from temperature and precipitation
data. 5 pts.
The key factors are:
a. It assumes that the species and species’ ecotypes are adapted to photoperiod
(latitude controlled) of the areas of newly suitable temperature.
b. It assumes that migration rate is not limiting—not all species will migrate at the
same rate and therefore new associations not represented by current community
c.
d.
e.
f.
types will be created. Migration rates may be intrinsically low, or existing
fragmentation of natural vegetation may create dispersal barriers that cannot be
overcome.
It assumes soils will be suitable but in fact, soil formation is a slow process and
may limit or alter the species which can establish.
It ignores possibly new aspects of disturbance regime (insect outbreaks, droughts,
fires, direct human action, etc.)
It ignores the micro-topography that could result in different species composition.
It ignores changes in the influence of chronic, long-term human action (pollution).
4. Critically evaluate the following statement: “If we know the photosynthetic light
saturation curve for a tree species, we should be able to predict the success of its
regeneration beneath a forest understory. “ 2 pts.
No—the photosynthetic light saturation curve certainly improves our understanding
regarding the potential capability of a given tree species to establish, growth and
survive beneath a forest understory. As shown by the classic trench experiments
other factors besides solar radiation often determine survival of tree seedlings in the
understory. These factors include soil moisture and nutrients (and competition for
these resources) and fungal pathogens in many or most cases. Other factors that
might be important for particular species could be herbivore impacts associated with
the understory environment, micro-site conditions required by certain tree species
(regeneration niche).
5. What are the genetic considerations in the design of forest pest control strategies? 3
pts.
There are several considerations related to the strategies. Most importantly, the
strategy has to consider the rates at which the plant and pest populations
reproduce. In contrast to tree species, insect pests have short generations and
therefore can quickly develop genetic adaptations to chemicals or other control
measures.
6. How can you predict whether wind will be a limitation on how a forest is managed?
3 pts.
- If the area is known to be windy, like most rain-shadow areas at mid-latitudes (e.g.
Front Range area), at the bottom of a mountain valley, at the ocean shore, or areas
affected continuously by Chinook winds or rarely by very strong winds (e.g. Routt
blowdown) wind will certainly have an influence on the germination, establishment,
and survival of tree seedlings. Foresters will have to adapt their management strategy
in order to cope with the effect of wind.
-Managers should recognize the different susceptibility of different species to the
effect of wind. Thus, if the management objective is to produce aspen, then wind is
much less of a consideration than if the objective is to produce lodgepole. For
example, in the Routt blowdown, aspen was less affected than the conifers.
-Managers should also recognize the different susceptibility of different stand
structures to the effect of wind (Even-aged stands are more susceptible than unevenaged stands to the effect of wind).
- If foresters are managing a windy area, the shape of the clearcuts is crucial for the
resulting type of airflow, which in turn will affect differently the neighboring stands.
A cone-shaped opening with the narrow end toward the prevailing winds will reduce
the risk of wind damage to the stands that are adjacent to the clearcuts. Clearcuts
promote turbulent airflow, which enhances the rocking motion. This reduces the
friction between roots and the soil, which is damaging for trees, more effectively than
laminar-unidirectional wind.
-In general, studying the climate and weather (wind maps) of the area planned for
management may provide useful hints of the wind conditions when choosing a site.
A reconnaissance in the field may give more insights about the effect of wind on the
vegetation. Morphological responses are the most easiest to recognize this effect: the
general physiognomy (i.e. narrow, tall trees, and limited root systems), flagging,
windsnap, uprooting, wind tatter, branch breakage, leaning, dwarfing, bole shape and
taper, and more swaying results in more root growth and buttresses. Additionally,
tree rings may show prolonged unidirectional growth or compression wood
(gymnosperms) or tension wood (angiosperms).
7. What role might fire play in maintaining the health of a forest? 2 pts.
Two key factors are:
a. Fire creates a range of stand ages…resulting in a more diverse landscape…with
benefits to wildlife and potentially reduction in the hazard of insect outbreaks and
possibly less extensive future fires…this depends on scale and severity of fire.
b. Fire creates opportunities for regeneration of some species that might decline or
disappear without fire.
Third factor could be:
c. Fire releases nutrients and accelerates biogeochemical cycling, often resulting in
increased primary productivity.
Or
d. Fire can kill fungal pathogens and diseases that could attack trees.