Key—second midterm
geog. 4371
1. In Veblen 1992 (pp. 168-174) I discuss the characteristics of treefall gaps that may be
important in creating different regeneration niches. List and briefly explain the importance of
the 4 broad categories of treefall gap characteristics that may create different regeneration
niches. 8 pts.
See pp. 168-174 for discussion under the following 3 categories:
Size, shape, and orientation
Gap size is related in that big trees may knock other trees down too (in a domino-like effect).
The availability of tree propagules and encroachment respond to treefalls may vary across gap
sizes.
The micro climatic conditions of gaps are size-, latitude-, shape-, and orientation-dependent.
By these way, the amount, intensity, and fluctuation of solar radiation that can reach the
interior of the gap, which subsequently determine the environmental conditions in terms of
microclimatic conditions (i.e. temperatures in the interior of the gap and at the soil surface; the
amount of moisture that is available for plant –within the gap and at soil surface—). These gap
characteristics may also influence the amount of precipitation that affect the quantity of
nutrients that are leached through the soil surface, and therefore the nutrients availability in the
soil for plants. At the same time, an environmental gradient between the center and the
periphery of gap may ameliorate the degree of intensity.
Time and periodicity of gap creation
The time and periodicity of the gap creation may be important for the timing of propagule
dispersion that may influence the recruitment rate potentiality of the different species. The
periodicity of gap formation may influence the species composition and degree of shadetolerance of the dominant species within the forest.
Influences of gap-creating species
Tree species may also influence the species composition of the understory by affecting their
recruitment, growth and survival rates. Therefore, a “relationship” between the mature
overstory species and the emerging understory seedlings may be possible to exist (i.e.
patterns of mature tree death, indirect biological interactions).
Environmental heterogeneity within the gap
All the gap characteristics mentioned above, including the effect of the treefall itself, increase
the spatial heterogeneity within the gap. The trunk of fallen trees may provide differences in a
variety of aspects (access to solar radiation, decaying rates that results in different
establishment rates, pathogens susceptibility, oxygen availability, dispersal symbiotic
relationships, drought suitability, litter allelopathy, and nutrient availability that encourage
regeneration for some species. The pit and mount created by the uprooting of trees expose
both mineral and upturned soils respectively.
Whereas these micro conditions may promote successful regeneration for certain species,
they also may inhibit the regeneration of others. In another words, the micro heterogeneity
allows for niche differentiation that often results in the specialization of species to specific gap
characteristics and hence, in species coexistence.
2. Explain the difference between Connell and Slatyer's "tolerance" and "facilitation"
mechanisms of succession. 2 pts.
Facilitation--early colonists modify site so that it becomes less suitable for subsequent
recruitment of early successional species, but more suitable for later successional species
Tolerance--early colonists modify site so it becomes less suitable for subsequent recruitment
of early successional species, but this has little or no effect on subsequent recruitment of later
successional species. so juveniles of later successional species tolerate the environmental
changes caused by the early colonists.
3. Very briefly, indicate how Clement's "organismal" view of the plant community is different
from Gleason's "individualistic concept of the plant association." 2 pts.
Clements— communities are highly organized and integrated (one genetic unit as a whole);
discrete, repeatable, and highly predictable (deterministic) units of vegetation.
Gleason— associations are loose (random) assemblages of species; chance is important in
determining species composition; not much interdependence among species; associations are
not highly organized or integrated; each species is independently distributed and associations
occur where their distributions overlap.
4. Explain the difference between the catastrophic regeneration mode and the continuous
regeneration mode. 1 pts.
Catastrophic regeneration follows large-scale disturbances; even-aged populations of shadeintolerant trees (fits early stages of Oliver’s model of whole stand replacement).
Continuous regeneration does not depend on disturbance; highly shade-tolerant species (in a
strict sense this is relatively rare).
5. Define "equilibrium model" as applied to successional theory. 1 pt.
It assumes long-term site stability of the environment (physical site, including climate) during
which succession progresses towards a stable endpoint.
6. Does the use of “subclimax” imply adoption of polyclimax theory or of monoclimax theory. 1
pt.
Monoclimax theory
7. Briefly explain why tree mortality rates should not be inferred from static age structure. 1
pts.
A static age structure is based on determining plant ages at one point in time rather than
following cohorts. In contast to a dynamic life table based on following a cohort over time, in a
static age structure table it is not known how much of the variation in each age class is the
result of past variation in tree recruitment as opposed to variation in mortality. Thus, it should
not be used to infer tree mortality rates, because these time-specific life tables assume that the
chances of recruitment and mortality are independent of the specific period of time.
8. In the modern hierarchical framework of causes of succession proposed by Pickett et al.
1987 (table 17-1 in Kimmins) what are the three “general causes of succession” and briefly
explain what is meant by “defining factors”. 2 pts.
1. Site availability (for invasion) due to coarse-scale disturbance.
2. Differential species availability due to dispersal differences, seed banks, bud bank, etc.
3. Differential species performance (i.e. ecophysiology, life history, competition, allelopathy,
etc.)
The defining factors refer to the particular factors that determine the outcome of the
intermediate-level processes and are discernible or quantifiable at specific sites. These are
the same as “initial conditions” and describe the local conditions of a particular site or
ecosystem.
9. List four advantages of using steady-state terminology instead of climax terminology to
describe compositional equilibrium. 2 pts.
No theoretical or causal implication
No implication of convergence towards a single climax type
Can by applied to overstory separately from understory
Can be applied as specified spatial and temporal scales
Can incorporate period, repeated disturbance
10. Explain the difference between realized niche and fundamental niche (Kimmins pp. 428429). 1 pt.
Fundamental niche is the maximum niche that the species can occupy in the absence of
competition from other species.
Realized niche is that portion of the fundamental niche a species does occupy because of
competitive exclusion from the rest of the fundamental niche.
11. In the context of the gap-phase mode of regeneration, explain the difference between the
sub-modes of “new recruitment” versus “reorganization.” 2 pts.
New recruitment refers to the establishment of new individuals from new seed dispersal and/or
new seed bank that arrived only after the tree-fall gap was formed.
In contrast, the reorganization sub-mode refers to a response—usually as release in heightgrowth—of “advance regeneration” that was established prior to the tree-fall gap. It includes
seedlings, saplings, vegetative shoots, growth of non-tree species, and lateral encroachment
from surrounding mature trees.
12. Explain the difference between a Deevey type I and type III survivorship curves. 2 pts.
Type I: low mortality, then synchronous collapse of the entire population (e.g. annual plants,
some bamboo spp, etc.)
Type III: Initial high death rate of juveniles followed by a longer period of time with lower
mortality rate of mature organisms (represented by a power curve model; e.g. most trees
under “normal” conditions).
13. List the 8 descriptors (or parameters) of a disturbance regime. 4 pts.
Spatial distribution
Frequency
Mean return interval
Size of area disturbed per event
Predictability (or variance of mean return interval)
Rotation period (or turnover time or disturbance cycle)
Magnitude (intensity or severity)
Disturbance interactions (synergism, timing of disturbances)
14. Explain the difference between a genet and a ramet. 1 pt.
Genet = a unit (plant) in the population arising from a seed from sexual reproduction.
Ramet = a unit (plant) in the population arising from vegetative reproduction; ramets (even
after being separated from parent plants) are genetically identical. Group of ramets from a
single parent (i.e. a clone).
15. Explain what F. Egler meant by “relay floristics” vs. “initial floristic composition” patterns of
succession. 2 pts.
Relay floristics model emphasizes reaction and seral stages resulting from successive waves
of plants occupying the site.
In contrast, the initial floristic composition model stresses the individualistic concept of the
plant association of H. Gleason and the importance of differential growth rates and longevities
of species. Many of the species that are dominant in later seral stages may be present early in
succession but may be less abundant or grow more slowly.