Fire and plants 2

1

• Fire and plants (2)
• Exploiting post-fire conditions
• Post-fire conditions are often quite different
  from pre-fire conditions, due to soil heating,
  nutrient changes, litter consumption, plant
  mortality, exposure to sun wind (from canopy
  mortality).
• A variety of plant responses to postfire
  conditions include
• increased productivity
• increased flowering
• increased seed dispersal
• synchronous release of canopy seeds
  (bradyspory/serotiny)
• germination of soil stored seeds
• synchronous sprouting
• improved seedling establishment.

2

• A. Productivity increased vigor and growth rate
  is commonly observed after fires.
• Several possible reasons
• warmer soils (more heat per day, longer growing
  seasonneed to consider interaction w/ moisture,
  however)
• increased available nutrientsinorganic formsin
  contrast to nutrients bound in biomass before
  burning
• thinning of competing vegetation in the
  understory and/or overstory
• removal of accumulated litter, esp. in grasses
  where litter can block light (clipping/grazing
  can have similar effects)
• consumption of allelopathic chemicals. Another
  way to look at productivity increase after fire
  is that the unburned state may actually reflect
  decreasing productivity.

3
Peterson, D.L., S.S. Sackett, L.J. Robinson, and
S.M. Haase. 1994. The effects of repeated
presecribed burning on Pinus ponderosa growth.
International Journal of Wildland Fire
4(4)239-247.
4
Basal Area control 40, both thinned treatments
12-14
Jason Kaye et al. (in review), Restoring
ecosystem function to ponderosa pine forests
after 120 years of fire exclusion.
5

• B. Flowering patterns
• Large-scale post-fire flowering is often
  reported (maybe relatively easy to notice),
  another patterns is not a single flush, but
  generally increased flowering for several years
  after fire.
• Mechanisms may be the same as productivity
  (vigorous plants flower more) may also be a
  pruning effect where fire damage stimulates
  flowering through plant hormones.
• C. Seed dispersal distances can increase as wind
  and water flow often increase following fire.
  Seedling establishment can be facilitated by
  litter shade removal.
• D. Synchronous release of canopy seeds
  (bradyspory/serotiny) caused by heating of sealed
  cones, produces a synchronous flush of seeds.

6

• E. Synchronous germination of soil-stored seeds
  
• Seeds stored in the soil seed bank germinate in
  response to environmental cues (light, moisture)
  and/or breaching of the seed coat to allow water
  to enter.
• Post-fire, many seeds may have been destroyed by
  heat, but the environment has changed litter
  removal and mortality may allow more light, heat,
  and moisture to penetrate (3 rain needed to wet
  thick ponderosa pine forest floor).
• Heat itself can degrade the seed coat and thus
  allow germination (but there is a balance between
  heat intensity duration vs. germination/mortalit
  ytoo cool, not much germination, too hot, not
  much survival).
• Chemicals in charcoal/ash may induce germination
  in some species (see Keeley/chaparral).
• Germination may not be due to fire directly, but
  be related to a post-fire flowering flush,
  resulting in more seeds.

7
Seed Germination Following Exposure to Powdered
Charred Wood (selected spp., ambient temp, in
light)
Keeley, J.E. 1987. Role of fire in seed
germination of woody taxa in California
chaparral. Ecology 68(2)434-443.
8
F. Simultaneous suckering or sprouting is
response of many plants to topkill, also leads to
even-aged stand (e.g., aspen, oak).
Barton, A.M. 2002. Intense wildfire in
southeastern Arizona transformation of a Madrean
oak-pine forest to oak woodland. Forest Ecology
and Management 165205-212.
9

• G. Increased seedling establishment
• Related to many of the same factors as increased
  productivity of survivors less litter, more
  nutrients.
• Suggested that herbivores, parasites, fungal
  diseases may be reduced (sterilizing effect of
  fire).
• Seed predation may decrease because seed
  predators may be satiated when many seeds are
  available simultaneously, allowing more seeds to
  germinate.

10

• Plant populations
• Factors that influence populations include
• density-dependent interactions within the
  population (intra-specific competition)
• density-dependent interactions with external
  factors (herbivory, inter-specific competition)
• other influences (disasters, disturbance,
  climate).

11
Census is the basic method of studying
population. Censuses over time (diachronic or
longitudinal study), especially differentiating
among cohorts, give quantitative data for setting
up survivorship curves. Plants stand still to
be counted and do not have to be trapped, shot,
chased, or estimated (Harper 1977 quoted in
Whelan, p. 148).
12
Measuring effects of a fire on population
13
Population data in analysis of effects of fire
frequency in CA chaparral (Paul Zedler) one
fire in 1979 stimulated population increase, as
expected (sprouting, seed germination). An
unusual second fire in 1980 led to striking
population reductions because it topkilled
resprouting plants at a time when root reserves
and the soil seed bank were depleted in response
to the 1979 fire.
14
What is a community?
15
What is a community? A group of co-existing
populations. Two (caricatured) views on how
they coexist (1) chance overlap of the ranges
of various populations, relatively random and
variable assemblage of species, (2) discrete unit
of organization, co-evolved and interacting.
There is merit to both views, examples of both
extremes. In a series of studies on the Santa
Catalinas (Tucson), Whittaker Nearing suggested
that plant species were distributed mostly
independently along an elevational gradient.
Alternatively, there are obvious tight links
between plants their mycorrhizal fungi, some
pollinators and their flowers (e.g., Ipomopsis
coloration/Ken Paige), herbivores and their
substrate, etc. Whelan distinguishes two
approaches to community studies (1)
reductionistic looks at specific interactions
between populations (e.g., already discussed pine
vs. oak in Barton 2002). (2) Holistic looks at
emergent or comprehensive community properties,
such as species richness or trophic structure.
16
Species richness changes are often studied after
fire, but can be complex species can be
replaced without changing richness (hence
fugacity index to look at spp. turnover)
natives can be replaced by immigrants, esp. those
with good dispersal adaptations (wings, awns)
new spp. can be from soil seed bank or
belowground rootswere they already part of the
community before fire? or do only aboveground
spp. count? species richness may vary with
quadrat size, sampling techniques. Species
composition changes are often studied in terms of
time since fire. Good example in SW is shift
from ponderosa pine to white fir and Douglas-fir
(more fire-susceptible species, esp. when small)
in the absence of fire. Biomass and other
changes in vertical and horizontal structure can
be compared.
17
Crawford, J.A., C.-H. A. Wahren, S. Kyle, and
W.H. Moir. 2001. Responses of exotic plant
species to fires in Pinus ponderosa forests in
northern Arizona. Journal of Vegetation Science
12261-268.
18
Fulé, P.Z., W.W. Covington, H.B. Smith, J.D.
Springer, T.A. Heinlein, K.D. Huisinga, and M.M.
Moore. 2002. Testing ecological restoration
alternatives Grand Canyon, Arizona. Forest
Ecology and Management 17019-41.