fs6 fc0 sc0 ls0 ws11">Nevertheless, in this study the interpretation of results was
made considering that pseudoreplication is the testing for
treatment effects with an error term inappropriate to the hy-
pothesis being considered (Hurlbert, 1984). Although we must
avoid the use of “pseudoreplicant s”, if t he va riab ility wit hin o ur
“small experimental units” is considerable (as in the case of this
study) we can assume certain significance of the estimations
resulting from “pseudoreplicants”. Despite criticism for pseu-
doreplication, nonreplicated studies might nevertheless produce
strong ecological evidence (Millar & Anderson, 2004; Hawkins,
1986 [cited by Hargrove & Pickering, 1992]). However, such
significance is related not to the treatments, but to the locations
where the treatments were applied.
Methodology
Study Area
The study area was located at 5 km to the west of Tapalpa town
(Jalisco state), in the west-central region of Mexico (Figure 1).
This area is located within the 19˚56' and 19˚58' North latitude;
103˚47' and 103˚51' West longitude (Benavides, 1987). The
Tapalpa Saw has the following general characteristics (Martínez
et al., 1990): Altitude: 1900 - 2400 m.a.s.l. Mean annual rainfall:
883.1 mm. Mean temperature: 16.6˚C (Minimum mean annual
9.1˚C, Maximum mean annual 24.3˚C). This region corre-
sponds to a temperate sub-humid climate (Benavides, 1987), and
is dominated by Pinus michoacana, Pinus oocarpa, Quercus spp,
and Alnus spp. The study area was mostly on north-facing slopes,
at an altitude of 2110 m.a.s.l. In average, the slope varied be-
tween 15% and 25%.
Experimental Design
The set of data, used in this work, resulted from measures
taken before and after applying two methods of prescribed fires
(March, 1991). These two method s were defined according to the
direction of the fire in relation to slope: 1) backing fire, and 2)
head fire. The experimental design was based on nine sample
plots of 20 × 30 m, where both methods were applied in three
sample plots re spectively . The remaining three sa mple plots were
used as control. In order to analyze statistically the effect of the
treatments on regenerations, the sample plots were divided into
24 smaller “sample units” (5 × 5 m each), which were considered
as pseudoreplicants.
Seedling Evaluation
The following factors were measured for each seedling, within
each sample unit: 1) number; 2) height; 3) status [live or dead];
4) vigor; 5) color; and 6) damage. It was not possible to identify
the corresponding species. These measures were made three
times: 1) Before burni ng. It had the purpose to evaluate the ori-
ginal seedlings in the sample units; 2) After burning. The se-
cond measures were made three weeks after burning, where all
the individuals per sample unit were evaluated; and 3) After
two years. In 1993 every seedling was recorded in all the sam-
ple units.
Data Analysis
The data analysis had two objectives: 1) To find out if there
was any significant difference between the two treatments in re-
lation to the control; and 2) to determine if there was any signi-
ficant difference bet ween the tre at ments. T his was achieve d ba sed
on a completely randomized design. Where differences were ana-
lyzed considering the number of seedlings resulting from each
treatment and the control. For this, each sample unit was con-
sidered as a pseudoreplicant. This means a total of 72 sample
units (replicants) per each treatment and control. The division
of the sample plots allowed us to diminish the variance (Steel &
Torrie, 1960). Also, an exploratory statistical analysis was ap-
plied, in order to show some characteristic of regeneration.
-106 -105-104 -103 -102
-106 -105-102
22
21
20
19
22
21
20
19
-104 -103
Figure 1.
Approximated location of the Tapalpa Saw, in Jalisco state, Mexico, were prescribed fires were applied.
1
0 Copyright © 2012 SciRes.
J. G. F. GARNICA
Results
General Fire Behavior
The fire behavior during the burnings was rather unstable in
both prescribed methods. There was a considerable variation in
the height o f flames, which was due to the di fferences in the spa-
ti al distribution of fuel load s over the study , and changes of wind
speed and direction.
Backing fire (March 25th, 1991). Using this backing fire
technique, the speed of spread of fire was approximately 24
m/h, under a windless condition. However, there were some gusts
of wind of 3 km/hour (from the east) that increase a lit bit fire
rate of spread. The wind had not been blowing in the same di-
rection as the down-slope direction in which the fire was se t (north
aspect). The average height of the flames was 0.5 m, which means
low fire intensity (Fuller, 1991). The maximum height of flames
was 2.4 m.
Head fire (March 26th, 19 91). The rate of spread of fire was
almost ten times faster than in the backing fire (276 m/h). At the
beginning there were gusts of wind between 3 and 4.5 km/hour
(from the east). In general, the average wind speed was 5.5 km/
hour. However, wind had not been blowing in the same direc-
ti on. The average height of flame was 1.5 m, whi ch corresponds
to a fire intensity of more than 300 kW/m (Luke & McArthur,
1977). The rising of temperature resulted in an increase of wind
speed up t o 7.5 and 9 km/hour, which, at the same time, increased
the speed of spread of fire. This caused that flames sweep more
quickly over the ground level, resulting in a decrease in soil
temperature (Whitaker, 1961), which reduces the risk of im-
pairment of seedling. However, fire intensity varies according
to the amount, moisture content, and structure of the fuel
(Chandler et al., 1983a).
Original Seedlings before Burning
Seedlings that originally were only in the treated plots were
evaluated. Table 1 shows the average number of seedlings, and
mean height that were found within the treated plots, before
burning. T he general mean number of indivi duals (within 72 m2)
was 7.2, which is eq uivalent to 995 seedlings per hectare, within
a range from 417 to 4028 individuals. Although, on average,
there were more individuals in the head fire plots (HFPs) (1157/
ha, SE = 662) than in the backing fire plots (BFPs) (833/ha, SE
= 263), this difference was not significant (F = 0.189; FCrit. =
7.708 [P < 0.05]).
Table 1.
Number and mean height of seedlings per plot, before burning, for both
treatments. Also the corresponding number of seedlings per hectare is
shown.
Treatment Plot Number
of seedlings Mean of number
of seedlings/haMean
height
1 9.5 1319 31.5
2 3 417 26.7
3 5.5 764 26.2
Backing fire
Mean 6 833 28.1
4 1.5 208 23.0
5 6 833 19.0
6 17.5 2430 24.5
Head fire
Mean 8.3 1157 22.2
Total mean 7.2 995.2 25.2
The mean height was 25.2 cm, within a range from 15.1 to
43 cm. The mean height of individuals in the BFPs was 28.1
cm (SE = 2.69), while for HFPs it was 22.2 (SE = 1.64). The
difference between the BFPs and HFPs was not significant (F =
6.46; FCrit. = 7.708).
Number of Seedlings after Burning
Immediately after burning 93.6% of the existing seedlings
and root sprouts were dead. The mean number of seedlings per
plot (600 m2) was 34 and the standard error was 21.1. This is
equivalent to 561 seedlings per hectare, a quite lower value than
that recorded in the original regeneration. As was expected, the
mean height of the seedlings was very similar to that before
burning ( 26.8 cm). The ra nge in heig ht was 10 - 160 cm, wit h 20 -
30 cm being the modal height class. Only 10 of the 202 seed-
lings recorded in the burnt plots survived to fire. All of them
were located in plot 1, within the backing fire plot.
Two years later a census of all the plots was carried out (Fe-
bruary 12-24, 1993). Both live and dead individuals were re-
corded. Table 2 shows a summary of the results of this survey.
On average the BFPs had a mean of 86 seedlings per plot. The
head fire plots had a very similar mean of 88 seedlings per plot,
while the control plots had an average of only 34 seedlings per
plot. However, plot 9 (control) showed a high density of seed-
lings (88). This can be explained because this plot had suffered
certain perturbations, such as trees harvesting, which moved
litter off the forest floor, allowing seeds to come into touch with
mineral soil. This did not occur in the other plots; therefore this
plot (9) was excluded from other comparisons.
Comparison of Seedlings before and after Burning
Comparison between treatments. Figure 2 shows a graphi-
cal comparison be tween the original regeneration (1991) and the
re generation produced two y ears after burning (1993). There was
a notably higher mean density of regeneration after two years.
Both treatments show almost the same production of regenera-
tion. The density of seedlings was very much less in the control
plots. Table 3 summarizes the corresponding averages of rege-
neration before and after burning, per plot. No data were collec-
ted from the control plots before burning. However, it is expec-
ted that the number of seedlings was very similar than the esti-
mated two years after burning the treated plots.
Table 2.
Number of seedlings per plot and per treatment, two years after burn-
ing.
Backing fire Head fire Control
Plot 1 2 3 4 5 6 7 8 9
Total767111347142 75 13 1 88
Mean 86 88 34
SE 13.2 28.2 27.3
S/ha 1267 1183 1883783 2367 1250 217 171467
Mean 1433 1467 567
SE = Standard error; S/ha = Seedlings per hectare.
Copyright © 2012 SciRes. 11
J. G. F. GARNICA
Figure 2.
Comparison of the mean number of seedlings/ha before and after pre-
scribed fires, following two burning treatments. No data were collected
from the control plots befor e burning.
Table 3.
Number of live seedlings/ha before after prescribed fires and two years
after prescribed fires.
Number of seedlings per hectare
Treatment Plot Before
burning After
burning After 2
years
1 1319 167 1267
2 417 0 1183
3 764 0 1883
Backing fire
Mean 833 56 1444
4 208 0 783
5 833 0 2367
6 2430 0 1250
Head fire
Mean 1157 0 1467
7 *** *** 217
8 *** *** 17 Control
Mean *** *** 117
***Non sampled.
The next step in the analysis of regeneration was to test sta-
tistically if the application of prescribed fires leads to a signify-
cantly greater seedling density than the control. For this several
analyses of variance were carried out (Table 4). These analyses
compared seedling density before and after for both BFPs and
HFP. In both cases the difference was not significant (one plot
showing the opposite trend to the other two). Considering the after
burning condition, both treatments and the control were com-
pared. Comparing between the head fire and control plots only
the difference of seedling density was significant (P < 0.05),
while comparing between the backing fire and control plots this
difference was highly significant (P < 0.01).
Plot 9 represents another, unintentional, control disturbance
to the forest (trees and soil) without burning. It is notable that
the density of seedlings in plot 9 is very close to the means of
both burning treatments. Therefore the effects of the burning treat-
ments on regeneration may not be different from any other form
of severe disturbance.
Comparison within treatments. Although the comparison be-
tween treatments resulted in a significant difference, according
to the nested ANOVA, such differences are explained not only
by the treatments themselves, but also by the difference between
plots withi n the treatments. Table 5 summarized the correspond-
ing nested analysis of variance, where the difference between
plots were highly significant (P < 0.01), while the difference
between treatments were just significant (P < 0.05).
The variation among plots (within treatments) represents the
“experi mental error”, while the v a riation among pseudoreplicants
(within plots) corresponds to the “sampling error” (Steel &
Torrie, 1960; Snedecor & Cochran, 1976). In other words, the
former is the variation among pseudoreplicants in different
plots treated alike. That is, variation among plots within treat-
ments. The latter is the variation among pseudoreplicants
treated alike within plots. Therefore, since different treatments
were applied to different plots and, consequently, pseudorepli-
cants, the variation among pseudoreplicants will be present in
comparisons among treatment means.
Characteristics of the New Regeneration
Pattern of distribution. Figure 3 shows the pattern of dis-
tribution of regeneration. It is easy to see that in burnt plots
regeneration is more evenly distributed than in control plots. In
burned plots, the most abundant number of individuals per pseu-
doreplicant was 1 - 5, and the maximum was 12 for the head fi re
and 22 for the backing fire. Most of the pseudoreplicants of the
two undisturbed control plots had no seedlings.
Height of seedlings. The height of the seedlings varied be-
tween 5 and 75 cm. However, most of the seedlings were be-
tween 5 and 30 cm. The distribution of number of seedlings per
height class in the plots treated with backing and head fire was
very similar, with the mode at 15 cm and t heir tendency is rather
to a Poisson distribution (Figure 4). Also, control plots showed
similar distribution, which was less skewed, the mode is at 35
cm. The normality of the se distributions was teste d according to
their skewness and kurtosis (Norusis, 1993). The results veri-
fied that the tendency of such distributions was not normal.
Therefore, to carry out the necessary analyses of variance the
data was transformed by square root.
Table 4.
Analysis of variance of different comparisons of seedling density between
BFPs, HFPs and control plots (P < 0.05).
ConditionComparison F P-value FCritical
Backing Before/After 3.21 0.150 7.71
Head Before/After 0.30 0.612 7.71
After Backing/Control36.55 0.009 10.1
After Head/Control 11.12 0.045 10.1
After Backing/Head0.07 0.798 7.71
Table 5.
Nested analysis of variance to compare the differences between treat-
ments and within treatments.
Source of variancedfSS MS Expected value F
Treatment 2 73.3736.7
22
01
24 63σσ++
2
2
σ
10.15*
Plot 5 18.083.62
22
01
24σσ+ 5.55**
Pseudoreplicant 184119.870.65 2
0
σ
Total 191
1
2 Copyright © 2012 SciRes.
J. G. F. GARNICA
N
UMBER O F SEEDLINGS
0
1 - 5
6 - 10
> 10
TREATMENTS
PLOTS
1 - 2 - 3
4 - 5 - 6
7 - 8 - 9
BACK ING FIRE
HEAD FIRE
CONTROL
3
4 5 6
7 8 9
1 2
PLOTS TREATMENTS
1 –2 –3BACKING FIRE
4 –5 –6HEAD FIRE
7 –8 –9CONTROL
NUMBER OF SEEDLINGS
1
4
2 3
7
56
8 9
Figure 3.
Pattern of distribution of regeneration per pseudoreplicant and plot,
showing 4 ranges of seedling frequency: 0; 1 - 5; 6 - 10; and >10 indi-
viduals.
Figure 4.
Number of seedlings per height class, for the three treatments (backing
fire, head fire, and contr ol). Plot IX is excluded from control.
The mean height of seedlings of all the plots was 17.5 cm.
Table 6 shows the means for height per plot and treatment.
Plots treated with the backing fire had the smallest mean height
(12.3 cm), while seedlings in the control plot s had a mean of 34.2
cm. This is the mean of all the seedlings not the mean of the
three plot means.
It is clear that control plots had, on average, the tallest seed-
lings. The corresponding analyses of variance showed that the
differences between the burning t reatments and control were just
significant when the BFPs and the control plots were compared.
These analyses considered the three control plots. The difference
between the BFPs and HFPs was significant as well (Table 7).
Condition of seedlings. Although it is important to have a
sufficient density of seedlings for forest management using na-
tural regeneration, it is also important to get healthy individuals,
which determine their quality. To define the quality of regen-
eration factors such as survival and damage were considerate.
In general, two years after burning, most of the survey ed seed-
lings were alive. Table 8 summarizes the percentage of alive and
dead seedlings for treatments. Both backing and head fire me-
thods showed a high percentage of live seedlings (99.2% and
98.9% respectively) (see also Figure 5), which is likely to be
proportional to the rate of seedling survival. Control plots had
85.3% seedling alive. Although there appe ars to be a difference in
Table 6. hts per plots and treatment, for regeneration after prescribed Mean heig
fires (treatment means ar e the means of all the seedlings).
the rate of sur vival between the plo ts with pres cribed fires and t h e
Treatment Plot
Number of Height mean Tr eatment mean
seedlings (cm) (cm)
Backing 1 76 11.6 12.3
Fire 2 71 11.5
3 113 13.3
Hd 4
1
Control 7
3
Tl 1
ea47 18.1
Fire 5 142 15.0 6.1
6 75 17.0
13 33.0
8 1 75.0 4.2
9 88 33.9
ota626 7.5
able 7. analysis of variance comparing differences in height between
Comparison F P-value FCritical
T
Results of
treatments, and treatments and the three contr ol pl ot s (P < 0.05).
Backing/Head 17.73 0.014 7.71
Blacking/Contro9.45 0.037 7.71
Head/Control 6.52 0.063 7.71
able 8. centage seedlings per treatments, two years after burn-
Treatment Total Alive Dead Alive %
T
Number and per
ing (Plot IX is included) .
Backing fire 260 258 2 99.2
Head fire 264 261 3 98.9
15 Control 102 87 85.3
Figure 5. of seedlings survival per treatment. Percentage
Copyright © 2012 SciRes. 13
J. G. F. GARNICA
control plot s, the corresponding analysis of variance did not cor-
damage, these
on
A Due to thants, used in
st
ods
en
able 9. damage to seedlings that were found and the sections of the
Section of the plant Type of dam age
trees of such species recoloniz rapidly in recently
roborate this. However, since plot 8 [control] had only a single
seedling, this was not a powerful statistical test.
Although seedlings showed different types of
ly affected a very low proportion of the seedlings (6.4% on
average). Table 9 shows a list of the types of damage that were
found, as well as the affected part of the plant. These types of
damage were n ot exclusive to cert ain treatment s. Although it wa s
not the purpose of this study to evaluate the type of damage and
its frequency, it would be important to analyze whether there is
any relationship between certain types of damage and the prac-
tice of prescribed fires.
Discussion and Conclusion
e variability within the pseudoreplic
udy, was considerable it could be assumed certain significance
of the resulting estimations. Therefore, despite that this was a
nonreplicated study; the results suggest strong ecological evi-
dence that prescribed fire enhance natural regeneration of Pinus
michoacana and Pinus oocarpa. Comparing between the rege-
neration before and after burning, in the treated plots, it is clear
that there was an improvement in the density of the natural re-
generation. The mean density was nearly doubled after burning.
However, this difference was not statistically significant, which
could reflect the high variance within the treated and the control
plots. Added to this, it is possible to consider that, according to
Keeley (1987), although opportunities for population expansion
increase after fire for some species, these opportunities increase
in the long absence of fire for others. For example, Acer ru-
brum seedlings respond negatively to fire, both in terms of
survival and reproduction (Reich & Abrams, 1990). In some
species, such as Pinus halepensis, a fter a fire the regeneration is
retarded during the first 2 - 3 years (Moravec, 1990). Therefore,
to estimate with more accuracy the fire effects on regeneration,
according to Bradstock and Myersough (1981) and Sirois and
Payette (1991), in future studies it will be necessary to record
other aspects, such as seed bearer density and damage or de-
struction of the seed supply [including the soil seed bank].
Compared with the undi sturbed control, both bur ning meth
hanced the mean density of natural regeneration. These results
are similar to the obtained in other studies. For example, St Pierre
et al. (1992) described that after fire the regeneration of jack
pine (Pinus banksiana) was increased. The reason of this is
because obligate post-fire seeding species tend to produce a
great proportion of viable seeds following fire (Bell et al.,
1993). Fire is the necessary trigger for general seed release for
many species (Teich, 1970; Enright & Lamont, 1989). Moreover,
T
Types of
plant affected.
Whole Direct damage (ca
e and grow
burnt sites (Savill & Evans, 1986). Van Lear and Waldrop
(1991) suggest that southern pines in the USA are not only fire
tolerant, but also they require fire for their self-perpetuation.
Nevertheless, I did not found any specific information on the
fire-dependency of the regeneration of P. michoacana and P.
oocarpa. Due to the frequency of fire in the study area, the
study of the fire-dependency of such species would be useful to
support the decision making process of forest managers.
Before treatments, the plots of both firing methods did not
dif fer significantly in their number of seedlings. Also, their mean
heights were not significantly different. This meant that the two
firing methods were applied to plots that were similar in their
regeneration. Due to the small mean size of the seedlings and
the similarity between P. michoacana and P. oocarpa at such
size, it was not possible to separate the two species. Neverthe-
less, because some of the enumerated “seedlings” were shoot
sprouts, it would be possible to define to which species they be-
long. Therefore, in future studies it would be better to different-
tiate whether the stems are seedlings or root sprouts. Further-
more, their distribution has to be considered. This information
will help to evaluate the original conditions of a forest stand,
and therefore to understand better the changes caused by fire.
Although it was considered that the prescribed fires were of
low intensity, they were sufficie ntly intense to kill almost all the
original regene ration. Hence, it is important that forest managers
determine if the regeneration is already sufficient, in which case
it could be bette r not to apply any prescribed fire. If more abund-
ant regeneration will come after a prescribed burning, the use of
fire may be justified.
After burning, plot 9 (control) showed a high density of seed-
li ngs, which did not corresp onded to the results in the other con-
trol plots. This can be explained because this plot was subjected
to certain perturbations, such as trees harvesting, which remo-
ved litter from the forest floor, allowing seeds to be in touch
with mineral soil. This favorably affected the establishment of
natu ral re ge nerat io n. Al tho ugh p resc ribe d fi re may resul t in abu nd-
ant seedlings of certain species (Pase & Lindenmuth, 1971),
this also depends on other factors, such as rainfall patterns
(Bradstock & Myerscough, 1981). In this study only the re-
generation after the two firing methods and the control were
considered.
Although the difference in the number of seedlings between
treatments was significant, there wa s also a high variation within
them. This may be because the conditions (light, moisture, seed-
bank etc.) varied from plot to plot (Steel & Torrie, 1960).
Hence, although in this study every effort was made to locate
the experimental plots in the same conditions (slope, amount of
fuel, number of trees, etc.), in future stud ies it wi ll be ne cess ary
to consider more factors in order to ensure homogeneity of
experimental plots. If we can defined an enough number of
replicates; a blocked design could be used in order to group
sample units based on a given criteria, trying that sample units
be selected randomly.
Although the intensity of fires has been recognized as an im-
po
used by cattle)
Curled
Shriveled
Needles
Roots
d
Fungus
Dried
Dried
Forked
Stem Deforme
rtant factor in subsequent regeneration (Whittaker, 1961), no
difference was found between the two firing methods in this
study. Thus the use of one or other methods may depend on
other criteria, such as safety, labor or cost. Some species have
the same regeneration response regardless of fire intensity (Ma-
lanson & O’Leary, 1982). However, excessively high soil tem-
peratures, produced by fire, are important with regard to the
1
4 Copyright © 2012 SciRes.
J. G. F. GARNICA
death or survival of seeds or other plant organs (Beadle, 1940).
The distribution of seedlings along the plots area was affec-
ted by the abundance of root sprout Analysis of Pseudoreplicants to Evaluate Natural Regeneration after Applying Prescribed Burns in a Temperate Forest of Mexico
Open Journal of Forestry
2012. Vol.2, No.1, 9-16
Published Online January 2012 in SciRes (http://www.SciRP.org/journal/ojf) http://dx.doi.org/10.4236/ojf.2012.21002
Copyright © 2012 SciRes. 9
Analysis of Pseudoreplicants to Evaluate Natural Regeneration
after Applying Prescribed Burns in a Temperate Forest of Mexico
José Germán F l ore s G arnica
Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias, Guadalajara, México
Email: flores.german@ inifa p.gob.mx
Received August 19th, 2011; revised J an ua ry 3rd, 2012; accepted January 10th, 2012
Although fire is one of the most important disturbing factors of forest in Mexico, little it is known on the
effects of fire on the particular Mexican forest ecosystems. This is remarked for the fact that the effects of
fires on vegetation vary among different types of forests. This lack of knowledge has constrained the use
of fire, as a silvicultural tool. Therefore, the purpose of this project was to evaluate the effects of fire on
regeneration, under burns. This work was carried out in a pine forest stand at Tapalpa Saw in Jalisco State,
Mexico, dominated by Pinus michoacana and Pinus oocarpa. The study evaluated the effects of two
techniques of prescribed burning: 1) backing, and 2) head fire. The sample plots were burned on 25 and
26 March 1991, before the rain season. One month before and 2 years after burning several measurements
were made in order to evaluate the effect of fire on regeneration. Due to the limitations to work with
“real” replicates (for treatments an control), original sample units (20 × 30 m) were divided into 5 × 5 m
smaller sample units, which were considered as pseudoreplicants. Therefore, such analysis did not avoid
introducing systematic error (bias) and minimize random error. Nevertheless, the variability within the
pseudoreplicants was considerable in order to assume certain significance of the resulting estimations.
Therefore, despite that this was a nonreplicated study; the results suggest strong ecological evidence that
prescribed fire enhance natural regeneration of Pinus michoacana and Pinus oocarpa. In general, it is
concluded that prescribed burning could be a valuable forest management tool in regions with similar
conditions to the study area, in order to improve regeneration. However, further research is needed before
prescribed fires can be applied with confidence in many Mexican forest conditions.
Keywords: Nonreplicated Study; Fire Effect Pinus michoacana; Pinus oocarpa; Forest Fires
Introduction
The successful mixing of fire and regeneration of some pine
species is not just a coincidence (Jenkins et al., 2011; VanLear
& Waldrop, 1991). As an example, there are serotinous trees,
which release a limited amount of seed year to year that need of
fire for general seed release (Vega et al., 2008; Enright & La-
mont, 1989). These trees needs to be exposed to temperatures
above 50˚C, as during a prescribed fire, in order that resin
bonds of cones break and release large quantities of seed for dis-
persal and reproductive growth if mineral soil is exposed (Teich,
1970; Givnish, 1981). In this way, serotiny should be a disad-
vantage where fires are less frequent (Givnish, 1981) Moreover,
some conifers that become established most readily on bare
mineral soil reproduce poorly because litter covers favorable
seedbeds (Land & Rieske, 2006; Smith, 1986; Johnston, 1971).
Thus, eliminating or decreasing litter of forest floor, through
prescribed fires, will result in better conditions for natural re-
generation (Arno, 1980). Moreover, through prescribed fires,
nitrogen and other nutrients of dead vegetation are released into
soil, which result in a improving of the seedbed (Haywood,
2007; Wade, 1989; DeBano, 1976). This favorable response of
regeneration to such temporal improved conditions corresponds
to “the law of population growth with limiting factor” (Ramade,
1984). Therefore, forest managers have tried to emulate the
natural advantages of fire to improve natural regeneration
(Nesmith et al., 2011; McNabb, 2001).
Although the use of fire, as a forest management tool, has been
practicing in many countries with similar conditions to Mexico
(Wells et al., 1979; Aguirre, 1981; Hudson & Salazar, 1981),
the use of prescribed fires in Mexico is very restricted (Flores,
2001; Toledo, 1988). This is due mainly to two major facts: 1)
most of the forest managers do not have a real knowledge of the
methodology and techniques that exist behind the use of fire;
and 2) very little is known about the effects of prescribed fires
in the particular conditions of the Mexican forests. Therefore,
wi t h th e i dea of contributing to knowledge about fire effects, the
purpose of this paper is to evaluate the impact of two tech-
niques of prescribed fires on regeneration.
As it is known, statistically, we require of some replications
in our experimental design, in order to support the significance
of the difference among some given treatments (William, 1992).
Also they are necessary in order to check that our results have
certain consistency and to estimate the experimental error. This
improves our precision by reducing the variation (standard
deviation) of a treatment mean. Moreover, the use of replica-
tions could help us to define if we have to estimate mo re var iable
experimental units. With the purpose of avoids introducing sys-
tematic error (bias) and minimize random error (types I and II
errors) (Heffner et al., 1996; Hurlbert, 1984). However, in this
study, the implementation of such replicates was time and cost
consuming, and, more relevant, risky. This limited the chance
to implement an orthodox experimental design in this study.
Therefore, alternatively, I divided out the original sample units
into smaller sample units, which were considered as “repli-
cants”. However, these were more properly considered as “pseu-
J. G. F. GARNICA
do replicants”. Which are used freque ntly in many ecological field
experiments, though not always consciously (Hurlbert, 1984).
in
e individuals of the
or
live. In contrast, the control
pl
ce or
in
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O’Leary, 1982). This is more likely if fires that are too frequent
for seed production (Smith, 1986). Also, the spatial distribution
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