Abies religiosa Seedling Limitations for Passive Restoration Practices at the Monarch Butterfly Biosphere Reserve in Mexico

Abstract

To recover the structure and functionality of a deforested ecosystem, two strategies of ecological restoration are considered: active restoration, which eliminates the disturbance agents and implements strategies to accelerate site recovery, and passive restoration, which eliminates disturbance agents, allowing natural regeneration to occur. Prior to choosing passive restoration, a field evaluation of the potential for natural regeneration is important. In this context, seedling and sapling density as well as patterns of recruitment and survival are appropriate indicators of restoration potential. In the present study, we deduced the potential of sacred fir (Abies religiosa) forest of the Monarch Butterfly Biosphere Reserve to recover by natural regeneration through seedling and sapling density and mortality, since A. religiosa is the dominant tree species in wintering sites of monarch butterfly. In 2015, we evaluated seedling density in 53 sites along an elevational gradient (3050–3550 m above sea level; m a.s.l.). There was a higher density of seedlings and saplings established in canopy gaps, compared to sites under dense forest canopy. Seedling recruitment was higher in sites at intermediate elevations (3050 to 3300 m a.s.l.) than in those at higher elevations. In a second survey, we studied A. religiosa seedling mortality over the dry season of 2016 to identify the environmental variables that cause the high seedling mortality and very low recruitment. Recently emerged seedling mortality was 49.2% at the end of the dry season (June 2016). The highest monthly mortality (14.3%) was recorded in April, a dry and warm month with the lowest values of moss thickness and soil moisture. We found no negative effects of moss layer on seedling mortality; indeed, moss appears to slow soil moisture reduction at the critical end of the warm and dry season. Soil and moss moisture values in April seem to be a critical factor for A. religiosa seedling recruitment, and we expect this condition will deteriorate under projected climatic change scenarios. Thus, the potential of MBBR A. religiosa forest to recover by passive restoration is highly constrained and will require management actions to achieve successful restoration outcomes.

Full text

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ORIGINAL RESEARCH
published: 13 May 2020
doi: 10.3389/fevo.2020.00115
Edited by:
Ryan G. Drum,
United States Fish and Wildlife
Service (USFWS), United States
Reviewed by:
Jordi Honey-Roses,
The University of British Columbia,
Canada
Jay E. Diffendorfer,
United States Geological Survey
(USGS), United States
*Correspondence:
Arnulfo Blanco-García
arnulfoblanco@yahoo.com.mx
Specialty section:
This article was submitted to
Conservation,
a section of the journal
Frontiers in Ecology and Evolution
Received: 31 December 2018
Accepted: 09 April 2020
Published: 13 May 2020
Citation:
Guzmán-Aguilar G,
Carbajal-Navarro A,
Sáenz-Romero C, Herrerías-Diego Y,
López-Toledo L and Blanco-García A
(2020) Abies religiosa Seedling
Limitations for Passive Restoration
Practices at the Monarch Butterfly
Biosphere Reserve in Mexico.
Front. Ecol. Evol. 8:115.
doi: 10.3389/fevo.2020.00115
Abies religiosa Seedling Limitations
for Passive Restoration Practices at
the Monarch Butterfly Biosphere
Reserve in Mexico
Gerardo Guzmán-Aguilar1, Aglaen Carbajal-Navarro1, Cuauhtémoc Sáenz-Romero2,
Yvonne Herrerías-Diego1, Leonel López-Toledo2and Arnulfo Blanco-García1*
1Facultad de Biología, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico, 2Instituto de Investigaciones
sobre los Recursos Naturales, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mexico
To recover the structure and functionality of a deforested ecosystem, two strategies
of ecological restoration are considered: active restoration, which eliminates the
disturbance agents and implements strategies to accelerate site recovery, and passive
restoration, which eliminates disturbance agents, allowing natural regeneration to occur.
Prior to choosing passive restoration, a field evaluation of the potential for natural
regeneration is important. In this context, seedling and sapling density as well as
patterns of recruitment and survival are appropriate indicators of restoration potential.
In the present study, we deduced the potential of sacred fir (Abies religiosa) forest of
the Monarch Butterfly Biosphere Reserve to recover by natural regeneration through
seedling and sapling density and mortality, since A. religiosa is the dominant tree
species in wintering sites of monarch butterfly. In 2015, we evaluated seedling density
in 53 sites along an elevational gradient (3050–3550 m above sea level; m a.s.l.).
There was a higher density of seedlings and saplings established in canopy gaps,
compared to sites under dense forest canopy. Seedling recruitment was higher in sites
at intermediate elevations (3050 to 3300 m a.s.l.)than in those at higher elevations.
In a second survey, we studied A. religiosa seedling mortality over the dry season
of 2016 to identify the environmental variables that cause the high seedling mortality
and very low recruitment. Recently emerged seedling mortality was 49.2% at the end
of the dry season (June 2016). The highest monthly mortality (14.3%) was recorded
in April, a dry and warm month with the lowest values of moss thickness and soil
moisture. We found no negative effects of moss layer on seedling mortality; indeed,
moss appears to slow soil moisture reduction at the critical end of the warm and
dry season. Soil and moss moisture values in April seem to be a critical factor for
A. religiosa seedling recruitment, and we expect this condition will deteriorate under
projected climatic change scenarios. Thus, the potential of MBBR A. religiosa forest to
recover by passive restoration is highly constrained and will require management actions
to achieve successful restoration outcomes.
Keywords: Abies religiosa, soil moisture, natural regeneration, seedlings, elevational gradient
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INTRODUCTION
The sacred fir (Abies religiosa) is a conifer native to Mexico. It
distributes in the mountainous areas (2100 to 3600 m a.s.l.) in
central Mexico, presenting monospecific forests between 3000
and 3300 m a.s.l. (Rzedowski, 2006). These forests occur in
locations that present very specific geographical, climatic and
ecological conditions (Pineda-López et al., 2013), particularly on
steep, humid and shaded slopes. One of the most emblematic
Abies religiosa forests in Mexico is found in the Monarch Butterfly
Biosphere Reserve (MBBR), which acts as refuge and habitat
for the monarch butterfly (Danaus plexipus L.) that evades the
winter conditions of Canada and the United States by annually
migrating south to a few mountainsides in central Mexico
(Honey-Rosés et al., 2018). The MBBR encompasses 56,259 ha
and was designated as a UNESCO World Heritage Site.
Currently, the Reserve is threatened by various political, social
and economic issues that lead to environmental degradation
associated with logging activities, expansion of the agricultural
frontier and overexploitation and inadequate management of
natural resources. This is despite the fact that the Monarch
Butterfly Reserve receives a considerable amount of financial
resources from national and international organizations to carry
out reforestation programs (Honey-Rosés et al., 2011). Between
2002 and 2010, the region received United States $9.2 million for
reforestation programs (SEMARNAT, 2011).
To recover the structure and function of a deforested
ecosystem, two strategies of ecological restoration are generally
considered: active restoration, which eliminates the agents of
disturbance and implements strategies to accelerate site recovery
(e.g., tree planting and soil conservation practices), and passive
restoration, which eliminates agents of disturbance in the area,
relying on natural regeneration (Holl and Aide, 2011).
The process of natural regeneration is one of the most
important issues in passive restoration, and can be seen
as a continuous cycle of ecological processes, such as the
development of seeds and their subsequent dispersal and
predation, or the germination and establishment of seedlings,
among others. The long-term success and dominance of tree
species depends on these ecological processes (Pérez-López et al.,
2013). Natural regeneration can be an appropriate option for
passive restoration of forests (Pensado-Fernández et al., 2014);
however, understanding the relationship between the structure
and dynamics of canopy vegetation with seedling density, are
crucial for predicting the likely effectiveness of passive restoration
strategies (Grime and Hillier, 2000).
Natural regeneration rates are highly variable depending on
the ecosystem, landscape context, land use history and passive
restoration may not always be successful (Lara-González et al.,
2009), taking longer to reach the goals established for the
restoration of a site than an active restoration. Such delays in
regeneration can sometimes be perceived as failures of passive
restoration. Lands subject to passive restoration can be seen in
developing countries as abandoned or unused land, which may
encourage local people to use these areas for livestock or other
activities. An advantage is that passive restoration is generally
perceived as a low-cost alternative, although in general it has
costs that are often not taken into account such as the purchase
of material (fences or barriers) to isolate the ground from
disturbance agents and payments for site surveillance (Zahawi
et al., 2014). It has the potential to achieve similar levels of
biodiversity and environmental services as an active restoration;
however, it is only feasible in certain places where the disturbance
was not so intense, natural communities are resilient and are far
from human communities (Holl, 1999;Zahawi and Augspurger,
1999;Muñiz-Castro et al., 2006;Suding and Hobbs, 2009;Aide
et al., 2010;Holl and Aide, 2011).
Due to the prevailing shade conditions throughout the
understory, the rates of natural regeneration in temperate forests
are reduced or even null in some cases. Temperate forests are
renewed by the dynamic of gap formation (which can have
both natural and artificial causes), where natural regeneration
processes are increased considerably. Natural regeneration in situ,
compared to traditional forest plantations, is an appropriate
option for ecological rehabilitation on degraded land, especially
if protected from livestock (Lara-González et al., 2009;Sánchez-
Velásquez et al., 2016).
Populations of tree species differ genetically along elevational
gradients, as a response to the selection pressure of temperature
and precipitation gradients (Rehfeldt, 1991;Ortiz-Bibian
et al., 2017). This makes it advisable to delineate elevational
zonings to guide seed and seedling movement in reforestation
programs. Castellanos-Acuña et al. (2014) reported a significant
morphological differentiation among populations of A. religiosa
along an elevational gradient: low-altitude populations have
shorter needles and longer cones than high-altitude and these
might have important consequences for seed production and
seedling quality.
Scientific literature on A. religiosa shade tolerance is at times
contradictory; according to Rzedowski (1978), the A. religiosa
is a shade tolerant species and canopy gaps contribute to
the regeneration of A. religiosa in the Cofre de Perote
National Park, in Veracruz, Mexico, and seedling density is
considerably greater in gaps than in the understory (Lara-
González et al., 2009). However, some authors consider that the
species can regenerate naturally in both clearings and understory
(Narakawa and Yamamoto, 2001;Sugita and Tani, 2001;Mori
and Takeda, 2002), while González et al. (1991) state that
the species grows in open places in smaller proportions than
in the understory.
Honey-Rosés et al. (2018) studied the drivers of forest cover
both inside and outside the MBBR using a combination of
remote sensing imagery and field-collected data. They found an
increase in forest cover of 5,673 ha occurred between 1986 and
2012: 71% of this recovery was attributed to natural regeneration
processes, while active restoration efforts only contributed 3.8%,
raising questions about the effectiveness of active restoration. The
rest (25%) was attributed to a combination of both techniques.
The authors conclude that due to the high potential for natural
regeneration in the reserve, management efforts should focus
on passive restoration activities instead of investing in active
restoration (Honey-Rosés et al., 2018).
While many forest managers may be attracted to the
idea of supporting natural regrowth via passive restoration,
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various biophysical conditions may impede the successful
recruitment of young seedlings in areas that are unhospitable
to forest regrowth. For example, Manzanilla (1974) suggests
a negative interaction of the moss layer thickness with
A. religiosa seedling mortality; since thicker layers will generate
a physical barrier that is responsible for the absence of natural
regeneration. Local forest technicians and landowners of the
MBBR support this assertion.
The objective of this study was to study factors that
may affect natural regeneration of A. religiosa seedlings at
MBBR, an essential consideration for implementing passive
ecological restoration. We studied regeneration capacity through
seedling density in response to elevation, canopy closure,
and other abiotic factors such moss layer thickness and soil
moisture. This research is intended to guide decision-making
regarding the implementation of adequate restoration and
conservation strategies at the Monarch butterfly overwintering
sites in the MBBR.
MATERIALS AND METHODS
Study Site
This study was carried out at Ejido de La Mesa, in the
municipality of San José Del Rincon, Estado de México (19◦340,
35.700 N and 100◦140, 30.200 W), in the central-western part of the
Mexican Transvolcanic-Belt.
In September and October of 2015, natural regeneration
of A. religiosa seedlings was monitored along an elevational
gradient (3050 to 3550 m a.s.l) in the MBBR. The transect
range was classified into two different elevational bands:
intermediate (3050–3300 m a.s.l.) and upper (3301–3550 m
a.s.l.), according to the elevational zoning of Castellanos-
Acuña et al. (2014). It was not possible to measure seedling
density at the lower elevational band (2800–3050 m a.s.l.),
since this is an area with a long history of impact by human
settlement and agricultural and livestock activities, and the
original A. religiosa trees at this low elevation remain only in
small forest fragments.
At both elevational bands (intermediate and upper), we
selected 53 sites with and without canopy gaps (canopy type):
25 sites in the intermediate band (11 under forest cover and 14
in gaps) and 28 sites in the upper band (9 under forest cover
and 19 in gaps). The area for each gap was different (<400m2):
the diameter not less than 15 ×15, nor more than 23 ×23 m
(resembling the size of an adult tree canopy). In closed canopy
sites a 15 m diameter circle was used (Supplementary Figure S1).
The selected sites presented slopes of less than 22◦(on steeper
slopes the effect of the gap decreases due to inclination of the
crowns of adjacent trees). Abundance, height and diameter of
seedlings (0–2 mm root collar diameter) and saplings (<5 cm
DBH) were measured throughout each site with canopy gaps and
within the 15 m circle for sites without canopy gaps. Additional
parameters measured in each site included: canopy cover, slope,
elevation and gap diameter. Percent cover of rocks, shrubs,
herbaceous plants, mosses and bare soil was recorded in three
2×2 m square quadrats per site.
Seedling Mortality Measurements
We conducted an additional survey in the same area, evaluating
the mortality of naturally regenerated A. religiosa seedlings
throughout the 2016 dry season (from February to early June).
Thirty quadrats of 4 m2(2 ×2 m) were delimited and
distributed in the same elevational bands (15 quadrats each in the
intermediate and upper bands). The quadrats were always located
beneath forest canopy (>60% of tree cover) with a minimum
distance of 50 m apart to avoid spatial autocorrelation. In each
quadrat, all of the recently emerged seedlings that appeared
to be less than 1-year-old (older seedlings show a lignified
stem) were individually labeled. Each month, we recorded
alive seedlings and carefully collected dead seedlings for dry
weight measurement.
In quadrats with presence of moss, the moss layer thickness
was measured monthly in three adjacent sites per quadrat (to
avoid disturbing seedlings and moss layer inside the quadrat).
We collected small samples of moss and soil adjacent from each
quadrat, weighed in situ, and packed fresh in zip sealed plastic
bags for subsequent drying in the laboratory to estimate the
relationship of volumetric moisture content.
Circular plots of 0.1 ha (17.8 m radius) were established to
count adult trees above each quadrat and we measured height
and diameter at breast height (DBH) of each tree recorded
and grouped in three categories (<25 cm, 25–45 cm, >45 cm)
according to Pineda-López et al. (2013) and Manzanilla (1974).
Canopy cover was estimated from hemispheric photographs
taken with Winscanopy (Regent Instruments Inc.), (Guay, 2014).
The samples of moss and soil were dried in an oven at 70◦C
for 5 days and weighed. Dead collected seedlings were divided
into their root and aerial parts, which were dried in an oven
for measurement of dry weight. The biomass allocation estimates
were done to asses if dead seedlings fail to reach the soil beneath
the moss layer. When dead seedlings were in moss, we recorded
if the roots penetrated the moss and made it into the soil below
since local forest technicians have claimed this is the main cause
of seedling mortality.
Data Analysis
Seedling and sapling density was analyzed using a generalized
linear model with a Poisson distribution. The independent
variables were elevational band (intermediate or upper), canopy
(gap or forest cover) and the interactions among these factors.
Linear regression or Spearman rank correlation tests were applied
to assess the relationship between the various environmental
variables and seedling density.
To determine temporal variation of A. religiosa seedling
mortality during 2016 dry season, we performed a repeated
measures ANOVA, with a post hoc Tukey paired test (the
dependent variable was the number of surviving seedlings per
month while elevational band was the independent variable). The
temporal comparison was conducted with paired Wilcoxon and
Kruskal-Wallis tests.
Moisture content of moss and soil was estimated through the
formula of gravimetric moisture: W%=Ma
Ms ×100, where
Ma is the weight of water lost following drying and Ms is the fresh
weight of soil or moss (Universidad Nacional de Córdoba, 1993),
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representing the percentage or weight of water in 1 g
of soil or moss.
The canopy photographs taken were analyzed with the
Winscanopy (Regent Instruments Inc.) (Guay, 2014) and
percentage of canopy cover was estimated.
In addition, linear and quadratic regressions were performed
between seedling mortality and moss layer thickness, moss
gravimetric moisture content and soil gravimetric moisture
content, to identify a threshold that promoted major
seedling mortality.
Finally, we applied a Cox proportional hazards model to
analyze the influence of environmental and forest structural
variables on seedling survival time. The independent variables
were soil organic matter content, moss cover, maximum moisture
content of moss, tree density, maximum and minimum thickness
of moss layer, minimum moisture content of moss and minimum
soil moisture content. A few seedlings disappeared from quadrats
during the study. These may have been eaten by herbivores
instead of dying but we included these individuals in the analyses.
All statistical analyses were performed with the packages R 3.1.3
and JMP 8.0 SAS Institute Inc.
RESULTS
Seedling and Sapling Density
There was a higher density of seedlings at the intermediate
elevational band compared to upper band. Most of the
individuals recorded in the intermediate band beneath the forest
were seedlings (68%) while canopy gaps harbored a higher
proportion of saplings than sites without canopy gaps (Figure 1).
In the upper zone, in addition to the lower overall density, only
3% of individuals were seedlings, with similar proportions in each
canopy type. However, there was a higher density of saplings in
gaps. The results of the generalized linear model were significant
for both parameters: elevational band (x2= 352.7, df = 10,
p<0.001) and canopy type (x2= 198.4, df = 10, p<0.001).
Saplings also showed significant differences: elevational band
(x2= 483, df = 10, p<0.001) and canopy type (x2= 243,
df = 10, p<0.001). However, there was no significant effect of
interactions between these independent variables.
Understory Conditions
The most common companion species at the sites were Acaena
elongata (a shrub of 0.3 to 1 m in height), Alchemilla procumbens
(a creeping grass of up to 30 cm in height), and Roldana
angulifolia (a shrub of 1 to 2.5 m in height).
Despite the small difference in tree coverage between the
gap and forest canopy types (Table 1), statistically significant
differences were found. The results suggest that A. religiosa
seedlings experience suitable conditions in the gaps (intermediate
levels of light) for initiation of the natural regeneration process.
The gaps presented a higher shrub and herb coverage than
sites without gap, while there was higher coverage of moss
beneath the forest canopy. In all four strata, significant differences
were present between sites with and without gaps (Table 1). No
differences in rock and bare soil coverage were observed between
gaps and without gaps sites.
A non-parametric correlation of Spearman ranks was
conducted between tree canopy openness and seedling density,
revealing a weak relationship (p= 0.009, rs= 0.351, n= 53).
No significant differences were found when correlating moss
cover with seedling density using the Spearman rank coefficient.
However, moss cover was negatively related to other understory
components (rocks, bare soil, and shrubs), and positively related
to herbs (Table 2).
Seedling Mortality Survey
Six-hundred sixty-one A. religiosa seedlings were marked and
monitored in 30 quadrats. In the upper elevational band, 15
quadrats were established and 378 seedlings monitored. In the
intermediate elevational band, 15 quadrats were established and
283 seedlings monitored. We found that mortality increased
during dry season reaching 48% (in either elevational band) when
FIGURE 1 | Seedling and sapling density by elevational band (intermediate 3050 to 3300 m a.s.l., upper band 3301 to 3550 m a.s.l.)and canopy type (with and
without canopy gap).
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TABLE 1 | Mean coverage (%) of different forest strata per canopy type (with and
without gap), and significance of the difference between canopy types in each
case.
Stratum Canopy type f p
Gap Without gap
Tree 91 98 38.9 0.001
Shrub 45 27 2.64 0.050
Herbs 73 53 3.67 0.010
Moss 55 79 4.62 0.006
Tree values came from hemispheric photos while the other data came from field
collection at each site.
TABLE 2 | Correlations between components of ground cover and seedling
density.
Seedling
density
% rocks % shrubs % herbs % moss % bare
soil
Seedling
density
0.92 0.40 0.81 0.47 0.31
% rocks −0.01 0.43 0.86 0.04* 0.39
% shrubs −0.12 −0.11 0.06 0.01* 0.47
% herbs 0.03 −0.02 0.26 0.00* 0.09
% moss 0.10 −0.29 −0.34 0.38 0.00*
% bare soil 0.14 0.12 0.10 −0.24 −0.41
The significance (n = 53) of the test is shown above the diagonal, while the
correlation coefficient is shown below.
FIGURE 2 | Abies religiosa mortality during the dry season between
elevational bands, (numbers show seedling mortality in each sampling month).
In March, 22 seedlings died in the upper band and 16 in the intermediate one.
the rainy season began. In either elevational band the highest
mortality occurred in April and the lowest in June (Figure 2).
Factors Associated With Seedling
Mortality
No significant differences were found between the two elevational
bands (Figure 2). A low proportion of the seedlings disappeared
in the month of April and May and these individuals apparently
FIGURE 3 | Causes of seedling mortality: this includes individuals with no
obvious cause of death and were used in biomass allocation estimate, and
“lost” refers to individuals where the entire plant was gone; letters show
significant differences as result of repeated measures ANOVA.
had been consumed by herbivores or had decomposed during the
interval between the two monitoring periods (Figure 3).
Biomass Allocation Estimate
The dead seedlings presented greater average aboveground
biomass compared to belowground biomass allocation, 65 and
35% in the upper band, and 68 vs. 32% at the intermediate band.
The ANOVA shows significant differences between this biomass
allocation, but no differences were found in this respect between
elevational bands. We also analyzed the correlation between
average aerial biomass and canopy cover of each site but there
is not a significant relation between these variables.
Moss Layer Thickness and Moisture
Content
The average initial thickness of the moss layer was 3.2 cm, and
this decreased to a minimum value in April (a warm and dry
month), at an average of 2.1 cm, before recovering quickly as
a consequence of the early rains in June. April was the only
statistically different month revealed in the repeated measures
ANOVA. February and June showed the highest average values
of thickness, with 3.1 and 2.9 cm, respectively (Figure 4). There
were no statistically significant differences in moss layer thickness
between elevational bands for any month.
Gravimetric Moisture Content of Moss
and Soil
The moss layer showed a higher water retention capacity,
containing up to 2.6 g of water/g of moss, as well as
rapid dehydration and rehydration with precipitation. For
both substrates (moss and soil), the lowest water content
was observed in April, with statistically significant differences
observed compared to the other months. The lowest thickness
of the moss layer and the highest seedling mortality was also in
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FIGURE 4 | Moss layer thickness variation throughout the dry season.
Different letters indicate significant (p<0.05) differences among months.
April. In contrast, the highest moisture content was observed
in June for both substrates because that month had the highest
rainfall (Figure 5).
The gravimetric moisture content of the moss showed a
positive relationship with canopy cover only in the wettest month
(June). The relationship of seedling mortality with monthly
moisture content (of moss or soil) is statistically significant and
shows clearly that lower humidity values are associated with
higher monthly mortality rates (Figure 6). A general trend is
evident: a mortality rate greater than 4% occurs when a critical
threshold of 1 g of water/g of moss, or 0.7 g of water/g of soil, is
reached during the dry season.
Proportional Risk Analysis
The Cox regression or proportional hazard analysis shows
that three parameters had an effect on A. religiosa seedling
mortality: soil organic matter content increases 29% the risk of
seedling mortality, and tree density surrounding the sites (1%)
and moss cover has a significant but weak effect in seedling
mortality (Table 3).
Forest Structure
There were many differences in adult trees surrounding 4 m2
quadrats between the two elevational bands. Sites in the
intermediate band showed an average tree density of 904/ha vs.
529 trees/ha in the upper band (f= 11.1, df = 1, p<0.002).
In the upper band, average tree height was 30.2 vs. 18.2 m in
the intermediate band (f= 26.0, df = 1, p<0.01). DBH was
significantly higher in the upper band (f= 13.0, df = 1, p<0.01),
while no significant differences were recorded in the canopy cover
(79.5% in intermediate vs. 76.8% in the upper band).
The tree diameter distribution showed that the average density
of trees with dbh less than 25 cm is clearly higher in the
intermediate band compared to the upper band, (f= 17.2, df = 1,
p<0.001). In the following two categories of DBH (25–45 cm and
>45.1 cm), the density decreased and was significantly higher in
the upper band (f= 9.1, df = 1, p<0.001) and (f= 12.4, df = 1,
p<0.001) (Figure 7).
DISCUSSION
In the present study, several variables were considered to have
affected the density of seedlings and saplings of A. religiosa,
one of which is elevation. Ortiz-Bibian et al. (2019) found that
populations of this species in the central part of their elevational
distribution (intermediate zone) exhibit a higher number of
viable seeds and greater germination capacity. This pattern could
explain the larger number of seedlings and juveniles we recorded
in the intermediate band.
FIGURE 5 | Gravimetric moisture content (GMC) of soil (right) and moss (left) over time; (different letters indicate statistical differences among the months as
revealed by the repeated measures ANOVA).
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FIGURE 6 | Quadratic regression of seedling mortality against: (left) moss moisture content, (right) soil moisture content. (Vertical black line indicates what appears
to be a critical humidity threshold in relation to seedling mortality).
TABLE 3 | Analysis of proportional risks (Cox Regression).
Source of risk for
seedling mortality
Chi
squared
Prob Risk
ratio
95
lower %
95
upper %
Minimum soil
moisture content
1.01 0.31 0.56 0.18 1.72
Maximum moss
thickness
0.09 0.76 1.02 0.90 1.15
Tree density 7.86 0.0051* 1.01 1.00 1.01
Bulk density 0.08 0.77 2.13 0.01 357.99
Soil organic matter
content
8.94 0.0028* 1.29 1.09 1.52
Moss cover 12.92 0.0003* 0.99 0.98 1.00
Likelihood ratio test = 49, df 6, p <0.001, n = 662, number of events = 233.
The observed higher seedling and sapling density in gaps
compared to forest is similar to the results of Lara-González et al.
(2009), who found that regeneration of A. religiosa is favored in
sites with greater canopy openness. Manzanilla (1974) reports
that the regeneration of A. religiosa occurs clumped in sites with
high availability of sunlight.
Regarding the size of the seedlings and saplings, diameter
showed a similar pattern (most individuals belonging to smaller
categories). However, there were notable differences between
elevational bands. At the intermediate band most of the
individuals recorded were seedlings (68% in forest and 45%
in gaps); while for the upper band, only 3% were seedlings
and 97% saplings. This suggests recruitment of seedlings to
saplings is limited in the intermediate elevation band. On
the other hand, seed limitation, either from a lack of seed
production, germination, or early post-germination survival
might be occurring at higher elevations. Likewise, in gaps at
the intermediate band, a vigorous germination process could be
underway, which would ensure that suitable plants are established
for the regeneration of the forest.
In relation to canopy type, individuals in gaps had greater size
and gaps had the highest percentage of shrub and herbaceous
plant coverage. This vegetation could therefore play a “nurse
plant” role, which might act to favor the establishment and
growth of A. religiosa.Sánchez-Velázquez et al. (2011), and also
Blanco-García et al. (2011) measured the effect of nurse plants
such as Baccharis conferta and Lupinus elegans in an A. religiosa
reforestation trial and documented lower mortality and higher
growth of Abies when growing under the canopy of these shrubs.
Bautista (2013) and Lara-González et al. (2009) reported that
morphological variables (number and length of lateral buds) and
natural regeneration (seedling density) of A. religiosa are favored
with increased canopy openness, as confirmed by the present
study. Even when canopy cover between gaps and forests was
slightly different (92 vs. 99%, respectively) these differences could
determine the suitability of the light conditions for Abies religiosa.
The Abies religiosa forests of Mexico are relatively dense
because of their closed canopies; the light that comes to the
ground is low and the understory is scarce, so the existence
of gaps is not common even though its contribution to forest
regeneration is very important. It has been observed that in open
areas the regeneration is more successful under the canopy of
some shrubs that act as nurse plants facilitating fir regeneration
(Lara-González et al., 2009).
Seedling Mortality Survey
The seedling mortality recorded in our survey was lower
compared to results reported for Abies pinsapo (Arista, 1993).
In the latter species, the possible factors contributing to seedling
mortality were high light intensity, low humidity and competition
with herbaceous plants. It was noted that seedlings died quickly
as soon as spring and summer began, possibly as a result of water
stress since they were exposed to full sunlight. The following
year, the same author (Arista, 1994) reported contrasting data
for another population of the same species, where seedlings less
than 1-year-old presented 45% survival in understory and 82%
in an open field, while older seedlings presented survival of 75%
in the forest and 83% in an open field. That study indicates low
humidity, light and extreme temperatures as the main factors that
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Guzmán-Aguilar et al. Abies religiosa Seedling Limitations
FIGURE 7 | Average density (individuals/ha) of adult trees in each diameter category per elevational band.
contribute to high mortality. Moreover, where humidity was not a
limiting factor, mortality was attributed to a possible fungal attack
or lack of mycorrhizae.
Ángeles-Cervantes and López Mata (2009) investigated
mortality in a cohort of A. religiosa seedlings in patches
affected and unaffected by fires, and found that an important
factor increasing Abies seedling mortality is desiccation. This is
attributable to the layer of moss and accumulated litter, which
prevents the root from reaching and penetrating the mineral
soil beneath. This could be one of the factors by which sites
with denser canopies (which have the highest percentage of
moss) present less natural regeneration, since even though moss
may constitute a suitable microsite for the germination of the
A. religiosa seed, thicker layers of moss actually behave as a barrier
for the longer-term persistence of the seedling. This concurs with
comments made by nursery managers located within the MBBR,
as well as forest technicians, who report that the presence of the
moss strongly causes mortality of A. religiosa seedlings and that
its partial removal might increase A. religiosa seedling survival.
Biomass Allocation Estimate
The difference in recorded biomass allocation may have an effect
on seedling mortality, since the failure of the root system to
supply water to the plant or to regenerate new roots will lead
to a vicious circle of water stress and depletion of carbohydrates,
which will cause a delay or a reduction of regrowth, or even the
death of the plant, since desiccation of the roots is considered to
have the most damaging effect on plant vitality (Brønnum, 2005).
Effect of Moss Layer
Our results suggest that the moss layer is not a primary limiting
factor for A. religiosa seedling survival. Manzanilla (1974) found
that in the A. religiosa forests, the thick layer of moss is
responsible for the low natural regeneration, since it acts as a
mechanical barrier reaching up to 30 cm in thickness in an
understory with abundant vegetation that prevents the seedling
root from reaching and anchoring to the mineral soil. Our study
did not find moss layers as thick as those reported in Manzanilla
(1974) and only 3.8% of dead seedlings roots failed to penetrate
the moss layer in our quadrats. Manzanilla (1974) suggests the
hypothesis that the negative interaction of the moss with the
seedlings will generate a physical barrier that is responsible for
the absence of natural regeneration. Local forest technicians and
land owners also support this assertion.
Similarly, there have been reports of positive and negative
effects on germination and recruitment generated by the organic
matter layer, since this layer usually reduces soil temperature
and water evaporation, increasing moisture in the soil and
promoting better conditions for germination. Nevertheless, it can
generate an allelopathic inhibition, reduce the incidence of light
or form a physical barrier to the penetration of the seedling roots
(Dechoum et al., 2015).
In contrast to the potential negative effects of moss on
seedling establishment described above, in our study sites,
the moss seems to provide a suitable environment for seed
germination and seedling establishment, which is very important
for the dispersion capability and establishment of woody
species (Dechoum et al., 2015). However, with the onset of
the dry season of the year, the moss loses its moisture very
quickly, causing thinning of the moss layer and a subsequent
loss of soil moisture, leaving the seedlings more exposed to
other agents that can potentially cause mortality, such as
temperature, solar radiation, lack of environmental humidity.
This effect of the humidity has been described by Chen et al.
(2015) as influencing the richness and abundance of moss
species during the transition from dry to wet periods, and its
variation due to the differential tolerance of some species to
this abiotic factor.
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Guzmán-Aguilar et al. Abies religiosa Seedling Limitations
In our study quadrats, soil moisture seems to be the abiotic
factor that most affects the mortality of A. religiosa seedlings both
directly and indirectly. For the genus Abies, availability of water
is very important at the seedling stage, since several species are
extremely sensitive to a moisture deficit in the substrate. Indeed,
it is considered the most important factor in the mortality of
coniferous seedlings within the first 5 years of growth (Van der
Salm et al., 2007; Rodríguez-Laguna et al., 2015). A clear example
is the high sensitivity to stress due to desiccation reported for
Abies prosera in a study conducted under controlled conditions
(Brønnum, 2005). In addition, it is essential to consider the
possible impact on ecosystems as a consequence of climate
change (Ledo et al., 2015), which is modifying the patterns and
frequency of the dry period and will likely have severe effects on
the recruitment of seedlings in the forests.
Finally, it is possible that critical environmental thresholds
(such as mortality greater than 4% per month with a reduction
of 1 g of water/g of moss or 0.7 g of water/g of soil) would be
lowered given projected climate change scenarios (Sáenz-Romero
et al., 2012). Higher temperatures and lower precipitation could
prevent the successful establishment of some tree species or
may limit their establishment to favorable years only, ultimately
changing the structure and functioning of the forest ecosystem
(Von Arx et al., 2013).
CONCLUSION
Abies religiosa seedlings are more abundant at intermediate
sites (3050 to 3300 m a.s.l.)than at upper (3301 to 3550 m
a.s.l.)elevations, where poor establishment and recruitment of
seedlings over the last 20 years have been observed. Additionally,
canopy gaps play a positive and very important role in seedling
recruitment, but a high proportion of seedling failure occurs at
intermediate elevations, and the consequent lack of recruitment
is an important issue that requires further research.
We found no evidence that the moss layer is responsible
for seedling mortality; indeed, it constitutes an excellent moist
microsite for seed establishment and germination, as well as
protecting the bare soil from excessive moisture loss through
evapotranspiration.
The most important factor increasing seedling mortality is
soil moisture in the critical warm and dry month of April. This
condition is likely to worsen under future scenarios of climatic
change, affecting the regeneration of the Abies religiosa forest.
The upper elevational range of the MBBR is experiencing
serious changes and active restoration might be needed to
maintain forest cover and the ecosystem services it provides for
inhabitants of the region, including the overwintering Monarch
butterfly colonies.
Passive restoration practice in MBBR is favored by a high
seed production and germination but constrained by a low
seedling density (especially beneath closed canopies in upper
elevation sites) and low recruitment (especially in intermediate
elevation sites). Forest management activities might be needed
to promote gap formation and improve seedling recruitment to
sapling stage.
AUTHOR CONTRIBUTIONS
AB-G and CS-R conceived the research project. GG-A, AC-N,
and AB-G carried out the field measurements and conducted the
statistical analysis. LL-T and YH-D provided helpful comments
during the development of the project and proposed and
conducted specific statistical analyses. All of the co-authors
revised and contributed to the manuscript. AB-G led the writing
of the manuscript.
FUNDING
Financial support was provided to AB-G by the UMSNH
Coordinación de la Investigación Científica; the Basic Research
CONACYT Fund (Ciencia Básica-2014-242985) and the UNAM
Laboratorio Nacional de Análisis y Síntesis Ecológica (LANASE
2018-293701).
ACKNOWLEDGMENTS
We thank Sr. Francisco Ramírez Cruz and Sra. Doña Petra Cruz-
Cruz, for the facilities for site selection and seedling measurement
at Ejido La Mesa. Without their help, this study would not have
been possible. The MBBR staff helped to select sites for fieldwork
and granted the license required to conduct the research. Thanks
to Marcos Sandoval-Soto, Beatriz Guerrero-Carmona, Nancy
Farías-Rivero, Miriam Linares, Jorge Herrera-Franco, Francisco
Loera-Padilla, and other UMSNH students for assistance with
taking measurements. We also thank two reviewers and an
English proofreader for improving the quality of this manuscript.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fevo.2020.00115/
full#supplementary-material
FIGURE S1 | Two different canopy types: on the left, a gap; on the right a site
without gap (beneath the forest).
REFERENCES
Aide, T. M., Ruiz-Jaén, M. C., and Grau, H. R. (2010). “What is the state
of tropical montane cloud forest restoration?,” in Tropical Montane Cloud
Forests Science for Conservation and Management, eds L. A. Bruijnzeel,
F. N. Scatena, and L. S. Hamilton (Cambridge: Cambridge University Press),
101–110.
Ángeles-Cervantes, E., and López Mata, L. Y. (2009). Supervivencia de una cohorte
de plántulas de Abies religiosa bajo diferentes condiciones postincendio. Boletín
Soc. Bot. México 84, 25–33.
Arista, M. (1993). Germinación de las Semillas y supervivencia de las plántulas de
abies pinsapo boiss. Acta Bot. Malacit. 18, 173–177.
Arista, M. (1994). Supervivencia de las plántulas de abies pinsapo boiss. en su
habitat natural. Anales Jardín Botánico de Madrid 2, 193–198.
Frontiers in Ecology and Evolution | www.frontiersin.org 9May 2020 | Volume 8 | Article 115

fevo-08-00115 May 11, 2020 Time: 19:23 # 10
Guzmán-Aguilar et al. Abies religiosa Seedling Limitations
Bautista, S. C. (2013). Estructura del bosque y arquitectura de brinzales de
Abies religiosa en Tlaxco, Tlaxcala. Ph. D thesis, Institución de Enseñanza e
Investigación en Ciencias Agrícolas, Mexico.
Blanco-García, A., Sáenz-Romero, C., Martorell, C., Alvarado-Sosa, P., and Lindig-
Cisneros, R. (2011). Nurse-plant and mulching effects on three conifer species
in a Mexican temperate forest. Ecol. Eng. 37, 994–998.
Brønnum, P. (2005). preplanting indicators of survival and growth of desiccated
abies procera bareroot planting stock. Scand. J. Forest Res. 1, 36–46.
Castellanos-Acuña, D., Sáenz, C., Lindig-Cisneros, R. A., and Silva-Farías, M. A.
(2014). Zonificación altitudinal provisional de Abies religiosa en una área
cercana a la reserva de la biosfera mariposa monarca, Michoacán. Revist.
Chapingo Ser. Cienc. Forest. Ambient. 20, 215–225. doi: 10.5154/r.rchscfa.2013.
11.041
Chen, S., Silk, J., Gao, J., Mao, L. F., and Shen, M. W. (2015). Latitudinal diversity
gradients in bryophytes and woody plants: roles of temperature and water
availability. J. Syst. Evol. 6, 535–545.
Dechoum, M. S., Zenni, R. D., Castellani, T. T., Zalba, S. M., and Rejmánek, M.
(2015). Invasions across secondary forest successional stages: effects of local
plant community, soil, litter, and herbivory on Hovenia dulcis seed germination
and seedling establishment. Plant Ecol. 216, 823–833.
González, G., Musalem, M., Zarate, A., Lara, G., and Velásquez, M. A. (1991).
Estudios de germinación en Oyamel (Abies religiosa [H.B.K.] Schl et Cham) bajo
ambientes naturales en Zoquiapan, México. Rev. Chapingo 15, 59–66.
Grime, J. P., and Hillier, S. H. (2000). “The contribution of seedling regeneration to
the structure and dynamics of plant communities, ecosystems and larger units
in the landscape,” in Seeds: The Ecology of Regeneration in Plant Communities,
2nd Edn, ed. M. Fenner (Wallingford: CAB International), 361–374.
Guay, R. (2014). WinSCANOPY 2014 for Canopy Analysis. WinSCANOPY Manual
Version 2014a.
Holl, K. D. (1999). Factors limiting tropical rain forest regeneration in abandoned
pasture: seed rain, seed germination, microclimate, and soil. Biotropica 31,
229–242.
Holl, K. D., and Aide, T. M. (2011). When and where to actively restore ecosystems?
Forest Ecol. Manag. 261, 1558–1563.
Honey-Rosés, J., Baylis, K., and Ramírez, M. (2011). A spatially explicit estimate
of avoided forest loss. Conserv. Biol. 25, 1032–1043. doi: 10.1111/j.1523-1739.
2011.01729.x
Honey-Rosés, J., Maurer, M., Ramirez, M. I, and Corbera, E. (2018). Quantifying
active and passive restoration in Central Mexico from 1986–2012: assessing the
evidence of a forest transition. Restor. Ecol. 26, 1180–1189.
Lara-González, R., Sánchez-Velásquez, L. R., and Corral-Aguirre, J. (2009).
Regeneración de Abies religiosa en claros del dosel versus sotobosque, parque
nacional cofre de perote, México. Agrociencia 43, 739–747.
Ledo, A., Cayuela, L., Manso, R., and Condes, S. (2015). Recruitment patterns
and potential mechanisms of community assembly in an Andean cloud forest.
J. Veget. Sci. 26, 876–888.
Manzanilla, H. (1974). Investigaciones Epidométricas Y Silvícolas En Bosques
Mexicanos De Abies Religiosa. Ciudad de México: Secretaría de Agricultura y
Ganadería.
Mori, A., and Takeda, H. (2002). Light-related competitive effects of overstory trees
on the understory conifer saplings in a subalpine forest. J. For. Res. 8, 163–168.
Muñiz-Castro, M. A., Williams-Linera, G., and Benayas, J. M. (2006). Distance
effect from cloud forest fragments on plant community structure in abandoned
pastures in Veracruz, Mexico. J. Tropical Ecol. 22, 431–440.
Narakawa, Y., and Yamamoto, I. S. (2001). Gap formation, microsite variation and
the conifer seedling occurrence in a subalpine old-growth forest, Central Japan.
Ecol. Res. 16, 617–625.
Ortiz-Bibian, M. A., Blanco-García, A., Lindig-Cisneros, R. A., Gómez-Romero,
M., Castellanos-Acuña, D., Herrerías-Diego, et al. (2017). Genetic variation in
Abies religiosa for quantitative traits and delineation of elevational and climatic
zoning for maintaining monarch butterfly overwintering sites in Mexico,
considering climatic change. Silvae Genet. 66, 14–23.
Ortiz-Bibian, M. A., Castellanos-Acuña, D., Gómez-Romero, M., Lindig-Cisneros,
R. A., Silva-Farías, M. A., and Sáenz-Romero, C. (2019). Variation among
Abies religiosa (H.B.K.) schl. et cham populations along an elevational
gradient of seed germination capacity. Revist. Fitotecn. Mexican. 42,
301–308.
Pensado-Fernández, J. A., Sánchez-Velásquez, L. R., Pineda-López, M., del, R.,
and Díaz-Fleischer, F. (2014). Plantaciones forestales vs. regeneración natural
in situ: el caso de los pinos y la rehabilitación en el parque nacional cofre de
perote. Bot. Sci. 92, 617–622.
Pérez-López, P., López-Barrera, F., García-Oliva, F., Cuevas-Reyes, P., and
González-Rodríguez, A. (2013). Procesos de regeneración natural en bosques
de encinos: factores facilitadores y limitantes. Biol. Public. Espec. 1, 18–24.
Pineda-López, M. R., Ortega-Solís, R., Sánchez-Velásquez, L. R., Ortiz-Ceballos, G.,
and Vázquez-Domínguez, G. (2013). Estructura poblacional de Abies religiosa
(Kunth) Schltdl. et Cham., en el ejido el conejo del parque nacional cofre
de perote, Veracruz, Mexico. Revist. Chapingo Ser. Cienc. Forest. Ambient. 3,
375–385.
Rehfeldt, G. E. (1991). “Gene resource management: using models of genetic
variation in silviculture,” in Proceedings of the Genetics/Silviculture Workshop,
Wenatchee.
Rodríguez-Laguna, R., Razo-Zárate, R., Fonseca-González, J., Capulín-Grande,
J., and Goche-Telles, R. (2015). Regeneración natural post-incendio de Abies
religiosa (H B K) Schl. et Cham, En El Parque Nacional ‘El chico’ Hidalgo. Revist.
Iberoameric. Cienc. 2, 11–22.
Rzedowski, J. (1978). La Vegetación de México. México: LIMUSA.
Rzedowski, J. (2006). Vegetación de México, 1st Edn, México: Comisión Nacional
para el Conocimiento y Uso de la Biodiversidad.
Sáenz-Romero, C., Rehfeldt, G. E., Duval, P., and Lindig-Cisneros, R. A. (2012).
Abies religiosa habitat prediction in climatic change scenarios and implications
for monarch butterfly conservation in Mexico. Forest Ecol. Manag. 275,
98–106.
Sánchez-Velásquez, L. R., Pineda-López, M., Avendaño-Yáñez, L., Lara-
González, R., Pensado-Fernández, J. A., Ortega-Solís, R., et al. (2016).
“Ecología, restauración y regeneración en la región del Cofre de Perote,” in El
Cofre de Perote Situación, perspectivas e importancia. eds H. V. Narave-Flores,
L. Garibay-Pardo, M. Chamorro-Zárate, L. R. Álvarez-Oseguera and Y. De la
Cruz-Elizondo (México: Universidad Veracruzana), 147–152.
Sánchez-Velázquez, L. R., Dominguez-Hernández, D., Pineda-Lopez, M., del, R.,
and Lara-González, R. (2011). Does Baccharis conferta shrub Act as a nurse
plant to the Abies religiosa seedling? Open Forest Sci. J. 4, 67–70. doi: 10.2174/
1874398601104010067
SEMARNAT (2011). Apoyos CONAFOR - Reserva de la BiosferaMariposa Monarca.
México: SEMARNAT.
Suding, K. N., and Hobbs, R. J. (2009). Threshold models in restoration and
conservation: a developing framework. Trends Ecol. Evol. 24, 271–279. doi:
10.1016/j.tree.2008.11.012
Sugita, H., and Tani, M. (2001). Differences in microhabitat related regeneration
patterns between two subalpine conifers, Tsuga diversifolia and Abies mariesii,
on Mount Hayachine, northern Honshu, Japan. Ecol. Res. 16, 423–433.
Universidad Nacional de Córdoba (1993). Determinación del Contenido Hídrico del
Suelo. Technical Report, NO. 8.
Van der Salm, C., de Vries, W., Reinds, G. J., and Dise, N. B. (2007). N Leaching
across European forests: derivation and validation of empirical relationships
using data from intensive monitoring quadrats. Forest Ecol. Manag. 238, 81–91.
Von Arx, G., Graf, E. P., Thimonier, A., and Rebetez, M. (2013). Microclimate in
forests with varying leaf area index and soil moisture: potential implications for
seedling establishment in a changing climate. J. Ecol. 5, 1201–1213.
Zahawi, R. A., and Augspurger, C. K. (1999). Early plant succession in abandoned
pastures in Ecuador. Biotropica 31, 540–552.
Zahawi, R. A., Reid, J. L., and Holl, K. (2014). Hidden costs of passive restoration.
Restorat. Ecol. 22, 284–287.
Conflict of Interest: The authors declare that the research was conducted in the
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The handling Editor and reviewer JD, declared their involvement as co-editors in
the Research Topic, and confirm the absence of any other ongoing collaboration.
Copyright © 2020 Guzmán-Aguilar, Carbajal-Navarro, Sáenz-Romero, Herrerías-
Diego, López-Toledo and Blanco-García. This is an open-access article distributed
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