Plant Ecol (****) ***:*** ***

DOI **.****/s*****-008-9488-9

Gap-scale disturbance processes in secondary hardwood

stands on the Cumberland Plateau, Tennessee, USA

Justin L. Hart Henri D. Grissino-Mayer

Received: 29 January 2008 / Accepted: 4 August 2008 / Published online: 23 August 2008

Springer Science+Business Media B.V. 2008

Abstract Disturbance regimes in many temperate, region. However, gap size was smaller in the

old growth forests are characterized by gap-scale developing stands, indicating that secondary forests

events. However, prior to a complex stage of contain a higher density of smaller gaps. The majority

development, canopy gaps may still serve as mech- of canopy gaps were projected to close by lateral

anisms for canopy tree replacement and stand crown expansion rather than height growth of

structural changes associated with older forests. We subcanopy individuals. However, canopy gaps still

investigated 40 canopy gaps in secondary hardwood provided a means for understory trees to recruit to

stands on the Cumberland Plateau in Tennessee to larger size classes. This process may allow over-

analyze gap-scale disturbance processes in develop- topped trees to reach intermediate positions, and

ing forests. Gap origin, age, land fraction, size, shape, eventually the canopy, after future disturbance

orientation, and gap maker characteristics were events. Over half of the trees located in true gaps

documented to investigate gap formation mechanisms with intermediate crown classi cations were Acer

and physical gap attributes. We also quanti ed saccharum, A. rubrum, or Liriodendron tulipifera.

density and diversity within gaps, gap closure, and Because the gaps were relatively small and close by

gap-phase replacement to examine the in uence of lateral branch growth of perimeter trees, the most

localized disturbances on forest development. The shade-tolerant A. saccharum has the greatest proba-

majority of canopy gaps were single-treefall events bility of becoming dominant in the canopy under the

caused by uprooted or snapped stems. The fraction of current disturbance regime. Half of the gap maker

the forest in canopy gaps was within the range trees removed from the canopy were Quercus;

reported from old growth remnants throughout the however, Acer species are the most probable replace-

ment trees. These data indicate that canopy gaps are

important drivers of forest change prior to a complex

stage of development. Even in relatively young

forests, gaps provide the mechanisms for stands to

develop a complex structure, and may be used to

J. L. Hart explain patterns of shifting species composition in

Department of Geography, University of North Alabama,

secondary forests of eastern North America.

Florence, AL 35632, USA

e-mail: abpnv4@r.postjobfree.com

Keywords Canopy gaps Disturbance

H. D. Grissino-Mayer

Forest development Mixed hardwoods

Department of Geography, The University of Tennessee,

Succession Tennessee

Knoxville, TN 37996, USA

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132 Plant Ecol (2009) 201:131 146

1998; Yamamoto and Nishimura 1999). Because

Introduction

canopy gaps are generally larger in old growth

remnants, many of the gaps in these forests close by

All forest ecosystems are subject to natural distur-

bance events that shape species composition and the height growth of subcanopy individuals rather than

lateral crown expansion of perimeter trees (Runkle

stand structure. In many forest types, gap-scale

1982). This gap-replacement process creates forests

disturbance processes are the dominant disturbance

mechanisms. Thus, canopy gap characteristics and with complex age and size structures, and patchy

species composition in the canopy (Lorimer 1980;

forest response have been studied in forests through-

out eastern North America to elucidate patterns, and Runkle 1982; Yetter and Runkle 1986; Runkle and

processes of gap-scale disturbances and forest vege- Yetter 1987). Although canopy gaps in secondary

forests are hypothesized to be smaller in size, they may

tation dynamics. The overwhelming majority of

canopy gap studies, however, have been conducted still act as a mechanism for canopy tree replacement,

in old growth remnants (e.g., Lorimer 1980; Barden and stand structural changes associated with older

forests (Clebsch and Busing 1989; Wilder et al. 1999;

1981; Runkle 1982; Cho and Boerner 1991; Runkle

2000). Throughout the Eastern Deciduous Forest Taylor and Lorimer 2003; Cole and Lorimer 2005).

Region, most forested land supports secondary stands The overarching goal of our study was to docu-

(secondary referring to all non-primeval forests prior ment the in uence of localized, natural disturbance

to a complex stage of development) composed of events on the development of secondary hardwood

mixed hardwood species (Cowell 1998; Rebertus and stands during the understory reinitiation stage of

Meier 2001). Few studies have analyzed gap-scale development. Our research was driven by four major

disturbances and forest response in secondary forests questions. Question 1: What are the patterns and

(but see Clebsch and Busing 1989; Dahir and Lorimer processes of canopy gap formation prior to a complex

1996; Wilder et al. 1999; Yamamoto and Nishimura stage of forest development? We hypothesized that

1999), and no such research has been conducted in most canopy gaps would be created by uprooted

mixed hardwood stands on the Cumberland Plateau. stems, as windthrow has been widely reported from

Undoubtedly, forest disturbance dynamics differ many old growth stands and visual observation of the

between old growth remnants and mature secondary forest revealed uprooted trees. Question 2: What

stands. Differences in disturbance characteristics are percentage of the forest is occupied by canopy gaps

attributed to variations in species composition, bio- and what are the shape, size, and age distributions for

mass arrangement, and tree-age distribution. As gaps in developing stands? We hypothesized that the

forests mature, the distance between large individuals land fraction of the forest in gaps would be within the

increases. Tree crowns separate into distinct catego- range of variability reported from old growth stands,

ries, creating a more complex vertical structure, and but the forest would contain a higher density of

species composition shifts to favor later-successional smaller gaps relative to older stands. Question 3: Do

species (Goebel and Hix 1996; Oliver and Larson small canopy disturbances in uence density and

1996; Goebel and Hix 1997). Forest response to diversity patterns in secondary stands? We hypoth-

disturbance events likely differs between old growth esized that larger gaps would support a higher

and secondary stands, because of differences in stand number of individuals as well as higher levels of

structure and species composition, and also because diversity because they should contain more microsite

of the ages of the oldest trees, as older trees are less heterogeneity, and the likelihood of documenting rare

able to respond to increase in available resources species should increase by sampling a larger spatial

resulting from disturbance events (Fritts 2001). area. Question 4: How do the gaps close, and what

In old growth forests, the spacing between large effects do they have on composition and structure in

individuals is greater than in secondary forests. Thus, developing stands? We hypothesized that most gaps

when a canopy tree is removed from an old growth would close by lateral crown expansion rather than

stand, the size of the canopy gap created should be height growth of subcanopy individuals and would

larger than a comparable disturbance during earlier cause the structure of the forest to move from a high

stages of forest development (Clebsch and Busing density of small trees to a lower density of larger

1989; Spies et al. 1990; Tyrell and Crow 1994; Runkle individuals, more typical of older stands.

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Plant Ecol (2009) 201:131 146 133

Methods acidic, highly leached, and low in fertility (Francis

and Loftus 1977; Smalley 1982; USDA 1995; Hart

Study area 2007). Depth to bedrock varies from 1 to 1.8 m and

slope gradients range from 15% to 60%. The

The study was conducted in the Pogue Creek Natural elevation of the study plots ranged from 260 to

Area (PCNA) located in Fentress County, Tennessee, 490 m amsl.

in the north-central portion of the state (Fig. 1). The Climate is classi ed as humid mesothermal with

PCNA is a 1,505 ha reserve managed by the State of moderately hot summers and short-mild to moder-

Tennessee, Department of Environment and Conser- ately cold winters (Thornthwaite 1948). Local

vation, Division of Natural Areas. The PCNA is topography strongly in uences microclimatic condi-

located on the Cumberland Plateau section of the tions. The average frost-free period is 160 days (from

Appalachian Plateaus physiographic province (Fenn- early-May to late-October) and the mean annual

temperature is 13 C. The July average is 23 C and

eman 1938). The underlying geology consists of

the January average is 2 C (USDA 1995). The area

Pennsylvanian sandstone, conglomerate, siltstone,

shale, and coal of the Crab Orchard and Crooked receives steady precipitation during the year with no

Forked Groups (Smalley 1986). The area has irreg- distinct dry season. Mean annual precipitation is

ular topography (Fenneman 1938) characterized by 137 cm and mean annual snowfall is 50 cm (USDA

long, narrow to moderately broad ridges and narrow 1995). Late spring and summer are characterized by

to moderately broad valleys (Smalley 1986). Soils are heavy rains that are often accompanied by moderate

Fig. 1 Map of the Pogue

Creek Natural Area,

Fentress County,

Tennessee. Shaded portion

of the Tennessee inset map

is the Cumberland Plateau

physiographic section

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134 Plant Ecol (2009) 201:131 146

above) canopy gaps were documented by recording

to severe thunderstorms and strong winds (Smalley

the number of paces across each. The fraction of land

1982).

area in canopy gaps was calculated by dividing the

Braun (1950) classi ed the area as part of the Cliff

transect distance in gaps by total transect length

Section of the Mixed Mesophytic Forest Region, but

(Runkle 1985; Runkle 1992). At each gap, physical

local topography in uences forest composition and

site characteristics, including percent slope, aspect,

true mesophytic species only dominate on protected

and elevation, were recorded . When walking tran-

sites. Regionally, forests are intermediate between

sects through a forest, large gaps are more likely to be

mixed mesophytic and Quercus Carya types (Hinkle

encountered than relatively small gaps, and sampling

1978; Hinkle 1989; Hinkle et al. 1993). Forest

estimators have been created to correct for sampling

vegetation patterns of the PCNA were quanti ed by

bias (see De Vries 1974; Pickford and Hazard 1978).

Hart and Grissino-Mayer (2008). The forest was

However, values obtained with the use of estimator

dominated by Carya ovata, Quercus rubra, Q. alba,

and Q. montana. The sparse sapling layer was equations and those obtained by simply dividing

dominated by Acer saccharum. The forest was transect distance in gaps by total transect length are

established in the late 1920s after the cessation of similar (Runkle 1985).

local logging operations. From eld observations and Gap area was determined for expanded and true

investigation of 17 tree cross sections from a previous gaps by, measuring length (largest distance from gap

study, no signs of re or other large-scale disturbance edge to gap edge) and width (largest distance

events were evident since the anthropogenic distur- perpendicular to the length). These measurements

bances of the 1920s (Hart 2007). Castanea dentata were tted to the formula of an ellipse (Runkle 1985;

Marsh was a forest component prior to the arrival of Runkle 1992). Although gap shapes can be highly

Cryphonectria parasitica (Murrill) M.E. Barr (chest- variable (Ferreira de Lima 2005), most gaps at the

nut blight). The blight reached the Cumberland PCNA had elliptical shapes, which is common for

Plateau in the 1920s, and by the end of the 1930s, forests of the southern Appalachian Highlands (Run-

most C. dentata in the region were dead. Thus, the loss kle 1982; Runkle 1992; Clinton et al. 1994). Thus,

of the species roughly coincided with stand initiation. tting the measurements to the formula of an ellipse

was appropriate for this study.

Field sampling Canopy gaps can be created by several different

mechanisms that remove overstory trees. Biotic and

Canopy gaps (n = 40) were located along transects abiotic forest conditions can be modi ed differently

throughout the reserve using the line intersect method by canopy disturbances that are caused by different

gap formation mechanisms. Differences between gap

(Runkle 1982; Runkle 1985; Veblen 1985; Runkle

1992). Gaps were de ned as environments where a origins may also in uence forest response. In order to

visible void space existed in the main forest canopy, analyze these patterns, gap formation mechanisms

were classi ed into one of the three categories (snag,

leaf height of the tallest stems was less than three-

fourths the height of the adjacent canopy, and gap uprooted stem, or snapped stem) according to gap

makers were present. We did not use a minimum gap origin (Clinton et al. 1993). The number of trees

involved in gap formation was also recorded to

size threshold to document the full range of canopy

gaps. Transects were established parallel to slope document the abundance of single-tree versus multi-

contour beginning at randomly selected points tree events.

Gap maker trees were taxonomically classi ed to

throughout the forest. All transects were located

along mid-slope positions. We sampled at mid-slope quantify any species-speci c overstory mortality

patterns and possible composition changes associated

positions, because the mid-slope forests of the reserve

are indicative of slope forests of the greater Cum- with small canopy disturbances. We measured gap

berland Plateau region and the majority of forested maker diameter at breast height (dbh, ca. 1.4 m above

the surface or root collar for downed individuals) and

land in the reserve occurs along mid-slopes. Total

length. Basal area (m2) was calculated for all gap

transect length and transect length in expanded

(boundary de ned by the base of surrounding canopy makers that could be accurately measured and totaled

by gap, to determine the amount of basal area lost per

trees (Runkle 1981)) and true (area unrestricted from

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Plant Ecol (2009) 201:131 146 135

gap maker death dates, tree rings were measured to

disturbance event. This information may be used to

the nearest 0.001 mm using a Velmex measuring

document the amount of biomass naturally removed

stage interfaced with Measure J2X software for all

from a stand through gap-scale processes. Direction

sampled gap makers. The measurement series were

of gap maker fall relative to slope (i.e., down, across,

visually compared to a reference Quercus chronology

or up slope) was also recorded and all gap makers

developed by Hart and Grissino-Mayer (2008) for the

were placed into one of four decay classes (1 4, with

site. We con rmed the graphical crossdating of all

4 being the most decayed) following criteria adapted

gap maker tree-ring series using the computer

from McCarthy and Bailey (1994).

software COFECHA, a quality-control program that

In each gap, we recorded species, crown class, and

uses segmented time series correlation analyses to

diameter of all trees C5 cm dbh to characterize forest

con rm the placements of all tree rings (Holmes

gap vegetation. Crown class categories (dominant,

1983; Grissino-Mayer 2001). In COFECHA, we

codominant, intermediate, and overtopped) were

tested consecutive 50-year segments (with 25-year

visually assessed based on the amount and direction

overlaps) on each undated gap maker series to the

of intercepted light (Oliver and Larson 1996). The

reference Quercus chronology. Once statistically

location of each of these individuals was also

con rmed, we assigned calendar years to all tree

recorded as being in either an expanded or true

rings in each individual undated measurement series.

canopy gap. All saplings (woody stems C1 m height,

\5 cm dbh) in the expanded gap area were tallied by All gap ages were con rmed using gap maker decay

classi cations.

species to characterize gap regeneration patterns. The

Canopy gaps can be caused by the removal of a

number of perimeter trees with dominant or codom-

single tree or a small cluster of trees. Because single-

inant positions in the canopy was documented for

tree gaps may result from the death of a large canopy

each gap, to analyze the number of trees required to

tree and multi-tree gaps may result from the deaths of

complete the canopy surrounding gaps, and the

relatively small trees, the amount of basal area lost

number of canopy individuals with the potential to

between single- and multi-tree gaps was statistically

close the void space through lateral crown expansion.

analyzed using a two-tailed t-test. This information

Tree core samples were collected to aid in the

may be useful to analyze the quantity of basal area

documentation of gap age. A minimum of nine trees

lost by small canopy disturbance events and applied

were cored (mean = 18.6) per gap resulting in the

to harvesting techniques that may mimic natural

collection of 742 cores. Tree core samples or cross

disturbance processes.

sections were also collected from all gap makers that

The rate of gap formation and closure may be

were not in an advanced state of decay (intact bark

balanced or may vary through time. Non-parametric

and no sapwood degradation), to aid with gap age

correlation techniques were used to analyze the

determination and to document the seasonal timing of

relationship between land fraction in gaps and gap

gap events, based on the amount of xylem produced

age. Gaps may be caused by a variety of formation

during the last year of growth. Dating the seasonality

mechanisms that differ in the way overstory vegeta-

of tree death and gap formation illustrates a new

tion is removed, and the mechanism of canopy tree

approach in dendroecology.

removal may in uence gap size. In order to deter-

mine, if a relationship existed between gap size and

Data analyses

gap origin, data were analyzed using a one-way

ANOVA. A Tukey honestly signi cant difference

Tree core and cross section samples were prepared

(HSD) test was used to compare mean expanded and

and processed for dating using the methods outlined

true gap sizes across origin categories to determine if

in Stokes and Smiley (1996). The samples were air-

gap size varied by gap formation mechanism.

dried, glued to wooden mounts, and sanded to reveal

Length and width of gaps were measured in the

the cellular structure of the wood (Orvis and

eld. Ratios were calculated for length to width

Grissino-Mayer 2002) before tree rings were dated

(L:W) of expanded and true gaps to document gap

with the aid of a stereo zoom microscope. All tree

shape characteristics. This information is useful to

cores were visually analyzed for radial growth

understand the variation in the shape of gaps created

releases to document gap age. In order to document

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136 Plant Ecol (2009) 201:131 146

subsequently blown down, was classi ed incorrectly.

by the disturbance and has implications for forest

However, measures were taken to avoid this issue,

response and microenvironmental changes within the

such as documenting the decay class of gap makers

gap environment.

For each gap, density and diversity (H0 ) measures and noting the position of the gap maker relative to

other downed logs. The number of gap maker trees

were calculated for saplings, trees, and total stems

involved with opening the canopy ranged from one to

(all woody stems C1 m height) to document forest

four. The majority (78%) of the canopy gaps involved

response to canopy disturbances. Gap size is believed

the death of only one individual. Of the nine multi-

to in uence stem density and diversity. Correlation

tree gaps, six (66%) resulted from uprooted stems

coef cients were calculated to determine if a rela-

including the gap that consisted of the removal of

tionship existed between gap size and density of

four canopy individuals, while the three other multi-

individuals in gaps. Regression techniques were then

tree gaps resulted from snapped boles.

used to model gap size and density relationships. In

We identi ed 50 gap maker trees in the 40 canopy

order to analyze the relationship between expanded

gaps studied. Most gap makers (n = 36, 72%) could

gap area and diversity patterns, correlation coef -

be identi ed to the species level; however, 4 (8%)

cients were calculated for sapling, tree, and total stem

could only be identi ed to genus and 10 (20%) were

diversity.

too decayed to be taxonomically classi ed. Of the 36

Canopy gaps can close by crown expansion of

gap makers that could be identi ed to species, 12

perimeter trees at canopy level or by the height

different species were represented. The most com-

growth of understory individuals. The likely closure

mon species that caused canopy gap formation was

mechanism, either by height growth or lateral crown

Quercus montana (n = 8). At the genus level, 50% of

expansion, of each gap was recorded in the eld to

all gap makers were Quercus.

document changes in forest structure following the

Diameter was measured at ca. 1.4 m above the

removal of canopy trees. Probable gap successors,

surface or root collar for 46 gap makers. Diameter

which are the individuals that will likely ll the

measurements could not be collected for four gap

canopy void, can often be determined in the eld

makers that were in a state of advanced decay.

(Barden 1979; Barden 1980; White et al. 1985;

Average gap maker diameter at breast height was

Yamamoto and Nishimura 1999). The documentation

38.38 cm 11.6 (SD). The minimum diameter of a

of replacement trees is useful to project the future

gapmaker was 19.5 cm and the maximum was 70 cm.

composition of the stand and to analyze the in uence

The gap maker with a diameter of 19.5 cm was

of canopy gaps on forest succession. In order to

involved in a multi-tree uprooting event that also

quantify recruitment following overstory removal,

included the death of an individual with a diameter of

crown class distributions were constructed for all

28 cm. Average basal area lost per gap was

trees located in true gap environments for the 15 most

0.16 m2 0.10 (SD). The minimum removed was

dominant species with canopy potential. These mea-

0.05 m2 and the maximum was 0.52 m2. Multi-tree

sures may be used to document future canopy trees

gaps (mean = 0.24 m2 0.13 (SD)) resulted in a

and recruitment patterns associated with gap-scale

larger amount (P \ 0.01) of basal area lost compared

disturbance processes.

to single-tree events (mean = 0.14 m2 0.08 (SD)).

Age was determined for all canopy gaps by the

identi cation of radial growth releases, crossdating

Results

the gap makers to document death dates, eld

Gap formation patterns and processes observation, and gap maker decay classi cation.

Gap ages ranged from 1 to 17 years with a mean of

Of the 40 gaps sampled, 8 (20%) were created by 7 years. Multiple gaps occurred in 13 years. The

snags, 16 (40%) were created by uprooted stems, and highest frequency of gap events during any one year

16 (40%) were created by snapped stems. Eventually, was ve, which occurred during 3 years (1999, 2002,

snag trees will fall, generally during mild to severe and 2003).

wind events, possibly causing further disturbance to Gap seasonality was determined for 17 gaps by

the forest. It is possible that a gap created by a snag, examining the amount of xylem produced during the

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Plant Ecol (2009) 201:131 146 137

last year of growth. Other gapmakers were too

decayed for this analysis. Of these 17 events, only

one occurred during the dormant season. For the

dormant season gap, the latewood portion of the last

ring was complete and buds were still present on the

tree. All other gap makers had partial rings, indicat-

ing that the gap events occurred during the growing

season. Because the majority of these individuals had

already completed the production of earlywood prior

to death, we surmise that these events occurred in the

middle or later part of the growing season.

Gap fraction and physical characteristics

Total transect length was 4.47 km, with 15% of the

total length in expanded gaps and true gaps, and 6% in

true canopy gaps only. When percentage values were

standardized at the hectare level, 1,500 m2/ha were in

expanded gaps and 600 m2/ha were in true gap Fig. 3 Mean sizes of expanded and true canopy gaps by gap

origin with standard deviations. Solid bar and different letter

environments. Total transect length in true canopy

indicate a signi cant (P \ 0.05) difference between gap

gaps was plotted by gap age to analyze patterns of gap

origins as detected by ANOVA and Tukey s post-hoc testing

formation and closure (Fig. 2). The largest amount of

The average L:W ratio of expanded gaps was

land area in true canopy gaps occurred in gaps that

1.58:1, with a maximum of 3.60:1 and a minimum of

were 2 years of age and no gap area occurred in gaps

1.01:1. Thus, the average expanded gap was 58%

aged 5, 6, 14, 15, or 16 years. A signi cant negative

longer than it was wide. Similar patterns were

relationship existed, where older gaps occupied a

observed for true gap areas, for which the mean ratio

smaller amount of land area relative to younger gaps.

Average expanded gap area was 213.34 m2 was 2.58:1. The maximum length of true gaps was

475% the width. The minimum L:W patterns of

108.44 (SD). The maximum expanded gap area was

587.91 m2 and the minimum was 47.10 m2. Average expanded and true gaps revealed circular over

true gap area when sampled was 42.78 m2 40.16 ellipsoidal shapes.

(SD), with a maximum of 157.84 m2 and a minimum of

1.14 m2. The size of expanded gaps created by Density and diversity within gaps

uprooted stems was signi cantly larger than that of

The mean number of canopy trees that bordered gaps

gaps created by snags (Fig. 3). No other size differ-

was 6.38 1.79 (SD). The maximum number of

ences between gap origins were signi cant.

perimeter trees was 12, and minimum number of trees

required to complete the canopy around a gap was 4.

In general, larger canopy gaps were bordered by a

higher number of canopy trees relative to smaller

gaps.

The average number of saplings in expanded gaps

was 54.48 28.47 (SD) with a maximum of 144 and

a minimum of 13 (Fig. 4). The mean number of trees

in expanded gaps was 22.73 7.99 (SD) with a

maximum of 44 and minimum of 11 individuals. The

average number of all stems C1 m height in

expanded gaps was 74.20 34.14 (SD). The highest

Fig. 2 Relationship between land fraction in true canopy gaps

number of stems in an expanded gap was 188 and the

and gap age in the Pogue Creek Natural Area in Tennessee

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138 Plant Ecol (2009) 201:131 146

Table 1 Density of saplings (C1 m height, \5 cm dbh) in

expanded canopy gaps in the Pogue Creek Natural Area in

Tennessee

Species Density/ Relative

ha density

Acer saccharum Marsh. 863.63 35.70

Fagus grandifolia Ehrh. 474.70 19.62

Acer rubrum L. 327.83 13.55

Asimina triloba (L.) Dunal 168.03 6.95

Magnolia acuminata (L.) L. 158.63 6.56

Fraxinus americana L. 88.13 3.64

Fig. 4 Mean number of saplings (C1 m height, \5 cm dbh),

trees (C5 cm dbh), and total stems (all stems C1 m height) Liriodendron tulipifera L. 49.35 2.04

with standard deviations in expanded canopy gaps in the Pogue Oxydendrum arboreum (L.) DC. 48.18 1.99

Creek Natural Area in Tennessee

Cornus orida L. 37.60 1.55

Ulmus rubra Muhl. 31.73 1.31

lowest number of individuals was 28. The highest

Nyssa sylvatica Marsh. 30.55 1.26

values for saplings and trees occurred in the same gap

Cercis canadensis L. 29.38 1.21

that was 10 years old and caused by the uprooting of

Tilia heterophylla Vent. 12.93 0.53

four trees.

Aesculus ava Ait. 11.75 0.49

The sum of all saplings in all expanded gaps was

Carpinus caroliniana Walt. 8.23 0.34

calculated by species and standardized at the hectare

Ilex opaca Ait. 8.23 0.34

level to document sapling establishment, and possible

Magnolia tripetala L. 8.23 0.34

species recruitment in gap environments. The most

Quercus montana Willd. 8.23 0.34

abundant species in the sapling layer of expanded

Carya ovata (P. Mill.) K. Koch 5.88 0.24

gaps was Acer saccharum followed by Fagus gran-

Ostrya virginiana (P. Mill.) K. Koch 5.88 0.24

difolia and Acer rubrum (Table 1). Together these

Sassafras albidum (Nutt.) Nees 5.88 0.24

three species comprised almost 69% of all saplings in

Ailanthus altissima (Mill.) Swingle 4.70 0.19

expanded gaps.

Betula lenta L. 4.70 0.19

Acer saccharum represented 29.18% of all trees in

Diospyros virginiana L. 4.70 0.19

true canopy gaps followed by A. rubrum and

Liriodendron tulipifera (Table 2). Collectively, these Quercus alba L. 4.70 0.19

three species represent over half of all trees in true Ulmus alata Michx. 3.53 0.15

canopy gaps. Dominance (based on basal area) was Amelanchier laevis Weig. 2.35 0.10

also calculated for all canopy gap trees. The most Carya tomentosa (Poiret) Nutt. 2.35 0.10

dominant species were A. saccharum and A. rubrum Quercus rubra L. 2.35 0.10

(Table 2). The Acer species were followed by a Ulmus americana L. 2.35 0.10

second tier of species that included L. tulipifera and Hamamelis virginiana L. 1.18 0.05

Carya ovata. No other species represented more than Magnolia macrophylla Michx. 1.18 0.05

6% of the basal area. Species and diameter of all Morus rubra L. 1.18 0.05

snags in true canopy gaps were also recorded. A total Quercus velutina Lam. 1.18 0.05

of 40 snags were documented and mean snag Total 2419.33 100.00

diameter at breast height was 10.89 cm 6.21

(SD). Of the 40 snags within true gaps, 12 different

species were represented with A. rubrum, A. saccha- sapling layer diversity was 2.22 and the minimum

rum and Q. montana being the most common (n = 8 was 0.78. Total species richness of the tree layer was

for all species). 28. Average diversity of all trees in expanded gaps

Expanded canopy gaps contained 34 different was 1.90 0.35 (SD) with maximum and minimum

species in the sapling layer. Mean sapling diversity values of 2.44 and 1.20, respectively. Mean total

(H0 ) was 1.43 0.42 (SD) (Fig. 5). Maximum diversity of all stems C1 m height was 1.95 0.36

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Plant Ecol (2009) 201:131 146 139

Table 2 Density and

Species Density/ Relative Dominance Relative

dominance measures for all

(m2/ha)

ha density dominance

trees (stems C5 cm dbh) in

true canopy gaps in the Acer saccharum 807.30 29.18 0.59 24.34

Pogue Creek Natural Area

Acer rubrum 391.95 14.16 0.32 13.15

in Tennessee

Liriodendron tulipifera 251.55 9.09 0.23 9.37

Carya ovata 146.25 5.29 0.21 8.68

Oxydendrum arboreum 146.25 5.29 0.14 5.91

Fagus grandifolia 175.50 6.34 0.13 5.48

Tilia heterophylla 122.85 4.44 0.10 4.15

Carya tomentosa 81.90 2.96 0.09 3.58

Carya glabra (P. Mill.) Sweet 64.35 2.33 0.08 3.38

Nyssa sylvatica 99.45 3.59 0.08 3.25

Fraxinus americana 70.20 2.54 0.07 2.95

Quercus alba 5.85 0.21 0.07 2.85

Cornus orida 111.15 4.02 0.06 2.59

Quercus montana 40.95 1.48 0.05 1.94

Magnolia acuminata 52.65 1.90 0.03 1.44

Quercus rubra 23.40 0.85 0.03 1.40

Ulmus rubra 40.95 1.48 0.03 1.16

Cercis canadensis 35.10 1.27 0.02 0.89

Carya cordiformis (Wangenh.) K. Koch 17.55 0.63 0.02 0.67

Ostrya virginiana 23.40 0.85 0.02 0.66

Diospyros virginiana 11.70 0.42 0.01 0.51

Sassafras albidum 11.70 0.42 0.01 0.49

Prunus serotina Ehrh. 5.85 0.21 0.01 0.31

Aesculus ava 5.85 0.21 0.01 0.25

Ulmus alata 5.85 0.21 0.00 0.21

Betula lenta 5.85 0.21 0.00 0.17

Magnolia tripetala 5.85 0.21 0.00 0.13

Ulmus americana 5.85 0.21 0.00 0.10

Total 2767.05 100.00 2.41 100.00

(SD). The highest total diversity value was 2.46 and

the lowest was 1.17. Interestingly, diversity patterns

differed by category (i.e., sapling, tree, and total). For

example, the gap with the lowest sapling diversity

was not the same gap with the lowest tree diversity.

However, the gap with the highest sapling and

highest total woody stem diversity values was an

exception.

Signi cant positive relationships were found for

the number of saplings (r = 0.54, P = 0.0003), trees

Fig. 5 Mean diversity for saplings (C1 m height, \5 cm dbh),

(r = 0.73, P \ 0.0001), and total stems (r = 0.62,

trees (C5 cm dbh), and total stems (all stems C1 m height)

P \ 0.0001) (Fig. 6). However, the largest gap did

with standard deviations in expanded canopy gaps in the Pogue

not contain the highest number of stems, which

Creek Natural Area in Tennessee

123

140 Plant Ecol (2009) 201:131 146

Fig. 7 Relationships between diversity values for saplings

Fig. 6 Relationships between the number of saplings (C1 m

(C1 m height, \5 cm dbh), trees (C5 cm dbh), and total stems

height, \5 cm dbh), trees (C5 cm dbh), and total stems (all

(all stems C1 m height) and expanded gap area in the Pogue

stems C1 m height) and expanded gap area in the Pogue Creek

Creek Natural Area in Tennessee

Natural Area in Tennessee

expanded area for all 40 gaps (213.34 m2). The gap

occurred in a gap of an intermediate size class (188

with the largest expanded area (587.91 m2) was

individuals/231.97 m2). A weak negative relationship

projected to close by understory height growth.

existed between sapling diversity and gap size

However, a relatively small gap (153.59 m2) was also

(r = -0.33, P = 0.04) (Fig. 7). A similar pattern

projected to close by height growth of a subcanopy

was also observed for total stem diversity (r = -

individual.

0.39, P = 0.01). Tree diversity showed no relation-

ship to expanded gap size. Shannon diversity (H0 ) is a Of the 10 successor trees documented, ve species

were represented (A. saccharum, A. rubrum, C. ovata,

dimensionless index such that gap size would not bias

Q. montana, and Quercus alba). Acer rubrum was the

the diversity measure.

most common gap successor (n = 3) followed by A.

saccharum (n = 2), C. ovata (n = 2), Q. montana

Gap closure and recruitment

(n = 2), and Q. alba (n = 1). Acer saccharum

represented 28.7% of trees with intermediate posi-

Of the 40 gaps studied, 10 were projected to close by

tions of all 15 selected species within true gap

height growth of understory individuals and the

environments (Table 3). Acer saccharum was fol-

remaining 30 gaps were projected to close by lateral

lowed by A. rubrum (13.45%) and L. tulipifera

branch growth of canopy trees surrounding the voids.

(13.45%), a noted gap-phase species. Collectively,

Mean expanded area of gaps likely to close via the

height growth of understory trees was 285.13 m2 these three species represented 55.6% of the inter-

mediate trees from the 15 selected species. A similar

137.58 (SD), which was ca. 34% greater than the mean

123

Plant Ecol (2009) 201:131 146 141

blowing down snag trees. Standing dead trees are

Table 3 Crown class distributions for all trees (stems C5 cm

dbh) in 40 true canopy gaps in the Pogue Creek Natural Area in often removed by mild to severe wind events, but the

Tennessee

potential for snags to be blown down varies by site

conditions (Jans et al. 1993). Further, snags that

Species Overtopped Intermediate

eventually fall likely alter the forest differently than

Density Relative Density Relative

gaps that are caused rapidly (Franklin et al. 1987;

density density

Krasny and Whitmore 1992; Clinton et al. 1994). The

Acer saccharum 72 36.36 64 28.70

eventual fall of a snag may cause additional forest

Acer rubrum 37 18.69 30 13.45

disturbance, possibly with a greater magnitude than

Liriodendron tulipifera 13 6.57 30 13.45

the initial event. Also, the bole and branches of

Carya ovata 4 2.02 21 9.42 standing dead trees may block sunlight from reaching

Tilia heterophylla 11 5.56 10 4.48 the understory, thereby, facilitating gap closure by

Oxydendrum arboreum 16 8.08 9 4.04 perimeter trees rather than subcanopy individuals.

Fraxinus americana 3 1.52 9 4.04 The percentage of single-tree gaps (78% of gaps

Carya glabra 1 0.51 9 4.04 sampled) was within the range of what has been

Fagus grandifolia 21 10.61 8 3.59 reported from old growth forests of the eastern USA

Carya tomentosa 5 2.53 8 3.59 (Runkle 1990). Of the multi-tree disturbance events,

Quercus alba 1 0.51 8 3.59 most were caused by uprooted stems. Windthrow

Quercus montana 1 0.51 6 2.69 gaps have the potential to cause more site modi ca-

Nyssa sylvatica 12 6.06 5 2.24 tion than gaps caused by other mechanisms, because

Quercus rubra 0 0.00 4 1.79 as the root network is lifted, microtopography (pits

and mounds) and soil characteristics are also modi-

Carya cordiformis 1 0.51 2 0.90

ed (Clinton et al. 1994; Beckage et al. 2000).

Total 198 100.00 223 100.00

Average diameter of gap maker trees was

38.38 cm at breast height and the average diameter

pattern was observed for overtopped positions, with of canopy trees (dominant and codominant crown

A. saccharum being the most abundant (36.36%) classes) that surrounded gaps was 38.83 cm 6.04

followed by A. rubrum (18.69%) and F. grandifolia (SD). This result is contrary to what has been

(10.61%). reported for old growth forests of the southern

Appalachians, where gap makers were signi cantly

larger than border trees (Runkle 1998). This pattern

may be related to the age of the forest. In second

Discussion

growth forests, canopy trees are within a narrower

Gap formation patterns and processes diameter range as their age (and diameter) structure is

not complex. Thus, in mature second growth forests,

The majority (80%) of the gaps documented origi- size does not indicate that one individual is more

nated from uprooted or snapped stems. Other studies likely to be removed from the canopy than another.

have also found these mechanisms to be the most

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