Vegetation Structure and Environmental Conditions of Forest Edges in Panama

Vegetation Structure and Environmental Conditions of Forest Edges in PanamaAuthor(s): Guadalupe Williams-LineraSource: Journal of Ecology, Vol. 78, No. 2 (Jun., 1990), pp. 356-373Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2261117 .Accessed: 27/03/2011 12:20

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Journal of Ecology (1990), 78, 356-373

VEGETATION STRUCTURE AND ENVIRONMENTAL CONDITIONS OF FOREST EDGES IN PANAMA

GUADALUPE WILLIAMS-LINERA*

Department of Botany, University of Florida, Gainesville, Florida 32611, U.S.A.

SUMMARY (1) Micro-environmental conditions, vegetation structure and tree mortality in five

forest edges ten months to twelve years old were studied in the tropical premontane wet forest of Panama.

(2) Along transects from clearings to the interior of the forest, the greatest change in temperature and relative humidity occurred between 2 5 and 15 m into the forest. The forest canopy was most open at the clearing-forest border. At the most recently cleared site, this open canopy extended farther into the forest edge than at sites where clearing took place earlier.

(3) Density and basal area of trees ( < 10 cm diameter at 1 3 m high) were twice as great at the forest edges compared with the interior of forests at the sites where the boundaries were created five to twelve years previously.

(4) Floristic composition was unchanged along transects from the forest boundaries to the interior of forests and light-demanding species were not more abundant at forest edges compared with the forest interior.

(5) The edge: interior ratio of trees that died after the edges were created was 14: 1. (6) Beyond 15-25 m into the forest, neither environmental conditions nor the forest

structure and tree mortality were influenced by proximity to the forest boundary. Between 0 and 15 m, however, vegetation structure changed with both distance from the forest boundary and time elapsed since clearing. This study indicates the ecological significance of edge vegetation as a buffer protecting forest vegetation from conditions in adjacent clearings.

INTRODUCTION When forests are cut and abrupt transitions of the vegetation result between clearings and the remaining forest, the environmental conditions at the boundary change markedly (Lee 1978; Lal 1987; Kapos 1989). Canopy removal increases the amounts of solar radiation and rainfall that reach the soil surface. The high relative humidity commonly observed in a forest interior decreases in clearings because of increased air temperature and wind velocity, and decreased rates of transpiration (Schulz 1960; Grubb & Whitmore 1966; Raynor 1969; Lawson, Armstrong-Mensah & Hall 1970; Lee 1978; Lawson, Lal & Oduro-Afriyie 1981; Fritschen 1985; Lal 1987). Environmental conditions at forest boundaries are intermediate between those prevalent in clearings and forest interiors. However, there is little information concerning the distance into the interior tropical forest at which these boundary conditions prevail.

Vegetation structure and floristic composition at the edge, and some distance into the interior of the forest, reflect changes in abiotic conditions (Wales 1972; Ranney, Bruner & Levenson 1981; Lovejoy et al. 1983; Miller & Lin 1985). For example, a structural analysis of temperate deciduous forest edges showed that basal area and stem density of trees and saplings decrease from the edge toward the interior for a distance of about 15 m (Ranney, Bruner & Levenson 1981). In an Acer rubrum (red maple) forest edge, leaf density was twice as large on the forest boundary as that in the interior (Miller & Lin 1985). In

* Present address: Instituto de Ecologia, Apartado Postal 63, Xalapa, Veracruz 91000, Mexico.

356

G. WILLIAMs-LINERA 357

temperate forests, trees at the forest boundary often develop low and thick branches on the side of the clearing, and also asymmetrical boles that lean toward the clearing (Wales 1972; Ranney 1977; Levenson 1981). Tropical forest boundaries have been described as marginal communities, unlike those of the forest interior. The former are characterized by dense strips of vegetation with many small trees, vines, woody climbers and heliophilous ground herbs (Longman & Jenik 1974). Within five years following the creation of a forest edge, forests in central Amazonia reportedly developed a zone of vines between 10 m and 25 m wide and secondary vegetation which acted as a windbreak, shaded the forest interior, decreased the sunlight and increased the humidity, compared with conditions immediately following clearing (Lovejoy et al. 1986). Although the tropical-forest edges are potentially important for forest preservation, and their occurrence has increased with deforestation, data on vegetation structure of these edges are lacking.

It has been suggested that the species composition of the vegetation of forest edges and the interiors of forests are different. Temperate-forest edges are reported to have more shade-intolerant individuals than those of the forest interiors (Wales 1972; Ranney, Bruner & Levenson 1981). In central Amazonia, secondary species invade forest edges and create a band of secondary vegetation among the trunks of dead and fallen trees (Lovejoy et al. 1984).

Proximity to the remaining forest may affect the species composition of vegetation in clearings over time. For instance, initial invasion of shrubs into the edge of a prairie may be a prerequisite for a subsequent successful invasion of hardwood forest species (Petranka & McPherson 1979). Plant species that produce fleshy fruits and frugivores that disperse their seeds are abundant along the edges of temperate forests (Thompson & Willson 1978). Trees of Stemmademia donnell-smithii (Apocynaceae), the fruits of which are dispersed by birds, produce more fruits at the edge of tropical pastures than in the forest interior (McDiarmid, Ricklefs & Foster 1977). Pastures adjacent to forest probably receive more propagules of tree species than pastures farther from the forest (Augspurger & Franson 1988). Continued agricultural activities are necessary to maintain pasture close to forest boundaries.

When isolated from the protection of neighbouring vegetation, forest trees at a newly created boundary have higher probabilities of dying than those in the forest interior because of their exposure to wind and other environmental conditions (Brokaw 1985; Lovejoy et al. 1986; Hubbell & Foster 1988; Lawton & Putz 1988). In the north-eastern United States forests of Abies balsamea, trees close to the clear-cut edges uproot more readily than trees within the forest. In general, increased mortality of trees occurs at a newly created edge following clearing (Gordon 1973; Sprugel & Bormann 1981).

This investigation was designed to study the effects of newly created forest boundaries on environmental conditions and the structure of tropical vegetation in Panama. The interrelationships between vegetation structure, tree mortality and environmental gradients were examined at the forest edges. In addition, the importance of cattle grazing and agricultural practices in the maintenance of the floristic composition and vegetation structure at the forest edge was investigated.

METHODS

Study area The study area is located near the border of the Province of Panama and the San Blas

Comarca, Republic of Panama, along the El Llano-Carti road (901 5'N and 901 8'N,

358 Tropical forest edges in Panama

78?55'W and 78?58'W) in the Tropical Premontane Wet Forest life zone (Tosi 1971). The altitude ranges from 220 m to 325 m a.s.l. Average temperature is 26 ?C but diurnal variation is greater than annual variation. Mean annual minimum and maximum temperatures are 21 ?C and 29 ?C, respectively. Annual average relative humidity is 86%. Mean annual rainfall is about 3500 mm; the drier season (< 100 mm month-1) extends from January to March. Soils are very acid (pH <4-6) and clayey, and contain large quantities of exchangeable Al and organic matter and low concentrations of exchangeable P, K, Ca and Mg.

Five clearings adjacent to undisturbed primary forest on the Pacific side of the continental divide were selected for this study. Details of the study sites are presented in Table 1. Site age was determined by questioning local inhabitants. The age of the edge is the time elapsed since the forest was cut. The disturbance history was similar in all the sites. The process of creating pastures begins with the cutting of several hectares of forest during the dry season and burning the area within a month after the vegetation dries. With the first rains (April and May), rice or corn is usually planted for one or two years depending on soil fertility. During the final crop, seeds of the exotic grass 'ratana' Ischaemum indicum (Houtt.) Merrill are sown, and the pasture that develops is grazed by cattle. Wire fences between the forest and the pasture were only present at the seven-year- old site. Cattle enter the edge of the forest where trails start or where wood has been extracted. Cattle did not penetrate beyond the immediate edge of the forest. There was no evidence of fire penetrating the edges of the forest at study sites.

Pasture-forest environmental gradients Maximum and minimum temperatures and relative humidity were measured along one

transect at each of the study sites. The transect was 70 m long, extending from 25 m into the pasture to 45 m into the forest. Sun shelters for the thermometers were constructed at 150 cm above the ground, using palm fronds. Eight weather shelters were placed at 10-m intervals along each transect, and one more was placed on each immediate pasture-forest border. Temperature and relative humidity (measured using a sling psychrometer) were recorded (at noon) three times a week for two weeks at each site during the dry season (January-March 1987). Measurements were made one site at a time; this explains some of the differences in absolute values of temperature and relative humidity among sites.

TABLE 1. Characteristics and disturbance history of the study sites near the border of the Province of Panama and the San Blas Comarca, Republic of Panama.

Cleared Edge age Altitude Slope area (years) (m) (?) Aspect (ha) Disturbance history of adjacent clearing

0 8 330 5-15 NW 5 rice field when the study was carried out 5 315 10-30 NE 20 rice and corn for two years then converted to pasture, 15 cattle,

1 horse 7 325 5-15 N 10 rice for a year then converted to pasture, 8 cattle, 2 horses

*10 340 10-25 SW 10 rice for one year then converted to pasture, 20-30 cattle, 4 horses most of the year

* 12 340 10-30 NW 20 * Ten-year-old and twelve-year-old sites were adjacent along the same ridge, separated by a deep forested

gorge.

G. WILLIAMS-LINERA 359

Overstorey density of trees was estimated by hemispherical canopy photographs (taken with a fish-eye lens placed 60 cm above the ground in February 1987). The photographs were taken at 0, 5, 15 and 25 m along two transects extending from the pasture-forest border into the forest at each study site. The negatives were analysed for canopy openness (percentage of the hemispherical image unobscured by vegetation) using a computerized canopy photograph analysis program (Becker, Erhart & Smith 1986).

Forest vegetation structure At each study site, vegetation was surveyed along four randomly located 10-m-wide

strips from the pasture-forest border to 20 m into the forest. The belt transects were divided into 5-m x 10-m plots parallel to the edge of the forest. In each plot number and diameter at breast-height (dbh: diameter at 1 3 m) of all trees and lianas ( > 5 cm dbh) were recorded. In a 5-m x 5-m subplot randomly selected in each of the 5-m x 10-m plots, the number and dbh of all woody plants (< 5 cm dbh but > 2 m tall) were recorded. Woody and herbaceous plants < 2 m tall were recorded by life form as a percentage cover in a randomly located 2-m x 2-m subplot in each of the 5-m x 1 0-m plots. All plants were identified. Forest vegetation height adjacent to pastures was measured with a Haga inclinometer.

To assess the depth of penetration of edge effects into the forest vegetation, the transects were extended to 60 m into the forest interior. Plots of 5 m x 5 m, as described above for plants < 5 cm dbh, were placed at intervals of 20 m along two transects. Point- quarter samples of trees (Cottam & Curtis 1956) were taken at 20-m intervals along each 20-m transect. The dbh and distance to nearest neighbouring tree were recorded in the size classes: trees 5-9 9 cm dbh, 10-19 9 cm dbh and ,20 cm dbh. Voucher specimens were deposited at the Herbarium of the University of Panama.

Effect of agricultural practices on the pasture vegetation at the boundary Permanent plots were established to assess the effect of cattle grazing and pasture

maintenance on near-edge vegetation in the pasture. Adjacent to edges created 5, 7, 10 and 12 years previously, two series of 2-m x 2-m plots were set up from the edge to 20 m into the pasture. To exclude cattle and agricultural activities a barbed-wire fence was built around one series of plots at each site, and the other was used as a control. The fence was located 1 m outside the plots. Percentage cover, average plant height by life form and floristic composition were recorded before construction of the exclosures (August- September 1986) and again in February, June and November 1987.

Effect of edge proximity on dead tree density To assess the effect of proximity to the forest boundary on tree mortality, all fallen,

broken, or standing dead trees in a 50-m x 60-m plot at each of the five study sites were counted. The short axis of the plot was the forest-pasture boundary. Each plot was divided into strips 5 m wide by 50 m long. In each strip the number of dead trees and the apparent mode of death (broken, uprooted, or died standing), diameter class (5-9 9, 10-19 9, 20-29 9 and > 30 cm dbh), and decay state of each were recorded.

The effects of edges on tree death cannot, however, be assessed using all dead trees, because some undoubtedly died prior to the creation of the edge. Therefore, decay classes were used to estimate when the trees had died. The decay classes used in this study (adapted from Maser & Trappe 1984) were as follows:


Class 1 mostly intact and undecayed trees, small twigs and even leaves present; Class 2 bark intact, branches present; Class 3 pieces of bark start to slough off but wood is still hard; Class 4 bark generally absent, wood starts to rot; Class 5 bole easily breaks down into small pieces of rotten wood when it is struck.

Classes 1 and 2 were assumed to include trees that died after the creation of the edges; classes 4 and 5 include trees that were presumed to have died prior to clearing, regardless of the time that had elapsed. Class 3 was used as an intermediate class for trees that had probably died before felling in the youngest clearings after the creation of the edges in the oldest clearings.

Data analyses To estimate distances at which the edge effect can be detected in both the clearings and

the forest, data of maximum temperature were fitted in a post-hoc piecewise linear regression model:

(ao; (Y < Yo) y = bo + b1D; (yo < D < Y1). (1)

Co; (y > Yi)

To estimate the distance into the forest up to which edge effects are manifest, canopy openness, basal area and density of woody plants ( < 5 cm dbh), and dead tree density were fitted in a post-hoc piecewise regression model:

y = bo + b1D; (y < y1). (2)

Both models are subject to the constraints that:

at y = yo, ao = bo + blDo, and at y = yl, co = bo + b1D1.

In model 1, D is distance along the pasture-forest gradient, yo is the estimated distance into the pasture at which maximum temperature starts to decrease, y, is the estimated distance into the forest where the variable is not affected by the proximity of the edge, ao is the predicted temperature at yo, and co is the predicted temperature at y . In model 2, D is the distance along the forest border and the interior of the forest, y, is the estimated distance into the forest up to which edge effects are manifest, and bo is the predicted value of the variable (canopy openness, basal area, density, or dead tree density) at the clearing- forest border. Piecewise linear models were fitted with SAS (SAS 1984) and SYSTAT (Wilkinson 1986).

ANOVA AND t-tests were performed with SYSTAT (Wilkinson 1986). All proportions were arcsine-square-root-transformed before being subjected to parametric tests. Means were compared using the Waller-Duncan Bayes LSD (Petersen 1985). Variation about the mean is reported as mean + one standard error, unless otherwise specified.

RESULTS

Pasture-forest environmental gradients In the pastures 25 m from the forest border the average maximum and minimum

temperatures during the dry season of 1987 were 1 5-3 1 ?C and 0-10 ?C higher,


TABLE 2. The effect of edge-age on the distance, estimated by regression model, to which the proximity to the forest boundary affects maximum temperature in the clearing (yo) and in the forest (yi), and predicted maximum temperature ao, at distance yo and co at distance yl. The clearing-forest border is at 0 m. Values in

parentheses are standard errors. Clearing Forest

Maximum Maximum Edge age Distance Yo temperature ao Distance YI temperature cO (years) (m) (OC) (m) (OC)

08 -94 (304) 279 69 (271) 257 5 -11.0(6 11) 299 25(375) 274 7 - 105 (384) 264 150 (394) 252 10 -07 (022) 276 65 (088) 252 12 -90 (1 73) 28 1 67 (1 64) 254

respectively, than corresponding values of 15-45 m into the forest. Midday relative humidity was 7% lower on the average in the clearing than 15-45 m into the forest.

Maximum temperature and the relative humidity progressively changed along pasture- forest interior transects in all sites. Because relative humidity is related to temperature, measurements of maximum temperature were used in a piecewise linear model to estimate the distance at which environmental conditions were affected by the proximity of the edge. The greatest change in maximum temperature (and consequently in relative humidity) occurred between 0 7 m and 10 5 m from the edge into the pasture and between 2 5 m and 15 m from the edge into the forest (Table 2). Beyond 15 m into the forest, changes in maximum temperature were not significant, irrespective of the time that has elapsed since clearing.

Along transects into the interior of the forest the canopy was most open at the clearing- forest border (Fig. 1). Piecewise linear regression indicated that the ten-month-old site had a higher percentage of canopy openness and that the effects of conditions at the edge were evident farther into the forest than at sites where clearing took place earlier (Table 3).

TABLE 3. The effect of edge age on the distance estimated by regression model, to which the proximity to the forest boundary is detected into the forest interior, and predicted values at the forest border of canopy openness, basal area and density of woody plants (< 5 cm dbh), and density of dead tree (5-m x 50-m strips parallel to

edge). Values in parentheses are standard errors. Distance (m) into forest (yi) and predicted Edge age (years)

Condition value 0 8 5 7 10 12

Canopy (%) 96 (0 86) 5 4 (0 15) 5 3 (0 42) 4 9 (0 53) 6 0 (0 39) ao 430(1 93) 31 8(066) 153(086) 128(1 72) 104(038)

Basal area (m2 ha-') yi 0.0 (1 06) 14 4 (2 15) 7 5 (2 34) 7.4 (0 78) 9 8 (1-31) ao 20(021) 51(044) 32(075) 61(054) 55(050)

Density (stems ha-') Yi 1-4 (1-66) 6-1 (1 20) 6-7 (2 80) 12 9 (3-5) 12 7 (0-97) ao 3515 (828) 7770(906) 6062(860) 8092 (1251) 9692 (783)

Dead trees (number 0-025 ha- ) y' 13.0 (1-00) 7-6 (1-77) 6.7 (0-60) 6-8 (0-93) 9 4 (1-41) ao 17-0 (1-21) 10.5 (2-63) 10-9 (2 17) 13 8 (2-72) 15 5 (2 12)

362 Tropicalforest edges in Panama

40 [

(a)

20 _

40: (b)

a1) 40 - (c )

a) .

" 40- (d)

20-

-0

40: (e)

20

40. (d, )

0 5 15 25 Distance into the forest (m)

FIG. 1. Changes in the percentage of canopy openness with distance into the forest at the sites in Panama where boundaries were (a) ten months, (b) five years, (c) seven years, (d) ten years, and

(e) twelve years old. Error bars represent one standard error.

Forest vegetation structure

Density and basal area of woody plants (< 5 cm dbh) decreased with distance from the pasture-forest border into the forest interior, except along the forest edge of the most recently cleared area (Fig. 2). Between 0 m and 20 m from the edge of the forest sites older than five years old showed the highest values for density and basal area of trees (< 5 cm dbh) (Fig. 2). Edge effects were estimated to extend 6-14 m into the forest interior (Table 3). Trees in the 5-9-9-cm-diameter class also showed higher values for density and basal area up to 20 m from the pasture-forest boundary than beyond 20 m into the interior of the forest at sites where the boundaries were created seven, ten and twelve years previously. The density and basal area of trees in other diameter classes (10-19-9 and > 20 cm dbh), however, did not change with distance into the forest (Table 4).

Stem density of woody plants (< 5 cm dbh and > 2 m tall) was 49-55% less in the interior of the forests than at the edges, except at sites which were cleared ten months and seven years previously (Table 4). At the most recently cleared site, saplings present when the field was created grew in height, and apparently there was little recruitment from seeds or seedlings.

Based on differences in stem density and basal area for woody plants (< 5 cm dbh), and temperature, relative humidity and openness of the canopy, a 'forest edge' was defined as a 20-m-wide strip of forest measured from the trunk of the trees > 10 cm dbh which were closest to the pasture. Data from 20-60 m from the forest edge were considered to be representative of the forest interior.

(a) (a)

T (b) 5 - ~~~~~~~(b) 5000: t 01111l

sooflfpL TlfllB (c) 5 0 0 0- ' iSlul (c)

E

0 C)~~~~~~~~~~~~d

(e) 5 (e)

5000:

0 5 10 15 2040 60510 150 i5204060 Distance into the forest (mi)

FIG. 2. Changes in density and basal area with distance into the forest in Panama for woody plants (< 5 cm dbh) at the sites where the boundaries were (a) ten months, (b) five years, (c) seven years,

(d) ten years and (e) twelve years old. Error bars represent one standard error.

TABLE 4. The effect of the time that has elapsed since clearing on stem density and basal area at the forest edge and within the forest. Values in parentheses are standard errors. Values at the forest edge are averages of sixteen plots for the ten- month and five, ten and twelve-year-old boundaries and thirteen plots for the seven- year-old site. Forest interior values are averages of six point-quarter samples (for

trees > 5 cm dbh).

dbh Stem density (stems ha-') Basal area (m2 ha- 1) category Edge age Edge Interior Edge Interior

(cm) (years) 0-20 m 20-60 m Edge: interior 0-20 m 20-60 m Edge: interior < 5 0 8 3621 (387) 3343(662) 1 08 2 30 (0-21) 1 60 (0 35) 1-44

5 6030(439) 3455(407) 1 75 3 45 (0 36) 1 86 (0-21) 1 85 7 5186 (482) 4157 (169) 1-25 2 44 (0 31) 1 88 (0 27) 1 30

10 6126 (564) 3044 (481) 2 01 3 55 (0 39) 1 38 (0 35) 2 57 12 5553 (692) 2749 (423) 2 02 2 77 (0-39) 1 47 (0 06) 1 88

5-9 9 0 8 709 (94) 636 (126) 1 11 2 96 (0-42) 2 97 (0-64) 1-00 5 811 (121) 782(102) 1-04 3-24(046) 3 32(047) 098 7 959 (100) 617 (105) 1 55 4 04 (0 44) 2 73 (0 46) 1 48

10 1036 (122) 676 (38) 1 53 4 31 (0-55) 2 85 (0 17) 1.51 12 1086 (167) 649 (54) 1 67 4 39 (0-73) 2 96 (0 24) 1 48

10-19-9 0 8 438 (52) 356 (24) 1 23 6 30 (0 75) 6-05 (0 56) 1 04 5 370 (74) 376 (47) 0 98 5-09 (1 05) 6 21 (0 91) 0-82 7 478 (68) 289 (35) 1 65 6 89 (1-07) 4 69 (0 54) 1-47

10 443 (82) 506(71) 0-88 646(1 15) 8 70(1 24) 0 74 12 681(118) 497(61) 1-37 920(1 51) 796(092) 1 16

)20 08 250(51) 211 (19) 1 18 25.03(7.77) 23-45(290) 1 07 5 312 (58) 243 (22) 1 28 35-74 (8 77) 36-25 (6-81) 0.99 7 239 (63) 202 (23) 1 18 2989(8-92) 26-54(4.89) 1 13

10 341 (56) 300 (37) 1-14 30-70 (6-93) 27 35 (4 56) 1-12 12 285 (53) 317 (29) 0-90 31 82 (7 97) 50 88 (9 68) 0-63


Within the forest, average density of trees (> 10 cm dbh) was 659 stems ha-1, basal area averaged 35-9 m2 ha-1, and canopy height was between 30 m and 35 m, with emergent trees 45-50 m tall. Woody plants (< 5 cm dbh) formed a very conspicuous layer 5-8 m tall along all forest edges, except at site of the most recent clearing. Dead branches, however, were observed on the side of tall trees facing the pasture at the forest edge.

Understorey percentage cover Coverage by plants (< 2 m height) was sparse; no significant differences were detected in relation to distance into the forest interior, irrespective of the age of the clearing (one-way ANOVA, P > 0-05). The average percentage cover was 15 8 (n = 97 plots; S.D. = 5.17). Cover by life-form was: tree seedlings 4 6%, palms 4 4%, Zingiberales and Araceae 3 2%, ferns 1[4%, herbaceous vines 0 9%, shrubs 0 5%, grasses 0-5%, liana seedlings 0 2% and herbs 0 1 %.

Life-forms Dicotyledonous trees represented 75-88% of the woody plants (<5 cm dbh) and

decreased in density with distance from the forest edge. Palms represented 4-5%, and lianas 2 8-13 0% of all individuals (< 5 cm dbh), respectively. Few shrubs were present in forest interiors (Fig. 3).

The tree: palm ratio of approximately 9: 1 was observed for woody plants (> 5 cm dbh) both in forest interiors and along forest edges. Lianas (>5 cm dbh), however, were recorded only at the boundaries of forests (Fig. 3).

Floristic composition A total of fifty-six families was represented by woody plants greater than 2 m tall

(Table 5). Species composition was similar in the forest interiors and the forest edges; light-demanding species such as Cecropia spp. (fourteen individuals out of 2738), Vismia macrophylla Kunth (four individuals), or Palicourea spp. (thirty-eight individuals) were not abundant along forest edges and in the forest interior. In the forest interior these species were restricted to gaps encountered along transects.

Effect of agricultural practices on the pasture vegetation at the boundary Pasture vegetation near forest boundaries five, seven, ten and twelve years old was

combined into three groups: planted Ischaemum indicum, herbaceous plants, and woody plants. The most common herbs were ferns, Lycopodium spp., sedges, grasses other than I. indicum and vines belonging to the families Passifloraceae, Rubiaceae, Smilacaceae and Sapindaceae. The most common woody plants were shrubs of the family Melastomata- ceae, Solanum spp., Vismia macrophylla, Cecropia spp. and Piper spp.

Within six to ten months, the percentage cover and height of the pasture vegetation groups, as well as the species composition, changed in the exclosures from which the cattle were excluded, unlike the controls plots which the cattle grazed (Fig. 4). Percentage cover of woody plants increased and I. indicum decreased significantly in the fenced plots in the pasture adjacent to the ten-year-old edges. All vegetation groups in pastures adjacent to the seven-year-old edge and ten-year-old edge, as well as herbs and ratana in pasture adjacent to the twelve-year-old edge, were taller in the fenced plots than in the non-fenced plots (t-tests, P < 0 02). The five-year-old pasture edge did not differ significantly in cover or in height between treatments. In all pastures, grazed plots supported fewer species than non-grazed plots.


<5 cm dbh >5 cm dbh 90 * (a) (a)

60 -

30 where thebounarieswee(a)temonth (b) fi , (b)

601

30-

90 ) twelve (c) 0

g- 60- 0

0E 30 -

60-

90 - e (e)

60-

30 -

TrePalm Liana Shrub Tree Palm Liana

FIG. 3. Changes in life-form proportions of woody plants <i5cm dbh (left) and >5cm dbh (right) at (w ) the forest edge (0-20 i), and (0) in the forest interior (20-60 i), at the sites in Panama where the boundaries were (a) ten months, (b) five years, (c) seven years, (d) ten years and

(e) twelve years old.

Effect of edge proximity on dead-tree density The density of dead trees decreased with distance into the forest. Dead trees (> 30 cm

dbh) were 2-6 times more abundant at the forest edges than in the forest. The ratio decreased for smaller-diameter categories (Fig. 5). In the forest, the average density of dead trees ( > 5 cm dbh) was 104 trees ha- 1. Trees in the forest died mainly by snapping of trunks (55%), by uprooting (33%), or while standing (12%). Most of the dead individuals were dicotyledonous trees (84%); palms represented 16% of all deaths.

Edge and interior dead trees were separated into the five decay classes listed earlier. When dead-tree density was examined in relation to decay classes there were differences between forest edge and the forest interior. When the first two classes of least decayed trees were compared, the average edge: interior ratio for all sites was 14: 1 for trees greater than 5 cm dbh. When the last two classes of most decayed trees were used, the number of


TABLE 5. Families and species of woody plants (> 2 m tall) in the study area near the border of the Province of Panama and the San Blas Comarca, Republic of Panama.

Nomenclature follows Croat (1978).

Number of Family Species individuals Percentage Palmae 196 7 16

Socratea durissima H. Wendl. 54 Iriartea gigantea H. Wendl. 24 Calyptrogyne spp. 21

Rubiaceae 179 6 54 Palicourea spp. 42 Faramea spp. 20 Pentagonia spp. 16 Cephaelis spp. 14 Psychotria spp. 11

Moraceae 161 5 88 Brosimum sp. 54 Sorocea spp. 13 Cecropia spp. 14 Perebea guianensis Aubl. 8

Guttiferae 145 5 30 Tovomita spp. 71 Rheedia spp. 18 Symphonia globulifera L. F. 7

Apocynaceae 122 4-46 Bonafousia undulata (Vahl.) Markgr. 89 Aspidosperma sp. 16 Stemmadenia sp. 5

Leguminosae 116 4-24 Inga spp. 53 Macrolobium sp. 15 Pterocarpus sp. 12 Bauhinia spp. 7

Sapotaceae 112 4 09 Manilkara zapota (L.) Van Royen 17 Pouteria spp. 9

Melastomataceae 93 3-40 Miconia chrysophylla Urb. 8 Miconia trinervia D. Don 4 Miconia simplex Triana 4 Miconia spp. (14 species) 70 Clidemia spp. 7

Annonaceae 83 3 03 Xylopia bocatorena Schery. 23 Annona spp. 9

Mystiricaceae 83 3 03 Dialyanthera otoba Warb. 24 Virola spp. 24

Burseraceae 75 2-74 Protium spp. 71 Tetragastris sp. 3

Euphorbiaceae 65 2-37 Mabea occidentalis Benth. 48 Richeria spp. 8

Lecythidaceae 62 2-26 Eschweilera pittieri R. Knuth 15 Gustavia dubia (Kunth) Berg 11 Gustavia sp. 9

G. WILLIAms-LINERA 367 TABLE 5 (continued).

Number of Family Species individuals Percentage

Myrtaceae 62 2-26 Eugenia spp. 22 Calyptranthes spp. 11I Myrcia spp. 6

Lauraceae 59 2-15 Ocotea cernua (Nees) Mez 5

Flacourtaceae 43 1-57 Ryania speciosa Vahl 25 Carpotroche platyptera Pittier 8 Casearia arborea (L. C. Rich.) Urban 4

Other families 371 13-55 Non-identified or non-collected individuals 711 25-97

Total 2738

dead trees at the edge with respect to the interior was roughly the same. The edge: interior ratios were 14:3, 11:2 and 9:0 for trees that died snapped, uprooted and standing, respectively, and which died most recently (decay classes 1 and 2). Decay classes are probably biased because woody decay rates are species specific and depend on bole diameter (Lang & Knight 1979; Harmon 1982). Nevertheless, when decay classes were not considered, average density of dead trees along all forest edges was 206 trees ha -', which is twice the average density found within the forest. Edge effects were estimated to extend farther into the forest at the 1 0-month-old site (1 3 m) than in the oldest edges (Table 3). These results strongly suggest that there is an increase in tree mortality when an edge is created.

(a) (C)

60 501 I fl~~~~~ 40

:05 (b) * (d)

50 ~~~~~~~~60-

0 0 6 10 15 0 6 10 15 Time since exclosure (months)

FIG. 4. Changes with time in cover and height of woody plants (El, 0), herbs (~ )and Ischaemum indicum (ratana grass) (U, U) in (a, c) fenced and (b, d) grazed plots in the pasture in Panama

adjacent to the edge created ten years previously.


I0

(b)

en 10

0~ o VI

a) Q k

E hE Z H !SE;

10 | (e) I0

5 30 60. Distance into the forest (m)

FIG. 5. Number of dead trees in 5-m x 50-m strips parallel to the forest edges at the sites where the boundaries were (a) ten months, (b) five years, (c) seven years, (d) ten years and (e) twelve years old. Diameter categories are: dead trees 5-9 9 cm dbh (E3); 10-19 9 cm dbh (E); 20-29 9 cm dbh

(0); and > 30 cm dbh (U).

DISCUSSION

Beyond 20 m into the forest from the pasture-forest border, environmental conditions and the structure of the vegetation were apparently not influenced by proximity of the forest boundary. The definition of a forest edge, however, depends upon the organisms under consideration. There is some evidence that forest boundaries affect animals farther into the forest than plants. For instance, studies on predation of bird nests showed 'an edge effect' that extended as far as 200-500 m into the forest (Wilcove, McLellan & Dobson 1986; Andren & Angelstam 1988), although Gates & Gysel (1978) found that 'the edge effect' on predation of bird nests extended only 45 m into the forest. Studies of plants in temperate-zone deciduous forests, indicate that the edge effects on basal area, stem density and leaf area density extend only 10-15 m beyond clearings (Ranney, Bruner & Levenson 1981; Miller & Lin 1985).

Edge effects on the micro-environment reach different distances into the forest, depending on the time that has elapsed since clearing and subsequent development of vegetation at the boundary. At these study sites, a change in environmental conditions occurred within 2-5-15 m of the boundary. The temperature, relative humidity, and openness of the canopy were similar to those of tropical wet forests beyond 15-25 m from the boundary of the forest. Kapos (1989), however, suggested that in tropical forests, the


edge effect on the microclimate in the understorey extended 40 m into the forest. Micro- environmental differences along transects at the sites in Panama are concordant with reports that maximum and minimum temperatures are higher, and that the relative humidity is lower in clearings and the borders of clearings and forests compared with corresponding data for the interiors of tropical forests (Schulz 1960; Grubb & Whitmore 1966; Lawson, Armstrong-Mensah & Hall 1970; Longman & Jenik 1974; Thompson & Pinker 1975; Lawson, Lal & Oduro-Afriyie 1981; Lovejoy et al. 1986; Kapos 1989).

Altered microclimatological conditions resulting from forest clearance affect vegetation structure at the boundary. For example, boundaries of clearings greater than five years old (except the one that is seven years old) support twice the density of woody plants in the forest interior. Within five years, most vegetation at the edge of the forest developed a wall of woody plants that insulated the forest from conditions of the clearing. These results are similar to reports from temperate deciduous forest edges, where edge values of stem density were about twice those in the interior of the forest (Ranney, Bruner & Levenson 1981).

The wall of vegetation was mostly composed of tree saplings and palms. Contrary to expectation, lianas (< 5 cm dbh) showed a similar proportion of plants along edges as in the forest interior. Lianas (> 5 cm dbh), however, were only found along edges of the forest. This contrasts with observations that liana seedlings are abundant and plants grow significantly taller along the edges of gaps than either in the centre of gaps or in the forest interior (Putz 1984).

Increased mortality rates of trees at the forest edge are probably related to major changes in micro-environmental conditions. In the 5-m band closest to the forest edge the number of dead trees was greatest. At the forest edge the number of trees that died and were standing was higher than the number in the interior. The ways in which trees died in the forest interior were similar to those observed on Barro Colorado Island, Panama, where most dead trees had snapped and smaller proportions had been uprooted or had died and were still standing (Putz & Milton 1982). Dicotyledonous trees and palms were equally likely to die when close to a newly created forest edge. Trees in the larger diameter classes, however, died in a greater proportion along the forest edges than in the interior of the forests. The increased number of dead trees and the fact that more trees died standing at the forest edge than in the interior of the forest is a reflection of the environmental stress experienced by trees isolated from the protection of neighbouring vegetation (Lovejoy et al. 1986; Waring 1987; Hubbell & Foster 1988; Lawton & Putz 1988).

Altered vegetation structure and increased rates of tree mortality confer a peculiar physiognomy on forest edges. Observed from the pasture side, forest-edge vegetation consisted of a well-defined 5-8-m-tall layer of understorey woody vegetation and tall trees. These two vegetation layers were easily distinguished. Crowns of woody plants, less than 8 m tall were growing toward the clearing. However, crowns of tall trees at the forest edge were growing away from the clearing, and branches were dead on the clearing side. Dead crowns of tall trees apparently differ from those in the temperate zone which grow laterally toward artificially created forest edges (Gysel 1951; Trimble & Tryon 1966; Levenson 1981; Ranney, Bruner & Levenson 1981). In high-altitude forests of Abies balsamea, however, death of exposed trees is common (Sprugel 1976; Sprugel & Bormann 1981). Also, in central Amazonia dead trees are abundant along forest edges (Lovejoy et al. 1984). The prevalence of dying trees along the edges of tropical forests (and high- altitude temperate forests) suggests that environmental conditions at the forest boundary are more severe in the tropics than in temperate regions. Alternatively, tropical forest


trees may be less adaptable to the new environmental conditions when isolated from neighbouring vegetation.

Development of vegetation at the edge of the forest is faster in tropical forests than in temperate forests. The understorey vegetation layer was well developed within five years following clearing and stem densities and basal areas of stems were double those of the forest interior. At the edges of temperate deciduous forest, in contrast, it takes about twenty years before stem densities and leaf densities are twice as large as those of the forest interior (Ranney, Bruner & Levenson 1981; Miller & Lin 1985).

Species composition at the forest edges and in the forest interior consisted mostly of shade-tolerant species with few light-demanding species. Tropical forests present 'a continuum of regeneration patterns' (Augspurger 1984; Brokaw 1987; Hubbell & Foster 1987). For convenience, tree species are classed into a few regeneration groups. Two common classifications are either primary and secondary tree species, or else light- demanding and shade-tolerant species (Whitmore 1975). The shade-tolerant species persist as suppressed seedlings or saplings in the understorey until a canopy gap opens above or near them and permits accelerated growth (Brokaw 1987). The light-demanding species germinate in gaps and grow rapidly. These species can rarely be found growing in the forest understorey, except perhaps under deciduous trees or close to gaps (Hubbell & Foster 1987).

Although vegetation at the forest edge was expected to be dominated by light- demanding plants, shade-tolerant species were abundant at both the forest edge and in the forest interior. Light-demanding species were abundant in the pastures but in the adjacent forest they were mostly growing in light gaps. In temperate deciduous forests, by comparison, species composition often differs markedly between the forest edge and the interior of the forest. Forest edges are more species-rich, and shade-intolerant species are abundant along forest boundaries (Wales 1972; Ranney, Bruner & Levenson 1981). Likewise, abundant regrowth of light-demanding species is observed along tropical forest boundaries in central Amazonia (Lovejoy et al. 1984). The scarcity of light-demanding plants on forest boundaries in Panama indicates that besides the requirement for increased light penetration, secondary species have other requirements for germination and establishment (Williams-Linera 1990). Quality of light (Vazquez-Yanes & Smith 1982; Vazquez-Yanes & Orozco-Segovia 1985) and soil disturbance (Wesson & Wareing 1969; Bell 1970; Putz 1983) may influence the presence of pioneer species at forest boundaries. In addition, the faster growth of existing plants in tropical forest boundaries may curtail the establishment of light-demanding species.

When cattle were excluded from pastures adjacent to forest edges, the general effect was a reduction in pasture grass and an increase in number, percentage cover and height of woody plants. Studies comparing grazed and ungrazed vegetation usually indicate that species diversity is higher in moderately grazed areas (Harper 1977; Collins 1987). Other authors, however, have reported that species diversity decreased as a function of increasing grazing by cattle (Waser & Price 1981), other large mammals (Belsky 1986) and small mammals (Dickinson & Kirkpatrick 1986). In this study, the fenced plots had a higher number of species than unfenced plots, and the pasture grass was shaded by woody species within six to ten months. Growth of secondary species was so fast that even the short period of cattle exclosure indicated a significant change. In the unfenced plots woody plants were browsed or cut when pastures were maintained. An implication of these observations is that abrupt pasture-forest boundaries are a result of grazing and pasture maintenance.


This study shows that vegetation structure at forest boundaries changes with both distance into the forest and over time. When edges are created, micro-environmental conditions in the forest close to the pasture margin are modified.

ACKNOWLEDGMENTS

Thanks are due to Jack Putz for support, encouragement and useful suggestions that greatly improved this manuscript, and P. Feinsinger, J. W. Ranney, N. V. L. Brokaw, J. R. Etherington and anonymous referees for valuable comments on the manuscript. A special thanks to Jack Ewel for helpful discussion. Many people helped in Panama, but I am particularly indebted to Gordon McPherson for helping me to identify plant species and Alan Smith for computerized analysis of hemispherical canopy photographs. I gratefully acknowledge the financial support received from the James Noyes Foundation through the Smithsonian Tropical Research Institute.

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Vegetation Structure and Environmental Conditions of Forest Edges in Panama