SEEDLINGS DYNAMICS IN UNDISTURBED AND ADJACENT FIRE DISTURBED FOREST IN THE GRAN SABANA, SOUTHERN VENEZUELA
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SEEDLINGS DYNAMICS IN UNDISTURBED
AND ADJACENT FIRE DISTURBED FOREST
IN THE GRAN SABANA, SOUTHERN VENEZUELA
Nelda Dezzeo, Saúl Flores
and Noemí Chacón
SUMMARY
In the Gran Sabana, southern Venezuela, the conversion of forests changed very little over the study period, which is associ-
large forest areas to a mosaic of forest, bush vegetation and sa- ated to the partial balance between initial tree seedlings mortal-
vanna (“savannization”) is a critical environmental issue associ- ity rate (44-66%) and newly recruited tree seedlings survival rate
ated with forest fires. Little is known about the behaviour of the (47-54%). At the end of the study ~80% of the recruited seedlings
seedling community in undisturbed and fire impacted forests in this in undisturbed forest corresponded to tree species, whereas in sec-
region. Recruitment, survival and growth of seedlings was followed ondary forest the proportions of tree (47%) and non-tree (53%)
over a 6 year period in permanent quadrats established in undis- seedlings were rather similar. Growth in height during the 6 year
turbed and adjacent fire-disturbed (secondary) forest. At the begin- period was considerably higher in secondary forest than in undis-
ning of the study, secondary forest showed lower (Pin their modes of life and the roles they The potential vegetation trunks on the forest floor, and the absence
play in the community (Bazzaz, 1991). of the region is considered to be evergreen of an organic surface layer (Table I), the
The regeneration process can be affect- montane forests (Huber, 1995), but the actu- latter being typical of undisturbed forest
ed by disturbances that frequently imply al vegetation is dominated by savannas with in the region, point to the strong impact
changes in environmental factors such as irregularly interspersed forest fragments. of fire. Due to the presence of large and
light, temperature, moisture, nutrients and This vegetation cover is associated with for- still standing burned tree trunks in sec-
wind levels, which influence abundance est fires without intentional land use change ondary forest, it is possible to suppose
and composition of seedlings and saplings by the sparse human population. Several of that before the fire event(s) this second-
in the forest understory (Whitmore, 1998; the savannas in the region are secondary, ary forest had a rather similar structure to
Barnes et al., 1998). originated and maintained by repeated fires that of undisturbed forest.
Numerous studies on the (Huber, 1990; Huber, 1995). According to
impact of disturbances on regeneration EDELCA (1986) the study region suffers Methods
have been carried out in tropical regions. from at least 2000 fires each year, and they
However, few of them have been focused are in great part started by the indigenous The selected areas of
on the long-term monitoring of seedlings population, who set fire for multiple pur- undisturbed and secondary forests were
and saplings (e.g. Uhl et al., 1988; Turner, poses. In many places grass savannas with separated ~600m from each other. The
1990; Nepstadt et al., 1996; Rettenmaier huge charred standing and fallen trunks of undisturbed forest had a closed canopy,
and Fölster, 1999; Rodríguez et al., 2004; the former forests provide evidence of the while the canopy of secondary forest was
Capers et al., 2005; Khumbongmayum et savannization process in the region (Fölster, very open. At each selected area of both
al., 2005; Dupuy and Chazdon, 2006), al- 1986; Huber, 1990). forests, 18 permanent quadrats of 1m2
though these studies are necessary to bet- Field work was conduct- each were set in May 1999. The quad-
ter understand the effect of perturbation ed in a site differentially affected by un- rats were established at least 5m apart to
on tropical forest dynamics. controlled forest fires occurring at least 5 guarantee independence between them.
The present work fo- years before the beginning of this study. The four corners of each quadrat were
cuses on what is really occurring with There is no registered information about marked by wooden stakes driven into the
the regeneration stratum in fire disturbed the fire history of the site, and the infor- ground. In each quadrat all individuals
forest and how is the behaviour of this mation gathered was obtained through in- >5cm and 2.5cm dbh/0.1ha 45 28
2200-4000mm and mean temperature of
17-24°C (Galán, 1984). Precipitation is
a
Number of trees 2.5-10cm dbh/0.1ha 340 103
distributed unevenly throughout the year,
a
Number of trees >10cm dbh/0.1ha 106 13
with a mean monthly rainfall >60mm
a
Basal area >10cm dbh (m2/ha-1) 40 2.0
during the dry season (Dec-Mar), which a
Basal area 5cm dbh/0.1ha 32 36
during this season (Walter and Medina, b
Total biomass (Mg/ha) 411 13
1971). Soils are derived from Precambrian a
Organic surface layer (Mg/ha) 227 0
sediments of the Roraima Group and are a
Maximum height of the trees (m) 30 16
in an advanced phase of weathering (Föl-
ster et al., 2001). a
From Dezzeo et al. (2004); b
from Dezzeo and Chacón (2005).
274 APR 2008, VOL. 33 Nº 4Every year (from May Table II 1999 to May 2005) censuses were con- Abundance* and tree seedling richness** in the sampled ducted to measure growth of the tagged area at the beginning (May 1999) and at the end (May 2005) plants, to count and tag newly recruited of the study seedlings, and to count the previously Variable Undisturbed forest Secondary forest tagged seedlings that had died. These re- cords give detailed information about the Tree seedlings abundance in 1999 26.6 ±9.0 a 1 5.3 ±3.3 b 1 fate of each individual plant in the sample Tree seedlings abundance in 2005 24.5 ±8.8 a 1 6.6 ±3.5 b 1 from one year to the next. In May 2004 Non-tree seedlings abundance in 1999 5.2 ±3.6 a A 14.9 ±11.4 b A the census could not be carried out. The Non-tree seedlings abundance in 2005 3.2 ±3.6 a B 11.3 ±5.0 b A collected data was used to evaluate spe- Tree richness in 1999 7.0 ±1.1 a + 3.5 ±2.0 b + cies richness of tree seedlings (number Tree richness in 2005 6.8 ±1.0 a + 3.8 ±2.3 b + of species per m2), seedling abundance by plant group (number of stems per * Mean number of plants/m 2 ±SD. ** Mean number of tree species/m 2 ±SD. Within a row, values m2), seedling dominance of tree species sharing the same lower case letter did not differ significantly. Within a column, values sharing the (number of stems of a species in rela- same number, capital letter, or plus symbol did not differ significantly (ANOVA, p1m in height during the Initial tree seedling abun- relative abundance. study period were not excluded from the dance was significantly higher (P
nificantly different (P
Higher tree seedling spe- midity and temperature change drastically
cies richness and corresponding higher because of increased insolation and wind
seedling abundance in undisturbed forest penetration (Camargo and Kapos, 1995;
compared to the secondary forest are not Kapos, 1989). This negative effect of an
surprising given the higher richness of open canopy is probably enhanced in sec-
tree species (Dezzeo et al., 2004) and the ondary forest due to the absence of the
higher seed numbers in the soil (Flores litter layer on the soils of this vegetation
and Dezzeo, 2005) of undisturbed forest. type. The presence of leaf litter in gaps or
Similar patterns of higher seedling species areas with very open canopy reduces the
richness and correspondingly higher seed- incoming radiation that reaches the soil
ling abundance have also been shown in surface and prevents large increases in soil
other old-growth tropical forests (Denslow, temperature (Molofsky and Augspurger,
1995; Nicotra et al., 1999; Rettenmaier 1992).
and Fölster, 1999). The tree seedling Another factor that could
abundance found in undisturbed forest be interfering with tree seedling estab-
Figure 4. Values of growth (mean ± SE per m 2) was very high compared to the range of lishment and tree seedling richness in the
in height (cm/year) of the initial censused tree values per m2 reported for 5-100cm tall secondary forest is the large abundance of
seedlings over time in undisturbed and secondary
tree seedlings in continuous forests (9.5- non-tree seedlings (Table II), particularly
forests.
17.2) and fragmented forests (5.5-15.3) in ferns and monocots grasses (Poaceae and
Brazil (Benítez-Malvido, 1998), and with Cyperacea), which can impose barriers for
initially labelled tree seedlings that sur- the range of values (0.7-6.4 plants per tree regeneration (Aide et al., 1996; Cabin
vived over the 6-year period was signifi- m2) reported for 10-50cm tall understory et al., 2002; Slocum et al., 2004).
cantly lower (Pmay be significantly greater for a seed be developed. According to Dezzeo et al. Dezzeo N, Chacón N, Sanoja E, Picón G (2004)
landing in the forest understory than for (2004) the organic surface layer plays an Changes in soil properties and vegetation
characteristics along a forest-savanna gradi-
a seed dispersed into a gap. Competition important role in maintaining the fertil- ent in southern Venezuela. For. Ecol. Manag.
with herbaceous species could be a signif- ity of the soils in undisturbed forests, and 200: 183-193.
icant factor leading to increased mortal- their absence on the soils of secondary Dupuy JM, Chazdon RL (2006) Effects of vege-
ity in gaps (Dupuy and Chazdon, 2006). forest is a critical factor determining the tation cover on seedling and sapling dynam-
However, this cannot explain the higher capacity of this vegetation type to recover ics in secondary tropical wet forests in Costa
tree seedling mortality found in second- from fire disturbance. The development of Rica. J. Trop. Ecol 22: 65-76.
ary forest, because of the mortality of strategies to control the use of fire or the EDELCA (1986) Resumen estadístico de los in-
cendios forestales en la cuenca alta del Río
non-tree seedlings was also very high. implementation of effective fire suppres- Caroní 1986. Mimeo. Caracas, Venezuela.
The better light condi- sion practices in the region is of funda- 23 pp.
tions in secondary forest have consider- mental importance for the forest conser- Flores S, Dezzeo N (2005) Variaciones tempora-
ably favoured the growth in height of the vation in the Gran Sabana. les en cantidad de semillas en el suelo y en
tree seedlings compared to the growth in lluvia de semillas en un gradiente bosque-
undisturbed forest (Figure 2). The positive Acknowledgments sabana en la Gran Sabana, Venezuela. Inter-
ciencia 30: 39-43.
influence of an open canopy (forest gap)
on seedling growth has been associated This article is a con- Fölster H (1986) Forest-savanna dynamics and
desertification processes in the Gran Sabana.
with relatively high levels of photosyn- tribution to the research project Atmos Interciencia 11: 311-316.
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DINÁMICA DE PLÁNTULAS EN UN BOSQUE NO PERTURBADO Y EN EL BOSQUE ADYACENTE PERTURBADO
POR EL FUEGO EN LA GRAN SABANA, SURESTE DE VENEZUELA
Nelda Dezzeo, Saúl Flores y Noemí Chacón
RESUMEN
En la Gran Sabana, al sureste de Venezuela, la conversión de ambos tipos de bosque variaron muy poco a lo largo del periodo
extensas áreas boscosas a un mosaico de bosque, vegetación ar- de estudio, lo cual está asociado con el balance parcial entre
bustiva y sabana (“sabanización”) es un asunto ambiental crí- tasa de mortalidad de plántulas marcadas al inicio del estudio
tico asociado con incendios forestales. Poco se conoce acerca (44-66%) y tasa de supervivencia de nuevas plántulas arbóreas
del comportamiento de la comunidad de plántulas en bosques no reclutadas (47-54%). Al final del estudio ~80% de las plántulas
perturbados y en bosques afectados por el fuego en la región. El reclutadas en el bosque no perturbado correspondió a especies
reclutamiento, supervivencia y crecimiento de plántulas fue segui- arbóreas, mientras que en el secundario la proporción de plán-
do durante un período de 6 años en cuadratas permanentes esta- tulas arbóreas (47%) y no arbóreas (53%) fue similar. El creci-
blecidas en bosque no perturbado y bosque adyacente perturba- miento en altura durante el período de 6 años fue considerable-
do por el fuego (secundario). Al comienzo del estudio, el bosque mente mayor en el bosque secundario que en el no perturbado
secundario mostró valores menores (PYou can also read
