When is a gap not a gap? Light levels and leaf area index in bamboo-filled gaps in a Chilean rain forest

6 Pages·2001·2.6 MB·English

by  Christopher H Lusk

Gayana 58(1): 25-30, 2001 ISSN 0016-5301
WHEN IS A GAP NOT A GAP? LIGHT LEVELS AND LEAF ÁREA INDEX
IN BAMBOO-FILLED GAPS IN A CHILEAN RAIN FOREST
CUANDO UN CLARO NO ES UN CLARO?: NIVELES LUMÍNICOS E
ÍNDICE DE ÁREA FOLIAR EN CLAROS OCUPADOS POR CHUSQUEA
QUILA, EN UN BOSQUE LLUVIOSO CHILENO
Christopher H. Lusk5
ABSTRACT RESUMEN
Bamboos of the genus Chusquea are aggressive Los bambús del género Chusquea son colonizadores agre-
colonisers oftree-fall gaps in the températe rain forests sivosdeclarosenlosbosqueslluviosostempladosenel sur
ofsouthernChile.Chusqueathicketsinhibitregeneration de Chile. Los matorrales de Chusquea, que se desarrollan
of tree species, suggesting that light levéis beneath traslaformacióndelos claros, suprimenlaregeneraciónde
bamboo-filled gaps may be lower than beneath tall las especies arbóreas, llevando a pensar que los niveles
forest. Two LAI-2000 canopy analysers were used to lumínicos bajo los bambú podrían ser inferiores a los que
compare diffuse light availability beneath five prevalecen bajo el bosque cerrado. Se usaron dos
Chusqueaquilathicketsandfiveneighbouringtallforest analizadores de doseles LAI-2000 para comparar disponi-
stands, in a low altitude (u 400 m a.s.l.) old-growth bilidad de luz difusa bajo cinco matorrales de Chusquea
températe rain forest in Parque Nacional Puyehue quila y cinco rodales aledaños de bosque cerrado, en un
(40°39'S). Leaf área Índices (LAI) of these two bosque lluvioso antiguo ubicado ubicado a una elevación
vegetation phases are also compared. Average diffuse deu400ms.n.m.enelParqueNacionalPuyehue(40°39'S).
light levéis beneath bamboo thickets (1.5 %) were only Además se comparó el índice de área foliar de estos dos
slightly lower than those beneath tall forest (1.7 %). tiposdevegetación. Elnivelpromediodelaluzdifusabajo
Similarly, mean leaf área index of Chusquea thickets losmatorralesdeC. quila(1,5 %)fuelevementemenorque
(6.1) was slightly higher than that calculated for tall el medido bajo el bosque cerrado (1,7 %). Asimismo, se
forest stands (5.3). However, measurement ofleaf área midió mayor índice de área foliar para los bambús (6,1)
index with the LAI-2000 is likely to overestimate the comparado con el bosque cerrado (5,3). Sin embargo, las
actual photosynthetic surface área of bamboo thickets, medicionesdeíndicedeáreafoliarconelLAI-2000proba-
due to retention of dead leaves by Chusquea. On the blemente sobreestiman lasuperficie asimilatoriaefectivade
other hand, LAI oftall forest stands is underestimated, los matorrales de bambú, debido a la retención de hojas
due to the omission offorest floor vegetation < 50 cm muertasporlosculmosde C. quila. Porotraparte, elíndice
tall. The abundance ofChusquea thickets in old-growth de área foliardel bosque es subestimado, debido a la omi-
forest, and their ability to retain sites by suppression of sióndelavegetacióndelpiso(<50cmenaltura). Laabun-
tree regeneration, suggests that a two-phase positive- dancia de los matorrales de Chusquea spp. en los bosques
feedback vegetation switch model is appropriate forthe antiguos, y su capacidad de retener los sitios mediante la
dynamics of these forests. supresión de laregeneración de los árboles, sugiere que un
modelo bifásico de retroalimentación positiva es apropiado
Keywords: Chusquea, diffuse light, forest dynamics, para la dinámica de estos bosques.
LAI-2000, positive feedback vegetation switch.
Palabras claves: Chusquea, dinámica forestal, LAI-
2000, luz difusa.
INTRODUCTION
*Departamento de Botánica, Universidad de Concep- In most forests, canopy gaps created by tree
ción, Casilla 160-C, Concepción, Chile. deaths provide the most favourable sites for
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Gayana 58(1), 2001
regeneration ofcanopy tree species (Pickett & White Eucryphia cordifolia Cav. with occasional emergent
m
1985). However, in some cases, intense competition Notlwfagus dombeyi (Mirb.) Oerst. up to 45 tall.
from understorey plants or lianas can limit
opportunities for establishment of tree seedlings in Instrumentaron
gaps (Hiura etal. 1996; Schnitzer etal. 2000). In ex- Diffuse light availability and leaf área index
treme cases, reversión ofgaps back to tall forest can (LAI)weremeasuredusingapairofLAI-2000canopy
be delayed for decades by vigorous growth of such analyzers (Li-COR, Lincoln, Nebraska). One
plants (Schnitzer et al. 2000). instrument was used to take measurements within the
In the températe forests of South America, forest, while the other was placed at the centre of a
m
bamboos of the genus Chusquea often form dense clearing ofabout 120 diameter, providing clearance
thickets in tree-fall gaps, especially on mesic low- of the 148°fieldofviewpercievedbythe sensors, and
altitude sites (Veblen 1982). Tree seedling densities programmed to take readings at 60 sec intervals.
beneath Chusquea thickets are often lower than Integration ofdata from the two instruments enabled
%
beneath neighbouring tall forest stands (Veblen estimation of diffuse non-intercepted irradiance in
1982; González 1999). Recruitment opportunities for the visiblerange, at sites within theforest.As theLAI-
tree species in gaps colonised by bamboos appear to 2000 operates only with diffuse light, measurements
largely restricted to elevated microsites on logs were carriedouton overcast days, or in late afternoon,
(Veblen et al. 1980a), and to brief periods after whenthesolardiscwasoutsidethefieldofview.Recent
%
periodic synchronous flowering and death of the workhasconfirmedthat diffuseirradiancemeasured
bamboos, atintervalsof15to25 years(Veblen 1982). with the LAI-2000 is a good surrogate of spatial
Itseemslikelythatbamboosoftencyclethroughmore variation in mean daily photosynthetic photon flux
&
than one generation in large gaps before eventually density (Machado Reich 1999) within aclimatically
ceding to tall forest. Chusquea bamboos therefore homogeneous área.
appeartohave aprofound influence on the dynamics The LAI-2000 estimates leafárea index (m2 per
m
of South American températe forests. 2landsurface)onthebasisofthegapfractionpresent
Dense thickets of Chusquea can attain a stan- inthe 148°fieldofview,assumingthatfoliageelements
ding biomass as high as 150 t ha-1, and an annual are randomly distributed in the canopy, and randomly
production of> 10 t ha"1 (Veblen et al. 1980b). The oriented with respect to azimuth (Welles & Norman
scarcity of regeneration beneath such thickets 1991).
suggest that the bamboos may cause greater light
extinction than tall forest, presumably reflecting Sampling
higher leaf área Índices. However, to date there are Five large gaps caused by múltiple tree falls
m
no data on light levéis beneath Chusquea stands, or were chosen subjectively, all at least 30 wide on
leaf área index of bamboo thickets. In this short the shortest axis. All gaps contained dense stands of
communication, I compare leaf área index and Chusquea quila > 2m tall, with scattered tree sapling
diffuse light availability beneath canopy gaps and shrubs occasionally overtopping the bamboo.
colonised by Chusquea quila Kunth and beneath Measurements with the LAI-2000 showed that
neighbouring closed canopies, in a low-elevation diffuse light availability above the bamboo was
evergreen rain forest in south-central Chile. generally > 35 % ofthat in the 2-ha clearing.
A line transect was run across each gap, and
measurements taken at random intervals. A total
METHODS
of 10to 12points was sampled in each gap. Ateach
point, measurements were made at a height of 50
Study área cm above ground, as low as is feasible with the
The study was carriedout in low-altitude (350 - LAI-2000. Light attenuation by herbaceous
m
400 a.s.l.) Valdivian rainforest in theAnticura sector vegetation in the understorey was therefore not
ofParque Nacional Puyehue (40°39'S, 72°H'W). The measured. This means that leaf área index is
m
forest canopy, 25 to 35 in height, is dominated by underestimated for tall forest, because of the
the broadleaved evergreens Laureliopsisphilippiana omission of the ground layer. Within 50m of each
(Looser) Schodde, Aextoxiconpunctatum R. et P. and gap, measurements were also made on a similar
26
Light extinction by Chasquea bamboos: Lusk, C.
transect of 10 to 12 points laid out under tall forest thicket + tall forest vegetation phases) was treated
with closed canopy. as a block. Data analysis was carried out using JMP
Foranalysis, eachpairoftransects(C/^^Mea Statistical Software (SAS Institute, Cary, NC).
Table 1.ANOVAtotestforeffectsofsiteandvegetationphaseonunderstoreydiffuselightlevéis(%)andleafáreaindex.
Tabla 1. ANDEVApara evaluar los efectos de sitio y de tipo de vegetación sobre lapenetración de luz difusa al piso
(%), y el índice de áreafoliar.
Dependent
variable
Gayana 58(1), 2001
RESULTS Light levéis were only slightly lower beneath
Chusquea quila thickets than in understories of
Diffuselightlevéis were slightlylowerbeneath neighbouring tall forest stands (Fig. 1). Insufficient
C. quila thickets than in tall forest understoreys (Fig. light availability per se is therefore possibly not
1). HoweverANOVAshowed no significant effect of a sufficient explanation of the failure of shade-
vegetation phase, ñor any evidence that site-to-site tolerant tree species to regenérate in gaps
variation had any significant influence on occupied by bamboo. The sudden onset of
understorey light levéis, ñor evidence of interaction shading resulting from the very rapid growth of
between site and vegetation phase (Table 1). bamboos (up to 9m per year according to Veblen
Leaf área index of C. quila thickets was on 1982) may also pose problems for tree seedlings.
average slightly higher than that of tall forest stands The juveniles of most Chilean rain forest trees
(Fig. 2).ANOVAshowedthatsite-to-sitevariationhad have leaf lifespans > 2 years (Lusk & Contreras
no significant influence on LAI (Table 1). There was 1999), and as production of new leaves is
a marginally significantly effect ofvegetation phase probably the most important mechanism of
(P = 0.07), and no evidence of interaction between acclimation to light environment, their foliage
site and vegetation phase. turnover rates may limit their ability to adjust
to sudden changes (cf. Mohammed & Parker
1999).
DISCUSSION Leaf área Índices measured for Chusquea
thickets were slightly higher than those measured
Leaf área Índices and forest understorey light for tall forest (Fig. 2). This is surprising, as
levéis reported here are similar to those documented interspecific variation in leaf área Índices of
for other températe forests (Schulze et al. 1994; woody plants is thought to be determined mainly
MacDonald & Norton 1992; Hollinger 1987). by leaf-level light compensation points (Givnish
However, the percentage of diffuse light reaching 1988), and the scarcity of Chusquea beneath den-
the understorey beneath tall forest stands in Parque se tree canopies suggests that it is less tolerant
Nacional Puyehue is slightly higher than valúes of shade than the seedlings of associated tree
typically reported for closed-canopy tropical species such as Aextoxicon punctatum and
rainforests (Canham et al. 1990; Torquebiau 1988). Laureliopsis philippiana.
Tree-falt
Bamboo
cBeback, fores
errcroachmerr
Fig.3.PositivefeedbackvegetationswitchmodelfordynamicsoftallforeststandsandChusqueathicketsinValdivianrainforest.
Fig.3.ModeloderetroalimentaciónpositivaparadinámicaderodalescerradosymatorralesdeChusqueaenelbosqueValdiviano.
28
Light extinction by Chusquea bamboos: Lusk, C.
How do Chusquea thickets then develop ACKNOWLEDGMENTS
such high leaf área Índices? The very rapid
growth rates documented by Veblen (1982) for This work was funded by FONDECYT grants
Chusquea spp. imply that light environments of 1980084 and 1000367.
individual Chusquea leaves will change rapidly
during the growing season, older leaves quickly
REFERENCES
becoming shaded by newer ones. The process of
leaf senescence and abscission may lag behind
the production of new leaves, and furthermore CanhamS,piCe.sD.&,J.PS..S.DeWnhsiltoe.w,1W9.9J0..PLlaitgth,tJr.eR.giRmuensklbee,neTa.At.h
inspection of Chusquea thickets shows that dead closed canopies and tree-fall gaps in températe
leaves are retained on culms for some time. andtropicalforests. Can. J. For. Res. 20: 620-631.
However, the LAI-2000 does not distinguish Givnish,T.J. 1988.Adaptationtosunandshade: awhole
between dead and live leaves, both of which cau- plant perspective. Austral. J. Plant Physiol. 15:
63-92.
se light extinction. The actual área of live leaves
González, M.E. 1999. Tree regeneration responses to
above compensation point is therefore almost mass flowering and death of the bamboo
certainly lower than the leaf área index of 6.1 Chusquea quila in south-central Chile. (Masters
estimated in this study, although it is difficult to Thesis) Dept. Geography, University of Colora-
gaugeTthheemaagbnuintuddaencoef thaisnderropre.rsistence of Hiura,dTo.,,J8.2Sapnp.o&Y. Konno. 1996.Age structure and
response to fine-scale disturbances of Abies
Chusquea thickets in many old-growth Chilean sachalinensis,Piceajezoensis,Piceaglehnii, and
rain forests (Veblen et al. 1996), their thorough Betula ermanii growing under the influence ofa
occupancy of site resources (as evident from dwarf bamboo understory in northern Japan.
high leaf área index and light extinction), and Can. J. For. Res. 26: 289-298.
Hollinger, D.Y. 1987. Photosynthesis and stomatal
their inhibition of tree regeneration, suggest that
conductance patterns of two fern species from
it is inappropriate to regard them as "seral" different forest understoreys. J. Ecol. 75: 925-
vegetation. In view of their abundance in the 935.
landscape, and the scarcity of tree regeneration Lusk, C.H. & O. Contreras. 1999. Foliage área and
beneath thickets, it seems likely that bamboos can crown nitrogen turnover in températe rain forest
cycle through more than one generation on sites juvenile trees of differing shade tolerance. J.
Ecol. 87: 973-983.
colonised after tree falls, thus retaining indivi- McDonald, D. & D.A. Norton. 1992. Light
dual sites for several decades. A positive-fee- environments in températe New Zealand
dback vegetation switch model (Wilson & Agnew podocarp rainforests. New Zealand J. Bot. 16:
1992) therefore seems appropriate for the 15-22.
dynamics of old-growth forests in the región. Machado,J.L.&P.B.Reich. 1999.Evaluationofseveral
measures of canopy openness as predictors of
Both vegetation phases of old-growth forest (tall photosynthetic photon flux density in a forest
forest and Chusquea thickets) appear to promote understorey. Can. J. For. Res. 29: 1438-1444.
their own continued occupancy of the site, Mohammed, G.H. &W.C. Parker. 1999. Photosynthetic
suppressing invasión by the other phase (Fig. 3). acclimation in eastern hemlock [Tsuga
canadensis (L.) Carr.] seedlings following
Disturbance by tree-falls effects a switch from
transfer of shade-grown seedlings to high light.
tall forest to Chusquea, and although episodes Oecologia 13: 117-124.
of bamboo dieback are assumed to provide the Pickett, S.T.A& P.S. White. 1985. TheEcology ofNa-
main opportunities for reestablishment of tall tural Disturbance and Patch Dynamics.
froartehsetr, thtihsanrevedresteersmwiinticshtimca,y dbeeppernodbaibniglistoinc SchnitzeArc,adS.eAm.,icJ.PWr.esDsa,llNienwg &YorWk..P Carson. 2000. The
impactoflianasontreeregenerationintropicalforest
stochastic variation in production of viable seed canopy gaps: evidence for an alternative pathway
by bamboos and trees. The switch back to tall ofgap-phase regeneration. J. Ecol. 88: 655-666.
forest could also be facilitated by encroachment Schulze,E.D.,FM.Kelliher,C.Kórner&R.Leuning.1994.
of neighbouring canopy tree crowns, reducing Relationships among máximum stomatal
conductance, ecosystem surface conductance,
light levéis and henee depressing the vigour of
carbónassimilationrateandplantnitrogennutrition:
bamboos.
aglobalecologyscalingexercise.Annu. Rev. Ecol.
29
Gayana 58(1), 2001
Syst. 25: 629-660. {Chusquea coleou and C. tenuiflora) in south-
Torquebiau, E.F. 1988. Photosynthetically-active central Chile. J. Ecol. 68: 397-404.
radiation environment, patch dynamics and Veblen,T.T.,C.Donoso,T.Kitzberger&A.J.Rebertus.
architecture in a tropical rainforest in Sumatra. 1996. Ecology of Southern Chilean and
Austral. J. Plant Physiol. 15: 327-342. ArgentineanNothofagusForests.In:Veblen,T.T.,
Veblen,T.T. 1982. GrowthpatternsofChusqueabamboos R.S. Hill & J. Read (Eds.), The Ecology and
in the understorey ofChilean Nothofagus forests Biogeography ofNothofagusForests, pages 293-
and their influences in forestdynamics. Bull. Torr. 353. Yale University Press, New Haven.
Bot. Club 109: 474-487. Welles, J.M. & J.M. Norman. 1991. Instrument for
Veblen,T.T.,F.M.Schlegel&B.Escobar. 1980a.Structure indirect measurement of canopy architecture.
and dynamics ofold-growthNothofagus forests in Agron. J. 83: 818-825.
the ValdivianAndes, Chile. J. Ecol. 68: 1-31. Wilson, J.B. &A.D.Q.Agnew. 1992. Positivefeedback
Veblen,T.T., F.M. Schlegel&B. Escobar. 1980b. Dry switches in plant communities. Adv. Ecol. Res.
matter production of two species of bamboo 23: 263-326.
Fecha de publicación: 30.08.2001
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