J. D. Sauerländer's Verlag, Bad Orb

Small-scale canopy disturbances, although less severe than large
disturbances in changing forest structure, may be more frequent
and affect a larger area over time (SPIES et al., 1990). The severity
of such disturbances can stimulate a change in the growth rates of
the surviving trees (OLIVER and LARSON, 1996) through increasing
the availability of the resources (light, soil moisture, nutrients,
microhabitats) and promoting plant growth (DENSLOW, 1980; CANHAM
and MARKS, 1985; RUNKLE, 1985; VEBLEN, 1992). The regeneration
dynamics are affected by the heterogeneity of resources in
forests (CANHAM et al., 1994). Nevertheless, advance regeneration,
present and growing slowly beneath a forest canopy, has a potential
advantage because they are in a position to respond to the creation
of canopy gaps following small-scale disturbances (OLIVER and
LARSON, 1996; MESSIER et al., 1999). This has been observed in
old-growth Nothofagus betuloides forests where an abundance of
understorey trees can persist in excess of one hundred years
(REBERTUS and VEBLEN, 1993a; VEBLEN et al., 1996) until the creation
of a gap when they can grow into the canopy (VEBLEN et al.,
1996).

The structure of the Nothofagus forests of Tierra del Fuego is
principally shaped by wind that acts as an agent of large and smallscale
disturbance (REBERTUS et al., 1997; PUIGDEFÁBREGAS et al.,
1999). The windthrow of individual trees creates canopy gaps
smaller than 200 m2 (REBERTUS and VEBLEN, 1993a; GUTIÉRREZ,
1994). In pure old-growth N. betuloides forests in Tierra del Fuego
discrete gaps might not be apparent, as they mostly occur as interwoven
gap complexes in the canopy layer (REBERTUS and VEBLEN,
1993a). Small canopy gaps (51 m2 on average) were observed in a
pure uneven-aged N. betuloides forest, while somewhat larger gaps
(107 m2) were recorded in a mixed uneven-aged N. betuloides –
Nothofagus pumilio forest (PROMIS, 2009). Wavelike patterns of
gap formation have been documented for pure N. betuloides, pure
N. pumilio and for mixed N. betuloides – N. pumilio forests in Tierra
del Fuego, in sites predisposed to wind disturbance (REBERTUS and VEBLEN, 1993b; REBERTUS et al., 1993; PUIGDEFÁBREGAS et al.,
1999). Moreover, a patch mosaic pattern has been found in these
Nothofagus forests forming multicohort stands with patches of
younger trees alongside larger patches of old-growth forest
(GUTIÉRREZ et al., 1991).

The small-scale disturbances mentioned above appear to be
important in the regeneration dynamics of southern South American
Nothofagus forests situated at higher elevations and at higher
latitudes where the diversity of tree species is low (VEBLEN et al.,
1996; POLLMANN and VEBLEN, 2004), as is the case for forests in
Tierra del Fuego. N. betuloides is capable of establishing beneath
small tree-fall canopy gaps (REBERTUS and VEBLEN, 1993a;
GUTIÉRREZ, 1994; ARROYO et al., 1996). Seedlings and saplings present
on the ground at the time of gap formation are also released by
the creation of these small-scale disturbances in the canopy
(VEBLEN et al., 1996; PROMIS, 2009). The successful establishment
and growth of young N. betuloides can be impeded where the
ground cover of understorey trees and shrubs such as Drymis winteri
and Maytenus magellanica is high (REBERTUS et al., 1993a;
GUTIÉRREZ et al., 1991; GUTIÉRREZ, 1994; VEBLEN et al., 1996).

There are still knowledge gaps regarding the effects of smallscale
disturbances on the regeneration dynamics of old-growth
N. betuloides forests. It is not known to which degree these small
canopy gaps increase the horizontal heterogeneity due to a gradient
in below-canopy solar radiation transmittances; and if there are differences
in solar radiation transmittances influencing the density
and growth rates of the young trees in old-growth forests. The
influence of canopy gaps on the browsing habits of Lama guanicoe
(guanaco) and the age distribution of the young trees in an oldgrowth
N. betuloides forest are also unknown.

The hypothesis central to this study was that the establishment
and growth of N. betuloides seedlings and saplings are affected by
both the occurrence of small-scale gap disturbances and the resulting
solar radiation gradient from the centre of a gap to areas beneath the undisturbed forest canopy. The objectives of this study
were to determine the effects of natural small-scale disturbances (1)
on the below-canopy solar radiation conditions, (2) on the regeneration
patterns (density and growth rates) of N. betuloides, and (3)
on the browsing damage caused by by Lama guanicoe.

KEY WORDS:
Old-growth forest; Nothofagus betuloides; canopy gaps; solar radiation
transmittances; regeneration; Tierra del Fuego.

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