Landscapes Forest and Global Change - ESA - Escola Superior ...

T.-C. Lin et al. 2010. Immediate effects of typhoon disturbance and artificial thinning on understory light environments
144
4. Discussion
There was a positive relationship between typhoon strength and increase of understory
light availabilities following typhoons. Typhoon Herb was a category three typhoon and the
strongest to impact Taiwan in the last 3 decades (Longshore 2008). It caused a decrease in the
canopy leaf area index from 2.99 to 2.40 or 20% on the transect (Lin et al. 1999). The 25%
artificial thinning should have also resulted in an approximately 25% removal of canopy leaf
area because all trees located in the thinned subplots were felled. However, the resulting
enhancement of understory light indices (approximately 42%) was significantly greater than that
which followed typhoon Herb (approximately 25%). Note that the magnitude of understory light
enhancement following typhoon Herb was comparable to the magnitude of canopy leaf removal
and was likely the result of the spatially distributed effect of typhoon disturbance on canopy
structure. In contrast, thinning removes whole clusters of trees to create gaps of 100m 2 that are
open to the sky and resulted in disproportional increases in understory light availabilities which
must be taken into consideration if artificial thinning is used in attempts to enhance understory
light availability to a pre-determined level because the reduction of tree basal area does not
provide a good estimate of increases in canopy transmittance (Hale 2003).
In addition to the disproportional increases in understory light availabilities, the much
greater understory light enhancement following 25% artificial thinning compared to the
strongest typhoon in the last three decades indicates that thinning of 25% or greater is likely to
create understory light environments that do not exist naturally in these forests. The light
availability following artificially thinning, 25%-36% of levels in the open, could lead to a very
different trajectory of forest regeneration.
The very different patterns of post-typhoon GSF for micro-sites that varied in pretyphoon
GSF (Fig. 2) may reflect differences in vulnerability of tree canopies. The increase in
GSF is certainly due to the opening of the forest canopies caused by typhoon disturbance. The
low light micro-sites were likely under taller and/or denser tree canopies than the high light
micro-sites. The decrease in GSF after typhoon disturbance in many high-light micro-sites
probably resulted from the larger effect of canopy closure between two repeated measurements
than the effect of typhoon disturbance in these high-light micro-sites. However, typhoons as
intense as Herb in 1996 can completely obliterate this effect of seasonal growth.
After 25% thinning there were still many micro-sites showing decreases in GSF (Fig. 1)
suggesting that the growth potential of micro-sites with high light availability was substantial.
Thus, if such micro-sites are not in or near the thinned subplots, the effect of seasonal growth
cannot be completely offset by thinning by 25% or less.
The very different responses to artificial thinning among micro-sites with similar prethinning
understory light indices (Fig. 2) resulted from their difference in distances relative to
the thinned subplots. Micro-sites near or on the edges of the thinned subplots certainly exhibited
large enhancements of understory light availability. Micro-sites further away were less affected,
and the effect of artificial thinning on understory light availability may even be smaller than the
effect of tree growth between the two repeated measurements.
In the natural forest at Fushan Experimental Forest, differences in topography and
species composition are possible causes for the variable impact of typhoon disturbance on
micro-sites of similar pre-typhoon light indices. The disruption to forest canopies and, in turn, to
understory light environments were scattered in space resulting in spatially variable impacts on
understory light availability. Thus, the effect of both typhoon disturbance and artificial thinning
on understory light environments exhibited large spatial variation.
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.