Functional Responses to Thinning
337
production. Both increase with the intensity of thinning, but leaf formation
increases faster than leaf shedding. The net result is a fast recovery of the
canopy leaf area, which can reach pre-thinning values in about 6 years.
Thinning improved the relative water availability and light intensity on
leaves, which promoted higher net photosynthesis and leaf conductance than
in control plots. Integrating these results at the canopy level, these differences
took place during the dry season, specially during August 1994, and disappeared in winter and spring when water was not a limiting factor. The increment of water availability, detected as increased predawn 'l'pd in the thinning
treatments, was mainly due to the redistribution of water between the remaining trees, and to the reduction of canopy interception. This increment
promoted higher GPP and NPP in the thinning treatments than in the control, which meant a fast growth of the remaining trees and recovery of the
tree canopy. However, water use efficiency did not increase because the increase in the photosynthetic rate was proportional to the increase in the
transpiration rate.
The water available to adult trees was possibly reduced through competition with the new resprouts after thinning, which, moreover, received high irradiation through gaps in the canopy. This fact was specially important in the
maximum thinning plot, where the high radiation allowed high photosynthetic rates, even 3 years after thinning.
References
Abrams MD, Mostoller SA (1995) Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during a drought. Tree Physiol
15:361-370
Aussenac G, Granier A (1988) Effects of thinning on water stress and growth in Douglas-fir. Can
J For Res 18:100-105
Breda N, Granier A, Aussenac G (1995) Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.). Tree Physiol 15:
295-306
Brix H, Mitchell AK (1986) Thinning and nitrogen fertilisation effects on soil and tree water
stress in a Douglas-fir stand. Can J For Res 16:1334-1338
Castell C, Terradas J, Tenhunen JD (1994) Water relations, gas exchange, and growth of resprouts
and mature plant shoots of Arbutus unedo 1. and Quercus ilex 1. Oecologia 98:201-211
Damesin C, Rambal S (1995) Field study of leaf photosynthetic performance by a Mediterranean
deciduous oak tree (Quercus pubescens) during a severe summer drought. New Phytol 131:
159-167
Ducrey M, Toth J (1992) Effects of clearing and thinning on height growth and girth increment
in holm oak coppices (Quercus ilex 1.). Vegetatio 991100:365-376
Epron D, Dreyer E (1993) Long-term effects of drought on photosynthesis of adult oak trees
[Quercus petrea (Matt.) Liebl.) and Quercus robur 1.] in a natural stand. New Phytol 125:
381-389
Goulden ML (1996) Carbon assimilation and water-use efficiency by neighboring Mediterranean-climate oaks that differ in water access. Tree PhysioI16:417-424
337
production. Both increase with the intensity of thinning, but leaf formation
increases faster than leaf shedding. The net result is a fast recovery of the
canopy leaf area, which can reach pre-thinning values in about 6 years.
Thinning improved the relative water availability and light intensity on
leaves, which promoted higher net photosynthesis and leaf conductance than
in control plots. Integrating these results at the canopy level, these differences
took place during the dry season, specially during August 1994, and disappeared in winter and spring when water was not a limiting factor. The increment of water availability, detected as increased predawn 'l'pd in the thinning
treatments, was mainly due to the redistribution of water between the remaining trees, and to the reduction of canopy interception. This increment
promoted higher GPP and NPP in the thinning treatments than in the control, which meant a fast growth of the remaining trees and recovery of the
tree canopy. However, water use efficiency did not increase because the increase in the photosynthetic rate was proportional to the increase in the
transpiration rate.
The water available to adult trees was possibly reduced through competition with the new resprouts after thinning, which, moreover, received high irradiation through gaps in the canopy. This fact was specially important in the
maximum thinning plot, where the high radiation allowed high photosynthetic rates, even 3 years after thinning.
References
Abrams MD, Mostoller SA (1995) Gas exchange, leaf structure and nitrogen in contrasting successional tree species growing in open and understory sites during a drought. Tree Physiol
15:361-370
Aussenac G, Granier A (1988) Effects of thinning on water stress and growth in Douglas-fir. Can
J For Res 18:100-105
Breda N, Granier A, Aussenac G (1995) Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.). Tree Physiol 15:
295-306
Brix H, Mitchell AK (1986) Thinning and nitrogen fertilisation effects on soil and tree water
stress in a Douglas-fir stand. Can J For Res 16:1334-1338
Castell C, Terradas J, Tenhunen JD (1994) Water relations, gas exchange, and growth of resprouts
and mature plant shoots of Arbutus unedo 1. and Quercus ilex 1. Oecologia 98:201-211
Damesin C, Rambal S (1995) Field study of leaf photosynthetic performance by a Mediterranean
deciduous oak tree (Quercus pubescens) during a severe summer drought. New Phytol 131:
159-167
Ducrey M, Toth J (1992) Effects of clearing and thinning on height growth and girth increment
in holm oak coppices (Quercus ilex 1.). Vegetatio 991100:365-376
Epron D, Dreyer E (1993) Long-term effects of drought on photosynthesis of adult oak trees
[Quercus petrea (Matt.) Liebl.) and Quercus robur 1.] in a natural stand. New Phytol 125:
381-389
Goulden ML (1996) Carbon assimilation and water-use efficiency by neighboring Mediterranean-climate oaks that differ in water access. Tree PhysioI16:417-424