Management of LAI

Except in natural ecosystems, the canopy is periodically managed by farmers, foresters, grazing animals, and agriculturists. All management operations including cutting, grazing pasture, thinning, fertilization, liming, mowing, pruning, species sawing, and high herbage use, affect LAI. As stocking rate increases in grazed pasture, the total consumption per hectare goes up, while the net primary productivity decreases. The LAI is usually 2-3 with lower stocking densities and 1-2 at higher densities. In both crops and young forests, fertilizer effects on LAI are significant, and should increase LAI up to 3 units. An example of LAI management in forests is thinning, which reduces stand basal area, density, and LAI. In some cases, especially in even-aged mono-species stands like coniferous plantations, the LAI reduction is proportional to the basal area removed. Nevertheless, in most cases, the percentage of basal area removed is not proportional to those of LAI. Thinning improves water balance, radiation penetration within the canopy of the remaining trees, soil biology, and organic matter mineralization as a result of microclimatic changes. Canopy recovery occurs over several years depending on the intensity of the thinning, tree age, and site fertility (Figure 2).

Managers control LAI to control productivity and water uptake, but in fact agricultural and forest managers should be interested in using estimates of LAI to gauge the vigor of cultures or plantations (crop or forest decline, pathogen attacks), to adjust management practices and thus produce optimum LAI (Figure 3).

Thinning

1995 1996 1997 1998 1999 2000 2001 2002 2003

Figure 2 Stand LAI recovery after thinning in a beech stand (RENECOFOR Network, Plot HET88). LAI was measured using a plant canopy analyzer (LAI 2000, Li-Cor, Nebraska, USA). Data from N. Breda.

Figure 3 Spatial variability of LAI in 60 ha of a managed beech forest (Hesse, France). Ground-based LAI measurements were distributed according to a systematic network (50 m x 50 m) using two cross-calibrated plant canopy analysers (LAI 2000, Li-Cor, Nebraska, USA). The scale ranges from 2 to 6, i.e., a similar range to that of the biomes presented in Figure 1. Variations in LAI are due to stand age, date, and intensity of last thinning, soil properties, and species composition. For further details, see Bouriaud O, Soudani K, and Breda N (2003) Leaf area index from litter collection: Impact of specific leaf area variability within a beech stand. Canadian Journal of Remote Sensing 29: 371-380.

600 m

Grassland

Forest

Figure 3 Spatial variability of LAI in 60 ha of a managed beech forest (Hesse, France). Ground-based LAI measurements were distributed according to a systematic network (50 m x 50 m) using two cross-calibrated plant canopy analysers (LAI 2000, Li-Cor, Nebraska, USA). The scale ranges from 2 to 6, i.e., a similar range to that of the biomes presented in Figure 1. Variations in LAI are due to stand age, date, and intensity of last thinning, soil properties, and species composition. For further details, see Bouriaud O, Soudani K, and Breda N (2003) Leaf area index from litter collection: Impact of specific leaf area variability within a beech stand. Canadian Journal of Remote Sensing 29: 371-380.

Grassland

Forest

Figure 4 Transpiration ratio (T/PET) as a function of LAI for forest and grasslands. For forest, symbols are for phenological periods: triangles: LAI increase during leaf expansion; black circles: LAI decrease during autumn; gray circles: maximum LAI for contrasting stands. Data from Granier A, Breda N, Biron P, and Villette S (1999) A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands. Ecological Modelling 116: 269-283 and Saugier B (1996) Evapotranspiration des prairies et des cultures. Comptes-rendus de l'Académie d'Agriculture de France 82: 133-153, for forests and grasslands, respectively.

Figure 4 Transpiration ratio (T/PET) as a function of LAI for forest and grasslands. For forest, symbols are for phenological periods: triangles: LAI increase during leaf expansion; black circles: LAI decrease during autumn; gray circles: maximum LAI for contrasting stands. Data from Granier A, Breda N, Biron P, and Villette S (1999) A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands. Ecological Modelling 116: 269-283 and Saugier B (1996) Evapotranspiration des prairies et des cultures. Comptes-rendus de l'Académie d'Agriculture de France 82: 133-153, for forests and grasslands, respectively.

600 m

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