Báo cáo lâm nghiệp: "Modelling the coppice stand structure: an ecophysiological approach"

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Modelling the coppice stand structure: an ecophysiological approach G.E. Scarascia-Mugnozza, R. Valentini E. Giordano and A. Del Lungo A. Del Lungo Oi i00 Viterbo, University of Tuscia, Italy Institute of Forest Biology, and the climate is mediterranean with a mean Introduction annual temperature of 17°C, a mean annual precipitation of 900 mm and a marked summer drought. The stand is managed as a coppice Limitations of water availability have seri- with standards, with 2400 sprouts on about impact on biomass production in ous 1200 stumps/ha and with 96 standards/ha. The mediterranean forest ecosystems. There latter are trees, managed on a longer rotation is also growing evidence about the than the coppice, to produce seeds for the influence of water deficits on the develop- regeneration of the stand. The rotation interval is 18 yr for the coppice and a multiple of it for ment of insect or pathogen problems that the standards. The mean annual aboveground eventually result in a general decline of biomass increment is 7 tons!ha-!!yr-!. these forests. Therefore, the analysis of description of the canopy structure A detailed the interactions between the water rela- accomplished by analyzing the crowns of was tions of a forest and its carbon cycle is of 12 representative trees divided into 3 different fundamental importance, in order to identi- diameter classes for the coppice (4, 10, 17 cm) and one for the standards (25 cm); these obser- fy the most suitable silvicultural tech- vations were made in September 1987, when niques to optimize productivity relative to the leaf area display was at its seasonal peak consumption (Jarvis, 1985). water and the stand was at the end of the rotation. For each model tree, leaf area measured on A promising approach consists of representative branches was significantly and the implementation of ecophysiological highly correlated (R 0.70) to branch length. 2 2 models that include the description of the Using this regression and knowing the length, structure of a forest stand together with location and orientation of each first order the relationships between physiological branch, the vertical distribution of leaf area of each model tree was derived. The orientation processes and environmental parameters. (azimuth, midrib and lamina angles) of an appropriate sample of leaves was also mea- sured, after stratification among trees and canopy layers (isebrands and Michael, 1986). Materials and Methods Ecophysiological (stomatal conductance and plant water potentia!), climatic and soil moisture data were collected throughout 4 growing sea- An established in experimental plot was a sons in the same experimental plot, by means Quercus cerris L. forest stand, in central Italy of a 20 m tall scaffold, an Ll 1600 porometer, a (42°23’N; 11 °55’E). Its elevation is 150 m a.s.l.
PMS pressure chamber, a SOLO 20 neutron The vertical distribution of leaf area for and various micrometeorological sen- probe the 4 average model trees was computed sors. every 0.5 m, taking into account the incli- nation and the length of the average branches for each layer (Fig. 1). Results The results of leaf orientation measure- ments showed that leaves were non-ran- domly distributed, except for the azimuth Large differences in total leaf area exist angle that was uniformly distributed. The among model trees of different diameter midrib and the lamina angles were nor- classes: the smallest sprouts, of the 4 cm mally distributed, according to the Kolmo- class, had an average total leaf area of gorov-Smirnov test. Lamina angle was 2.8 m per tree; the intermediate, 13.2 m 2 ; 2 88° + 4.27 for standards and coppice the largest sprouts, 87.3 m and the aver- ; 2 pooled together, indicating that the lamina age standard had a leaf area of 150.3 m . 2 were generally not tilted. The midrib angle Over the whole stand, standard leaf area was higher for the standards than for the represented approximately 50% of the leaf coppice (120° ± 6.39 vs 93° ± 7.46) area displayed by the stand. The leaf area indicating that, in the former, leaves were index (LAI) for the whole stand was 4.5, generally inclined towards the ground, whereas for each average model tree, whereas in the coppice they were pointing from the smallest sprout to the standard upwards. was, respectively, 0.3, 3.9, 3.3 and 3.5.
n is obviously related to the leaf The information about canopy structure number and the response of physiological parame- displayed by the tree at a given layer. area ters to environmental conditions have each leaf, the orientation is computed For the basis of the observed average and been incorporated into a detailed ecophy- on siological model of light interception and standard deviation values. Light intercep- water consumption by the forest stand tion is then estimated separately for the (Fig. 2). The important components of the sunlit and the shade leaves; in the first model consist of the subroutines de- case, the light level is computed according to the projection of the leaf on the plane scribing light interception and stomatal perpendicular to solar radiation, whereas it conductance of the stand. is assumed that the shade leaves receive The former is based on a Montecario algorithm that simulates the characteris- diffuse light that is extinguished according to Lambert-Beer’s law. Stomatal conduc- tics of a population of n leaves for each tance computation is based on its relation- average model tree within each layer; the
with 2 ecophysiological parameters ships Acknowledgments that have been found to be closely related to it: photosynthetic photon flux density Research work supported by CNR, Italy. Special grant LP.R.A. Subproject 1 Paper no. 23i6. (PPFD) and predawn water potential of the trees (P ): P c7 . vn-c PPFD’^ o t1I References - rr Isebrands J.G. & Michael D.A. (1986) Effects of leaf morphology and orientation on solar radia- Discussion and Conclusions tion interception and photosynthesis in Popu- lus. In: Proc. Conf. Crown and Canopy Struo- ture (Fujmori T. & Whitehead D., eds.), lbaraki, Detailed process-based models have Japan, pp. 359-381 much to offer for a better knowledge of Jarvis P.G. (1985) Increasing productivity and forest ecosystem functioning and for their value of temperate coniferous forest by manipu- management (Landsberg, 1986). correct lating site water balance. In: Weyerhaeuser The model that we have implemented is Science Symp. Forest Potentials (Ballard R. & based on a few fundamental physiological Farnum P., eds.), Weyerhauser Co., Tacoma, pp. 39-74 processes, such as light interception and Kaufmann M.R. (1982) Leaf conductance as a stomatal conductance, as influenced by function of photosynthetic photon flux density light regime and predawn water potential and absolute humidity difference from leaf to (Kaufmann, 1982). Transpirational results air. Plant Physiol.. 69, 1018-1022 from the model showed close correspon- Landsberg J.J. (1986) In: Physiological Ecolo- dence to values determined independently gy of Forest Production. Academic Press, New York, pp. 193 (Fig. 3).