Báo cáo lâm nghiệp: "Ecophysiology of 8 woody multipurpose species from semiarid northeastern Mexico"

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Ecophysiology of 8 woody multipurpose species from semiarid northeastern Mexico J.H.A. Boerboom 2 H. Stienen M.P. Smits N. Reid J. Landa 34 1 2 1 Bicontrol GmbH, PF 1630. 6500 Mainz, F.R.G., Institut GmbH. 1630, Landbouw 2 Universiteit Wageningen, Uakgroep Bosteelt & Boscecologie, The Netherlands, Department 3 of Plant Ecology, University of New England, Armidale, Australia, and Facultad 4 de Ciencias Forestales, UANL, Linares N.L., Mexico Introduction selection of the species most apt for silvicultural management under specified conditions. The water use/productivity The Tamaulipan thornscrub is a low dry ratio may be a useful be index for screen- forest and shrubland complex covering ing potential forestry species because about 20 million hectares of the Gulf drought frequently limits growth. coastal plain in northeastern Mexico and southern Texas, the region is called the MatorraL The Tamaulipan thornscrub has Materials and Methods been grazed by domestic stock for about 4 centuries and is heavily exploited by sub- The study was conducted in the Matorral on the sistence farmers for fuel, timber and other UANL campus, 10 km south of Linares resources (Reid et al., 1989). In order to (24°47’N, 99°32’W), in spring and summer prevent further desertification and sustain 1988. Mean annual rainfall is 749 mm. agricultural, livestock and forestry produc- Droughts occur in midsummer and winter. Annual free evaporation is 3 times the average tion, the Matorral must be conserved and, precipitation (Carstens, 1987). where necessary, enriched with desirable The study site was a representative area of multipurpose trees and shrubs (MPTS). the Matorral growing on a Vertisol. Photosyn- The Facultad de Ciencias Forestales of thesis and transpiration were recorded with a portable Li-6200 (McDermitt, 1987). Compar- the Universidad Autonoma de Nuevo Leon able data for each species were collected by (UANL, Mexico), is applying a holistic taking 20 sequential measurements under agroforestry approach to some of the identical climatic conditions and by monitoring region’s landuse problems (Stienen, the diurnal changes of all species days of on extreme (wet, dry) environmental conditions. 1989). Native shrubs and trees have been The water use efficiency of photosynthesis selected for agroforestry trials to date on (WUEP) was calculated as photosynthetic the basis of the products and expected s-1) 2 m- ’ (pmol over transpiration rate rate productivity. Knowledge of their ecophy- (mmol Water potentials were taken ) 1 s. 2 m- ’ siological characteristics would facilitate with a pressure bomb. Climatological data and
h above the tree crown and 6 h within the soil water, expressed as the percentage of dry weight, were monitored throughout the canopy in summer, indicating that condi- study. Photosynthetically active radiation (PAR, tions were unsuitable for the development ) 1 s- ’ 2 pmol.m- was measured over 1 h periods of shade leaves. above and below the canopy. Eight native spe- cies were tested and assigned a relative rank The ecophysiological data for the 8 spe- from 1 to 8 for each of the parameters on the cies are presented in TableI and as a basis of the degree of drought adaptation. case study in Figs. 1 and 2. Photosynthe- These ranks are summarized in a final drought sis and transpiration rates are means of adaptation ranking. the data for the most humid and driest days, and were used to calculate water use efficiency. The values were generally Results and Conclusions 20-40% higher under wet conditions, even WUEP, thus none of the species Relative humidity fell as low as 20% when showed evidence of extraordinary drought midday temperatures were 40°C. In May, adaptation. The 2 Amyris species had the before the first rains, soil moisture content best WUEP ratios and Diospyros texana 16% at 10 cm depth, 17% at 30 cm was and Celtis pallida the lowest. Fig. 2 pre- and 18% at 90 cm. After 149 mm of rain in sents diurnal curves for the 2 species that June, values were twice as high in the exhibited the greatest contrast in INUEP. upper soil horizons but were invariant at Species differed in their abilities to achieve 90 cm depth. By August, soil moisture had a high rate of photosynthesis with moder- again dropped to the May levels, around ate water use in the mornings, and only the permanent wilting point. the more drought-resistant species re- opened stomata in the afternoons. The Periods of light saturation for C0 as- 2 water potentials of the latter species fell to similation (PAR>500 , 1 s 2 m- ’ ,umol Long - 7.0 MPa under dry conditions, while the and Hallgren, 1985) lasted for more than 8
Shaded leaves low in the canopy contin- less drought-adapted species registered ued to photosynthesize after fully isolated MPa, indicating that water values> -3.5 leaves had closed their stomata. Relative potential was inversely proportional to humidity inside the canopy was at times WUEP (cf. Schulze, 1986) and that the over 10% higher than above the crown. more drought-adapted species tolerated higher internal water stress. The species exhibited various foliar The mean diurnal photosynthetic rates strategies for coping with drought. Heliet- ta parviiolia deactivated chlorophyll, with a of all species were relatively low (Table I). The highest instantaneous rate (29.7 pmol consequent yellow discoloration of the leaves under dry conditions. Gochnatia of C02!m-2!s-1) was achieved by Cordia boissieri on April 29 at 3:10 pm. The other hypoleuca avoided direct insolation by orienting the leaf surface vertically, in the water-wasting species (Condalia hookeri, Celtis pallida and Diospyros texana) also plain of the incoming sunlight. Amyris texana partially rolled its leaves, while A. registered high rates under the same madrensis did not show any leaf re- conditions, while the remaining species sponse. The remaining species dropped exhibited 20-30% less photosynthesis but their leaves to varying degrees and were higher WUEPs.
therefore ranked low with References to regard drought adaptation. Carstens A. (198!7) Struktur eines Matorrals im The summary ranking of drought resis- semiariden-subhumiden Nordosten Mexikos tance in the 8 species is presented in und auswirkungen von behandlungen zu seiner Table I. Amyris madrensis performed best, bewirtschaftung. Beitr. Trop. Subtrop. Landwirt while Diospyros texana showed the least Forstw. 27 adaptation to drought. Species with a Long S.P. & Hallgren J.E. (1985) Measurement of C0 assimilation plants in the field and the 2 summary rank of 6-8 exhibited conserva- laboratory. In: Techniques in Bioproductivity tive water use strategies, partially opening and Photosynthesis. 2 edn. (Coombs J., et aL, their stomates under dry conditions. None eds.). UNEP, Pergamon Press, Oxford, pp. 62- of the species can be recommended for 94 planting in drier regions with the exception McDermitt D.K. (1987) Photosynthesis mea- surement system-performance comparison of of Amyris madrensis for wood production the Li-6200 and the Li-6000. Licor Inc., Applica- and A. texana as a forage shrub. These tion Note 6200-1. Licor, Lincoln, NE, U.S.A. species merit further evaluation in agro- Reid N., Marroquin J. & Beyer-Miinzel P. (1989) forestry trials under semiarid conditions. Utilization of shrubs and trees for browse, fire- wood and timber in the Tamaulipan thornscrub, northeastern Mexico. For. Ecol. Manage. (in press) Schulze E.D. (1986) Carbon dioxide and water Acknowledgments vapor exchange in response to drought in the atmosphere and in the soil. Annu. Rev. Plant. Physiol. 37, 247-274 The authors wish to thank the W. German Department for Economic Collaboration for the Stienen H. (1989) The agroforestry potential of funds to purchase the equipment used in this combined production systems in northeastern study. Mexico. Agrofor. Syst. 7, (in press)