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University of Arizona (1972)

ENVIRONMENT AND SEASON IN A TROPICAL DECIDUOUS FOREST IN NORTHWESTERN MEXICO

Krizman, Richard Donald,

Titre : ENVIRONMENT AND SEASON IN A TROPICAL DECIDUOUS FOREST IN NORTHWESTERN MEXICO

Auteur : Krizman, Richard Donald,

Université de soutenance : University of Arizona

Grade : Doctor of Philosophy (PhD) 1972

Résumé
During each of the four seasons of the year, with special emphasis at the time of the onset of the summer monsoon, micro environmental parameters were monitored near Alamos, Sonora, Mexico, in the northern edge of the tropical deciduous forest. The vegetation of this area is characterized as being season ally, spring and early summer, deciduous. Forest stature is very low, 10-15 m, and is composed of two tree strata. The A-stratum canopy is discontinuous and consists of widely spaced individuals or clumps. The B-stratum is nearly continuous and is composed of numerous codominant species of short trees. Giant cacti constitute a significant component. Woody lianas are common while epiphytes are rare. Two shrub strata exist. C-stratum shrubs, composed of a small number of species, are 2-5 m tall, often multiple stemmed and may ob tain coverages of 50%. D-stratum shrubs are less than 1 m tall, are often in consociation, and make up coverages of 65%. An ephemeral herbaceous E-stratum exists, mainly following the summer rains. The monitoring of nine thermistors on an 18 m tall mast showed air temperature profiles which changed in character during the seasons of the year. In all seasons nocturnal mean temperatures at crown height were warmer than those near the ground. Diurnal temperatures were observed to switch ; in the leafless seasons ground-level temperatures were warmer and during the remainder of the year crown-level tempera tures were warmer. Surface soil temperatures in the forest reached high levels and fluctuated widely in the leafless positions of the year. With the onset of the summer rains and subsequent foliage development, maximum surface temperatures dropped by 30°C and daily fluctuations were one fifth of earlier values. Mean daily relative humidity (computed from readings taken every 2 hrs for two days) was lowest in the spring, 31%. This value rose to 867o during the monsoon season. Light reaching the substrate approached the value of open light when the forest was leafless (907o). In late summer this value dropped to 3-57o. It then increased through autumn and winter as the forest sheds its leaves. Soil moisture was near field capacity during the monsoon period and reached far below the permanent wilting point in the leafless seasons

Mots clés : Forests and forestry — Mexico — Sonora (State) ; Forest ecology — Mexico — Sonora (State) ; Forest influences.

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