Notes on Wind-Related Growth Patterns of Paramo Plants in Venezuela

Abstract

In the paramo (tundra) of the Venezuelan Andes, persistent northeast winds, even though gentle, are correlated with distinctive plant growth patterns. A brittle-leaved, stiff-branched shrub (Hesperomeles) develops wind-swept forms, apparently because the wind snaps off leaves. Shrubs with flexible stems and leaves (Hypericum) are not generally injured by the wind, but flowering on the windward side of such shrubs may occur a month later than on the leeward side. Plants with flexible but extremely dense foliage, e.g., Aciachne, a cushion grass, apparently filter moisture from the fog-laden wind during the wet season; because of this process, clones of Aciachne grow in ribbons toward the prevailing wind. ALTHOUGH THERE IS a large body of literature on the tundra ecosystems of temperate mountains and the Arctic (Billings and Mooney 1968), comparatively little is known about the ecology of the pairamo of the northern Andes. In the Venezuelan Andes (90 N), the tree line occurs between 2700 and 3200 m, depending on exposure and human interference. The upper limit of plant life occurs at about 4880 m (Vareschi 1956). Discussions of the vegetation and plant ecology of the Venezuelan paramo have been presented by Cuatrecasas (1968) and Vareschi (1953, 1956, 1958). The Venezuelan pairamo is characterized by daily fog and rain during the wet season (April to October), and there is little sunshine after 10:00 a.m. During the dry season, there is often high solar radiation for much of the day, much less fog, and very little precipitation. Each of the wettest months receives 120-150 mm of precipitation; mean monthly precipitation during the dry season ranges from 5-10 mm. Annual precipitation varies from 750 to 1200 mm, depending on exposure and elevation. Mean maximum temperature at Mucubaji (3550 m) is 8.40 C; mean minimum temperature is 1.30 C. Mean temperatures decrease approximately 0.70 C for every 100 m rise in elevation. Mean wind speed early in the morning and at night,at 3550 m (Mucubaji) is about 5 km/hour. During the afternoon, mean wind speed rises to 13 or 14 km/hour. Wind speed is slightly higher in the wet season than in the dry season. Gentle northeast winds are a nearly constant factor in the environment during the wet season. Violent winds are rare, except on the highest peaks, such as Pico Espejo (4765 m). These climatic data are taken from Nieto and Garcia (1968). The gentleness of pairamo winds is evidenced by the presence of tall, columnar rosette plants of Espeletia spp. These may grow to over 2 m tall on even the most exposed ridges (fig. IA). The wooly leaves of the terminal rosette are retained on the stem for years, forming a coat 1-2 dm thick. No obvious differential weathering on windward versus leeward sides was noted for these sheaths of dead leaves. It is apparent, however, that for other plant forms, even the gentle p-aramo winds do induce distinctive growth patterns.