Gap Dynamics of a Tropical Cloud Forest in Northeastern Mexico

Abstract

Gap dynamics in a tropical cloud forest in Tamaulipas, Mexico were studied by describing the treefall regime and by measuring the temporal and spatial occurrence of gaps (1-yr-old lightgaps produced by treefalls) and patches (older lightgaps). The study plots comprised protected (4 ha) and exposed (2 ha) zones, where the frequency of gap and patch size per unit area was determined. The number of trunks on the forest floor was counted and their length and diameter were recorded. The causes of treefalls were determined. Fallen tree species were identified and a comparison of the specific composition between fallen and standing trees was made for the community. The results suggest that gaps play an important role in determining an environmental mosaic within the tropical cloud forest. Species-producing gaps are those dominating the canopy layer in approximately 80 percent of the treefalls. An average of 19 patches ha-' was registered, comprising 2 5 percent of the study site. The mean patch size as fitted to a lognormal distribution was estimated as 4.71 ? 0.6. Turnover rates were estimated as 158 yr for areas disturbed by natural causes and 90 yr for areas disturbed by joint causes (i.e., natural plus man-induced). The natural and the actual (natural plus man-induced) gap production rates were 0.5 and 0.63 gap ha-' yr-1. This means that an average of 1.27 percent of the forest area is disturbed every year by natural causes and that 2.22 percent of the area is disturbed by joint causes. FOREST REGENERATION PROCESSES frequently depend upon natural disturbance regimes. Wind, fire, and hurricanes kill trees and open gaps in the forest canopy (Hartshorn 1978, Bormann & Likens 1979, Lorimer 1980). The structural changes produced by treefalls create an environmental mosaic in the community determined by a gapage-dependent process (Watt 1947; Whitmore 1975, 1978; Halle et al. 1978; Oldeman 1978; Hartshorn 1980; Runkle 1981; Nakashizuka 1984a; Brokaw 1985; and Martinez-Ramos 1985). The spatial and temporal heterogeneity of forest structure depends on the rate of gap production as well as on gap sizes, shapes, and locations. According to the way a treefall occurs, gap formation provides a range of resources for the differential establishment of species within the community (Denslow 1980, Brokaw 1982, Putz et al. 1983, Nakashizuka 1984b, Moore & Vankat 1986, Veblen 1986). This paper describes the gap dynamics of a tropical cloud forest in northeastern Mexico. The work includes the study of the treefall regime which considers the factors influencing the way trees die and its effect on the species composition of the forest canopy. This paper also presents the temporal and spatial occurrence of gaps and patches based on the estimation of gap production and turnover rates, as well as on a description of patch sizes. STUDY AREA The study was conducted in the Biosphere Reserve El Cielo, a tropical cloud forest community which covers 10,000 ha (between parallels 23?12'N and 23?03'N; 99?18'W) at an altitude of 1250 m. The reserve is in the Sierra de Cucharas at the eastern slope of the Sierra Madre Oriental in the state of Tamaulipas, Mexico. The mean monthly rainfall in the region is 252.2 mm. A dry season occurs from late November to March, although there is considerable atmospheric humidity because of the frequent occurrence of mists. The mean monthly temperature is 13.03?C, with a minimum of 5.84?C and a maximum of 22.6?C. The climate is tropical-humid with a short, dry season (Puig 1976). The forest is greatly influenced by the easterly winds and polar air masses producing the cold winter winds locally called nortes. It is also affected by frequent, damaging hurricanes 1:34% of the total hurricanes that reach Mexican eastern coasts affect the state of Tamaulipas (Jauregui 1967)1. The cloud forest is in a mountainous region of volcanic origin. Soils are lithosols 10 cm deep with a sandy loam texture; rendzinas can sometimes be found 10 to 40 cm deep with a clay texture, both with a high organici-matter content (Puig et al. 1983). The vegetation has been described as a mixture of tropical and temperate species (Sharp et al. 1950, Hernandez X. et al. 1951, Sharp 1953, Rzedowski 1978, Lonard & Ross 1979) and more recently it was quantitatively described as a forest with abundant tropical tree species but with temperate canopy trees structurally dominating the community (Puig et al. 1983). This forest was I Received 10 October 1986, revision accepted 30 March 1987. 2 Present address: Centro de Investigaciones Biol6gicas de Baja California Sur, Apdo. Postal 128, La Paz 23060, Baja California Sur, Mexico. 178 BIOTROPICA 20(3): 178-184 1988 This content downloaded from 207.46.13.48 on Wed, 12 Oct 2016 05:31:54 UTC All use subject to http://about.jstor.org/terms