Tree Species Distributions in Relation to Stream Distance in a Mid-Montane Wet Forest, Puerto Rico

Caribbean Journal of Science, Vol. 45, No. 1, 52-63, 2009 Copyright 2009 College of Arts and Sciences University of Puerto Rico, Mayagüez Tree species distributions in relation to stream distance in a mid-montane wet forest, Puerto Rico Tamara Heartsill Scalley1,3 , Todd A. Crowl1 , and Jill Thompson2 1 Ecology Center and Watershed Science, Utah State University, Logan, Utah 84322-5210; Corresponding author: [email protected] 2 Institute for Tropical Ecosystem Studies, University of Puerto Rico, Rio Piedras, Puerto Rico 00931-1910 3 Current address: International Institute of Tropical Forestry, USDA Forest Service, Jardín Botánico Sur, 1201 Calle Ceiba, Río Piedras, Puerto Rico 00926-1115 A BSTRACT. — Riparian zones are dynamic areas adjacent to flowing freshwater that connect aquatic and terrestrial environments. We measured individual tree distances relative to two permanent streams in the Luquillo Forest Dynamics Plot, Puerto Rico, to determine if tree species exhibit distinct affinities for riparian zones in a mid-montane tropical forest. In addition, we also calculated stem density, species richness, and diversity indices in 20 x 20 m randomly selected quadrats at different distances from the streams. We found that no single species in the study site was predominantly associated with riparian zones; however, almost all species were represented by a few stems in close proximity to streams. Multivariate (Fuzzy Set Ordination) ordination of plot species composition in relation to distance to streams indicated that there is greater variation in species composition closer to streams, and less variation in species composition farther from streams. This study suggests that in mid-montane wet forest, riparian zones include all tree species found throughout the forest landscape. R ESUMEN. — Las zonas ribereñas son áreas de gran dinamismo adyacentes a cuerpos de agua dulce que conectan a los ambientes terrestres con los acuáticos. Para determinar si hay especies de árboles que se encuentran exclusivamente asociadas a la zona ribereña en un bosque húmedo de montaña, medimos la distancia de árboles individuales en relación a dos quebradas permanentes en la Parcela de Dinámica Poblacional del Bosque de Luquillo, Puerto Rico. También calculamos cantidad de tallos, riqueza de especies e índice de diversidad en cuadrantes 20 x 20 m que estaban a diferentes distancias de las quebradas. No encontramos especies que estuvieran exclusivamente en las zonas ribereñas, sin embargo, casi todas las especies estaban representadas cerca de las quebradas. Encontramos una densidad más alta de árboles cerca de las quebradas. Un análisis de ordenación multivariado (Fuzzy Set Ordination) a base de composición de especies y distancia a las quebradas de cuadrantes 20 x 20 m indicó que la composición de especies en cuadrantes cerca de las quebradas era más variada que la composición de especies de los cuadrantes a distancias mayores de 50 metros de las quebradas. Este estudio sugiere que en bosques húmedos de montaña las zonas ribereñas incluyen todas las especies de árboles que se encuentran en el paisaje. K EYWORDS. — Luquillo Forest Dynamic Plot , Puerto Rico , riparian , streams , trees Introduction and making available new areas for colo- nization (Scatena and Lugo 1995). Species Throughout the landscape, freshwater that can quickly colonize or reestablish in and terrestrial systems connect in dynamic these areas are capable of dominating ripar- and complex areas called riparian zones ian zones. In general, riparian species are (Naiman and Decamps 1990, Malanson those that have a suite of life history and 1993, Naiman and Décamps 1997, Tabacchi physiological attributes that allow success et al. 1998). In montane streams, storm in the transitional and disturbance-prone flows produce periodic accumulations of environment of montane riparian zones organic matter and nutrients from upland (Tabacchi et al. 1990, Kellman and Tackab- areas, often altering the stream channel erry 1993). In some temperate biomes these 52 TREE DISTRIBUTION IN A PUERTO RICAN WET FOREST 53 communities are composed of species that (1996) tropical climate system. The LFDP are distinct from the surrounding non- ranges in elevation from 333 to 428 m above riparian landscape (Andersson et al. 2000, sea level, and has annual mean precipita- Hylander et al. 2002, Shafroth et al. 2002). It tion of 3,500 mm almost evenly distributed is often presumed that riparian zones har- throughout the year (Heartsill Scalley et al. bor higher species richness and diversity 2007). due to resource availability, however, there Two perennial streams, Prieta, 2nd order, are few estimates of how many plant spe- and Toronja, 1st order, flow through the cies are found exclusively in riparian zones LFDP from east to west ( Fig. 1 ). Average (Metzger et al. 1997, Decocq 2002, Sabo et al. width of the stream channels is 3 m and av- 2005). Whether distinct riparian plant com- erage depth is 0.5 m (Heartsill Scalley et al. munities exist in mid-montane tropical wet 2001). These are high gradient streams with forests, such as the Luquillo Experimental small floodplain areas. The streams are not Forest, is an open question. incised, and due to the presence of large In the context of a wet forest, we ask (1) boulders, stream channel elevations can be Is there a distinct group of tree species as- near the surrounding bank elevations. Both sociated with riparian zones? and (2) Are Prieta and Toronja streams are boulder and there gradients in community metrics such bedrock lined, and carry small amounts of as stem density or species richness as a silt, clay, and sand. Common disturbances function of distance from streams? We hy- in the Luquillo Mountains and the LFDP in- pothesize that in this wet forest landscape, clude tree falls, landslides, and hurricanes riparian zones may not provide a suffi- (Scatena and Larsen 1991, Scatena and Lugo ciently different abiotic or environmental 1995, Reagan and Waide 1996). Tree falls template to result in exclusively riparian create small gaps in the forest canopy, while tree species or community characteristics. landslides produce larger disturbed areas To answer these questions, we analyzed in- with exposed mineral soil. Hurricanes pro- dividual tree species distribution patterns duce large-scale effects on the forest land- and plot-scale community characteristics scape, including decrease in canopy cover, in the 16 ha Luquillo Forest Dynamics Plot high precipitation and stream flows, and (LFDP) which has two permanently flow- high wind velocity that increases the occur- ing streams, and where the tree species rence of tree falls and landslides. Details on composition and previous land-use prac- the effects of hurricane Hugo on the LFDP tices have been well described and docu- can be found in Brokaw and Grear (1991), mented (Thompson et al. 2002, Uriarte et al. Thompson et al. (2002) and Uriarte et al. 2004). 2004. The forest type in the LFDP is known Methods as “Tabonuco” after the dominant tree, Dacryodes excelsa , which occurs below 600 Study area m asl in the Luquillo Mountains (Brown The 16 ha Luquillo Forest Dynamics Plot et al. 1983). Current species composition in (LFDP; southwest corner 18 20’ N, 65 49’ the LFDP is related to past land-use history W), is situated within the Luquillo Experi- and soil type, which have resulted in de- mental Forest in northeastern Puerto Rico finable species-composition patterns in the ( Fig. 1 ) which is part of the Luquillo Long forest landscape of the Luquillo Experimen- Term Ecological Research site and the Cen- tal Forest (Zimmerman et al. 1995, Thomlin- ter for Tropical Forest Science network of son et al. 1996, Reed 1998, Thompson et al. long term plots of the Smithsonian Tropi- 2002). The present species composition pat- cal Research Institute. The LFDP is di- tern includes species brought for restoring vided into 400 quadrats each 20 x 20 m in forest canopy, those left as remnants from a 500 x 320 m grid. The plot is classified as agricultural activities, ruderal species, and Subtropical wet forest in the Holdridge life non- native species that have invaded the zone system (Ewel and Whitmore 1973), plot (Taylor 1994, Thompson et al. 2002). and Tropical montane forest in Walsh’s Prior to 1932, land-use activities within the 54 T. HEARTSILL SCALLEY, ET AL. Fig. 1. Luquillo Forest Dynamics Plot (LFDP), with study streams Toronja and Prieta in relation to elevation contours and human disturbance due to previous land uses within the 16 hectare plot, located in the Luquillo Experimental Forest, northeast Puerto Rico. LFDP area included selective logging, and human disturbance and less canopy cover subsistence agriculture. However, the forest ( Fig. 1 ; Thompson et al. 2002). surrounding the LFDP has remained protected since 1934 when the United States Luquillo forest dynamics plot dataset Department of Agriculture, Forest Service purchased the area. The LFDP was divided We used data from the 1989 rapid tree into land-cover classes based on the amount damage inventory taken after hurricane of canopy cover recorded in aerial photo- Hugo, ( http://luq.lternet.edu/data/ graphs in 1936 (Thompson et al. 2002). These lterdb60/metadata/lterdb60.htm ) as well photographs reveal that the area south of as the plot inventory, started in 1990. Dur- Prieta stream showed the greatest canopy ing this inventory, all trees >10 cm in di- cover as a result of minimal human distur- ameter at a height of 130 cm from the bance, while the area north of Prieta stream, ground (DBH) were measured, identified encompassing the Toronja stream and cov- to species, and mapped in x,y coordinates ering the rest of the LFDP, had greater within the grid. The dataset is described TREE DISTRIBUTION IN A PUERTO RICAN WET FOREST 55 in http://luq.lternet.edu/data/lterdb119/ nearest stream. In addition, we conducted lin- metadata/lterdb119.htm and Thompson ear regressions between mean elevation per et al.

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