Forest structure and biodiversity on a steep geophysical gradient: the cloud forest lee marginRobert O. Lawton, Ronald M. Welch, Salvi Asefi, Deepak K. Ray, Udaysankar S. Nair, Kerry N. Rabenold & William A. Haber

INTRODUCTION

Tropical montane cloud forests are central elements of the complex of montane environments that lie at the core of most tropical biodiversity hotspots. Cloud forests, defined by frequent, prolonged and predictable immersion in cloud, are centers of endemism for many taxa. Although lowland rain forests are richer in species at spatial scales less than 0.1km2, cloud forests and associated formations are as species rich at scales of 10km2 and above. This contrast between the spatial organization of biodiversity in lowland and montane regions is poorly understood, but must be addressed in any theory explaining global gradients in biodiversity and ecosystem structure. This project is designed to address, in the well-known montane forests of Monteverde, in the Cordillera de Tilarán of Costa Rica, interrelationships among the critical components of this problem: (1) the geophysical gradient at the cloud forest margin, (2) the pattern of tree and bird species distribution, and (3) the physical structure of the forest.

We use GOES and MODIS imagery, regional atmospheric modeling, and ground observation to establish diurnal and seasonal patterns of cloud immersion.

Persea spp.Ocotea spp.

MAPPING TREE AND BIRD DISTRIBUTIONS

Tree distributions and abundances are determined from a series of 20 x 50 m plots in which all individuals >10 cm dbh are measured and identified. Bird distributions and abundances are derived from auditory census points co-located with tree plots.

Serial replacement of closely related tree species along the upper Pacific slope of the Cordillera de Tilarán seems common. Above are the distributions of three closely related species in each of three general of the Lauraceae.

Bird communities also change dramatically on the upper Pacific slope. To the right the first axis scores from a detrended correspondence analysis of bird community composition are plotted against distance from the continental divide, a proxy for the frequency of cloud immersion.

Beilsmiedia spp. Persea spp. Ocotea spp.

(a) Upper Pacific slope of Costa Rica’s Cordillera de Tilarán during dry season (b) Cloud forests ( lower montane rain forests; ~1500-1800 m) are obscured in (a) by clouds. (c) Evergreen lower montane wet forests lie just below the clouds (~1300-1500m), and semi-deciduous premontane and lower montane moist forests occupied the now deforested slopes below. During the rainy season, convective thunderstorms wet the whole area, but during the dry season (Jan-mid-May) cloud and mist are the only water inputs.

The clouds visible above in (a) are the evaporating lee edge of an orographic cloud bank created by trade winds flowing from the Caribbean, across the lowlands of northern Costa Rica, and over the Cordillera.

THE SETTING: Monteverde, Costa Ricab

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MAPPING THE FREQUENCY OF CLOUD IMMERSION

Photos from fixed observation points yield local cloud base heights, which are used to assess atmospheric models and satellite retrievals.

Regional atmospheric modeling (with CSU RAMS) allows the evaluation of the influences of changes in land use and sea surface temperature on the spatiotemporal distribution of orographic cloud. Shown to the right are maps of the Cordillera de Tilarán showing the frequency of cloud in the forest from RAMS simulation of two weeks in March 2003, based on the current distribution of land uses. 1 km inner grid and 1 km DEM. Model runs with better spatial resolution will be used to produce more detailed maps for use with tree and bird distribution data.

Cloud frequency during the dry season of Costa Rica from GOES imagery. Although this is from all clouds, trade wind orography creates the patches of high cloud frequency on the windward slopes of mountains.

January 20021015 local time

March 20021015 local time

Dry season 2003

MAPPING FOREST STRUCTURE

Winter cold fronts spilling down the Great Plains and across the Gulf of Mexico produce storms known as “temporales” or “nortes” in Central America. These storms accelerate the trade wind flow, bringing winds >100 kph across the continental divide in the Cordillera de Tilarán. As a result of the consistency of direction of the trade wind flow across the mountains there is much local variation in wind exposure. This is related to variation in tree density and in measures of tree size – dbh, height, and crown breadth. Local variation in crown breadth of upper canopy trees on the windward (right) and leeward (left) side of Cerro Roble (1699m) can be readily distinguished in the IKONOS image to the right. This variation in crown breadth, and the correlations between crown size, dbh, and tree height provide a way to examine spatial variation in forest structure in concert with patterns of species distribution using high resolution remote sensing.

Atmospheric modeling allows evaluation of the impact of regional land use changes on the spatiotemporal distribution of orographic cloud. To the right is an east (right) to west (left) vertical cross section of the RAMS simulated cloud water mixing ratio (gms water/kg air) above the Cordillera de Tilaran and adjacent lowlands at midday March 9, 1999 for a completely deforested landscape (a) and a completely forested one (b). The implications for species distributions are obvious.

Ground-based observations, remote sensing using GOES and MODIS, and atmospheric modeling are used to characterize the spatiotemporal distribution of orographic clouds