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Epiphyte biomass and associated canopy water storage capacity may vary greatly in tropical montane forests depending on climate, forest structure, and stand age. This study compares old-growth and secondary forests in the upper montane belt of the Cordillera de Talamanca (Costa Rica) with respect to biomass of non-vascular and vascular epiphytes and their effect on water fluxes in the canopies of an old-growth forest, an early-successional stand (10–15 years of age), and a mid-successional stand (c. 40 years). Irrespective of stand age, epiphyte communities were strongly dominated by non-vascular plants (70–99% of total epiphytic biomass). Epiphyte biomass in the old-growth forest (3400 kg ha−1) was more than 20 times that of the youngest stand (160 kg ha−1) and more than six times that of the intermediate stand (520 kg ha−1). Consequently, the water storage capacity of non-vascular epiphytes and canopy humus increased from 0.06 mm in the early-successional, via 0.18 mm in the mid-successional, to 0.97 mm in the old-growth stand. Thus, the recolonization by epiphytes of tropical successional forests after clear-cutting, and the restoration of epiphytic water storage capacity will require many decades if not centuries.
Although soil resources are widely considered as a major factor that reduces the productivity, stature, and diversity of tropical montane cloud forests (TMCF), systematic comparisons of soil resources within and between TMCF are lacking. This study combines published reports on TMCF soils with new data on the soils and forest structure of the Luquillo Mountains in Puerto Rico to assess the current state of knowledge regarding global and local-scale variation in TMCF soils. At the global scale, soils from 33 TMCF sites and over 150 pedons are reviewed. Compared to soils in humid lowland tropical forests, TMCF soils are relatively acidic, have higher organic matter content, and are relatively high in total nitrogen and extractable phosphorus. Across all sites, significant correlations also exist between mean annual precipitation and soil pH and base saturation, but not between any soil chemical factor and canopy height, site elevation, or air temperature. Although comparisons between TMCF are limited by inconsistent sampling protocols, analysis of available data does indicates that lower montane cloud forests (LMCF) have taller canopies, higher soil pH, lower soil nitrogen, and higher C/N ratios than upper montane cloud forests (UMCF). Within an UMCF in NE Puerto Rico, the abundance of soil nitrogen, carbon, and potassium accounted for 25% to 54% of the variation in canopy height. However, as much as 68% of the variation in stand height could be accounted for when site exposure, slope gradient, and the percent coverage of surface roots were also included in the analysis. […]
Between 5 March and 10 May 2001, the performance of three types of passive fog gages (wire harp WH, standard fog collector SC, and Juvik gage JU) was compared at a wind-exposed Puerto Rican elfin cloud forest site. The gages were used to determine the timing and duration of fog and of fog-induced crown drip. Gage to canopy conversion factors were derived from the ratio between throughfall, and fog collected by the respective gages during periods with fog-only and negligible evaporation loss. The fog gages indicated very similar timing and fog duration, presumably because dense fog prevailed for 75–80% of the time and winds were generally sufficiently strong (4–5 m s−1). Resulting gage-to-canopy factors were 0.12 for WH and SC, and 0.10 for JU. Estimating fog interception by the canopy using these conversion factors gave very similar values for the three gage types (mean rates of 0.15 ± 0.01 mm hour−1). Throughfall typically started about 5 hours after the fog gages indicated the beginning of a fog event. This time lag probably reflects the filling of the storage capacity of the canopy, because wet canopy evaporation was negligible. Integrating the estimated fog interception rates over the time lag suggested an average value of 0.4 mm for the canopy storage capacity during events with fog-only.
This volume represents a uniquely comprehensive overview of our current knowledge on tropical montane cloud forests. 72 chapters cover a wide spectrum of topics including cloud forest distribution, climate, soils, biodiversity, hydrological processes, hydrochemistry and water quality, climate change impacts, and cloud forest conservation, management, and restoration. The final chapter presents a major synthesis by some of the world's leading cloud forest researchers, which summarizes our current knowledge and considers the sustainability of these forests in an ever-changing world. This book presents state-of-the-art knowledge concerning cloud forest occurrence and status, as well as the biological and hydrological value of these unique forests. The presentation is academic but with a firm practical emphasis. It will serve as a core reference for academic researchers and students of environmental science and ecology, as well as practitioners (natural resources management, forest conservation) and decision makers at local, national, and international levels.
Annual precipitation on the Caribbean island of Puerto Rico has decreased steadily during the twentieth century, on average by 16%. The reduced rainfall manifested itself in the form of regular water rationings for millions of inhabitants during the 1990s. This chapter examines the link between the reduction in precipitation and the land-cover change using a combination of energy balance measurements and meso-scale atmospheric modeling. The explanation of the reduction in precipitation proved to be different than expected. Based on measurements made earlier over rain forest and pasture in Amazonia, a forest-covered island was expected to be cooler because of the higher transpiration of forest compared to grassland. The opposite proved to be the case: transpiration by a coastal wetland forest was less than that of an adjacent, well-watered grassland. In addition, the forest's albedo was 8% lower than that for the grassland. Together, these two factors caused the sensible heat flux over the forest to be twice that over the grassland. The surface energy balance observations over forest and grassland were used in a meso-scale atmospheric circulation model (RAMS) to simulate the meteorological effects of island-wide deforestation. The simulations indicated that the development of a sea breeze during the day dominates the climate on the island. In model runs in which the island was assumed to be completely covered with forest, the sea breeze was considerably stronger than when the vegetation had been transformed to grassland. Along the sea breeze front, convergence caused upward air motions. […]
Between 10 July and 25 October 2001, fog interception (CWI) by a Puerto Rican elfin cloud forest at 970 m.a.s.l. was studied. Values of CWI were estimated from the wet-canopy water budget as the sum of throughfall, stemflow, and interception loss, minus rainfall corrected for the effect of slope. CWI was also estimated from throughfall measurements during periods of fog-only. Timing and duration of fog were measured using a wire harp that was protected against rainfall. Estimated rates of CWI correlated significantly (p < 0.01) with collections by the fog gage. Gage to canopy factors were determined from the slopes of these relationships and were considered to integrate differences in fog-catching efficiency of the wire harp and the forest canopy. Applying these factors to the fog gage record yielded an estimated 1.1–1.4 mm day−1 of fog CWI, equivalent to 10–12% of mean daily corrected rainfall (11.3 mm day−1). The wet-canopy water budget method gave the best results when rainfall amounts were small compared to fog, but error bands of estimated CWI became too wide to give meaningful results when rainfall was equal to or larger than the amount of fog. Furthermore, solving the water budget for large rainstorms gave negative values of CWI, possibly because of underestimated stemflow.
Epiphytic vascular plants and bryophytes constitute an important component of cloud forest canopies. Because of their different characteristics compared with leaves and other tree structural elements, epiphytes can be expected to behave differently in terms of their ability to intercept and store rain and cloud water, whereas losses through evaporation and drip may also occur at different rates. The water dynamics of epiphytes were studied in a windward lower montane cloud forest in northern Costa Rica. The exposed site experienced frequent horizontal precipitation (fog and wind-driven rain) as well as strong winds. In situ epiphyte wetting experiments were conducted at different levels within the 20-m canopy during a series of fog events using pre-weighed branches with known epiphyte biomass, while making simultaneous measurements of fog density and drop-size spectrum on a tower extending above the canopy. Rates of water loss via evaporation from pre-wetted epiphyte-laden branches suspended at different heights within the canopy were determined on dry days. Storage capacities were determined by gravimetric means, both in the field and under controlled conditions. Total epiphyte biomass of the forest was estimated through systematic sampling of three emergent trees and five sub-canopy trees in combination with a diameter survey of four plots of 1000 m2 each. Fog interception rates by epiphyte-laden branches differed with position in the canopy, with an average rate of 54.7 ml hour−1 kg−1 of oven-dry biomass. Absorption rates were correlated with fog liquid water content and initial moisture content of the sample. […]
Mike Bonell, Chief of the Hydrological Processes and Climate Section, The UNESCO Division of Water Sciences,
L. A. Bruijnzeel, Senior Lecturer/Associate Professor of Eco-Hydrology, The Department of Hydrology and Geo-Environmental Sciences, Vrije Universiteit, Amsterdam
This section contains four chapters providing a critical assessment of existing guidelines (also called best management practices) that have been designed to minimise adverse effects on soils and streams during timber harvesting and land clearing operations in areas with tropical forest, as well as under rainfed cropping in deforested tropical steeplands. In addition, the main factors hampering the widespread application of these best management practices are examined in some detail and areas indicated requiring additional research.
Cassells and Bruijnzeel provide an overview of guidelines aiming to minimise adverse impacts on the residual vegetation, soils and streams during tropical timber harvesting operations. Research and land management experience over many decades have demonstrated that poorly planned or managed logging operations generally have deleterious environmental impacts. Most commonly, logging and the associated road construction lead to problems with erosion and sedimentation, and thus to a reduction in the quality of streamwater and aquatic habitat. At the same time, there is considerable evidence indicating that, provided forest managers and planners respect broad land capability limits, appropriately managed logging operations can be compatible with the maintenance of good quality water supplies. In this regard, experience has shown that particular attention needs to be given to the careful location of roads, extraction trails and stream crossings; minimising ground disturbance and maintaining effective ground cover; as well as maintaining undisturbed buffer strips (as stressed by Connolly and Pearson; Hamilton) around key streams and waterways.