Elfin Cloud Forest Research Articles

Carbon fluxes associated with fog in an elfin cloud forest in Anaga (Tenerife, Canary Islands)

The role of fog in the relict laurel forests of the Macaronesia is still not fully understood and it is even more relevant in the Canary Islands, where precipitations are rather scarce and summers are dry. Eddy covariance CO2 exchange (FCO2) measurements in a “brezal de tejo” elfin cloud forest in Tenerife (Canary Islands), over a four-weeks period during the wet season, showed that the vegetation can sustain a carbon assimilation rate in an environment of fog, leading to higher median water use efficiency values (19.5 μmol mg−1 ≤ WUE ≤ 68.3 μmol mg−1) than during no-fog periods (WUE = 1.8 μmol mg−1), being highest under thick fog (WUE = 68.3 μmol mg−1). Median FCO2 values across the day were slightly greater under foggy versus no-fog conditions during the study period. These increments were more prominent under a low light environment, especially in the morning, thus highlighting the important difference between cloud immersion versus high clouds. A greater proportion of light diffusivity due to fog may enhance carbon assimilation, although additional concomitant factors may intervene during thick fog conditions. Also, net carbon assimilation was initiated earlier under foggy conditions. During cloud immersion and depleted light, the maximal CO2 exchange, FCO2_max, diurnal values were higher than those observed when fog was absent. Additionally, FCO2_max remained high (FCO2_max ≈ 13 μmol m−2 s−1) for large vapour pressure deficit conditions (VPD > 0.6 kPa). All this is indicative of a profligate water use strategy of the vegetation, presumably due to low stomatal control. Fog was present during 36.9 % of the time and, as a consequence, the estimated mean daily settling flux of water droplets was 1.186 mg m−2 s−1. For the entire period measured, the cumulative settling flux was 1.30 mm compared to the 97.6 mm of rainfall and the 36.6 mm of throughfall, and the canopy was wet at least 33.3 % of the time. The results suggest that such an anisohydric behaviour of the vegetation permits a continuous growth with maximal water use efficiency, while the frequent presence of fog and wetting of the canopy would minimize the risk of cavitation associated with such non-conservative water use response.

Hydrometeorology of tropical montane cloud forests: emerging patterns

AbstractTropical montane cloud forests (TMCF) typically experience conditions of frequent to persistent fog. On the basis of the altitudinal limits between which TMCF generally occur (800–3500 m.a.s.l. depending on mountain size and distance to coast) their current areal extent is estimated at ∼215 000 km2or 6·6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog‐affected forest may occupy as much as 2·21 Mkm2. Four hydrologically distinct montane forest types may be distinguished, viz. lower montane rain forest below the cloud belt (LMRF), tall lower montane cloud forest (LMCF), upper montane cloud forest (UMCF) of intermediate stature and a group that combines stunted sub‐alpine cloud forest (SACF) and ‘elfin’ cloud forest (ECF). Average throughfall to precipitation ratios increase from 0·72 ± 0·07 in LMRF (n= 15) to 0·81 ± 0·11 in LMCF (n= 23), to 1·0 ± 0·27 (n= 18) and 1·04 ± 0·25 (n= 8) in UMCF and SACF–ECF, respectively. Average stemflow fractions increase from LMRF to UMCF and ECF, whereas leaf area index (LAI) and annual evapotranspiration (ET) decrease along the same sequence. Although the data sets for UMCF (n= 3) and ECF (n= 2) are very limited, the ET from UMCF (783 ± 112 mm) and ECF (547 ± 25 mm) is distinctly lower than that from LMCF (1188 ± 239 mm,n= 9) and LMRF (1280 ± 72 mm;n= 7). Field‐measured annual ‘cloud‐water’ interception (CWI) totals determined with the wet‐canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm (n= 15). Field measured values also tend to be much larger than modelled amounts of fog interception, particularly at exposed sites. This is thought to reflect a combination of potential model limitations, a mismatch between the scale at which the model was applied (1 × 1 km) and the scale of the measurements (small plots), as well as the inclusion of near‐horizontal wind‐driven precipitation in the WCWB‐based estimate of CWI. Regional maps of modelled amounts of fog interception across the tropics are presented, showing major spatial variability. Modelled contributions by CWI make up less than 5% of total precipitation in wet areas to more than 75% in low‐rainfall areas. Catchment water yields typically increase from LMRF to UMCF and SACF–ECF reflecting concurrent increases in incident precipitation and decreases in evaporative losses. The conversion of LMCF (or LMRF) to pasture likely results in substantial increases in water yield. Changes in water yield after UMCF conversion are probably modest due to trade‐offs between concurrent changes in ET and CWI. General circulation model (GCM)‐projected rates of climatic drying under SRES greenhouse gas scenarios to the year 2050 are considered to have a profound effect on TMCF hydrological functioning and ecology, although different GCMs produce different and sometimes opposing results. Whilst there have been substantial increases in our understanding of the hydrological processes operating in TMCF, additional research is needed to improve the quantification of occult precipitation inputs (CWI and wind‐driven precipitation), and to better understand the hydrological impacts of climate‐ and land‐use change. Copyright © 2010 John Wiley & Sons, Ltd.

Hydrometeorology and structure of tropical montane cloud forests under contrasting biophysical conditions in north‐western Costa Rica

AbstractTropical cloud forests have received increasing attention because of their significance for freshwater supply. This study aimed to understand hydro‐meteorological gradients in relation to spatial changes in forest structure in north‐western Costa Rica. Seven climate stations (measuring rainfall, horizontal precipitation, throughfall, temperature and soil moisture) were installed along a 2·5 km transect between 1200 and 1500 m.a.s.l. on the Atlantic (windward) slope and the Pacific (leeward) slope of the Tilarán mountains. Forest structure was investigated on seven 10 × 50 m plots. Epiphytic vegetation was assessed on six trees at 1450 m and at 1200 m on the Pacific slope. Annual rainfall ranged from 3690 mm on the leeward slope to 6390 mm on the windward side. Horizontal precipitation was 3560 mm at the ridge, where it exceeded rainfall during the dry season, compared to 330 mm and 28 mm at the lowest windward and leeward plots, respectively. Throughfall remained below rainfall on the lower slopes but exceeded rainfall on the ridge. Soil water content ranged between 70% and 80% on the ridge top, where waterlogging occurred frequently. The studied forests were classified as lower montane rain forest, lower montane cloud forest and elfin cloud forest. The greatest canopy heights and basal areas occurred on the leeward slope between 1200 and 1450 m and at the lowest windward plot. Tree heights remained below 15 m on the ridge, where stilt roots occurred frequently. Near the ridge, epiphyte abundance and species richness were greater, compared to the lower leeward slope. These findings prove the importance of horizontal precipitation in the study area, confirm the important role of epiphytes as indicators for moisture gradients and elucidate the variability of forest structure under the given biophysical conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Comparison of passive fog gauges for determining fog duration and fog interception by a Puerto Rican elfin cloud forest

AbstractBetween 5 March and 10 May 2001, the performance of three types of passive fog gauges (wire harp WH, standard fog collector SC and Juvik gauge JU) was compared at a wind‐exposed Puerto Rican elfin cloud forest site. The gauges were used to determine the timing and duration of fog, as well as fog‐induced crown drip using gauge to canopy conversions factors. The latter were calculated as the average ratio between throughfall, and fog collected by the respective gauges during periods with fog‐only and negligible evaporation loss. The fog gauges indicated very similar timing and fog duration, presumably because dense fog prevailed for 75–80% of the time whereas winds were generally sufficiently strong (4–5 m s−1) and from a nearly constant direction. Resulting gauge to canopy factors were 0·12 for WH and SC, and 0·10 for JU. Estimating fog interception by the elfin forest canopy using these conversion factors gave very similar values for the three gauge types (mean rates of 0·15 ± 0·01 mm h−1). Throughfall typically started about 5 h after the fog gauges indicated the beginning of a fog event. This time lag probably reflects the filling of the canopy storage capacity. Integrating the estimated fog interception rates over the time lag suggested an average value of 0·4 mm for the canopy storage capacity during fog‐only events. The present results are expected to apply to sites with comparable wind and fog conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Fog Water Collection in a Subtropical Elfin Laurel Forest of the Garajonay National Park (Canary Islands): A Combined Approach Using Artificial Fog Catchers and a Physically Based Impaction Model

AbstractFog precipitation has long been assumed as an additional water source in the relic laurel ecosystems of the Canary Islands, located at 500–1400 m MSL. However, to what extent fog water can contribute to the laurel forest water balance is not yet clear. Combining data from artificial fog catchers and a physically based impaction model, the authors evaluated the potential contribution of fog water captured by needle-leafed Erica arborea L. trees in a selected watershed of the Garajonay National Park (La Gomera Island) for a 2-yr period (February 2003–January 2005). Fog water collection was measured with artificial catchers at four micrometeorological stations placed at 1145, 1185, 1230, and 1270 m MSL. Average fog water collection was only significant at the highest measurement site (one order of magnitude greater than at lower altitudes), totaling 496 L m−2 yr−1 during the 2-yr period. The average fog water yield in the first and second annual periods ranged between 0.2–5.0 and 0.1–2.1 L m−2 day−1, respectively. Rainfall exhibited seasonality, distinguishing between rainy and dry seasons, while fog water collection was distributed more evenly throughout the year. Regarding fog water captured by the vegetation, the impaction model predicted a significant amount of fog water potentially collected by a single E. arborea tree, on the order of 1810–2090 L yr−1. Taking tree population density into account, the yearly average water contribution to the soil surface by wind-driven fogs was 251–281 mm, whereas annual rainfall was 635 and 1088 mm, respectively. The hourly course of micrometeorological variables shows a 58% reduction in global radiation under foggy conditions and a concomitant 3°–6°C mean temperature decrease compared to fog-free periods. Thus, limiting evapotranspiration may also be a relevant effect of fog in this subtropical elfin cloud forest.

Effects of Wind on the Allometry of Two Species of Plants in an Elfin Cloud Forest

ABSTRACTThigmomorphogenesis includes the effects of mechanical perturbation on plant growth. To test whether thigmomorphogenesis is evident at different scales within plants, we investigated the effect of wind on allometric relationships between specific plant parts. We chose two species from the elfin cloud forest of Puerto Rico that have contrasting growth habits, the shrub Clibadiun erosum (Asteraceae) and the palm Prestoea acuminata var. montana (Arecaceae), and subjected them to barrier‐protected and wind‐exposed treatments. For C. erosum, we compared the allometry of stems and branches against three allometric models that predict that plant height or branch length increases at the 1, 2/3, and 1/2 power of stem diameter. Only the geometric similarity model (scaling exponent of 1) seemed to hold when plants were exposed to the wind. We found relatively fewer leaves per number of branches produced and fewer leaves per increment of branch diameter in the plants of C. erosum exposed to the wind. Mean petiole length ratios (petiole length/basal radius) of P. acuminata were higher on leaves of barrier‐protected plants for both simple and compound leaves, indicating that petioles were stouter and mechanically safer in the wind‐exposed plants. We suggest that alteration of the allometric relationships of plant parts, organs, or plant modules (stems and branches of C. erosum and leaves of P. acuminata) and alteration of the number of plant parts (leaves and branches of C. erosum) are adaptive responses resulting from the mechanical perturbation induced by wind in the elfin forest.

Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest

The Luquillo Mountains of northeastern Puerto Rico harbours important fractions of tropical montane cloud forests. Although it is well known that the frequent occurrence of dense fog is a common climatic characteristic of cloud forests around the world, it is poorly understood how fog processes shape and influence these ecosystems. Our study focuses on the physical characteristics of fog and quantifies the fogwater input to elfin cloud forest using direct eddy covariance net flux measurements during a 43-day period in 2002. We used an ultrasonic anemometer–thermometer in combination with a size-resolving cloud droplet spectrometer capable of providing number counts in 40 droplet size classes at a rate of 12.5 times per second. Fog occurred during 85% of the time, and dense fog with a visibility < 200 m persisted during 74% of the period. Fog droplet size depended linearly on liquid water content ( r 2 = 0.89 ) with a volume-weighted mean diameter of 13.8 μ m. Due to the high frequency of occurrence of fog the total fogwater deposition measured with the eddy covariance method and corrected for condensation and advection effects in the persistent up-slope air flow, averaged 4.36 mm day −1, rainfall during the same period was 28 mm day −1. Thus, our estimates of the contribution of fogwater to the hydrological budget of elfin cloud forests is considerable and higher than in any other location for which comparable data exist but still not a very large component in the hydrological budget. For estimating fogwater fluxes for locations without detailed information about fog droplet distributions we provide simple empirical relationships using visibility data.

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

AbstractThe deposition of fog to a wind‐exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind‐induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface‐normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain‐free conditions (0·11 ± 0·05 mm h−1) and rainy conditions (0·24 ± 0·13 mm h−1) were about three to six times the eddy‐covariance‐based estimate (0·04 ± 0·002 mm h−1). Under rain‐free conditions, water‐budget‐based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ∼40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ∼770 mm year−1 for the summit cloud forest just below the ridge top (according to the water budget method) and ∼785 mm year−1 for the cloud forest on the lower windward slope (using the eddy‐covariance‐based deposition rate corrected for estimated vertical flux divergence). Copyright © 2006 John Wiley &amp; Sons, Ltd.

Effect of water, temperature and fertilizers on soil nitrogen net transformations and tree growth in an elfin cloud forest of Colombia

(Accepted 31st July 1999)In the ‘elfin’ cloud forest of Serrania de Macuira, exchangeable ammonium and nitrate, and the rates of soil nitrogen mineralization and nitrification were measured in soil samples under different water, temperature and mineral nutrient additions. The effects of nitrogen, phosphorus and nitrogen plus phosphorus fertilization on radial trunk growth were measured in three tree species. In the cloud forest soils, concentrations of ammonium were much higher than those of nitrate. Nitrate was higher in samples collected during the afternoon than during the morning, probably as a result of leaching during the night or nitrification during the day. When samples were incubated under different water and temperature treatments, rates of nitrogen mineralization and nitrification increased more with changes in soil water content than with changes in temperature. Nitrification was significantly increased in soils amended with ammonium or with ammonium plus phosphorus, suggesting that nitrification is substrate-limited. Fertilization with nitrogen and phosphorus resulted in significantly increased girth increments in Guapira fragrans (Dum. -Cours.) Little and Rapanea guianensis Aublet. Myrcianthes fragrans (Sw.) D.C. did not respond to the fertilization. The results of this study support the hypothesis that the characteristics of montane rain forest in small and large tropical mountains (the ‘Massenerhebung’ effect) are greatly controlled by soil water conditions and related soil nitrogen availability.

Ecophysiology of Cecropia schreberiana saplings in two wind regimes in an elfin cloud forest: growth, gas exchange, architecture and stem biomechanics

The hypothesis that the short, stunted development of elfin cloud forest trees is influenced by strong winds was tested by studying growth, gas exchange and biomechanics of potted saplings of Cecropia schreberiana Miq. ex. C. peltata L. (Cecropiaceae) exposed to two natural wind regimes (exposed and protected) at high elevation sites in the Luquillo Experimental Forest, Puerto Rico. The wind-exposure treatment produced several thigmomorphogenetic responses, including reductions in plant stature and crown area, changes in allocation patterns, and increased root to shoot ratio, leaf abrasion and leaf epinasty. Wind-exposure decreased maximum photosynthetic rate and respiration on an area basis, but not on a leaf-mass basis. Wind-exposed plants had lower apparent quantum yields, and higher light compensation points than wind-protected plants. Photosynthetic nitrogen-use efficiency was lower in wind-exposed plants, but such plants had higher leaf nitrogen concentration than wind-protected plants. There were no effects of treatments on stomatal conductance, transpiration rate and water-use efficiency. Stems of wind-exposed plants had lower second moment of area, apparent modulus of elasticity, flexural stiffness and stem density, but higher water content than wind-protected plants. Tissue-density-specific stiffness and the calculated critical height were not affected by the treatments. Wind-exposed plants were biomechanically less predisposed to bending and failing under their own weight than wind-protected plants because their safety factors were smaller, indicating that maintenance of an ontogenetically less developed structure enables plants to cope with wind loading. Windward trees showed a lower scaling component of the allometric relationship between diameter and height than leeward trees.

Soil Respiration in the Clud Forest and Dry Deciduous Forest of Serrania de Macuira, Colombia

Soil respiration was studied in the elfin cloud forest and in the dry deciduous forest of Serrania de Macuira, Guajira Peninsula, Colombia. At the end of the 1985 rainy season, soil respiration was significantly higher (P < 0.001) in the cloud forest (mean daily CO2 evolution = 511 mg CO2 m-2 hr-') than in the dry deciduous forest (251 mg CO2 m-2 hr-'). In both forests, carbon dioxide evolution was higher during the night than during the day. The differences between day and night values in the cloud forest were attributed to the daily variation in fog interception. Day and night values differed more in the cloud forest than in the deciduous forest. High soil respiration rates in the cloud forest as compared to the deciduous forest were attributed to the relatively high temperature (24?C), and water content of the soils (31%), and to the abundant earthworms (64 earthworms m-2).

Climatic factors and tree stature in the elfin cloud forest of Serrania de Macuira, Colombia

Cloud cover, solar radiant flux density, temperature and relative humidity regimes were studied along the altitudinal gradient of Serrania de Macuira (865 m), Guajira, Colombia. The regional climate at Macuira is characterized by the presence of small cumulus clouds, persistent trade winds from the northeast, and low and highly seasonal rainfall. During the day time, the clouds remain above the highest peaks of the Serrania, decreasing radiation, air temperature, relative humidity and evaporation in relation to the lowlands. At night the clouds come into contact with the forest, supplying water by means of mist and fog interception. Owing to this pattern, the cloud forest trees on top of the mountain experience low air saturation deficits only during the night, early morning and late afternoon. This shows that the low stature of trees in this forest is not the result of an uninterrupted low saturation deficit that would inhibit mineral pumping by means of transpiration. The soil temperature gradient in Macuira was steeper than that reported for higher and more massive mountains of the tropics. This steeper gradient is the result of high temperatures in the dry lowlands around the mountain. Strong winds seem to be the cause of the reduction in tree stature in the more exposed slopes and ridges, but are not the general explanation for small trees in this forest. Wind speed is much lower in gullies than on ridges and slopes, nevertheless, trees are also small. We think that the climatic factor that has a more pronounced effect on the physiognomy of this forest is cloud water. The effects of high water content of the soil on soil chemistry, would be worth investigating as a general explanation for the small stature of these trees.

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira, Guajira, Colombia

Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m-2 day-1 during cloudy days and between 532 and 944 g m-2 day-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.

Crassulacean acid metabolism in tropical dicotyledonous trees of the genusClusia

The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genusClusia sampled at three sites in the state of Falcon in northern Venezuela is characterized.Clusia leaves have a somewhat succulent appearance. Unlike leaves of many other CAM plants, which are uniformly built up of very large isodiametric cells, there are distinct layers of palisade and spongy mesophyll, with individual cells being smaller. There is no specialized water storage tissue. δ13C values indicate thatC. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at ∼800 m altitude, performs C3 photosynthesis (δ13 −27.1‰). However,C. rosea in the tall cloud forest on Cerro Santa Ana (∼600m altitude), andC. rosea andC. alata in the dry forest on Serrania San Luis (∼900 m altitude) perform CAM (δ13C −14.1 to −19.2‰). InC. alta andC. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240 mmol 1−1 for malid acid and from 35 to 112 mmol 1−1 for citric acid. The sum of the changes in malate and citrate levels accounts for the changes of titratable protons measured. With a day-night change of titratable protons of 768 mmol 1−1 in one of the analyses,C. rosea showed the highest value yet encountered in a CAM plant. Oscillations of free sugars (fructose, glucose, sucrose) and of starch were also analysed in the CAM performingClusia species. Carbon skeletons of the precursors involved in nocturnal malate and citrate synthesis largely derive from free sugars and not from polyglucan. Unlike some other CAM plants, there is no clear and quantitative correlation between day-night changes of organic acid levels and cell sap osmolality.