Home About us Contact
|
Home About us Contact | ||
|
|
||
Dense Canopies (dense + canopy)
Selected AbstractsSeasonal snowpack dynamics and runoff in a cool temperate forest: lysimeter experiment in Niigata, JapanHYDROLOGICAL PROCESSES, Issue 20 2005Andrew C. Whitaker Abstract Seasonal snowpack dynamics are described through field measurements under contrasting canopy conditions for a mountainous catchment in the Japan Sea region. Microclimatic data, snow accumulation, albedo and lysimeter runoff are given through the complete winter season 2002,03 in (1) a mature cedar stand, (2) a larch stand, and (3) a regenerating cedar stand or opening. The accumulation and melt of seasonal snowpack strongly influences streamflow runoff during December to May, including winter baseflow, mid-winter melt, rain on snow, and diurnal peaks driven by radiation melt in spring. Lysimeter runoff at all sites is characterized by constant ground melt of 0·8,1·0 mm day,1. Rapid response to mid-winter melt or rainfall shows that the snowpack remains in a ripe or near-ripe condition throughout the snow-cover season. Hourly and daily lysimeter discharge was greatest during rain on snow (e.g. 7 mm h,1 and 53 mm day,1 on 17 December) with the majority of runoff due to rainfall passing through the snowpack as opposed to snowmelt. For both rain-on-snow and radiation melt events lysimeter discharge was generally greatest at the open site, although there were exceptions such as during interception melt events. During radiation melt instantaneous discharge was up to 4·0 times greater in the opening compared with the mature cedar, and 48 h discharge was up to 2·5 times greater. Perhaps characteristic of maritime climates, forest interception melt is shown to be important in addition to sublimation in reducing snow accumulation beneath dense canopies. While sublimation represents a loss from the catchment water balance, interception melt percolates through the snowpack and contributes to soil moisture during the winter season. Strong differences in microclimate and snowpack albedo persisted between cedar, larch and open sites, and it is suggested further work is needed to account for this in hydrological simulation models. Copyright © 2005 John Wiley & Sons, Ltd. [source] Analytical models for the mean flow inside dense canopies on gentle hilly terrainTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 634 2008D. Poggi Abstract Simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments. Laser Doppler anemometry (LDA) measurements are conducted in a neutrally-stratified boundary-layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy. The hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy (H/L , 1 and Lc/L , 1, where H is the hill height, L the half-length, and Lc the canopy adjustment length-scale). The LDA data suggest that the often-criticized linearizations of the advective terms, turbulent-shear-stress gradients and drag force appear reasonable except in the deep layers of the canopy. As predicted by a previous analytical model, the LDA data reveal a recirculation region within the lower canopy on the lee slope. Adjusting the outer-layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model's performance. For the velocity perturbations in the deeper layers of the canopy, a new analytical model, which retains a balance between mean horizontal advection, mean pressure gradient and mean drag force but neglects the turbulent-shear-stress gradient, is developed. The proposed model reproduces the LDA measurements better than the earlier analytical model, which neglected advection but retained the turbulent-shear-stress gradient in the lower layers of the canopy and near the hill top. This finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small. In essence, the newly-proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean-momentum equation. We briefly discuss how to integrate all these findings with recent advances in canopy lidar remote-sensing measurements of general topography and canopy height. Copyright © 2008 Royal Meteorological Society [source] Turbulent flows on forested hilly terrain: the recirculation regionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 625 2007D. Poggi Abstract A number of analytical and numerical studies employing first-order closure principles have suggested that canopy flows on gentle sinusoidal hills feature a recirculation region, situated on the lee side, that can dramatically affect scalar transfer between the biosphere and the atmosphere. To date, the onset of this region, and its effects on bulk flow properties, have not been experimentally investigated. We study the applicability of first-order closure schemes jointly with the properties of this recirculation region, using detailed laser Doppler anemometry (LDA) measurements. These experiments are conducted in a neutrally stratified boundary-layer flow within a large flume over a train of gentle and narrow hills. The canopy is composed of an array of vertical cylinders with a frontal-area index concentrated in the upper third, to resemble a tall hardwood forest at maximum leaf area. The LDA measurements are recorded for both sparse and dense canopies. We find that, while the onset of a recirculation region is ambiguous in the sparse-canopy case, it is well delineated in the dense-canopy case. This finding constitutes the first experimental evidence confirming the analytical and numerical model predictions concerning this region in dense canopies on gentle hills. Moreover, we show that the presence of the recirculation region can explain the anomalous pressure variation across the hill (first reported in numerical simulations) using an ,effective hill shape' function. Detailed momentum-flux measurements show, surprisingly, that the effective mixing length leff within the canopy and in the inner layer is not significantly affected by the recirculation region. We expected leff to be comparable to the size of the vortex responsible for the recirculation zone, but the measurements show that leff maintains its canonical canopy turbulence shape. Using laser-induced fluorescence (LIF) measurements, we find that the recirculation region is not characterized by a classical ,rotor', but by a highly intermittent zone with alternating positive and negative velocity values in the lower layers of the canopy. These LIF measurements may explain why leff maintains its canonical canopy turbulence shape in the recirculation region. The LIF measurements also show that the main mechanism for scalar transfer within the recirculation region is a sequence of accumulation,ejection episodes that are quasi-periodic in nature. Copyright © 2007 Royal Meteorological Society [source] Ecophysiological controls over the net ecosystem exchange of mountain spruce stand.GLOBAL CHANGE BIOLOGY, Issue 1 2007Comparison of the response in direct vs. diffuse solar radiation Abstract Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m2 m,2) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO2 to PPFD. NEE and stomatal conductance at the canopy level (Gcanopy) was estimated from half-hourly eddy-covariance measurements of CO2 and H2O fluxes. In addition, daily courses of CO2 assimilation rate (AN) and stomatal conductance (Gs) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer. We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves. Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels. [source] Increasing representation of localized dung beetles across a chronosequence of regenerating vegetation and natural dune forest in South AfricaGLOBAL ECOLOGY, Issue 3 2002Adrian L. V. Davis Abstract Aim Species assemblages with high proportions of localized taxa occur in regional islands with a history of strong eco-climatic separation from adjacent systems. Current disturbance in such islands of relictualism or endemism disrupts the distinctive local character in favour of regionally distributed taxa with a wider range of tolerances. However, rehabilitation of the system should restore the localized biota. Thus, we used biogeographical composition to assess progress towards restoration of the dung beetle fauna associated with such an island of endemism following dredge-mining. Location The study was conducted in natural coastal dune forest and a 23-year chronosequence of regenerating dune vegetation in the Maputaland centre of endemism, KwaZulu-Natal, South Africa. Methods Dung beetles were trapped in eight stands of regenerating vegetation of different ages (< 1 year to ~21 years) and in four stands of natural dune forest with differing ecological characteristics defined by measurements of vegetative physiognomy and microclimate. Species groups defined from multivariate analysis of biogeographical distribution patterns and vegetation associations were used to demonstrate quantitative compositional changes in the dung beetle assemblages across the chronosequence to natural forest. Results Three biogeographical groups were defined. One group comprised species widespread in southern Africa or both southern and east Africa. The other two groups were endemic, one to the east coast and the other to Maputaland. There was a general trend from dominance by regionally distributed dung beetle taxa to dominance by locally distributed taxa across the chronosequence of regenerating vegetation from grassland, to open Acacia karroo thicket, to dense A. karroo- dominated woodland. However, this trend was linked closely to the relative physiognomic and microclimatic similarity between the regenerating vegetation and natural forest. Thus, proportions of locally distributed taxa were lower in older chronosequence woodland (~18,~21 years) with its low canopy cover and open understorey than in dense early chronosequence woodland (~9,~12 years), which is physiognomically and microclimatically closer to species-diverse natural forest with its dense canopy and understorey. Overall, the present dung beetle community comprises five species groups. Single widespread (21 spp.) and endemic groups (14 spp.) showed similar patterns of association with early chronosequence grassland and open thicket stands. A single widespread (3 spp.) and two endemic shade-associated groups (3 and 11 spp.) showed differing patterns of association centred, respectively, in late chronosequence woodland, natural forest, or all shaded stands. Main conclusions At 23 years, vegetative regeneration is still at an early stage, but abundant activity of most, although not all species recorded in natural forest, is recovered with the closure of the woodland canopy at ~9 years. Compositional differences with respect to natural forest vary closely with vegetative physiognomy and its effect on the microclimate. Therefore, full compositional recovery is dependent on the re-establishment of natural forest physiognomy and microclimate. [source] Analytical models for the mean flow inside dense canopies on gentle hilly terrainTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 634 2008D. Poggi Abstract Simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments. Laser Doppler anemometry (LDA) measurements are conducted in a neutrally-stratified boundary-layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy. The hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy (H/L , 1 and Lc/L , 1, where H is the hill height, L the half-length, and Lc the canopy adjustment length-scale). The LDA data suggest that the often-criticized linearizations of the advective terms, turbulent-shear-stress gradients and drag force appear reasonable except in the deep layers of the canopy. As predicted by a previous analytical model, the LDA data reveal a recirculation region within the lower canopy on the lee slope. Adjusting the outer-layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model's performance. For the velocity perturbations in the deeper layers of the canopy, a new analytical model, which retains a balance between mean horizontal advection, mean pressure gradient and mean drag force but neglects the turbulent-shear-stress gradient, is developed. The proposed model reproduces the LDA measurements better than the earlier analytical model, which neglected advection but retained the turbulent-shear-stress gradient in the lower layers of the canopy and near the hill top. This finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small. In essence, the newly-proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean-momentum equation. We briefly discuss how to integrate all these findings with recent advances in canopy lidar remote-sensing measurements of general topography and canopy height. Copyright © 2008 Royal Meteorological Society [source] Effects of forest management on epiphytic lichen diversity in Mediterranean forestsAPPLIED VEGETATION SCIENCE, Issue 2 2010Gregorio Aragón Abstract Question: What are the responses of epiphytic lichens to the intensity of management along a large environmental gradient in Mediterranean Quercus forests? Location: Central Spain. Methods: This study was carried out on 4590 trees located in 306 forest stands dominated by Quercus faginea or Quercus ilex ssp. ballota. The effect of forest management and other predictor variables on several species diversity indicators were studied. Variables modelled were total species richness, cyanolichen richness and community composition. A large number of predictor variables were included: forest fragmentation (patch size, stand variability), climate and topographic (altitude, slope, sun radiation, annual rainfall and mean annual temperature) and intensity of management. General linear models and constrained ordination techniques were used to model community traits and species composition, respectively. Results: Total richness and especially cyanolichens richness were significantly and negatively affected by the intensity of management. Lichen composition was influenced by management intensity, climatic and topographic variables and stand variability. Conclusions: In Mediterranean forests, human activities related to forestry, agricultural and livestock use cause impoverishment of lichen communities, including the local disappearance of the most demanding species. The conservation of unmanaged forests with a dense canopy is crucial for lichen diversity. [source] | |||||||||||||