Abstract: The decline in aboveground wood production after canopy closure in even-aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations of Eucalyptus saligna near Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shifft in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven be reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well. We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6 1/2 years (when trees were ~25m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production. The forest closed its canopy in 1-2 years, with peak aboveground wood production, coinciding with canopy closure, of 1.2-1.8 kg C.m-2yr-1. Aboveground wood production declined from 1.4 kg C.m-2yr-1 at age 2 to 0.60 kg C.m-2yr-1 at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C.m-2yr-1 at age 2 to 3.2 kg C.m-2yr-1 at age 6. Aboveground woody respiration declined from 0.66 kg C.m-2yr-1 at age 2 to 0.22 kg C.m-2yr-1 at age 6 and TBCA declined from 1.9 kg C.m-2yr-1 at age 2 to 1.4 kg C.m-2yr-1 at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photsynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutritient availablity declined more slowly than did our control treatment, which was fertilized only during stand establishment.

Abstract: Fine roots are a key component of carbon (C) flow and nitrogen (N) cycling in forest ecosystems. However, the complexity and heterogeneity of the fine root branching system have hampered the assessment and prediction of C and N dynamics at ecosystem scales. We examined how root morphology, biomass, and chemistry differed with root branch orders (1-5 with root tips classified as first order roots) and how different root orders responded to increased C sink strength (via N fertilization) and reduced carbon source strength (via canopy scorching) in a longleaf pine (Pinus palustris L.) ecosystem. With increasing root order, the diameter and length of individual roots increased, whereas the specific root length decreased. Total root biomass on an areal basis was similar among the first four orders but increased for the fifth order roots. Consequently, total root length and total root surface area decreased systematically with increasing root order. Fine root N and lignin concentrations decreased, while total non-structural carbohydrate (TNC) and cellulose concentrations increased with increasing root order. N addition and canopy disturbance did not alter root morphology, but they did influence root chemistry. N fertilization increased fine root N concentration and content per unit area in all five orders, while canopy scorching decreased root N concentration. Moreover, TNC concentration and content in fifth order roots were also reduced by canopy scorching. Our results indicate that the small, fragile, and more easily overlooked first and second order roots may be disproportionately important in ecosystem scale C and N fluxes due to their large proportions of fine root biomass, high N concentrations, relatively short lifespans, and potentially high decomposition rates.

Abstract: Amazonia contains vast stores of carbon in high-diversity ecosystems, yet this region undergoes major changes in precipitation affecting land use, carbon dynamics, and climate. The extent and structural complexity of Amazon forests impedes ground studies of ecosystem functions such as net primary production (NPP), water cycling, and carbon sequestration. Traditional modeling and remote-sensing approaches are not well suited to tropical forest studies, because (i) biophysical mechanisms determining drought effects on canopy water and carbon dynamics are poorly known, and (ii) remote-sensing metrics of canopy greenness may be insensitive to small changes in leaf area accompanying drought. New spaceborne imaging spectroscopy may detect drought stress in tropical forests, helping to monitor forest physiology and constrain carbon models. We combined a forest drought experiment in Amazonia with spaceborne imaging spectrometer measurements of this area. With field data on rainfall, soil water, and leaf and canopy responses, we tested whether spaceborne hyperspectral observations quantify differences in canopy water and NPP resulting from drought stress. We found that hyperspectral metrics of canopy water content and light-use efficiency are highly sensitive to drought. Using these observations, forest NPP was estimated with greater sensitivity to drought conditions than with traditional combinations of modeling, remote-sensing, and field measurements. Spaceborne imaging spectroscopy will increase the accuracy of ecological studies in humid tropical forests.

Abstract: The Ituri Forest, Democratic Republic of Congo (formerly Zaire) is an example of a closed canopy forest showing extreme depletion in 13C. δ13C values for plants from the canopy top, from gaps in the canopy, and from the subcanopy average −29.0±1.7‰, −30.4±0.9‰, and −34.0±1.5‰, respectively. The δ13C of forest mammals show these differences, with the subcanopy browsers (okapi, dwarf antelope) having δ13C values for tooth enamel much more negative than subcanopy frugivores who derive their food from the canopy top, and from folivores and omnivores living in gap or clearing areas. Nitrogen isotopes in plants from this ecosystem have an average δ15N value of 5.4±1.8‰ and do not show significant differences at the 95% confidence interval between plants from the canopy top, from gaps in the canopy, and from the subcanopy. The δ18OSMOW values of surface waters in the study area are between −2.0 and −2.7. The δ18OPDB for tooth enamel ranged from −3 to +7‰.

Abstract: Climate change is likely to produce more frequent and longer droughts in the Mediterranean region, like that of 1994, which produced important changes in the Quercus ilex forests, with up to 76% of the trees showing complete canopy dieback. At the landscape level, a mosaic of responses to the drought was observed, linked to the distribution of lithological substrates. Damage to the dominant tree species (Q. ilex) and the most common understorey shrub (Erica arborea) was more noticeable on the compact substrates (breccia) than on the fissured ones (schist). This result was consistent with observations documenting deeper root penetration in schist than in breccia materials, allowing the plants growing on fissured substrates to use water from deeper soil levels. Smaller plants were more vulnerable to drought than larger plants in the trees, but not in the shrubs. Overall, Q. ilex was more affected than E. arborea. The resilience of the system was evaluated from the canopy recovery 1 year after the episode. Stump and crown resprouting was fairly extensive, but the damage pattern in relation to substrate, plant size, and species remained similar. The effect of recurrent drought episodes was studied on vegetation patches of Q. ilex located on mountain slopes and surrounded by bare rock. We observed that plants that resprouted weakly after a previous drought in 1985 were more likely to die or to produce poor regeneration in 1995 than plants that had resprouted vigorously. Vegetation patches located on the lower part of the slope were also less damaged than patches situated uphill. The study provides evidence of relevant changes in forest canopy as a consequence of extreme climate events. The distribution of this effect across the landscape is mediated by lithological substrate, causing patchy patterns. The results also support the hypothesis that recurrent droughts can produce a progressive loss of resilience, by depleting the ability of surviving plants to regenerate.

Abstract: [1] This paper addresses the effects of canopy physical processes on snow mass and energy balances in boreal ecosystems. We incorporate new parameterizations of radiation transfer through the vegetation canopy, interception of snow by the vegetation canopy, and under-canopy sensible heat transfer processes into the Versatile Integrator of Surface and Atmosphere (VISA) and test the model results against the Boreal Ecosystem-Atmosphere Study (BOREAS) data observed at South Study Area, Old Jack Pine. A modified two-stream radiation transfer scheme that accounts for the three-dimensional geometry of vegetation accurately simulates the transferring of solar radiation through the vegetation canopy when the leaf and stem area index is reduced to match the observed. VISA produces higher-than-observed surface albedo in wintertime. Implementation of a snow interception model that explicitly describes the loading and unloading of snow and the melting and refreezing of snow on the canopy into VISA reduces the fractional snow cover on the canopy and the surface albedo. VISA overestimates the downward sensible heat fluxes from the canopy to the snow surface, which leads to earlier snow ablation and a shallower snowpack than the observed. Explicitly including a canopy heat storage term in the canopy energy balance equation decreases the spuriously large amplitude of the diurnal canopy temperature variation and reduces the excessive daytime sensible heat flux from the canopy downward to the snow surface. Sensitivity tests reveal that the turbulent sensible heat flux below the vegetation canopy strongly depends on the canopy absorption coefficient of momentum. During spring the daytime temperature difference between the snow surface and the vegetation canopy forms a strongly stable atmospheric condition, which results in a larger absorption coefficient of momentum and a weak turbulent sensible heat flux. The modeled excessive downward sensible heat flux from the vegetation canopy to the snow surface is considerably reduced through the stability correction to the canopy absorption coefficient of momentum.

Abstract: Two canopy properties, leaf area index (LAI) and covered ground (CoverGnd), were estimated using hemispherical photography of three oak (Quercus pyrenaica) and eight pine (Pinus sylvestris) forest plots in Sierra de Guadarrama (central Spain). Pulses from airborne laser scanner (Lidar) that hit the surface on the exact location (within centimeter resolution) of the photographs were analyzed and separated by different radius size (from 0.5 to 20 m). The correlation between Lidar and hemispherical photography estimates of canopy properties was highly significant, but was affected by the type of forest and the radius size. CoverGnd was better estimated using a small radius size (2.5 m, equivalent to one fourth of canopy height), while LAI was better estimated using a larger radius size (7.5–12.5 m, equivalent to the entire canopy height). In general, the smaller the tree, the shorter the radius was that must be used to select Lidar data, and the best Lidar estimator of canopy properties was the percentage of canopy hits. Overall oak canopies showed better results than pine forest. The poorer estimation in pine forest plots was probably due to the larger foliage and branch clumping of pine versus oak canopies. Lidar data could be used to produce high-resolution regional maps of the canopy properties studied.

Abstract: Nondestructive monitoring and diagnosis of plant N status is necessary for precision N management. The present study was conducted to determine if canopy reflectance could be used to evaluate leaf N status in rice (Oryza saliva L.). Ground-based canopy spectral reflectance and N concentration and accumulation in leaves were measured over the entire rice growing season under various treatments of N fertilization, irrigation, and plant population. Analyses were made on the relationships of seasonal canopy spectral reflectance, ratio indices, and normalized difference indices to leaf N concentration and N accumulation in rice under different N treatments. The results showed that at each sampling date, leaf N concentration was negatively related to the reflectance at the green band (560 nm) while positively related to ratio index, with the best correlation at jointing. However, the relationships between leaf N accumulation and reflectance at green band and ratio index were consistent across the whole growth period. The ratio of near infrared (NIR) to green (R 810 /R 560 ) was especially linearly related to total leaf N accumulation, independent of N level and growth stage. Tests of the linear regression model with different field experiment data sets involving different plant densities, N fertilization, and irrigation treatments exhibited good agreement between the predicted and observed values, with an estimation accuracy of 96.69%, root mean square error of 0.7072, and relative error of -0.0052. These results indicate that the ratio index of NIR to green (R 810 /R 560 ) should be useful for nondestructive monitoring of N status in rice plants.

Abstract: We combined a detailed field study of canopy gap fraction with spectral mixture analyses of Landsat 7 ETM1 satellite imagery to assess landscape and regional dynamics of canopy damage following selective logging in an eastern Amazon forest. Our field studies encompassed measurements of ground damage and canopy gap fractions along multitemporal sequences of post-harvest regrowth of 0.5-3.5 yr. Areas used to stage har- vested logs prior to transport, called log decks, had the largest forest gap fractions, but their contribution to the landscape-level gap dynamics was minor. Tree falls were spatially the most extensive form of canopy damage following selective logging, but the canopy gap fractions resulting from them were small. Reduced-impact logging resulted in consistently less damage to the forest canopy than did conventional logging practices. This was true at the level of individual landscape strata such as roads, skids, and tree falls as well as at the area-integrated scale. A spectral mixture model was employed that utilizes bundles of field and image spectral reflectance measurements with Monte Carlo analysis to estimate high spatial resolution (subpixel) cover of forest canopies, exposed nonphotosynthetic vegetation, and soils in the Landsat imagery. The method proved highly useful for quantifying forest canopy cover fraction in log decks, roads, skids, tree fall, and intact forest areas, and it tracked canopy damage up to 3.5 yr post-harvest. Forest canopy cover fractions derived from the satellite observations were highly and inversely correlated with field-based canopy gap fraction. Subsequent regional-scale estimates of forest gap fraction were derived from the combi- nation of field- and satellite-based measurements. A 450-km 2 study of gap fraction showed that approximately one-half of the canopy opening caused by logging is closed within one year of regrowth following timber harvests. This is the first regional-scale study utilizing field measurements, satellite observations, and models to quantify forest canopy damage and recovery following selective logging in the Amazon.

Abstract: The vertical accretion of salt marshes is mainly due to flow reduction and wave damping by vegetation. However, the details of the hydrodynamics are only partially understood, and have been studied mainly in the laboratory. This study presents detailed field investigations of the water flow in a Spartina maritima salt-marsh in the Ria Formosa, a shallow, meso-tidal lagoon in Southern Portugal. Detailed velocity profiles were obtained within and above the 30 cm high canopy using a high-precision velocimeter. Results show that the influence of the bottom becomes negligible a few centimetres above the bed, and that the flow depends on the vegetation density at each level of the canopy. When the canopy is partially emergent or is only slightly submerged, the upward increase of horizontal velocity is roughly linear. A more drastic flow reduction exists when the canopy is well submerged, with a slow, nearly constant velocity in the denser part of the canopy and a faster, logarithmic shaped velocity pro...

Abstract: We develop an analytical model for atmospheric boundary-layer flow over a hill that is covered with a vegetation canopy. The slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of Hunt. Perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill. In the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag. The flow there can be calculated from linearized dynamics, which show that the maximum streamwise winds are where the perturbation pressure is at a minimum, i.e. near the crest of the hill. Deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag, here the nonlinear canopy drag. This nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest, i.e. on the upwind slope of the hill. In the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy, leading to separation with a region of reversed flow when the canopy is sufficiently deep. Coupling between the out-of-phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill, while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed-up over the hill. Finally, we find that there is no formal limit process where the solutions with a canopy yield the well-known solutions for flow over a rough hill. This finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers. Copyright © 2004 Royal Meteorological Society

Abstract: We describe the three-dimensional structure of an old-growth Douglas-fir/western hemlock forest in the central Cascades of southern Washington, USA. We concentrate on the vertical distribution of foliage, crowns, external surface area, wood biomass, and several components of canopy volume. In addition, we estimate the spatial variation of some aspects of structure, including the topography of the outer surface, and of microclimate, including the within-canopy transmittance of photosynthetically active radiation (PAR). The crowns of large stems, especially of Douglas-fir, dominate the structure and many aspects of spatial variation. The mean vertical profile of canopy surfaces, estimated by five methods, generally showed a single maximum in the lower to middle third of the canopy, although the height of that maximum varied by method. The stand leaf area index was around 9 m2 m−2, but also varied according to method (from 6.3 to 12.3). Because of the deep narrow crowns and numerous gaps, the outer canopy surface is extremely complex, with a surface area more than 12 times that of the ground below. The large volume included below the outer canopy surface is very porous, with spaces of several qualitatively distinct environments. Our measurements are consistent with emerging concepts about the structure of old-growth forests, where a high degree of complexity is generated by diverse structural features. These structural characteristics have implications for various ecosystem functions. The height and large volume of the stand indicate a large storage component for microclimatic variables. The high biomass influences the dynamics of those variables, retarding rates of change. The complexity of the canopy outer surface influences radiation balance, particularly in reducing short-wave reflectance. The bottom-heaviness of the foliage profile indicates much radiation absorption and gas exchange activity in the lower canopy. The high porosity contributes to flat gradients of most microclimate variables. Most stand respiration occurs within the canopy and is distributed over a broad vertical range.

Abstract: We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost-benefit theories of leaf life span. Nitrogen content per unit mass (Nmass) and light- and CO2-saturated photosynthetic rates per unit mass (Pmass) of upper canopy leaves decreased with annual precipita- tion, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower Pmass ,N mass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar δ 15 N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high Pmass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.

Abstract: The periodicity, synchrony, and causes of variability in the demography of tree leaves in ecosystems with relatively aseasonal climates, such as tropical rain forests, is still poorly understood. To address this issue, we surveyed the timing of birth and death of .40 000 leaves of 1445 individuals of 23 evergreen rain forest species in several late primary and early secondary successional plant communities at San Carlos de Rio Negro, Venezuela, in the northern Amazon basin. In all species, the mortality rate generally in- creased with leaf age. However, in many species, deceleration of death rates with extreme leaf age was noted. In general, for each species, the age structure of leaf populations and the frequency distribution of leaf life span were broad. Species differed substantially in their leaf demography. Measured in their native habitats, seven species common to disturbed open sites had shorter median life spans (0.7 yr) than five species common to open but infertile Bana primary communities (1.9 yr average) or six species common to two tall primary forest communities (Tierra Firme and Caatinga), when measured in high-light conditions in the canopy (2.0 yr average). Variation in light availability had consistent effects on leaf life span in all species. Species native to Tierra Firme forest had average leaf life spans of 3.2, 1.9, and 1.6 yr, respectively, in deeply shaded understory microsites, in small gaps, and in sunlit mature tree canopies. Species native to Caatinga forest had average leaf life spans of 4.2, 3.4, and 2.5 yr, respectively, in these same microsite types. Two species common in gaps and in disturbed sites had much longer leaf life span in shaded understory than in open, disturbed microsites. For all species, responses were similar when trees were planted in sites differing in light availability, as when trees naturally established across light gradients. The rate of leaf production, the risk of leaf mortality, and the leaf life span were not periodic or related consistently to seasonality of climate. Negligible relationships existed between the mild annual dry season and either leaf production or leaf mortality in all species. Thus, leaf phenology and demography were essentially aseasonal in this tropical forest environment.

Abstract: Growth and photosynthesis of grapevine ( Vitis vinifera L.) planted on two sloping cool climate vineyards were measured during the early growth season. At both vineyards, a small difference in mean minimum air temperature (1‐ 3 ∞ C) between two microsites accumulated over time, producing differences in shoot growth rate. The growth rates of the warmer (upper) microsite were 34‐63% higher than the cooler (lower) site. Photosynthesis measurements of both east and west canopy sides revealed that the difference in carbon gain between the warmer and cooler microsites was due to low temperatures restricting the photosynthetic contribution of east-facing leaves. East-facing leaves at the warmer microsite experienced less time at suboptimal temperature while being exposed to high irradiance, contributing to an average 10% greater net carbon gain compared to the east-facing leaves at the cooler microsite. This chilling-induced reduction in photosynthesis was not due to net photo-inhibition. Further analysis revealed that CO 2 - and light-saturated photosynthesis of grapevines was restricted by stomatal closure from 15 to 25 ∞ C and by a limitation of RuBP regeneration and/or end-product limitation from 5 to 15 ∞ C. Changes in photosynthetic carboxylation efficiency implied that Rubisco activity may also play a regulatory role at all temperatures. This restriction of total photosynthetic carbon gain is proposed to be a major contributor to the temperature dependence of growth rate at both vineyards during the early season growth period.

Abstract: Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers).