Canopy Changes Research Articles

Detection of initial damage in Norway spruce canopies using hyperspectral airborne data

Current broadband sensors are not capable of separating the initial stages of forest damage. The current investigation evaluates the potential of hyperspectral data for detecting the initial stages of forest damage at the canopy level in the Norway spruce (Picea abies (L.) Karst) forests of Czech Republic. Hyperspectral canopy reflectance imagery and foliar samples were acquired contemporaneously for 23 study sites in August 1998. The sites were selected along an air pollution gradient to represent the full range of damage conditions in even-aged spruce forests. The changes in canopy and foliar reflectance, chemistry and pigments associated with forest damage were established. The potential of a large number of spectral indices to identify initial forest damage was determined. Canopy hyperspectral data were able to separate healthy from initially damaged canopies, and therefore provided an improved capability for assessment of forest physiology as compared to broadband systems. The 673-724 nm region exhibited maximum sensitivity to initial damage. The nine spectral indices having the highest potential as indicators of the initial damage included: three simple band ratios, two derivative indices, two modelled red-edge parameters and two normalized bands. The sensitivity of these indices to damage was explained primarily by their relationship to foliar structural chemical compounds, which differed significantly by damage class.

Canopy recovery after drought dieback in holm‐oak Mediterranean forests of Catalonia (NE Spain)

AbstractClimate 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.

Reflectance assessment of seasonal and annual changes in biomass and CO 2 uptake of a Mediterranean shrubland submitted to experimental warming and drought

We aimed to evaluate how the remote sensing vegetation indices NDVI and PRI responded to seasonal and annual changes in an early successional stage Mediterranean coastal shrubland canopy that was submitted to experimental warming and drought simulating predicted climate change for the next decades. These conditions were obtained by using a new non-intrusive methodological approach that increases the temperature and prolongs the drought period by using roofs that automatically cover the vegetation after the sunset or when it rains. On average, warming increased air temperature by 0.7 °C and soil temperature by 1.6 °C, and the drought treatment reduced soil moisture by 22%. We measured spectral reflectance at the canopy level and at the individual plant level seasonally during 4 years. Shrubland NDVI tracked the community development and activity. In control and warming treatments, NDVI increased with the years while it did not change in the drought treatment. There was a good relationship between NDVI and both community and individual plant biomass. NDVI also decreased in summer seasons when some species dry or decolour. The NDVI of E. multiflora plant individuals was lower in autumn and winter than in the other seasons, likely because of flowering. Shrubland PRI decreased only in winter, similarly to the PRI of the most dominant species, G. alypum. At this community scale, NDVI was better related than PRI to photosynthetic activity, probably because photosynthetic fluxes followed canopy seasonal greening in this complex canopy, which includes brevideciduous, annual and evergreen species and variable morphologies and canopy coverage. PRI followed the seasonal variations in photosynthetic rates in E. multiflora and detected the decreased photosynthetic rates of drought treatment . However, PRI did not track the photosynthetic rates of G. alypum plants which have lower LAIs than E. multiflora. In this community, which is in its early successional stages, NDVI was able to track biomass, and indirectly, CO 2 uptake changes, likely because LAI values did not saturate NDVI. Thus, NDVI appears as a valid tool for remote tracking of this community development. PRI was less adequate for photosynthetic assessment of this community especially for its lower LAI canopies. PRI usefulness was also species-dependent and could also be affected by flowering. These results will help to improve the interpretation of remote sensing information on the structure and physiological status of these Mediterranean shrublands, and to gain better insight on ecological and environmental controls on their ecosystem carbon dioxide exchange. They also show the possibility of assessing the impacts of climate change on shrubland communities.

Forest stand structure and dynamics at Riding Mountain National Park, Manitoba, Canada

We used multivariate analysis to model boreal forest stand structure and dynamics at Riding Mountain National Park, Manitoba based on data from 202 sampled stands. Eight forest stand-types were recognized based on canopy composition: black spruce on peat substrates, jack pine — black spruce, bur oak, eastern deciduous (green ash — American elm — Manitoba maple), balsam fir, trembling aspen — paper birch — mountain maple, trembling aspen — balsam poplar, and white spruce. The first four stand-types occur in edaphically distinct environments, while the four remaining boreal mixedwood stand-types occur in edaphically similar environments. We found that the composition and abundance of advance regeneration were best predicted by current canopy composition (redundancy = 54.4%); this reflects both the limited dispersal of conifer seeds and the strong vegetative reproductive capacity of hardwoods. Biotically-controlled site factors such as bareground, herb and shrub cover, ungulate browsing intensity, and stand age were also reasonably good predictors of advance regeneration (redundancy = 31.7%). Edaphic variables such as soil pH, conductivity, particle size, organic horizon depth and slope proved to be poor predictors of advance regeneration, however (redundancy = 18.1%). Size-class ordination indicated that many stand-types have relatively short successional trajectories, suggesting limited change in forest canopy composition over time. There are two exceptions: in the jack pine — black spruce stand-type, black spruce will increase over time, and in the trembling aspen — paper birch — mountain maple stand-type, eastern deciduous species (green ash, American elm, Manitoba maple, and bur oak) are forecast to become increasingly dominant. We also describe a synoptic model of mixedwood boreal forest stand dynamics for the Riding Mountain area. The model includes a number of factors that we consider to be critical determinants of forest dynamics, such as seed source availability, small and large-scale disturbances, species life-history characteristics, and environmental gradients. Our succession model is more similar those described for eastern than western Canada, which may reflect the lower frequency of catastrophic fires in the Riding Mountain area compared to boreal forests further west. Our model emphasizes that successional trajectories do not converge towards a single self-perpetuating “climax”. Instead, successional vectors may diverge, converge or remain cyclical, and multiple potential pathways are possible for each stand-type. Our results also illustrate that species assemblages, and the propensity for canopy change in the absence of fire, are governed by the cumulative and synergistic effects of climate, topography, disturbance frequency, size and intensity, edaphic conditions, and the proximity of parental seed sources. Fire suppression in the southern boreal forest has resulted in a paradigm shift in disturbance regime, from large, synchronous catastrophic fires to small-scale, asynchronous gap formation. A major challenge for boreal forest ecologists is to determine the long-term consequences of this paradigm shift on the composition, structure and health of boreal forest stands and landscapes.

Modeling and Measurement of Structural Changes at a Landscape Scale in Dryland Areas

We present a technique to quantify and model the intensity of structural changes produced by management of dry grazing lands at a landscape scale. The technique is illustrated with the analysis of digitized black–white (b/w) imagery and an application to the study of changes induced by grazing gradients. Structural changes in patchy vegetation canopies were studied in the Patagonian Monte (Chubut, Argentina) at two resolution scales by means of linear transects in the field (50 m) and others drawn on aerial b/w photographs (2–5 km) of grazed paddocks. Spatial series of plant cover values along transects in the field and on photographs were analyzed with standard techniques of spectral analyses, including auto-correlation spectra and Fourier transforms. In order to test the internal consistency of the techniques used, synthetic plant canopies with patches of varying cover and size were generated by means of a stochastic model of plant growth under different stocking rates or after varying periods of recovery. The behavior of the simulation model is consistent with the observed dynamics of plant canopies in semiarid environments. There is a consistent relation between the number and geometric properties of plant patches (patch number, patch size, patch connectivity) and the signal/noise ratios of the Fourier decomposition describing plant density data. Signal/noise ratios corresponding to plant cover data in paddocks with different grazing treatment are consistent with the assumptions derived from modeled canopies, as well as those estimated from optical density of b/w aerial photographs of paddocks. We tested the hypotheses that patch arrangements as quantified by the signal/noise ratios vary in accordance with grazing gradients in paddocks with a permanent corner-located watering point. The use of digitized b/w images allows inspecting permanent changes over time periods when other types of images were not yet available.

Elephant-induced change in woody vegetation and its impact on elephant movements out of a protected area in Zimbabwe

This study, tested the hypothesis that vegetation structure and relative abundance of certain plants in remnant forest fragments differ from forests found within protected areas, and that this situation may be a causal factor in elephant dispersal. It monitored the vegetation structure and relative abundance of species in remnant forest fragments in communal land areas (CLs) in the vicinity of the Sengwa Wildlife Research Area (SWRA) and compared results with those from SWRA. Changes in woodlands with elephants are well documented, and although considered generalist feeders elephants may be selective and therefore eliminate preferred woody species locally (Wing and Buss 1970; Anderson 1973; Thomson 1975; Afolayan 1975; Jachmann and Bell 1979; Barnes 1983; Guy 1989). The present study assessed canopy changes from 1966-1993 aerial photography, and species known to be preferred by elephants were monitored for 3 years in transects in the CLs. Species differences can in part be attributed to high elephant densities, fluctuations in numbers in the late 1970s when culling took place, and fire. At present elephant are feeding primarily in the riverine valleys in the CLs, this provides for the least conflict with humans. In the past elephant would have dispersed as food resources became depleted, this may be what is occurring as they move into remnant forest mosaics outside protected areas. Management must consider effects of unchecked elephant population growth with reference to the removal of preferred tree species and the motivation of the animals to come in conflict with crops lands as they search for favoured indigenous species.

Change detection and landscape structure mapping using remote sensing

Satellite remote sensing has long held promise as a powerful method of detecting forest canopy changes and mapping landscape structure over vast, often multi-jurisdictional forest areas. Landsat Thematic Mapper (TM) spectral response, for example, can be related accurately to changes in physiology and cover at a range of small to intermediate mapping scales. These data have been available continuously for almost 20 years; many areas have earlier satellite image archives stretching back to the 1970s. When considering spatially-explicit changes to landscapes—caused by natural and human disturbances—over this time period, digital, synoptic, and repeatable satellite remotely-sensed data are emerging as the observational media of choice that forest managers must possess and use wisely. In this paper, successful use of satellite remote sensing in two of Canada’s Model Forests is described. First, in the Fundy Model Forest in southeastern New Brunswick, a 15-year TM image sequence was used to detect area changes associated with different harvesting and silvicultural practices. Second, in the Foothills Model Forest in west-central Alberta, grizzly bear habitat maps have been created from multi-scene TM land cover mosaics. These map products constitute critical information on landscape change and configuration required to answer key management questions. The paper concludes with a prognosis for the future role of satellite remote sensing in sustainable forest management as data quality continues to improve (i.e., increasing spatial, spectral, temporal, and radiometric resolutions), and methods are brought into the purview of forest managers and practitioners. Key words: remote sensing, landscape change, forest fragmentation, human disturbance, management information needs, habitat

Dry Season Habitat Use by Lizards in a Tropical Deciduous Forest of Western Mexico

Tropical deciduous or dry forests are characterized by strong seasonality in precipitation, typically having a prolonged dry season lasting at least four months (Bullock, 1986; Murphy and Lugo, 1986). As the dry season progresses, the forest canopy changes from a relatively homogeneous dense cover of the wet season to a more variable and open canopy (Lott et al., 1987; Bullock and Solis-Magallanes, 1990). As deciduous trees lose their leaves, more sunlight penetrates the canopy and creates a variety of forest gaps and thermal microenvironments on or near the ground. In contrast, forest habitat near seasonally dry watercourses (arroyos), remains green throughout the dry season, providing shaded habitats with dense canopy cover (Lott et al., 1987; Bullock and Solis-Magallanes, 1990). The dry season in a tropical deciduous forest represents a challenging time of year for most ground dwelling vertebrates. Water and energy resources are at the yearly low (Janzen and Schoener, 1968; Lister and Garcia, 1992), and thermal options may also be reduced, especially in the more shaded habitats of arroyos. Many vertebrates show reduced activity or seasonal changes in habitat use during the dry season (Beck and Lowe, 1991; Ceballos, 1995). Other species may move away from upland habitats and into arroyos or watercourses, which are used as dry season refuges (Ceballos, 1990). Those that remain active tend to be juveniles or adults of smaller species (Fleming and Hooker, 1975; Lister and Garcia, 1992; Casas-Andreu and Gurrola-Hidalgo, 1993; Fitzgerald et al., 1999). We know little about how dry forest lizards respond to seasonal changes in habitat structure or of the patterns of activity exhibited by lizards that remain active during the dry season. Our study examines how lizard habitat use and activity (i.e., species found to be active, timing, and amount of activity) are influenced by the variability in forest structure during the dry season in a tropical deciduous forest in southwest Mexico. We address the following questions: (1) During the dry season, does lizard activity differ between arroyo and upland habitats? (2) Is lizard activity related to forest canopy cover during the dry season?

Changes in stand composition and structure between 1981 and 1996 in four Front Range plant communities in Colorado

We conducted a study analyzing the vegetation dynamics in four communities along an elevational gradient in the Front Range of the Colorado Rocky Mountains and compared our results with previous research conducted in 1981. The objective of this study was to determine whether the successional plant community trajectories hypothesized by the original investigator (Marr) in 1953 were consistent with plant community parameters measured in subsequent years. The ponderosa pine and Douglas-fir/ponderosa pine communities’ herbaceous composition remained relatively constant between 1981 and 1996; however, a few individual species in both communities experienced significant changes in frequency over time. These individual species changes, along with dominant shifts in the forest canopy, were inconsistent with Marr’s original successional hypotheses for these stands that stated the ponderosa pine stand would remain dominated by ponderosa pine and grassy openings and the Douglas-fir/ponderosa pine stand would eventually have equal dominance between the two tree species. The aspen herbaceous community experienced the most significant change between 1981 and 1996. Significant decreases in species richness and diversity were recorded along with large changes in species composition and frequencies of individual species. These changes were associated with successional changes in the forest canopy, which was congruent with Marr’s successional hypothesis for this stand. The kobresia meadow herbaceous community showed the least change between 1981 and 1996 among the four communities; however, there was variation in frequency percentages for a few individual species over time and a small turnover in species composition. Marr’s hypothesis that this stand represented a climax community is consistent with our results.

Hillslope topographic and hydrologic effects on overland flow and erosion

Variability in surface condition occurs at different spatial and temporal scales. Locally, surface conditions interact with soil components and determine the dynamic of surface sealing, biomass, and roughness. At the hillslope scale, conditions at the surface vary with the topographic position that can cause different hydrologic regimes and erosion. This paper presents our efforts in quantifying how hillslope position and moisture condition affected runoff generation and sediment production. In the field, we monitored runoff and sediment productions from a hillslope under natural conditions. Sediment production under seepage condition was quantified in a laboratory dual-box system under simulated rainfall. Field results showed that variability in runoff and sediment production could be attributed to topographically induced and localized surface variations in addition to seasonal changes in rainfall pattern and crop canopy. Based on rainfall pattern and changes in surface conditions, three different stages of runoff and sediment productions were identified in an annual cycle. Laboratory experiments in soil moisture gradient (i.e., seepage vs. drainage) effects on erosion showed that drainage conditions limited sediment detachment while seepage conditions enhanced rilling. These results showed a close linkage between surface moisture condition and erosion process, and consequently, the dominant sediment regime. These different methodologies improved the understanding of runoff and erosion processes occurring at the hillslope and the role of topographic position and hydrology on these processes.

Fungal and bacterial enzyme activities in Alnus nepalensis D. Don

Fungal and bacterial numbers and their enzymes activities in terms of enzymes, namely cellulase, amylase and invertase were estimated in two forest stands of alder (open and closed forest). The fungal and bacterial population numbers were higher in the closed forest than in the open ones. Consequently, the different enzyme activities were also greater in the closed forest. A correlation coefficient was calculated between fungal population numbers, bacterial population numbers, moisture content, pH, total nitrogen, weight loss, cellulose and total sugars and the enzymes activities. Invertase activity showed a positive correlation ( P < 0.05) with litter soluble sugars and total nitrogen but negatively with weight loss. Amylase and cellulase activities were correlated significantly with fungi and bacteria and moisture content of litter. Cellulase also correlated significantly but negatively with the cellulose ( P < 0.01). The results of the investigation indicated that changes in forest canopy has an effect on fungal and bacterial population numbers and microbial enzymes activities.

No evidence of saturation of the xanthophyll cycle pool size vs. integrated quantum flux density relationship in tree canopies

The content of xanthophyll cycle carotenoids per unit of total chlorophyll (VAZ/Chl) or per unit of total carotenoids (VAZ/C) generally scales positively with long term quantum flux density (Qint) in plant canopies. However, in natural conditions, the light constantly fluctuates, but the limits and the speed of acclimation responses have not been studied extensively. To determine the extent of foliar adjustment of xanthophyll cycle carotenoid pool size and the kinetic constants of acclimation, we provided extra light (500 µmol m-2 s-1 for 12 h photoperiod) to 24 individual leaves of Tilia cordata Mill. and Populus tremula L. along the light gradient in mature tree canopies (4-25 m). We expected the VAZ/Chl and VAZ/C vs. Qint relationships to saturate in time as the leaves reach a new equilibrium with the changed incident irradiance conditions. The leaves were monitored for 11 days, during which both the epoxidation state of the leaves as well as VAZ/Chl steadily increased. In 11 days, the epoxidation state of the uppermost canopy leaves reached from 50% to 99%, but that of the lower canopy leaves to 75%, indicating a limited potential for acclimation. There were also up to three-fold increases in VAZ/Chl in the upper canopy, but non-significant changes in the lower canopy. When all leaves were examined simultaneously, the kinetic constants of acclimation were directly proportional to the previous leaf light environment. However, there was no evidence of saturation of VAZ/Chl vs. Qint relationships in time, suggesting that the xanthophyll cycle pool did not reach the poise with the changed leaf light environment during 11 days. We conclude that foliar xanthophyll cycle pool size is generally not in steady-state under natural conditions, and that the biochemical acclimation capacity is directly proportional to long-term leaf light environment.

A novel light stress-enhanced one-helix chlorophyll a/b-binding protein from Arabidopsis thaliana

The content of xanthophyll cycle carotenoids per unit of total chlorophyll (VAZ/Chl) or per unit of total carotenoids (VAZ/C) generally scales positively with long term quantum flux density (Qint) in plant canopies. However, in natural conditions, the light constantly fluctuates, but the limits and the speed of acclimation responses have not been studied extensively. To determine the extent of foliar adjustment of xanthophyll cycle carotenoid pool size and the kinetic constants of acclimation, we provided extra light (500 µmol m-2 s-1 for 12 h photoperiod) to 24 individual leaves of Tilia cordata Mill. and Populus tremula L. along the light gradient in mature tree canopies (4-25 m). We expected the VAZ/Chl and VAZ/C vs. Qint relationships to saturate in time as the leaves reach a new equilibrium with the changed incident irradiance conditions. The leaves were monitored for 11 days, during which both the epoxidation state of the leaves as well as VAZ/Chl steadily increased. In 11 days, the epoxidation state of the uppermost canopy leaves reached from 50% to 99%, but that of the lower canopy leaves to 75%, indicating a limited potential for acclimation. There were also up to three-fold increases in VAZ/Chl in the upper canopy, but non-significant changes in the lower canopy. When all leaves were examined simultaneously, the kinetic constants of acclimation were directly proportional to the previous leaf light environment. However, there was no evidence of saturation of VAZ/Chl vs. Qint relationships in time, suggesting that the xanthophyll cycle pool did not reach the poise with the changed leaf light environment during 11 days. We conclude that foliar xanthophyll cycle pool size is generally not in steady-state under natural conditions, and that the biochemical acclimation capacity is directly proportional to long-term leaf light environment.

Seasonal Variations in Radiation and Energy Balance in a Deciduous Broad-Leaved Forest

This study was conducted to document the effects of canopy change on radiation and energy balances in a deciduous broad-leaved forest. Global solar radiation was highly variable with a maximum close to 28.7 MJ m−2 day−1 and a minimum of about 0.7 MJ m−2 day−1. The high daily variation of net radiation was the result of the frequency and duration of rainfall. The mean annual albedo in this study site was 0.126. Seasonal variation of albedo was characterized by steadily decreasing values with leaf-fall, but at the time when new leaves sprouted the trend did turn upwards. The mean annual radiation efficiency at this study site was 0.564, and the seasonal variation of the radiation efficiency was dependent upon the net long-wave radiation rather than albedo. This study also illustrated the seasonal variation of the proportion of the actual evaporation to the equilibrium evaporation as a means for estimating the latent heat flux from forest using the equilibrium evaporation model. Seasonal variation of the proportion of the actual evaporation to the equilibrium evaporation reflected the physiological and productive activities of trees. Latent heat flux dominated the energy balance and, in particular, reached about 96% of net radiation in July.

Photosynthetic acclimation of the liana Stigmaphyllon lindenianum to light changes in a tropical dry forest canopy.

Tropical plant canopies show abrupt changes in light conditions across small differences in spatial and temporal scales. Given the canopy light heterogeneity, plants in this stratum should express a high degree of plasticity, both in space (allocation to plant modules as a function of opportunity for resource access) and time (photosynthetic adjustment to temporal changes in the local environment). Using a construction crane for canopy access, we studied light acclimation of the liana Stigmaphyllon lindenianum to sun and shade environments in a tropical dry forest in Panama during the wet season. Measured branches were randomly distributed in one of four light sequences: high- to low-light branches started the experiment under sun and were transferred to shade during the second part of the experiment; low- to high-light branches (LH) were exposed to the opposite sequence of light treatments; and high-light and low-light controls , which were exposed only to sun and shade environments, respectively, throughout the experiment. Shade branches were set inside enclosures wrapped in 63% greenhouse shade cloth. After 2 months, we transferred experimental branches to opposite light conditions by relocating the enclosures. Leaf mortality was considerably higher under shade, both before and after the transfer. LH branches reversed the pattern of mortality by increasing new leaf production after the transfer. Rates of photosynthesis at light saturation, light compensation points, and dark respiration rates of transferred branches matched those of controls for the new light treatment, indicating rapid photochemical acclimation. The post-expansion acclimation of sun and shade foliage occurred with little modification of leaf structure. High photosynthetic plasticity was reflected in an almost immediate ability to respond to significant changes in light. This response did not depend on the initial light environment, but was determined by exposure to new light conditions. Stigmaphyllon responded rapidly to light changes through the functional adjustment of already expanded foliage and an increase in leaf production in places with high opportunity for carbon gain.

Canopy Analysis as a Technique to Characterize Defoliation Intensity on Sandhills Range

Characterization of relationships between grazing and vegetation responses is difficult. Rapid and accurate measurement of pasture canopy characteristics would help clarify these relationships if canopy changes are directly related to grazing variables. The objectives of this study were (1) to evaluate use of the LI-COR LAI-2000 for quantification of changes in canopy density and architecture in response to defoliation by cattle, (2) to determine if changes in leaf area index (LAI) measured with the LAI-2000 are related to stocking rate, and (3) to determine advantages and drawbacks of the LAI-2000 for monitoring grazing impacts on canopy density and architecture. Leaf area index and mean foliage tilt angle were measured before and after defoliation by cattle (Bos taurus L.) in June, July, and August under 9 grazing treatments on Nebraska Sandhills range. Differences in LAI could be attributed to certain grazing treatments at various points throughout the season. Grazing treatment had little impact on mean foliage tilt angle. Change in LAI (delta LAI) had a significant negative relationship with stocking rate (P < or = 0.0001). The relationship detected for delta LAI versus stocking rate predicted LAI reductions of between 0.14 and 0.40 for the range of stocking rates studied; stocking rate accounted for 62% of the decrease in LAI caused by grazing. When configured for the Sandhills canopy, the LAI-2000 provided a rapid and precise method for quantification of the degree of defoliation associated with grazing.

Vegetation changes in forests of the Krkonoše Mts. over a period of air pollution stress (1980–1995)

Species composition of the understory of spruce (Picea abies (L.) Karsten) and beech (Fagus sylvatica L.) forests subjected to intense air pollution stress in the Krkonose Mts., Czech Republic, showed directional changes over the last 15 years. The changes were documented with repeated observations of 29 permanent plots, initiated in 1980 and analysed with constrained ordination methods (Canonical Correspondence Analysis). Ground layer changes were mainly associated with the loss of canopy foliage from air pollution stress. The increased amount of the light at ground level resulted in increased herb cover. The initially dominant species (e.g., Calamagrostis villosa (Chaix) J. F. Gmelin, Deschampsia flexuosa (L.) P. B., Vaccinium myrtillus L., Athyrium distentifolium Tausch) were those that increased in cover. Moss cover, and moss and herb richness, declined over the course of the study. Thus, the changes in tree canopy are accompanied with changes in the forest understory, which in turn can affect the dynamics of these forests.

Monitoring of forest changes using unsupervised methods: A case study from boreal forest on mineral soils

An unsupervised clustering method was presented for monitoring rapid forest changes such as cuttings, in large areas. The work was based on the hypothesis that most of the important changes in the forest canopy can be detected using space‐born remote sensing information. Multitemporal Landsat TM data covering boreal forest were utilized. The results showed that clustering of changes was not very accurate without prior radiometric calibration. In this study a relative regression calibration was combined with Studentization of the spectral difference features. The change detection accuracy at pixel level was not acceptable, therefore forest stands were used as classification units. It is proposed that if forest stands are not available, spectral segments can be used as observation units for classification. The stand‐level change detection accuracy using the Kernel density linkage clustering method varied from 87.6% with a three‐year interval to 93.1% with a one‐year interval.

Evaluation of a measurement method for forest vegetation in a large-scale ecological survey

We evaluate a field method for determining species richness andcanopy cover of vascular plants for the Forest Health MonitoringProgram (FHM), an ecological survey of U.S. forests. Measurementsare taken within 12 1-m2 quadrats on 1/15 ha plots in FHM.Species richness and cover are determined for four height classes(strata) within each quadrat and aggregated by stratum over the entireplot. We estimated (1) the agreement between experienced trainers andinexperienced technicians who collected the data on this survey(accuracy) and (2) the agreement among the technicians (precision) forresults on species richness and cover from 3 test plots at 3 timeintervals. The methods appear to be highly precise, although somediscrepancies with the values obtained by the trainers were found.Trainers found significantly more species in the ground stratum (0–0.6 m) and measured significantly more cover in the uppermost stratum(>4.9 m). The proportion of variation due to measurement error andtemporal variability was less than 13% for species richness (all strata)and cover (all but one stratum). This indicates that the method issuitable for monitoring changes in species richness and canopy coverfor a large-scale synoptic monitoring project such as FHM.

Carbon dioxide concentrations within forest canopies—variation with time, stand structure, and vegetation type

AbstractVertical CO2 profiles (between 0.02 and 14.0 m) were studied in forest canopies of Pinus contorta, Populus tremuloides, and in a riparian forest with Acer negundo and Acer grandidentatum during two consecutive growing seasons. Profiles, measured continuously during 1‐ to 13‐day periods in four to five stands differing in overstorey canopy area index (CAI &lt; 4.5; including leaves, branches and stems), were well stratified, with highest [CO2] just above the forest floor. Canopy [CO2] profiles were influenced by stand structure (CAI, presence of understorey vegetation), and were highly dependent on vegetation type (deciduous and evergreen). A doubling of CAI in Acer spp. and P. tremuloides stands did not show an effect on upper canopy [CO2], when turbulent mixing was high. However, increasing understorey biomass in Acer spp. stands had a profound effect on lower canopy [CO2]. In open stands with a vigorous understorey layer, higher soil respiration rates were offset by increased understorey gas exchange, resulting in [CO2] below those of the convective boundary layer (CBL). Midday depletions up to 20 ppmv below CBL values could be frequently observed in deciduous canopies. In evergreen canopies, [CO2] stayed generally above the CBL background values, [CO2] profiles were more uniform, and gradients were smaller than in deciduous stands with similar CAI. Seasonal changes of canopy [CO2] reflected changes in soil respiration rates as well as plant phenology and gas exchange of both dominant tree and understorey vegetation. Seasonal patterns were less pronounced in evergreen than in deciduous forests.