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Studies have reported divergent results on the effect of soil fertility and canopy opening on understory density and growth of sugar maple (AS; Acer saccharum Marsh.) and American beech (FG; Fagus grandifolia Ehrh.). The main objective of this study was to evaluate the effect of a gradient of canopy opening and soil fertility on the density and growth of AS and FG saplings in southwestern Quebec, Canada. We investigated 56 stands containing both AS and FG that were subjected to different disturbance history types (DHTs) (UF, unmanaged forest; PC, partial cut; and CC, clearcut) on various soil types. AS and FG absolute and relative sapling density varied greatly among the 56 stands; however, no significant effects of DHT, soil nutrient availability, or their interaction were found. Both species responded positively in terms of radial growth to canopy openings, with FG growth being slightly better than AS growth in PC stands compared with other canopy treatments. Contrary to our hypothesis, AS did not show significantly higher growth than FG following clear-cutting. These results do not support the idea that AS abundance and growth could be promoted by increasing the intensity of the canopy opening during harvest, at least on the generally acidic and base-poor soils that were investigated. Key words: Fagus grandifolia, Acer saccharum, soil nutrient availability, light availability, saplings, clearcut, partial cut. Des etudes font etat de resultats divergents concernant l'effet de la fertilite du sol et de l'ouverture du couvert forestier sur la densite et la croissance de l'erable a sucre (AS; Acer saccharum Marsh.) et du hetre d'Amerique (FG; Fagus grandifolia Ehrh.) sous couvert. L'objectif principal de cette etude etait d'evaluer l'effet d'un gradient d'ouverture du couvert forestier et de la fertilite du sol sur la densite et la croissance des gaules d'AS et de FG dans le sud-ouest du Quebec, au Canada. Nous avons echantillonne 56 peuplements contenant des AS et des FG ayant ete soumis a differents types d'historique de perturbation (THP) (FN, forets non amenagees; CP, coupes partielles; CT, coupes totales) sur differents types de sol. La variation des densites absolue et relative des gaules d'AS et de FG entre les 56 peuplements etait importante, mais nous n'avons trouve aucun effet significatif du THP, de la disponibilite des nutriments du sol ou de leur interaction. La croissance radiale des deux especes a reagi positivement a l'ouverture du couvert, mais la croissance de FG etait legerement superieure a celle d'AS dans les CP comparativement aux autres traitements du couvert forestier. Contrairement a notre hypothese, la croissance d'AS n'etait pas significativement plus grande que celle de FG a la suite d'une CT. Ces resultats n'appuient pas l'idee qu'on puisse favoriser l'abondance et la croissance d'AS en augmentant l'intensite des ouvertures par la coupe, a tout le moins sur les sols generalement acides et pauvres en bases qui ont ete etudies. [Traduit par la Redaction] Mots-cles: Fagus grandifolia, Acer saccharum, disponibilite des nutriments du sol, disponibilite de la lumiere, gaules, coupe totale, coupe partielle., Introduction Throughout the last century, forest management has changed the structure and composition of temperate deciduous stands within the forested landscapes of Quebec. Most notably, results from several experiments have [...]

The effects of the permanent removal of the canopy-forming alga Fucus serratus was studied in terms of both functioning and diversity on a mid-low rocky shore ecosystem in the south-west English Channel (48°N 43.686', 3°W 59.282'). Ecosystem functioning was examined as net or gross primary productivity (NPP or GPP) and respiration (Resp) measured through C[O.sub.2] fluxes. Diversity was examined as number and composition of species. Measurements were performed in situ, during emersion times, without altering target assemblages. The experiment was designed with two treatments [control (C) or canopy removed (CR)] and five replicates, and was conducted over an 18-month period (from February 2006 to August 2007) to integrate the seasonal variability. The mean GPP and Resp were severely reduced in CR treatment compared to control throughout the survey. The mean NPP was not affected at first, due to the development of opportunistic green algae, but was drastically reduced after 9 months of experiment. The canopy removal affected neither the number of species nor their distribution among trophic groups, and the algal community was only slightly affected. The abundance and biomass of mobile invertebrates, however, were greatly reduced in the absence of canopy. This indicates an important effect of the dominant alga on the higher trophic levels of the community. At this tidal level, the canopy did not seem to affect the community by dampening the environmental stress but by providing food, habitat or both., Introduction Canopy-forming algae are often found in high biomass and abundance on rocky shores and may be responsible for the major part of carbon fluxes in these systems as they [...]

Fire has an important role for regeneration of many boreal forest tree species, and this includes both wildfire and prescribed burning following clear-cutting. Depending on the severity, fire can have a variety of effects on above- and belowground properties that impact tree seedling establishment. Very little is known about the impacts of ground fire severity on post-fire seedling performance, or how the effects of fire severity interact with those of canopy structure. We conducted a full-factorial experiment that manipulated surface-burn severity (no burn; light, medium, or heavy burn; or scarification) and canopy (closed forest or open clear-cut) to reveal their interactive effects on ecophysiological traits of establishing broadleaf and conifer seedlings in a Swedish boreal forest. Medium and heavy surface burns increased seedling growth, photosynthesis, respiration, and foliar N and P concentrations, and these effects were most apparent in open clear-cuts. Growth rates of all species responded similarly to surface-burn treatments, although photosynthesis, foliar P, and specific leaf area were more responsive to burning treatments for broadleaf species than for conifers. Our study demonstrates that the positive impacts of fire on tree seedling physiology are dependent on a minimum severity threshold and are more effective when combined with clear-cutting. Key words: boreal forest, fire severity, fire management, foliar nutrients, photosynthesis, respiration, scarification, tree seedling regeneration. Le feu joue un role important pour la regeneration de plusieurs especes d'arbre de la foret boreale et cela inclut les feux de foret et le brulage dirige apres une coupe a blanc. Selon la severite, le feu peut avoir une variete d'effets sur les proprietes aeriennes et souterraines qui ont un impact sur l'etablissement des semis. On connait mal les impacts de la severite des feux de surface sur la performance des semis apres feu ou la fa^on dont les effets de la severite du feu interagissent avec ceux de la structure du couvert forestier. Nous avons realise une experience selon un plan factoriel complet dans laquelle nous avons fait varier la severite d'un brulage de surface (aucun brulage; brulage leger, moyen ou severe; ou scarifiage) et le couvert forestier (ferme ou ouvert a la suite d'une coupe a blanc) dans le but de reveler leurs effets interactifs sur les caracteristiques ecophysiologiques des semis de coniferes et de feuillus qui s'etablissent dans une foret boreale suedoise. Les brulages de surface moyen et severe ont augmente la croissance, la photosynthese, la respiration et la concentration foliaire de N et P des semis mais ces effets etaient davantage apparents dans les coupes a blanc. Le taux de croissance de toutes les especes a reagi de fa^on similaire aux traitements de brulage de surface bien que la photosynthese, le P foliaire et la surface foliaire specifique aient reagi davantage aux traitements de brulage chez les feuillus que chez les coniferes. Notre etude demontre que les impacts positifs du feu sur la physiologie des semis d'arbre dependent d'un seuil de severite minimum et sont plus efficaces lorsque le feu est combine a la coupe a blanc. [Traduit par la Redaction] Mots-cles: foret boreale, severite du feu, gestion du feu, nutriments foliaires, photosynthese, respiration, scarifiage, regeneration de semis d'arbre., Introduction It is well recognized that fire is a major driver of regeneration patterns in boreal forest systems (Zackrisson 1977; Engelmark 1984; Payette 1992; Lampainen et al. 2004; Nilsson and [...]

Canopy-forming seaweeds are important foundation species or ecosystem engineers in intertidal habitats. By limiting a variety of abiotic stresses during low tides, algal canopies improve the performance of many understory organisms. The reduction of heat stress through substrate shading and moisture retention has received considerable attention in marine biology. However, the thermal influence of canopies during winter has not been empirically evaluated. Using intertidal fucoid canopies (Ascophyllum nodosum) from Atlantic Canada, we did a field experiment contrasting canopy-covered and no-canopy areas to test the hypothesis that canopies limit low temperatures during winter low tides. During 35 days between January and March, mid-intertidal temperature was often negative near the time of the lowest daily tides, on average more than 1 °C lower on bare substrate than under full canopy cover. The difference between both canopy treatments was higher around spring tides than around neap tides. Temperature on bare substrate was once even up to 10 °C lower than under a full canopy. Previous studies have shown that single occurrences of lethal negative temperatures and frequent occurrences of sublethal temperatures kill intertidal organisms every winter. Thus, our study suggests that, in addition to their bioprotective role during summer, canopy-forming seaweeds might also play a relevant facilitative role during winter., Author(s): Ricardo A. Scrosati [sup.1] , Julius A. Ellrich [sup.1] Author Affiliations: (Aff1) 0000 0004 1936 7363, grid.264060.6, Department of Biology, St. Francis Xavier University, , B2G 2W5, Antigonish, NS, [...]

No known research has examined the concentration and flux of dissolved organic carbon (DOC) for stemflow across temporal scales from within single storm events to seasonal and annual scales or employed UV-Vis spectral metrics to examine the chemical character of dissolved organic matter (DOM) of stemflow drainage. Thus, our study examined stemflow DOC concentration and flux and DOM character from American beech (Fagus grandifolia Ehrh.) and yellow poplar (Liriodendron tulipifera L.) to address this data gap. Intrastorm stemflow DOC concentrations and fluxes diminished by an order of magnitude over time for American beech but remained relatively constant for yellow poplar. Stemflow DOM aromaticity, however, generally increased and [E.sub.2]: [E.sub.3] ratios generally decreased as events progressed, suggesting transport of different compounds at different moments. Although less enriched in DOC than yellow poplar, American beech stemflow DOC fluxes were double per annum. Differential interspecific stemflow DOM characteristics are ascribed to (i) significant differences in [SUVA.sub.254] values (aromaticity) for the leafless season and annually and (ii) significantly higher [E.sub.2]: [E.sub.3] and [S.sub.R] ratios for yellow poplar stemflow than for American beech annually and across leafed and leafless seasons. Our results suggest that stemflow significantly affects the amount and chemical character of carbon flux to the forest floor that may engender hot spots around tree boles. Resume: Aucune recherche connue n'a porte sur la concentration et le flux de carbone organique dissout (COD) associe au ruissellement sur le tronc a differentes echelles temporelles: soit pendant des tempetes particulieres ou a des echelles saisonniere et annuelle, ni utilise des mesures spectrales dans l'ultraviolet et le visible pour etudier les caracteristiques chimiques de la matiere organique dissoute (MOD) associee au ruissellement sur le tronc. Par consequent, notre etude a porte sur le flux et la concentration de COD associe au ruissellement sur le tronc ainsi que sur les caracteristiques de la MOD chez le hetre a grandes feuilles (Fagus grandifolia Ehrh.) et le tulipier de Virginie (Liriodendron tulipifera L.) afin de combler cette lacune. Pendant les tempetes, le flux et la concentration de COD associe au ruissellement sur le tronc ont diminue avec le temps d'un ordre de grandeur chez le hetre a grandes feuilles mais sont demeures relativement constants chez le tulipier de Virginie. Cependant, l'aromaticite de la MOD associee au ruissellement sur le tronc a generalement augmente et le rapport [E.sub.2]: [E.sub.3] a generalement diminue a mesure que les evenements progressaient, ce qui indique que differents composes seraient transportes a differents moments. Bien que moins riche en COD, le flux de COD associe au ruissellement sur le tronc du hetre a grandes feuilles etait sur une base annuelle deux fois plus eleve que celui du tulipier de Virginie. Les differences interspecifiques dans les caracteristiques de la MOD associee au ruissellement sur le tronc sont dues a (i) d'importantes differences dans les valeurs de [SUVA.sub.254] (aromaticite) pendant la saison sans feuilles et annuellement et (ii) des poids moleculaires relatifs du COD (rapports [E.sub.2]: [E.sub.3] et [S.sub.R]) associe au ruissellement sur le tronc significativement plus eleves chez le tulipier de Virginie que chez le hetre a grandes feuilles annuellement et pendant les saisons avec et sans feuilles. Nos resultats indiquent que le ruissellement sur le tronc a un effet significatif sur la quantite et les caracteristiques chimiques du flux de carbone vers la couverture morte et qu'il peut engendrer des points chauds autour du tronc des arbres. [Traduit par la Redaction], Introduction Canopy-derived hydrologic fluxes travel to the forest floor along two flow paths: throughfall and stemflow (Levia and Frost 2003; Levia and Frost 2006). Throughfall is the partition of incident [...]

Light penetration measurements have not been possible in turfgrass environments due to the large size of commercial light bars relative to the low height of the turfgrass canopy. A light bar was designed and constructed for use in turfgrass canopies mowed as short as 1.3 cm. The instrument uses four miniature solar cells connected in parallel and mounted to a high density polyethylene (HDPE) bar measuring 3.5-mm wide, 6-mm tall, and 200-mm long. The constructed light bar was calibrated against a Li-Cor (Li-Cor, Lincoln, NE) pyranometer sensor and obtained a relationship of Y = [1.0293e.sup.16.039x] with an [R.sup.2] value of 0.96. Data demonstrate measured differences in light penetration through an annual ryegrass (Lolium multiflorum Lam.) canopy maintained at three mowing heights. Light penetration measured at the base of annual ryegrass canopies maintained at cutting heights of 3.2, 5.7. and 8.3 cm amounted to 5.36 to 7.48%, 1.21 to 3.43%, and 0.10 to 0.40%, of full sunlight, respectively. This device should allow researchers the ability to assess canopy light penetration in low-growing vegetation including turfgrasses. doi: 10.2135/cropsci2009.06.0359

We evaluated whether two indices of light availability resolved differences among microsites within deeply shaded understories ( Nous avons evalue si deux indices de disponibilite de la lumiere detectaient les differences entre les microsites situes sous des couverts tres ombrages (moins de 12 % de la densite de flux de photons photosynthetiques (DFPP) au-dessus du couvert) et si des differences marquees de structure du couvert forestier influencaient la facon dont les deux indices etaient relies a des mesures directes du rayonnement photosynthetiquement actif (RPA) incident. Le RPA incident a ete mesure a l'aide de photodiodes au phosphure d'arseniure de gallium placees a plusieurs endroits dans deux blocs de foret adjacents au Michigan, Etats-Unis. Un bloc etait domine par un conifere sempervirent, la pruche du Canada (Tsuga canadensis (L.) Carriere), et l'autre par un feuillu caducifolie, l'erable a sucre (Acer saccharum Marsh.). Les deux indices testes etaient l'ouverture du couvert mesuree a l'aide de l'analyseur du couvert vegetal LAI-2000 de LI-COR, et le pourcentage de la DFPP au-dessus du couvert, mesure sous le couvert par temps nuageux (%DFPP). L'ouverture du couvert et le %DFPP n'ont pas predit efficacement la moyenne a long terme de la DFPP journaliere. Toutefois, les deux indices ont predit efficacement la moyenne a long terme de la mediane journaliere de la DFPP, une mesure alternative du RPA incident mesure directement qui reduit la contribution relative des taches de lumiere. Les relations entre les deux indices et la moyenne de la mediane journaliere de la DFPP etaient differentes selon que le bloc etait domine par la pruche ou par l'erable a sucre. Par consequent, on devrait tenir compte de l'effet de la structure du couvert quand ces deux indices sont utilises pour tirer des conclusions au sujet des differences de disponibilite de la lumiere entre des blocs de foret, particulie rement sous des couverts tres ombrages ou de faibles ecarts de disponibilite de la lumiere sont importants. [Traduit par la Redaction], Introduction Light is one of the key resources that control the growth and survival of plants in the understory of forests. For example, the growth and survival of saplings increase [...]

In mixedwood boreal forests of western Canada, stands classified as 'pure deciduous' by forest inventories sometimes contain a few large white spruce (Picea glauca (Moench) Voss) trees among or emerging from the canopy. These trees are important as regeneration seed sources and for habitat structure. Neither their abundance nor the characteristics of stands in which they occur have previously been quantified. Of 275 'pure aspen' stands in northeastern Alberta, 19.6% contained at least one such spruce detectable in an aerial photograph. These trees were found in stands across the range of sampled canopy heights, densities, age classes, and stand sizes and were often present in the interior of stands, not just on the perimeter. The frequency of 3 ha cells containing at least one spruce was related to (i) stand shape and size, (ii) amount of mature white spruce in adjacent forest, (iii) canopy height, (iv) stand age, and (v) stand density. We conclude that such trees are relatively abundant and widely distributed within the boreal mixedwood forests of Alberta. There is presently no provision to maintain this landscape element within managed forests. More information is needed to determine if or how they should be considered in forest management planning. Dans la foret boreale mixte de l'Ouest canadien, il arrive que les peuplements classes comme << feuillus purs >> dans les inventories forestiers contiennent quelques grosses tiges d'epinette blanche (Picea glauca (Moench) Voss) qui font partie ou emergent de la canopee. Ces arbres sont importants comme source de graines pour la regeneration et pour la structure de (habitat. Toutefois, leur abundance et les caracteristiques des peuplements dans lesquels ils sont presents n'ont jamais ete quantifiees. Parmi 275 << peupleraies pures >> du nord-est de (Alberta, 19,6 % contenaient au moms une telle epinette detectable a partir de photographies aeriennes. Ces arbres ont ete detectes dans des peuplements couvrant toute la gamme de hauteurs de canopee, de densites, de classes d'age et de tailles de peuplement echantillonnees; ils etaient souvent presents a finterieur des peuplements et non seulement en peripherie. La frequence des cellules de 3 ha a contenant au moms une epinette a ete reliee a (i) la forme et la taille du peuplement, (ii) la quantite d'epinettes blanches matures dans la foret adjacente, (iii) la hauteur de la canopee, (iv) Page du peuplement et (v) la density du peuplement. Nous concluons que de tels arbres sont relativement abondants et largement distrbbues dans la foret boreale mixte de l'Alberta. Il n'y a presentement aucune mesure visant a maintenir cet element du paysage dans les foress amenagees. Plus de renseignements sont necessaires pour determiner si ou comment ces arbres devraient etre consideres dans la planification de l'amenagement forestier. [Traduit par la Redaction], Introduction In the boreal mixedwood forests of western Canada, the canopies of upland mesic sites are dominated by some combination of Populus species (typically trembling aspen, Populus tremuloides Michx., but [...]

Landscape heterogeneity influences large herbivores by altering their feeding rates, but as herbivores attempt to maximize feeding rates they also create spatial heterogeneity by altering plant growth. Herbivore feeding rates thus provide a quantitative link between the causes and consequences of spatial heterogeneity in herbivore-dominated ecosystems. The fractal geometry of plant canopies determines both the density and mass of twigs available to foraging herbivores. These properties determine a threshold distance between plants ([d.sup.*]) that distinguishes the mechanisms regulating herbivore intake rates. When [d.sup.*] is greater than the actual distance between plants (d), intake is regulated by the rate of food processing in the mouth. But when [d.sup.*] < d, intake is regulated by the rate at which the herbivore encounters new plants. Alterations to plant geometry due to past browsing could change the rate at which herbivores encounter and process bites of plant tissue, modify [d.sup.*] relative to d, and thus change intake rates and the distribution of mechanisms regulating it across landscapes. We measured changes in the geometry of aspen (Populus tremuloides) and balsam fir. (Abies balsamea) saplings along gradients of moose browsing from 2001 to 2005 at Isle Royale National Park, Michigan, USA. For aspen saplings, fractal dimension of bite density, bite mass, and forage biomass responded quadratically to increasing moose browsing and were greatest at ~3-4 g x [m.sup.-2] x [yr.sup.-1] consumption. For balsam fir, in contrast, these same measures declined steadily with increasing moose browsing. The different responses of plant canopies to increased browsing altered [d.sup.*] around plants. In summer, [d.sup.*] [greater than or equal to] d for aspen saplings at all prior consumption levels. Food processing therefore regulated summer moose feeding rates across our landscapes. In winter, changes in bite mass due to past browsing were sufficient to cause [d.sup.*] < d for aspen and balsam fir. Therefore, travel velocity and food processing jointly regulated intake rate during winter. Browsing-induced changes in the small-scale geometry of plant canopies can determine intake rate at larger spatial scales by changing [d.sup.*] relative to d and, hence, which mechanisms determine intake rate, essentially altering how herbivores sense the distribution of their food resources. Key words: Abies balsamea; Alces alces; allometry; aspen; balsam fir; browsing; fractal dimension; herbivory; moose; plant geometry; Populus tremuloides; threshold distance.

We resurveyed the under-and overstory species composition of 94 upland forest stands in southern Wisconsin in 2002 2004 to assess shifts in canopy and understory richness, composition, and heterogeneity relative to the original surveys in 1949-1950. The canopy has shifted from mostly oaks (Quercus spp.) toward more mesic and shade-tolerant trees (primarily Acer spp.). Oak-dominated early-successional stands and those on coarse, nutrient-poor soils changed the most in canopy composition. Understories at most sites (80%) lost native species, with mean species density declining 25% at the 1-[m.sup.2] scale and 23.1% at the 20-[m.sup.2] scale. Woody species have increased 15% relative to herbaceous species in the understory despite declining in absolute abundance. Initial canopy composition, particularly the abundance of red oaks (Quercus rubra and Q. velutina), predicted understory changes better than the changes observed in the overstory. Overall rates of native species loss were greater in later-successional stands, a pattern driven by differential immigration rather than differential extirpation. However, understory species initially found in early-successional habitats declined the most, particularly remnant savanna taxa with narrow or thick leaves. These losses have yet to be offset by compensating increases in native shade-adapted species. Exotic species have proliferated in prevalence (from 13 to 76 stands) and relative abundance (from 1.2% to 8.4%), but these increases appear unrelated to the declines in native species richness and heterogeneity observed. Although canopy succession has clearly influenced shifts in understory composition and diversity, the magnitude of native species declines and failure to recruit more shade-adapted species suggest that other factors now act to limit the richness, heterogeneity, and composition of these communities. Key words: biotic homogenizution; oak forest; shade tolerance; species loss; succession; understory.

Abstract: Within a canopy, spatial variation in leaf traits may be determined by light and nutrient availabilities. Such environmentally caused changes in leaf traits may be an important cause of [...]

Potted experiments of rice (Oryza sativa L.) plants were conducted to produce various scales of brown planthopper [Nilaparvata lugens (Stal), Homoptera:Delphacidae] and leaffolder [Cnaphalocrosis mediualis Guenee (Lep., Pyralidae)] infestations, respectively, for canopy hyperspectral reflectance measurements, and then to identify spectral characteristics (SCs) associated with insect infestations leading to the establishment of spectral models for severity assessment. By linear correlation intensity analysis, correlation coefficients (r) along the spectral domain of 350 to 2400 nm were determined and narrow bands related to infestation severity were selected as SCs. The reflectance at green light (490-560 nm) maximum ([R.sub.GREEN]), red light (640-740 nm) minimum ([R.sub.RED]), and near-infrared (740-1300 nm) peak ([R.sub.NIR]) were also considered. For canopies infested with brown planthopper, r value at 426 nm was the highest (r = 0.878 **). Among the calculated spectral indices using two SCs, the determination coefficient of [R.sub.NIR]/[R.sub.RED] ratio was the highest ([R.sup.2] = 0.922, P < 0.001). For leaffolder infested canopies, the most negative r value located at 757 nm (r =-0.613 *) in active tillering stage but shifted to 445 nm (r = -0.928 **) in heading stage. The index [R.sub.NIR]-[R.sub.RED] in the active tillering increased [R.sup.2] value to 0.422 (P < 0.001), while no increase in [R.sup.2] was found in the examined Sis in heading stage. Models with more than two SCs yielded from multiple linear regression analysis exhibited a further improvement for discriminating infestation severity.

Abstract: Many old-growth deciduous forests in eastern North America exhibit dynamics that suggest succession from dominance by oaks (Quercus spp.) to dominance by maples (Acer spp.). We examined this phenomenon [...]

A better understanding of forest subcanopy flows is needed to evaluate their role in the horizontal movement of scalars, particularly in complex terrain. This paper describes detailed measurements of the canopy structure and its variability in both the horizontal and vertical directions at a deciduous forest in complex terrain (the Harvard Forest, Petersham, Massachusetts). The effects of the trunks and subcanopy shrubs on the flow field at each of six subcanopy array locations are quantified. The dynamics of the subcanopy flow are examined with pragmatic methods that can be implemented on a small scale with limited resources to estimate the stress divergence, buoyancy, and pressure gradient forces that drive the flow. The subcanopy flow at the Harvard Forest was driven by mechanisms other than vertical stress divergence 75% of the time. Nocturnal flows were driven predominantly by the negative buoyancy of a relatively cool layer near the forest floor. The direction of the resulting drainage flows followed the azimuth of the longest forest-floor slope. Similar results were found at a much flatter site at Borden, Ontario, Canada. There was no clear evidence of flow reversals in the subcanopy in the the of ridges or hills at the Harvard Forest even in high wind conditions, contrary to some model predictions.

The dispersion and capture of differently shaped particles within a Zostera marina L. (eelgrass; Zosteraceae) bed were examined to understand submarine pollination and other dispersals. During periods of moderate flow in the canopy, the capture rate of 'spherical' (the shape of ancestral pollen) and 'filamentous' (the shape of eelgrass pollen) particles was greater for particles released at the top of the canopy (3.07 and 4.53% x [10.sup.-5] [cm.sup.-2] of collector; i.e., percentage of particles captured normalized to collector area) and greater for filamentous than for spherical particles (4.51% x [10.sup.-5] [cm.sup.-2] vs. 2.01% x [10.sup.-5] [cm.sup.-2]). Estimates of the horizontal P (Joseph-Sendner diffusion velocity) and the vertical diffusivity (Gaussian K) of filamentous particles were small (P [approximately equal to] 4 x [10.sup.-4] m/s; K [approximately equal to] [10.sup.-4] [m.sup.2]/s) compared to theoretical values that do not consider plant canopies. These findings support the concept that eelgrass canopies modify the fluid dynamics (i.e., reduced turbulent mixing) within their canopies. These results indicate that 1000-10000 Z. marina pollen are required to pollinate a single flower. Similarly, it was estimated that under some conditions, the probability of particle impaction on eelgrass vegetation approaches certainty. These results provide insight into the evolution of filamentous pollen and submarine pollination, as well as dispersal and other mass transport phenomena within macrophyte canopies. Key words: canopy flow; dispersion; mixing; particle capture; particle transport; seagrass; submarine pollination; Zosteraceae; Zostera marina.

Shade-avoidance responses were examined for two species common to the coastal redwood forest, Sequoia sempervirens and Satureja douglasii. Sequoia seedlings demonstrated a shade-avoidance response when given end-of-day far-red light by increased hypocotyl, epicotyl, and first-node extension, and greater total number of needles and reduced anthocyanin concentration. Thus, Sequoia seedlings respond as sun-adapted plants. Satureja has several leaf monoterpene chemotypes that occur in different light environments including the redwood forest, and the types responded differently to the light treatments. The pulegone type responded to end-of-day far-red light as a sun-adapted plant with significant extension growth, increased leaf area and chlorophyll, and reduced anthocyanin. The isomenthone type responded as a shade-tolerant plant and did not exhibit extension growth nor a change in other parameters with end-of-day far-red light. However, the carvone and bicyclic types had variable responses depending on the parameter studied, which indicated genetic variation for these traits. Key words: end-of-day far-red light; Lamiaceae; phytochrome; Satureja; Sequoia; shade avoidance; Taxodiaceae.

Species responses to disturbance (mortality of dominant individuals within a community) influence many aspects of that ecological community. To trace the responses of vegetation to one particular type of disturbance, I sampled vegetation in 250 canopy gaps in 1976/1977, 1983, and 1990/1991. These gaps were located in three sites in the southern Appalachians of eastern North America: Great Smoky Mountains National Park, Walker Cove Research Natural Area, and Joyce Kilmer Wilderness Area. Each gap was sampled thrice for sapling composition (stems ≥ 1 cm in diameter at breast height). Additional measurements included the extension growth of border trees into the gap, recent mortality rates of border trees, deterioration rates of gap makers that had been standing dead in earlier samples, and the composition of understory plots. The mean extension (branch) growth of border trees was 12 cm/yr, with slower growth by longer branches. Gap makers that had been tall stumps tended to deteriorate, although 35% stayed intact over the 14 yr of the study. Border-tree mortality averaged 0.60%/yr, with higher rates for larger stems and with much interspecific variation. Stem density of saplings in gaps increased during the first sampling interval and decreased during the second as self-thinning counteracted increased establishment. Basal area increased during both sampling intervals. The death of border trees increased basal area per unit gap area. The four main species (Acer saccharum, Tsuga canadensis, Fagus grandifolia, and Halesia carolina) showed different patterns of correlation to gap size and age. Species in general showed more correlations with gap age for the first sample than afterwards; gap size was more consistently related to species importance. Species patterns also were affected by the presence or absence of border-tree mortality. The stands studied seem nearly at equilibrium. Some small changes are likely to occur, but the species present dominated all size classes: gap saplings, border trees, other canopy trees not related to gaps, and understory saplings. Species differed in their relative growth rates in the understory and in gaps of different sizes. Species also differed in their survival rates in the understory and in the canopy. Key words: Acer saccharum; border trees; canopy gaps; Fagus grandifolia; Halesia carolina; long-term studies; old-growth forests; repeated disturbance, impacts on vegetation/species; southern Appalachians (USA); tree species, relative growth and survival; Tsuga canadensis., INTRODUCTION Many studies have documented the response of species to gaps formed in the forest canopy by the death of a large branch, a whole tree, or a small group [...]

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation of the experiment). Measurements in the control plots indicated that the interval of stand development from age 6 to 23 yr was marked by a peak in basal area and leaf area of pin cherry at about age 17 yr, followed by a steady decline in P. pensylvanica dominance thereafter. Fertilization increased and prolonged the dominance of P. pensylvanica, indicating that nutrient limitation accelerates the demise of this species during the second and third decades of stand development. All species in the plots responded to fertilization with increased foliar nutrient (N, P, and K) concentrations and often higher specific leaf area (area:mass ratio), and these responses were most pronounced for P. pensylvanica. Although the light-response curve for photosynthesis of P. pensylvanica was altered by fertilization, with higher rates at low light levels, photosynthesis of its principal competitor, Betula papyrifera, was not affected. The marked growth response of P. pensylvanica was accompanied by changes in its canopy architecture, as the trees had more leaf area per unit stem basal area, and proportionally more of this leaf area was in the upper canopy. In contrast, height and leaf area of B. papyrifera were similar in the control and fertilized plots. Seed deposition of P. pensylvanica also increased in the fertilized plots during one year of high seed production. Thus, the performance in competition of P. pensylvanica was improved by the removal of apparent nutrient limitations on its physiological performance, canopy growth, and ability to compete for light. Leaf area index of the fertilized plots was only slightly higher than the control plots, and the same was true for stand basal area. The removal of nutrient limitation increased the intensity of one-sided competition for light by concentrating the dominance among the largest trees; consequently, very high mortality of suppressed stems of all species occurred. The increased dominance of the fast-growing P. pensylvanica contributed to increases in aboveground net primary productivity (ANPP) in the fertilized plots. Some of this ANPP response was probably associated with reduced C allocation to roots in some of the fertilized plots, but this pattern was not consistent across all the stands. The results indicate that the outcome of interspecific competition in mesic forests, where co-limitation by light and soil resources prevails, depends upon the effect of site quality upon the relative intensity of one-sided competition (for light). Key words: birch; canopy architecture; co-limitation; competition; leaf area index; northern hardwood forests; photosynthesis; pin cherry; productivity; Prunus pensylvanica; resorption; specific leaf area., INTRODUCTION Natural vegetation exhibits a complex three-dimensional structure (i.e., spatial distribution of leaves, roots, and stems) that influences its function both in ecosystem energetics and material cycling, and in the [...]

1 Canopy-level C[O.sub.2] exchange and biomass were measured monthly for 9 consecutive months during 1990 on grazed and non-grazed portions of a Danish salt marsh dominated by Spartina anglica. The empirical measurements were used to calibrate a model of canopy photosynthesis and soil respiration that was subsequently integrated to provide estimates of annual net ecosystem production (NEP). 2 Standing total and green biomass on the non-grazed site reached maxima during September of 1.9 [+ or -] 0.2 and 1.1 [+ or -] 0.2 kgm-2 ([+ or -] 1 SD, n = 3), respectively. Corresponding maxima (0.8 [+ or -] 0.1 and 0.5 [+ or -] 0.05 kg [m.sup.-2]), were achieved during July at the grazed site. 3 Sediment organic matter as a percentage of dry weight was 31.6% and 5% at the non-grazed and grazed sites, respectively. Accounting for the higher sediment bulk density at the grazed site meant that concentrations of organic carbon were 0.027 and 0.021 g [cm.sup.-3] at non-grazed and grazed sites, respectively. Thus, the fraction of NEP removed by grazing animals significantly reduced the accumulation of sediment organic matter. 4 Estimate of NEP varied between 17 and 38 mol [m.sup.-2] [year.sup.-1] of carbon at the grazed site and between 5 and 38 mol [m.sup.-2] [year.sup.-1] at the non-grazed site, depending on assumptions about canopy geometry. Soil respiration, 36 mol [m.sup.-2] [year.sup.-1], did not differ between sites. Canopy respiration was less (21 mol [m.sup.-2] [year.sup.-1]) at the grazed site than at the non-grazed site (28 mol [m.sup.-2] [year.sup.-1]), and accounted for 22-29% and 2841% of gross photosynthesis at grazed and non-grazed sites, respectively. The decrease in canopy respiration compensated the grazed canopy for losses to herbivores. 5 A canopy photosynthesis model was used to calculate a theoretical envelope of canopy biomass, termed the production window, inside of which NEP is positive and outside of which NEP is negative. The production window is bisected by the optimum canopy biomass, which is the theoretical biomass at which NEP is maximized. The seasonality of NEP is determined by the trajectory of actual canopy biomass through the production window. It was shown that canopy biomass on the grazed site was close to the theoretical optimum biomass late in the season, while the non-grazed canopy biomass was supraoptimal. Keywords: grazing, model, net ecosystem production, photosynthesis, respiration

This study identifies patterns in the gap disturbance regime along a successional gradient in the southern boreal forest and uses this information to investigate canopy composition changes. Gaps were characterized in hardwood, mixed-forest, and conifer stands surrounding Lake Duparquet in northwestern Quebec. From 39 to 80 gaps were evaluated along transects established in each of these stands. The abundance of gap makers and gap fillers and total regeneration was evaluated by species, as well as the size of each gap encountered along the transects. The percentage of the forest in canopy gap was calculated directly from the proportion of the transect in gap and by using gap area and line-intercept techniques. Changes in composition were evaluated from gap-maker and gap-filler distributions and by using transition matrices based on species mortality and regeneration in canopy gaps. The percentage of the forest in canopy gap ranges from 7.1% in a 50-yr-old forest dominated primarily by aspen to 40.4% in a 234-yr-old fir-dominated forest. Gap events are due to individual or small-group tree mortality in the early successional forest but become species-specific events controlled by spruce budworm outbreaks in the later stages of succession. Due to the high latitude, direct light only reaches the forest floor in the very largest gaps of the conifer-dominated stands. However, these gaps form slowly as budworm-caused mortality occurs over a number of years, whereas in aspen-dominated stands gaps are formed quickly by the snapping of tree stems. Balsam fir is the most abundant gap-filling species; however, its abundance is negatively correlated to gap size in all stand types. Markovian transition matrices suggest that in the young aspen-dominated forests small gaps lead to species replacement by more shade-tolerant conifers but that in the oldest forests the larger gaps will result in maintenance of the intolerant species and an increase in the abundance of cedar. Key words: balsam fir; boreal forest disturbance regimes; canopy composition; canopy gaps; canopy turnover times; cedar; gap characteristics along a successional gradient; gap size distributions; gap makers and gap fillers; regeneration of trees and forest; spruce budworm-caused gaps; trembling aspen., INTRODUCTION In forests where large-scale disturbances are rare, stand dynamics are controlled by the creation of gaps due to single or multiple overstory-tree mortality. It is within these openings that [...]

1 Aspects of epiphyte population dynamics were studied in a Mexican humid montane forest using repeated photographs of branches to follow the fate of 5124 individuals of 44 epiphyte species over 2 years. The diameter of the supporting branch and the size of the longest leaf of bromeliads were measured. 2 Bromeliads tended to grow on smaller, ferns and orchids on thicker, and Peperomia on intermediate branches. Although most plants showed a non-random distribution, there was much overlap. Small individuals of Tillandsia and Catopsis were more frequently found on small branches than were large plants. 3 The probability of falling with a branch within a year decreased with increasing branch size, from 28% for twigs < 1 cm in diameter to 0% for branches > 12cm. 4 In Tillandsia, mortality from factors other than branchfall declined from 0.33 for plants < 2 cm to about 0.06 for plants > 15 cm. If branchfall was excluded, mortality of Tillandsia was higher on thick than on thin branches, which may have been caused by the decreasing light level towards the centre of tree crowns. 5 Overall probabilities for all plants registered were 0.840 (survival), 0.082 (birth), 0.071 (mortality from branchfall) and 0.096 (mortality from other factors). This resulted in a negative population balance if branchfall was included, and an almost balanced one if branchfall was excluded. 6 The distribution of epiphytes among branches and trees and the rates of recruitment, growth and mortality of these subpopulations must be determined to understand the population dynamics. Keywords: canopy, disturbance, epiphyte, population dynamics. Tillandsia

Wavelike oscillations are a common form of air motion in the forest canopy at night. This paper investigates the canopy wave phenomenon using a two-dimensional inviscid linear wave model taking into account the drag force exerted on the wave wind components by plant elements and the plant-air heat exchange induced by temperature wave oscillations. The model appears to have adequately reproduced the salient features of a wave event in a boreal aspen forest. The wave dynamics are investigated as functions of parameters of the background states expressed in analytical form. It is shown that canopy waves are generated by wind shear near the treetops and share features of a Kelvin-Helmholtz disturbance. Because it is located close to the inflection point of the mean wind, the ground exerts a strong stabilizing effect on the wave motions, particularly in a sparse forest. The main role of the canopy drag in the wave dynamics is the creation of the inflection point; its damping effect on wave oscillations themselves is limited to disturbances of wavelengths shorter than that of the fastest growing waves. Wavelength, phase speed, and period of the fastest growing waves, those that are most likely to dominate observations, appear insensitive to static stability.

Surface roughness and canopy cover are important factors in preventing soil erosion. There is limited information on how soil surface roughness changes as a function of natural rainfall erosivity and canopy cover by plants. We hypothesized that canopy cover, tillage systems, and cumulative rainfall erosivity (CRE) would have unique effects on roughness. We tested this hypothesis on a Miami silt loam soil (fine-silty, mixed, mesic Typic Hapludalf) using a portable laser microtopographer. Tillage treatments of conventional (moldboard plowing + disking), chisel plowing, and chisel plowing + dragging a chain produced three initial roughness levels. Surface cover was none (fallow) or soybean [Glycine max (L.) Merr.]. Random roughness (RR), standard deviation (SD), tortuosity (T), and fractal roughness functions, expressed by the fractal index (D) and the crossover length (l), were calculated from microtopography data. Chisel tillage had the greatest initial values of surface roughness, followed by chisel + chain and conventional tillage, as measured by the l index. All indices but D generally decreased with CRE. The RR and SD indices decreased quadratically with CRE, with decreases of 38 and 36%, respectively, from initial values after 200 units of CRE, while the T and l indices decreased exponentially, with decreases of 40 and 60%, respectively, from initial values after 200 units of CRE. Soybean cover lowered soil surface roughness 7% less than fallow, as measured by the 1 index. The l index was 50, 71, and 205% more sensitive to changes in CRE than RR, SD, and T indices, respectively. The fractal roughness functions, with D and l indices calculated, were the best approaches to characterize surface roughness at small scales, such as existing plant rows, mainly due to l index sensitivity to changes in CRE.

ERS-1 Windscatterometer measurements over Siberia were investigated to determine the potential of coarse resolution radar data for land cover mapping and monitoring. Two indices have been introduced: the Slope Index calculates the incidence angle dependence of the radar measurements, the Radar Backscatter Index (RBSI) combines Slope Index and radar backscatter intensity. Using the RBSI, good correlations to canopy density were found and the freeze/thaw-transition can be determined. Promising correspondence was found with five-day interval time series of normalized radar cross sections and crop yield estimates. Index Terms - Boreal Forest, canopy density, ERS Windscatterometer, freeze/thaw cycle, Kazhakstan agriculture, radar vegetation monitoring, Siberia.

Open boreal forests present a challenge in understanding remote sensing signals acquired with various solar and view geometries. Much research is needed to improve our ability to model the bidirectional reflectance distribution (BRD) for retrieving the surface information using measurements at a few angles. The geometric-optical bidirectional reflectance model presented in this paper considers four scales of canopy architecture: tree groups, tree crowns, branches and shoots. It differs from the Li-Strahler's model in the following respects: 1) the assumption of random spatial distribution of trees is replaced by the Neyman distribution which is able to model the patchiness or clumpiness of a forest stand; 2) the multiple mutual shadowing effect between tree crowns is considered using a negative binomial and the Neyman distribution theory; 3) the effect of the sunlit background is modeled using a canopy gap size distribution function that affects the magnitude and width of the hotspot; 4) the branch architecture affecting the directional reflectance is simulated using a simple angular radiation penetration function; and 5) the tree crown surface is treated as a complex surface with microscale structures which themselves generate mutual shadows and a hotspot. All these scales of canopy architecture are shown to have effects on the directional distribution of the reflected radiance from conifer forests. The model results compare well with a data set from a boreal spruce forest.

In modeling forest canopies, several scattering mechanisms are taken into account. 1) volume scattering; 2) surface-volume interaction; 3) surface scattering from forest floor. Depending on the structural and dielectric characteristics of forest canopies, the relative contribution of each mechanism in the total backscatter signal of an imaging radar can vary. In this paper, two commonly used first-order discrete scattering models, distorted born approximation (DBA) and radiative transfer (RT) are used to simulate the backscattered power received by polarimetric radars at P-, L-, and C-bands over coniferous and deciduous forests. The difference between the two models resides on the coherent effect in the surface-volume interaction terms. To demonstrate this point, the models are first compared based on their underlying theoretical assumptions and then according to simulation results over coniferous and deciduous forests. It is shown that by using the same scattering functions for various components of trees (i.e., leaf, branch, stem), the radiative transfer and distorted Born models are equivalent, except in low frequencies, where surface-volume interaction terms may become important, and the coherent contribution may be significant. In this case, the difference between the two models can reach up to 3 dB in both co-polarized and cross-polarized channels, which can influence the performance of retrieval algorithms.

Estimates of direct and indirect light regime were derived from hemispherical canopy photographs for four Florida sand pine (Pinus clausa) scrub stands. Shrub foliage-height density and sand pine regeneration were tallied in 4 [m.sup.2] cells around photo points. We found negative correlations between shrub foliage-height density and understory light regime in all four stands. Sand pine regeneration was spatially associated with higher surface light levels and reduced shrub foliage-height density within only one of our four stands, a recently burned (early 1970s) peninsular site. We found more consistent evidence of shade intolerance in our two Ocala sand pine stands. Sand pine regeneration was abundant in high light patches within an open, recently burned site; however, in an adjacent 60-yr-old, closed-canopy stand, regeneration was lacking despite a local seed source. Our two Choctawhatchee sand pine stands were characterized by higher surface light regime values than the two peninsular stands and sand pine seedling growth rates were lower than in the one peninsular stand that had regeneration. The closed-canopy stand had a sparse shrub layer; regeneration was ample in this stand, but did not covary with light regime or shrub foliage-height density. The open-canopy stand was in a coastal dune setting with stunted canopy trees exceeding 100 yr of age. Sand pine regeneration in this stand was not sufficiently abundant to test for spatial association with light regime or shrub layer., INTRODUCTION Within its present range, sand pine [Pinus clausa (Chapm. ex. Engelm.) Vasey ex. Sarg] has been separated into two geographically disjunct varieties. Ocala sand pine (Pinus clausa var. clausa [...]

The effect of leaf aging on photosynthetic capacities was examined for upper canopy leaves of five tropical tree species in a seasonally dry forest in Panama. These species varied in mean leaf longevity between 174 and 315 d, and in maximum leaf life span between 304 and 679 d. The light-saturated C[O.sub.2] exchange rates of leaves produced during the primary annual leaf flush measured at 7-8 mo of age were 33-65% of the rates measured at 1-2 mo of age for species with leaf life span of < 1 yr. The negative regression slopes of photosynthetic capacity against leaf age were steeper for species with shorter maximum leaf longevity. In all species, regression slopes were less steep than the slopes predicted by assuming a linear decline toward the maximum leaf age (20-80% of the predicted decline rate). Maximum oxygen evolution rates and leaf nitrogen content declined faster with age for species with shorter leaf life spans. Statistical significance of regression slopes of oxygen evolution rates against leaf age was strongest on a leaf mass basis ([r.sup.2] = 0.49-0.87), followed by leaf nitrogen basis ([r.sup.2] = 0.48-0.77), and weakest on a leaf area basis ([r.sup.2] = 0.35-0.70). Key words: leaf age; leaf longevity; leaf nitrogen content; photosynthetic capacities; tropical canopy trees.

The cagelike, steel gondola lofted by a construction crane swings slowly next to the 30-meter-high crown of an Anacardium excelsum tree. Purple flowers from an immense network of vines cover [...]

Pinon-juniper woodlands in the semiarid western USA have expanded as much as fivefold during the last 150 yr, often accompanied by losses of understory vegetation and increasing soil erosion. We conducted this study to determine the differences in soil morphology between canopy and intercanopy locations within a pinon (Pinus edulis Engelm.)-juniper [Juniperus monosperma (Engelm.) Sarg.] woodland with uniform parent material, topography, and climate. The woodland studied, located near Los Alamos, NM, has a mean tree age of 135 yr. We examined soil morphology by augering 135 profiles in a square grid pattern and comparing soils under pinon and juniper canopies with intercanopy soils. Only two of the 17 morphological properties compared showed significant differences. The B horizons make up a slightly greater proportion of total profile thickness in intercanopy soils, and there are higher percentages of coarse fragments in the lower portions of canopy soil profiles. Canopy soils have lower mean pH and higher mean organic C than intercanopy soils. Regression analysis showed that most soil properties did not closely correspond with tree size, but total soil thickness and B horizon thickness are significantly greater under the largest pinon trees, and soil reaction is lower under the largest juniper trees. Our findings suggest that during the period in which pinon-juniper woodlands have been expanding, the trees have had only minor effects on soil morphology.

Photosynthesis, transpiration, and leaf area distribution were sampled in mature Quercus virginiana and Juniperus ashei trees to determine the impact of leaf position on canopy-level gas exchange, and how gas exchange patterns may affect the successful invasion of Quercus communities by J. ashei. Sampling was conducted monthly over a 2-yr period in 12 canopy locations (three canopy layers and four cardinal directions). Photosynthetic and transpiration rates of both species were greatest in the upper canopy and decreased with canopy depth. Leaf photosynthetic and transpiration rates were significantly higher for Q. virginiana (4.1-6.7 [[micro]mol] C[O.sub.2][multiplied by][m.sup.-2][multiplied by][s.sup.-1] and 1.1-2.1 mmol [H.sub.2]O[multiplied by][m.sup.-2][multiplied by][s.sup.-1]) than for J. ashei (2.1-2.8 [[micro]mol] C[O.sub.2][multiplied by][m.sup.-2][multiplied by][s.sup.-1] and 0.7-1.0 mmol [H.sub.2]O[multiplied by][m.sup.-2][multiplied by][s.sup.-1]) in every canopy level and direction. Leaves on the south and east sides of both species had higher gas exchange rates than leaves on the north and west sides. Although Quercus had a greater mean canopy diameter than Juniperus (31.3 vs. 27.7 [m.sup.2]), J. ashei had significantly greater leaf area (142 vs. 58 [m.sup.2]/tree). A simple model combining leaf area and gas exchange rates for different leaf positions demonstrated a significantly greater total canopy carbon dioxide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g C[O.sub.2][multiplied by][tree.sup.-1][multiplied by][d.sup.-1], respectively). Total daily water loss was also greater for Juniperus (125 vs. 73 L[multiplied by][tree.sup.-1][multiplied by][d.sup.-1]). Differences in leaf gas exchange rates were poor predictors of the relationship between the invasive J. ashei and the codominant Q. virginiana. Leaf area and leaf area distribution coupled with leaf gas exchange rates were necessary to demonstrate the higher overall competitive potential of J. ashei. Key words: Canopy-level gas exchange; Cupressaceae; Fagaceae; gas exchange; Juniperus ashei; leaf area; Quercus virginiana; semiarid savannah.

The growth and physiological characteristics of canopy trees are expected to differ systematically from those of understory trees on the basis of size-dependent aspects of biomechanics, resource availability, and life history. Although such differences have previously been noted, there has been relatively little effort to quantify these in terms of interspecific allometric relationships. Asymptotic maximal height ([H.sub.max]) is advocated as a measure of the size of dicotyledonous woody plants for this purpose. Height diameter (H-D) relationships in 38 species within six genera of Malaysian rain forest trees are well described by an asymptotic model, and thus provide a basis for estimating [H.sub.max] using static observational data. Three important aspects of tree growth strategies are shown to be predictable on the basis of these values: average tree growth rates are positively related to [H.sub.max], while wood density and the initial allometric slope of (species-specific) H-D relationships are negatively related to [H.sub.max]. These patterns may be explained by an association of low light levels with slow growth and high density wood in understory species; the latter property may in turn allow for relatively high allometric slopes of H-D relationships in saplings of small-statured species. Analyses that control for phylogenetic differences provide evidence that such interspecific allometric patterns are the product of convergent evolution. These results are consistent with the idea that much ecological variation within species-rich taxa of southeast Asian rain forest trees is related to differentiation along a vertical axis of tree size. Key words: allometry; asymptotic height; canopy trees; Malaysia; rain forest trees.

1 We studied stratification of tree species by analysing the allometry and demography of 14 abundant species in a warm-temperate rain forest on Yakushima Island, southern Japan. 2 We regressed relationships between trunk diameter at breast height (d.b.h.) and top height of trees using an expanded allometry which incorporates an upper limit to height. As trees grew up towards the canopy, individuals with the same d.b.h. were taller if they came from species with larger maximum height. This appeared to reflect a trade-off between height growth and lateral growth of crowns. Two canopy dominants continued to gain in d.b.h. after they had reached the canopy, while their height remained almost constant. 3 We examined size-specific patterns of relative growth rate of d.b.h. from a decade of field censuses. By fitting a Gompertz-type growth curve to size-specific growth of individual trees, we derived a projected potential maximum size for each species. The projected maximum size (both d.b.h. and height) was similar to the observed value for most species. Three canopy species exhibited larger projected maximum d.b.h. than observed values; the difference seemed to be due to mortality during the long period that each of these species resides in the canopy. 4 Understorey species recruited equally well as larger-growing species amongst the 14 studied species, although their tree mass increments per unit time were smaller. This suggests that understorey species have more effective recruiting capacity relative to vegetative growth. Keywords: architecture, diameter-height curve, maximum size, mortality, recruitment rate, size growth rate, tree community

1 Tree-falls and branch-falls of all trees with a diameter at breast height (d.b.h.) of over 10 cm were studied in 12 ha of primary rain forest at Nouragues, French Guiana. Consequences in terms of the number of damaged trees and number and sizes of resulting canopy gaps were investigated. Spatial distribution of tree-falls and spatial relation with pre-existing canopy gaps were analysed. Forest turnover rates were calculated on different bases. 2 Of the 6314 trees initially present in the plot 1.5% had fallen and 1.3% lost at least one major branch after one year. In total, 4.9% of the standing basal area had been felled or damaged. 3 The major cause of tree-fall and tree damage is falling trees and limbs: 75% of the fallen and 83% of the damaged trees were felled or damaged by others ('domino-wise' felled or damaged trees). Of the remaining ('initiating') fallen trees, 38% were uprooted and 62% were snapped-off above the ground. Similar percentages were observed for domino-wise felled trees. 4 The spatial distribution of the 38 trees which initiated events was random. Pre-existing canopy gaps did not increase the chances of surrounding trees to initiate a tree- or branch-fall. However, initiating tree- and branch-falls were found clustered at around 50 m from large gaps, while few gaps occurred at around 25 m from old gaps. 5 Some 35% of the fallen trees did not create canopy gaps, and 42% of the damaged trees were not involved in the creation of a canopy gap. Initiating tree-fall that resulted in a canopy gap had no larger d.b.h. than initiating tree-fall not resulting in a canopy gap. In contrast, gap-creating branch-fall trees had a larger d.b.h. than those that did not create gaps. 6 Gap size increased with the number and basal area of the trees involved. Gap size measured following the Brokaw (1982a) definition was not very relevant in quantifying the direct consequences of gap creation, and a modified gap definition is proposed. 7 Both the forest turnover rate based on the number of fallen trees and the turnover rate based on the area of canopy gaps underestimate the actual disturbance regime of the forest. To quantify the disturbance regime of a forest, the basal area of the annually fallen and damaged trees deserves more attention. Keywords: branch-falls, canopy gaps, forest dynamics, French Guiana, Monte Carlo test, spatial pattern, tree-falls, tropical rain forest, turnover rate

Overhead, at a height, perhaps of a hundred feet, is an almost unbroken canopy of foliage formed by the meeting together of these great trees and their interlacing branches; and this canopy is usually so dense that but an indistinct glimmer of the sky is to be seen, and even the intense tropical sunlight only penetrates to the ground subdued and broken up into scattered fragments . . . it is a world in which man seems an intruder, and where he feels overwhelmed . . .

Aster macrophyllus, a temperate forest understory species of the northeastern United States, inhabits a broad range of light habitats. Plants receiving several minutes of direct san in canopy gap and forest edge habitats occasionally wilt, a response indicative of water stress. We compared two alterative scenarios for patterns of evaporative load and stomatal conductance for plants in large (0.15 ha) tree canopy gaps and small (3 [m.sup.2]) herbaceous subcanopy gaps: 1) evaporative loads are typically moderate and stomatal conductance is largely governed by light intensity; or 2) evaporative loads are often substantial, mandating stomatal closure to prevent excessive transpiration. In all eases evaporative loads were elevated by light intensity above 25% of full sun. This was accompanied by substantial stomatal closure. Transitions from low to moderate light intensity (< 13% full sun) caused little increase in leaf evaporative load, and stimulated increases in stomatal conductante. Very brief periods of high light also stimulated stomatal opening. Light environments in the small herbaceous subcanopy gaps differ greatly in their patterns of evaporative load from day to day.

A study of the methods used to detect changes in the forest canopy in north central Minnesota shows that digital monitoring is able to detect changes in the forest stands and pixel based change information provides useful sub-stand information about canopy changes. A stand is a piece of land in which conditions do not change with respect to specific management objectives. The change maps formed by Landsat Thematic Mapper show that the variations in the data obtained from digital change classification are not errors, but reflect sub-stand information that can either be incorporated into the study or corrected for.

Yet another continent of life remains to be discovered, not upon the earth, but one to two hundred feet above it. . . . There awaits a rich harvest for [...]

A study of the canopy influence on rainfall fields' microscale structure conducted in the tropical forests of French Guyana using a network of rain gauges reveals that microscale rainfall is continuous over the open deforested meadow region, while the rainfall field under the canopy of the forested area is unstructured. The random rainfall distribution in the forested area is due to the vegetation there, but if the canopy retention capacity is saturated, the throughfall field will become continuous.