Gap Formation Rate Research Articles

Out of steady state: Tracking canopy gap dynamics across Brazilian Amazon

AbstractCanopy gaps are evidence of disturbances on forest landscapes. A forest stand is in constant flux, with long stretches of biomass accumulation punctuated by episodic disturbances. We used multitemporal airborne laser scanning data to compare the gap dynamics of four Amazon forest sites. We assessed gap dynamics over 1.9–3.8 years between 2017 and 2020 at sites in the central, central eastern, southeastern, and northeastern regions of the Brazilian Amazon, over areas ranging from 590 to 1205 ha at each site. Gap size ranged from a minimum of 10 m2 to a maximum of about 10,000 m2. We analyzed four stages of gap dynamics: formation, expansion, persistence, and recovery based on two consecutive airborne laser scanning surveys. The gap fraction at our study sites varied between 1.26% and 7.84%. All the sites have similar proportion of gaps among gap size classes. What notably differed among sites was not the gap size‐distribution, but the relative importance of stages of gap dynamics. Expansion and persistence rates ranged from 12 to 118 m2 ha−1. The gap formation rate (formation + expansion) was lower than the recovery rate for three of the four study sites. In contrast, the southeastern site has 1.44 times more area in formation and expansion compared to gap recovery. Over the 2–4 years interval of our study, no site was close to steady state. Multitemporal analyses of large areas over many years are needed to improve our understanding of tropical forest dynamics.

Influence of gap-scale disturbance on developmental and successional pathways in Quercus-Pinus stands

Quercus-Pinus forests of the eastern USA cover millions of hectares and span a variety of ecoregions. Understanding the influence of natural disturbance on developmental and successional pathways is important for managers that wish to sustain Pinus spp. in these mixtures. Quantifying developmental and successional patterns in this forest type can help assess the need to actively manage natural processes and inform silvicultural prescriptions to achieve management goals. Little research has been conducted on natural, gap-scale disturbance processes in Quercus stands with strong components of Pinus taeda, Pinus virginiana, and Pinus echinata. We examined 60 canopy gaps in a Quercus-Pinus forest on the southern Cumberland Plateau in Alabama to document gap formation, closure, and other characteristics and to analyze the influence of localized disturbance on development and succession. The majority of gapmaker trees (56%) were Pinus individuals and 44% were hardwoods. Most gaps (58%) closed by height growth of subcanopy trees. The majority of these gap filler taxa were hardwoods: Quercus (39%), Carya (14%), Nyssa sylvatica (12%), and other hardwoods (15%), with Pinus representing 14%. The number of Pinus gapmakers and the number of gaps projected to fill by subcanopy recruitment of hardwoods indicated the forest was in the latter stages of a composition shift from Pinus to a much stronger Quercus component. Significant positive relationships existed between gap size and sapling diversity (r2=0.15, P=0.002), tree diversity (r2=0.21, P=0.0002), and total stem diversity (r2=0.29, P<0.0001) indicating a positive relationship may exist between gap size and diversity on xeric ridge tops where shade-tolerant species are less competitive. We speculated the ridge top positions contributed to the relatively high gap formation rates noted in this study. Pinus composition was found to be patchy, indicating a gap-based approach may be used to manage for Pinus recruitment in hardwood dominated systems.

Can the life-history strategy explain the success of the exotic trees Ailanthus altissima and Robinia pseudoacacia in Iberian floodplain forests?

Ailanthus altissima and Robina pseudoacacia are two successful invasive species of floodplains in central Spain. We aim to explain their success as invaders in this habitat by exploring their phenological pattern, vegetative and sexual reproductive growth, and allometric relations, comparing them with those of the dominant native tree Populus alba. During a full annual cycle we follow the timing of vegetative growth, flowering, fruit set, leaf abscission and fruit dispersal. Growth was assessed by harvesting two-year old branches at the peaks of vegetative, flower and fruit production and expressing the mass of current-year leaves, stems, inflorescences and infrutescences per unit of previous-year stem mass. Secondary growth was assessed as the increment of trunk basal area per previous-year basal area. A. altissima and R. pseudoacacia showed reproductive traits (late flowering phenology, insect pollination, late and long fruit set period, larger seeds) different from P. alba and other native trees, which may help them to occupy an empty reproductive niche and benefit from a reduced competition for the resources required by reproductive growth. The larger seeds of the invaders may make them less dependent on gaps for seedling establishment. If so, these invaders may benefit from the reduced gap formation rate of flood-regulated rivers of the study region. The two invasive species showed higher gross production than the native, due to the higher size of pre-existing stems rather than to a faster relative growth rate. The latter was only higher in A. altissima for stems, and in R. pseudoacacia for reproductive organs. A. altissima and R. pseudoacacia showed the lowest and highest reproductive/vegetative mass ratio, respectively. Therefore, A. altissima may outcompete native P. alba trees thanks to a high potential to overtop coexisting plants whereas R. pseudoacacia may do so by means of a higher investment in sexual reproduction.

USING GAPS INN-BODY TIDAL STREAMS TO PROBE MISSING SATELLITES

We use N-body simulations to model the tidal disruption of a star cluster in a Milky-Way-sized dark matter halo, which results in a narrow stream comparable to (but slightly wider than) Pal-5 or GD-1. The mean Galactic dark matter halo is modeled by a spherical Navarro-Frenk-White (NFW) potential with subhalos predicted by the LCDM cosmological model. The distribution and mass function of the subhalos follow the results from the Aquarius simulation. We use a matched filter approach to look for "gaps" in tidal streams at 12 length scales from 0.1 kpc to 5 kpc, which appear as characteristic dips in the linear densities along the streams. We find that, in addition to the subhalos' perturbations, the epicyclic overdensities (EOs) due to the coherent epicyclic motions of particles in a stream also produce gap-like signals near the progenitor. We measure the gap spectra - the gap formation rates as functions of gap length - due to both subhalo perturbations and EOs, which have not been accounted for together by previous studies. Finally, we project the simulated streams onto the sky to investigate issues when interpreting gap spectra in observations. In particular, we find that gap spectra from low signal-to-noise observations can be biased by the orbital phase of the stream. This indicates that the study of stream gaps will benefit greatly from high-quality data from future missions.

Quantification of repeated gap formation events and their spatial patterns in three types of old-growth forests: Analysis of long-term canopy dynamics using aerial photographs and digital surface models

Treefall gaps are important elements of forest dynamics, as they contribute substantially to regeneration and forest turnover rates. Repeated gap formation, i.e. the formation of new gaps in areas where gaps have previously occurred then closed within decadal timeframes, is also believed to occur frequently and to contribute significantly. However, most gap studies have focused on the relationships among gap size, associated light levels and regeneration processes. In contrast, little reliable, quantitative information on the frequency of repeated gap formation events and their spatial patterns is available. To quantify repeated gap formation events in old-growth forests, we re-analyzed our previously published data on long-term canopy dynamics in three forest types (warm-temperate, cool-temperate and subalpine coniferous forests) in Japan, using digital surface models developed from aerial photographs taken in different years. Estimated repeated gap formation rates were 2.6%/32years (0.081%/year), 3.8%/43years (0.088%/year) and 8.3%/43years (0.193%/year) in the warm-temperate, cool-temperate and subalpine forest, respectively. The size distributions of repeated gap areas were skewed towards small areas, did not differ among the three forest types and closely fitted a power-law function. The mean repeated gap area also did not differ among forest types. The repeated gap areas were generally distributed along the edges of old gaps in the warm-temperate and cool-temperate broadleaved forests, according to a torus-translation test. Within the old gaps, continuous and/or rapid height growth was detected in sites with repeated gap events, and dramatic changes in canopy height were observed in sites adjacent to the edges of old gaps, which could have been due to the lateral expansion of branches and their subsequent destruction. These analyses revealed that the temporal and spatial canopy dynamics of the studied forest communities were very diverse and more dynamic than previously reported. Hence, future analyses of data obtained from Large-scale and Long-term Monitoring sites (LLMs) established in various types of forest communities around the world should pay attention to repeated gap formation events and their spatial patterns.

Gap dynamics in a near-natural spruce forest at Mt. Brocken, Germany

We analyzed indices of gap dynamics, i.e. gap formation and gap closure rates as well as turnover rates, in a montane conifer forest in the temperate zone and asked: (1) Are the characteristics of gap dynamics comparable with those of other temperate forests? (2) Do gap dynamics exhibit temporal trends, and (3) if so, are they related to recent climate change? We also addressed methodological issues concerning the differences observed at differing temporal and spatial scales of the used datasets. Thus, we asked further how the indices of gap dynamics are influenced (4) by differences in the length of observation periods and (5) by differences in gap size categories included in the study?The study area was located in the near-natural Picea abies forests in the core zone of the Harz National Park on the north-eastern slopes of Mt. Brocken, Germany. From this area, aerial photographs from the last 60yrs (1945, 1991, 2000, 2003) were analyzed by GIS techniques. Total gap area, number of gaps, gap size and area-based turnover rate, rotation time, gap formation and gap closure rate were calculated and correlated with climate variables.The total gap area varied between 10.9ha and 19.0ha for the study area of 225.2ha between the different years. Similarly, the median of the gap size was 155.6m2 and 87.0m2 in 1945 and 2003, respectively. A rotation time of 228yrs was calculated for the study area. Area-based turnover rates and gap formation rates increased with time, while mean gap size decreased. The most frequently recorded gaps were those in the category <100m2. In particular, small gaps at the level of single trees (≤40m2) became more frequent in the last decade and showed a higher gap formation rate than larger gaps. Temperature was significantly related to both turnover and gap closure rates.In conclusion, for the near-natural spruce stands at Mt. Brocken indices of gap dynamics were of a magnitude comparable to those described from other forest types in the temperate zone. However, the indices of gap dynamics increased with time, which coincided with increasing temperatures over the last few decades.

Identification of gaps in mangrove forests with airborne LIDAR

Mangrove forests change frequently due to disturbances from tropical storms, frost, lightning, and insects. It has been suggested that the death and regeneration of trees in small gaps due to lightning may play a critical role in mangrove forest turnover; however, the large-scale quantification of spatial pattern and areas of gaps is lacking for investigating this issue. Airborne light detection and ranging (LIDAR) technology provides an effective way for identifying gaps by remotely obtaining direct measurements of ground and canopy elevations. A method based on an alternative sequential filter and black top-hat mathematical morphological transformation was developed to extract gap features. Comparison of identified gap polygons with raw LIDAR measurements and field surveys shows that the proposed method successfully extracted gap features in mangrove forests in Everglades National Park. There are 400–500 lightning gaps per square kilometer in mangrove forests at the study sites. The distribution of gap sizes follows an exponential form and the area of gaps with sizes larger than 100 m2 account for 55–61% of the total area of gaps. The area of gaps in the mangrove forest in Everglades National Park is about 4–5% of the total forest area and the average gap formation rate is about 0.3% of the total forest area per year, indicating that lightning gaps play an important role in mangrove forest dynamics.

Long-term canopy dynamics analyzed by aerial photographs and digital elevation data in a subalpine old-growth coniferous forest

:We analyzed the long-term canopy dynamics of a 2-ha permanent plot in subalpine old-growth coniferous forest over 43 y using digital surface models (DSMs) and a digital elevation model (DEM). The models contoured canopy and ground surface elevation, respectively. Abies mariesii, A. veitchii, Betula ermanii, Picea jezoensis var. hondoensis, and Tsuga diversifolia were the main species in the forest canopy. Canopy surface DSMs (2.5- × 2.5-m grids) were constructed of the area including the 2-ha plot using aerial photographs from 1959, 1969, 1979, 1989, and 2002, and a DEM was constructed from ground survey data collected in 1991. Canopy height profiles were obtained by calculating the difference between the canopy and the ground surface, and the status of the forest in each grid cell for each year was classified as gap or closed canopy, depending on whether the canopy height was ≤ 25 m or > 25 m, respectively. Tree census data were collected in 2000. The threshold value was decided by comparing the gap from digital elevation data with the result of the field survey. The total gap area in 1959 was greater than 1 ha, indicating that some disturbances had occurred in this plot, probably related to the Isewan Typhoon. A large change occurred during 1969–1989, when mean canopy closure rates were significantly higher than mean gap formation rates. Abies mariesii and B. ermanii tended to occur in the canopy layer in grid cells that contained gaps in 1959 and closed canopy in 2002. The presence of A. veitchii in the canopy layer was also associated with the change from gap to closed canopy, although not significantly so. These results suggest that Abies spp. regenerate more effectively than the other species by establishing seedling or sapling banks before gap formation. Large-scale disturbances, such as the Isewan Typhoon, do not favour the regeneration of spruce over subalpine fir, and species other than spruce are responsible for recovery following such disturbances, according to our analyses of long-term canopy dynamics.

Long‐term canopy dynamics in a large area of temperate old‐growth beech (Fagus crenata) forest: analysis by aerial photographs and digital elevation models

Summary Long‐term canopy dynamics in a large area of temperate old‐growth beech forest in the Daisen Forest Reserve, south‐western Japan (11.56 ha studied over 43 years), were investigated using digital elevation models (DEMs) of the canopy surface, constructed from aerial photographs taken in the growing season (i.e. with foliage) in 1958, 1978, 1992 and 2001. A ground surface DEM at the same resolution (a 2.5 × 2.5 m grid) was constructed using aerial photographs taken when foliage was absent (winter 2002). Canopy height data were obtained by calculating differences in elevation between the canopy and the ground surface, and a canopy height profile was constructed. Topographic data for a 4‐ha plot, located within the 11.56‐ha area, were obtained via a ground survey and used to validate the ground surface DEM derived from aerial photographs. Canopy height class distributions changed significantly over the 43 years. The total number of gaps, defined as areas where canopy height was ≤ 15 m, decreased but total gap area increased over time. Total gap area in 2001 was twice that of 1958. The density of gaps decreased as gap size increased. Gap formation rates increased from 0.47% year−1 (1958–78) to 1.30% year−1 (1992–2001), with a mean of 0.77% year−1, and substantially exceeded closure rates, which fluctuated from 0.28% year−1 (1958–78) to 0.54% year−1 (1978–92), with a mean of 0.39% year−1. Gaps generally expanded and became connected to each other. Temporal variation in gap formation and closure might be correlated with the frequency and severity of typhoon disturbances but, if the observed trends continue, this old‐growth beech stand may become an open stand. The long‐term dynamics of this forest type appear to be far from equilibrium.

Forest gap dynamics and the Ising model

The vegetation height in forest ecosystems is spatially heterogeneous. Canopy gaps (sites with low vegetation) are formed by treefalls, and they recover to canopy sites (with high vegetation) either by growth of small trees or by branch extension of surrounding trees. The dynamics of canopy gaps have been studied using a spatial Markov chain with nearest neighbor interaction. (1) If the canopy recovery rate is constant and if the gap formation rate for a site increases exponentially with the number of neighboring gap sites, the equilibrium distribution is the same as the one generated by the Ising model in statistical mechanics. Here, we extend the equivalence to the situation in which both the gap formation and canopy recovery depend on the neighborhood, as shown in recent forest data. (2) We develop a statistical test of whether a given spatial pattern is a random sample from the Ising model. The test is based on the conditional probability of configurations on a partial lattice. We apply the method to vegetation height data from the Ogawa forest reserve, Japan, measured on a 5 × 5 m grid in 1976, 1981, 1986, and 1991. The spatial pattern of the original forest data deviates significantly from the Ising model. We examine whether a larger sampling distance or the removal of the effects of the topography can reduce this deviation.

Gap Formation by Planets in Turbulent Protostellar Disks

The processes of planet formation and migration depend intimately on the interaction between planetesimals and the gaseous disks in which they form. The formation of gaps in the disk can severely limit the mass of the planet and its migration toward the protostar. We investigate the process of gap formation through magnetohydrodynamic simulations in which internal stress arises self-consistently from turbulence generated by the magnetorotational instability. The simulations investigate three different planetary masses and two disk temperatures to bracket the tidal (thermal) and viscous gap opening conditions. The results are in general qualitative agreement with previous simulations of gap formation but show significant differences. In the presence of MHD turbulence, the gaps produced are shallower and asymmetrically wider than those produced with pure hydrodynamics. The rate of gap formation is also slowed, with accretion occurring across the developing gap. Viscous hydrodynamics does not adequately describe the evolution, however, because planets capable of producing gaps also may be capable of affecting the level of MHD turbulence in different regions of the disk.

Gap dynamics in Allegheny northern hardwood forests in the presence of beech bark disease and gypsy moth disturbances

To determine changes in gap dynamics over a 6-year period in forests affected by beech bark disease and gypsy moth infestations, we conducted line transect surveys of gaps and gap makers in three Allegheny hardwood forests in central New York. Gap makers were defined as any trees whose death or loss of >50% of the branches caused an opening in the canopy, and were divided into three categories: decline (standing live trees with >50% branch loss), standing dead, and treefall. Total land area in gaps increased significantly ( p≤0.00) from 19.7 to 31.9% over the 6-years of the study. American beech ( Fagus grandifolia Ehrh.) represented 52.4% of the gap makers in 1990 and 1996. Of the total beech gap makers, 30% were decline and 53% were treefall gap makers in 1990, but by 1996, only 8% were decline as compared to 76% treefall gap makers. In contrast, the majority of oak ( Quercus spp.) gap makers were standing dead in 1990 and 1996. Although the rate of gap formation slowed at the end of this study, the predominance of beech in the subcanopy and regeneration layers poses persistent challenges for forest managers.

Dynamic steady state of patch‐mosaic tree size structure of a mixed dipterocarp forest regulated by local crowding

A patch age‐ and tree size‐structured simulator was applied to demonstrate the landscape dynamics of a lowland mixed dipterocarp forest, using census data over a 3 year interval from two 1 ha plots in northern West Kalimantan, Indonesia (Western Borneo). Tree growth rate and recruitment rate were estimated as functions of tree size and local crowding. The effect of local crowding was assumed to be one‐sided through light competition, where the basal area for all trees larger than a target tree inside the circle of 10 m radius around the target was employed as the index of crowding. Estimated parameters were similar between the two plots. Tree mortality was expressed by descending function of tree size with asymptotic mortality for large trees corresponding to the gap formation rate. One parameter specifying the survival of trees at gap formation, which was required for the landscape‐level simulation of a shifting‐gap mosaic, was left undetermined from plot census data. Through simulation, this parameter was estimated so as to best fit the observed among‐patch variation in terms of local basal area. The overall time course of simulation and tree size structure were not sensitive to this parameter, suggesting that one‐sided competition along the vertical forest profile is a stronger determinant of average forest structure than among‐patch horizontal heterogeneity in this forest. Simulated dynamic steady state successfully reproduced the observed forest architecture in the gap‐dynamic landscape. It took about 400 years for a vacant landscape to be replaced by a steady‐state architecture of forest. Sensitivity analysis suggests that steady‐state basal area and biomass are most sensitive to changing gap formation rate and intrinsic size growth rate.

Variation in canopy gap formation among developmental stages of northern hardwood stands

Trends in gap dynamics among pole, mature, and old-growth northern hardwood stands were investigated on eight sites in the Porcupine Mountains of western upper Michigan. Recent gaps (created between 1981 and 1992) were identified using permanent plot records of tree mortality, while older gaps (1940–1981) were identified using stand reconstruction techniques. Although canopy gaps were somewhat more numerous in pole and mature stands, gaps were &lt;25% as large as those in old-growth stands because of smaller gap-maker size, and the proportion of stand area turned over in gaps was only about half as large. Gap makers in younger stands generally had mean relative diameters (ratio of gap-maker DBH to mean DBH of canopy trees) &lt;1.0 and were disproportionately from minor species such as eastern hophornbeam (Ostryavirginiana (Mill.) K. Koch). Gap makers in old-growth stands had mean relative diameters &gt;1.5 and were predominantly from the dominant canopy species. Even in old-growth forests, most gaps were small (mean 44 m2) and created by single trees. Based on the identity of the tallest gap tree in each gap, nearly all shade-tolerant and midtolerant species have been successful in capturing gaps, but gap capture rates for some species were significantly different from their relative density in the upper canopy. The tallest gap trees of shade-tolerant species were often formerly overtopped trees, averaging more than 60% of the mean canopy height and having mean ages of 65–149 years. Canopy turnover times, based on gap formation rates over a 50-year period, were estimated to average 128 years for old-growth stands dominated by sugar maple (Acersaccharum Marsh.) and 192 years for old-growth stands dominated by hemlock (Tsugacanadensis (L.) Carrière). While these estimates of turnover time are substantially shorter than maximum tree ages observed on these sites, they agree closely with independent data on mean canopy residence time for trees that die at the average gap-maker size of 51 cm DBH. The data support previous hypothetical explanations of the apparent discrepancy between canopy turnover times of &lt;130 years for hardwood species and the frequent occurrence of trees exceeding 250 years of age.

Canopy Gap Characteristics and Drought Influences in Oak Forests of the Coweeta Basin

Canopy gaps in southern Appalachian mixed—Quercus forest were characterized to assess the impact of the 1985—1988 record drought on patterns of tree mortality in relation to topographic variables and changes in overstory composition. Using permanent transects, we samples 68 canopy gaps within the Coweeta Basin. Among 1—5 yr old gaps, the most common gap type was the 1—yr—old single—tree snag, accounting for 49% of all gaps sampled; 65% of all gaps occurred within 2 yr of the drought peak. Gap area ranged from 40 to 850 m2, averaged 239 m2, with a median of 152 m2. Analysis of 1988 color infrared (IR) aerial imagery yielded a gap formation rate of 0.8 gaps°ha—1°yr—1 in mixed—Quercus stands affecting 2.0% of the area in this forest type in 1988. The most frequent gap—forming species were Quercus coccinea, Q. rubra, and Q. velutina, respectively. Evidence suggests that moisture stress brought on by severe drought increases the susceptibility of Quercus species of the shoe—string fungus Armillaria mellea, which may result in significant shifts in canopy composition in these forests.

Community dynamics of Ogawa Forest Reserve, a species rich deciduous forest, central Japan

Forest community dynamics were studied for 4 years in a 6 ha permanent plot of species rich, old-growth, temperate deciduous forest in Ogawa Forest Reserve, central Japan. The gap formation rate, recruitment, mortality, gain and loss rate in basal area during 4 years were 42 m2 ha−1 yr−1, 1.74% yr−1, 1.19% yr−1, 1.12% yr−1 and 0.88% yr−1, respectively. The turnover time calculated from them ranged from 58 to 240 years. Both the mortality and mortality factors were size dependent; trees in middle size class had smallest mortality, and the proportion of the trees killed by disturbances increased with size. Gap creations were concentrated in a particular year, suggesting a large heterogeneity in time. Spatial distribution of recruited trees were biassed to the old gaps (older than 4 years), especially that of the species with Bell-shaped dbh distribution (shade intolerant) strongly associated with the gaps. Recruitment in tree stems and the loss of basal area, thus had the larger variability than mortality of stems and this forest, and the species with L-shaped dbh distribution seemed to going to increase the importance in the future if the present trend continues to be held. The turnover time of population is positively correlated with the maximum dbh size of the species, indicating the slow change of the population of large sized species.

Population dynamics of coniferous and broad‐leaved trees in a Japanese temperate mixed forest

AbstractThe growth and survival of coniferous and broad‐leaved trees were followed over a 5‐yr period in a temperate old‐growth mixed forest in Japan, and dynamic features of the forest were studied in relation to the life history of the dominants, the coniferous Abies homolepis and the broad‐leaved Fagus crenata. During this period, the gap formation rate was 31m2 ha−1yr−1, the mortality of trees &gt; 2m high was 1.7%/yr, and the rate of loss in basal area 1.4%/yr. These values were much higher than the recruitment, 0.3%/yr, and the total growth of surviving and new trees, 0.6%/yr, owing to the inhibition of regeneration by understorey dwarf bamboo (Sasa borealis).A transition matrix model based on DBH size classes predicts that the basal area of the forest will decrease by 14% in 50 yr, but that the DBH distribution of trees &gt; 10 cm diameter will change little. Equilibrium DBH distributions assuming recruitment being equal to mortality, were quite different between broad‐leaved and coniferous trees, reflecting different survivorship curves of the two dominants.The composition and structure of the forest may change depending on the pattern and frequency of disturbances, or episodic events, notably the synchronous death of Sasa borealis.

Canopy gaps in Douglas-fir forests of the Cascade Mountains

Types and rates of mortality were measured and canopy gap formation rates were estimated from 5- to 15-year records of mortality in 34 permanent plots in mature (100- to 150-year-old) and old-growth (&gt;200-year-old) Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco)/western hemlock (Tsugacanadensis (Raf.) Sarg.) forests in western Oregon and Washington. Gap surveys were conducted in a mature and an old-growth stand, and characteristics of 40 gaps and regeneration were measured. Most canopy trees died without disrupting the forest in both mature (87.6%) and old-growth stands (73.3%). The amount of forest area per year representing new gaps was 0.7% in mature stands and 0.2% in old-growth stands. The gap survey found a higher proportion of gaps in the mature stand than in the old-growth stand. Most regeneration (&gt; 1 m tall) in gaps was western hemlock; Douglas-fir regeneration did not occur. The ratio of seedling density in gaps to density under canopies was about 3 for the mature stand and about 9 for the old-growth stand. Seedling density was correlated with measures of gap age but not gap size. The study suggests that gap disturbances and vegetative responses are important processes in the dynamics of these forests. However, gap formation rates and vegetative responses appear to be slow relative to other forest types. In addition to gap size, canopy structure and disturbance severity are important determinants of gap response.

Synchrony of Regeneration, Gaps, and Latitudinal Differences in Tree Species Diversity

Treefall gaps are necessary for establishment and/or growth to maturity of many forest trees. It therefore seems reasonable to hypothesize that characteristics of gaps influence both persistence of populations and coexistence of species in forests. The purpose of this essay is to suggest one way that gap characteristics might result in differences in tree species diversity between tropical and temperate zone forests. Latitudinal gradients in species diversity seem unlikely to be a result of differences in the physical or distributional characteristics of gaps in different forest types. Gap formation rates (fraction of ground area converted to new gaps per year or fraction of canopy trees or canopy area dying per year) are similar for a variety of forests in tropical (Brokaw 1985a, Denslow 1987) and temperate (Runkle 1985) zones. Gap size varies widely within each region, but average sizes and the range of sizes are similar (Runkle 1982, 1985, Denslow 1987). In both regions gaps occur most years and throughout the year, although they may be more common some months than others (Brokaw 1982b, Romme and Martin 1982). Latitudinal differences might exist in the ability of species to respond to gap conditions. That is, two gaps with the same size, shape, and mechanism of formation represent different regeneration opportunities in temperate zone and tropical forests. One mechanism, discussed by Poulson and Platt (1989), is that within-gap heterogeneity in light levels, especially in large gaps, will be greater in temperate than tropical zones, resulting in greater potential opportunities for withingap specialization for different light levels in temperate than tropical forests. I argue that winter imposes a synchronization of responses that reduces the potential diversity of species possible in temperate zone forests. I will discuss this relationship using the model of Chesson and Warner (1981), Warner and Chesson (1985), and Chesson (1986). The storage effect model, as presented in the above papers, may be described as follows. Species can persist in a community if their adults are long lived and if their reproductive success is such that population increases in scattered good years offset declines suffered in more frequent bad years. The regenerative consequences of good years are stored in long-lived individuals that survive through many years with little successful reproduction. A low-density species must experience good years frequently enough that extinction does not occur. High variance in reproduction over time, as a result of environmental variability or intrinsic factors, increases the chance that favorable reproductive periods will be encountered, at least occasionally, by rare species. The model also assumes that species at low densities can increase greatly in good years but decline only slightly in bad years, whereas abundant species can decline to a greater extent in any given year. Tropical forests are characterized by a broad interplay among seasonal variations in gap characteristics and climate. Thus arrival in gaps (before or soon after formation) as well as survival and growth to maturity (see Martinez-Ramos et al. 1989, Schupp et al. 1989) have to be evaluated relative to seasonal phenology of tree species. In tropical forests, seasonal variation occurs in arrival of seeds at a site. The seasonal rhythm of both fruitfall and seed germination has been well documented (Foster 1982, Garwood 1983). Differences among species in these properties could create some of the variability predicted by the storage effect model to result in enhanced species diversity. Being present on a site before or soon after a gap is created results in a large advantage over later arrivals. Growth rates are higher and mortality lower for plants establishing before or soon after treefall (Denslow 1987). Uhl et al. (1988) found that 4 yr after gap formation, advance regeneration accounted for 97% of all trees 1 m tall in five artificial single-treefall gaps and 83% of all trees in a multiple-treefall gap in Venezuela. Brokaw (198 5b, 1987) found that successful stems all were established prior to or within 1-3 yr following gap formation. Despite the importance of new seed dispersal into gaps and the production by pioneer species of lightrequiring seeds (Whitmore 1989), the great majority of tropical rain forest woody species are present to some extent in the understory before gaps form (Hubbell and Foster 1986a, b, Brokaw and Scheiner 1989). Although such advance regeneration is present contin-

A methodology for estimating canopy disturbance frequency and intensity in dense temperate forests

Analysis of the frequency of past moderate and high-intensity disturbances has been hindered in forests of complex age structure by methodological problems. A methodology is proposed for developing a disturbance chronology in such stands by identifying the probable date of canopy accession for each sample tree. Canopy accession dates are based on an evaluation of radial growth pattern and early growth rates of existing canopy trees. Canopy disturbance intensity is defined as the percentage of sample trees with canopy accession events in each decade. Rotation periods for disturbances of various intensities are calculated from the chronology. The method was evaluated using 893 increment cores from 70 plots in northern hardwood stands of western Upper Michigan. The estimated average disturbance rate for all plots and decades was 5.7–6.9% of land area per decade, with an implied average canopy tree residence time of 145–175 years. These estimates are similar to those obtained by on-site estimates of canopy tree residence time and studies in the literature on the rate of gap formation. Problems in radial increment analysis and possible solutions are discussed.