Gap Dynamics Research Articles

Episodic, patchy disturbances characterize an old-growth Picea abies dominated forest landscape in northeastern Europe

The conventional theory of boreal forest dynamics recognizes two distinct disturbance regime types, small-scale gap dynamics and dynamics driven by large stand-replacing disturbances. We used satellite imagery and extensive field measurements to examine the landscape-level pattern and impact of an extensive disturbance episode that occurred in the early 2000s in primeval forest dominated by Picea abies in the Arkhangelsk region, Russia, due to drought and subsequent bark beetle (Ips typographus) outbreak. We also quantified forest age structures and deadwood characteristics at the landscape-level to study how such disturbances shape forest structures over larger scales. Satellite image analysis revealed that disturbance patches covered about 16% of the land area in the 12km×12km landscape studied. The size of the disturbance patches was strongly skewed toward small ones (median size 0.12ha) and they were distributed across the landscape with some tendency of aggregation. The landscape forest matrix was dominated by old-growth forest. The dominant trees in the forest were established prior to 1850, and approximately half of the forest had established prior to 1800. However, the patchy occurrence of younger forest suggests that the landscape previously was subject to patchy disturbance similar to the recent one. This conclusion also gained support from historical records. We conclude that the structure and dynamics of the studied primeval forest landscape was driven by the combined impact of small-scale “background” mortality (classical gap dynamics) and infrequent episodes of patchy intermediate severity and scale disturbances.

Twenty-three years of stand dynamics in an old-growth Chamaecyparis forest in central Japan

Structures and dynamics of old-growth coniferous stands are affected by several types of disturbances including typhoons. We report the forest dynamics of four old-growth Chamaecyparis stands in central Japan that differ in the disturbance history of typhoons over a period of 23 years. The stem number, basal area and mortality were examined. In a predominant stand of C. obtusa (Sieb. et Zucc.) Endl., 24 % of the C. obtusa canopy trees died, mainly as a result of the severe damage of a strong typhoon that caused a single tree-fall gap and the following gap enlargements. In this stand, the total basal area decreased to 76.5 % of the initial value, although the mortality declined in recent years. In contrast, the other three stands decreased only slightly in the stem numbers (0.0–5.6 %) and increased in the basal areas of C. obtusa canopy trees. It is confirmed that the stand-level ingrowths of 300-year-old C. obtusa canopy trees could contribute to the increase in the stock of each stand. Our results support an idea that the dynamics of old-growth Chamaecyparis forests were greatly affected by typhoons. The stand structures will be gradually changed (with the processes of gap dynamics) and C. obtusa will continue to be dominant, potentially over hundreds of years.

The Sectoral Structures in Romania, its Regions and the EU Countries – Key Features of Economic and Social Cohesion

The economic structure by sectors plays a very important part in the dynamics of development gaps at sub-national level, because it influences the income level and distribution in the regions/sub-regions of a country, as many empirical studies showed. In the perspective of increasing globalization, difficult recovery from economic crises, and persistent inter-country, inter-regions and intra-regional development gaps, the goals of cohesion policy in the EU member states in the future programming period, 2014- 2020, envisage, among others, the best use of regional/local human, natural and capital resources in order to increase the living standard of all citizens and diminish the development imbalances.The paper presents an analysis of the sectoral gross value added dynamics in Romania, its regions and other EU countries with the help of various structural coefficients, in the attempt to identify patterns of sectoral convergence or divergence, structural resilience or fragility and the likely impact of economic crises.

The Role of Two Frictions in Geographic Price Dispersion: When Market Friction Meets Nominal Rigidity

This paper empirically investigates and theoretically derives the implications of two frictions, market friction and nominal rigidity, on the dynamic properties of intra-national relative prices, with an emphasis on the interaction of the two frictions. By analyzing a panel of retail prices of 45 products for 48 cities in the U.S., we make two major arguments. First, the effect of each type of friction on the dynamics of intercity price gaps is quite different. While market frictions arising from physical distance and transportation costs contribute significantly to volatile and persistent movements of intercity price disparities, nominal rigidity is associated with higher persistence, but not with a greater volatility of the intercity price disparity. This empirical evidence is different from what is predicted by standard theoretical models based on price stickiness. Second, the strength of the marginal effect of a market friction hinges on the extent of nominal rigidity, in a counteracting manner. The marginal effect of a market friction dwindles as the extent of price stickiness increases. We provide an alternative theoretical explanation for this finding by extending the state- dependent pricing(SDP)model of Dotsey et al.(1999) and show that our two-city model with nominal rigidity andd market frictions can successfully explain the salient features of the dynamic behavior of intercity price differences.

Characterizing forest structural types and shelterwood dynamics from Lorenz-based indicators predicted by airborne laser scanning

In this study, Lorenz curve descriptors of tree diameter inequality were used to characterize the dynamics of forest development in a shelterwood-managed Pinus sylvestris (L.) dominated area. The purpose was to stratify the forest area into forest structural types (FST) from airborne laser scanning (ALS)-based wall-to-wall predictions of the chosen indicators: Gini coefficient (GC) and Lorenz asymmetry (LA). A clear boundary at GC = 0.5 was found, which separated even-sized (below) and uneven-sized (above) areas. Furthermore, a need for including LA in the characterization of the uneven-sized areas was detected, to distinguish bimodal from reverse J-shaped stands. Beta regression was used for the ALS predictions, yielding RMSEs of 19.67% for GC and 11.01% for LA. Based on our results, we concluded that forest disturbance decreases GC, whereas seed regeneration increases GC and, therefore, gap dynamics are characterized by shifts between either side of the GC = 0.5 threshold. In even-sized stands, GC decreases toward maturity owing to self-thinning occurring at the stem exclusion stage. In uneven-sized stands, the skewness of the Lorenz curve indicates understory development, as ingrowth decreases LA. The possible applications of the resulting FST map are discussed; for instance, in identifying areas needing silvicultural treatments or evaluating forest recovery from disturbances.

Climate change and fire management in the mid-Atlantic region

In this review, we summarize the potential impacts of climate change on wildfire activity in the mid-Atlantic region, and then consider how the beneficial uses of prescribed fire could conflict with mitigation needs for climate change, focusing on patters of carbon (C) sequestration by forests in the region. We use a synthesis of field studies, eddy flux tower measurements, and simulation studies to evaluate how the use of prescribed fire affects short- and long-term forest C dynamics. Climate change may create weather conditions more conducive to wildfire activity, but successional changes in forest composition, altered gap dynamics, reduced understory and forest floor fuels, and fire suppression will likely continue to limit wildfire occurrence and severity throughout the region. Prescribed burning is the only major viable option that land managers have for reducing hazardous fuels in a cost-effective manner, or ensuring the regeneration and maintenance of fire-dependent species. Field measurements and model simulations indicate that consumption of fine fuels on the forest floor and understory vegetation during most prescribed burns is equivalent to <1–3years of sequestered C, and depends on pre-burn fuel loading and burn intensity. Overstory tree mortality is typically low, and stands have somewhat reduced daytime C uptake during the next growing season following burns, but may also have reduced rates of ecosystem respiration. On an annual basis, net ecosystem productivity is negative the first year when consumption losses are included, but then positive in following years, and stands can reach C neutrality within <2–3years. Field data and model simulations suggest that increases in prescribed burning in fire-prone areas would have little appreciable effect on long-term forest C dynamics in some fire-prone forest types. Large-scale conversion to young pine plantations for fiber and biofuels will potentially increase the risk of wildfires, as had occurred previously in the late-19th and early-20th centuries in the region.

Landscape‐scale forest disturbance regimes in southern Peruvian Amazonia

Landscape-scale gap-size frequency distributions in tropical forests are a poorly studied but key ecological variable. Currently, a scale gap currently exists between local-scale field-based studies and those employing regional-scale medium-resolution satellite data. Data at landscape scales but of fine resolution would, however, facilitate investigation into a range of ecological questions relating to gap dynamics. These include whether canopy disturbances captured in permanent sample plots (PSPs) are representative of those in their surrounding landscape, and whether disturbance regimes vary with forest type. Here, therefore, we employ airborne LiDAR data captured over 142.5 km2 of mature, swamp, and regenerating forests in southeast Peru to assess the landscape-scale disturbance at a sampling resolution of up to 2 m. We find that this landscape is characterized by large numbers of small gaps; large disturbance events are insignificant and infrequent. Of the total number of gaps that are 2 m2 or larger in area, just 0.45% were larger than 100 m2, with a power-law exponent (alpha) value of the gap-size frequency distribution of 2.22. However, differences in disturbance regimes are seen among different forest types, with a significant difference in the alpha value of the gap-size frequency distribution observed for the swamp/regenerating forests compared with the mature forests at higher elevations. Although a relatively small area of the total forest of this region was investigated here, this study presents an unprecedented assessment of this landscape with respect to its gap dynamics. This is particularly pertinent given the range of forest types present in the landscape and the differences observed. The coupling of detailed insights into forest properties and growth provided by PSPs with the broader statistics of disturbance events using remote sensing is recommended as a strong basis for scaling-up estimates of landscape and regional-scale carbon balance.

Spatial distribution of &lt;i&gt;Guaiacum sanctum&lt;/i&gt; (Zygophyllaceae) seedlings and saplings relative to canopy cover in Palo Verde National Park, Costa Rica

The spatial distribution of individuals is a fundamental property of most species and constitutes essential information for the development of restoration and conservation strategies, especially for endangered plant species. In this paper we describe the spatial distribution of different size classes of the endangered tropical tree Guaiacum sanctum and the effect of canopy cover on spatial aggregation. Adult G. sanctum were located and mapped in a 50 ha plot in Palo Verde National Park, Costa Rica. Seedlings, saplings and juveniles were mapped to the nearest centimetre and permanently marked in three 50 x 50 m subplots. Within each subplot spatial aggregation was assessed using Ripley's K statistic and canopy opening readings were performed every 5 m using a densitometer. Kriging spatial interpolation and Monte Carlo simulations were used to determine if average canopy cover differed among size classes. Individuals of G. sanctum were spatially aggregated at all size classes with seedlings being the most frequent size class in all subplots. Seedlings were found predominantly in areas with significantly higher canopy cover. In contrast, juveniles were more likely found in areas with higher light availability. The high number of seedlings, saplings, and juveniles relative to adults suggests that populations of G. sanctum in PVNP are expanding. Light availability and canopy structure are important factors shaping the spatial distribution of this species. The contemporary demographic structure of G. sanctum is dependent on forest gap dynamics and changes in human disturbance during the past 25 years.

Sprouting extends the regeneration niche in temperate rain forests: The case of the long-lived tree Eucryphia cordifolia

Sprouting is a widespread trait of geographical, ecological and taxonomical scales that allows resource exploitation, space colonization, vegetative reproduction and post-disturbance persistence. It is considered that persistence is the most frequent functional role of sprouting in woody plants. Here we propose that niche differentiation between sprouts and saplings extends the regeneration niche to suboptimal conditions for sexual recruitment. To test this hypothesis, we sampled root suckers (sprouts) and saplings of the long-lived tree species Eucryphia cordifolia Cav. in an even-aged coastal temperate rain forest. Canopy openness was measured over each recruit as a proxy of regeneration niche. Sapling survival was checked after two years. The entire forest canopy openness range was used as an estimation of niche availability. Light and microclimatic conditions were recorded for different canopy openness. Sprouts and saplings occupied a narrow range of the canopy openness near to the closest extreme of the gradient (4.8–14.1%). The distribution of saplings was explained by the lower survival of small saplings under more open canopies, as a consequence of the interactive effect of higher evaporative demand and radiation load in more opened canopies. Although the niches of sexual and vegetative recruits slightly overlapped (31% of the interquartile range on average), sprouts inhabit the more opened sites. Therefore, sprouts extend the regeneration niche to suboptimal conditions for sapling establishment. Our results contribute toward explaining the high colonization ability of E. cordifolia in this kind of forest, whose composition is strongly driven by gap dynamics.

Canopy Gap Dynamics in a Second-Growth Appalachian Hardwood Forest in West Virginia

ABSTRACT We examined canopy gaps on the 3,100 ha West Virginia University Research Forest (WVURF), a 70–80 year-old, second-growth, Appalachian hardwood forest near Morgantown, West Virginia. The objectives of this study were: (a) to describe gap characteristics (size, age, fraction, regeneration and gap-makers) of the forest as a whole, and (b) to assess whether gap characteristics varied by slope position (cove, midslope, ridge), slope aspect (NE, NW, SE, SW), and forest type (cove hardwood, mesic oak, xeric oak). Sixty transects were established, with a total length of 22,043 m. Eighty canopy gaps were identified. Average gap size was 98.6 (±134.17) m2. Gap age ranged from 2–29 years old with a mean age of 16.4 (±6.3) yrs. Overall, 2.7% of the forest was composed of gaps. There were no differences in gap size by aspect or slope position. Cove hardwoods had larger gap sizes than mesic and xeric oak forests. There were no differences in gap age based on slope position, aspect, or forest type. Gap fractio...

タイ・メクロン流域の熱帯季節林におけるギャップダイナミックス―種数－面積曲線に及ぼす環境傾度と植物多様性―

タイ・メクロン流域の熱帯季節林におけるギャップダイナミックス―種数－面積曲線に及ぼす環境傾度と植物多様性―

Fire and gap dynamics over 300 years in an old‐growth temperate forest

AbstractQuestionsWhat are the long‐term patterns of wildfire occurrence and gap dynamics in an old‐growth deciduous forest? Are there temporal patterns in fire and gap dynamics over the last ca. 300 yrs? How is drought related to fire occurrence? Are there temporal interactions between gap dynamics and fire?LocationLilley Cornett Woods Appalachian Research Station, Southeastern Kentucky, USA. LCW; 37°05′ N, 83°00′ W.MethodsWe cross‐dated and analysed annually‐resolved tree‐ring data from 35 tree cross‐sections in an old‐growth deciduous forest to reconstruct historical fire and canopy disturbance and explore connections among these processes. Canopy disturbance patterns as indicated by tree growth release within this collection [fire history collection: (FHC)] were compared to cores from 26 trees collected in 1983 for the purposes of climate reconstruction [climate collection: (CC)].ResultsInitiation dates in the FHC ranged from ca. 1670 to 1925. Thirty‐three fire scars were identified from 1678 to 1956. The mean interval between fire events was 9.3 yrs, and there were many more fires after 1800 than before that date. Gap dynamics, as reconstructed through growth release detection, were relatively constant through the FHC record and were supported by a similar result in the CC. The mean number of years between detected release events was 5.2 yrs. Many individual trees, and the mean growth chronology for the FHC, indicate that many oak trees exhibit growth release after long periods of suppression and, after a final release, exhibit a step‐change in growth rate suggesting canopy accession.ConclusionsFire and gap dynamics occurred through much of the last ca. 350 yrs in this old‐growth forest. There was not evidence to support that these two processes were temporally linked – gap dynamics were ostensibly independent of fire occurrence. Even so, we posit that these two processes may have a synergistic effect on long‐term dynamics, wherein fire ‘filters’ the seedling pool and gap openings provide canopy accession opportunities. We also note several instances where release events are associated with stand‐wide growth increases suggesting large‐scale canopy accession. These events could influence the overstorey composition of the forest for centuries.

Gap formation and carbon cycling in the Brazilian Amazon: measurement using high-resolution optical remote sensing and studies in large forest plots

Background: The dynamics of gaps plays a role in the regimes of tree mortality, production of coarse woody debris (CWD) and the variability of light in the forest understory. Aims: To quantify the area affected by, and the carbon fluxes associated with, natural gap-phase disturbances in a tropical lowland evergreen rain forest by use of ground measurements and high-resolution satellite images. Methods: We surveyed two large forest inventory plots of 114 and 53 ha of the Tapajós National Forest (TNF) in the Brazilian Amazon during 2008 and 2009, respectively. We mapped all gaps and collected data on light availability, CWD stocks and tree mortality in the field. Gap location, canopy openness (CO) and leaf area index (LAI) estimated in the field were compared with two IKONOS–2 high-resolution satellite images acquired at approximately the time of the field measurements. Results: In the two large plots (167 ha total area) we found 96 gaps. The gaps represented 1.42% of the total area and gaps <1-year-old accounted for 0.81% of the plot area. In TNF, the production of CWD in recent gaps was 0.76 Mg C ha−1 year−1 and the mean tree mortality was 2.38 stems ha−1 year−1. The area of gaps estimated using thresholds of light intensity measured by remote sensing optical instruments was twice as large as the gap areas measured on the ground. We found no significant correlation between spectral remote sensing images and CO or LAI, probably due to the high degree of shadow in the high-resolution satellite images. Conclusions: We present the first statistics of CWD production based on gap size in the tropical forest literature. Tree mortality and CWD flux and the forest floor light environment were closely related to gap area. However, less than 30% of the annual tree mortality and CWD flux was associated with gaps, and gaps were difficult to detect using remote sensing methods because of the high proportion of shadow in the images. These results highlight the need for permanent plots in long-term carbon studies.

Composition and habitat use of small mammals in old-growth mountain forests

Old-growth mountain forests in the Pyrenees have natural gap dynamics, a well-developed shrub layer and a large amount of dead wood. Small mammal communities in two types of old-growth forests, silver fir and mountain pine, were studied in July and September in 2006 and 2007. Four species were trapped: bank vole (Myodes glareolus), wood mouse (Apodemus sylvaticus), garden dormouse (Eliomys quercinus) and common shrew (Sorex araneus). Bank voles and wood mice were most commonly trapped (78% of all captures). There were no differences in community composition in the two forest types, although the bank vole was more often captured in the silver fir than in the mountain pine forest. Mammals were more frequently captured at trap stations with high shrub cover, high tree regeneration cover and low herbaceous cover. Our results show that forest structure and, to a lesser extent, forest type determine small mammal community structure, and specifically fine-scale occurrence patterns, in these old-growth forests.

Quantitative easing, credibility and the time-varying dynamics of the term structure of interest rate in Japan

A key issue in current research about quantitative easing monetary policy (QEMP) is the ability of this strategy to impact the term structure of interest rates. Using a dynamic model for the yield curve with time-varying-parameters to the Japanese data, we provide three insights. First, the expectations hypothesis of the term structure of interest rates is generally supported even during the QEMP period. Second, the estimation results reveal that the contribution of macroeconomic variables on the variation of the yield curve is relatively small, especially during the QEMP period. As for the feed-back effect, the yield curve factors contribute only marginally to inflation variation. However, they account for more relevant part of output gap dynamics. Third, the monetary policy shock has a significant effect on yield curve level factor only during the high interest rate periods. However, the decline in the level factor during the QEMP period, while insignificant, indicates a strengthening credibility of the Bank of Japan and thus the effectiveness of its policy.

Small‐scale disturbance and stand structure dynamics in an old‐growth Picea abies forest over 54 yr in central Sweden

AbstractQuestionsCan assumptions of the existence of spatially distinct patches (delineated structurally homogeneous parts of the forest, being either areas consisting of canopy trees or areas without canopy trees but in an early or later regenerative phase) and of directional development over time of the vegetation in such patches, as implied by current theory of storm gap dynamics, be verified by remapping previous study sites?LocationNatural, unmanaged boreo‐nemoral spruce‐dominated forest in eastern central Sweden.MethodsBy re‐mapping three plots, ca, 50 yr after the first inventory, we studied the structure and dynamics of gaps (patches without canopy tree cover) and major tree populations. The old and new maps allowed us to compare two independent assessments of the forest dynamics: one based on tree population changes and one on changes in gap area over time.ResultsThe current population structure could partly be described through the earlier‐encountered structures of the different tree populations and consecutive processes of recruitment and mortality. However, the re‐mapping exercise showed that spatially delineated patches did not develop directionally over time, nor was their development spatially discrete.ConclusionsPatch dynamics proceeds in such a way that the fate of a single patch may depend on the development of neighbouring patches. As gaps may partly close or merge into larger gaps, and as gap disappearance rate is a function of actual gap size, performance of an initially delimited patch is largely determined by developments in neighbouring patches and cannot be predicted from its momentary patch characteristics. Consequently, we propose an ‘open matrix model’ to describe the changes in a boreo‐nemoral spruce forest, rather than a ‘storm gap dynamics’ model.

Axisymmetric whole pin life modelling of advanced gas-cooled reactor nuclear fuel

Thermo-mechanical contributions to pellet–clad interaction (PCI) in advanced gas-cooled reactors (AGRs) are modelled in the ABAQUS finite element (FE) code. User supplied sub-routines permit the modelling of the non-linear behaviour of AGR fuel through life. Through utilisation of ABAQUS’s well-developed pre- and post-processing ability, the behaviour of the axially constrained steel clad fuel was modelled. The 2D axisymmetric model includes thermo-mechanical behaviour of the fuel with time and condition dependent material properties. Pellet cladding gap dynamics and thermal behaviour are also modelled. The model treats heat up as a fully coupled temperature-displacement study. Dwell time and direct power cycling was applied to model the impact of online refuelling, a key feature of the AGR. The model includes the visco-plastic behaviour of the fuel under the stress and irradiation conditions within an AGR core and a non-linear heat transfer model. A multiscale fission gas release model is applied to compute pin pressure; this model is coupled to the PCI gap model through an explicit fission gas inventory code. Whole pin, whole life, models are able to show the impact of the fuel on all segments of cladding including weld end caps and cladding pellet locking mechanisms (unique to AGR fuel). The development of this model in a commercial FE package shows that the development of a potentially verified and future-proof fuel performance code can be created and used.

Gap Dynamics and Structure of Two Old-Growth Beech Forest Remnants in Slovenia

ContextDue to a long history of intensive forest exploitation, few European beech (Fagus sylvatica L.) old-growth forests have been preserved in Europe.Material and MethodsWe studied two beech forest reserves in southern Slovenia. We examined the structural characteristics of the two forest reserves based on data from sample plots and complete inventory obtained from four previous forest management plans. To gain a better understanding of disturbance dynamics, we used aerial imagery to study the characteristics of canopy gaps over an 11-year period in the Kopa forest reserve and a 20-year period in the Gorjanci forest reserve.ResultsThe results suggest that these forests are structurally heterogeneous over small spatial scales. Gap size analysis showed that gaps smaller than 500 m2 are the dominant driving force of stand development. The percentage of forest area in canopy gaps ranged from 3.2 to 4.5% in the Kopa forest reserve and from 9.1 to 10.6% in the Gorjanci forest reserve. These forests exhibit relatively high annual rates of coverage by newly established (0.15 and 0.25%) and closed (0.08 and 0.16%) canopy gaps. New gap formation is dependant on senescent trees located throughout the reserve.ConclusionWe conclude that these stands are not even-sized, but rather unevenly structured. This is due to the fact that the disturbance regime is characterized by low intensity, small-scale disturbances.

Treefall Gap Dynamics in a Tropical Rain Forest in Papua New Guinea

Treefall gaps play important roles in tropical rain-forest ecology. But studies rely increasingly on models, remote sensing, and a few intensively studied research sites, mostly in the neotropics. We studied the basic parameters of gap dynamics (size, causes, and frequency) of treefall gaps in a lower montane primary forest on the southern flank of the central range of Papua New Guinea. We found 40 treefall gaps formed on 10.4 km of transect sampled annually over 3 yr. Mean proportion of forest under new gaps was 0.015/yr. Mean area of treefall gaps ≤1 yr old was 312 m2, and gap area was positively correlated with diameter of the fallen tree. Mostly only large trees (diameter at breast height [DBH] = 53 cm) fell as snapped (n = 23) or uprooted (n = 17), creating both single (n = 34) and multiple (n = 6) treefall gaps. There was no strong directionality in the bearings of treefalls. This study provides some of the first information on gap dynamics in Papua New Guinea, where such data can be used to inform sustainable forestry harvesting practices.

Stand age structural dynamics of conifer, mixedwood, and hardwood stands in the boreal forest of central Canada

To study the effects of stand development and overstory composition on stand age structure, we sampled 32 stands representing conifer, mixedwood, and hardwood stand types, ranging in ages from 72 to 201 years on upland mesic sites in northwestern Ontario. We defined the stages of stand development as: stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamics, and gap dynamics. Stand age structure of conifer stands changed from bimodal, bimodal, reverse-J, and bimodal, respectively, through the stages of stand development. Mixedwood and hardwood stands revealed similar trends, with the exception of missing the canopy transition/gap dynamic stage in mixedwoods. Canopy transition/gap dynamic stage in hardwoods showed a weaker reverse-J distribution than their conifer counterparts. The results suggest that forest management activities such as partial and selection harvesting and seed-tree systems may diversify standard landscape-level age structures and benefit wildlife, hasten the onset of old-growth, and create desired stand age structures. We also recommend that the determination of old-growth using the following criteria in the boreal forest: 1) canopy breakdown of pioneering cohort is complete and stand is dominated by later successional tree species, and 2) stand age structure is bimodal, with dominating canopy trees that fall within a relatively narrow range of age and height classes and a significant amount of understory regeneration.