Gaps Of Different Sizes Research Articles

Effect of gap size in grass cover on the percentage and rate of dandelion achene germination

AbstractIn Central Europe, Taraxacum officinale Weber ex Wiggers (dandelion) is an economically important admixture in grasslands. Its spread is supported by its ability to germinate in gaps in grass stands. In a 5‐year experiment, we examined the effect of gap size on the germination rate and percentage of dandelion achenes. Each year, achenes were sown in late spring and mid‐summer in 90 square plots of six sizes, ranging from 2.5 × 2.5 cm to 80 × 80 cm. The plots were placed on a 2 × 6 m experimental area covered with low‐cut lawn. The course of germination varied among individual replicates depending on the weather. In each replicate, however, the rate of germination increased, and the percentage of germinated achenes decreased with increasing gap size. Differences in germination paralleled differences in the microclimate on the soil surface of plots where humidity decreased and temperature increased with plot size. The ultimate cause of this difference was the variance among gaps of different sizes in proportion to which the total area of the gap surface is insolated during the day. The colonization of lawns by dandelion is supported by the formation of small gaps in grass stands.

Impact of pore sizes and defects on C-S-H/epoxy bonding in wet conditions: A molecular dynamics analysis

Despite the recent advances in understanding the water ingress at the interface of calcium silicate hydrate and epoxy, there are still open questions regarding the pore size and defects' dependence on the structure, dynamic and mechanical properties of the interfaces. In this paper, molecular dynamics simulations are carried out for perfect and defective interface models consisting of C-S-H and epoxy resins, respectively. Gaps of different sizes between epoxy and C-S-H are filled with water molecules, with a maximum spacing of 2.79 nm, to investigate the effect of water ingress on the structural, dynamic, and mechanical properties of the interface. The results indicate that in the interface formed by the defective structure, the calcium ions responsible for providing strength have been transformed into a free state due to the influence of water molecules. Consequently, they lose their ability to bridge between the epoxy and silicate tetrahedra, leading to the inability to establish interfacial strength. The defective structure, with its larger surface area and increased interaction with water, is more susceptible to degradation, especially noticeable in the tensile simulations. Furthermore, the interactions between water and C-S-H are the primary factors influencing the microstructure and dynamic behavior of the interface. As the number of water molecules increases, the interface strength weakens. This work provides new insight into understanding the bonding mechanism of the C-S-H/epoxy interface with water ingress. It contributes to the design and development of FRP reinforced concrete composites.

Regeneration and growth of tree seedlings and saplings in created gaps of different sizes in a subtropical secondary forest in southern China

Gap-based silvicultural systems are of particular interest in secondary forest management because gaps are associated with regeneration and tree species diversity. However, whether using a range of gap sizes to influence tree regeneration is effective remains unclear. In this study, gaps of different sizes were created to determine their effects on tree regeneration and growth in a secondary Pinus massoniana Lamb. forest in Hunan Province, China. A robustly designed experiment was used with over nine years of repeated measurements. Seedling densities were highest in small gaps, and seedling diversity increased with increasing gap size. Sapling densities and diversity were highest in medium gaps. Within medium and large gaps, saplings were heterogeneously distributed along gradients from opening to closed forest canopy, suggesting gap partitioning of saplings. Emergence rates of three dominant species (Pinus massoniana Lamb, Schima superba Gardn. et Champ., and Cyclobalanopsis glauca (Thunb.) Oerst.) were roughly equal in medium gaps. Regeneration of P. massoniana and C. glauca was spatially diverse as gap sized increased. However, there was little evidence of S. superba divergence within and among gaps, which indicated differential tolerance to shade. A nonlinear mixed-effects model was used to detect effects of microenvironmental variables on diameter–biomass relations. The model suggested that gaps combined with solar radiation, soil moisture, and soil temperature had various effects on sapling biomass accumulation. For saplings with the same basal diameter, estimated average aboveground biomass of S. superba and C. glauca was highest in medium gaps, whereas that of P. massoniana was highest in large gaps. The study suggests that tree regeneration and growth requirements are often not met with a single gap size alone. In practice, a single tree/group selection with medium gaps hybrid approach may be required in subtropical secondary forests to increase regeneration and ecosystem resilience, while also considering other factors such as microenvironmental variables, shade tolerance, and others.

Natural regeneration in logging gaps of different sizes in Subri River Forest Reserve (Ghana)

Subri River Forest Reserve (SR) is the most extensive forest area in Ghana with an accompanying rich floral species. Over the years, logging from both legally prescribed and illegal operations remain the predominant forest disturbance in SR. Gap creation following logging is crucial in determining tree species composition and diversity. Hence, the study evaluated the composition and diversity of naturally regenerated tree species in logging gaps of different sizes and, again examined the roles of these tree species in fulfilling the economic and ecological agenda of sustainable forest management after logging in SR. Twelve gaps were randomly selected: 4 each were grouped into small size (≤ 200 m2), medium size (201–300 m2), and large size (≥ 300 m2). Data were gathered from 1 m2 circular area at gap centres and repeatedly inside 1 m width strip along 20 m individual N-S-E-W transects. Species diversity differed significantly between gap sizes. Higher diversity indices were measured in large size gaps. Gap sizes shared similar species. There were significant differences among various height groupings of tree species across all three gap sizes. Pioneers preferred medium to large size gaps, while shade-tolerant tree species preferred small size gaps for their abundance. Vulnerable and Lower Risk Near Threatened tree species under Conservation Status and, Premium and Commercial tree species under Utilisation Status preferred small size gaps for their proliferation and conservation. Therefore, we recommend the single tree-based selective logging for ensuring creations of small to medium size (200–300 m2) gaps through adjustments to the logging permit process, revision of Allocation Quota Permit, strict adherence to the 40-year polycyclic selection system, along with more dedicated enforcement and monitoring. Changes along these protocols would tremendously facilitate natural regeneration of different suites of timber species resulting in the improvement of the overall biodiversity conservation associated with the forest, more sustainable forest harvests and more income to those who receive permits.

Correlation-based imputation method for estimating missing well log data in a Hungarian groundwater well

Incomplete well logging datasets measured in boreholes are frequently encountered especially in old wells. The main goal of the research, described in this paper, is to refill a data matrix with synthetic data that is missing for some reason and how the imputation procedure was to determine well the measured and estimated values fit in the case of data gaps of different sizes, and how the clay content and porosity change as a result. There can be many reasons for the lack of data, such as not measuring the desired parameters in a given depth zone. To fill this data gap, correlation-based imputation method was used which was applied in MATLAB software development system. A case study involving a Hungarian thermal water well is shown to demonstrate the reliability of the multi-linear correlation based method, which can be fruitfully applied in other wells and investigation areas.

Effects of Gap Size and Cardinal Directions on Natural Regeneration, Growth Dynamics of Trees outside the Gaps and Soil Properties in European Beech Forests of Southern Italy

The present study was focused on how cardinal directions inside gaps of different sizes (small, 200 m2; medium, 400 m2; and large, 600 m2) can affect soil characteristics and tree regeneration. Additionally, the effects of gap size on the growth dynamics of trees outside the gaps were evaluated. The study was carried out in a European beech stand located in Aspromonte National Park (Southern Apennines, Calabria, Italy). Microclimatic variables, physical, chemical, and biochemical soil properties, natural regeneration density, and growth trees outside the gaps density of natural regeneration were assessed. This study provided evidence for an important effect of cardinal points on micro-environmental parameter variability, nutrient cycle, physic-chemical soil properties, water availability, and biological processes such as trees growth and regeneration. The European beech natural regeneration was most abundant in the south part of the gaps. Thus, we can state that cardinal points affect the trees natural regeneration in a species-specific manner. The new microclimatic conditions due to the gap opening had positive effects on the tree growth located along the gap edge, especially in the trees sampled on the edges of the medium gaps. On the contrary, the trees located in the forest recorded a productivity coherent with the period prior the gap opening. In medium-sized gaps, the combination and interaction of microclimatic and soil parameters (humification and mineralization process and microbial activity) created the best conditions for beech natural regeneration and favored an increase in the productivity of the trees at the edge of the gaps.

Response of plant diversity and soil physicochemical properties to different gap sizes in a Pinus massoniana plantation.

As one means of close-to-nature management, forest gaps have an important impact on the ecological service function of plantations. To improve the current situation of P. massoniana plantations, three different sizes of forest gaps (large gaps, medium gaps and small gaps) were established to observe whether gap setting can improve the soil fertility and plant diversity of forest plantations. The results showed that compared with the control, the soil organic matter content of different soil layers increased significantly in the medium forest gap and large forest gap. The content of soil organic matter in the surface layer of the middle gap had the largest increase (80.64%). Compared with the control, the content of soil-available potassium between different soil layers decreased significantly by 15.93% to 25.80%. The soil hydrolysable nitrogen reached its maximum under the medium gap. Soil moisture showed significant changes among different gap treatments, different soil layers and their interaction, decreasing significantly in large gaps and small gaps but increasing significantly in medium gaps. The soil bulk density decreased significantly compared with the control, and the surface soil reached the minimum in the medium gap. There were different plant species in forest gaps of different sizes, and shrub layer plants were more sensitive to gap size differences than herb layer plants. The plant diversity indices of the shrub layer increased significantly and showed a maximum under the medium gap. The plant diversity of the herb layer showed the opposite trend, and the Shannon-Wiener index, Simpson index and Pielou index were significantly lower than those of the control. RDA showed that different gap treatments had significant effects on the distribution of plants under the forest. Soil available potassium, soil moisture and soil bulk density affected the distribution and diversity of plants under the forest, serving as the limiting factors of plant growth. In forest management, if we strictly consider the improvement of plant diversity and soil physicochemical properties, these results suggest that a medium gap should be established in a plantation for natural restoration.

Use of direct seeding and seedling planting to restore Korean pine (Pinus koraiensis Sieb. Et Zucc.) in secondary forests of Northeast China

The poor natural regeneration of Korean pine represents a substantial hurdle limiting the restoration of secondary forests to mixed broad-leaved Korean pine forests (MBKPFs). Active restoration based on direct seeding or planting seedlings is accordingly considered to be a prerequisite for promoting Korean pine recruitment in secondary forests. To evaluate the most effective practices for restoring Korean pine in secondary forests, we studied the effects of microsites associated with gaps of different sizes and within-gap locations on direct seeding (rodent access/exclusion and soil/litter cover) and seedling planting (1-, 2-, and 3-year-old seedlings). We found that an average of 82.4% of sown seeds were eaten by rodents, with only 9.9% germinating and surviving after two growing seasons. Among the germinated seedlings, 37.6% were subsequently excised by rodents, whereas 6% died due to other natural causes. Furthermore, compared with the control sites (forest understorey), we detected no significant differences among the different microsites with respect to an effective reduction in rodent damage or promotion of seed germination, whereas rodent exclusion was more effective in protecting seeds from predation than the effect of microsite. With regards to planted seedlings, we found that the proportion of survival and relative root collar diameter growth were highest in the center (64.1%) and edge (60.0%) sites of large gaps, and that the survival of planted seedlings (43.7%) was considerably higher than that of seedlings derived from directly sown seeds (9.9%). Collectively, our findings indicate that, owing to seed predation and seedling excision by rodents, direct seeding with either mesh or soil cover would not represent an effective strategy for promoting Korean pine recruitment in secondary forests. Accordingly, for the purposes of restoring secondary forests to MBKPFs, we recommend planting 3-year-old Korean pine seedlings in large gaps within secondary forests.

Landscape forest loss changes sunfleck dynamics in forest fragments of southern Bahia, Brazil

AbstractLandscape-scale habitat loss can change the floristic composition of forest fragments, affecting the survival of specific groups of plants, as shade-tolerant and emergent trees. This increasing in tree mortality creates forest canopy gaps of different sizes that ultimately determine the solar radiation available in the forest understorey. We conducted a study aiming to assess how the loss of forest cover at landscape level (i.e. deforestation) affects the sunfleck dynamics, a proxy of light regime in forest understorey. We expected that fragments located in landscapes with less forest cover have a high number of larger canopy gaps and, consequently, long-lasting sunflecks. In each forest fragment, a 100 per 50 m plot was established, and in each plot, we took 10 hemispherical photographs. The images were analysed using the Gap Light Analyzer software. The sunflecks were divided into six temporal classes. We evidenced that landscape-scale deforestation increased the frequency of all sunfleck intervals >8 min, particularly the long-lasting (> 32 min) sunflecks. We propose that the increasing frequency of long-lasting sunflecks reduces suitability of microhabitat to some shade-tolerant species in local fragments, a potential proximal mechanism contributing to compositional shifts of tree assemblages observed in forest fragments within deforested landscapes.

Effects of gap size on natural regeneration and micro-environmental soil conditions in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst) dominated mixed forest

The study focused on the effects of gap size on natural regeneration of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) and micro-environmental soil conditions in gaps of different sizes under temperate mixed forest in the Czech Republic. Six gaps comprising two for small (≥ 200 m<sup>2</sup>), medium (≥ 500 m<sup>2</sup>) and big (≥ 900 m<sup>2</sup>) each were selected. Ten circular 1 m<sup>2</sup> subsampling plots were established at 2 m intervals along individual North-South-East-West transects, including one at the gap centre. Regeneration was monitored in 2014 and repeatedly in 2019. Soil conditions were only measured in 2019. Gap size was found to be a significant parameter for European beech natural regeneration in 2014. Besides, the quick occupation of European beech in gaps at natural beech zone provoked its prolific regeneration compared to Norway spruce in 2014. However, in 2019 the recent threat of weather variabilities was responsible for the general abysmal growth performance of natural regeneration. Division of gap microsites into different within-gap positions based on prevailing light or shade conditions was helpful in assessing the significant variations of soil conditions within-gap positions and among gap sizes. Soil temperature and moisture significantly influenced the regeneration of European beech and Norway spruce, respectively.

Effect of gap size on tree species diversity of natural regeneration - case study from Masaryk Training Forest Enterprise Křtiny

Forest gaps remain the optimal forest management practice in modern forestry. Upon all the physical properties of forest gaps, the ‘gap size’ feature stands out as an essential property. The effect of gap size on tree species composition and diversity of natural regeneration in forest gaps of different sizes was investigated. Eight research forest gaps were selected from the Training Forest School Enterprise, also called Masaryk Forest in Křtiny, a temperate mixed forest in the Czech Republic. By given gap sizes, small (< 700 m<sup>2</sup>) and large gaps (≥ 700 m<sup>2</sup>) were defined. Forty-one (41) regeneration microsites (RSs) of 1 m<sup>2</sup> circular area at 2 m intervals were demarcated within each forest gap. These RSs served as data collection points. From the total of eleven (11) species enumerated, large gaps obtained higher species composition (10) and diversity (Simpson = 0.5 1-D; Shannon = 1.0 H and Pielou’s evenness = 0.5 J indices) records, yet, small gaps presented favourable conditions for prolific natural regeneration significantly. Light-adapted species demonstrated no significant difference (P > 0.05) between small and large gaps, however, intermediate and shade-tolerant species were significantly higher (P < 0.05) in small gaps. There were progressive declines in height growth of natural regeneration from 0–20 cm to 21–50 cm and 51+ cm in small and large gaps at R<sup>2 </sup>= 99% and 88%, respectively. <br /> The development of herbaceous vegetation in small and large gaps had positive and negative effects on the natural regeneration of Fagus sylvatica and Abies alba species, respectively.

Seedling recruitment in response to artificial gaps: predicting the ecological consequence of forest disturbance

Artificial gaps are widely created for forest management. However, the interactive effects of gap sizes and within-gap locations on seedling regeneration are rarely tested. Therefore, whether there are lower and upper size limits for artificial gaps remains controversial. Here, we aimed to test the hypothesis that the effects of within-gap positions on seedling regeneration are dependent on gap sizes. Newly germinated wild seedlings of shade-intolerant Quercus mongolica scattered in different positions within artificially created gaps of different sizes were surveyed to test our hypothesis. Large gaps promoted seedling growth more than medium and small gaps. The increase in Q. mongolica seedling growth in large gaps can be largely attributed to the dormancy breaking of the apical buds and increased net photosynthetic rates. More importantly, we showed that the effects of within-gap position were prominent only in medium gaps, i.e., the effect of within-gap position appears to be dependent on gap size. We show an integrative effect of gap size and within-gap location on seedling performance of shade-intolerant oaks, suggesting future studies should evaluate the influence of within-gap location in conjunction with the role of gap size. We therefore suggest that gap creation in silvicultural systems should highlight the lower and upper size limits of forest gaps, which will vary with the species and regions studied.

Preliminary assessment of effect of disturbance on natural regeneration in gaps of different sizes

The study focused on natural regeneration of European beech (Fagus sylvatica), Norway spruce (Picea abies) and European larch (Larix decidua) within very small and four times bigger size gaps following a disturbance at a mixed temperate forest in the Czech Republic. In spring 2013, experimental gap design starts, when 1 m2 circular sampling plots along transects were delineated within four selected naturally occurring canopy openings with size below 20 m2. In December 2013, these initial canopy openings were artificially enlarged by felling to 226 m2 for small and 904 m2 for big gaps. Regeneration was monitored in the next two consecutive growing seasons after disturbance. Light conditions were measured before and after disturbance. Results indicated that four times larger gaps increased twice levels of light conditions, and that diffuse light starts to equilibrate to direct light there. Large gaps were favouring larch regeneration. Beech regeneration was predominant, independently on gap size as the study area belongs naturally to Beech Forest Vegetation Zone, however, the decline of spruce regeneration was presumably linked to drought. Gap size explained variation of larch regeneration in gaps. Contrarily, gap size could not be associated with the prolific regeneration of beech and abysmal regeneration performances of spruce in gaps.

Regeneration Dynamics Following the Formation of Understory Gaps in a Slovakian Beech Virgin Forest

The frequency and size of canopy gaps largely determine light transmission to lower canopy strata, controlling structuring processes in the understory. However, quantitative data from temperate virgin forests on the structure of regeneration in gaps and its dynamics over time are scarce. We studied the structure and height growth of tree regeneration by means of sapling density, shoot length growth and cumulative biomass in 17 understory gaps (29 to 931 m2 in size) in a Slovakian beech (Fagus sylvatica L.) virgin forest, and compared the gaps with the regeneration under closed-canopy conditions. Spatial differences in regeneration structure and growth rate within a gap and in the gap periphery were analyzed for their dependence on the relative intensities of direct and diffuse radiation (high vs. low). We tested the hypotheses that (i) the density and cumulative biomass of saplings are higher in gaps than in closed-canopy patches, (ii) the position in a gap influences the density and height growth of saplings, and (iii) height growth of saplings increases with gap size. Sapling density and biomass were significantly higher in understory gaps than under closed canopy. Density of saplings was positively affected by comparatively high direct, but low diffuse radiation, resulting in pronounced spatial differences. In contrast, sapling shoot length growth was positively affected by higher levels of diffuse radiation and also depended on sapling size, while direct radiation intensity was not influential. Conclusively, in this forest, regeneration likely becomes suppressed after a short period by lateral canopy expansion in small gaps (<100 m2), resulting in a heterogeneous understory structure. In larger gaps (≥100 m2) saplings may be capable even at low plant densities to fill the gap, often forming a cohort-like regeneration layer. Thus, gaps of different sizes imprint on the resulting canopy structure in different ways, enhancing spatial heterogeneity.

Analyzing Spatial Distribution Patterns of European Beech (Fagus sylvatica L.) Regeneration in Dependence of Canopy Openings

The use of natural regeneration techniques is one of the key elements of modern (close-to-nature) forestry. In natural forests, changes in canopy cover, such as the emergence and successive re-closure of canopy gaps are particularly important, as they influence the light availability on the forest floor. Creating canopy gaps of different size is a promising silvicultural tool allowing the regulation of the light availability in managed forests in order to control regeneration composition and development. In this study, we used terrestrial laser scanning data to investigate the relationship between canopy-gap dimensions and emerging natural regeneration along a gradient of management in forests dominated by European beech (Fagus sylvatica, L.). We analyzed the spatial distribution and height of regeneration patches in dependence of gap characteristics. Mean regeneration height decreases progressively from the gap polygon over a transition zone towards the area under the canopy, while the tallest regeneration plants were placed in positions midway between center and gap edge, and not directly in the gap center as we initially assumed. The centers of regeneration patches were not displaced when compared to the associated canopy gap centers, as has been reported in other studies conducted on the northern hemisphere for various tree species. The observed patterns did not depend on management strategies, indicating that regeneration responded equally to naturally created gaps and gaps that were caused by logging. We conclude that establishment and development of shade-tolerant European beech regeneration in forest stands is driven by gap openings, but not necessarily direct radiation. If at all, pronounced direct radiation mainly occurs at the northern edge of large gaps. Neither regeneration patch center, nor regeneration tree height pointed in that direction. Our study suggests that in the investigated beech-dominated forests the effect of increased light availability at the northern edge of a gap is overruled by other factors increasing towards the gap edge, such as increased belowground competition of the overstory trees.

Dynamics of gaps and large openings in a secondary forest of Northeast China over 50 years

The gap dynamics in the studied secondary forest were comparable to those of other temperate forests; large openings were filled within 30 years by afforestation; large and medium gaps closed 30–40 years after they formed. Gaps have important roles in forest regeneration and plant succession. However, it is difficult to determine gap dynamics over long time periods at regional scales. We studied how the dynamics of gaps and large openings (oversized “gaps”) changed in a secondary temperate forest over 50 years. We computed the dynamic indices of gaps (16–3257 m2) and large openings (>3257 m2) using remote sensing techniques applied to six satellite images that were taken approximately every 10 years. Additionally, number-based gap closure ratios were calculated at each interval. Gap dynamics were comparable in magnitude to those calculated for other temperate forests, and 60% and 53% of the large and medium gaps had closed within 30–40 and 20–30 years, respectively. The small gaps closed within 10 years, based on ground-level surveys, and 79.2% of large openings that existed in 1964 were covered by artificial forests in 1994. Gaps of different sizes closed within 40 years due to natural regeneration. Large openings had closed within 30 years via afforestation. These findings can be used for evaluating recovery status and for predicting succession times in secondary forest structures driven by gap formation.

Use of canopy gap openings to restore coniferous stands in Mediterranean environment

In Mediterranean regions, climate change increasingly affect tree species distributions. Conifer forests under continuing disturbance show a more rapid shift to dominance by beech and other temperate broadleaves. Thus, there is an urgent need to conserve coniferous vegetation to avoid local extinction. Gap opening has profound effects on the structure and dynamics of most forests and may represent a sustainable way to restore coniferous ecosystems in Mediterranean habitats. What kind of artificial canopy opening is the most sustainable and effective means for restoring coniferous ecosystem functions? We explored the efficacy of artificial gaps in regeneration and dynamics of coniferous in Mediterranean environment. We examined how regeneration of different tree species is associated with soil environmental conditions and how gaps of different sizes influence the ecology and management of Mediterranean forest. Specifically, we analyzed gap disturbance in silver fir and black pine stands, as they dominate central and southern Italian forests. We demonstrated a specificity between gap size and coniferous species regeneration, indicating that small gaps (about 200 m2) favor silver fir regeneration, while black pine, depending on its subspecies, regenerates both in small and medium gaps (about 500 m2). Further, we found that gap characteristics (age and shape) and suitable substrate availability are the primary factors affecting seedling establishment. Our results provide functional information to design a silvicultural system useful to manage the natural regeneration of Mediterranean forest minimizing the environmental and visual impact.

Introduction of suitable species for planting in gaps of different size (case study: Loveh forest, Golestan, Iran)

This study was carried out in Loveh forest, north of Iran. We measured the quantitative characteristics of seedlings including collar diameter (measured at 1 cm above the soil surface), height, and current annual height increment and calculated the mean annual increments of height and diameter. Ten gaps with the same climatic, edaphic and topographic conditions were selected. In each gap, two transects, 2 m wide, were laid along the longest (North-South) and the shortest (West-East) axis. Nine sampling microplots, 4 m<sup>2</sup> area, were established: one at the centre, four at the edges and four under the canopy within the transects. Results showed that the highest frequency of seedlings was seen in medium-size gaps (200–500 m<sup>2</sup>). Quantitative characteristics of oak seedlings were better in small and medium-size gaps. There was a significant difference in the length of spring shoots, mean annual increment of height and mean annual increment of diameter between the species. Hornbeam, wild service and wild cherry seedlings are recommended for faster gap recovery, as the results showed that these species had the highest values of studied variables.

Effects of canopy gap size on growth and spatial patterns of Chinese pine (Pinus tabulaeformis) regeneration

This study investigates the short-term influences of canopy gap size on the growth and spatial patterns of Chinese pine (Pinus tabulaeformis) regeneration established before (REBG) and after (REAG) gap creation. We established four gap sizes based on the mean height (H) of the canopy trees, comprising minor (0.75H), small (1.00H), medium (1.25H), and large (1.50H) gaps, and three closed canopy reference plots (10×10m) in a plantation in northern China. Seven years after gap creation, each gap was divided into three sections according to the mean crown radius of the border trees, including the gap central area (section B), the sun-exposed gap edge (section C) and the less sun-exposed gap edge (section D). A quantitative analysis revealed no difference in the density of REBG and REAG among the different gap sizes or sections; however, their heights significantly increased with increasing gap size. Significantly taller and shorter regeneration were generally found in section B and under the closed canopy, respectively, whereas those in sections C and D showed no notable differences in height. The spatial patterns of REAG and REBG were mostly random and there were no spatial associations between these two age classes of regeneration within individual gaps. Among gaps of different sizes, the maximum density of REAG occurred most often along the northeastern edges of gaps with a diameter of less than 1.5H, whereas the highest density of REBG appeared in the center and along the southeastern edges of minor gaps and the northeastern edges of large gaps. The spatial patterns in height of the regeneration were similar to those of abundance. Our exploratory analysis revealed that gap expansion along the northeastern edge will likely be necessary to encourage canopy gap closure and successful recruitment in the study area. Further continued monitoring might be required to determine the long-term efficacy of gap creation and such expansion.

Preserving the variance in imputed eddy-covariance measurements: Alternative methods for defensible gap filling

The high utility of eddy covariance (EC) data has made it the cornerstone of carbon dynamics research for more than two decades. However, a substantial number of measurements from EC data can be missing for various reasons. Robust gap-filling methods are required to estimate carbon budgets from net ecosystem exchange measurements of CO2 (NEE) with high precision and accuracy. While the gap-filled methods used have provided unbiased estimates of annual NEE, little research has been done on preserving the variance structures associated with gap-filled flux data. In this project, we used EC data from a longleaf pine ecosystem located in the southeast US to investigate variance preservation in gap-filling methods.We used three non-linear regression approaches to impute artificially created gaps of different sizes via light and temperature response curves: 1) “traditional” fixed monthly window, 2) moving window, and 3) moving window with parameter prediction using physiological drivers. The results of gap-filling simulations showed that the variability of NEE estimates made with moving window and parameter prediction methods were closer to that of observed NEE, whereas the traditional method had overall lower variability. The average root mean square errors (RMSE) of predictions was lower for moving window and parameter prediction (3.38 and 3.22, respectively), versus that of the traditional method (3.42) over one year, including both daytime and nighttime data across all gap sizes. The variances associated with moving window and parameter prediction methods were 52% and 57%, respectively, of the observed variance, versus that of the traditional (51%), while the average of first-order autocorrelation coefficients was 0.76 for each method compared to 0.58 for observed. The results showed that the moving window approaches provided better estimates (lower RMSEs and more similar variance) at annual scales, yet underestimated the observed variance. These results contribute toward the development of a framework of standardized gap-filling approaches which maintain variation inherent in EC data. Moreover, these results call for further research on potential environmental drivers and their interactions for inclusion in gap-filling models, as well as exploration of sampling size of estimation windows and averaging time (half hour) of flux data to promote variance maintenance and decrease the autocorrelation of predictions.