Sapling Height Growth Research Articles

High potential but little success: ungulate browsing increasingly impairs silver fir regeneration in mountain forests in the southern Swiss Alps

Abstract Sustainable regeneration of silver fir (Abies alba Mill.) is important for maintaining and improving the future resilience and protective capacity of mountain forests in the southern Swiss Alps. However, increasing densities of wild ungulates, such as red and roe deer, can hamper the regeneration success of silver fir. A comprehensive understanding of forest–wildlife interactions is essential to plan concrete measures for improving the adaptation potential of mountain forests. Here, we monitored the dynamics of natural silver fir regeneration in mountain protective forests. This study encompassed 30 unfenced permanent plots across two sites, Lostallo and Soazza, located in Valle Mesolcina in the southern Swiss Alps. These sites were selected for their comparable ecological characteristics related to silver fir regeneration, yet contrasting densities of mature, seed-bearing silver fir trees. We recorded the number of seedlings (<10 cm height) and saplings (10–200 cm height), and repeatedly assessed height growth, frequency, and intensity of ungulate browsing on saplings in the summers 2016, 2019, and 2022. In 2022, we additionally recorded the presence of browsing on other tree species in the unfenced plots and the height growth increments of silver fir saplings in pre-existing fences. On average, 509 silver fir seedlings/ha were present in the study area, but there was a lower density of saplings (382 saplings/ha). Between 2016 and 2022, the browsing frequency on silver fir saplings increased from 33 per cent to nearly 100 per cent, accompanied by a significant rise in browsing intensity. Furthermore, sapling height growth from 2019 to 2022 was significantly reduced or even negative in the unfenced plots as compared to the growth of saplings in the fenced plots. The observed seedling density indicated a good potential for natural regeneration of silver fir, irrespective of the differences in the density of seed-bearing trees between the two sites. However, the increased ungulate browsing impaired successful regeneration in the considered mountain forests in the southern Swiss Alps. Thus, without an effective control of ungulate browsing, silver fir may not be able to substantially contribute to the transition to more resilient and future-proof mountain forests.

Effects of Gap Size on Natural Regeneration in Picea asperata Forests of Northern China

Our study aimed to assess the impacts of varying forest gap sizes on the density, growth, and spatial patterns of seedlings and saplings in spruce (Picea asperata) forests in the Saihanba region, Hebei Province, China. Twenty-four forest gaps were surveyed and categorized into six classes based on the gap size. A one-way ANOVA was used to compare differences in the density, height, and ground diameter of seedlings and saplings among six gap classes. Ripley’s K function was used to explore the spatial patterns of regeneration establishment in each class. The findings of our study indicated that the forest gap size did not significantly influence the density of seedlings or the ground diameter growth of saplings, whereas it significantly influenced the height growth of saplings. In smaller gaps, natural regeneration occurred primarily in the gap edges. As the gap size increased, the natural generation began to shift from the edge areas to the gap centers. Large forest gaps had the highest percentages of random distribution patterns across all spatial scales. Aggregated distributions were observed at distances less than 1 m in all gap size classes, whereas uniform distributions tended to occur in the small gaps at distances of 2–4 m. Our findings indicated that larger forest gaps, ranging from 60 to 120 m2, were more conducive to spruce regeneration. The results can inform the development of targeted strategies for understory afforestation and the artificial promotion of natural regeneration in spruce forests.

Height-diameter models of regenerated saplings of Larix gmelinii based on dummy variable and quantile regression.

Based on data collected from 2054 saplings of Larix gmelinii forest in 55 fixed plots in 2018-2019 in Cuigang Forestry Station, Daxing'anling area, we classified the stand density index (SDI) into four classes, i.e., Class Ⅰ (SDI1<1863 plants·hm-2), Class Ⅱ (1863 plants·hm-2≤SDI2<2155 plants·hm-2), Class Ⅲ (2155 plants·hm-2≤SDI3<2459 plants·hm-2) and Class Ⅳ (SDI4≥2459 plants·hm-2) by using the quartile method. We constructed a dummy variable model and quantile regression model for the height-breast diameter of saplings of L. gmelinii with dummy variable method introduced SDI. The results showed that among the five selected representative non-linear tree height curve models, the Richards model fitted the best, with Ra2, RMSE and MAE of 0.7637, 0.8250 m and 0.5696 m. The dummy variable model including the SDI constructed based on the Richards model showed a 1.3% increase in Ra2 compared with the base model, while RMSE, MAE, and AIC decreased by 2.1%, 1.5%, and 11.2%, respectively. When the quantile τ was 0.5, Ra2 of quantile regression model was the maximum, and RMSE, MAE, AIC was the minimum, being 0.7612, 0.8294 m, 0.5657 m, and -767.19, respectively. Compared with SDI1, sapling height in SDI2-SDI4 was increased by 5.6%, 5.6%, and 11.3%, suggesting reasonable that regulation of stand density was conducive to increase the height growth of saplings in regeneration.

Effects of hurricane canopy gaps on longleaf pine and upland oak sapling growth

Longleaf pine forests and woodlands are an ecologically important ecosystem that once dominated much of the southeastern U.S. Frequent fire is well-known to be a driver of forest dynamics in these systems, but far less is known about how wind disturbance may reorganize plant communities. To gain a better understanding of how hurricane impacts may alter dynamics in a second-growth longleaf pine woodland, we investigated growth response of saplings from three competing upland species (Pinus palustris, Quercus falcata, Q. margaretta) following Hurricane Michael (2018) in southwest Georgia, United States. We used maximum likelihood to model height and radial growth of saplings as species-specific Michaelis-Menten functions of light availability and examined the effect of sapling size, soil water holding capacity, and annual precipitation using a model comparison approach. The best model for both height and radial growth included species-specific growth rates, a power function for sapling size, and an effect of 2-year lagged precipitation. Overall, we found that height growth rate increased with irradiance for all species, but asymptotic height growth rate for all species was approached at low irradiance. Rates of radial growth also increased with irradiance and were more strongly differentiated among species. Radial growth in longleaf pine and southern red oak responded most strongly to increasing irradiance. Our findings suggest that increases in light availability from hurricanes has direct effects on altering growth dynamics of saplings. Further, because stem radius is a strong predictor of survival from surface fires, hurricane damage may also have an indirect effect on sapling dynamics, by altering survival in future fires. Our results suggest that hurricanes can reinforce the dominance of longleaf pine and potentially play a role in stabilizing community structure. Understanding the effects of common disturbances like wind and fire—together and individually—can reveal important mechanisms that shape the structure and function of fire-prone ecosystems.

Deer browsing and light availability limit post oak (Quercus stellata) sapling growth and post-fire recovery in a xeric woodland

Historically, oaks dominated the canopies of large regions of eastern North American forests, but at many sites current recruitment is insufficient to sustain oak dominance. Intense browsing by white-tailed deer (Odocoileus virginianus) and light limitation due to expansion by fire-intolerant tree species may cause poor oak recruitment. In a xeric woodland in Kansas USA, we used a deer exclusion experiment in natural canopy gaps to quantify effects of browsing and light availability on post oak (Quercus stellata) sapling performance, meaning growth and survival. Two weeks after beginning the experiment, a prescribed fire occurred that, due to natural burn heterogeneity, affected 29 of 31 experimental canopy gaps and top-killed 78.6% of experimental saplings. This allowed us to examine post-fire recovery in a sub-set of saplings. Our hypotheses were 1) ambient deer browsing intensity reduces post oak sapling performance, 2) sapling performance increases with gap size and light availability, 3) deer browsing effects on sapling performance vary with light availability, and 4) re-growth rate of saplings that were top-killed by fire is affected by light, deer browsing and the interaction of light and browsing. After five years, deer exclusion increased sapling growth in height and aboveground biomass. Sapling height growth was positively correlated with light. Deer effects on sapling performance generally were consistent across light environments. However, there was a weak pattern toward more negative effects of browsing in high light than in low light environments for saplings that re-sprouted post-fire. Protection from deer shortened the time required for saplings to re-attain their pre-fire height from five to three years. We conclude that deer browsing reduces post oak recruitment in canopy gaps in these xeric woodlands. Further, under current browsing intensity, fire return intervals less than five years will strongly limit oak recruitment.

Climate and ungulate browsing impair regeneration dynamics in spruce-fir-beech forests in the French Alps

Key messageDifferent components of water balance and temperature reduce density and height growth of saplings of Picea abies (L.) H. Karst (Norway spruce), Abies alba Mill. (silver fir) and Fagus sylvatica L. (European beech) in mixed uneven-aged forests in the French Alps and Jura mountains. Ungulate browsing is an additional pressure on fir and beech that could jeopardise the renewal of these species in the future.ContextThe uncertainty in tree recruitment rates raises questions about the factors affecting regeneration processes in forests. Factors such as climate, light, competition and ungulate browsing pressure may play an important role in determining regeneration, forest structures and thus future forest composition.AimsThe objective of this study was to quantify sapling densities and height increments of spruce, fir and beech and to identify dominant environmental variables influencing them in mixed uneven-aged forests in the French Alps and Jura mountains.MethodsSapling height increment and density were recorded in 152 plots, and non-linear mixed models were obtained to establish relations between them and environmental factors known to affect regeneration, namely altitude, slope, aspect, canopy openness, soil characteristics, temperature, precipitation and ungulate browsing.ResultsRegeneration density, varying from 0 to 7 saplings per m 2, decreased with sapling height and was also negatively affected for spruce by PET, but positively for fir by precipitation and for beech by mean annual soil water content. Height increment reached up to 50 cm annually, increasing with sapling height and canopy openness and decreasing under high maximum summer temperatures for spruce and beech. The statistical effect of different environmental variables varied slightly among species but trends were quite similar. Additionally, ungulate browsing was high, with fir being the most intensely browsed, followed closely by beech, while spruce was rarely browsed.ConclusionsAll these results suggest that more temperature warming and a decrease in water availability could negatively impact sapling growth and density in the three species, with possible reduction of forest renewal fluxes. The observed increase of ungulate populations leading to increased browsing could be particularly detrimental to fir saplings.

Do trait responses to simulated browsing in Quercus robur saplings affect their attractiveness to Capreolus capreolus the following year?

With the rise of large herbivore populations in most northern hemisphere forests, browsing is becoming an increasingly important driver of forest regeneration dynamics. Among other processes affecting the regeneration, the concept of plant-herbivore feedback loops holds that browsed saplings are more subject to subsequent herbivory. This phenomenon is interpreted as a consequence of compensatory growth following browsing since fast growth is generally associated with higher digestibility and lower defense against herbivores. However, studies linking browsing-induced trait variations to subsequent attractiveness to herbivores are still lacking, especially in the forest context. In this study, we experimentally examine the existence of a feedback loop between oak ( Quercus robur L.) and roe deer ( Capreolus capreolus ) and investigate its underlying morphological and chemical traits. We simulated single and repeated roe deer browsing on nursery-grown oak saplings and measured the changes in sapling height growth, lateral branching, leaf traits and winter shoot traits over two years. We conducted winter feeding trials with tame roe deer one year after the first treatment to test the effect of simulated browsing on sapling attractiveness. Simulated browsing reduced sapling height growth but had no effect on branching. Simulated browsing had no effect on leaf traits after half a year, but decreased the phenolic content and increased the fiber content of winter shoots the following winter. Contrary to our predictions, roe deer preferentially browsed control saplings over saplings previously browsed. After two years, repeated browsing promoted fast carbon acquisition leaf traits (high chlorophyll, high specific leaf area and low fiber content), reduced leaf phenolic content and increased leaf digestibility. We showed that a reduction in 1-year-old oak sapling height growth following browsing, combined with increased structural defense at the expense of chemical defense in winter shoots the following winter, was correlated with reduced browsing pressure, thereby challenging the feedback loop hypothesis. However, we also demonstrated that repeated browsing promoted fast carbon acquisition leaf traits in 2.5-year-old saplings, which tend to support the existence of a feedback loop on older and more intensively browsed saplings. As such, our study provides empirical evidence that morphological and physiological trait responses to browsing influence oak sapling attractiveness, but that the direction and magnitude of this effect depend on the ontogenic stage of the sapling and on the number of browsing events. • We examine the morphological and chemical response of oak saplings to browsing. • Browsing reduces sapling height growth and modifies shoots chemical composition. • We show with feeding trials that roe deer prefers unbrowsed saplings. • Oak trait responses to browsing decrease the probability of subsequent herbivory.

Response of Ziziphus mucronata and Acacia nilotica saplings to increasing clipping intensity in a southern African savanna

Many woody plants respond to intense clipping through an increase in growth parameters and nutritive value. However, optimal clipping intensities that result in peak plant growth and nutritive value have not been determined. We studied the response of Ziziphus mucronata and Acacia nilotica saplings to four clipping intensities. Parts of the main stem and each lateral branch were removed to simulate 0%, 30%, 60%, and 90% clipping intensity. After four months of resprouting five sapling growth parameters were determined viz. height, basal stem diameter, length of the longest shoot, root length, and number of new shoots. In addition, foliar crude protein (CP), neutral detergent fibre (NDF), acid detergent fibre (ADF), total phenolic (TP), and condensed tannin (CT) concentrations were determined. Shoot clipping did not stimulate sapling height and basal diameter growth, while shoot length responded to an increase in clipping intensity with a weak hump-shape in both species. Ziziphus mucronata increased root growth in response to increasing clipping intensity, while A. nilotica showed no clear trend. The largest number of new shoots was produced at 60% and 30% clipping intensities for Z. mucronata and A. nilotica respectively. Ziziphus mucronata produced more new shoots than A. nilotica. The most severe (90%) clipping intensity produced the highest CP and lowest NDF content in the two species, reduced TP and CT content in A. nilotica, but not in Z. mucronata. We conclude that an increase in clipping intensity produced varying responses in sapling growth parameters and nutritive values in Z. mucronata and A. nilotica.

Tree diversity promotes growth of late successional species despite increasing deer damage in a restored forest.

The role of tree diversity in restored forests and its impact on key ecological processes like growth and resistance to herbivory has become increasingly important. We analyzed height growth and white-tailed deer Odocoileus virginianus browsing damage to saplings of 16 broadleaved tree species in a large-scale (13ha) reforestation experiment in Maryland, USA, where we manipulated tree diversity in 70 1,225-m2 plots. After four growing seasons, higher plot-level tree richness led to increased deer browsing damage (i.e., associational susceptibility). Despite increased deer damage to saplings in mixed plots, tree richness had no overall effect on sapling height growth. However, diversity-height relationships were related to species functional traits. Light demanding species with large leaves and faster growth rates had reduced heights in mixtures, whereas shade-tolerant, slower-growing species generally had either increased or unchanged height growth in diverse tree communities, likely related to increased canopy closure in mixtures relative to monocultures. We show that tree diversity can improve growth of late successional species despite exacerbated mammalian herbivore damage. By facilitating the establishment of species with a range of life-history strategies, increased tree diversity may enhance ecosystem multi-functionality in the early stages of forest restoration.

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 (&lt;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.

Effects of sulfuric, nitric, and mixed acid rain on Chinese fir sapling growth in Southern China

The influence of acid rain on plant growth includes direct effects on foliage as well as indirect soil-mediated effects that cause a reduction in root growth. In addition, the concentration of NO3- in acid rain increases along with the rapid growth of nitrogen deposition. In this study, we investigated the impact of simulated acid rain with different SO42-/NO3- (S/N) ratios, which were 1:0, 5:1, 1:1, 1:5 and 0:1, on Chinese fir sapling growth from March 2015 to April 2016. Results showed that Chinese fir sapling height growth rate (HGR) and basal diameter growth rate (DGR) decreased as acid rain pH decreased, and also decreased as the percentage of NO3- increased in acid rain. Acid rain pH significantly decreased the Chlorophyll a (Chla) and Chlorophyll b (Chlb) content, and Chla and Chlb contents with acid rain S/N 1:5 were significantly lower than those with S/N 1:0 at pH 2.5. The chlorophyll fluorescence parameters, maximal efficiency of Photosystem II photochemistry (Fv/Fm) and non-photochemical quenching coefficient (NPQ), with most acid rain treatments were significantly lower than those with CK treatments. Root activities first increased and then decreased as acid rain pH decreased, when acid rain S/N ratios were 1:1, 1:5 and 0:1. Redundancy discriminant analysis (RDA) showed that the Chinese fir DGR and HGR had positive correlations with Chla, Chlb, Fv/Fm ratio, root activity, catalase and superoxide dismutase activities in roots under the stress of acid rain with different pH and S/N ratios. The structural equation modelling (SEM) results showed that acid rain NO3- concentration and pH had stronger direct effects on Chinese fir sapling HGR and DGR, and the direct effects of acid rain NO3- concentration and pH on HGR were lower than those on DGR. Our results suggest that the ratio of SO42− to NO3− in acid rain is an important factor which could affect the sustainable development of monoculture Chinese fir plantations in southern China.

Regeneration response to canopy gap size in a Chinese pine plantation: Species diversity patterns, size structures and spatial distributions

Chinese pine (Pinus tabuliformis Carr.) is one of the most commercially important conifers in northern China because of its important economic and ecological value. However, most Chinese pine plantations have suffered degradation following fertility declines, pests and disease outbreaks. In this study, to evaluate whether gap creation can help achieve canopy recruitment for sustainable management, we analyzed the species diversity patterns of woody plant regeneration, growth status and the spatial distribution of Chinese pine regeneration in three control plots (C, 10×10m) and four classes of gaps seven years after gap creation. The diameters of the gap classes were 1.00 (G-I, 10.0m), 1.25 (G-II, 12.5m), 1.50 (G-III, 15.0m), and 1.75 (G-IV, 17.5m) times the mean canopy height. There were no differences in diversity patterns between the tested gap sizes, between gaps and under the canopy, this finding may be attributed to trees and shrubs require a relatively long time to become established. Although gap size had no influence on the density of Total (all regeneration), it significantly decreased and increased the density of seedlings (height≤100cm) and saplings (height>100cm), respectively, along the gap size gradient, which likely resulted from the differences between these two categories of regeneration in terms of shade tolerance and resource requirements. Similarly, the height, ground diameter and height growth of Total, seedlings and saplings in the current year showed positive responses to gap size. All gap size classes showed approximately normal height and ground diameter distributions, with a longer tail toward larger individuals with greater gap sizes. These results suggested that Chinese pine can be managed by gap creation in plantations, especially when combined with gradual gap expansion. In addition, Total and seedlings showed mostly random patterns across all scales, with aggregation at smaller scales, and the percentage of random patterns of saplings was greater than that of seedlings for all gap sizes. These differences in spatial patterns may have been associated with the survival strategies of tree species and the density-dependent processes. Further monitoring and gap expansion will likely be required to determine the long-term efficacy of gap creation to ensure successful canopy recruitment and to maintain sustainable management.

From the individual to the landscape and back: time‐varying effects of climate and herbivory on tree sapling growth at distribution limits

Summary As herbivory can modulate climate‐induced shifts in species distribution, disentangling the relative importance of herbivory and climate on plant growth can help to predict and manage future changes in vegetation, such as those occurring at treeline areas. An individual‐based hierarchical Bayesian time‐series model (individual‐based model; IBM) was developed to estimate the time‐varying impact of climate and herbivory on individual pine sapling height growth in woodland and treeline ecosystems of southern Europe during a 16‐year period. The performance of the IBM was compared to a linear mixed‐effects (LME) model to test for potential inferential effects when individual variability is marginalized. Time‐varying models were also compared to constant parameter approaches. Model fitting and posterior predictive checking suggests a better statistical performance of individual‐scale, time‐varying inference. LME modelling overestimated herbivory effects and underestimated environmental stochastic effects, and model validation indeed suggested severe overfitting in the LME model relative to the IBM strategy. These results reveal a potential failure of common aggregation strategies to correctly resolve the effects of climate and herbivory variability at the individual scale. Moreover, ignoring the time‐varying nature of the effects may preclude the correct estimation of the temporal scale of climate and herbivory impacts. In general, we found stronger individual‐ and time‐averaged effects of summer precipitation relative to the effects of herbivory, particularly at the treelines. Also, individual pine responses showed effects of the same sign more consistently in the case of precipitation. This suggests that precipitation is more pervasive at the population level, while herbivory act as a spatially aggregating force through individual‐level damage. Synthesis. Our results suggest that accounting for individual and temporal variability in ecological inference greatly improves the assessment of the relative importance of climate and herbivory on species distribution shifts. Strong effects of precipitation at the treeline could allow tree upward expansion, although increasing ungulate populations and associated browsing damages might limit positive climatic responses of pine saplings in the near future. In this context, the spatially heterogeneous effect exerted by herbivory could result in diverse vegetation structures in ecotones, adding a new dimension to the predictions on climate‐driven vegetation shifts.

The impact of overstory density on sapling height growth in the Missouri Ozarks: implications for interspecific differentiation during canopy recruitment

Successful canopy recruitment is one of the most important components of sustainable forestry practices. For many desirable species in oak-dominated forests, insufficient sapling growth is a common limitation to successful recruitment. The objectives of this study were to (i) examine the impact of overstory density on sapling growth in the Missouri Ozarks, (ii) investigate the potential for overstory retention to promote compositional shifts via interspecific differences in sapling height growth, and (iii) compare the use of mean and near-maximum growth rates to quantify the impact of overstory density on sapling growth and height differentiation among species. We found that the periodic annual height increment of saplings decreased with increasing overstory density for all species groups in this study (red oaks (Quercus spp.), white oaks (Quercus spp.), hickories (Carya spp.), sassafras (Sassafras spp.), blackgum (Nyssa spp.), dogwood (Cornus spp.), red maple (Acer spp.), ashes (Fraxinus spp.), and elms (Ulmus spp.)). There was evidence of interspecific differentiation in growth rates during the sapling stage, and the observed differences were more pronounced at low overstory densities. Increasing overstory densities either reduced or eliminated the differences in growth among species. Although red oaks displayed the greatest maximum growth rates of all species under low overstory densities (&lt;5 m2·ha−1), the growth advantage of red oaks was reduced with increasing overstory density. This may provide opportunities to shift species composition toward white oaks using partial harvesting regimes in the Missouri Ozarks. However, white oaks had little to no advantage in height growth over many competing species when overstory density exceeded about 10 m2·ha−1. This implies that the probability of recruitment under overstory densities greater than about 10 m2·ha−1is likely to decline for all oaks in the Missouri Ozarks. We found that using the 90th quantile of height growth rates to evaluate the impact of overstory density on sapling growth had two potential advantages over using the mean growth rate: (i) it provided better models of the limiting effects of overstory density on sapling height growth, and (ii) the focus was on the growth rates of stems that were most likely to recruit into the canopy.

Role of planting stock size and fertilizing in initial growth performance of rowan (Sorbus aucuparia L.) reforestation in a mountain frost hollow

The aim of study: (1) to compare the survival rate, growth performance and nutrition of large and common-sized planting stock of rowan (Sorbus aucuparia L.) on a frost-exposed site and (2) to assess whether fertilizing had any effect on the plantations.Area of study: The Jizera Mts., an area heavily disturbed by air pollution situated on the Czech-Polish border close to GermanyMaterials and methods: Two types of planting stock were tested in a mountain frost hollow on an acidic mountain humic podsol: (a) the bare-rooted saplings 131–140 cm tall and (b) common-sized containerized transplants 26–35 cm. One half of the saplings and common-sized transplants were left untreated and the other half were fertilized with a low dose (30 g per tree) of a slow release fertilizer based on methylene urea and potassium magnesium phosphate. Growth performance and nutrition of plantations were investigated.Main results: Due to serious deformations and stem breakages inflicted by snow and frost, the prospects of common-sized transplants seem much worse than those of saplings. The height growth of saplings was significantly more rapid than that of common-sized transplants. As for growth, neither the saplings nor common-sized transplants did significantly respond to fertilizing. The effects of fertilizing on nutrition of rowans were unconvincing. The extreme temperature events during growth seasons and snow deformations in winters might be the decisive factors influencing growth performance of rowans under referred conditions.Research highlights: On the frost-exposed sites, the height of taller saplings might partly compensate for a missing shelter of forest stand since the terminal leaders are above ground-frost zone.Key words: mountain ash; sapling; common-sized transplants; nutritional status; temperature.Abbreviations: CS – Control Saplings; CT – Control Transplants; FS – Fertilized Saplings; FT – Fertilized Transplants

Response of tree regeneration to experimental gap creation and deer herbivory in north temperate forests

Structural heterogeneity has become a goal of contemporary forest management, yet the effect of incorporating variable sized canopy openings characteristic of older forests on ecosystem services is still largely unknown. Single-tree selection silviculture reduces tree species diversity, and group-selection harvests often produce inconsistent results in maintaining the proportion of species with low or intermediate shade tolerance. It is unclear how much variability is related to inherent growth rate differences among shade tolerance classes, asymmetric competition, sprouting behavior, herbivory, and other factors. We conducted an experiment to control several of these factors. The northern hardwood study area in north-central Wisconsin included 15 replicates of each of 3 sizes of experimental gaps (50m2, 200m2, and 380m2). Ten main plots (80×80m2) were fenced to exclude deer. Vertical height growth of saplings and stump sprouts was monitored for two years pre-treatment and four years post-treatment.Overstory gaps significantly increased height growth rates, but there was no significant difference between rates of the very shade-tolerant Acer saccharum and several midtolerant species in any gap size. Saplings dominated the regeneration layer in small gaps. Stump sprouts were more abundant and grew faster than saplings in large gaps, but after 4years, A. saccharum advance regeneration still predominated in the upper height classes. Deer had limited effects on sapling development or species composition because tall advance regeneration was beyond their reach, but they severely affected the sprout layer. In unfenced plots, the unpalatable Ostrya viriginiana had the tallest sprouts. Overall, midtolerant species made up about 16% of the gap regeneration layer and appear to be increasing their proportion over time. Height growth rates of many saplings and sprouts exceeded 50cm per year, suggesting that successful gap capture would be likely for both shade-tolerance groups under current environmental conditions. The non-significant difference in growth rates between shade-tolerant and midtolerant species across the light gradient could change as more time elapses since gap creation. However, our findings after four years are consistent with other studies in suggesting that there may be no consistent trends in the relative growth responses of shade-tolerant and midtolerant tree species to increased light and gap size.

Shelterwood cutting to release coniferous advance growth and limit aspen sucker development in a boreal mixedwood stand

Stratified mixtures of pioneer hardwoods sheltering shade-tolerant conifers are commonly encountered in the southern boreal forest. We used two-step shelterwood cutting to release conifer advance growth and limit trembling aspen (Populus tremuloides Michx.) development in a stratified mixed aspen–conifer stand in Quebec, Canada. This paper presents 10-year regeneration dynamics after the establishment cut applied with different cutting intensities (0%, 35%, 50%, 65%, and 100% basal area removal). Aspen suckering was proportional to cutting intensity (p<0.001) and its survival was limited under all four densities of residual cover. After 10years, aspen density was ⩽600 stems/ha in the 0%, 35%, and 50% cuts, 1600stems/ha in the 65% cut and 4700stems/ha in the clearcut. Adequate protection of balsam fir (Abies balsamea (L.) Mill.) and spruce (Picea glauca (Moench) Voss and Picea mariana (Mill.) BSP) advance regeneration strongly contributed to limiting aspen development. The release treatment affected conifer sapling height growth, stem diameter, crown length growth and live crown ratio. Generally, the growth reaction to canopy removal was better in the clearcut than in partial cuts. Balsam fir response was good in the two initial height classes studied (130–300 and 301–500cm), but a significant spruce response occurred only in the smallest class. The final cut will be necessary to assess the overall effect of two-step shelterwood cutting on species composition and growth of the new cohort.

The Effect of Experimental Prescribed Fire on White Pine Regeneration

We examined the effects of prescribed fire on white pine (Pinus strobus) advance regeneration in small experimental plots in mixed hardwood-pine forests in southwestern Virginia during the fall and spring. Fires killed all newly germinated seedlings in both spring and fall. Saplings (ranging from 0.7 - 3.4 m in height) all suffered some degree of crown scorch, but sapling survival was higher on plots burned in the fall (54%) compared to those burned in the spring (20%). After one growing season following the fires, seedling density had recovered to a level similar to that of control plots for fall burns, but was still significantly lower than control plots for spring burns. After one growing season, the amount of crown mortality for saplings was reduced with increasing height and root collar diameter. Height growth of saplings after one growing season following fires was not different between burned and control plots. Fall burns may be less detrimental to white pine regeneration because of reduced mortality to saplings and by providing microsite conditions during the period of seed rain that are conducive to recover seedling regeneration.

Low intensities of red deer browsing constrain rowan growth in mature boreal forests of western Norway

Browsing by cervids plays a key role in structuring forest ecosystems and dynamics. Many boreal forest systems are managed for timber resources, and at the same time the wild cervid populations are also harvested. Thus, the determination of sustainable densities of cervids for the purpose of forest and game management is challenging. In this study we report on a red deer (Cervus elaphus) exclosure experiment in the mature forests of Western Norway. Ten pairs of exclosures and browsed plots were initiated in 2008. The rate of browsing and height growth of marked individuals was recorded annually, and the total densities of all tree species assessed over the following 4 y. We found that height growth of rowan (Sorbus aucuparia) saplings (1 m tall), the most numerous tree species at the site, was prevented when 20% of the shoots were browsed. Outside of the exclosures, net height growth of rowan saplings tended to be positive when trees were below 40 cm in height, but growth was constrained in rowan saplings over this height. The density of rowan also increased in both treatments, showing that recruitment was occurring, but the increase was greater where browsed than in the exclosure. The increase in density of rowan, combined with the curtailment of height growth in the presence of red deer, serves to create a carpet of short stature rowan saplings. This has parallels with the browsing lawn concept, but it seems to occur in interaction with snow depth; individuals protruding above the snow layer are likely to be browsed during the winter, whilst smaller individuals are protected during this season, when browsing is at its peak.

The effect of age on height growth in even-sized saplings of Fagus sylvatica L.

After controlling for the effects of size and light, partial regression revealed that height growth of common beech saplings was negatively affected by sapling age. Common beech (Fagus sylvatica L.) saplings were studied along gradients of light availability (4–82 % of full sunlight), initial size (9–290 cm), and age (2–25 years) to examine the interactive effect of these variables on saplings’ annual height growth. Although age was non-significant as a main effect in a linear model, sapling age had a significant interaction with the other variables. After controlling for the effects of size and light, partial regression revealed that height growth was negatively affected by sapling age. Observed growth decline in older common-sized saplings may be explained not as effect of age per se, but as indirect age-related effect probably induced through plastic response of saplings to past growth conditions.