Influence of Light on Natural Regeneration of Pedunculate Oak (Quercus roburL.) in the Maksimir Forest Park in Zagreb

U park-šumi Maksimir, u mješovitoj sastojini hrasta lužnjaka i običnoga graba (Carpino betuli-Quercetum roboris/Anić 1959/ Rauš 1969), analiziran je utjecaj svjetla na brojnost i kvalitetu mladog hrastovog naraštaja. Pokusna ploha obuhvatila je dvije pomladne jezgre u razvojnom stadiju mladika i površinu između jezgri u fazi ponika te mlađeg i starijeg pomlatka. Analiza podataka pokazala je da se prirodno pomlađivanje na malim površinama u park-šumi Maksimir odvija uspješno. Broj biljaka po jedinici površine je zadovoljavajući i iznosi 8,3 kom/m2. Prema vrsti smjese to je mješovita sastojina hrasta lužnjaka i običnoga graba s manjim udjelom divlje trešnje (PrunusaviumL.), klena (Acer campestreL.), lipe (Tiliasp.) i mliječa (A. platanoidesL.). Prednost mješovitih sastojina ogleda se u biološkoj i ekološkoj ravnoteži, a to su glavni ciljevi gospodarenja u park-šumi Maksimir. No, kvaliteta mladog na raštaja hrasta lužnjaka je upitna. Posljedica visoke gustoće biljaka na pom-lad noj površini je visok prosječni koeficijent vitkosti mladog hrastovog naraštaja. Velik udio deformiranih debala, loše razvijenih krošnji i prevelika zastupljenost korovske vegetacije posljedica su nenjegovanja pomlatka. Utvrđene su prosječne godišnje relativne vrijednosti difuznog i izravnog svjetla u pojedinim razvojnim stadijima hrasta lužnjaka. Ovisnost broja biljaka u različitim razvojnim stadijima o svjetlosnim uvjetima pojedinog mikrostaništa potvrđena je ?2testom. Dobivena je jaka pozitivna linearna ko re lacija između prosječnog visinskog prirasta hrasta lužnjaka i vrijednosti izravnog svjetla (r = 0,5809).In a forest park Maksimir, in a mixed forest stand of Pedunculate Oak and Common Hornbeam (Carpino betuli-Quercetum roboris/Anić 1959/ Rauš 1969), influence of light conditions on number and quality of young oak growth was investigated. Experimental plot covered two regeneration gaps and the space between, including the trees in a different stage of development (seedlings, saplings and young trees). Measurements were taken in the winter 2006 and spring 2007 in a single plot within the stand. The plot was divided in 105 sub-plots of 1.5 m x 1.5 m and on each sub-plot height (cm), tree length (cm), ground level diameter (mm) and last five height increments (cm) were measured. Parallel to the measurement, evaluation of stem quality, crown form and tree health state was conducted. Spatial distribution of old grown trees at the plot area, together with crown projections, was recorded. At each sub-plot a hemispherical photograph was taken. Average annual relative values of diffuse and direct light for different development stages of Pedunculate Oak were determined. Depending on the values of diffuse and direct light, four microsites (marked: A – D) with different light conditions were defined. Results indicate that natural regeneration in naturally occurring gaps could be considered successful. Average number of trees per square meter was 8.3. Species composition of naturally occurring young trees in the gap indicates a continuation of the same forest community, namely mixed forest stand of Pedunculate Oak and Common Hornbeam with a smaller share of Wild Cherry (Prunus aviumL.), Hedge Maple (Acer campestreL.), Norway Maple (A.platanoidesL.) and Lime (Tiliasp.). Mixed forest stands are of great biological and ecological value, but also attractive to the park visitors. Maintaining them and their stability is a main management goal in this forest park. However, in naturally occurring gaps in the absence of silvicultural treatments the quality of young trees is questionable. High density of young growth at our plot resulted with high tree slenderness coefficient of 97,7. Share of deformed tree stems was significant (30.3 %), as well as share of badly developed tree crowns (44.3 %). Great abundance of weed vegetation was recorded and can be attributed to the lack of silvicultural treatments during regeneration. Chi-square test showed statistically significant dependence of number of young oak growth in different development stages with respect to the light conditions at microsite. Lower values of direct and diffuse light (microsite C) correspond with great number of oak seedlings. Surviving of oak seedling in low light conditions confirms the fact that in first few years oak is shade tolerant. However, great abundance of oak saplings at microsites A (lower values of direct and higher values of diffuse light) and D (higher values of direct and lower values of diffuse light) indicates that oak, when it arrives to the stage of saplings, favours higher light conditions. This is further corroborated at microsite B (with high values of direct and diffuse light) where young oak trees were most abundant. Quality of oak saplings and young trees was better at microsite B. Young oak trees grown in high light conditions obtained greater height increment and stem verticality. High positive correlation is obtained between Pedunculate Oak average height increment and average values of direct light (r = 0,5809)

Influence of Light on Natural Regeneration of Pedunculate Oak (Quercus roburL.) in the Maksimir Forest Park in Zagreb

U park-šumi Maksimir, u mješovitoj sastojini hrasta lužnjaka i običnoga graba (Carpino betuli-Quercetum roboris/Anić 1959/ Rauš 1969), analiziran je utjecaj svjetla na brojnost i kvalitetu mladog hrastovog naraštaja. Pokusna ploha obuhvatila je dvije pomladne jezgre u razvojnom stadiju mladika i površinu između jezgri u fazi ponika te mlađeg i starijeg pomlatka. Analiza podataka pokazala je da se prirodno pomlađivanje na malim površinama u park-šumi Maksimir odvija uspješno. Broj biljaka po jedinici površine je zadovoljavajući i iznosi 8,3 kom/m2. Prema vrsti smjese to je mješovita sastojina hrasta lužnjaka i običnoga graba s manjim udjelom divlje trešnje (PrunusaviumL.), klena (Acer campestreL.), lipe (Tiliasp.) i mliječa (A. platanoidesL.). Prednost mješovitih sastojina ogleda se u biološkoj i ekološkoj ravnoteži, a to su glavni ciljevi gospodarenja u park-šumi Maksimir. No, kvaliteta mladog na raštaja hrasta lužnjaka je upitna. Posljedica visoke gustoće biljaka na pom-lad noj površini je visok prosječni koeficijent vitkosti mladog hrastovog naraštaja. Velik udio deformiranih debala, loše razvijenih krošnji i prevelika zastupljenost korovske vegetacije posljedica su nenjegovanja pomlatka. Utvrđene su prosječne godišnje relativne vrijednosti difuznog i izravnog svjetla u pojedinim razvojnim stadijima hrasta lužnjaka. Ovisnost broja biljaka u različitim razvojnim stadijima o svjetlosnim uvjetima pojedinog mikrostaništa potvrđena je ?2testom. Dobivena je jaka pozitivna linearna ko re lacija između prosječnog visinskog prirasta hrasta lužnjaka i vrijednosti izravnog svjetla (r = 0,5809).In a forest park Maksimir, in a mixed forest stand of Pedunculate Oak and Common Hornbeam (Carpino betuli-Quercetum roboris/Anić 1959/ Rauš 1969), influence of light conditions on number and quality of young oak growth was investigated. Experimental plot covered two regeneration gaps and the space between, including the trees in a different stage of development (seedlings, saplings and young trees). Measurements were taken in the winter 2006 and spring 2007 in a single plot within the stand. The plot was divided in 105 sub-plots of 1.5 m x 1.5 m and on each sub-plot height (cm), tree length (cm), ground level diameter (mm) and last five height increments (cm) were measured. Parallel to the measurement, evaluation of stem quality, crown form and tree health state was conducted. Spatial distribution of old grown trees at the plot area, together with crown projections, was recorded. At each sub-plot a hemispherical photograph was taken. Average annual relative values of diffuse and direct light for different development stages of Pedunculate Oak were determined. Depending on the values of diffuse and direct light, four microsites (marked: A – D) with different light conditions were defined. Results indicate that natural regeneration in naturally occurring gaps could be considered successful. Average number of trees per square meter was 8.3. Species composition of naturally occurring young trees in the gap indicates a continuation of the same forest community, namely mixed forest stand of Pedunculate Oak and Common Hornbeam with a smaller share of Wild Cherry (Prunus aviumL.), Hedge Maple (Acer campestreL.), Norway Maple (A.platanoidesL.) and Lime (Tiliasp.). Mixed forest stands are of great biological and ecological value, but also attractive to the park visitors. Maintaining them and their stability is a main management goal in this forest park. However, in naturally occurring gaps in the absence of silvicultural treatments the quality of young trees is questionable. High density of young growth at our plot resulted with high tree slenderness coefficient of 97,7. Share of deformed tree stems was significant (30.3 %), as well as share of badly developed tree crowns (44.3 %). Great abundance of weed vegetation was recorded and can be attributed to the lack of silvicultural treatments during regeneration. Chi-square test showed statistically significant dependence of number of young oak growth in different development stages with respect to the light conditions at microsite. Lower values of direct and diffuse light (microsite C) correspond with great number of oak seedlings. Surviving of oak seedling in low light conditions confirms the fact that in first few years oak is shade tolerant. However, great abundance of oak saplings at microsites A (lower values of direct and higher values of diffuse light) and D (higher values of direct and lower values of diffuse light) indicates that oak, when it arrives to the stage of saplings, favours higher light conditions. This is further corroborated at microsite B (with high values of direct and diffuse light) where young oak trees were most abundant. Quality of oak saplings and young trees was better at microsite B. Young oak trees grown in high light conditions obtained greater height increment and stem verticality. High positive correlation is obtained between Pedunculate Oak average height increment and average values of direct light (r = 0,5809)

WHICH ECOPHYSIOLOGICAL FACTORS INFLUENCE REPRODUCTION OF FOREST TREES AND WHETHER TREES PRODUCED SEED MORE FREQUANTLY AND ABUNDANTLY IN THE PAST?

U radu je raspravljeno o problematici definiranja i upotrebe termina „puni“ ili „obilan urod“ na temelju usporedbe strane i domeće literature. Predstavljene su dosadašnje spoznaje o plodonošenju šumskoga drveća u Hrvatskoj. Prikazan je utjecaj nekih ekofizioloških čimbenika na razvoj rasplodnih organa (cvjetova i plodova) šumskoga drveća i predstavljene su dosadašnje spoznaje o mogućem utjecaju klimatskih promjena (npr. sve učestalija sušna razdoblja i povećanje količine CO2 u atmosferi) na rasplodni ciklus šumskoga drveća. Naposljetku, obilan urod predstavljen je kao određena evolucijska prednost koja je pojedinim vrstama osigurala opstanak sve do današnjih dana.Masting, or an abundant fruiting of forest trees in intermittent intervals in a broader geographical area, represents an evolutionary advantage which has ensured survival of certain forest tree species up till today. Due to the selection pressure, surviving individuals are the ones which underwent initiation and differentiation of flowers under the influence of strong interactive control of climatic factors and nutrients which a particular individual tree had at its disposal. In other words, initiation and differentiation of a large number of flowers occurs only under the influence of both, sufficient amount of nutrients and favourable climatic conditions, which is also a basic requirement for the occurrence of masting. The advantage of masting occurrence is reflected in an increased success of pollination, decreased populations of pests affecting reproductive organs of forest trees, and dissemination of particular amounts of seed to greater distances from parent trees. Particular phases of forest tree fruit development take place under the influence of a specific interaction of physiological processes within the plant organism (hormonal control, mineral nutrition and carbohydrate metabolism) which are stimulated, and to a certain degree controlled, by the climatic factors (air temperature,amount of precipitation, relative humidity of air, etc.). Under favourable climatic conditions, a portion of nutrients which forest trees have at their disposal gets used in fruit development, which is reflected in great amount of initiated anddifferentiated flowers. The amount of nutrients which would be invested in fruit development depends greatly on the site quality. Trees grown in high quality environment, as opposed to those grown in lower site quality, replenish their nutrient reserves more rapidly despite having identical climatic growth conditions, thanks to a more intensive photosynthetic activity, which in turn results in an increased seed production

Quality assessment of high density digital surface model over different land cover classes

Background and Purpose: Recent research on generation of digital surface models (DSMs) using image matching methods revealed a great potential of DSM application in forestry, especially in forest inventory. However, research dealing with DSM generation from digital aerial images are still lacking in Croatia. Therefore, the main objective of this study was to present the workflow for generating high density DSM from colour infrared (CIR) digital stereo aerial images using area-based image matching algorithm. Materials and Methods: The high density DSM was generated from colour infrared digital aerial stereo images using Dense DTM algorithm of PHOTOMOD software - an area-based image matching algorithm which operates on the principle of cross-correlation approach. To evaluate the quality of the generated DSM, an agreement assessment with manual stereo measurements was conducted over three different land cover classes (forests, shrubs, grasslands) using the same images as for DSM generation. Results: The good vertical agreement between the generated DSM and stereo measurement was achieved for all three land cover classes present at the research area. The highest vertical agreement was obtained for the grassland land cover class (RMSE=0.36), slightly lower for forest (RMSE=0.62), whereas the lowest vertical agreement was obtained for shrub land cover class (RMSE=0.83). Conclusions: The results of this research are very promising and suggest that the high density DSM generated from digital aerial stereo images and by using the proposed methodology has the potential to be used in forestry, primarily in forest inventory. Therefore, further research should be focused on generation of CHM by subtracting available DTM from the high density DSM and on the examination of its potential for deriving various forest attributes

Biogeochemical Modelling vs. Tree-Ring Measurements - Comparison of Growth Dynamic Estimates at Two Distinct Oak Forests in Croatia

Background and Purpose: Biogeochemical process‑based models use a mathematical representation of physical processes with the aim of simulating and predicting past or future state of ecosystems (e.g. forests). Such models, usually executed as computer programs, rely on environmental variables as drivers, hence they can be used in studies of expected changes in environmental conditions. Process‑based models are continuously developed and improved with new scientific findings and newly available datasets. In the case of forests, long-term tree chronologies, either from monitoring or from tree-ring data, offer valuable means for testing modelling results. Information from different tree cores can cover a wide range of ecological and meteorological conditions and as such provide satisfactory temporal and spatial resolution to be used for model testing and improvement. Materials and Methods: In our research, we used tree-ring data as a ground truth to test the performance of Biome-BGCMuSo (BBGCMuSo) model in two distinct pedunculate oak forest areas, Kupa River Basin (called Pokupsko Basin) and Spačva River Basin, corresponding to a wetter and a drier site, respectively. Comparison of growth estimates from two different data sources was performed by estimating the dynamics of standardized basal area increment (BAI) from tree-ring data and standardized net primary productivity of stem wood (NPPw) from BBGCMuSo model. The estimated growth dynamics during 2000-2014 were discussed regarding the site-specific conditions and the observed meteorology. Results: The results showed similar growth dynamic obtained from the model at both investigated locations, although growth estimates from tree-ring data revealed differences between wetter and drier environment. This indicates higher model sensitivity to meteorology (positive temperature anomalies and negative precipitation anomalies during vegetation period) than to site-specific conditions (groundwater, soil type). At both locations, Pokupsko and Spačva, BBGCMuSo showed poor predictive power in capturing the dynamics obtained from tree‑ring data. Conclusions: BBGCMuSo model, similar to other process-based models, is primarily driven by meteorology, although site-specific conditions are an important factor affecting lowland oak forests’ growth dynamics. When possible, groundwater information should be included in the modelling of lowland oak forests in order to obtain better predictions. The observed discrepancies between measured and modelled data indicate that fixed carbon allocation, currently implemented in the model, fails in predicting growth dynamics of NPP. Dynamic carbon allocation routine should be implemented in the model to better capture tree stress response and growth dynamics

Forest carbon allocation modelling under climate change

Carbon allocation plays a key role in ecosystem dynamics and plant adaptation to changing environmental conditions. Hence, proper description of this process in vegetation models is crucial for the simulations of the impact of climate change on carbon cycling in forests. Here we review how carbon allocation modelling is currently implemented in 31 contrasting models to identify the main gaps compared with our theoretical and empirical understanding of carbon allocation. A hybrid approach based on combining several principles and/or types of carbon allocation modelling prevailed in the examined models, while physiologically more sophisticated approaches were used less often than empirical ones. The analysis revealed that, although the number of carbon allocation studies over the past 10 years has substantially increased, some background processes are still insufficiently understood and some issues in models are frequently poorly represented, oversimplified or even omitted. Hence, current challenges for carbon allocation modelling in forest ecosystems are (i) to overcome remaining limits in process understanding, particularly regarding the impact of disturbances on carbon allocation, accumulation and utilization of nonstructural carbohydrates, and carbon use by symbionts, and (ii) to implement existing knowledge of carbon allocation into defence, regeneration and improved resource uptake in order to better account for changing environmental conditions. © The Author(s) 2019. Published by Oxford University Press

Forest carbon allocation modelling under climate change

Carbon allocation plays a key role in ecosystem dynamics and plant adaptation to changing environmental conditions. Hence, proper description of this process in dynamic vegetation models is crucial for the simulations of the impact of climate change on carbon cycling in forests. Here we review how carbon allocation modelling is implemented in 31 dynamic vegetation models to identify the main gaps compared to our theoretical and empirical understanding of carbon allocation. We found that a hybrid approach based on combining several principles and/or types of carbon allocation modelling prevailed in examined models. The analysis revealed that although the number of carbon allocation studies over the last 10 years has substantially increased, some background processes are still insufficiently understood, and some issues in models are frequently oversimplified or even omitted. Hence, current challenges for carbon allocation modelling in forest ecosystems are (i) to overcome remaining limits in process understanding, particularly regarding the impact of disturbances on carbon allocation, accumulation and utilisation of non-structural carbohydrates, and carbon use by symbionts, and (ii) to implement existing knowledge to mechanistic description of carbon allocation in models that would integrate the impact of environmental conditions, disturbances, and seasonal variation in carbon allocation, or (iii) to improve more simplistic models by accounting for the impact of crucial factors affecting carbon allocation in particular environment

Evaluation of the Terrestrial Ecosystem Model Biome-BGCMuSo for Modelling Soil Organic Carbon under Different Land Uses

Soil organic carbon (SOC) is a mandatory pool in national inventory reports on greenhouse gas (GHG) emissions and removals to the UNFCCC. Hence, its accurate assessment is important. Modelling SOC changes for national GHG reports is encouraged, but the uncertainty related to this pool still presents a significant challenge; thus, verifying modelling results with field observations is essential. We used the process-based model Biome-BGCMuSo and assessed its suitability for use in Croatia’s GHG reporting. We modelled SOC stocks in the top 30 cm of the mineral soil layer (SOC30) for four different land-use (LU) categories (Deciduous/Coniferous Forest, Grassland and Annual Cropland) distributed in three biogeographical regions (Alpine, Continental and Mediterranean) and compared them with results of a national soil survey. A total of 573 plot level simulations were undertaken and results were evaluated at three stratification levels (LU, LU × biogeographical region, and plot). The model reproduced the overall country mean of SOC30 with no overall bias, and showed good performance at the LU level with no significant (p < 0.05) difference for all LUs except Deciduous Forest (11% overestimation). At finer stratifications, the model performance considerably worsened. Further model calibration, improvement and testing, as well as repeated soil survey are needed in order to assess the changes in SOC30 and to evaluate the potential of the Biome-BGCMuSo model for use in GHG reporting