Assessing the effects of management on forest growth across France: insights from a new functional-structural model

Under-Canopy Regeneration of Scots Pine (Pinus sylvestris L.) as Adaptive Potential in Building a Diverse Stand Structure

Under-canopy natural regeneration of Scots pine (Pinus sylvestris L.) stand is able to create stands with a complex structure, which are characterised by greater resistance to extreme phenomena related to climate change. The main aim of the work was to analyse the potential of pine undergrowth, its role in the stand, and its usefulness in further breeding plans to create stands of various structure, with greater stability and tolerance to stress factors, including greater resistance to climate change. The study was carried out in north-eastern Poland, in pine stands thinned by strong (hurricane level) wind in 2002. The study area covers 225.2 ha and is located in 29 stands. Naturally regenerated pine saplings with a minimum height of 0.5 m, characterised by a high density, growing under the upper cover of the stand, with varying degrees of thinning, were qualified for the study. On average 7.820 pine saplings were recorded in the area of one hectare. Over 65% of the examined saplings were determined to be of very good or good quality, prognostic for further development and the creation of the main stand in the future. The principle of increasing the structural diversity of stands is the method of increasing the adaptability of forests to environmental changes.

Electromagnetic Induction Is a Fast and Non-Destructive Approach to Estimate the Influence of Subsurface Heterogeneity on Forest Canopy Structure

The spatial forest structure that drives the functioning of these ecosystems and their response to global change is closely linked to edaphic conditions. However, the latter properties are particularly difficult to characterize in forest areas developed on karst, where soil is highly rocky and heterogeneous. In this work, we investigated whether geophysics, and more specifically electromagnetic induction (EMI), can provide a better understanding of forest structure. We use EMI (EM31, Geonics Limited, Ontario, Canada) to study the spatial variability of ground properties in two different Mediterranean forests. A naturally post-fire regenerated forest composed of Aleppo pines and Holm oaks and a monospecific plantation of Altlas cedar. To better interpret EMI results, we used electrical resistivity tomography (ERT), soil depth surveys, and field observations. Vegetation was also characterized using hemispherical photographs that allowed to calculate plant area index (PAI). Our results show that the variability of ground properties contribute to explaining the variability in the vegetation cover development (plant area index). Vegetation density is higher in areas where the soil is deeper. We showed a significant correlation between edaphic conditions and tree development in the naturally regenerated forest, but this relationship is clearly weaker in the cedar plantation. We hypothesized that regular planting after subsoiling, as well as sylvicultural practices (thinning and pruning) influenced the expected relationship between vegetation structure and soil conditions measured by EMI. This work opens up new research avenues to better understand the interplay between soil and subsoil variability and forest response to climate change.

Tree Mortality Risks Under Climate Change in Europe: Assessment of Silviculture Practices and Genetic Conservation Networks

General Context: Climate change can positively or negatively affect abiotic and biotic drivers of tree mortality. Process-based models integrating these climatic effects are only seldom used at species distribution scale.

Objective: The main objective of this study was to investigate the multi-causal mortality risk of five major European forest tree species across their distribution range from an ecophysiological perspective, to quantify the impact of forest management practices on this risk and to identify threats on the genetic conservation network.

Methods: We used the process-based ecophysiological model CASTANEA to simulate the mortality risk of Fagus sylvatica, Quercus petraea, Pinus sylvestris, Pinus pinaster, and Picea abies under current and future climate conditions, while considering local silviculture practices. The mortality risk was assessed by a composite risk index (CRIM) integrating the risks of carbon starvation, hydraulic failure and frost damage. We took into account extreme climatic events with the CRIMmax, computed as the maximum annual value of the CRIM.

Results: The physiological processes' contributions to CRIM differed among species: it was mainly driven by hydraulic failure for P. sylvestris and Q. petraea, by frost damage for P. abies, by carbon starvation for P. pinaster, and by a combination of hydraulic failure and frost damage for F. sylvatica. Under future climate, projections showed an increase of CRIM for P. pinaster but a decrease for P. abies, Q. petraea, and F. sylvatica, and little variation for P. sylvestris. Under the harshest future climatic scenario, forest management decreased the mean CRIM of P. sylvestris, increased it for P. abies and P. pinaster and had no major impact for the two broadleaved species. By the year 2100, 38–90% of the European network of gene conservation units are at extinction risk (CRIMmax=1), depending on the species.

Conclusions: Using a process-based ecophysiological model allowed us to disentangle the multiple drivers of tree mortality under current and future climates. Taking into account the positive effect of increased CO2 on fertilization and water use efficiency, average mortality risk may increase or decrease in the future depending on species and sites. However, under extreme climatic events, our process-based projections are as pessimistic as those obtained using bioclimatic niche models.

Drought elicits contrasting responses on the autumn dynamics of wood formation in late successional deciduous tree species

Research on wood phenology has mainly focused on reactivation of the cambium in spring. In this study we investigated if summer drought advances cessation of wood formation and if it has any influence on wood structure in late successional forest trees of the temperate zone. The end of xylogenesis was monitored between August and November in stands of European beech and pedunculate oak in Belgium for two consecutive years, 2017 and 2018, with the latter year having experienced an exceptional summer drought. Wood formation in oak was affected by the drought, with oak trees ceasing cambial activity and wood maturation about 3 weeks earlier in 2018 compared with 2017. Beech ceased wood formation before oak, but its wood phenology did not differ between years. Furthermore, between the 2 years, no significant difference was found in ring width, percentage of mature fibers in the late season, vessel size and density. In 2018, beech did show thinner fiber walls, whereas oak showed thicker walls. In this paper, we showed that summer drought can have an important impact on late season wood phenology xylem development. This will help to better understand forest ecosystems and improve forest models.

Developing Growth Models of Stand Volume for Subtropical Forests in Karst Areas: A Case Study in the Guizhou Plateau

Forest stand volume is one of the key forest structural attributes in estimating and forecasting ecosystem productivity and carbon stock. However, studies on growth modeling and environmental influences on stand volume are still rare to date, especially in subtropical forests in karst areas, which are characterized by a complex species composition and are important in the global carbon budget. In this paper, we developed growth models of stand volume for all the dominant tree species (groups) (DTSG) in a subtropical karst area, the Guizhou Plateau based on an investigation of the effects of various environmental factors on stand volume. The Richards growth function, space-for-time substitution and zonal-hierarchical modeling method were applied in the model fitting, and multiple indices were used in the model evaluation. The results showed that the climatic factors of annual temperature and precipitation, as well as the site factors of stand origin, elevation, slope gradient, topsoil thickness, site quality degree, rocky desertification type and rocky desertification degree, have significant influences on stand volume, and the topsoil thickness and site quality degree have the strongest positive effect. A total of 959 growth equations of stand volume were fitted with a five-level stand classifier (DTSG–climatic zone–site quality degree–stand origin–rocky desertification type). All the growth equations were qualified, because all passed the TRE test (≤30%), and the majority of the R2 ≥ 0.50, above 70% of the RMSE were between 5.0 and 20.0, and above 80% of the P ≥ 75%. These findings provide updated knowledge about the environmental effect on the stand volume growth of subtropical forests in karst areas, and the developed stand volume growth models are convenient for forest management and planning, further contributing to the study of forest carbon storage assessments and global carbon cycling.

Environmental control of carbon allocation matters for modelling forest growth

We aimed to evaluate the importance of modulations of within-tree carbon (C) allocation by water and low-temperature stress for the prediction of annual forest growth with a process-based model. A new C allocation scheme was implemented in the CASTANEA model that accounts for lagged and direct environmental controls of C allocation. Different approaches (static vs dynamic) to modelling C allocation were then compared in a model-data fusion procedure, using satellite-derived leaf production estimates and biometric measurements at c. 10⁴ sites. The modelling of the environmental control of C allocation significantly improved the ability of CASTANEA to predict the spatial and year-to-year variability of aboveground forest growth along regional gradients. A significant effect of the previous year's water stress on the C allocation to leaves and wood was reported. Our results also are consistent with a prominent role of the environmental modulation of sink demand in the wood growth of the studied species. Data available at large scales can inform forest models about the processes driving annual and seasonal C allocation. Our results call for a greater consideration of C allocation drivers, especially sink-demand fluctuations, for the simulations of current and future forest productivity with process-based models.

Wood phenology, not carbon input, controls the interannual variability of wood growth in a temperate oak forest

Although the analysis of flux data has increased our understanding of the interannual variability of carbon inputs into forest ecosystems, we still know little about the determinants of wood growth. Here, we aimed to identify which drivers control the interannual variability of wood growth in a mesic temperate deciduous forest. We analysed a 9-yr time series of carbon fluxes and aboveground wood growth (AWG), reconstructed at a weekly time-scale through the combination of dendrometer and wood density data. Carbon inputs and AWG anomalies appeared to be uncorrelated from the seasonal to interannual scales. More than 90% of the interannual variability of AWG was explained by a combination of the growth intensity during a first 'critical period' of the wood growing season, occurring close to the seasonal maximum, and the timing of the first summer growth halt. Both atmospheric and soil water stress exerted a strong control on the interannual variability of AWG at the study site, despite its mesic conditions, whilst not affecting carbon inputs. Carbon sink activity, not carbon inputs, determined the interannual variations in wood growth at the study site. Our results provide a functional understanding of the dependence of radial growth on precipitation observed in dendrological studies.

Stand density, tree social status and water stress influence allocation in height and diameter growth of Quercus petraea (Liebl.)

Even-aged forest stands are competitive communities where competition for light gives advantages to tall individuals, thereby inducing a race for height. These same individuals must however balance this competitive advantage with height-related mechanical and hydraulic risks. These phenomena may induce variations in height-diameter growth relationships, with primary dependences on stand density and tree social status as proxies for competition pressure and access to light, and on availability of local environmental resources, including water. We aimed to investigate the effects of stand density, tree social status and water stress on the individual height-circumference growth allocation (Δh-Δc), in even-aged stands of Quercus petraea Liebl. (sessile oak). Within-stand Δc was used as surrogate for tree social status. We used an original long-term experimental plot network, set up in the species production area in France, and designed to explore stand dynamics on a maximum density gradient. Growth allocation was modelled statistically by relating the shape of the Δh-Δc relationship to stand density, stand age and water deficit. The shape of the Δh-Δc relationship shifted from linear with a moderate slope in open-grown stands to concave saturating with an initial steep slope in closed stands. Maximum height growth was found to follow a typical mono-modal response to stand age. In open-grown stands, increasing summer soil water deficit was found to decrease height growth relative to radial growth, suggesting hydraulic constraints on height growth. A similar pattern was found in closed stands, the magnitude of the effect however lowering from suppressed to dominant trees. We highlight the high phenotypic plasticity of growth in sessile oak trees that further adapt their allocation scheme to their environment. Stand density and tree social status were major drivers of growth allocation variations, while water stress had a detrimental effect on height in the Δh-Δc allocation.

Functional-structural plant models: a growing paradigm for plant studies

A number of research groups in various areas of plant biology as well as computer science and applied mathematics have addressed modelling the spatiotemporal dynamics of growth and development of plants. This has resulted in development of functional-structural plant models (FSPMs). In FSPMs, the plant structure is always explicitly represented in terms of a network of elementary units. In this respect, FSPMs are different from more abstract models in which a simplified representation of the plant structure is frequently used (e.g. spatial density of leaves, total biomass, etc.). This key feature makes it possible to build modular models and creates avenues for efficient exchange of model components and experimental data. They are being used to deal with the complex 3-D structure of plants and to simulate growth and development occurring at spatial scales from cells to forest areas, and temporal scales from seconds to decades and many plant generations. The plant types studied also cover a broad spectrum, from algae to trees. This special issue of Annals of Botany features selected papers on FSPM topics such as models of morphological development, models of physical and biological processes, integrated models predicting dynamics of plants and plant communities, modelling platforms, methods for acquiring the 3-D structures of plants using automated measurements, and practical applications for agronomic purposes.