Long-term Forest Growth Research Articles

Sensitivity of long-term productivity estimations in mixed forests to uncertain parameters related to fine roots

Forest growth models are increasingly being used in forestry and ecology research as predictive tools to help developing practical guidelines and to improve understanding of the drivers of forest ecosystem functioning. Models are usually calibrated using parameters directly obtained or estimated from empirical field observation, and hence are subject to uncertainty. Thus, output accuracy depends on input parameters precision and on how influential is each parameter on model behaviour. Hence, it is important to analyse parameter-related uncertainty and its effects on model outputs. This can be done by performing sensitivity analyses, which allow to explore the influence of one or several calibration parameters on model outputs. As studies on tree root parameters are particularly scarce, the aim of the present work was to evaluate the influence of parameters related to fine roots on estimations of long-term forest growth patterns in pure and mixed forests, using FORECAST (a hybrid forest growth model) as a virtual lab. The fine root parameters assessed were biomass, turnover rate, and nitrogen content. The analysis was performed by simulating monospecific stands of two contrasting species (Pinus sylvestris L. and Fagus sylvatica L.), and mixed stands formed by both species. In all cases, FORECAST showed good capability to contain uncertainty propagation during the first and middle stages of stand development (<40 years). After that moment, model output uncertainty steadily increased, but it reached different maximum uncertainty levels depending on stand type. Simulations of the less nutrient demanding P. sylvestris manifested very little sensitivity when growing in monospecific stands. However, F. sylvatica monospecific stands showed intermediate sensitivity, but significant species interactions occurred in mixed stands that determined the biggest impact detected of uncertainty related to fine root parameters over model outputs. All things considered, FORECAST displayed an interesting capability to capture some of the interspecific interactions that are key in mixed forests functioning. Our results suggest an acceptable model performance under uncertain parameterization but also caution against expecting accurate quantitative estimations of forest growth, especially when considering long-term scenarios in complex mixed stands.

3PG-MT-LSTM: A Hybrid Model under Biomass Compatibility Constraints for the Prediction of Long-Term Forest Growth to Support Sustainable Management

Climate change is posing new challenges to forestry management practices. Thinning reduces competitive pressure in the forest by repeatedly reducing the tree density of forest stands, thereby increasing the productivity of plantations. Considering the impact of thinning on vegetation and physiological and ecological traits, for this study, we used Norway spruce (Picea abies) data from three sites in the PROFOUND dataset to parameterize the 3-PG model in stages. The calibrated 3-PG model was used to simulate the stand diameter at breast height and the stem, root, and leaf biomass data on a monthly scale. The 3PG-MT-LSTM model uses 3-PG simulation data as the input variable. The model uses a long short-term memory neural network (LSTM) as a shared layer and introduces multi-task learning (MTL). Based on the compatibility rules, the interpretability of the model was further improved. The models were trained using single-site and multi-site data, respectively, and multiple indicators were used to evaluate the model accuracy and generalization ability. Our preliminary results show that, compared with the process model and LSTM algorithm without MTL and compatibility rules, the hybrid model has higher biomass simulation accuracy and shows a more realistic biomass response to environmental driving factors. To illustrate the potential applicability of the model, we applied light (10%), moderate (20%), and heavy thinning (30%) at intervals of 10, 15, 20, 25, 30 years. Then, we used three climate scenarios—SSP1-2.6, SSP2-4.5, and SSP5-8.5—to simulate the growth of Norway spruce. The hybrid model can effectively capture the impact of climate change and artificial management on stand growth. In terms of climate, temperature and solar radiation are the most important factors affecting forest growth, and under warm conditions, the positive significance of forest management is more obvious. In terms of forest management practices, less frequent light-to-moderate thinning can contribute more to the increase in forest carbon sink potential; high-intensity thinning can support large-diameter timber production. In summary, moderate thinning should be carried out every 10 years in the young-aged forest stage. It is also advisable to perform light thinning procedures after the forest has progressed into a middle-aged forest stage. This allows for a better trade-off of the growth relationship between stand yield and diameter at breast height (DBH). The physical constraint-based hybrid modeling approach is a practical and effective tool. It can be used to measure long-term dynamic changes in forest production and then guide management activities such as thinning to achieve sustainable forest management.

Storing carbon or growing forests?

Forest managers should promote the long-term growth of forests rather than maximize their short-term accumulation of carbon. Contemporary economic and political interests favor rapidly storing carbon in global forests. Against this background, forest managers are expected to contribute to mitigating planetary climate change by sharply increase forest carbon stocks. Building up the global forest growing stock too rapidly ignores the long-term cycles that govern forest growth dynamics. A further flaw in the strategy stems from the fact that anticipated changes in future climate argue against indiscriminately maximizing the carbon stock over the next two or three decades.4 A range of forest practices from planting site specific species to more comprehensive landscape management offer a path to better long-term forest growth. We claim that past climate policies have taken a narrow view that favors rapidly accumulating forest carbon stocks to the detriment of management options that focus on improving long term forest growth and ecosystem health.

Broad-scale and long-term forest growth predictions and management for native, mixed species plantations and teak in Costa Rica and Panama

Native tree species and species mixtures are key elements for biodiversity conservation by forest plantations. Yet, introduced species planted in monoculture still dominate plantation forests in many regions around the world and especially in the tropics. In Costa Rica and Panamá, Tectona grandis (teak) is the most planted species, occupying 49% and 64% of the forest plantation area. Here, we analyzed growth performance of four neotropical native species (Dalbergia retusa, Dipteryx oleifera, Hieronyma alchorneoides, Vochysia guatemalensis) in mixture with teak or as an alternative to teak plantations. For the first time, we parametrized a forest growth model for these native, neotropical tree species that was based on a large database which covers different climate, soil and management conditions as well as prolonged monitoring. We parametrized the mixture version of the process-based forest growth model Physiological Principles Predicting Growth (3-PGmix). We then developed management scenarios for pure and mixed plantations, where we simulated mixtures of T. grandis with D. oleifera and D. retusa as well as a mixture of D. oleifera, H. alchorneoides and V. guatemalensis as an alternative to teak plantations. With average 9% of model error, 3-PGmix qualified as a tool for making growth predictions for these native tree species and teak. Except for the very fast growing V. guatemalensis, growth of the native species at harvest age was lower than previously suggested by other studies in pure plantations. With two exceptions, all species showed good growth in mixture and a potential for implementation. Mixture simulations were highly sensitive to the 3-PG fertility rating input parameter. In all our simulated mixtures, we observed a trade-off between volume production and dbh growth, where dbh growth was inversely related to the tree density of the most productive species in the mixture. For mixtures of natives with of a close to baseline thinning T. grandis plantation, diameter growth of the natives was strongly reduced by the presence of teak. When comparing the tree-by-tree mixtures to their respective monocultures, the time of comparison was important, since, in monoculture, fast-growing species can be replanted during the long rotation periods of the slower growing species. This can reverse findings of increased volume production during early ages of plantation development.

Global sensitivities of forest carbon changes to environmental conditions.

The responses of forest carbon dynamics to fluctuations in environmental conditions at a global scale remain elusive. Despite the understanding that favourable environmental conditions promote forest growth, these responses have been challenging to observe across different ecosystems and climate gradients. Based on a global annual time series of aboveground biomass (AGB) estimated from radar satellites between 1992 and 2018, we present forest carbon changes and provide insights on their sensitivities to environmental conditions across scales. Our findings indicate differences in forest carbon changes across AGB classes, with regions with carbon stocks of 50-125MgC ha-1 depict the highest forest carbon gains and losses, while regions with 125-150MgC ha-1 have the lowest forest carbon gains and losses in absolute terms. Net forest carbon change estimates show that the arc-of-deforestation and the Congo Basin were the main hotspots of forest carbon loss, while a substantial part of European forest gained carbon during the last three decades. Furthermore, we observe that changes in forest carbon stocks were systematically positively correlated with changes in forest cover fraction. At the same time, it was not necessarily the case with other environmental variables, such as air temperature and water availability at the bivariate level. We also used a model attribution method to demonstrate that atmospheric conditions were the dominant control of forest carbon changes (56% of the total study area) followed by water-related (29% of the total study area) and vegetation (15% of the total study area) conditions. Regionally, we find evidence that carbon gains from long-term forest growth covary with long-term carbon sinks inferred from atmospheric inversions. Our results describe the contributions from the atmosphere, water-related and vegetation conditions to forest carbon changes and provide new insights into the underlying mechanisms of the coupling between forest growth and the global carbon cycle.

Broad-Scale and Long-Term Forest Growth Predictions and Management for Native, Mixed Species Plantations and Teak in Costa Rica and Panama

Broad-Scale and Long-Term Forest Growth Predictions and Management for Native, Mixed Species Plantations and Teak in Costa Rica and Panama

Subalpine forest dynamics reconstructed throughout the last 700 years in the Central Pyrenees by means of tree rings and pollen

Understanding how climate has modulated forest growth and composition in the past is necessary to predict the influence of the ongoing climate warming on the dynamics of mountain forests. We studied the past dynamics of subalpine Pyrenean forests during the last 700 years by assessing the relationships between sedimentary pollen and tree-ring records, and their link with climatic drivers. We compared the pollen record and the montane pollen ratio, an integrative index obtained from sedimentary pollen that allows inferring past altitudinal variations in the montane–subalpine ecotone, with tree-ring width from mountain pine ( Pinus uncinata) subalpine forests located in Central Pyrenees. To assess climate–growth associations, we related the dendrochronological data with instrumental meteorological records (1901–2010) and temperature reconstructions from the Pyrenees and Northern Hemisphere. Few robust associations were found between arboreal pollen taxa and tree-ring width series of the surrounding forests. However, significant correlations were found between the montane pollen ratio and tree-ring width series from nearby forests (located less than 10 km apart). This relationship could be potentially useful to infer long-term forest growth changes at decadal to centennial scales using the montane pollen ratio. On the contrary, our results show that tree radial growth has mainly been constrained by low temperatures although the growth sensitivity to climate has considerably varied over the last 700 years. Similar results were obtained for the last century as growth variability of these high-elevation forests is still driven by low temperatures, but a relaxation of this constrain in recent decades has been detected.

An individual-based model of long-term forest growth and carbon sequestration in planted mangroves under salinity and inundation stresses

We developed a spatially explicit individual-based model of forest development trajectories of monospecific Rhizophora mucronata plantations. The model incorporates stochastic initial seedling spacing, propagule dispersal, recruitment, and mortality. We simulated and compared the growth, development and accumulation of carbon stocks of restored mangroves between optimal and sub-optimal settings. Salinity is considered as a stressor, while flooding effects are described as an inundation stress factor. In the optimal setting, the simulated mangrove population accumulated large aboveground carbon stocks (of around 140 T/ ha) after 100 years. Under sub-optimal conditions, we observed delayed maturity by at least 10 years near the salinity threshold and the carbon stock decreased through time towards much lower values (25 T/ha). More importantly, the continuous presence of stressors may lead to forest population collapse (at 50 yrs post-planting) probably as a result of the accumulated effects of physiological stresses. Thus, restored mangrove populations that are located in highly saline and frequently inundated sites, may eventually collapse even though they may appear to be healthy in the early stages of forest development. Our results imply that current and future restoration practices should carefully consider site selection in order to ensure viable long-term forest development and to have an optimum contribution to carbon sequestration. Keywords—mangroves, individual-based model, growth, forest development, restoration trajectory, salinity, inundation, carbon sequestration

Modeling annualized occurrence, frequency, and composition of ingrowth using mixed-effects zero-inflated models and permanent plots in the Acadian Forest Region of North America

Forest tree ingrowth is a highly variable and largely stochastic process. Consequently, predicting occurrence, frequency, and composition of ingrowth is a challenging task but of great importance in long-term forest growth and yield model projections. However, ingrowth data often require different statistical techniques other than traditional Gaussian regression, because these data are often bounded, skewed, and non-normal and commonly contain a large fraction of zeros. This study presents a set of regression models based on discrete Poisson and negative binomial probability distributions for ingrowth data collected from permanent sample plots in the Acadian Forest Region of North America. Models considered here include regular Poisson, zero-inflated Poisson (ZIP), zero-altered Poisson (ZAP; hurdle Poisson), regular negative binomial (NB), zero-inflated negative binomial (ZINB), and zero-altered negative binomial (ZANB; hurdle NB). Plot-level random effects were incorporated into each of these models. The ZINB model with random effects was found to provide the best fit statistics for modeling annualized occurrence and frequency of ingrowth. The key explanatory variables were stand basal area per hectare, percentage of hardwood basal area, number of trees per hectare, a measure of site quality, and the minimum measured diameter at breast height of each plot. A similar model was developed to predict species composition. All models showed logical behavior despite the high variability observed in the original data.

Restoration of ecosystems for biodiversity and carbon sequestration: Simulating growth dynamics of brigalow vegetation communities in Australia

Restoration of abandoned and degraded ecosystems through enhanced management of mature remnant patches and naturally regenerating (regrowth) forests is currently being used in the recovery of ecosystems for biodiversity protection and carbon sequestration. Knowledge of long-term dynamics of these ecosystems is often very limited. Vegetation models that examine long-term forest growth and succession of uneven aged, mixed-species forest ecosystems are integral to the planning and assessment of the recovery process of biodiversity values and biomass accumulation. This paper examined the use of the Ecosystem Dynamics Simulator (EDS) in projecting growth dynamics of mature remnant brigalow forest communities and recovery process of regrowth brigalow thickets. We used data from 188 long-term monitored plots of remnant and regrowth forests measured between 1963 and 2010. In this study the model was parameterised for 34 tree and shrub species and tested with independent long-term measurements. The model closely approximated actual development trajectories of mature forests and regrowth thickets but some inaccuracies in estimating regeneration through asexual reproduction and mortality were noted as reflected in stem density projections of remnant plots that had a mean of absolute relative bias of 46.2 (±12.4)%. Changes in species composition in remnant forests were projected with a 10% error. Basal area values observed in all remnant plots ranged from 6 to 29 m 2 ha −1 and EDS projections between 1966 and 2005 (39 years) were 68.2 (±10.9)% of the observed basal area. Projected live aboveground biomass of remnant plots had a mean of 93.5 (±5.9) t ha −1 compared to a mean of 91.3 (±8.0) t ha −1 observed in the plots. In regrowth thicket, the model produced satisfactory projections of tree density (91%), basal area (89%), height (87%) and aboveground biomass (84%) compared to the observed attributes. Basal area and biomass accumulation in 45-year-old regrowth plots was approximately similar to that in remnant forests but recovery of woody understorey was very slow. The model projected that it would take 95 years for the regrowth to thin down to similar densities observed in original or remnant brigalow forests. These results indicated that EDS can produce relatively accurate projections of growth dynamics of brigalow regrowth forests necessary for informing restoration planning and projecting biomass accumulation.

Carbon dioxide exchange and canopy conductance of two coniferous forests under various sky conditions

Sky conditions play an important role in the Earth's climate system and CO(2) uptake by plants. We used eddy covariance and meteorological data, including global and diffuse photosynthetic photon flux density (PPFD), recorded over the 2008 and 2009 growing season at two Sitka spruce [Picea sitchensis (Bong.) Carr.] forest sites in northern Britain, in order to establish relationships between physiological properties under diverse sky conditions, i.e. (1) sunny, (2) cloudy, and (3) overcast, and several canopy activity-related properties. These properties are: (1) response to PPFD, (2) photosynthetic light use efficiency, and (3) canopy stomatal conductance. We found that Sitka spruce forests utilise PPFD in a more efficient way when solar radiation is dominated by diffuse radiation. Furthermore, our results show that diffuse radiation enhances canopy stomatal conductance, an effect which may be the result of both blue light enrichment within the canopy and the reduction in vapour pressure deficit during cloudy and overcast weather. Diffuse radiation does not only influence short-term (hourly, daily, monthly) canopy activity but also long-term forest growth.

Effects of irrigation and N fertilization on growth and structure of Pinus radiata stands between 10 and 29 years of age

Temporal change in stem volume, basal area, diameter, and height are reported for stands of Pinus radiata growing near Canberra, ACT, Australia between the ages of 10 and 29 years. The stands were subjected to either a single fertilization (F) at age 10 years, irrigation (I), irrigation after a single fertilization at age 10 years (IF), irrigation with continuous liquid fertilization (IL), or no treatment (Control (C)). At age 15 years all the stands were thinned. At the same time, irrigation ceased in a duplicate IF stand, and irrigation and liquid fertilization ceased in a duplicate IL stand. Stem growth responded markedly to irrigation, with a strong interactive effect when combined with N fertilization. Over a 19 year treatment period (ages 10–29 years) the percentage gross underbark volume responses, relative to the control which had an annual growth rate of 19.3 m 3 ha −1 year −1, were F (98%), I (132%), IF (184%), and IL (229%). Standing stem volumes (m 3 ha −1) at the end of the study were C (340), F (328), I (438), IF (624), and IL (776). The IF stand continued to grow at rates considerably greater than the I or F for the entire experiment, indicating efficient N cycling with increased water supply over a period of nearly 20 years. In non-irrigated stands, added N only increased growth during wet periods, and the response declined to be similar to the control within 5–8 years. Irrigation increased growth in the larger trees proportionally more than the smaller trees, resulting in an increased diameter distribution compared with the non-irrigated stands. Cessation of irrigation in combination with thinning after 5 years of treatment immediately reduced annual basal area growth to levels similar to the control for at least 2 years, but did not lead to tree mortality. Thinning reduced stand basal area increment for 2–3 years in treatments without adequate N supply, even if they were irrigated. The study demonstrates the importance of monitoring long-term growth to understand the changing interactions between water and N availability. For example, initial responses to irrigation only were markedly attenuated over time because of increasing N limitations, while irrigated and fertilized stands showed little or no change in the response pattern. Understanding the temporal change in water–N interactions has implications for modelling long-term forest growth, and forest management.

Modelling short-term CO 2 fluxes and long-term tree growth in temperate forests with ASPECTS

The net ecosystem exchange (NEE) of CO 2 between temperate forests and the atmosphere governs both carbon removal from the atmosphere and forest growth. In recent years, many experiments have been conducted to determine temperate forest NEE. These data have been used by forest modellers to better understand the processes that govern CO 2 fluxes, and estimate the evolution of these fluxes under changing environmental conditions. Nevertheless, it is not clear whether models capable of handling short-term processes, which are mostly source-driven, can provide an accurate estimate of long-term forest growth, which is potentially more influenced by sink- and phenology-related processes. To analyse the interactions between short- and long-term processes, we developed the ASPECTS model, which predicts long-term forest growth by integrating, over time, hourly NEE estimates. Validation data consisting of measurements of NEE by eddy-covariance and forest carbon reservoir estimates were obtained from mixed deciduous and evergreen experimental forests located in Belgium. ASPECTS accurately estimated both: (1) the NEE fluxes for several years of data; and (2) the amount of carbon contained in stems, branches, leaves, fine and coarse roots. Our simulations demonstrated that: (1) NEE measurements in Belgian forests are compatible with forest growth over the course of the 20th century; and (2) that forest history and long-term processes need to be considered for accurate simulation of short-term CO 2 fluxes.

Potential sampling bias in long-term forest growth trends reconstructed from tree rings: A case study from the Italian Alps

Tree-ring studies of long-term growth trends have often produced controversial results. In such studies, the largest-diameter trees in a stand are usually sampled. We assessed the influence of stand dynamics on long-term growth trends by examining the past diameters of all the trees living in two uneven-aged subalpine Norway spruce ( Picea abies (L.) Karst.) stands in the Italian eastern Alps, as reconstructed from ring widths. The trees were ordered according to diameter, and groups of 12 trees (the 12 largest, the 12 smallest, etc.) were formed. Different diameter groups have different increment curves. In both stands, the 12 largest trees in 1992 have not had consistently faster growth rates than smaller trees. This indicates that changes in diameter ranking order have occurred in the past and may be expected in the future. During stand development, changes occur in the relative position of individual trees, as ordered by diameter. The largest-diameter trees, at any time, may not always have been the largest trees and may not continue to be so. In a given year, the largest trees on average grow slower than other trees, which will become the largest in the future. The mean chronologies of the trees that were among the largest, prior to the harvest, and which presently (in 1992) are no longer in the top 12, and the mean chronologies of the trees that have moved up into the top 12 show very different growth trends. If analysed out of context, they would be interpreted differently, leading to different conclusions on long-term growth trends. When only the 12 largest-diameter trees are sampled, a bias may be present, as the trees may not have been open grown and free of competition in the past. Consequently, studies of long-term growth may be seriously affected by bias attributable to stand dynamics and sampling strategies. In future studies, the growth patterns of all diameter classes in a stand should be assessed, rather than restricting the sampling to the largest diameters.

Harvesting effects on phosphorus availability in a mixed eucalypt ecosystem in southeastern Australia

Native eucalypt forests in the foothills of eastern Victoria, Australia, are being managed for sustained production of hardwood timbers on at least 80-year rotations. The likely effects of intensive harvesting on soil phosphorus reserves were investigated. Distribution of P in a mixed eucalypt ecosystem was determined and removals in sawlogs and residual roundwood were estimated. Availability of soil P was studied in detail by determining fluoride, acid-soluble and organic forms, plus P adsorption isotherms and P levels following sequential extractions under optimal soil: water ratios and extraction times.The above-ground biomass had a P content of 21.2 kg ha−1, and removals of P in harvesting were estimated to range from 1.5–3.0 kg ha−1, depending on harvesting intesting intensity. It was estimated that a further 8 kg P ha−1 would have been lost by burning logging residue to prepare a seedbed for natural regeneration. Soil phosphorus available for long-term forest growth was estimated to be 57 kg ha−1 based on results of sequential soil extractions under optimal conditions. This is a new approach to estimating long-term available P in forests, and we have described this estimate by the term ‘forest-accessible phosphorus’.The combined losses from harvesting and burning represented 18% of the forest-accessible phosphorus. On this basis it was concluded that these eucalypt ecosystems could be intensively harvested for at least five rotations before a measurable change in site productivity would be detected as a result of P depletion.