Height–diameter modeling of tree species in boreal and mixed forests using a mixed-effects approach and stand-level variables

Above-ground biomass (AGB) plays a pivotal role in assessing a forest’s resource dynamics, ecological value, carbon storage, and climate change effects. The traditional methods of AGB measurement are destructive, time consuming and laborious, and an efficient, relatively accurate and non-destructive AGB measurement method will provide an effective supplement for biomass calculation. Based on the real biophysical and morphological structures of trees, this paper adopted a non-destructive method based on terrestrial laser scanning (TLS) point cloud data to estimate the AGBs of multiple common tree species in boreal forests of China, and the effects of differences in bark roughness and trunk curvature on the estimation of the diameter at breast height (DBH) from TLS data were quantitatively analyzed. We optimized the quantitative structure model (QSM) algorithm based on 100 trees of multiple tree species, and then used it to estimate the volume of trees directly from the tree model reconstructed from point cloud data, and to calculate the AGBs of trees by using specific basic wood density values. Our results showed that the total DBH and tree height from the TLS data showed a good consistency with the measured data, since the bias, root mean square error (RMSE) and determination coefficient (R2) of the total DBH were −0.8 cm, 1.2 cm and 0.97, respectively. At the same time, the bias, RMSE and determination coefficient of the tree height were −0.4 m, 1.3 m and 0.90, respectively. The differences of bark roughness and trunk curvature had a small effect on DBH estimation from point cloud data. The AGB estimates from the TLS data showed strong agreement with the reference values, with the RMSE, coefficient of variation of root mean square error (CV(RMSE)), and concordance correlation coefficient (CCC) values of 17.4 kg, 13.6% and 0.97, respectively, indicating that this non-destructive method can accurately estimate tree AGBs and effectively calibrate new allometric biomass models. We believe that the results of this study will benefit forest managers in formulating management measures and accurately calculating the economic and ecological benefits of forests, and should promote the use of non-destructive methods to measure AGB of trees in China.

The impact of frequent water deficits on dominant tree species in boreal forests has received increased attention, particularly towards addressing the global climate change scenarios. However, the impacts of coupled light intensity and water deficit in the regeneration and growth of Larix gmelinii seedlings, a dominant species in China's boreal forests, are still unclear. We conducted a dual-factor controlled experiment with four light intensities (natural sunlight, 50% shading, 75% shading, and 90% shading) and three soil water conditions (80%, 60%, and 40% soil saturated water content). The results showed that the coupling of light and water has a significant effect on the growth and development of Larix gmelinii seedlings. In 40% of the saturated soil moisture content, net photosynthetic rate, transpiration rate, chlorophyll a, and total phenol-leaf were significantly lower than the same light conditions under 80% soil saturated water content. Under the coupling treatment of 60% soil saturated water content and 50% shading treatment, the plant height increment, net photosynthetic rate, stomatal conductance, transpiration rate, chlorophyll a, and phenolic compound content were significantly higher than those of other coupling treatments; however, more than 75% shading inhibited photosynthetic parameters, chlorophyll a, total flavonoid-leaf, and total flavonoid-branch. Our results have important implications for forest management practices; they provide a scientific reference for the early growth of Larix gmelinii seedlings under the coupling of light and water and promote the survival and growth of seedlings.

Background Climate change has become one of the significant global challenges confronting all the people in the world. Climate change, particularly rising atmospheric carbon dioxide (CO2) concentration (ca) and temperature, and changes in water availability have also affected tree growth of forest ecosystems worldwide. Several studies about the effects of climate change on tree growth and physiological responses have been reported. However, these are mostly based only on experiments with isolated trees or seedlings grown under intensive and short-term exposure to one or two climatic factors. Since long-term and gradual impacts of climate change on tree growth and physiological responses could be different from the short-term effects, there is an urgent need to investigate how tree species respond to elevated atmospheric CO2 and temperature and water availability changes at larger scales and over more extended periods. This study is complicated because tree growth rates vary among genotypes and change as trees age and because climatic conditions, such as temperature, precipitation and humidity, and atmospheric CO2, have been changing over time. There are also indications that tree responses to climate change may change with time as the trees grow. Moreover, since tree growth (biomass) is a product of physiological processes, the physiological processes are affected by the structure of the organs or tissues where the processes occur. In turn, the structures of the organs are determined by the products of the previous physiological processes. To understand the mechanisms on how long-term climate change affects tree physiological processes and growth, we have to understand the relationships among the climate, tree growth and physiological responses, and how long tree growth trend will remain alongside the elevating CO2 concentration before it declines, considering that the forest ecosystems are one of the most important contributors to the global CO2 assimilation, which can effectively counteract the global warming. Hypothesis and objective My research has been focused on studying the long-term tree growth and water use efficiency in response to rising atmospheric CO2 concentration, in combining with other environmental factors, and their influences on climate change in the future. I hypothesised that tree growth is affected by biological effects, such as tree species and ages, and non-biological effects, namely locations, temperature, precipitation and humidity. Thus, all my experiments' main objectives were to confirm my hypothesis and quantify how those biological and non-biological factors would influence tree growth and water use efficiency in the context of both spatial and temporal scales. The goals and objectives of this research were: To determine the effect of long-term climatic conditions on tree growth and physiological processes. The objectives were to determine how climatic factors would influence tree growth and physiological responses; to determine the key climatic controls of the change in tree growth and physiological responses, and determine how each of the key climatic factors and their interactions affect tree growth and physiological responses. To determine the variation of the climate-tree relationship among tree species. The objective was to determine the phenotypic and genotypic variation of the effects of climate change on tree growth and physiological responses among tree species, To determine the acclimatization of tree species in response to climate change. The objectives were to determine the responses of tree species to climate change over time and determine tree species' responses to climate change before and after they are exposed to specific climatic conditions. Materials and methods To achieve the objectives, tree ring technologies were adopted. Trees record relevant information in their annual rings, represent important natural archives of climate changes, and provide archives of tree growth responses to the past climate variation. With tree ring width growth, information from tree-ring stable isotope compositions were used to better understand the dynamic relationships among the climate, tree growth, and physiological responses. My current research was commenced with seven tree species sampled from five different forests in China, covering both subtropical and boreal climatic conditions. The long-term tree-ring chronology was established by applying tree ring width measurement and cross-dating verified by COFECHA program; therefore, the basal area increments (BAI) were calculated sequentially. Meanwhile, the intrinsic water-use efficiency (iWUE) was calculated by measuring carbon isotope composition (δ13C) in tree ring samples. Tree ring δ13C relationships with BAI and atmospheric CO2 concentration were also quantified. Results and discussion From this study, we have gained further understanding of the relationships among long-term climate change, tree growth and physiology, as a basis for future projection of silvicultural manipulations under different climate change scenarios. In Chapter 2, both BAI values of the two tree species (Pseudolarix amabilis and Cryptomeria japonica, sampled from a subtropical monsoon forest located in eastern China), continuously increased with the rising of CO2 concentration until the atmospheric CO2 concentration tipping points were reached (the tipping points of Pseudolarix amabilis and Cryptomeria japonica were in year 1997 and 1996 when atmospheric CO2 concentration reached 365.1 ppm and 636.0 ppm respectively), after which tree growth started to decline with the rising CO2, while iWUE exerted a continuous increase trend with the increasing CO2 concentration. In Chapter 3, the results showed a decreasing trend in relative humidity over the past 70 years in a subtropical forest of south-east China with rising CO2 concentrations and temperature and the initial increasing tree growth for both Pinus massoniana and Cryptomeria japonica from the rising CO2, which peaked when CO2 concentration reached 330 ppm and 385 ppm in year 1974 and 2008 respectively, but decreased thereafter with increasing water limitation. Tree iWUE showed the same continuing increase trend as the two species in Chapter 2. In Chapter 4, three tree species (Cinnamomum micranthum, Pinus massoniana and Cunninghamia lanceolata) were sampled in two nearby subtropical forests of south-east China. The tree-growth also initially increased with the rising Ca, then decreased with the increasing Ca. The tipping points among the three species slightly varied but all happened between year 1995 and 1999. In addition, iWUE continuously increased with the rising Ca regardless the tipping points of BAI with the Ca. In Chapter 5, two species (Larix gmelinii Rupr and Betula platyphylla) were sampled in a boreal forest of north-east China, the results were similar to the previous chapters, while iWUE showed consistent increase during the entire growth period for both species, BAI reached the tipping points when Ca reached 366 ppm in year 1998 for Larix gmelinii Rupr, and 353.5 ppm in year 1989 for Betula platyphylla. In summary, the experimental results demonstrated that tree growth of BAI showed a continuous increase among all sampled tree species with the rising CO2 concentration until the CO2 concentration tipping points were passed. The trees’ responses were both species and site dependent. After reaching the critical points, tree growth started to decline even with the rising CO2 concentration, while iWUE exerted a continuously increasing trend with the increasing CO2 concentration, which biologically proofed that the decreased BAI was not dominated by tree age, but due to the rise of Ca and warming induced water limitation. The series of tree ring studies reported in this thesis has highlighted that there would be non-linear tree growth responses to the increasing Ca of the tree species in both subtropical and boreal forests, with the initial increases in tree growth detected as the atmospheric CO2 increased, but the tree growth peaked when the critical tipping points of Ca were reached and then declined thereafter. However, tree WUE continued to increase with the rising Ca, initially due to the increasing photosynthesis and tree growth, then later due to the warming induced water limitation. Unfortunately, the tipping points of Ca for tree species in both subtropical and boreal forests were reached between 1974 and 2008, and tree growth decreased with the rising Ca once the CO2 tipping points were passed, leading to a positive feedback to climate change.

The boreal forest represents the terrestrial biome most heavily affected by climate change. However, no consensus exists regarding the impacts of these changes on the growth of tree species therein. Moreover, assessments of young tree responses in metrics transposable to forest management remain scarce. Here, we assessed the impacts of climate change on black spruce (Picea mariana [Miller] BSP) and jack pine (Pinus banksiana Lambert) growth, two dominant tree species in boreal forests of North America. Starting with a retrospective analysis including data from 2591 black spruces and 890 jack pines, we forecasted trends in 30-year height growth at the transitions from closed to open boreal coniferous forests in Québec, Canada. We considered three variables: (1) height growth, rarely used, but better-reflecting site potential than other growth proxies, (2) climate normals corresponding to the growth period of each stem, and (3) site type (as a function of texture, stoniness, and drainage), which can modify the effects of climate on tree growth. We found a positive effect of vapor pressure deficit on the growth of both species, although the effect on black spruce leveled off. For black spruce, temperatures had a positive effect on the height at 30years, which was attenuated when and where climatic conditions became drier. Conversely, drought had a positive effect on height under cold conditions and a negative effect under warm conditions. Spruce growth was also better on mesic than on rocky and sub-hydric sites. For portions of the study areas with projected future climate within the calibration range, median height-change varied from 10 to 31% for black spruce and from 5 to 31% for jack pine, depending on the period and climate scenario. As projected increases are relatively small, they may not be sufficient to compensate for potential increases in future disturbances like forest fires.

Population dynamics and individual growth dynamics of Larix gmelinii under non-stand replacing fire

Seedling establishment on fallen logs is a major regeneration system for tree species in boreal forests. Seedling survival on fallen logs is affected not only by the microsite environment but also by the genetic factors of individuals. To quantify the genetic effects on seedling longevity, we identified seedlings using a number tag system and collected needles of Picea jezoensis and Abies sachalinensis established on fallen logs in spring 2006. Survival or death of each seedling was investigated during 2006-2012. We genotyped seedlings with microsatellite markers and calculated individual-based multilocus heterozygosity (MLH) for each seedling. A Cox proportional hazards model was applied to evaluate the effects of MLH on seedling longevity of the two species considering the fallen log conditions. The model indicated that MLH positively affected seedling longevity in P. jezoensis, whereas the effects of MLH were not significant in A. sachalinensis. Here, we discuss differences in the effects of MLH on seedling longevity between the two species, considering species characteristics and MLH frequency distribution.

The effect of stand age on biodiversity in a 130-year chronosequence of Populus tremula stands

At northern latitudes, a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We investigated the impact of high (H) and moderate (M) daytime air relative humidity (RH) on water relation parameters and foliar nutrient status in saplings of silver birch, to elucidate the interactions between water and nutrient uptake. A ~40% lower daytime vapour pressure difference between the leaf interior and atmosphere caused significantly (P 0.05) between the treatments. Our results suggest that the 32.3% higher total foliar P in the H treatment could be partly induced by 28.2% greater transpiration-driven mass flow of water-soluble P and/or by 25.4% higher number of absorptive root tips. Our results confirm that elevated daytime atmospheric humidity increases the potential for night-time water flux and might also facilitate the uptake of mineral nutrients in silver birch, a dominating deciduous tree species in boreal forests.

兴安落叶松针叶解剖结构变化及其光合能力对气候变化的适应性

Conifers are dominant tree species in boreal forests, but are susceptible to attack by bark beetles. Upon bark beetle attack, conifers release substantial quantities of volatile organic compounds known as herbivore-induced plant volatiles (HIPVs). Earlier studies of broadleaved plants have shown that HIPVs provide information to neighbouring plants, which may enhance their defences. However, the defence responses of HIPV-receiver plants have not been described for conifers. Here we advance knowledge of plant–plant communication in conifers by documenting a suite of receiver-plant responses to bark-feeding-induced volatiles. Scots pine seedlings exposed to HIPVs were more resistant to subsequent weevil feeding and received less damage. Receiver plants had both induced and primed volatile emissions and their resin ducts had an increased epithelial cell (EC) mean area and an increased number of cells located in the second EC layer. Importantly, HIPV exposure increased stomatal conductance and net photosynthesis rate of receiver plants. Receiver-plant responses were also examined under elevated ozone conditions and found to be significantly altered. However, the final defence outcome was not affected. These findings demonstrate that HIPVs modulate conifer metabolism through responses spanning photosynthesis and chemical defence. The responses are adjusted under ozone stress, but the defence benefits remain intact.

ABSTRACTThe growth rate of most tree species in boreal forests will increase with changing climate. This increase is counterbalanced by an increased risk of damage due to extreme weather events. It is believed that the risk of storm damage will increase over time, especially if forests continue to be managed as they are today. In this study, a new landscape-level hybrid forest growth model 3PG-Heureka was developed and simulations were performed to predict the damage caused by storm events in Kronoberg county, over a period of 91 years (2010–2100) with different alternative management regimes under various climatic scenarios (historic, RCP4.5 and RCP8.5). The results indicate that damage caused by storm events could drastically reduce the annual volume increment and annual net revenue obtained from forest landscapes if current forest management regimes are used. These problems can be reduced by adopting alternative management strategies involving avoiding thinning, shorter rotation periods and planting alternative tree species. Alternative management strategies could potentially improve annual volume increments and net revenue obtained while reducing storm-felling. Planting Scots pine instead of Norway spruce across the landscape to minimize storm damage is predicted to be less effective than reducing rotation periods.

Using long-term data to reveal the geographical variation in timing and quantity of pollen and seed production in silver and pubescent birch in Finland: Implications for gene flow, hybridization and responses to climate warming

Dahurian larch (Larix gmelinii) is the dominant tree species in boreal forests, and its photosynthetic response to climate warming is important in modeling and predicting carbon cycling for boreal forest ecosystems. In 1983, seedlings of L. gmelinii from 11 provenances were transplanted into two common gardens with different climate conditions (control and warming climate). Forty years after the transplant, we investigated the response of leaf photosynthetic capacity to climate warming and its variation among provenances. The warming treatment significantly increased the maximum net photosynthetic rate (Pmax-a), photosynthetic nitrogen use efficiency (PNUE), maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), triose phosphate utilization rate (TPU), mesophyll conductance (gm), leaf nitrogen content (Narea), and chlorophyll content (Chlm). Pmax-a was significantly positively associated with Vcmax, Jmax, TPU, gm, and Narea, and the slope of the linear regression between Pmax-a and Vcmax, Jmax, and TPU was greater in the warming treatment. The responses of Pmax-a, PNUE, Vcmax, Jmax, TPU, Narea, and Chlm to warming differed among provenances. As the aridity index of the original site increased, the magnitude of the warming treatment’s effect on Pmax-a, Vcmax, Jmax, and TPU represented a varying form of a bell-shaped curve. Overall, the warming treatment improved the photosynthetic capacity of L. gmelinii, but the extent of the improvement varied among provenances. These findings provide insights into the mechanisms underlying the responses of L. gmelinii to climate warming.

Questions(1) How is the abundance of individual epiphytic macrolichen species affected by host tree species and stand development, where canopy succession takes place in prolonged absence of stand‐replacing disturbance? (2) Do the associations between individual epiphytic macrolichen species and host tree species change through stand development?LocationThe central boreal forest of Canada (49°23′ N–49°36′ N, 89°31′ W–89°44′ W).MethodsWe estimated the percentage cover of epiphytic macrolichens on individual trees in 51 post‐fire successional stands of varying tree species composition, ranging from 7 to 209 yr since fire. We analysed how epiphytic macrolichen species cover on individual host tree species and community composition change with time since fire and host tree species using GLM and permutational multivariate ANOVA.ResultsCover of individual epiphytic macrolichen species on the host tree species increased continuously, peaked at different time scales or gradually declined through stand development in which canopy succession took place. Some lichen species exhibited exclusive association to particular host tree species. Moreover, stand age‐related trends of the cover of some epiphytic macrolichen species differed with host species. Multivariate analysis indicated that host tree species at each stand development stage supported different epiphytic macrolichen species composition.ConclusionEpiphytic macrolichen species dynamics is strongly influenced by time since fire and host tree species identity. Host‐specific preferences and their interaction with time since fire suggest that epiphytic lichen composition on a particular host tree species is not merely a random sample of the local species pool, but rather related to their time since fire‐dependent preferences for their hosts.

Methane emissions from plant foliage may play an important role in the global methane cycle, but their size and the underlying source processes remain poorly understood. Here, we quantify methane fluxes from the shoots of Scots pine trees, a dominant tree species in boreal forests, to identify source processes and environmental drivers, and we evaluate whether these fluxes can be constrained at the ecosystem-level by eddy covariance flux measurements. We show that shoot-level measurements conducted in forest, garden, or greenhouse settings; on mature trees and saplings; manually and with an automated CO2-, temperature-, and water-controlled chamber system; and with multiple methane analyzers all resulted in comparable daytime fluxes (0.144 ± 0.019 to 0.375 ± 0.074 nmol CH4 g-1 foliar d.w. h-1). We further find that these emissions exhibit a pronounced diurnal cycle that closely follows photosynthetically active radiation and is further modulated by temperature. These diurnal patterns indicate that methane production is associated with diurnal cycle of sunlight, indicating that this production is either a byproduct of photosynthesis-associated biochemical reactions (e.g., the methionine cycle) or produced through nonenzymatic photochemical reactions in plant biomass. Moreover, we identified a light-dependent component in stand-level methane fluxes, which showed order-of-magnitude agreement with shoot-level measurements (0.968 ± 0.031 nmol CH4 g-1 h-1) and which provides an upper limit for shoot methane emissions.