Direct response of tree growth to soil water and its implications for terrestrial carbon cycle modelling

Wood growth constitutes the main process for long‐term atmospheric carbon sequestration in vegetation. However, our understanding of the process of wood growth and its response to environmental drivers is limited. Current dynamic global vegetation models (DGVMs) are mainly photosynthesis‐driven and thus do not explicitly include a direct environmental effect on tree growth. However, physiological evidence suggests that, to realistically model vegetation carbon allocation under increased climatic stressors, it is crucial to treat growth responses independently from photosynthesis. A plausible growth response function suitable for global simulations in DGVMs has been lacking. Here, we present the first soil water‐growth response function and parameter range for deciduous and evergreen conifers. The response curve was calibrated against European larch and Norway spruce in a dry temperate forest in the Swiss Alps. We present a new data‐driven approach based on a combination of tree ring width (TRW) records, growing season length and simulated subdaily soil hydrology to parameterize ring width increment simulations. We found that a simple linear response function, with an intercept at zero moisture stress, used in growth simulations reproduced 62.3% and 59.4% of observed TRW variability for larch and spruce respectively and, importantly, the response function slope was much steeper than literature values for soil moisture effects on photosynthesis and stomatal conductance. Specifically, we found stem growth stops at soil moisture potentials of ?0.47 MPa for larch and ?0.66 MPa for spruce, whereas photosynthesis in trees continues down to ?1.2 MPa or lower, depending on species and measurement method. These results are strong evidence that the response functions of source and sink processes are indeed very different in trees, and need to be considered separately to correctly assess vegetation responses to environmental change. The results provide a parameterization for the explicit representation of growth responses to soil water in vegetation models.     

Annual cycle of phytoplankton in Ace Lake,an ice covered,saline meromictic lake

The annual cycle of phytoplankton in saline, meromictic Ace Lake (68°2S.4S, 78°11.1E) in the Vestfold Hills, Antarctica, was studied from January, 1979 to January 1980. Ace Lake has permanent gradients of temperature, salinity, dissolved oxygen, and hydrogen sulphide, and is ice covered with up to 2 m of ice for 10–12 months each year. The phytoplankton community had low diversity, consisting of only four species, all flagellates — a prasinophyte Pyramimonas gelidicola McFadden et al., a cryptophyte of the genus Cryptomonas; an unidentified colourless microflagellate, and an unarmoured dinoflagellate. These were restricted to the oxic zone of the lake from the surface to 10 m.The phytoplankton had a cycle of seven months of active growth over spring and summer. Low numbers of cells survived in the water column over winter. Spring growth was initiated below the ice by increased light penetration through the ice into the lake, enhanced at the time by the removal of surface snow which accumulated on the ice over winter. Peak phytoplankton biomass production was by the shade adapted P. gelidicola and occurred at the interface of the oxic and anoxic zones where substantial available nitrogen as ammonia is found.The three dominant phytoplankton species displayed distinct vertical stratification over the oxic zone. This stratification was not static and developed over spring as the flagellates migrated to preferred light climate zones. Mean cell volume of two of the flagellates varied significantly over the year. Minimum volumes were recorded in winter and volume increased progressively over spring to reach maximum mean cell volume in summer. Mean cell volume was positively correlated with light intensity (maximum ambient PAR at the respective depth for date of sample). Low cell volume in winter may be related to winter utilization of carbohydrate reserves by slow respiration, and may represent a survival mechanism.     

Age and growth rate determinations of an intertidal bivalve, Phacosoma japonicum, using internal shell increments

A venerid bivalve Phacosoma japonicum (Reeve) occurring commonly in the Japanese coastal area preserves periodic growth lines in the shell cross-section. Long-term shell growth patterns of this species have been traced for many individuals on the intertidal flat of the Seto Inland Sea, west Japan. Sclerochronological analysis of these individuals and specimens collected monthly shows that several growth cessation marks within their shells are formed during the winter of each year prior to spawning. Hence the marks were used for age and growth rate determinations. As large individuals showed little shell growth for more than two years after the formation of 7 or 8 annual increments, this species probably has a lifespan of more than ten years. Shell growth patterns of this species based on annual increments can be accurately approximated by a von Bertalanffy curve. The number of microgrowth increments formed during a year tends to decrease with age, although it varies markedly among specimens of the same age. Furthermore, even in summer during rapid shell growth, the microgrowth increments do not represent daily and/or sub-daily tidal rhythms in many specimens. The results of this study and those by several authors strongly suggest that the annual increments are the key for age and growth rate determinations of both living and fossil bivalve species.     

Student's tutorial on bloom hypotheses in the context of phytoplankton annual cycles

Phytoplankton blooms are elements in repeating annual cycles of phytoplankton biomass and they have significant ecological and biogeochemical consequences. Temporal changes in phytoplankton biomass are governed by complex predator–prey interactions and physically driven variations in upper water column growth conditions (light, nutrient, and temperature). Understanding these dependencies is fundamental to assess future change in bloom frequency, duration, and magnitude and thus represents a quintessential challenge in global change biology. A variety of contrasting hypotheses have emerged in the literature to explain phytoplankton blooms, but over time the basic tenets of these hypotheses have become unclear. Here, we provide a “tutorial” on the development of these concepts and the fundamental elements distinguishing each hypothesis. The intent of this tutorial is to provide a useful background and set of tools for reading the bloom literature and to give some suggestions for future studies. Our tutorial is written for “students” at all stages of their career. We hope it is equally useful and interesting to those with only a cursory interest in blooms as those deeply immersed in the challenge of understanding the temporal dynamics of phytoplankton biomass and predicting its future change.     

滇西北玉龙雪山不同海拔云南松(Pinus yunnanensis)径向生长对气候因子的响应

云南松(Pinus yunnanensis)是重要的造林树种,在我国西南地区广泛分布。研究不同海拔云南松径向生长对气候变化的响应,有助于了解气候变化背景下云南松的敏感性和适应性。在滇西北丽江玉龙雪山不同海拔采集了云南松树木年轮样品,采用传统的树木年轮方法制作了不同海拔云南松树轮宽度标准化年表,并分析了不同海拔云南松径向生长与气候因子的相关性。结果表明:1)低海拔样点云南松具有较快的年平均生长速率。2)不同海拔云南松对气候因子的响应模式一致,树轮宽度与当年5—6月的降水量、帕尔默干旱指数(PDSI)和相对湿度呈正相关,与同期温度呈负相关。3)不同海拔的云南松径向生长对气象因子的响应程度不一样,即低海拔样点云南松树轮宽度与当年5月份的干旱指数、相对湿度、降水量相关系数较高;而高海拔样点的云南松树轮宽度与5—6月的降水、相对湿度、干旱指数的相关系数较低。研究表明春末夏初的水分条件是玉龙雪山云南松径向生长的主要限制因子,且低海拔地区云南松生长受水分限制更为严重,区域气候变暖和干旱化趋势可能对低海拔地区云南松的生长产生持续的负面效应。研究结果可为探讨气候变化下云南松的适宜分布区、以及云南松人工林的经营和可持续管理提供参考。     

Leaf vein fraction influences the Péclet effect and 18O enrichment in leaf water

The process of evaporation results in the fractionation of water isotopes such that the lighter ¹⁶O isotope preferentially escapes the gas phase leaving the heavier ¹⁸O isotope to accumulate at the sites of evaporation. This applies to transpiration from a leaf with the degree of fractionation dependent on a number of environmental and physiological factors that are well understood. Nevertheless, the ¹⁸O enrichment of bulk leaf water is often less than that predicted for the sites of evaporation. The advection of less enriched water in the transpiration stream has been suggested to limit the back diffusion of enriched evaporative site water (Péclet effect); however, evidence for this effect has been varied. In sampling across a range of species with different vein densities and saturated water contents, we demonstrate the importance of accounting for the relative ‘pool’ sizes of the vascular and mesophyll water for the interpretation of a Péclet effect. Further, we provide strong evidence for a Péclet signal within the xylem that if unaccounted for can lead to confounding of the estimated enrichment within the mesophyll water. This has important implications for understanding variation in the effective path length of the mesophyll and hence potentially the δ¹⁸O of organic matter.     

Spatiotemporal variability of stone pine (Pinus pinea L.) growth response to climate across the Iberian Peninsula

Climate warming and increasing aridity have impacted diverse ecosystems in the Mediterranean region since at least the 1970s. Pinus pinea L. has significant environmental and socio-economic importance for the Iberian Peninsula, so a detailed understanding of its response to climate change is necessary to predict its status under future climatic conditions. However, variability of climate and uncertainties in dendroclimatological approach complicate the understanding of forest growth dynamics. We use an ensemble approach to analyze growth-climate responses of P. pinea trees from five sites along a latitudinal gradient in Spain over time. The growth responses to April-June precipitation totals were stronger in the north than in the south. Since the 1950s, the sensitivity of growth to April-June precipitation increased in the north and decreased in the south. Meteorological drought usually started in May in the southern sites, but in June-July in the northern sites. The water deficit in the southern sites is thus greater and more limiting for tree growth, and this likely accounts for the lower growth sensitivity during these months. Our results indicate that P. pinea has a high degree of plasticity, suggesting the species will withstand changing climatic conditions. However, growth response to drought regimes varies among P. pinea populations, suggesting that different populations have different capacities for acclimation to warmer and drier climate, and this may influence future vegetation composition.     

Influence of climatic conditions and industrial emissions on spruce tree-ring Pb isotopes analyzed at ppb concentrations in the Athabasca oil sands region

This study investigates Pb isotope ratios at low concentrations (parts per billion; ppb) in tree rings and soils in the Northern Athabasca Oil Sands Region (NAOSR), western Canada, to evaluate if: (1) climatic conditions influence on tree-ring Pb assimilation; and (2) such low Pb content allows inferring the regional Pb depositional history.Our results reflect the influence of winter snow cover and the importance of minimum temperature and precipitation in spring and summer on the bioavailability of Pb and its passive assimilation by trees in sub-arctic semi-humid climatic conditions. Winter conditions can influence the state of root systems that subsequently impacts the following growth period, while spring and summer conditions likely control microbial processes and water source, and may thus impact Pb assimilation by trees. Thus, the results of tree-ring Pb concentrations show interesting correlation with cumulated snow from November of the previous year to February (ρ = 0.53; P < 0.01; n = 36). Likewise, the ²⁰⁶Pb/²⁰⁷Pb ratios inversely correlate with minimum temperature from April to September (ρ = −0.67; P < 0.01; n = 40) and precipitation from May to August (ρ = −0.42; P < 0.01; n = 36). The isotopic results also suggest that the effects of climatic variations are superimposed by regional industrial Pb deposition: Western North American Aerosols (WNAA) and fugitive dust from the oil sands mining operations appear to be the most likely sources.Importantly, this study suggests that even at low Pb concentrations, tree-ring Pb isotopes are modulated by climatic conditions and potential input of regional and long-range transport of airborne Pb. These interpretations open the possibility of using Pb isotopes as an environmental tool for inferring the pollution history in remote regions, and improving our understanding of its natural cycle through the forest environment.