Forest fine-root production and nitrogen use under elevated CO2: contrasting responses in evergreen and deciduous trees explained by a common principle

Despite the importance of nitrogen (N) limitation of forest carbon (C) sequestration at rising atmospheric CO₂ concentration, the mechanisms responsible are not well understood. To elucidate the interactive effects of elevated CO₂ (eCO₂) and soil N availability on forest productivity and C allocation, we hypothesized that (1) trees maximize fitness by allocating N and C to maximize their net growth and (2) that N uptake is controlled by soil N availability and root exploration for soil N. We tested this model using data collected in Free‐Air CO₂ Enrichment sites dominated by evergreen (Pinus taeda; Duke Forest) and deciduous [Liquidambar styraciflua; Oak Ridge National Laboratory (ORNL)] trees. The model explained 80–95% of variation in productivity and N‐uptake data among eCO₂, N fertilization and control treatments over 6 years. The model explains why fine‐root production increased, and why N uptake increased despite reduced soil N availability under eCO₂ at ORNL and Duke. In agreement with observations at other sites, the model predicts that soil N availability reduced below a critical level diminishes all eCO₂ responses. At Duke, a negative feedback between reduced soil N availability and N uptake prevented progressive reduction in soil N availability at eCO₂. At ORNL, soil N availability progressively decreased because it did not trigger reductions in N uptake; N uptake was maintained at ORNL through a large increase in the production of fast turnover fine roots. This implies that species with fast root turnover could be more prone to progressive N limitation of carbon sequestration in woody biomass than species with slow root turnover, such as evergreens. However, longer term data are necessary for a thorough evaluation of this hypothesis. The success of the model suggests that the principle of maximization of net growth to control growth and allocation could serve as a basis for simplification and generalization of larger scale forest and ecosystem models, for example by removing the need to specify parameters for relative foliage/stem/root allocation.     

兼具凝集素活性的芥菜几丁质酶BjCHI1

利用毕赤酵母表达系统表达芥菜几丁质酶基因BjCHI1及其两个衍生基因BjCHI2和BjCHI3,获得相应的蛋白质。经FPLC纯化后,测定了3种蛋白质的几丁质酶活性,发现它们均能降解CM-chitin-RBV和胶状几丁质。以CM-chitin-RBV为底物时的Km值分别为0.799mg/mL、0.544mg/mL和0.793mg/mL,差别甚微。而以胶状几丁质为底物时的Km值分别为0.281mg/mL、0.388mg/mL和1.643mg/mL,表现一定的差别,说明几丁质结合域影响了酶对不溶性底物的亲和力。3种蛋白中,只有BjCHI1在33μg/mL以上浓度具有凝集素活性,而BjCHI2和BjCHI3的浓度即使高达800μg/mL也无凝集素活性,表明2个几丁质结合域是BjCHI1具有凝集素活性的必需条件,这是植物中发现的第一个兼有几丁质酶和凝集素活性的蛋白质。      

Geochemistry of Persististrombus latus Gmelin from the Pleistocene Iberian Mediterranean realm

De Torres, T., Ortiz, J.E., Arribas, I., Delgado, A., Julià, R. & Martín‐Rubí, J.A. 2009: Geochemistry of Persististrombus latus Gmelin from the Pleistocene Iberian Mediterranean realm. Lethaia, Vol. 43, pp. 149–163. In this paper the organic and inorganic geochemistry of fossil and extant Persististrombus latus are compared, together with other strombid species (Lentigo lentiginosus, Lobatus gigas, Strombus alatus, Lobatus raninus, Laevistrombus canarium and Tricornis latissimus). Using a large sample of well‐preserved fossil P. latus shells from the Mediterranean realm, we examined the warming period of sea water in the Middle Pleistocene. A mineralogical study of the shells demonstrates the continuous presence of calcite and a complex organic matter distribution, which was well preserved in many cases, thereby making the U/Th dating of strombid shells unreliable. U/Th analysis of coral samples and amino acid racemization dating of pelecypod shells confirmed that P. latus entered the Mediterranean realm in MIS 7 and 5. The oscillations of the δ¹⁸O values reflect annual growth periods and provide a mixing of the first signal record (primary growth) and successive overgrowths. □Amino acid racemization, Mediterranean Sea, Persististrombus latus, shell mineralogy, U/Th, δ¹⁸O.     

Serotonergic mechanisms mediating spawning and oocyte maturation in the zebra mussel,Dreissena polymorpha

Summary

The zebra mussel, Dreissena polymorpha, is a freshwater biofouling bivalve unintentionally introduced in the 1980s into North America from Europe. Oocyte maturation (germinal vesicle breakdown, GVBD) and spawning of the zebra mussel can be triggered with serotonin (5-hydroxytryptamine, 5-HT). In pharmacological experiments to characterize the receptor mediating spawning, the serotonin receptor agonists 8-OH-DPAT, TFMPP, and 1-(1-naphthyl)piperazine were effective at stimulating spawning; whereas, 2-methylserotonin and alpha-methylserotonin had no effect. In experiments with antagonists of serotonin receptors ketanserin and propranolol had no effect; mianserin, NAN-190, and cyproheptadine had partial inhibitory effects; and methiothepin was a very effective antagonist. Metergoline had mixed agonist/antagonist properties. Ergotamine was the most effective activator of spawning in females. Compared to serotonergic receptors in other organisms, the receptors that activate spawning in zebra mussels resemble 5HTlym, 5HTdro2 and human 5HT1Dβ, which are receptors that may act both by inhibiting adenylyl cyclase and by activating phospholipase C. In zebra mussels, 5-HT and 8-OH-DPAT activate GVBD in gonad fragments, a process also initiated by manual dissection of gonad fragments. GVBD can be inhibited by pre-treatment of ovaries with forskolin and theophylline, suggesting an inhibitory role for cyclic AMP. The Ca²⁺ ionophore A23187 can trigger GVBD and polar body formation. Thus, oocyte maturation in zebra mussels may be initiated via serotonergic receptors simultaneously inhibiting adenylyl cyclase and activating Ca²⁺ mechanisms.     

Leaf stomatal responses to vapour pressure deficit under current and CO2-enriched atmosphere explained by the economics of gas exchange

Using the economics of gas exchange, early studies derived an expression of stomatal conductance ( g ) assuming that water cost per unit carbon is constant as the daily loss of water in transpiration ( f _e) is minimized for a given gain in photosynthesis ( f _c). Other studies reached identical results, yet assumed different forms for the underlying functions and defined the daily cost parameter as carbon cost per unit water. We demonstrated that the solution can be recovered when optimization is formulated at time scales commensurate with the response time of g to environmental stimuli. The optimization theory produced three emergent gas exchange responses that are consistent with observed behaviour: (1) the sensitivity of g to vapour pressure deficit ( D ) is similar to that obtained from a previous synthesis of more than 40 species showing g to scale as 1 −  m  log( D ), where m   ∈  [0.5,0.6], (2) the theory is consistent with the onset of an apparent 'feed-forward' mechanism in g , and (3) the emergent non-linear relationship between the ratio of intercellular to atmospheric [CO₂] ( c _i/ c _a) and D agrees with the results available on this response. We extended the theory to diagnosing experimental results on the sensitivity of g to D under varying c _a.     

Does the disturbance hypothesis explain the biomass increase in basin‐wide Amazon forest plot data?

Positive aboveground biomass trends have been reported from old-growth forests across the Amazon basin and hypothesized to reflect a large-scale response to exterior forcing. The result could, however, be an artefact due to a sampling bias induced by the nature of forest growth dynamics. Here, we characterize statistically the disturbance process in Amazon old-growth forests as recorded in 135 forest plots of the RAINFOR network up to 2006, and other independent research programmes, and explore the consequences of sampling artefacts using a data-based stochastic simulator. Over the observed range of annual aboveground biomass losses, standard statistical tests show that the distribution of biomass losses through mortality follow an exponential or near-identical Weibull probability distribution and not a power law as assumed by others. The simulator was parameterized using both an exponential disturbance probability distribution as well as a mixed exponential–power law distribution to account for potential large-scale blowdown events. In both cases, sampling biases turn out to be too small to explain the gains detected by the extended RAINFOR plot network. This result lends further support to the notion that currently observed biomass gains for intact forests across the Amazon are actually occurring over large scales at the current time, presumably as a response to climate change.