Tree carbon allocation explains forest drought-kill and recovery patterns.

The mechanisms governing tree drought mortality and recovery remain a subject of inquiry and active debate given their role in the terrestrial carbon cycle and their concomitant impact on climate change. Counter-intuitively, many trees do not die during the drought itself. Indeed, observations globally have documented that trees often grow for several years after drought before mortality. A combination of meta-analysis and tree physiological models demonstrate that optimal carbon allocation after drought explains observed patterns of delayed tree mortality and provides a predictive recovery framework. Specifically, post-drought, trees attempt to repair water transport tissue and achieve positive carbon balance through regrowing drought-damaged xylem. Furthermore, the number of years of xylem regrowth required to recover function increases with tree size, explaining why drought mortality increases with size. These results indicate that tree resilience to drought-kill may increase in the future, provided that CO2 fertilisation facilitates more rapid xylem regrowth.

Uncertainty in the response of terrestrial carbon sink to environmental drivers undermines carbon-climate feedback predictions.

Terrestrial ecosystems play a vital role in regulating the accumulation of carbon (C) in the atmosphere. Understanding the factors controlling land C uptake is critical for reducing uncertainties in projections of future climate. The relative importance of changing climate, rising atmospheric CO2, and other factors, however, remains unclear despite decades of research. Here, we use an ensemble of land models to show that models disagree on the primary driver of cumulative C uptake for 85% of vegetated land area. Disagreement is largest in model sensitivity to rising atmospheric CO2 which shows almost twice the variability in cumulative land uptake since 1901 (1 s.d. of 212.8 PgC vs. 138.5 PgC, respectively). We find that variability in CO2 and temperature sensitivity is attributable, in part, to their compensatory effects on C uptake, whereby comparable estimates of C uptake can arise by invoking different sensitivities to key environmental conditions. Conversely, divergent estimates of C uptake can occur despite being based on the same environmental sensitivities. Together, these findings imply an important limitation to the predictability of C cycling and climate under unprecedented environmental conditions. We suggest that the carbon modeling community prioritize a probabilistic multi-model approach to generate more robust C cycle projections.

FOREST ECOLOGY. Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models.

The impacts of climate extremes on terrestrial ecosystems are poorly understood but important for predicting carbon cycle feedbacks to climate change. Coupled climate-carbon cycle models typically assume that vegetation recovery from extreme drought is immediate and complete, which conflicts with the understanding of basic plant physiology. We examined the recovery of stem growth in trees after severe drought at 1338 forest sites across the globe, comprising 49,339 site-years, and compared the results with simulated recovery in climate-vegetation models. We found pervasive and substantial "legacy effects" of reduced growth and incomplete recovery for 1 to 4 years after severe drought. Legacy effects were most prevalent in dry ecosystems, among Pinaceae, and among species with low hydraulic safety margins. In contrast, limited or no legacy effects after drought were simulated by current climate-vegetation models. Our results highlight hysteresis in ecosystem-level carbon cycling and delayed recovery from climate extremes.

Mandibular centricity: centric relation.

Centric relation can be a confusing term because it continues to evolve in meaning. This article presents a discussion of the historical aspects of centric relation. Guidelines to decide when to use centric relation in clinical dentistry are included.

Short TE proton MRS and neurofibromatosis type 1 intracranial lesions.

PURPOSE: The intracranial lesions of neurofibromatosis type 1(NF-1) have variable pathology and growth based on molecular genetics. Because of this variable pathology and growth, the lesions are followed by sequential MRI. Our hypothesis was that MR spectroscopy (MRS) could provide a noninvasive neurochemical biopsy of NF-1 lesions, thereby distinguishing the different lesions, monitoring their variable growth, and having added value when compared with MRI. METHOD: Nineteen patients fulfilling the National Institutes of Health criteria for NF-1 were followed with sequential MRI and short TE proton MRS. MRI monitored the lesions by observing the area of prolonged T2, mass effect, and degree of enhancement. MRS monitored the lesions by following the level of neurons, cellularity, and a by-product of the inositol signaling pathway. A comparison was made between the MRI and MRS findings to determine if MRS provided added value. Sixty-nine spectra were obtained in 24 resions. RESULTS: MRI was able to identify hamartomas, gliomas, and indeterminate lesions. MRS was able to distinguish three distinct spectra when compared with the cellularity of normal deep white matter (DWM): a hamartoma spectrum with a choline/creatine (CHO/CRE) ratio below 1.5, a transitional spectrum with a CHO/CRE ratio above 1.5 and below 2.0, and a glioma spectrum with a CHO/CRE ratio above 2.0. On comparing MRS and MRI, MRS provided added value by identifying changes in cellularity while MR images were stable, identifying spectra that could distinguish hamartomas from gliomas, and identifying a transitional spectrum that could progress or regress into glioma or hamartoma spectrum. CONCLUSION: MRS was able to identify three distinct spectra in NF-1 lesions when compared with the cellularity of normal DWM, thereby providing a neurochemical means to characterize lesions.

Fabricating swinglock removable partial denture frameworks.

A technique has been described which outlines the relief of the master cast and the wax-up and positioning of the attachments for swinglock removable partial denture frameworks. The advantages of the technique are that it reduces the bulk of the casting, provides an effective method of waxing and positioning the attachments, and reduces the amount of time needed for wax-up.