On predicting the response of forests in eastern North America to future climatic change

Our ability to accurately predict the response of forests in eastern North America to future climatic change is limited by our knowledge of how different tree species respond to climate. When the climatic response of eastern hemlock is modeled across its range, we find that the assumed climatic response used in simulation models is not sufficient to explain how this species is presently responding to climate. This is also the case for red spruce growing in the northern Appalachian Mountains. Consequently, simulations of future change to forests that include eastern hemlock and red spruce may need to be improved. We suspect that similar findings will be made when other tree species are studied in detail using tree-ring analysis. If so, our present understanding of how individual tree species respond to climate may not be adequate for accurately predicting future changes to these forests. Tree-ring analysis can increase our understanding of how climate affects tree growth in eastern North America and, hence, provide the knowledge necessary to produce more accurate predictions.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Extreme <Superscript>15</Superscript>N Depletion in Seagrasses

Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8?‰ δ¹⁵N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ¹⁵N values ranging from +2.95 to ?12.39?‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ¹⁵N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ¹⁵N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ¹⁵N values were correlated with Halodule uninervis δ¹⁵N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant ¹⁵N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ¹⁵N values. This study reports some of the most extreme, negative δ¹⁵N values ever noted for seagrass (as low as ?12.4?‰) and some of the most highly spatially variable (values varied over 15.4?‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.     

Climate change, reproductive performance and diet composition of marine birds in the southern California Current system, 1969–1997

We studied the effects of low-frequency climate change on the reproductive performance of 11 species of marine bird in the southern California Current system, 1969–1997. Reproductive performance of Brown Pelican (Pelecanus occidentalis) and Double-crested Cormorant (Phalacrocrax auritus) in southern California demonstrated an increase in the 1970s and early 1980s, attributable to recovery from organochlorine contamination (primarily DDE). Brandt's Cormorant (Phalacrocorax penicillatus) in central California was the only species to demonstrate a secular increase in performance through time, a pattern that remains unexplained. Ashy Storm-petrel (Oceanodroma homochroa) and Pelagic Cormorant (Phalacrocorax pelagicus) demonstrated curvilinear patterns of change, with decreasing reproductive performance in the past decade. All other species including Western Gull (Larus occidentalis), Pigeon Guillemot (Cepphus columba), Xantus's Murrelet (Synthiloboramphus hypoleucus), Common Murre (Uria aalge), Cassin's Auklet (Ptychoramphus aleuticus) and Rhinoceros Auklet (Cerorhinca monocerata) showed diminishing reproductive performance through time. Patterns of change for the murre and auklets were not significant, presumably because of a lack of reproductive variation for these species, which display a conservative breeding effort (i.e. single-egg clutches). Changes in the birds' abilities to provision young and maintain chick survival during May–July each year appeared most closely related to overall changes in reproductive performance. Dietary change indicated a decline in use of juvenile rockfish (Sebastes spp.) by marine birds in central California. There was also significant interannual variability in consumption of juvenile rockfish and the euphausiid Thysanoessa spinifera. Patterns of change in marine bird reproductive performance were generally concordant between southern and central California after considering the period of recovery for Brown Pelican and Double-crested Cormorant. The decline in reproductive performance and changes in diet composition do not appear directly related to the polarity reversal of the Pacific Decadal Oscillation in 1976/1977. Instead, reproductive performance and dietary characteristics indicate substantial change in the late 1980s, suggesting another regime-shift at that time.