Tree-ring isotopes suggest atmospheric drying limits temperature–growth responses of treeline bristlecone pine

Altitudinally separated bristlecone pine populations in the White Mountains (California, USA) exhibit differential climate-growth responses as temperature and tree-water relations change with altitude. These populations provide a natural experiment to explore the ecophysiological adaptations of this unique tree species to the twentieth century climate variability. We developed absolutely dated annual ring-width chronologies, and cellulose stable carbon and oxygen isotope chronologies from bristlecone pine growing at the treeline (~3500 m) and ~200 m below for the period AD 1710-2010. These chronologies were interpreted in terms of ecophysiological adaptations to climate variability with a dual-isotope model and a leaf gas exchange model. Ring widths show positive tree growth anomalies at treeline and consistent slower growth below treeline in relation to the twentieth century warming and associated atmospheric drying until the 1980s. Growth rates of both populations declined during and after the 1980s when growing-season temperature and atmospheric vapour pressure deficit continued to increase. Our model-based interpretations of the cellulose stable isotopes indicate that positive treeline growth anomalies prior to the 1980s were related to increased stomatal conductance and leaf-level transpiration and photosynthesis. Reduced growth since the 1980s occurred with a shift to more conservative leaf gas exchange in both the treeline and below-treeline populations, whereas leaf-level photosynthesis continued to increase in response to rising atmospheric CO2 concentrations. Our results suggest that warming-induced atmospheric drying confounds positive growth responses of apparent temperature-limited bristlecone pine populations at treeline. In addition, the observed ecophysiological responses of attitudinally separated bristlecone pine populations illustrate the sensitivity of conifers to climate change

Meta-analysis Reveals Different Competition Effects on Tree Growth Resistance and Resilience to Drought

Drought will increasingly threaten forest ecosystems worldwide. Understanding how competition influences tree growth response to drought is essential for forest management aiming at climate change adaptation. However, published results from individual case studies are heterogeneous and sometimes contradictory. We reviewed 166 cases from the peer-reviewed literature to assess the influence of stand-level competition on tree growth response to drought. We monitored five indicators of tree growth response: mean sensitivity (inter-annual tree ring width variability); association between inter-annual growth variability and water availability; resistance; recovery; and resilience to drought. Vote counting did not indicate a consistent effect of competition on mean sensitivity. Conversely, higher competition for resources strengthened the association between water availability and inter-annual growth rates. Meta-analysis showed that higher competition reduced resistance (p < 0.001) and improved recovery (p < 0.05), but did not consistently affect resilience. Species, site and stand characteristics, and drought intensity were insignificant or poor predictors for the large variability among the investigated cases. Our review and meta-analysis show that competition does not affect the response of tree growth to drought in a unidirectional and universal way. Although density reduction (thinning) can alleviate growth declines during drought, the effects on growth after stress are uncertain. The large variability among investigated cases suggests that local-scale processes play a crucial role in determining such responses and should be explicitly evaluated and integrated into specific strategies for adaptation of forests to climate change

Basic wood density and moisture content of 14 shrub species under two different site conditions in the Chilean Mediterranean shrubland

Aim of study: The aim of this study is to provide information on species-specific basic wood density (g cm(-3)) and moisture content (%) in Mediterranean shrublands.Area of study: The study covers two sites of the sclerophyllous shrubland in central Chile, Cortaderal (34 degrees 35'S 71 degrees 29'W) and Miraflores (34 degrees 08'S 70 degrees 37'W), characterized by different climatic and topographic conditions.Materials and methods: The sampling area covers 4,000 m(2) over four plots at two sites. Shrub species were identified and size-related attributes such as height and crown size measured. A total of 322 shrubs were sampled at 0.3 m aboveground to determine basic wood density and moisture content. Species-specific differences and similarities were analyzed by multiple pairwise comparisons (post-hoc tests) and by ordination and hierarchical clustering.Main results: We found high variation across species in wood density (0.46-0.77 g cm(-3)) and moisture content (41.6-113.1%), with many significant differences among species in wood density and among sites in moisture content. Because intraspecific variability could not be explained by shrub size and pronounced differences in wood density (0.49-0.64 g cm(-3)) also occurred between species of the same genus (e.g., Baccharis linearis and Baccharis macraei), our results suggested that phylogenetic affinity may be less important than adaptation to local conditions.Research highlights: The values presented here were variable according to the type of species and environmental conditions, necessitating the determination of basic wood density (BWD) and moisture content at site - and species-specific level. The provided BWD estimates allow converting green volume to aboveground biomass in shrubland areas and are an essential source of information for estimating the carbon stocks

Intra‐Annual Climate Anomalies in Northwestern North America Following the 1783–1784 CE Laki Eruption

The 1783–1784 CE Laki eruption in Iceland was one of the largest, in terms of the mass of SO2 emitted, high-latitude eruptions in the last millennium, but the seasonal and regional climate response was heterogeneous in space and time. Although the eruption did not begin until early June, tree-ring maximum latewood density (MXD) reconstructions from Alaska suggest that the entire 1783 summer was extraordinarily cold. We use high-resolution quantitative wood anatomy, climate model simulations, and proxy systems modeling to resolve the intra-annual climate effects of the Laki eruption on temperatures over northwestern North America. We measured wood anatomical characteristics of white spruce (Picea glauca) trees from two northern Alaska sites. Earlywood cell characteristics of the 1783 ring are normal, while latewood cell wall thickness is significantly and anomalously reduced compared to non-eruption years. Combined with complementary evidence from climate model experiments and proxy systems modeling, these features indicate an abrupt and premature cessation of cell wall thickening due to a rapid temperature decrease toward the end of the growing season. Reconstructions using conventional annual resolution MXD likely over-estimate total growing season cooling in this year, while ring width fails to capture this abrupt late-summer volcanic signal. Our study has implications not only for the interpretation of the climatic impacts of the Laki eruption in North America, but more broadly demonstrates the importance of timing and internal variability when comparing proxy temperature reconstructions and climate model simulations. It further demonstrates the value of developing cellular-scale tree-ring proxy measurements for paleoclimatology. © 2020. American Geophysical Union. All Rights Reserved.Climate Program Office6 month embargo; first published online 14 December 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]