Physiological, biochemical, and anatomical responses of Araucaria araucana seedlings to controlled water restriction.

Water stress triggers acclimation responses and can damage plants, which varies by species and stress levels. Ongoing climate change is projected to result in longer and more intense water stress conditions leading to an alarming increase in drought-induced forest decline. The aim of this study was to evaluate the physiological responses of leaves and stem wood anatomy from Araucaria araucana pot-grown three-year old seedlings, a conifer tree from northwestern Patagonia. Plants were subjected to moderate and severe water restriction regimes and compared to well-watered controls. Severe water stress reduced relative leaf water content and triggered an accumulation of free proline in leaves, regardless of age. Epicuticular wax extrusions increased in apical leaf stomata while photosynthetic pigments decreased, resulting in differential oxidative damage. The concentration of phenolic compounds was not affected by water restrictions. Plants exposed to restricted water regimes showed diminished middle leaf biomass and expansion (~60% of total leaves), increased stem wood density, and experienced 7% and 30% mortality rates under moderate and severe water stress, respectively. Our findings suggest that under moderate water stress, analogous to short-term droughts, A. araucana seedlings activate physiological mechanisms that allow them to withstand short periods of drought, while more severe water stress and longer droughts can be severely harmful.

Variations in the intrinsic water-use efficiency of north Patagonian forests under a present climate change scenario: tree age, site conditions and long-term environmental effects.

The carbon isotope composition (δ13C) in tree rings were used to derive the intrinsic water-use efficiency (iWUE) of Araucaria araucana trees of northern Patagonia along a strong precipitation gradient. It is well known that climatic and ontogenetic factors affect growth performance of this species but little is known about their influence in the physiological responses, as iWUE. Thus, the main objective of this study was to assess the physiological reactions of young and adult trees from two open xeric and two moderately dense mesic A. araucana forests to the increases in atmospheric CO2 (Ca) and air temperature during the 20th century, and to relate these responses with radial tree growth. The results indicated that the iWUE and the intercellular CO2 concentration (Ci) increased 33% and 32% in average during the last century, respectively, but carbon isotope discrimination (∆13C) was more variable between sites and age classes. Trees from xeric sites presented greater iWUE and lower ∆13C and Ci values than those from mesic sites. In general, iWUE was strongly related with Ca and was significantly affected by mean summer maximum temperature. ∆13C from mesic sites seemed to be mainly affected by summer maximum temperature, while trees from xeric conditions did not show any influence. Tree age also presented a significant effect on iWUE. Adult trees showed higher iWUE values than young trees, indicating an incidence of the tree age and/or height, mainly in closed mesic forests. Moreover, some trees presented positive relationships between iWUE and radial tree growth, while others presented negative or no relationships, indicating that other factors may negatively influence tree growth. Broadly, the results demonstrate the incidence of climatic, environmental and ontogenetic variability in the tree responses; however, more studies are needed to better understand which forests will be more affected by actual and future climate changes.

Stable oxygen isotopes (delta18(O)) in Austrocedrus chilensis tree rings reflect climate variability in northwestern Patagonia, Argentina.

The stable oxygen isotope (delta (18)O) composition of Austrocedrus chilensis (D. Don) Endl. (Cupressaceae) tree rings potentially provide retrospective views of changes in environment and climate in the semi-arid lands of Patagonia. We report the development of the first annually resolved delta (18)O tree-ring chronology obtained from natural forests of the foothills of the northwestern Patagonian Andes. The isotope record spans between 1890 and 1994 AD. We explore the probable links between this record and the climate of the region. Air temperatures during summer conditions are significantly, but not strongly, inversely correlated with annual delta (18)O values from Austrocedrus tree rings. The strongest correlations are between the southern oscillation index (SOI) and the tree rings. The existence of millennial-age Austrocedrus trees in northern Patagonia provides interesting possibilities for examining these climate-related isotopic signals over most of the last 1,000 years.

Climate variability 50,000 years ago in mid-latitude Chile as reconstructed from tree rings.

High-resolution proxies of past climate are essential for a better understanding of the climate system. Tree rings are routinely used to reconstruct Holocene climate variations at high temporal resolution, but only rarely have they offered insight into climate variability during earlier periods. Fitzroya cupressoides-a South American conifer which attains ages up to 3,600 years-has been shown to record summer temperatures in northern Patagonia during the past few millennia. Here we report a floating 1,229-year chronology developed from subfossil stumps of F. cupressoides in southern Chile that dates back to approximately 50,000 14C years before present. We use this chronology to calculate the spectral characteristics of climate variability in this time, which was probably an interstadial (relatively warm) period. Growth oscillations at periods of 150-250, 87-94, 45.5, 24.1, 17.8, 9.3 and 2.7-5.3 years are identified in the annual subfossil record. A comparison with the power spectra of chronologies derived from living F. cupressoides trees shows strong similarities with the 50,000-year-old chronology, indicating that similar growth forcing factors operated in this glacial interstadial phase as in the current interglacial conditions.