Recent decadal drought reverts warming‐triggered growth enhancement in contrasting climates in the southern Andes tree line

Abstract

Aims Rising temperature and declining summer precipitation due to the 1970s‐climate shift in southern South America have reduced forest productivity at dry sites. Here, we worked with the most widespread Southern Hemisphere tree line species, Nothofagus pumilio, across contrasting climatic conditions and determined whether rising atmospheric CO2 concentrations as well as warmer and drier climatic conditions provoked by the 70s‐climatic shift have been causing systematic changes in tree line growth rates and intrinsic water‐use efficiency (iWUE). Location 36–54°S, southern Andes. Time period 1950–2010. Major taxa studied Nothofagus pumilio. Methods We worked at five disparate climatic tree line locations, spanning 18 degrees of latitude; at each location, we sampled trees at four different elevations, including tree line elevation. We quantified the variation in annual tree‐ring width (TRW) as a function of climate, elevation, tree age, size, annual CO2 concentrations and location, using linear mixed‐effects models and interpreted TRW trends in relation to iWUE and isotope (δ13C and δ18O) signalling. Results Across locations, the patterns of tree line growth occurring in the 1980–2010 period exhibited a clear and significant negative trend, in contrast to the previous 1950–1980 period. We found an increase in iWUE and δ18O across time and locations. Given that an increase in δ18O indicates a decrease in stomatal conductance, we assert that drought‐induced stomatal closure appears to be causing the reduction in growth. Main conclusions We show unequivocal evidence that warmer and drier summer conditions translated into a decrease in growth rates along the elevational tree line of the southern Andes, reverting previous growth improvements linked to climate warming. An improvement in iWUE at all locations is most likely explained by decreased stomatal conductance given the rising δ18O signal. An iWUE–growth decoupling may act as an ecological strategy to respond to drought.

Inhabited subsurface wet smectites in the hyperarid core of the Atacama Desert as an analog for the search for life on Mars

“…As already mentioned, the biomass isotopic ratio of δ 13 C measured in the biomass (i.e., TOC) of the clay-rich samples was − 23.4‰. Assuming a mean δ 13 C of − 8‰ for the fixed atmospheric CO 2 90,91 , the observed δ 13 C value corresponded to a fractionation of ca. 15‰, which is within the range (δ 13 C from − 11 to − 26‰) of those described for microorganisms assimilating CO 2 by the Calvin cycle 92 .…”

Radial Growth Patterns Associated with Tree Mortality in Nothofagus pumilio Forest

“…Wood density, together with other hydraulic functional traits such us higher hydraulic capacitance of the leaves and stems, higher ability to recover hydraulic conductivity, and tighter stomatal control of evaporative losses, are important to survive to droughts, as shown in surviving Austrocedrus chilensis vs. dying Nothofagus dombeyi trees growing at the same site after the 1998-1999 drought in northern Patagonia [33,83]. In recent decades, particularly from 1980 onwards, increased water use efficiency and rising delta 118 O in lower and upper elevation N. pumilio forests have likely resulted from drying environmental conditions [32]. However, greater plasticity in ecophysiological traits (e.g., stomatal conductance) of low elevation forest may become an advantage for these forests under a warmer climate [84,85].…”

“…Warming-induced growth reductions and forest decline are expected for the 21st century, especially in warmer and drier areas [14,30]. In Patagonian forests, growth reductions of dominant tree species since the 1980s have been related to the positive phase of the Southern Annular Mode [16,31,32]. Extreme droughts and decreasing precipitation in northern Patagonia have induced extensive forest dieback concurrent with negative trends in radial growth of Austrocedrus chilensis, Nothofagus dombeyii and Nothofagus pumilio [33][34][35][36][37].…”

Global fading of the temperature–growth coupling at alpine and polar treelines

“…Although during the 20th century tree growth has positively responded to rising temperatures in most treeline sites (Salzer et al, 2009;Wilmking et al, 2004), recent investigations in these heat-limited environments have detected weak or even negative associations between growth and temperature due to nongrowing season conditions (Fajardo et al, 2019;Hofgaard et al, 2019;Piper et al, 2016).…”

“…Although during the 20th century tree growth has positively responded to rising temperatures in most treeline sites (Salzer et al, 2009; Wilmking et al, 2004), recent investigations in these heat‐limited environments have detected weak or even negative associations between growth and temperature due to nongrowing season conditions (Fajardo et al, 2019; Hofgaard et al, 2019; Piper et al, 2016). In addition, there is the divergence problem which describes a loss of responsiveness of tree growth to rising growing season temperatures in subarctic regions after the 1960s (Briffa et al, 1998).…”