Growth decline and divergent tree ring isotopic composition (δ¹³C and δ¹⁸O) contradict predictions of CO₂ stimulation in high altitudinal forests

Abstract

Human-induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low-latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high-elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000 m asl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual-resolution) and isotopic composition (decadal-resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood (13) C discrimination, resulting from increasing water use efficiency (20-60%), coinciding with rising atmospheric CO2 . Changes in (13) C discrimination were not followed, however, by shifts in tree ring δ(18) O, indicating site- and species-specific differences in water source or uptake strategy. Our results indicate that CO2 stimulation has not been enough to counteract warming-induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high-elevation ecosystems to atmospheric change.

Drought Influence over Radial Growth of Mexican Conifers Inhabiting Mesic and Xeric Sites

“…For instance, A. durangensis (Figures 3 and 4) showed high sensitivity to water deficit derived from high temperatures and high evapotranspiration rates, which had previously been observed in other fir species in drought-prone areas from Europe [11]. Our results indicate that dominant species in mesic habitats, such as fir species, respond to water deficit, particularly to atmospheric drought, which is supported by studies based on photosynthesis and water-use efficiency in other fir forests of Mexico [52] and the Mediterranean Basin [53]. Consequently, we expect that water-use efficiency inferred from carbon isotope data would be lower for A. durangensis than in the pine species studied at the xeric habitat.…”

Quantifying the impact of drought on soil-plant interactions: a seasonal analysis of biotic and abiotic controls of carbon and nutrient dynamics in high-altitudinal grasslands

“…Here, temperature increases happening concurrently with water deficit, caused significant reductions in plant productivity (up to 24 % above and 17 % below ground, see Online Resource 6), particularly, in the late growing season. Climate fluctuations, as well as seasonal variations in water regime, and drought stress, have been shown to alter plant productivity, phenological cycles, reproductive traits, and patterns of resource use across biomes (Gomez-Guerrero et al 2013;Hueso et al 2011;Jentsch et al 2009;Kreyling et al 2008;Silva and Anand 2013a, b;van Ruijven and Berendse 2010). Our results show that in climate sensitive Tibetan grasslands, these plant-soil interactions represent a major factor determining responses to drought through alteration in C and nutrient dynamics.…”

“…This shows that the combination of higher temperatures and lower precipitation would have drastic synergistic impacts Under drought stress, different ecosystems may have different levels of resistance and resilience. Most existing evidence suggests that temperate grasslands and forests respond to drought similarly, with reductions in productivity, reproductive success and, when the stress is persistent, alteration in structure and species composition (Breda et al 2006;Gomez-Guerrero et al 2013;Kahmen et al 2005;Leithead et al 2010;Lewis et al 2004). Mirroring responses expected for vegetation, microbial productivity (MBC) declined in drought treatments.…”

“…IPCC 2007). Numerous studies have quantified the impacts of climate fluctuations through analyses of physiological responses, and vegetation distribution/displacement over the past century, in response to changes in atmospheric composition and climate (Silva et al 2009(Silva et al , 2011Leithead et al 2010Leithead et al , 2012Gomez-Guerrero et al 2013;Silva and Anand 2013a, b;Silva and Horwath 2013). Despite evidence for a coherent process of global change presented by these studies, little is known about the underlying mechanisms controlling responses to climate variability.…”

Simulating Water-Use Efficiency of Piceacrassi folia Forest under Representative Concentration Pathway Scenarios in the Qilian Mountains of Northwest China

“…This inference is different from the study from Sang and Su [42], who found that the precipitation is the most sensitive factor affecting the WUE of P. schrenkiana forest (site elevations ď2440 m). This difference may be caused by the fact that forest carbon assimilation is more sensitive to the CO 2 concentration in relatively higher elevation regions [49,50], like the forest sites (2770-3250 m) in this study.…”