Increasing atmospheric CO2 concentrations outweighs effects of stand density in determining growth and water use efficiency in Pinus ponderosa of the semi-arid grasslands of Nebraska (U.S.A.)

Giovanna Battipaglia,Tala Awada,Rahel Aus Der Au,Michele Innangi,Matthias Saurer,Paolo Cherubini

doi:10.1016/j.gecco.2020.e01274

Abstract

This study investigated the impacts of environmental (e.g., climate and CO2 level) and ecological (e.g., stand density) factors on the long-term growth and physiology of ponderosa pine (Pinus ponderosa) in a semi-arid north American grassland. We hypothesized that ponderosa pine long-term growth patterns were positively influenced by an increase in atmospheric CO2 concentrations and a decrease in stand density. To test this hypothesis, comparison of long-term trends in tree-ring width and carbon and oxygen stable isotopic composition of trees growing in dense and sparse forest stands were carried out at two sites located in the Nebraska National Forest. Results indicated that tree-ring growth increased over time, more at the sparse than at the dense stands. In addition, the carbon and oxygen isotopic ratios showed long-term increases in intrinsic water use efficiency (WUEi), with little difference between dense and sparse stands. We found a clear trend over time in ponderosa pine tree growth and WUEi, mechanistically linked to long-term changes in global CO2 concentration. The study also highlighted that global factors tend to outweigh local effects of stand density in determining long-term trends in ponderosa pine growth.Finally, we discuss the implications of these results for woody encroachment into grasslands of Nebraska and we underlined how the use of long-term time series is crucial for understanding those ecosystems and to guarantee their conservation.

Highlights

Spatiotemporal changes in grassland ecosystems processes are strongly influenced by natural disturbances, climatic variability and human activities, such as grazing management
We investigated the impact of stand density, climate variability, and increasing atmospheric CO2 concentration on long-term growth and physiological processes of ponderosa pine trees in the semi-arid Nebraska grasslands, using tree-ring stable isotopes
We found an increase in ponderosa pine growth starting from 1960 at both sites, and in particular at the sparse one

Summary

Introduction

Spatiotemporal changes in grassland ecosystems processes are strongly influenced by natural disturbances, climatic variability and human activities, such as grazing management. In Nebraska, as well as in other semi-arid regions of the Great Plains, one of the consequences of these changes is a shift in vegetation cover, such as the transformation of savanna-like ponderosa pine (Pinus ponderosa) ecosystems into dense forests, and its expansion into adjacent grasslands from historical grasslands-woodlands ecotones (e.g., Eggemeyer et al, 2006, 2009; Msanne et al, 2017). Lawson) is a tree species with one of the largest distribution area in North America (Sala et al, 2005), and recently it has expanded into grasslands from its original forest sites Factors driving this phenomenon are still debated. The role of increasing CO2 concentrations, especially increasing plant WUEi and growth, in the physiological adaptation of plants to newly established climatic conditions in semi-arid ecosystems remains poorly understood (Archer et al, 1995; Davis et al, 1999; Dickie et al, 2007; Polley et al, 2003; Van Auken, 2009; Classen et al, 2010; Sullivan et al, 2017)

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