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The greening of the Northern Great Plains and its biogeochemical precursors

Over a century of CO2 increases has caused plants in the Northern Great Plains to become increasingly water-use efficient and more photosynthetically active. Yet, this increase in plant-level productivity has likely come at the cost of reduced nitrogen availability across the NGP.

The Northern Great Plains during a sweet clover bloom

Vegetation greenness has increased across much of the global land surface over recent decades. This trend is projected to continue – particularly in northern latitudes – but future greening may be constrained by nutrient availability due to an increased demand from CO2 enrichment. In this study, we examined the relative strengths of CO2 versus climate on leaf-level chemistry over a century across a greening landscape.

NGP Greening over the last 20 years

To do this, we combined satellite measurements of greening and a 135‐year record of plant carbon and nitrogen concentrations and stable isotope ratios (δ13C and δ15N) from four plant species in the Northern Great Plains (NGP).

We documented significant greening over the past two decades with the highest proportional increases in net greening occurring in the driest and warmest areas. In contrast to the climate‐dependency of greening, we find spatially uniform increases in leaf‐level intercellular carbon and water use efficiency that track rising atmospheric CO2. Despite large spatial variation in greening, we find sustained and climate‐independent declines in foliar nitrogen over the last century. Parallel declines in foliar δ15N and increases in carbon:nitrogen ratios point to diminished nitrogen availability as the likely cause.

Plant-level trade-offs between productivity and nitrogen are temporally mismatched
This simultaneous increase in greening and decline in foliar nitrogen across our study area points to increased nitrogen use efficiency over the last two decades. Further, our results point towards and impending bottleneck, where eventually plant nitrogen use efficiency will be insufficient to sustain future greening trends in NGP grasslands.

Read an interview with Jack Brookshire here!

Also, this paper received a commentary from Joe Craine following publication here.

Authors: E.N. Jack Brookshire, Paul C. Stoy, Bryce Currey, Bruce Finney


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