Wednesday, March 9, 2016

Declines in tree nutrient concentration over past 25 years

I've been trying to catch up on journals lately. Apparently, I hadn't read anything from Global Change Biology over the past 2 years. Must have been distracted. No time like the present...

Here's one that struck me as amazing.

Researchers in Europe resampled forest leaves from 1992 - 2009 across a large number of plots in Europe. At each site for a subset of species they assessed nutrient concentrations and leaf mass--a pretty simple and standard measurement. Doing this allowed them to examine the trajectory of nutrient concentrations (and contents). Nutrient concentrations in leaves are critical to determining tree productivity as well as interactions with herbivores, so knowing whether concentrations are going up or down is critical to modeling the future productivity of these forests.

Here's the simplified result: almost all nutrient concentrations were declining. 20 nutrients had declining concentrations. 2 were increasing.

Here's an example of the pattern for beech. white bars are concentrations, grey contents.

The authors focus on P nutrition the most, emphasizing the role of N deposition in promoting P limitation. Yet, even N concentrations were declining. These declines must be more than just N deposition causing imbalances, especially since N deposition has been declining over the time period. 

The authors suggest elevated atmospheric CO2 might also be playing a role, as well as droughts and warming, but this paper mostly describes the pattern, which is fine.

The big question is: What is causing this massive, continental decline in nutrient concentrations?

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