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Peer Review

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Abstract:
Changes to forest production drivers (light, water, temperature, and site nutrient) over
the last 55 years have been documented in peer-reviewed literature. The main objective
of this paper is to review documented evidence of the impacts of climate change trends
on forest productivity since the middle of the 20th century. We first present a concise
overview of the climate controls of forest production, provide evidence of how the main
controls have changed in the last 55 years, followed by a core section outlining our
findings of observed and documented impacts on forest productivity and a brief
discussion of the complications of interpreting trends in net primary production
(NPP). At finer spatial scales, a trend is difficult to decipher, but globally, based on both
satellite and ground-based data, climatic changes seemed to have a generally positive
impact on forest productivity when water was not limiting. Of the 49 papers reporting
forest production levels we reviewed, 37 showed a positive growth trend, five a negative
trend, three reported both a positive and a negative trend for different time periods, one
reported a positive and no trend for different geographic areas, and two reported no
trend. Forests occupy
52% of the Earth’s land surface and tend to occupy more
temperature and radiation-limited environments. Less than 7% of forests are in strongly
water-limited systems. The combined and interacting effects of temperature, radiation,
and precipitation changes with the positive effect of CO2, the negative effects of O3 and
other pollutants, and the presently positive effects of N will not be elucidated with
experimental manipulation of one or a few factors at a time. Assessments of the greening
of the biosphere depend on both accurate measurements of rates (net ecosystem
exchange, NPP), how much is stored at the ecosystem level (net ecosystem production)
and quantification of disturbances rates on final net biome production.