[MCN] US forests now more vulnerable to climate than to logging

[Lance Olsen]

Forest Ecology and Management
Volume 360, 15 January 2016, Pages 242-252
doi:10.1016/j.foreco.2015.10.042

Forest disturbance across the conterminous United States from 
1985-2012: The emerging dominance of forest decline
Warren B. Cohen et al

Highlights
*We characterized annual rates of forest disturbance between 1985 and 2012.
*Landsat time series were visually interpreted, with support of ancillary data.
*A probability design was used to scale estimates and provide uncertainties.
*Harvest was the most important disturbance agent class prior to the mid-90s.
*Forest decline is now more important than harvest as the dominant agent class.

Abstract
http://www.sciencedirect.com/science/article/pii/S037811271500599X

Evidence of shifting dominance among major forest disturbance agent 
classes regionally to globally has been emerging in the literature. 
For example, climate-related stress and secondary stressors on 
forests (e.g., insect and disease, fire) have dramatically increased 
since the turn of the century globally, while harvest rates in the 
western US and elsewhere have declined. For shifts to be quantified, 
accurate historical forest disturbance estimates are required as a 
baseline for examining current trends. We report annual disturbance 
rates (with uncertainties) in the aggregate and by major change 
causal agent class for the conterminous US and five geographic 
subregions between 1985 and 2012. Results are based on human 
interpretations of Landsat time series from a probability sample of 
7200 plots (30 m) distributed throughout the study area. Forest 
disturbance information was recorded with a Landsat time series 
visualization and data collection tool that incorporates ancillary 
high-resolution data. National rates of disturbance varied between 
1.5% and 4.5% of forest area per year, with trends being strongly 
affected by shifting dominance among specific disturbance agent 
influences at the regional scale. Throughout the time series, 
national harvest disturbance rates varied between one and two 
percent, and were largely a function of harvest in the more heavily 
forested regions of the US (Mountain West, Northeast, and Southeast). 
During the first part of the time series, national disturbance rates 
largely reflected trends in harvest disturbance. Beginning in the 
mid-90s, forest decline-related disturbances associated with 
diminishing forest health (e.g., physiological stress leading to tree 
canopy cover loss, increases in tree mortality above background 
levels), especially in the Mountain West and Lowland West regions of 
the US, increased dramatically. Consequently, national disturbance 
rates greatly increased by 2000, and remained high for much of the 
decade. Decline-related disturbance rates reached as high as 8% per 
year in the western regions during the early-2000s. Although low 
compared to harvest and decline, fire disturbance rates also 
increased in the early- to mid-2000s. We segmented annual 
decline-related disturbance rates to distinguish between newly 
impacted areas and areas undergoing gradual but consistent decline 
over multiple years. We also translated Landsat reflectance change 
into tree canopy cover change information for greater relevance to 
ecosystem modelers and forest managers, who can derive better 
understanding of forest-climate interactions and better adapt 
management strategies to changing climate regimes. Similar studies 
could be carried out for other countries where there are sufficient 
Landsat data and historic temporal snapshots of high-resolution 
imagery.

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Study co-author Dr Amanda Bates, from Ocean and Earth Science at the 
University of Southampton, said: "In 100 years from now, 100 per cent 
of species in many communities will be lost and replaced by new 
species able to tolerate warmer conditions, leading to a 
redistribution of species across the globe."

http://www.eurekalert.org/pub_releases/2015-11/uos-tso111015.php

and/or:

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature16144.html

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