MSGID: 1:317/3 642e4ae3   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Gone for good? California's beetle-killed, carbon-storing pine forests   
   may not come back    
    Massive climate-stoked infestations are also unlikely to recur, thanks to   
   lower tree-stand density    
      
     Date:   
         April 5, 2023   
     Source:   
         DOE/Los Alamos National Laboratory   
     Summary:   
         Ponderosa pine forests in the Sierra Nevada that were wiped out   
         by western pine beetles during the 2012-2015 megadrought won't   
         recover to pre-drought densities, reducing an important storehouse   
         for atmospheric carbon.   
      
      
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   FULL STORY   
   ==========================================================================   
   Ponderosa pine forests in the Sierra Nevada that were wiped out by western   
   pine beetles during the 2012-2015 megadrought won't recover to pre-drought   
   densities, reducing an important storehouse for atmospheric carbon.   
      
      
   ==========================================================================   
   "Forests store huge amounts of atmospheric carbon, so when western pine   
   beetle infestations kill off millions of trees, that carbon dioxide goes   
   back into the atmosphere," said Zachary Robbins, a postdoctoral at Los   
   Alamos National Laboratory.   
      
   Robbins is corresponding author of a new paper published in the journal   
   Frontiers in Environmental Science about carbon stored in living ponderosa   
   pines in the Sierra Nevada of California.   
      
   "We also found that because so many trees died during the megadrought,   
   there's much less risk of another huge die-off this century because the   
   bark beetles will have fewer host trees," he said.   
      
   A mixed bag of impacts It's a mixed bag, however. Western pine beetle   
   outbreaks driven by climate change will continue to occur, limiting   
   forest regeneration after the drought.   
      
   "Some carbon loss won't be resequestered in trees, but fewer trees on   
   the landscape dampens the severity of western pine beetle outbreaks,"   
   Robbins said.   
      
   "The forest seems to reach an equilibrium at a certain point."   
   The beetle needs a minimum density of trees to support explosive   
   population growth. "Our paper found that as forests reach a certain   
   threshold of ponderosa pine density, they become exponentially more   
   likely to have western pine beetle-driven die-off," Robbins said.   
      
   Complex interactions among climate change, weather, trees and beetles   
   are decimating ponderosa forests in California and elsewhere. During   
   the 2012-2015 drought, an estimated 129 million trees of various species   
   died in the Sierra Nevada.   
      
   Carbon sink or carbon source?  As carbon sinks, forests sequester, or   
   store, more than a tenth of the greenhouse-gas emissions in the United   
   States. Trees can use the increasing carbon in the air to grow and thus   
   fix more carbon in the form of wood.   
      
   "If there's no disturbance, such as a beetle outbreak, then the carbon   
   gets stored, but when the beetle comes with the higher temperatures of   
   climate change, their populations develop more rapidly," said Chonggang   
   Xu, a senior scientist at Los Alamos and a co-author of the paper. "Fewer   
   beetles die in the winter because of warmer minimum temperatures."   
   Add in climate-driven drought, and "it all contributes to more frequent   
   beetle outbreaks," Xu said.   
      
   "The ecosystem has been fundamentally changed by the effects of   
   climate change," Xu said. "That means the forest can't recover to the   
   pre-megadrought carbon level."  During an outbreak, millions of trees die   
   and begin releasing carbon back to the atmosphere through microbial decay.   
      
   Carbon-budget buster Xu said it's critical for future carbon budgeting to   
   include the effects on forest regeneration caused by beetle outbreaks and   
   other disturbances such as drought and fire. Carbon budgeting involves   
   understanding how to manage carbon emissions to prevent catastrophic   
   warming.   
      
   Rising temperatures also compound the effects of drought. "In the past,   
   a three-year drought would have substantial impact on tree mortality,"   
   Xu said.   
      
   "But in a warmer future, a two-year drought could have similar   
   consequences."  Robbins, Xu and their collaborators simulated forests   
   and beetle dynamics at 31 sites where ponderosa pines predominate in the   
   central and southern Sierra Nevada. To study tree growth, regeneration   
   and background mortality, they modeled insect mortality and phenology   
   (cyclical biological events), tree defense and insect attacks.   
      
   They considered two periods, 2006-2018 and 2018-2100. The team used data   
   from observations and modeling to simulate the climate for each site   
   for 2006-2018 and they used climate model data alone for the 2018-2100   
   projections.   
      
   Outbreaks in California and beyond The work has implications for ponderosa   
   forests beyond California.   
      
   "Many of the interactions we describe here are very similar to those   
   across the West, where you have warming temperatures helping beetle   
   populations and you have dense forests of stressed trees, which will   
   continue to be stressed by future drought," Robbins said. "That will   
   increase the risk of outbreaks. Our paper is specific to California,   
   but it's a fundamental problem and we expect it to be reflected in   
   many different forests throughout the western United States."  Xu and   
   Robbins said their results reflect what land managers have known for   
   many years: Overstocked pine stands are particularly susceptible to   
   beetle outbreaks. As droughts increase in severity, managers may have   
   to scale back forest density through active management to mitigate the   
   increased risk.   
      
   "Our evidence shows there's a lot to be gained by those proactive   
   management processes," Robbins said.   
      
   In a related 2022 paper, Robbins, Xu and collaborators published a   
   new modeling framework to assess the risk bark beetles pose in many   
   forest ecosystems under climate change. Adding together the effects   
   of compromised tree defenses (15% to 20%) and increased bark beetle   
   populations (20%), the team had determined that 35% to 40% more ponderosa   
   pines would die from beetle attacks for each degree Celsius of warming.   
      
       * RELATED_TOPICS   
             o Plants_&_Animals   
                   # Trees # Nature # Drought #   
                   Insects_(including_Butterflies)   
             o Earth_&_Climate   
                   # Forest # Global_Warming # Climate # Rainforests   
       * RELATED_TERMS   
             o Mountain_pine_beetle o Forest o Forestry o   
             Climate_change_mitigation o Beetle o Carbon_dioxide o Fisher   
             o Ocean_acidification   
      
   ==========================================================================   
   Story Source: Materials provided by   
   DOE/Los_Alamos_National_Laboratory. Note: Content may be edited for   
   style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Zachary J. Robbins, Chonggang Xu, Alex Jonko, Rutuja Chitra-Tarak,   
         Christopher J. Fettig, Jennifer Costanza, Leif A. Mortenson,   
         Brian H.   
      
         Aukema, Lara M. Kueppers, Robert M. Scheller. Carbon stored in live   
         ponderosa pines in the Sierra Nevada will not return to pre-drought   
         (2012) levels during the 21st century due to bark beetle outbreaks.   
      
         Frontiers in Environmental Science, 2023; 11 DOI: 10.3389/   
         fenvs.2023.1112756   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/04/230405161420.htm   
      
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