Abstract and Figures

Drought is a global issue that is exacerbated by climate change and increasing anthropogen-ic water demands. The recent occurrence of drought in California provides an important opportunity to examine drought response across ecosystem classes (forests, shrublands, grasslands, and wetlands), which is essential to understand how climate influences ecosystem structure and function. We quantified ecosystem resistance to drought by comparing changes in satellite-derived estimates of water-use efficiency (WUE = net primary productivity [NPP]/evapotranspiration [ET]) under normal (i.e., baseline) and drought conditions (ΔWUE = WUE 2014 − baseline WUE). With this method, areas with increasing WUE under drought conditions are considered more resilient than systems with declining WUE. Baseline WUE varied across California (0.08 to 3.85 g C/mm H 2 O) and WUE generally increased under severe drought conditions in 2014. Strong correlations between ΔWUE, precipitation, and leaf area index (LAI) indicate that ecosystems with a lower average LAI (i.e., grasslands) also had greater C-uptake rates when water was limiting and higher rates of carbon-uptake efficiency (CUE = NPP/LAI) under drought conditions. We also found that systems with a baseline WUE ≤ 0.4 exhibited a decline in WUE under drought conditions, suggesting that a baseline WUE ≤ 0.4 might be indicative of low drought resistance. Drought severity, precipitation, and WUE were identified as important drivers of shifts in ecosystem classes over the study period. These findings have important implications for understanding climate change effects on primary productivity and C sequestration across ecosystems and how this may influence ecosystem resistance in the future.
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vvArticle e015611v


234
56789
1United States Forest Service, Rocky Mountain Research Station, 240 West Prospect Road, Fort Collins, Colorado 80524 USA
2School of Biological, Earth and Environmental Science, The University of New South Wales, Sydney, New South Wales 2052 Australia
3Laboratory of Ecology (iEcolab), Andalusian Institute for Earth System Research, Andalusian Center for Environmental Research,
University of Granada, Avda. Mediterráneo s/n, Granada 18006 Spain
4United States Geological Survey, Fort Collins Science Center, Fort Collins, Colorado 80526 USA
5The Natural Capital Project, The Stanford Woods Institute for the Environment, Stanford University, 371 Serra Mall, Stanford,
California 94305 USA
6Embrapa Informática Agropecuária, Av. André Tosello, 209, Campus Unicamp, 13083-886, Campinas, SP, Brazil
7Department of Environmental Science and Policy, University of California, Davis, One Shields Avenue, Davis, California 95616 USA
8Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstr. 43, 10115 Berlin, Germany
9Department of Earth System Science, University of California, Irvine, California 92697 USA
Citation:
       

Abstract.
             
 

           

 2014   

 2   

   


     




Key words:        

Received
Copyright:     

E-mail: 
vvArticle e015612v www.esajournals.org
 
IntroductIon
    
      
  

   

    
     
       
      
      
       
       


      
     


        





    
       
      

      
     
  
    
       
    
     
  
2
     2
     
    


      

  

    
     
        
      
      

     
       
     




 
  
   
      
    




    
    
  

     
    
       


       
       



     
    

      
     
      
       

       
  

        
      
    
     
  

vvArticle e015613v www.esajournals.org
 
    
      
      



    
       
     

         
   
      
    


  

 

 
      
    




    


Methods
Study site


   
      
    


  



     

 
       


Ecosystem class
     
     
     
 
   



       

     
 


     


 


    


      
    
      
      
     
 
 
   





        

    



     
     
      
        
     
     
      
       
vvArticle e015614v www.esajournals.org
 
      
   


Drought indices and climate conditions
  
      
     
     
       
     
      
        
       



       
     
     

         
      

        
     


      
             


              

vvArticle e015615v www.esajournals.org
 

     

Ecosystem measures of C, ET, and LAI
     

      
    
  
      
  
  

 
     
    

   
  
     
    

R
(Rm      

      


    T  

       

        
     
      
       
      

    

  
RRm



     
      





      
     
       
 

 
 

  
      
        

    

        

      
       


      

       

 
  
 
 
        
 22    
        


      

       

        
     
       
 
     
(1)
GPP
=
Tmax
×
m(Tmin)
×
FPA R
×
PAR
×
0.45
(2)
PSNnet
=
GPP
Rlr
(3)
NPP
=
(PSNnet)
Rg
Rm
vvArticle e015616v www.esajournals.org
 
     
 

    
   
      
  




      
        

        
      


Water- use and carbon- uptake efficiency
       



        
)

       
        
  
         

        

      
    
 
  
          
        
      

       
   
        
      
    
      
    R2  

     
     
          
        

      

       

     
        
  
      
      

     

MOD16 (ET) and MOD17 validation (NPP and GPP)
  

       
       

      
     

      

  
     

     

      

  
       



Study design
      
      
     
(4)
WUE
=
NPP
ET
(5)
CUE
=
NPP
ET
vvArticle e015617v www.esajournals.org
 

      

      
    


      

n
     
 
     
   
       
     
      
       

      
 

     



     

      

     
       
     

        


      

      

      

   
       
       
ρ

    


      λ





    

         


  

(m)



 R2R2R2
   470     
   475     


  1710     


  1280     


  1300     
   275     


  405     


  1160     


  2015     


  2700     
Notes:   R2                

vvArticle e015618v www.esajournals.org
 
        
     


 

 

 
      
       
     

 







   

      
 
 
      
         


  
       
      

      

       

      
 
 

     
  

results
MOD16 (ET) and MOD17 validation (NPP and GPP)
      
R2P
(R2P

 r     
R2P

     

       
 R2     
(R2 R2
R2        
  R2    
     R2  
       

Baseline conditions


     
  

     



    
      
    
     
      
     
      



    
         
          
          
          
Note: 
vvArticle e015619v www.esajournals.org
 

     

     
       
 


     


      
     

2      
               

    m
 m22

 
Px
vvArticle e0156110v www.esajournals.org
 
Temporal and spatial patterns in drought




   
       
   
     
       

        

   
 
      


Quantifying drought resistance with ΔWUE
     
      
        
      


        


    


      
      
  
       
  
    
  R2   P   
      
     
     
  R2   P  

     P  
       
              



vvArticle e0156111v www.esajournals.org
 


Changes in ecosystem class
      
  
    

     
   


2014
2014

vvArticle e0156112v www.esajournals.org
 

    



   
      
 
     



       

dIscussIon
ΔWUE as an indicator of drought resistance
    



     
     
  


   

      

 2

      

  
      
2014  
     
(yR2     
  
        


2014
 
 2       

   Z P
    
    
    
    
    
Note: 
     

vvArticle e0156113v www.esajournals.org
 
      

    

    
     
       
      


  
      


   
     
 
      
   
      
    
     
      




       
      


Ecosystem resistance to drought
 
     
      
      
    
  
          
      
       
       

     
      

       
  




2013
   
2002     
    
    
    

   

   Z P

    
    
    
    

(2014)
   
    
    

    
    
    
    

(2014)
   
    
    

    
    
    
    

(2014)
   
    
    
Note:     

vvArticle e0156114v www.esajournals.org
 

  

      
    


  
     
     



     
     
        
  
    




       

      
      
      


  2O) Source (
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
 4
  
  
  
  
  
  
  
  
   
  
  
  
  
Note: 
vvArticle e0156115v www.esajournals.org
 

       



       


     
  
     
   



      
      
     

      
 


   

     

Implications for climate change

  
2
     

      


     
        
 

    
        


      

     
      

       
      
     
  

      
     

    
       
    
  
     

Research limitations
   
    

     


  
     
      



  
      
      

     
        

  
  

     
      


conclusIons


     


     

vvArticle e0156116v www.esajournals.org
 

      




       
      




     
  

    
     

      

AcknowledgMents
       
       
     

      


       
     
  
       
         
      
      
     


     
      
  
     

         

lIterAture cIted

     



 


       
      

        
      

  
       
2






      


 
 




     



   

 
      
  

          
   



  
    
      




         
     


vvArticle e0156117v www.esajournals.org
 
        
     
      


       
      
  

          
       


  

         
    
 
       

  
      
in
    
     
      
      


     
     
     





     


      


        

     


      


   in     
       
 
     
  

        
 




  


  
   


    

   






     
     
       


 

      
      

      
      
 

         

    
      



   
      



            

      
  

vvArticle e0156118v www.esajournals.org
 
  


   
     
  

      
 

      


      
    


     
        
     

     
      

          
       
      


    

      

    



 
   

        
22


       

  
     

      
       
  



     


     
    
   



 







  


       
    
      
     

         
        
      
     

         
      
     

        



      
 

   


     
     
    

      


         
      


vvArticle e0156119v www.esajournals.org
 



   



  

           
         
 2


supportIng InforMAtIon
        

... Vegetation responses to drought are controlled by physical processes at the landscape level such as soil water routing and retention, groundwater interactions, and evaporative demand [9][10][11][12] and by plant community composition and stress-tolerance thresholds of species, populations, and individuals [13][14][15] . Variability across spatial and temporal scales in plant responses to drought can include differences in water-use efficiency, stomatal regulation, hydraulic characteristics, and structural adjustments to root networks and leaf area 7,[15][16][17][18] . For example, plant species and functional groups differ in their approaches to balancing trade-offs between regulating leaf-water potentials and maintaining stomatal conductance, with consequences for susceptibility to drought-induced carbon starvation and/or hydraulic failure 15,19 . ...
... Drought sensitivity was generally high in forests with both high climatic water deficit and low AET (indicating chronic water limitation on productivity, Fig. 4), and forest drought sensitivity was highest in dry forests and woodlands such as pinyon-juniper woodlands (Fig. 2). Dry forests are commonly considered drought-tolerant, and their spectral responses to drought may in part reflect adaptations that allow tree species to persist through periods of soil water scarcity by sacrificing short-term productivity 13,15,17,28,46,47 . Indeed, some dry forest types may expand their ranges as climates become drier, displacing more droughtvulnerable vegetation types according to some climate projections 48,49 . ...
... However, prolonged drought stress can cause profound impacts on wet forests up to and including mortality, in part because tree species have fewer adaptive responses (e.g., cavitation resistance) to cope with moisture deficits 14 . Such species tend to invest more resources into competing for light (e.g., via height and leaf area) than hydraulic architecture to withstand water stress 47,50 and may show reduced water-use efficiency under drought 17 . Thus, long-term (i.e., multi-year) droughts and extreme drought conditions (which were not observed in this study) are likely to affect forest growth rates and carbon storage in the wetter forests of the region and to elevate vulnerability to large wildfires and insect outbreaks 34 . ...
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Climate change is anticipated to increase the frequency and intensity of droughts, with major impacts to ecosystems globally. Broad-scale assessments of vegetation responses to drought are needed to anticipate, manage, and potentially mitigate climate-change effects on ecosystems. We quantified the drought sensitivity of vegetation in the Pacific Northwest, USA, as the percent reduction in vegetation greenness under droughts relative to baseline moisture conditions. At a regional scale, shrub-steppe ecosystems—with drier climates and lower biomass—showed greater drought sensitivity than conifer forests. However, variability in drought sensitivity was considerable within biomes and within ecosystems and was mediated by landscape topography, climate, and soil characteristics. Drought sensitivity was generally greater in areas with higher elevation, drier climate, and greater soil bulk density. Ecosystems with high drought sensitivity included dry forests along ecotones to shrublands, Rocky Mountain subalpine forests, and cold upland sagebrush communities. In forests, valley bottoms and areas with low soil bulk density and high soil available water capacity showed reduced drought sensitivity, suggesting their potential as drought refugia. These regional-scale drought-sensitivity patterns discerned from remote sensing can complement plot-scale studies of plant physiological responses to drought to help inform climate-adaptation planning as drought conditions intensify.
... years, under the combined and cumulative effect of climatic and anthropogenic factors, they have become particularly sensitive to extreme climate variability (Malone et al. 2016;Le Page and Zribi 2019;Fragaszy et al. 2020). In the particular context of Mediterranean arid and semi-arid agroecosystems, drought is the most doubtful climatic hazard (Cook et al. 2016;Ouatiki et al. 2019;Fragaszy et al. 2020;Meliho et al. 2020;Hadri et al. 2021). ...
... In these agroecosystems, the response of natural and anthropogenic units to drought is controlled by many processes. These processes include non-climatic factors namely, topographic factor, soil type, soil water conveyance and retention, interactions with underground water, the existence or lack of irrigation-related inputs Malone et al. 2016;Hoylman et al. 2019). Among these factors, the dominance of poorly evolved soils and the progressive degradation of the vegetation cover is the real factors of sensitivity and fragility of agrosystems in the face of the impacts of drought. ...
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Several drought indices are commonly used to quantify the severity and magnitude of agricultural drought. A comprehensive assessment of temporal and spatial convergences is important. This study evaluates the spatio-temporal concordances of eight drought indices, between October and April, over the period 2003 to 2021 under the semi-arid conditions of two watersheds in Morocco. These include three monovariate, two bivariate, and three composite indices. Taylor diagram, linear regression and Random Forest clustering were used to evaluate and compare the overall performance of the indices. The results indicate that spatio-temporal variability and spatial heterogeneity of the different agrosystems have little influence on the concordances of composite indices while TCI and VCI have a variable spatio-temporal concordance depending on the spatial extent considered. Nevertheless, depending on the spatial extent considered and the agroclimatic characteristics of each agrosystem, the seasonal correlations between the composite indices and precipitation anomalies, net primary productivity and cereal yield anomalies vary from 0.52 to 0.79, 0.23 to 0.78 and from 0.33 to 0.73, respectively. The correlations between the simple indices and precipitation anomalies, net primary productivity, and cereal yield anomalies range, respectively from 0.33 to 68 0.13 to 0.72, and from 0 to 0.84. In general, the composite indices showed the best agreement between them and reference indicators but the VCI seems better suited for the assessment of the seasonal profitability of studied agrosystems.
... Plant species in semi-arid regions such as the Mediterranean Basin are generally drought-adapted and have developed drought escape, tolerance or avoidance resistance mechanisms, notably via increased carbon uptake efficiency (Nardini et al., 2014;Aslam et al., 2015). Yet, under climate change, the drought resistance threshold of Mediterranean plant species and communities may be exceeded, reducing ecosystem productivity and threatening plant diversity (Malone et al., 2016;Rodriguez-Ramirez et al., 2017). ...
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1. Pollinators are declining globally, with climate change implicated as an important driver. Climate change can induce phenological shifts and reduce floral resources for pollinators, but little is known about its effects on floral attractiveness and how this might cascade to affect pollinators, pollination functions and plant fitness. 2. We used an in situ long‐term drought experiment to investigate multiple impacts of reduced precipitation in a natural Mediterranean shrubland, a habitat where climate change is predicted to increase the frequency and intensity of droughts. Focusing on three insect‐pollinated plant species that provide abundant rewards and support a diversity of pollinators (Cistus albidus, Salvia rosmarinus and Thymus vulgaris), we investigated the effects of drought on a suite of floral traits including nectar production and floral scent. We also measured the impact of reduced rainfall on pollinator visits, fruit set and germination in S. rosmarinus and C. albidus. 3. Drought altered floral emissions of all three plant species qualitatively, and reduced nectar production in T. vulgaris only. Apis mellifera and Bombus gr. terrestris visited more flowers in control plots than drought plots, while small wild bees visited more flowers in drought plots than control plots. Pollinator species richness did not differ significantly between treatments. Fruit set and seed set in S. rosmarinus and C. albidus did not differ significantly between control and drought plots, but seeds from drought plots had slower germination for S. rosmarinus and marginally lower germination success in C. albidus. 5. Synthesis. Overall, we found limited but consistent impacts of a moderate experimental drought on floral phenotype, plant reproduction and pollinator visits. Increased aridity under climate change is predicted to be stronger than the level assessed in the present study. Drought impacts will likely be stronger and this could profoundly affect the structure and functioning of plant‐pollinator networks in Mediterranean ecosystems.
... The water use efficiency (WUE) of an ecosystem is the ratio of carbon sequestration to water consumption, which couples the terrestrial carbon and water cycles and closely links photosynthesis and evapotranspiration (ET) processes in the ecosystem [1,2]. Although there are many ways to express the WUE of an ecosystem, the ratio of the net primary production (NPP) to ET is widely used [3]. The variation of WUE controlled by both biotic and abiotic factors [4,5] and recent research showed that a vapor pressure deficit (VPD) and canopy conductance are two dominant factors of WUE in response to drought [6]. ...
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Soil moisture (SM) significantly affects the exchange of land surface energy and the stability of terrestrial ecosystems. Although some conclusions have been drawn about the effects of SM on the ecosystem water use efficiency (WUE), the influence mechanism and the quantitative assessment framework of SM on WUE are still unclear. This study provides an analysis framework for the feedback relationship between SM and WUE based on the dependence of the evaporation fraction on SM and output datasets from remote sensing and the Global Land Data Assimilation System. The results show that the range of WUE of terrestrial ecosystems of China was0.02–19.26 g C/kg H2O in the growing season with an average value of 1.05 g C/kg H2O. They also show a downward trend in43.99% of the total area. In the evapotranspiration (ET) pathway, SM negatively affected WUE, and the sensitivity coefficient ranged from −18.49 to −0.04. In the net primary production (NPP) pathway, the sensitivity coefficient ranged from −68.66 to 43.19. Under the dual effects of the ET and NPP pathways, the influence of SM on WUE was negative in 84.62% of the area. Variation in SM led to significant WUE variability. Generally, the percentage change in WUE (ΔWUE) ranged from 0% to 190.86%, with an average value of 28.02%. The maximum ΔWUE ranged from 0% to 758.78%, with an average value of 109.29%. The WUE of forest ecosystems showed strong resistance to SM variation, whereas that of non-forest vegetation was more sensitive to SM variation. This analytical framework provides a new perspective on the feedback relationship between WUE and SM in terrestrial ecosystems.
... Trend analysis provides insight into the ecological history of an area over specific intervals and can be used to frame hypotheses with respect to resistance, persistence or vulnerability to change (Folke et al., 2004;Hughes et al., 2017;Malone et al., 2016). Previous trend analysis studies have documented evidence of multidecadal poleward expansion of deciduous shrubs into the Canadian (McManus et al., 2012) and Siberian tundra Frost et al., 2014) as a result of warming temperatures. ...
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Riparian ecosystems provide critical habitat for many species, yet assessment of vegetation condition at local scales is difficult to measure when considering large areas over long time periods. We present a framework to map and monitor two deciduous cover types, upland and riparian, occupying a small fraction of an expansive, mountainous landscape in north-central Wyoming. Initially, we developed broad-scale predictions of predominant woody vegetation types by integrating Landsat data into species distribution models and combining subsequent outputs into a synthesis map. Then, we evaluated a 35-year Landsat time series (1985–2019) using the Mann-Kendall test to identify significant trends in the condition of upland and riparian deciduous vegetation and assessed the rate and direction of change using the Theil-Sen estimator. Finally, we used plot level data to assess the utility of the framework to detect bottom-up controls (ungulate browse pressure and management actions) on vegetation condition. The synthesis map had an overall correct classification rate of 87% and field data indicated deciduous vegetation within 45 m of coniferous forest faces increased pressure of conifer expansion. The trend assessment identified consistent patterns operating at the landscape scale across both upland and riparian deciduous vegetation; a predominant greening trend was observed for 12 years followed by a 9-year browning trend, before switching back to a greening trend for the last 13 years of the study. Our results indicate trends are driven by the climate of the measurement period at the landscape scale. Although we did not find conclusive evidence to establish a strong link between browse pressure and satellite data, we highlight examples where prevailing trends can be overridden by local disturbance or management intervention. This framework is transferable to other understudied riparian environments throughout western North America to provide insight on ecohydrological processes and assess global and local stressors across broad spatiotemporal scales.
... For South China, however, moderate and extreme drought reduced annual WUE and severe drought slightly increased annual WUE, suggesting a 'turning point' in WUE-drought relationship (Liu et al 2015). In contrast, positive WUE responses to severe and extreme drought occurred across land biomes in California (Malone et al 2016) and in the forest ecosystem in Southwest China (Zhou et al 2018). Spatially coherent increases in intrinsic WUE were also observed over the Northern Hemisphere during severe droughts that affected Europe, Russia and the United States in 2001-2011 (Peters et al 2018). ...
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Warming has resulted in increases in frequency, intensity and/or duration of droughts in most land regions over the globe. Nevertheless, knowledge on how ecosystem water use efficiency (WUE) responds to extreme drought stress and whether the responses are affected by drought timing is still limited. In this study, we examined the changes in ecosystem WUE under extreme drought years over Northern Eurasia during 1982-2011 and further assessed WUE responses to droughts with separate groupings designed to characterize the timing of extreme drought stress. We found that drought timing indeed influenced the responses of ecosystem WUE under extreme drought years. Negative impacts of extreme drought stress during the dry season on ecosystem WUE were more remarkable than those from extreme drought stress during the wet season. Particularly, impacts of droughts on ecosystem carbon-water interactions differed among ecosystem types due to the specific hydrothermal condition of each biome. The information provided by our analyses plays an importance role in identifying water use strategies of terrestrial vegetation in response to drought stress and will help improve our understanding and predictions of the response of ecosystem WUE to global environmental change.
... The slope of the linear regression between δ BP and δ pre is −0.6, indicating that 60% of the initial gradual changes in productivity, negative or positive, were offset at the BP. Given that almost the entirety of California has experienced equally strong drought conditions (Williams et al., 2015), these patterns suggest that California ecosystems have a wide range of drought resistance and resilience (Malone et al., 2016). Larger initial decreases in productivity mean a higher recovery potential when the environmental stress is relieved. ...
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The 2012–2016 California Drought severely impacted natural vegetation across a wide range of environmental gradient. Although several studies have reported an increase in plant water stress and mortality, the spatiotemporal variations of ecosystem productivity responses and the associated environmental and biological drivers remain unclear. Here, using Enhanced Vegetation Index from the Moderate resolution imaging spectrometer, we found that 45% of the natural ecosystems showed an abrupt change (breakpoint [BP]) in productivity during 2012–2016. There were three major contrasting temporal patterns of productivity responses: (i) a steady increase under higher temperature followed by a decline due to accumulated moisture depletion (high elevation forest) or temperature decrease (high elevation nonforest), (ii) gradual decline during the drought followed by a rapid recovery within 1 year after drought stress was partially relieved, and (iii) both a gradual decline and an abrupt decline. The magnitude of abrupt changes was negatively correlated (r = −0.80, p < 0.001) with initial gradual changes. Overall, changes during BP offset, on average, 57% of the preceding gradual responses. The spatial variability in ecosystem response patterns is driven by both environmental and biological factors. Particularly, for forests, positive BP was driven by increasing rainfall and decreasing temperature, while negative BP was mainly driven by the precipitation anomaly. By 2019, 33% of the natural vegetation have recovered to the level of EVI in 2010. Ecosystem responses to multiyear droughts can influence ecosystem dynamics in a complex pattern. Multiple ecohydrological factors should be considered to understand and predict the long-term drought impacts on ecosystems.
... This result confirms Jones (1985) who showed that plants with lower LAI tended to increase their WUE. A more recent study completed in California ecosystems also supports that conclusion (Malone et al. 2016). Furthermore, whole tree transpiration is generally known to increase with LAI (Bréda and Granier 1996;Cregg et al. 2000) and therefore WUE and total leaf area are important parameters to consider when examining plant adaptations to water stress. ...
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With continued global change as a result of land use changes, invasive species and changing climatic patterns, it is becoming increasingly important to understand the adaptability of Senegalia senegal provenances to maximize resilience in managed and natural populations of this species. The objective of this study is to investigate Senegalia senegal geno-typic differences in water use efficiency (WUE) measured by stable 13 C isotope composition in foliage according to their ploidy levels. Secondary objectives are to discuss inherent adaptive variation related to soil pH, survival, growth indexes, gum arabic yield and WUE within provenance in the climate change context. A Senegalia senegal progeny trial, in Dahra, Senegal was used in this study: 443 adult trees consisting of 60 families nested within 4 provenances were assessed in this study. Results showed significant differences in gum yield among provenances (P = 0.0002) and families (P < 0.0001). Diamenar and Ngane provenances showed overall similar annual gum yield despite a lower tree survival rate of Ngane than Diamenar. Growth traits, especially stem volume index and crown area index were larger on Ngane provenance, which also displayed significantly higher foliar WUE and lower leaf area index (LAI) than the other provenances. WUE was positively correlated with gum yield (P = 0.0302), but the coefficient of determination was only 2%. Foliar δ 13 C varied significantly (P < 0.0001) between diploids (− 27.91‰) and polyploids (− 27.12‰). However, within each provenance no significant difference was found. Only 15% of isotope compositions could be explained by ploidy level variation. Differences found in growth and gum yield may be attributed to genotype-specific variation. However, a significant correlation between soil pH and tree survival rate was found (P = 0.0051; r = 0.60). This study confirmed a possible improvement of the gum arabic sector through genotype based selection. Ngane and Diamenar seem to be more profitable to grow in Dahra than the other tested provenances. Future research should investigate the effect of soil pH, other soil physical and chemical properties, and management activities to improve site quality on tree survival and gum yields among provenances. Further, more research is needed to clarify inherent traits underlying drought tolerance in the field and gum yield performance.
... In addition to these landscape-scale physical processes, drought resistance in forests can arise from physiological traits (Figure 2) that vary among individuals, populations, and species. In California, for example, Malone et al. (2016) found that drought event resistance was strongest in vegetation with the highest baseline wateruse efficiency, suggesting adaptation to chronic water limitation. ...
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Disturbance refugia – locations that experience less severe or frequent disturbances than the surrounding landscape – provide a framework to highlight not only where and why these biological legacies persist as adjacent areas change but also the value of those legacies in sustaining biodiversity. Recent studies of disturbance refugia in forest ecosystems have focused primarily on fire, with a growing recognition of important applications to land management. Given the wide range of disturbance processes in forests, developing a broader understanding of disturbance refugia is important for scientists and land managers, particularly in the context of anthropogenic climate change. We illustrate the framework of disturbance refugia through the individual and interactive effects of three prominent forest disturbance agents: fire, drought, and insect outbreaks. We provide examples of disturbance refugia and related applications to natural resource management in western North America, demonstrate methods for characterizing refugia, identify research priorities, and discuss why a more comprehensive definition of disturbance refugia is relevant to conservation globally.
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Assessing ecosystem risk is crucial to ecosystem management and to achieving the Paris Agreement’s 1.5 °C or 2.0 °C global warming targets, but the spatial pattern and drivers of ecosystem risk are highly controversial. By simulating China’s ecosystem water use efficiency using the Lund–Potsdam–Jena global vegetation model, the ecosystem risk at the eco-geographical and vegetation-type scales was assessed in this study to identify the regions at risk under global warming, and the impacts of the species richness (ASR) and micro-meteorological factors on the ecosystem risk were investigated in the identified regions using the structural equation model (SEM). Our results indicate that about 30.5% and 32.2% of the ecosystems in China will face risk under global warming targets of 1.5 °C and 2.0 °C, respectively. The ecosystem risk is spatially heterogeneous and biome-specific. The ecosystem risk will mainly occur in the mid-temperate humid/sub-humid region, warm temperate humid/sub-humid region, northern subtropical humid region, mid-subtropical humid region, and the Tibetan Plateau region in China, of which cropland, grassland, and deciduous broadleaf forests will account for 39%, 17%, and 14%, respectively. The results of the SEM analysis indicate that the micro-meteorological factors and ASR explain 48% of the variation in the ecosystem risk, and the micro-meteorological factors and ASR have similar but opposite influences on the ecosystem risk. For the ecosystems identified as at risk under global warming, the negative total effect of the micro-meteorological factors on the ecosystem risk is mainly due to the dominant role of the shortwave radiation in regulating the ecosystem risk; and the positive total effect of the ASR on the ecosystem risk indicates that the ecosystems with higher species richness may face more severe damage when the risk occurs. Therefore, we should carefully balance the effect of the ASR on the ecosystems’ abilities to adapt to changes and ecosystems’ abilities to recover from damage. Our results contribute to precise ecosystem management in China and provide evidence to support the idea that properly optimizing biodiversity can prevent and reduce the damage to ecosystems caused by global warming.
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Drought is an intermittent disturbance of the water cycle that profoundly affects the terrestrial carbon cycle. However, the response of the coupled water and carbon cycles to drought and the underlying mechanisms remain unclear. Here we provide the first global synthesis of the drought effect on ecosystem water use efficiency (WUE = gross primary production (GPP)/evapotranspiration (ET)). Using two observational WUE datasets (i.e., eddy-covariance measurements at 95 sites (526 site-years) and global gridded diagnostic modelling based on existing observation and a data-adaptive machine learning approach), we find a contrasting response of WUE to drought between arid (WUE increases with drought) and semi-arid/sub-humid ecosystems (WUE decreases with drought), which is attributed to different sensitivities of ecosystem processes to changes in hydro-climatic conditions. WUE variability in arid ecosystems is primarily controlled by physical processes (i.e., evaporation), whereas WUE variability in semi-arid/sub-humid regions is mostly regulated by biological processes (i.e., assimilation). We also find that shifts in hydro-climatic conditions over years would intensify the drought effect on WUE. Our findings suggest that future drought events, when coupled with an increase in climate variability, will bring further threats to semi-arid/sub-humid ecosystems and potentially result in biome reorganization, starting with low-productivity and high water-sensitivity grassland.