The magnitude and pace of photosynthetic recovery after wildfire in California ecosystems

Abstract

Wildfire modifies the short- and long-term exchange of carbon between terrestrial ecosystems and the atmosphere, with impacts on ecosystem services such as carbon uptake. Dry western US forests historically experienced low-intensity, frequent fires, with patches across the landscape occupying different points in the fire-recovery trajectory. Contemporary perturbations, such as recent severe fires in California, could shift the historic stand-age distribution and impact the legacy of carbon uptake on the landscape. Here, we combine flux measurements of gross primary production (GPP) and chronosequence analysis using satellite remote sensing to investigate how the last century of fires in California impacted the dynamics of ecosystem carbon uptake on the fire-affected landscape. A GPP recovery trajectory curve of more than five thousand fires in forest ecosystems since 1919 indicated that fire reduced GPP by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>157.4</mml:mn> <mml:mspace width="0.166667em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.166667em"/> <mml:mn>7.3</mml:mn> </mml:mrow> </mml:math> g C m <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> y <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mspace width="0.333333em"/> <mml:mtext>mean</mml:mtext> <mml:mspace width="0.166667em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.166667em"/> <mml:mspace width="0.333333em"/> <mml:mtext>SE,</mml:mtext> <mml:mo> </mml:mo> <mml:mi>n</mml:mi> <mml:mo>=</mml:mo> <mml:mtext>1926</mml:mtext> </mml:mrow> </mml:math> ) in the first year after fire, with average recovery to prefire conditions after <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>12</mml:mn> </mml:mrow> </mml:math> y. The largest fires in forested ecosystems reduced GPP by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>393.8</mml:mn> <mml:mspace width="0.166667em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.166667em"/> <mml:mn>15.7</mml:mn> </mml:mrow> </mml:math> g C m <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> y <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ( n = 401) and took more than two decades to recover. Recent increases in fire severity and recovery time have led to nearly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>9.9</mml:mn> <mml:mspace width="0.166667em"/> <mml:mo>±</mml:mo> <mml:mspace width="0.166667em"/> <mml:mn>3.5</mml:mn> </mml:mrow> </mml:math> MMT CO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msub> </mml:math> (3-y rolling mean) in cumulative forgone carbon uptake due to the legacy of fires on the landscape, complicating the challenge of maintaining California’s natural and working lands as a net carbon sink. Understanding these changes is paramount to weighing the costs and benefits associated with fuels management and ecosystem management for climate change mitigation.