Variation in stem mortality rates determines patterns of above‐ground biomass in <scp>A</scp>mazonian forests: implications for dynamic global vegetation models

Abstract

<jats:title>Abstract</jats:title><jats:p>Understanding the processes that determine above‐ground biomass (<jats:styled-content style="fixed-case">AGB</jats:styled-content>) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing and evaluating dynamic global vegetation models (<jats:styled-content style="fixed-case">DGVM</jats:styled-content>s). <jats:styled-content style="fixed-case">AGB</jats:styled-content> is determined by inputs from woody productivity [woody net primary productivity (<jats:styled-content style="fixed-case">NPP</jats:styled-content>)] and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody <jats:styled-content style="fixed-case">NPP</jats:styled-content>, control variation in <jats:styled-content style="fixed-case">AGB</jats:styled-content> among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in <jats:styled-content style="fixed-case">AGB</jats:styled-content> and woody <jats:styled-content style="fixed-case">NPP</jats:styled-content> with the predictions of four <jats:styled-content style="fixed-case">DGVM</jats:styled-content>s. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of <jats:styled-content style="fixed-case">AGB</jats:styled-content>, which is consistent with the importance of stand size structure for determining spatial variation in <jats:styled-content style="fixed-case">AGB</jats:styled-content>. The relationship between stem mortality rates and <jats:styled-content style="fixed-case">AGB</jats:styled-content> varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences <jats:styled-content style="fixed-case">AGB</jats:styled-content>. In contrast to previous findings, we find that woody <jats:styled-content style="fixed-case">NPP</jats:styled-content> is not correlated with stem mortality rates and is weakly positively correlated with <jats:styled-content style="fixed-case">AGB</jats:styled-content>. Across the four models, basin‐wide average <jats:styled-content style="fixed-case">AGB</jats:styled-content> is similar to the mean of the observations. However, the models consistently overestimate woody <jats:styled-content style="fixed-case">NPP</jats:styled-content> and poorly represent the spatial patterns of both <jats:styled-content style="fixed-case">AGB</jats:styled-content> and woody <jats:styled-content style="fixed-case">NPP</jats:styled-content> estimated using plot data. In marked contrast to the observations, <jats:styled-content style="fixed-case">DGVM</jats:styled-content>s typically show strong positive relationships between woody <jats:styled-content style="fixed-case">NPP</jats:styled-content> and <jats:styled-content style="fixed-case">AGB</jats:styled-content>. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in <jats:styled-content style="fixed-case">DGVM</jats:styled-content>s.</jats:p>