Can a general theory of forest structure and dynamics based on metabolic scaling laws provide insights to forest managers?

Background/Question/Methods

The metabolic theory of ecology (MTE) integrates allometric and biomechanical principles to explain emergent properties of plant communities. Extensions of MTE to forests have shown how tree-level allometric relations scale-up to predict forest-wide attributes such as tree size distributions and mortality rates. The power of the MTE is that these scaling relationships are thought to be general rules that hold for all forests. The essential underpinning of the MTE, namely the strong expression of allometric scaling in trees, has long been a core aspect of forest management. For example, empirically-derived relationships between tree density, growth, and mortality inform the “thinning rules” that foresters apply to achieve site specific management objectives. At the same time, the criticisms regarding the applicability of predictions based on the MTE are also mirrored in the forestry literature. For example, MTE assumes the competition for space is the dominant process governing stand dynamics. No account is made for the size asymmetric competition for light nor is there a place for density independent causes of mortality. Similar debates have occurred in forestry regarding the generality of the “3/2 power” thinning law. However the potential value of general expectations based on the fundamental physiology and architecture of trees should not be dismissed lightly. In era of unprecedented and unpredictable changes in the environment, the validity of guidelines based on past measurements is diminished. Therefore we explore the potential of MTE predictions to inform management using long-term data from two temperate forests. Specifically we ask if can use deviations from the expected MTE behavior to provide an early warning of novel events that may threaten management goals.  

Results/Conclusions

In both examples, we calculated the annual mortality rate as a function of stem diameter for periods that spanned the emergence of forest decline. In one case, the decline was due to the arrival of an invasive pathogen in a California conifer forest; in the other the decline was related to a suite of stressor that impacted a northern hardwood forest in New Hampshire. In our preliminary analysis, we see a marked increase in the mortality rate (i.e., above the MTE prediction) in the mid-sized trees with the deviations more apparent in the northern hardwood stand. These deviations are reflected in the size distribution exponent. We propose an approach that relies on the more easily measured variable, size distribution, as a means for managers to detect the initial stages of forest decline.