Assessing the performance of tree trenches and tree boxes

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The health of an urban, suburban, rural, or natural forest is rarely limited to individual species alone. An assessment of forest health should be related to both the individual tree and the collection of trees, including interactions between trees.

Many metrics and methods have been developed for assessment of individual tree health. The concept of “resilience” at the individual and canopy levels is the core of the assessment tools. The majority of these evaluative methods and metrics focus on the response of the individual or evaluative unit to a disturbance regime to quantify the “resilience.” The type and capacity of response to the given disturbance and the time it takes to return to the initial qualitative equilibrium state indicate the overall resilience to the disturbance or pressure. Eichorn and Roskams (2013) cite various sources indicating that this return to “equilibrium” is not always return to the initial state, stating that, “open systems will reorganize at critical points of instability.” Determining the critical thresholds for certain pressures, disturbances, and changes the system or individual can tolerate before it cannot recover can provide a proxy for tree and forest health.

The resilience of the tree individuals and canopy is often difficult to quantify directly for multiple pressures. Rather, indirect measures are often employed for inventory and monitoring of tree health. Measurements and metrics can also be taken both directly (e.g. assessing growth rings from a core) and indirectly (e.g. remote sensing of canopy leaf area). Direct and indirect methodologies are discussed and compared below. It is suggested that the base of monitoring, evaluation, and correlation of forest health be that of overall forest resilience, rather than individual tree health. The foundation of the assessment focuses on the health of the individual as a component of the collection of individuals in the forest canopy. Eichhorn and Roskams (2013) suggest using two levels of monitoring and implementation:

• Level 1 – a large-scale systemic network of the trees within the defined forest area or region; and,
• Level 2 – an individual- or stand-based approach using intensive monitoring plots

These levels are not distinct in their interactions and the information gained at each level can inform the interactions and information at the other level. Interactions at each of these levels may also be correlated with and inform forest health and interactions at the national or global scale. We suggest future strategies and policy efforts to standardize, create, and implement a larger national, and possibly global, forest assessment tool for monitoring, assessing, and evaluating the health of our forest. Per the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests, implementation within the MPCA tree monitoring focuses on the following objectives of Minnesota tree condition monitoring, as a subset of the national and global forest system (Eichorn and Roskams (2013): to contribute to a Minnesota-wide early warning system and to a better understanding of tree vitality, including relationships to stress factors and ecosystem disturbances;

• to provide a periodic information on the spatial and temporal variation of tree condition in relation to stress factors;
• to currently document and evaluate the major environmental challenges in Minnesota such as the impact of climate change on forest ecosystem stability;
• to gain information about the impact of biotic and abiotic stressors on crown and tree condition;
• to provide baseline data on the distribution, occurrence, and harmfulness of biotic agents or co-occurring factors in total or parts of Minnesota;
• to validate models regarding stress or risk for trees;
• to contribute to decision support for forest policy and forest practice with regard to ecological sustainability of forest management.

The methodologies presented hereafter focus on these objects in order to establish a framework for a comprehensive tree monitoring system that can be added to as new methodologies and assessment tools emerge.

Methodologies for assessing tree health

Assessment of tree and forest health can be measured directly or indirectly at either the Level 1 (overall forest) or Level 2 (individual or stand) scales. Indicators of tree condition found in monitoring efforts may be assessed via qualitative and quantitative methods for assessing morphology and architecture (canopy, trunk, fruit, roots, etc.), forest composition, biotic/abiotic agents, growth rate, and age. Eichhorn and Roskams (2013) give an overview of indicators of tree health that may be focused on within a monitoring program at both Level 1 and Level 2, and the targeted areas for assessment and evaluation. A summary of variables they suggest as indicators of tree condition is presented below.

• Primary production
  • Defoliation - estimate the leaf/needle loss in the assessable tree crown relative to a reference standard; estimates range from 0 to 100 in 5 percent classes