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
  • Apical shoot architecture - estimate the shoot development in relation to the standard of an adaptable tree crown from 0 to 100 in 5 percent classes
  • Fructification - estimate fruits in whole tree crown from 0 to 100 in 5 percent classes
  • Fruit biomass - calculate biomass (mass per hectare) of fruits in litterfall traps from a stand
  • Diameter growth - measure diameter growth, in centimeters, of a stand
• Ecosystem disturbances
  • Determine occurrence and diagnose symptoms and signs of biotic and abiotic agents of whole trees
  • Record number of trees from a plot absent as a result of removal or mortality and cause(s) of absence
• Ecosystem internal regulation
  • Measure or estimate tree age, in years, from a sample of trees
  • Measure tree-related stand structure from a sample of trees

The proposed MPCA tree and forest monitoring system and protocol presented hereafter focuses mainly on direct measurements of individual trees at a Level 1 scale, as a proxy for Level 2 interactions, using the Eichhorn-Roskams (2013) methodology described above.

Direct measurement methods typically employ tree architecture and morphology as a measure and indicator of tree health. The measurements are broken down by foliation (defoliation) of the upper crown/canopy, apical shoot architecture, and fructification. The following table breaks down the areas of evaluation and assessment within each category of tree area.

Qualitative and quantitative evaluation and assessment of trees, by area of tree.
Link to this table.

Area of tree	Qualitative and quantitative evaluation and assessment
Foliage	Leaves, related to ability of tree to capture light for metabolic processes (photosynthesis), particularly noting: overall canopy area openings in the canopy and areas of the openings quality of leaves, noting any structure or color change leaf drop, patchiness, or mortality
Apical shoots and overall tree architecture and morphology	Trunk quality and location of any damage, disruptions and disturbance type of response noted to disturbance (e.g. scab, open wound, healing, etc.) bark quality compared to known standards, noting any quality and quantity differences from normal presence of insects, insect-related activity, and infection Branches overall divergence from normal branching pattern (e.g. no limbs on one side of tree) branch mortality or abscission, presence of and location per normal growth patterns no leaf out and bud-related structures set, presence of and location per normal patterns presence of insects, insect-related activity, and infection
Fructification	Fruit production, as an indicator of reproductive success and health, infection or stress-related response quality of fruit, noting any damage, infection or pest indications quantity of fruit. Can be difficult to interpret results, as fruit abundance or deficit can indicate stress (succession-related response of reproductive proliferation and seed band inundation prior to mortality) or success/health (succession-related response of population growth due to abundant resources; excess resources and metabolic byproducts applied seed production).
Roots	Difficult to assess without disturbing tree, with exception of aerial root structures (not found in MN species) Note any presence and location of roots and root structures above soil finished grade

Recommended monitoring protocol

This section provides a recommended monitoring protocol. The goal of monitoring is to identify issues or concerns that require a response. Responses to monitoring findings are presented in the Operation and maintenance of tree trenches and tree boxes section of this Manual. Specific troubleshooting responses are presented in [table]. This series of responses is mainly on a Level 1, or individual basis, with limited Level 2 assessment suggested via remote sensing technologies. The collection, interpretation, and response to the larger forest (Level 2) health, requires collection and collaboration of results between individuals. Large-scale comparative and predictive metrics and an ecosystem-scale assessment system is suggested for future investigation to provide a standardized system for collection and comparison of Level 1 monitoring information.

Frequency of monitoring

Annual tree monitoring is suggested for the crown and apical shoots (all above-ground structures). It is suggested that this could be performed with rapid assessment tools by citizens’ monitoring programs, watersheds, municipalities, or other groups. Standardization of the monitoring information collected, date of collection, procedure for collection, and reporting of monitoring for comparison of results between individuals and over larger areas is suggested.

Positions of Collecting Monitoring Information

Two points should be established and noted on a map and/or (preferred) GPS (Global Positioning System) device for consistent results collection. One point (Point A) should be directly underneath the tree for assessment of leaf area coverage and canopy diameter. The second point (Point B) provides quantitative and qualitative results of tree height and vertical leaf coverage, and is suggested to be at least 50 feet from trunk to provide a fixed location for consistent evaluation as the tree grows. A GIS (Geographic Information system) coverage and GPS-based system would aid in consistent evaluation and database assembly for large-scale evaluation and monitoring assessment and responses. Leaf and other tree debris can also be evaluated at any location underneath and surrounding the tree for gathering additional information regarding tree illness, injury, or stress responses.

Collected Information

Following information gathered should be assembled into a standard format. The information gathered focuses on the above-ground portion of the tree and surface root presence, on an individual/single-tree basis as the standard unit of measurement. Please note: fructification can give ambiguous indications of overall health, and it should not be used as an independent indication of tree health. Fructification can be used in conjunction with the above-ground structural assessment as a component of the Level 1 (individual or stand) monitoring, but will be more effective as a Level 2 (larger-scale forest assessment) to indicate ecosystem-wide response to stressors, such as insect invasion and long-term drought or reduced soil moisture availability.

Monitoring program

Level 1 monitoring

Level 1 Tree Monitoring focuses on the individual or small unit of individual trees for a field-based assessment tool. This program allows for increased ability to be performed by a layperson or expert, but does not utilize less-accessible programs and methods such as GIS-based or model-based systems that would be more useful at a Level 2 analysis. Other methodologies – electrical conductivity within sap, chlorophyll fluorescence, glucose presence - for Level 1 monitoring were encountered in the literature review, but the tools and/or knowledge required to perform the analyses or monitoring or the detailed level of information gather were prohibitive for generalized use in this level of monitoring program. (Martinez-Trinidad, et al. 2010). It is suggested that this monitoring methodology be standardized in a rapid assessment form for data collection and comparison throughout Minnesota to better perform the Level 2 analyses and provide a greater degree of statistical confidence in results due to standardized methodology.