Comparison of Tree Condition and Value for City Parks and Stephen F. Austin State University in Nacogdoches, Texas, U.S.

DISCUSSION

Similar tree studies on the SFASU campus have been conducted previously. The initial study in 1971 was based on a portion of the campus and recorded 38 species (Nixon and Mims 1971), 30 species in 1992 (Creech et al. 1994), 37 species in 1994 study (Perkins 1996), and 30 species in 2008. Many of the trees lost or damaged in 1994 were located near new sidewalk construction and utility lines. Also, many of the species reported in 1994 were removed and not replaced. They included the native species Prunus caroliniana (cherry laurel) and the exotic species Ligustrum sp. (privet), Eucalyptus camaldulensis var. camaldulensis (river red gum), E. ovata (swamp gum), E. robusta (swamp mahogany), E. camldulensis var. obtusa (river red gum), and Prunus mexicana (Mexican plum).

For urban forest management, several aspects should be considered, including the desire and needs of the community, the urban forest structure, inventory and monitoring, and dialogue among owners, managers, and users (Dwyer et al. 2000). Trees dead or in poor condition should be considered for removal and replanting. Developing a management plan for these issues requires adopting an expanded set of goals incorporating environmental processes and conditions at a larger scale than the individual tree. One of the most important tree problems of both SFASU campus and city park trees was trunk decay, progressive deterioration of woody tissue in both living and dead trees (Nicholas and Crawford 2003). Wood decay in a living tree in the middle of a park could result in limb or trunk breakage during adverse weather. Trees with a low rating of trunk condition have a high rating of insect and disease.

As trunk decay increases, as noted by missing bark and evident decay at the base or on the trunk, care must be taken in further analysis of decay. Indicators of decay of branches, the trunk, and the root collar need to be carefully assessed for signs and symptoms of decay and pathogens. Indicators include fungal conks, cavities or openings, and carpenter ant (Camponotus spp.) nests (Luley 2012). Kane et al. (2001) and Kane and Ryan (2004) review strength loss and compare methods between estimation of loss as a function of stem hollow percentage cautioning that deciding on hazardous and non-hazardous trees is complicated, and “Hazard tree assessment is an art as much as it is a science . . . It is better to approach hazard tree assessment as risk management” (Kane et al. 2001). Methods of searching for decay include use of the IML-RESISTOGRAPH^® to detect hollow areas of the trunk or branches and calculate strength loss (Johnstone et al. 2007; Johnstone et al. 2010) and the PiCUS^® Sonic Tomograph for quantification of decay (Gilbert and Smiley 2004). Pokorny (2003) presents urban tree risk assessment incorporating a tree risk management plan (Hauer and Johnson 2003; Pokomy and Albers 2003) including risk inspections; and detection and assessment of hazardous defects in trees (Albers et al. 2003) including decay detection devices benefits and limitations. Therefore, it is important to identify decay in the urban forest and the care needs to be taken in the evaluation of the trees selected for removal. All defective trees cannot be detected, corrected, or eliminated. Root problems and internal defects are not easily discernible and inspections need to be made to determine change in the CTLA rating prompting additional management. Albers et al. (2003) stress seven categories of defects for trees: decayed wood; cracks; root problems; weak branch unions; cankers; poor tree architecture; and dead trees, tops, and branches. Risk assessment guidelines for these categories are presented, including moderate and high risk of failure along with commonly found defects by species.

Trees with a lower CTLA rating can be revisited on an inspection schedule. Pokorny and Albers (2003) recommend an annual visit for very high hazard areas and one to two years for high hazard areas. Moderate and low hazard areas range from three to five years for individual tree inspections. Tree inspections are needed as tree structure and vigor change over time and risk inspections provide a continuous source of tree resource data as a foundation for a comprehensive management system (Pokorny and Albers 2003).

In overall health condition, SFASU campus trees were significantly better compared to city park trees for the 12 major species (n ≥ 30) except for Q. stellata and U. americana (Table 1).

As a guide to achieve biodiversity for shade trees, the 10-20-30 rule is often used. The rule states that no more than 10% of the urban forest should be of the same species; no more than 20% of the forest should be of the same genus, and no more than 30% of the community’s tree inventory should be a single family. These guidelines result in a more biologically diverse planting (Santamour 2004). In both the SFASU campus and the city parks, the species and genus composition exceeded these guidelines (i.e., P. taeda on the SFASU campus and C. illinoinensis in the city parks). Tree plantings need to be organized with consideration to growing space, species composition, and long-term use of either the campus or city parks. Planting organizers need to take into consideration increasing species diversity and examine the current urban forest for additional management, including Celtis laevigata (sugarberry) and Q. nigra. An examination of tree condition (Table 1; Table 2) indicates species that need additional maintenance.

Using the 10-20-30 rule as a guide and associating the results with both Shannon–Wiener and Simpson’s indices, the biodiversity of both SFASU campus trees and city parks trees were not adequate. The 10-20-30 rule was developed with the goal or preventing or avoiding catastrophic losses. This rule may not consider cultivars or species within the same genus. Recommendations include diversification at the generic, familial, and even ordinal level (Raupp et al. 2006). Santamour (2004) and Raupp et al. (2006) point out that new introduced pests can virtually eliminate urban forest trees, as with Dutch elm disease on American elm (Ulmus americana) and chestnut blight (Cryphonectria parasitica) on American chestnut (Castanea dentate). Other examples include the introduced emerald ash borer (Agrilus planipennis) on ash (Fraxinus spp.) (Vannatta et al. 2012) and Asian longhorned beetle (Anoplophora glabripennis) on maple (Acer spp.). Recommendations include diversifying the species of trees planted in urban areas (Raupp et al. 2006). Increasing biodiversity increases the urban forest’s capacity to recover from drought, wind damage, and management actions (Fischer et al. 2006). Promotion of biodiversity increases both the social and ecological benefits of the urban forest and planning needs to incorporate both native species and cultivars that are not invasive (Alvey 2006). Care must be taken with nonnative and invasive species as urban forests often serve as an introduction point for these species [e.g., Chinese tallow (Triadica sebifera)] (Webster et al. 2006). Chinese tallow may have a competitive advantage with growth and reproduction over other species (Rogers and Siemann 2005; Schaefer 2009). With the high percentage of P. taeda on the university campus, measures can be taken to increase species diversity in new planting while keeping the overstory pine. The university and the city could introduce new species or increase the plantings of some existing species. Throughout the campus and parks there are areas underneath the overstory canopy with an absence of trees, indicating the need for additional plantings. Advanced planning is necessary since it may take three to eight years to produce a 5 cm caliper tree.

Measuring the benefits of urban trees include both monetary and public or social values (Scott and Betters 2000; Miller 2007). An urban forest can enhance real estate values, reduce heating and cooling expenses, and improve aesthetics (McPherson 1992; McPherson et al. 1994). Although the SFASU campus trees were found in better health condition than those of the city parks, SFASU campus trees average economic value was less than that of city parks due to a smaller average size and different species composition.

Based on the CTLA tree valuation, the SFASU forest is an asset valued at more than $27 million to the university, and would increase to $35 million for all trees in optimum condition. However, there is $7 million reduction in potential value (29%) because of imperfect condition. A distinctive characteristic of urban forests is that when they mature, more maintenance is required. The results indicate that many campus trees currently receive less maintenance than needed. McPherson et al. (1999) indicate investment in the urban forest offers return to the citizens, and continued investment is needed. Planning and managing for the future will require careful planning and the development of a long-term urban forest management plan. A concerted effort is underway to remove hazard trees from the campus and to increase tree maintenance based on the results of this study.

The replacement value method is extremely useful when planning for tree removal, site cleanup, restoration, preparation, and additional maintenance expenditures. The demands to quantify the value of trees become even more important when trees are threatened by disease. Ellison (2005) indicated the need to balance the benefits of risk reduction with the associated costs of lost tree value and financial expenditure. Evaluating and mapping the tree population and identifying the interface between trees and targets prioritize risk assessments. The measurements of the tree condition in the current study can be used in risk assessment. Trees with lower condition values (Table 1; Table 2) indicate trees at greater risk. Risk includes dead or dying branches (Structure) while trunk defects include cracks and sporophores (Trunk). These trees require careful monitoring to ascertain further loss of condition. The value of these trees added to an ArcGIS 10.1 database provides the basis for future comparisons on an annual examination. As both of the study areas investigated are high traffic areas, both the proximity of the trees and target evaluation need to be considered. More frequent inspections of areas of high public access need to be considered with updates to the hazard rating data (Hickman et al. 1995; Ellison 2005). Hazard rating of campus and park trees needs to include the “prudent person” concept (Sharon 1987) for one that looks ahead and carefully provides for the future. Values, such as the cost to maintain declining trees and the removal of trees that have been killed by disease, can be derived based on the calculated dollar values. If the values people are paying for the trees need to be evaluated, another modeling method, predictive modeling, would be better to reflect those values (Martin et al. 1989).

More specialized software used for urban forest evaluation and management have been developed and widely used for professional GIS application with powerful database and GIS mapping techniques. Some advanced software, including Tree-Pro™ and CITYgreen, are designed to archive tree data and allow for tree evaluation in a more convenient way. Longcore (2004) conducted a case study in Los Angeles, California, U.S., in 2004, with CITYgreen software. Based on a geographic representation of the study area, Longcore analyzed the benefits of trees and greenspace and pointed out some limitations, including that the tree growth model seems to work well on young trees, but calculates unrealistically large future tree sizes for mature trees. Currently trees on the SFASU campus and in Nacogdoches city parks are being evaluated using i-Tree Design to determine benefits of the urban forest on rainfall interception, energy savings, benefits to air quality and carbon dioxide reduction. Abd-Elrahman et al. (2010) used i-Tree ECO to develop an urban forest inventory. This software, using the standards set by the ISA, can track, value, and rate trees and generate reports containing assessed value, species, location, hazard rating, DBH, height, and maintenance history. With that, trees would be evaluated and managed more easily and efficiently.