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ŠUMARSKI LIST 11-12/2019 str. 62     <-- 62 -->        PDF

Assigning priority to tracks depends on the goal of the survey. The UFMO method was designed for use in urban forests where recreational, aesthetic, and health functions are most important. Therefore, it is reasonable to protect visitors from damaged trees that may fall or may harm the visitor or her/his property. Accordingly, the higher priority tracks have paths with a higher number of visitors and are close to real estate. In our study, we chose paths that were closer to the inventoried ISM plots, which made the ISM and the UFMO methods spatially comparable. Furthermore, ISM is designed to repeat the inventory on the same plots and to calculate trends from past measurements. If we want to calculate trends with the UFMO method, we must monitor the same tracks in an agreed time span, and the tracks must be chosen randomly, otherwise the results are biased and not representative of the whole population. Where the whole population in the UFMO method does not actually mean the urban forest, but only the trees alongside the surveyed paths.
Furthermore, roads and trails are not generally established randomly within an area but are generally found in areas with suitable terrain. There may well be a correlation between terrain and site factors that can influence tree response to insects and pathogens. More importantly, roads and trails receive much higher levels of public use than elsewhere within an urban forest, and consequently trees in the vicinity of roads and trails are more likely to be damaged or stressed as a result of the more frequent use, which could produce biased results. However, these drawbacks must be weighed against the increase in coverage made available by transects (Hiby and Krishna, 2001) and other advantages that the UFMO method offers. We believe that our results would be replicable in transects off the path, e.g. more management options would also be found on an off-path transect. But still when we perform a health survey along forest trails, the results are more useful for urban forest management organisations which are generally more concerned about human safety than monitoring forest health.
Because the UFMO method avoids patches of difficult terrain (e.g. dense vegetation), the transects can be completed faster and with less man-power, thereby reducing the costs of monitoring (Walters, 2010). Although the UFMO method exceeded the ISM method in almost all chosen performance measures, the UFMO method still has several weaknesses. Typically, walking paths, roads, or game trails are chosen to represent the non-linear transect. This leads to bias in the habitat surveyed. Therefore, it is important to consider the focus of the monitoring before implementing transects as the survey method and to plan transects accordingly.
Both the ISM and UFMO methods record the causal agents of tree damage. However, the UFMO method looks one step ahead and only records the damage that requires management measures. This makes the UFMO method much more practical and directly linked with management planning. The UFMO method could be further improved if only the trees that require management measures were recorded. In addition, causal agents could be left out, i.e. only the GPS location and management measure would be noted. If we want to gain deeper insight into urban forest health, an inventory of all damaging agents is not only desirable but required. Taking photos of damaged trees and damaging agents is desired but not required.
There are other methods available to detect and monitor urban tree damage, e.g. remote sensing using visible spectre (Verlič, 2015; Verlič et al., 2015), remote sensing using a novel hyperspectral camera from a UAV or aircraft (Näsi et al., 2018), remote sensing using airborne laser scanner data (Rahman and Rashed, 2015), or citizen science (Roman et al., 2017). Citizen science was successfully used in the LIFE ARTEMIS project, where the UFMO method was used to survey alien plants in the Tivoli, Rožnik and Šiška Hill Landscape Park in MOL by involving volunteers (Marinšek et al., 2018). The numerus low-cost remote sensing technologies based on individual tree analysis and calibrated remote sensing imagery offer great potential for affordable and timely assessment of the health condition of vulnerable urban forests (Rahman and Rashed, 2015; Näsi et al., 2018). However, such novel methods using up-to-date technologies usually do not suggest specific management measures for a specific tree nor do they identify the cause of the damage, which is very often the deciding factor in determining which management measures are needed. Here, the UFMO and the ISM methods are superior to these novel methods.
In conclusion, the comparison results, in general, favoured the UFMO method and non-linear transects over ISM and a systematic grid for monitoring urban forest health. Although the UFMO method demonstrated several advantages over ISM in our study, there is still much room for improvement of the UFMO method. Finally, the decision about which method to use depends primarily on the purpose of the surveying, which dictates the core design of the surveying method, and secondly on the economic performance. Furthermore, the ISM monitoring method can be supplemented with the UFMO surveying method in order to capitalize on the potential synergies of combining both approaches.
The study was performed in the framework of the EMoNFUr Project – Establishing a Monitoring Network to Assess Lowland Forest and Urban Plantations in Lombardy and