Comparison of two methods for monitoring urban forests health

Abstract

We compared the performance of two methods for monitoring urban forest health. The first was based on a systematic grid (ISM), and the second on non-linear transects (UFMO). Both methods were tested during July and August 2013 in the Rožnik urban forest in the Municipality of Ljubljana (MOL). We assessed crown condition and damaging agents on 15 ISM plots, surveying an area of 92 a (are = 100 m2) in 1,640 minutes. By comparison, the UFMO method was used to survey an area of 518 a in 1,700 minutes. The performance of the ISM and the UFMO methods was 17.8 min/a and 3.28 min/a, respectively. According to the time/area performance measure, the UFMO method performed 5.4 times better than the ISM method. The UFMO method recorded 1.5 times more damaging agents per hour, 2.7 times more trees per hour, and 13.4 times more dead standing trees per hour. It also suggested 7.0 times more management measures per hour. However, the density of the data gathered was 7.1 times higher for the ISM method. According to the chosen comparison measures, the overall performance of the UFMO method exceeded the ISM method in all chosen performance measures expressed in relative time except the amount of data gathered. We conclude that, for the same sampling cost, the ISM approach produces an unbiased, but imprecise, estimate of overall forest health, while the UFMO method produces a biased, but more precise, estimate. We discuss possible improvements and further limitations of the UFMO method with an emphasis on the differences between the two methods of monitoring and surveying forest health. We conclude, that the ISM monitoring method can be supplemented with the UFMO surveying method to capitalize on the potential synergies of combining both approaches.