Nothofagus alessandrii forests in Chile 231


TABLE 1 Land-use classes in the study area of the Maule region in central Chile estimated using remote-sensing data (Fig. 1).


ID Land-use class


1 Open shrublands & grasslands


2 Pine plantations 3 Shrublands 4 Native forests Description


Areas without trees, with scarce vegetation cover & seasonal grassland &/or shrubland species, especially Vachellia caven


Young & adult industrial exotic Pinus radiata plantations (with closed canopies)


Areas with low woody canopy cover dominated by shrubs & native or exotic invasive tree species


Areas associated with the coastal Maulino forest ecosystem dominated by Nothofagus glauca, with few frag- ments dominated by Nothofagus alessandrii


5 Urban 6 Water


7 Farmlands


8 Clear cut or young pine plantations


Human settlements (small towns & villages)


Major water bodies Crops & rain-fed vineyards


Industrial Pinus radiata plantations recently harvested or planted within the last 5 years&with an open canopy


Pinus radiata invasions inside N. alessandrii forest fragments before the mega-fire


To develop a pre-fire land-cover map, we selected pre-fire images from Sentinel-1 and Sentinel-2 from 19 January 2017, 1 week before the Las Máquinas mega-fire. Within this map we assessed P. radiata invasion inside the N. ales- sandrii forest fragments by counting the number of pixels classified as pine (corresponding mainly to dominant pine trees) inside the native forest. Thenwe depicted the coverage of the invasion in ha. We reclassified shrubland predictions into native forest classes as we know from field experience that these forest stands do not have dominant shrubland species.


Forest composition and structure, and post-fire responses of burnt N. alessandrii forests


variables described in the previous section for the analysis, including optical (Sentinel-2 bands and indices), structural (Sentinel-1) and topographical (digital terrain model and topographical position index) information. We collected 90 training and 30 validation polygons per


class (first seven classes of Table 1) using visual interpre- tation of high-resolution Google Earth images and a 2016 land-cover map (Zhao et al., 2016). We then used Google Earth to train and validate a random forest classifier (Breiman, 2001) using the collected samples. We tuned and assessed model performance using the overall accuracy metric obtained from the confusion matrix. Classifying clear cuts or young pine plantations (identi-


fied as class 8 in Table 1) was spectrally complex as these classes are often confused with crops, shrublands and natur- ally non-vegetated areas. To overcome this, we used the Google Earth integration of the LandTrendr algorithm (Kennedy et al., 2018) to segment and assess time since last disturbance using normalized difference vegetation index time series analysis. We analysed annual data for 1990–2016. We assumed that disturbances during 2011– 2016 occurring inside the shrubland and grassland classes corresponded to the ‘clear cuts or young pine plantations’ class and reclassified them accordingly.Wedid not consider fire occurrences during 2011–2016, so there could be small assignment errors to class 8. Finally, we applied a majority filter to improve the classification and to obtain per-class coverages (in ha and per cent, respectively).


We assessed the survival and post-fire responses of a second-growth N. alessandrii forest affected by the Las Máquinas mega-fire. This remnant, corresponding to one of the nine N. alessandrii forest populations, is located in El Desprecio (The Neglected) property owned and pro- tected by a private company dedicated to forest plantations. The mega-fire occurred in summer 2017 and burnt almost 100% of this N. alessandrii population. In April 2017 (i.e. 2 months after the fire), we set nine


250-m2 circular plots (8.9-m radius) in stands with low, moderate and high dNBR-based severity levels. In these plots we recorded all tree species (native and exotic) and measured the diameter at breast height of all individuals with a diameter at breast height $5 cm. We classified the survival status of the trees as either dead (stems and canopy burnt without green foliage) or alive (green foliage in the stems and/or crowns, and/or basal vegetative sproutings). In each plot we set 12 circular subplots of 3.14 m2 (1-m ra- dius) to count the number of tree seedlings (0.05–2.00 m in height) of the various species and to assess their regener- ation origin (i.e. seeds or basal sprouts of trees).Weassessed the survival status of trees and seedling recruitment again 2 years after the fire, in April 2019.


Results


Fire severity, and land use and land cover surrounding N. alessandrii forests


Of the 172 ha of N. alessandrii stands burnt by the 2017 mega-fire (corresponding to 55% of the total area), 15%(25 ha) was burnt by a low-severity fire and 47%(81 ha) and 38% (66 ha) were burnt by moderate- and high-severity fires, re- spectively. The land-use and land-cover map of the first seven classes (Table 1) had an overall accuracy of 87%.


Oryx, 2023, 57(2), 228–238 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605322000102


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