Relating modelled habitat suitability for Abies balsamea to on-the-ground species structural characteristics in naturally growing forests

Modelling tree habitat suitability (HS) is a common practice to assess tree species distribution across a broad range of spatial scales. However, it is seldom used to test the extent that modelled HS-scores (probability of species occurrence) can represent on-the-ground measures of species-structural characteristics. In this study, we compare four parametric and non-parametric models generated with the R-package, sdm, to assess the potential for these models to estimate tree species distribution of balsam fir [Abies balsamea (L.) Mill.] in naturallygrowing forests across an extensive landscape. Central to this development are inventory plot data of species presence-absence and four abiotic factors linked to plant growth and distribution. The study’s abiotic factors include: (1) photosynthetically active radiation; (2) growing degree-days; (3) relative extractable soil water content; and (4) near-surface wind speed, all expressed spatially at 30-m resolution. To gauge whether modelled HS can explain structural characteristics in balsam fir-dominated stands, field-based estimates of site index (SI) and cumulative aboveground biomass (ABG) were compared against independently-derived HS-scores. The results showed that: (i) random forest was the most successful at representing species distribution of balsam fir among the four methods considered; (ii) overall growing conditions for balsam fir was observed to be most favourable on north-facing slopes, particularly in the northwest part of the target landscape, where near-surface air temperatures are cooler, soils are moderately wetter, and wind speeds are lower; (iii) tree-based calculations of SI were partially characterised by patterns in modelled HS-scores, due to scale differences (i.e., from individual tree to 30-m grid cells) and an inadequate number of sample trees; and (iv) patterns of cumulative AGB were more accurately represented by species HS. Modelled HS-scores, as potential indicators of tree species habitat preference, AGB, and species distribution, can offer key ecological information essential to inform forest management and conservation planning at the landscape level.