Guide to reading the interpretive narrations

We highly encourage reading the interpretive narrations to enable a better understanding of the suite of information sources we have compiled about each species. We do include terms in the interpretations that we need to define, however. All of these points are further discussed in the About the Atlas.

The DISTRIB model predicts suitable habitat as depicted by importance values (IV, based on basal area and number of tree stems) for tree species under current (~2000) and future (~2100) climates. It does not predict where the species will be at 2100, but rather where suitable habitat for that species may be by then. The model outputs produce ratios of future to current IVs, which are the basis for the change classes (see below).
SHIFT model
The SHIFT model calculates the likelihood of colonization, for each 1x1 km cell, for suitable habitats designated by DISTRIB-II, over an approximately 100-year time period consisting of multiple generations, depending on the tree species. Merging SHIFT outcomes with DISTRIB-II outcomes provides the power to evaluate the potential for the species to migrate naturally into the new habitat projected by the DISTRIB-II model. For many species, habitat may expand greatly (especially under high emissions), but there is virtually no chance of much of that area to get colonized naturally. SHIFT also projects whether the species may be suitable to ‘infill’ which is noted in the narratives (see below).
Model reliability
Not all species models are equal - we therefore need to know about the "model reliability" of each species modeled by DISTRIB-II. Model reliability (MR) is generally higher for common species than for rare species. We calculate a MR score based on combining five model output variables into a single rating, and then reclass this score into three classes - high, medium and low – terms used in the narratives. More confidence should be placed on models with medium and high MR than the low MR species.
Change class
Class of potential change in habitat suitability by 2100 according to the ratios of future (2070-2099) suitable habitat for an average of 3 climate models to the actual (2001-2016) suitable habitat, using either lower emissions (RCP 4.5) or higher emissions (RCP 8.5). Classes follow ratios as as follows for common species (species occupied >10% of cells (%Cell > 10): No Change=ratio 0.8-1.2; Small increase=ratio 1.2-2.0; Large increase=ratio >2.0; Small decrease=ratio 0.5-0.8; Large decrease=ratio <0.5. When <10% of the cells within the region are occupied by the species, it was categorized as ‘rare’ and the change classes were expanded, so that No change=ratio 0.6-4.0; Small increase=ratio 4.0-8.0; Large increase=ratio >8.0; Small decrease =ratio 0.2-0.6; Large decrease =ratio <0.2. Because many species occupy <10% of the cells, often the narrative reports an increase or decrease in suitable habitat, but then states something like “though still classed as 'No change' because it has <10% area”.
The DISTRIB-II and the SHIFT models cannot take into account the multitude of biological and disturbance factors affecting species distribution (insect outbreaks, fire, etc). We therefore use a scoring system based on the available literature to account for these ‘modification factors’. Our scoring system gauges the effect of 9 biological and 12 disturbance components in modifying the interpretations of the species response outcomes; combining these forms the basis of an Adaptability score. Higher relative biological and disturbance scores yield higher adaptability. These scores (ranging from 1.7 for balsam fir to 8.5 for red maple) were classified into low (<3.4 ), medium (3.4-5.1), and high (>5.1) adaptability, used in the narratives.
Narrowly/widely distributed
The narrow vs. wide designation was based on the percent of the eastern US occupied by the species (Wide =>10% occupied, Narrow <10%). The majority of species (76% of species) do occupy <10% of the eastern US.
The sparse vs. dense categorization was based on the density of forest inventory (FIA) plots recording presence for the species. Dense species had >= 40% of FIA plots among grid cells with importance value > 0 for the species reported presence; sparse species had <40% of plots recording presence. In other words, ratio the number of FIA plots with species present over all FIA plots for those cells recording the species – that percentage dictates this categorization. Dense species are therefore frequently found within their range and sparse species not as much.
High/low IV
The high vs. low importance value (IV) was based on the mean IV where present (High=>6, Low <6). The breakpoint of 6 was chosen because it is the approximate median value over all species.
This abundance categorization is based on the weighted sum of importance value across the entire eastern US, so that it takes into account both the area of occupation of the species and the importance of the species where it occurs. For these species, this FIAsum score ranges from 0.23 for rock elm to 664.8 for loblolly pine. We rank species with FIAsum scores > 75 as ‘abundant’, scores < 5 as ‘rare’, and in between 5 and 75 as ‘common’.
Suitable habitat
Suitable habitat is that territory deemed from the DISTRIB-II model to have some level of habitat that is suitable for the species to survive.
Usually mentioned last in the narratives, this rating provides an overall estimate of capability for the species to cope with the changing climate within the eastern US. Capability class is based on the Change Classes at RCP 4.5 or 8.5 and the Adaptability of the species to the added disturbances likely under climate change. Classes range from Very Good to Good to Fair to Poor to Very Poor for those species with adequate models. First, the Change Class is considered, followed by Adaptabiltiy to arrive at an initial capability. Then Abundance is used to modify classes so that if the species is Abundant, we increase capability by one class (e.g., poor to fair, or good to very good); if species is Rare, we decrease capability by one class (e.g., poor to very poor); if species is Common, there is no change in capability. The idea is that more abundant species have a greater likelihood to find refugial sites into the future, whereas rare species are less likely to do so.
The species is projected, according to SHIFT, to be a good species to plant to ‘fill in’ suitable habitats, most often in the general proximity of the current range of the species.
The species is projected, according to SHIFT, to colonize new areas outside its current range; most often to the north.
Colonization potential
Based on SHIFT outputs, the potential for the species to colonize new suitable habitats as designated by DISTRIB-II. This can occur either as ‘infill’ or ‘migrate’.
RCP 4.5/RCP 8.5
We used a range of models and scenarios to capture projections of current (1981-2010) and future (2070-2099) temperature and precipitation. These were selected from three General Circulation Models (GCM), and their averages. From these, we used Representative Concentration Pathways (RCP) RCP 4.5 to represent relatively lower future emissions and RCP 8.5 to represent higher emissions. Increases of end-of-century mean annual temperatures from these models over the eastern US average from 3.4°C (6.1°F) for RCP 4.5 to 5.6°C (10.1°F) for RCP 8.5. This is a lot, and further information can be found in our storymap “Climate Pressures in the 21st Century”.