We located our study within the Sangre de Cristo Mountains, New Mexico and Colorado, USA (Figure 1). Forested area within this range is approximately 150 km by 420 km, a measured area of 4,620,000 ha bounded by 34.85° to 38.45° N latitude, -104.43° to - 106.00° W longitude. This includes the areas east of Poncha Pass and south of the Arkansas River, CO, incorporating numerous sub-ranges south to the Santa Fe, NM, area, generally to the east of the Rio Grande River. Elevations range from approximately 2000 m in the fringing pinyon-juniper woodland to 4374 m at Blanca Peak. With increasing elevation, forest types range from pinyon-juniper woodland and ponderosa pine, through mixed- conifer and spruce-fir, with occasional stands of lodgepole pine (Pinus contorta Douglas ex Loudon) and more frequent Rocky Mountain bristlecone pine (Pinus aristata Engelm.), and high peaks above treeline (Allen and Peet 1990). Forest types are determined by the
dominant species present at plot sampling (USDA Forest Service 2018 (2)). This area is the southeastern-most part of the Southern Rockies ecoregion (Bailey et al. 1994). It has undergone fire exclusion, leading to changing fire regimes and in some areas other recent
disturbance including wind-driven blowdown and insect-driven mortality events (Veblen et al. 2000; Litschert et al. 2012). Tree ring demography work within the study area
demonstrates the effects of fire exclusion on the mixed-conifer forest, with substantial recruitment following the last widespread fire (Supplementary Figure S-1). In the near future, this area is expected experience hotter, drier conditions from climate change (McDowell et al. 2015). Summaries of climate, elevation, and fire history variables by forest type are available in Table 1.
Constraining the analysis to this portion of the southern Rocky Mountains allows for a more precise interpretation of the utility of the index we developed. Limiting the forest types and ecosystems under investigation reduces the complexity of interpreting variation across space within forest types. Focusing on mixed-conifer dynamics further allows for a close look at climate-disturbance interactions in a forest type subject to substantial
anthropogenic effects from fire exclusion (Falk et al. 2011; O’Connor et al. 2017). Results of this study are broadly applicable across western US interior forests with similar forest types.
Forest Inventory and Analysis
We used data from the Forest Inventory and Analysis National Program (FIA), a US federal program that surveys all forested land in the United States using standardized
protocols (USDA Forest Service 2018 (2)). We used FIA plot, condition, tree, and seedling/regeneration data from Colorado and New Mexico encompassing eleven forest types (Burrill et al. 2017; USDA Forest Service 2018 (1)). Using QGIS (QGIS Development Team 2016), we selected plots within the study area encompassing the New Mexico-
Team 2018), we extracted all relevant data for this set of plots. We used data from the first sampling of plots starting in 2003 (CO) or 2005 (NM), through 2017 for analysis. We excluded plot data sampled prior to implementation of total seedling counts, as well as plot re-measurements. A limited number of re-measured plots were available after the change to seedling sampling protocols; we used the first sampling instance from these plots and excluded the later sampling.
According to FIA protocols, trees of ≥ 12.7 cm (5 in) diameter (at breast height or root crown, DBH or DRC) are recorded at four 7.3 m (24 ft) diameter subplots near each plot point sampled. We compared trees with regeneration, the set of small young stems as defined by individuals comprising the FIA seedling data set. Seedlings per the FIA are conifer stems at least 15.2 cm (6 in) in length and less than < 2.5 cm (1 in) diameter and hardwood stems at least 30.5 (12 in) in length and less than < 2.5 cm (1 in) in diameter, were recorded within a 2.1 m (6.8 ft) diameter microplot in each of the four subplots (Burrill et al. 2017). Woodland species (including Gambel oak (Quercus gambelii Nutt.) were counted as one individual seeding where stems emerged from one point.
We refer to products derived from the FIA seedling data as ‘regeneration’ or ‘regeneration group’, as seedling tends to refer to very young individuals only, and in Rocky Mountain conifers a stem may take decades to reach 40 cm in height (Mast et al. 1998; Nigh and Everett 2007). Following a release event clearing space these stems may emerge from the understory and so may be considered a potential future of the stand.
A total of 30,359 individual trees and 20,815 seedling stems were surveyed across all plots included in the study (Table 2). The smaller area for seedling sampling combined with trees tending towards a mass-reproduction strategy means that recorded seedlings tend to
consist of fewer species in greater numbers than recorded trees, with high variance across the study space (Koenig and Knops 2000). Normalizing the CMTI by individuals recorded for each demographic group reduces the influence of variable reproduction by focusing on the community-level functional tolerance of recent recruitment.
The unique unit of analysis is the FIA plot-condition: an area with consistent forest type and other condition data within which sub-plot sampling of species composition occurs. Forest type is an assigned value based on species presence and dominance at a plot, and can vary across sub-plots at a sampling site. Location and elevation is tied to the plot level, and exact locations are not available due to site protection and privacy concerns. On federal lands, plot locations are adjusted by a random factor up to 1.6 km (1 mi) from their actual location; up to 20% of private land locations may be “swapped” within the county between ecologically-similar plots (Burrill et al. 2017). Within the study area, approximately 50% of plots were federal, and 50% private or Native American tribal land, and potentially subject to swapping. As a result explicitly mapped findings are accurate only to ±1.6 km, and up to 10% of all sites are accurate to within-county distances. Forest type and other data values used for analysis are retained for swapped sites. No significant difference in index value by forest type was found based on land ownership (public vs. private/tribal lands) despite FIA swapping of plot locations, and so all plots were included in the analysis.
Community Mean Tolerance Index
We developed the novel Community Mean Tolerance Index (CMTI), which is applied across demographic groups to find mean site values depending on the community composition of species functional tolerances. Using trait-based species-level relative
tolerance for drought and shading, the CMTI can track ecosystem trends driving regeneration success.
Drought and shade tolerance align with specific climate or disturbance drivers, notably precipitation and temperature for drought tolerance and time since fire for shade tolerance (Niinemets and Vallardes 2006; Clark et al. 2016; O’Connor et al. 2017). The CMTI reflects the recent site-specific growing conditions, and the relative ability for species reproduction, seedling survival and recruitment into the regeneration group under those conditions. Species composition shifts are quantifiable and become apparent when analyzed as a vector of summed tolerances sampled across time, or in the absence of repeated
measures, across demographic groups at a given site.
The CMTI is a weighted average of the functional tolerance of all individuals of all species within a plot or site. It is calculated as a sum of species tolerance values times the number of each species, normalized by the total number of individuals, with an individual index value derived for each demographic group (mature trees and regeneration groups), and tolerance type (drought or shade). The relative proportion of species was used rather than basal area or other metrics of dominance as the primary concern was shifting patterns of regeneration and for seedlings, tally data are more available and meaningful than basal area.
The CMTI is described by the following equation, as derived from species-specific drought (TolD) or shade (TolS) tolerance values, the sum across species of the relative proportional influence of the number of individuals of each species by demographic group at a unique plot (n Species site-group), times the individual species-specific tolerance value, and divided by the total number of individuals by demographic group at a unique plot (n Totalsite- group). For drought tolerance the equation is as follows: