Impacts of changing climate and disturbance regimes on forest ecosystem resilience in the Southern Rocky Mountains
Climate-driven forest disturbances, particularly drought-induced tree mortality and large high-severity fires from increasingly warm and dry conditions, are altering forest ecosystems and the ecosystem services society depends on (e.g., water supplies) in the Southern Rockies and across the Western U.S. We will combine unique, long-term place-based ecological data, diverse methods (e.g., paleo, remote-sensing, and modeling), new methods on forest demography, masting, and cellular-level tree growth, and networking approaches to understand the current and future drivers and impacts of drought- and fire-related disturbances on forest ecosystems. Our decades-long collaborations with local managers, Tribes, and diverse research colleagues enable us to co-produce science that informs natural resource management and has many societal impacts.

Statement of Problem
Increasingly extreme drought, heat waves, fires, and forest mortality are transforming forest ecosystems across the western United States. Forests in the U.S. provide drinking water for more than 125 million people, they are the largest terrestrial carbon sink, and they provide a wide range of additional ecosystem services. Fires and drought are converting large areas of forest to non-forest and post-fire flooding and debris flows are affecting communities and riparian ecosystems downstream. These rapid changes are disrupting the key ecosystem services that forests provide to society. Models project that warming will continue to amplify these extreme events, particularly in the southwestern U.S. Understanding how Southern Rockies ecosystems respond to climate is essential for effective natural resource management. The inclusion of these effects and forecasts in management planning and implementation can improve efficiency, reduce societal surprises, and enhance ecosystem resilience.
Why this Research is Important
Ongoing and projected changes in forest ecosystems have direct implications for society and natural resource managers in the western U.S. National parks and national forests contain large areas of forests that support critical wildlife habitat and provide important ecosystem services. The projected and observed emergence of warmer and drier conditions over the past 25 years in the Southwest has been forcing resource managers and policy makers to change how they think about the sustainability and management of regional ecosystems. Therefore, preparing for and adapting to ongoing ecological changes is a societal and institutional challenge, as well as an ecological one, and the need for science-based adaptive management is increasingly recognized at all levels of land management and government.

Objectives
The overall goal of this project is to investigate effects of climate variability and land use on long-term ecosystem dynamics and changing disturbance regimes, with a focus on forests of the southern Rocky Mountains. To accomplish this goal, we will build upon our unique, long-term, place-based data sets from forested landscapes of northern New Mexico that are part of larger, regional to continental networks. Our specific research objectives are:
- To investigate patterns and drivers of long-term changes in fire regimes, including area burned and fire severity, by placing recent climate-driven extreme events and trends in a multi-century perspective using the North American tree-ring fire-scar network (NAFSN).
- To assess vegetation dynamics and resilience in response to climate and disturbance (fire and drought) by leveraging 33 years of repeated measurements of permanent vegetation plots across environmental gradients in Bandelier National Monument.
- To examine the drivers and dynamics of changing tree mortality and forest ecosystem resilience across ecological gradients in response to hotter drought, insect outbreaks, and management.
- To develop ecological forecasts of the regional synchronization of seed production (mast years), informing climate-driven seed limitations on post-fire tree regeneration, seed collection for reforestation, and management of threatened species (e.g., piñon jay).
- To quantify seasonal climate drivers of the phases of cell production that make up tree ring growth by combining 33 years of bi-weekly tree growth data in Bandelier and cellular-level xylogenesis analyses, which will improve models of tree growth, paleo reconstructions, and forecasting of carbon sequestration and forest mortality.


Methods
Climate drivers of changing fire regimes
We will leverage the new North American Fire Scar Network (NAFSN) to reconstruct area burned in prior centuries in the Southwest using new methods in development (synchrony and tessellations). We will then compare historical versus modern annual area burned (1984 – 2023) using Monitoring Trends in Burn Severity (MTBS) data to place modern fires within the historical range of variability and quantify climate drivers. In addition, we will use Neutral Landscape Models to simulate high severity fire patch size distributions and compare this to historical occurrence of low severity fire from NAFSN sites in the Jemez Mountains. Both projects will provide a greater understanding of whether climate driven contemporary fires are uncharacteristically severe or extensive relative to historic (pre-1900) fire regimes.
Climate drivers of vegetation change
We will analyze a subset of 166 existing vegetation transects distributed throughout Bandelier National Monument and the adjoining Santa Fe National Forest, some of which have been read annually for more than 30 years. Centimeter-resolution species-level ground cover and overstory canopy cover will be resurveyed along 50-meter permanent transects using the basal line intercept method, targeting plots that have not be re-surveyed since recent (2011) fires and drought mortality. Multivariate analysis and scaling techniques will be used to assess spatiotemporal changes in plant community composition across these vegetation transects in relation to drought, disturbance, and thin and burn treatments. Generalized linear models will be used to relate these changes to potential environmental drivers, including climate, soils, and topography.
Climate and management influences on forest mortality and ecosystem resilience
We will assess multiple indicators of forest ecosystem resilience through time and untreated versus treated stands (e.g., thinning and prescribed fire). Understory plant community measures, tree-ring growth, and annual mortality rates will be compared across a network of paired – treated versus untreated - 1 to 0.1 ha permanent plots. Trees are monitored annually for survival (per the methods developed in Sequioa NP and utilized by A. Das). Understory vegetation is measured annually in (24) 1 m2 quadrats distributed throughout each stand. Percent cover, as well as species richness and diversity will be calculated and compared for each plot. Increment cores will be taken from trees across paired plots and resilience metrics calculated from tree-ring widths, which are strong predictors of tree mortality risk from drought, competition, and insect susceptibility.
Climate influence on regional synchronization of tree seed production
Mast years are reconstructed using a novel dendrochronological technique - the cone-scar method. A generalized linear model predicting mast years as a function of precipitation and temperature during cone initiation (September) and pollination (following May) will be cross validated for the period of 2004-2023 at an established network of sites in Colorado and New Mexico. Subsequent predictions will be disseminated via a website to land managers, Tribes, and other partners - and forecasts will be iteratively validated using standardized, user-provided observations. The MASTREE+ data network will be used to develop forecast models for other conifer species identified by partner needs.
Climate drivers of cellular-level tree ring growth
We will leverage tree microcore samples previously collected to quantify patterns and drivers of variability of the four phases of cell growth that form tree rings. These data span an elevation gradient from the upper to lower local limits of ponderosa pine, therefore providing an opportunity to understand the influence of heat and moisture stress on tree growth processes. Standard processing and data extraction and analysis methods will be performed on each microcore. We will use general additive models to quantify and compare the timing and duration of cell growth through cambial initiation, enlarging, cell wall thickening, and maturation among sites and compare with climate data to identify environmental controls. These data can be used to understand the sub-annual process and influences on tree growth which can improve paleo-climate and fire reconstructions, as well can provide calibration/validation data for mechanistic tree growth models, such as the Vaganov Shaskin model, that are valuable for projecting future tree growth.


Synthesis of the new North American tree-ring fire-scar network: using past and present fire-climate relationships to improve projections of future wildfire
Contemporary fires are less frequent but more severe in dry conifer forests of the southwestern United States
Trees have similar growth responses to first-entry fires and reburns following long-term fire exclusion
Pre-fire assessment of post-fire debris flow hazards in the Santa Fe Municipal Watershed
Multi-decadal vegetation transformations of a New Mexico ponderosa pine landscape after severe fires and aerial seeding
Climate-driven forest disturbances, particularly drought-induced tree mortality and large high-severity fires from increasingly warm and dry conditions, are altering forest ecosystems and the ecosystem services society depends on (e.g., water supplies) in the Southern Rockies and across the Western U.S. We will combine unique, long-term place-based ecological data, diverse methods (e.g., paleo, remote-sensing, and modeling), new methods on forest demography, masting, and cellular-level tree growth, and networking approaches to understand the current and future drivers and impacts of drought- and fire-related disturbances on forest ecosystems. Our decades-long collaborations with local managers, Tribes, and diverse research colleagues enable us to co-produce science that informs natural resource management and has many societal impacts.

Statement of Problem
Increasingly extreme drought, heat waves, fires, and forest mortality are transforming forest ecosystems across the western United States. Forests in the U.S. provide drinking water for more than 125 million people, they are the largest terrestrial carbon sink, and they provide a wide range of additional ecosystem services. Fires and drought are converting large areas of forest to non-forest and post-fire flooding and debris flows are affecting communities and riparian ecosystems downstream. These rapid changes are disrupting the key ecosystem services that forests provide to society. Models project that warming will continue to amplify these extreme events, particularly in the southwestern U.S. Understanding how Southern Rockies ecosystems respond to climate is essential for effective natural resource management. The inclusion of these effects and forecasts in management planning and implementation can improve efficiency, reduce societal surprises, and enhance ecosystem resilience.
Why this Research is Important
Ongoing and projected changes in forest ecosystems have direct implications for society and natural resource managers in the western U.S. National parks and national forests contain large areas of forests that support critical wildlife habitat and provide important ecosystem services. The projected and observed emergence of warmer and drier conditions over the past 25 years in the Southwest has been forcing resource managers and policy makers to change how they think about the sustainability and management of regional ecosystems. Therefore, preparing for and adapting to ongoing ecological changes is a societal and institutional challenge, as well as an ecological one, and the need for science-based adaptive management is increasingly recognized at all levels of land management and government.

Objectives
The overall goal of this project is to investigate effects of climate variability and land use on long-term ecosystem dynamics and changing disturbance regimes, with a focus on forests of the southern Rocky Mountains. To accomplish this goal, we will build upon our unique, long-term, place-based data sets from forested landscapes of northern New Mexico that are part of larger, regional to continental networks. Our specific research objectives are:
- To investigate patterns and drivers of long-term changes in fire regimes, including area burned and fire severity, by placing recent climate-driven extreme events and trends in a multi-century perspective using the North American tree-ring fire-scar network (NAFSN).
- To assess vegetation dynamics and resilience in response to climate and disturbance (fire and drought) by leveraging 33 years of repeated measurements of permanent vegetation plots across environmental gradients in Bandelier National Monument.
- To examine the drivers and dynamics of changing tree mortality and forest ecosystem resilience across ecological gradients in response to hotter drought, insect outbreaks, and management.
- To develop ecological forecasts of the regional synchronization of seed production (mast years), informing climate-driven seed limitations on post-fire tree regeneration, seed collection for reforestation, and management of threatened species (e.g., piñon jay).
- To quantify seasonal climate drivers of the phases of cell production that make up tree ring growth by combining 33 years of bi-weekly tree growth data in Bandelier and cellular-level xylogenesis analyses, which will improve models of tree growth, paleo reconstructions, and forecasting of carbon sequestration and forest mortality.


Methods
Climate drivers of changing fire regimes
We will leverage the new North American Fire Scar Network (NAFSN) to reconstruct area burned in prior centuries in the Southwest using new methods in development (synchrony and tessellations). We will then compare historical versus modern annual area burned (1984 – 2023) using Monitoring Trends in Burn Severity (MTBS) data to place modern fires within the historical range of variability and quantify climate drivers. In addition, we will use Neutral Landscape Models to simulate high severity fire patch size distributions and compare this to historical occurrence of low severity fire from NAFSN sites in the Jemez Mountains. Both projects will provide a greater understanding of whether climate driven contemporary fires are uncharacteristically severe or extensive relative to historic (pre-1900) fire regimes.
Climate drivers of vegetation change
We will analyze a subset of 166 existing vegetation transects distributed throughout Bandelier National Monument and the adjoining Santa Fe National Forest, some of which have been read annually for more than 30 years. Centimeter-resolution species-level ground cover and overstory canopy cover will be resurveyed along 50-meter permanent transects using the basal line intercept method, targeting plots that have not be re-surveyed since recent (2011) fires and drought mortality. Multivariate analysis and scaling techniques will be used to assess spatiotemporal changes in plant community composition across these vegetation transects in relation to drought, disturbance, and thin and burn treatments. Generalized linear models will be used to relate these changes to potential environmental drivers, including climate, soils, and topography.
Climate and management influences on forest mortality and ecosystem resilience
We will assess multiple indicators of forest ecosystem resilience through time and untreated versus treated stands (e.g., thinning and prescribed fire). Understory plant community measures, tree-ring growth, and annual mortality rates will be compared across a network of paired – treated versus untreated - 1 to 0.1 ha permanent plots. Trees are monitored annually for survival (per the methods developed in Sequioa NP and utilized by A. Das). Understory vegetation is measured annually in (24) 1 m2 quadrats distributed throughout each stand. Percent cover, as well as species richness and diversity will be calculated and compared for each plot. Increment cores will be taken from trees across paired plots and resilience metrics calculated from tree-ring widths, which are strong predictors of tree mortality risk from drought, competition, and insect susceptibility.
Climate influence on regional synchronization of tree seed production
Mast years are reconstructed using a novel dendrochronological technique - the cone-scar method. A generalized linear model predicting mast years as a function of precipitation and temperature during cone initiation (September) and pollination (following May) will be cross validated for the period of 2004-2023 at an established network of sites in Colorado and New Mexico. Subsequent predictions will be disseminated via a website to land managers, Tribes, and other partners - and forecasts will be iteratively validated using standardized, user-provided observations. The MASTREE+ data network will be used to develop forecast models for other conifer species identified by partner needs.
Climate drivers of cellular-level tree ring growth
We will leverage tree microcore samples previously collected to quantify patterns and drivers of variability of the four phases of cell growth that form tree rings. These data span an elevation gradient from the upper to lower local limits of ponderosa pine, therefore providing an opportunity to understand the influence of heat and moisture stress on tree growth processes. Standard processing and data extraction and analysis methods will be performed on each microcore. We will use general additive models to quantify and compare the timing and duration of cell growth through cambial initiation, enlarging, cell wall thickening, and maturation among sites and compare with climate data to identify environmental controls. These data can be used to understand the sub-annual process and influences on tree growth which can improve paleo-climate and fire reconstructions, as well can provide calibration/validation data for mechanistic tree growth models, such as the Vaganov Shaskin model, that are valuable for projecting future tree growth.

