“We have a different climate coming in the future and we have to think differently” — Jim Worrall

Nearly every mature spruce tree has been killed by spruce beetle in this area of the Rio Grande National Forest in southwest Colorado. (Credit: U.S. Forest Service; photo: Brian Howell)

Click here to read the report. Here’s an excerpt:

Abstract. This analysis quantifies the topoclimate niche of 14 tree species in southwestern Colorado and predicts the 2060 niche distribution for each species. It draws on comprehensive, high-resolution vegetation datasets, a precise climate downscaling model, GCMs and RCPs used by IPCC, a foremost decision-tree learning algorithm, and advanced analytical techniques. The models accurately predict recent species distributions at high resolution based on reference climate, slope, and aspect. The results are presented as spatially explicit change zones to enhance utility in management. Results can be used to: (a) determine site-specifically the most appropriate management actions for climate adaptation of vegetation, (b) focus efforts where they have the greatest likelihood of long-term success, and (c) identify potential climate refugia.

Introduction
In the southern Rocky Mountains, it is increasingly evident that weather, insects, diseases, stand conditions, and fire will interact to transform forests as the climate changes. We have already seen widespread changes. Fires have been larger and more severe (van Mantgem et al. 2013, Westerling et al. 2006). Piñon ips responded to the turn-of-the-century drought by killing piñon on over 2.9 million acres in the 4-corner states (Breshears et al. 2005). In Colorado, sudden aspen decline impacted 1.2 million acres (17% of the aspen cover type) (Worrall et al. 2015), mountain pine beetle killed trees on 3.4 million acres, and spruce beetle has impacted 1.6 million acres to date (Howell et al. 2016). These agents kill stressed trees, often building their populations to kill trees in the absence of stress.

These large-scale disturbances provide a strong reminder of the powerful influence of climate on vegetation. The Forest Service and other agencies increasingly mandate extensive consideration of climate change in project, landscape, and forest planning. While vulnerability assessments and other elements provide a good overview of potential climate change impacts and general adaptation measures, they do not provide the quantitative, spatially explicit projections needed to adapt vegetation management to climate change. Impacts to tree species will vary greatly across the landscape – from habitat lost to new habitat emerging. Our management today should be quite different among these locations.

Bioclimate models offer an approach to develop spatially explicit projections of climate change impacts. By analyzing the relationship between known presence/absence of a species and reference climate (which led to the current distribution) at each point, they can predict the likelihood that a given climate will be suitable. Predicted distributions based on grids of reference climate match very well with known distributions. Grids of projected future climate then result in spatially explicit projections of future suitability. Bioclimate models have been extensively used and tested in research (Fettig et al. 2013, Gray & Hamann 2012, Hamann & Wang 2006, Iverson et al. 2008, Rehfeldt et al. 2006, Rehfeldt et al. 2014a, Sáenz-Romero et al. 2012, Worrall et al. 2013). Their application in management has been limited due to the coarse scale of mapping (~ 1 km resolution), lack of topographical response, and the complexity of results. Recent work has addressed these issues: methods for mapping at a 90-meter pixel scale suitable for landscape analysis, incorporation of topographic variables to increase fine-scale accuracy, and a method for projecting change zones that are directly applicable to management (Rehfeldt et al. 2015).

Here we report the methods, results, and some management implications of bioclimate modeling and change projections for 14 tree species in southwestern Colorado. The objectives of this phase of the project were to: (a) develop bioclimate models for dominant tree species of southwestern Colorado based on local data, incorporating topographic variables, and with results presented at a scale useful to management, and; (b) interpret the models by projecting change zones for the species (Lost, Threatened, Persistent, and Emergent) to make them useful for management.

From The Cortez Journal (Jim Mimiaga):

In the past 20 years, Southwest Colorado forests have been in the line of fire of insect epidemic and disease.

The pattern is a clue of a drying climate that could produce a much different landscape 60 years from now in the Dolores watershed, said Jim Worrall, a forest pathologist with the U.S. Forest Service.

He presented the results of a new Forest Service study during a recent meeting of the Dolores Watershed and Resilient Forest Collaborative in Cortez.

First, it was the ongoing spruce beetle epidemic in the eastern San Juan Mountains, Worrall said, then a sudden aspen decline, which focused in the La Plata Mountains.

About the same time, the round-headed pine beetle moved farther north then before, ravaging forests in northwestern Montezuma County. Add to that an emerging budworm infestation in the Dolores Valley.

Many of these forest problems can be attributed to a drying trend from the mid-1980s that culminated in a turn-of-the-century drought with record temperatures in 2001-2003, Worrall said.

“It was a climate change-type drought, and it occurred across the interior West,” he said. “Put in context, you have to go back 800 years in the tree ring data to find a drought that severe.”

Climate studies forecast that the severe dry conditions of 2002 could be the norm by mid-century, Worrall said. Other research shows they could be the norm by the 2030s.

Worrall and his team developed bioclimate models with data from 14 tree species in Southwest Colorado to tease out what the landscape might look like for the future of the Dolores River watershed.

Computer modeling of the watershed used 850,000 tree-location data points and accounted for tree type-location or absence, topography, historic climate and climate variables. It then used algorithms to predict the likelihood of the species being present there in the future.

The modeling shows significant future changes. It predicts that by 2060, a drying climate will have eliminated ponderosa pines from current locations. They will move to higher elevations, possibly replacing spruce-fir stands. Oak brush is also shown moving into higher elevations.

A lot of the stands of the Utah juniper variety would be lost to a drier climate, models show, but aspen stands will likely persist because of to their resistance to drought…

That could guide future management decisions, Worrall said. For example, the round-headed pine beetle is impacting ponderosa forests in the Lake Canyon area of western Montezuma County, the farthest the beetle has ever been seen at that concentration north of New Mexico.

Local foresters are thinning the forest there to try and stop or slow down the beetle’s damaging progress.

“Long-term ponderosa may not be the future, so where there are good piñon stands, that component should be preserved instead of hydro-axing it,” Worrall said.

Protecting seed sources is an important aspect of planning for the future, agreed Bruce Short, a silviculturist from Mancos.

“We need to be proactive in thinking about how we manage what we have in the next 50 years,” he said. “Should we start taking seeds from one area to another based on future suitability?”

The forest crystal ball also reveals potential erosion problems in the future, added Mike Preston, general manager for the Dolores Water Conservancy District, which manages McPhee Reservoir.

“If the ponderosa pine zone comes apart, and there are no re-emergent species to replace it, it could impact water quality,” he said.

Pinus ponderosa subsp. ponderosa. Photo credit Wikimedia.

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