Jonathan D. Coop

CONSTRAINTS ON TREE SEEDLING ESTABLISHMENT IN MONTANE GRASSLANDS OF THE VALLES CALDERA, NEW MEXICO

Montane and subalpine grasslands are prominent, but poorly understood, features of the Rocky Mountains. These communities frequently occur below reversed tree lines on valley floors, where nightly cold air accumulation is spatially coupled with fine soil texture. We used field experiments to assess the roles of minimum temperature, soil texture, grass competition, and ungulate browsing on the growth, photosynthetic performance, and survival of transplanted ponderosa pine (Pinus ponderosa) seedlings at 32 sites straddling such reversed tree lines in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains, New Mexico (USA). Seedling growth increased most strongly with increasing nighttime minimum temperatures away from the valley bottoms; seedlings experiencing the coldest temperatures on the caldera floor exhibited stunted needles and often no measurable height growth. Based on the chlorophyll fluorescence ratios PhiPSII and Fv/Fm, we found that low minimum temperatures, low soil moisture, and fine soil texture all contributed to photoinhibition. Neighboring herbs had only minor negative effects on seedlings. We found no effect of ungulates, but golden-mantled ground squirrels (Spermophilus lateralis) caused substantial seedling mortality. Second-year seedling survival was highest on sandy soils, and third-year survival was highest at sites with higher minimum temperatures. We conclude that differential tree seedling establishment driven by low minimum temperatures in the valley bottoms is the primary factor maintaining montane grasslands of the VCNP, although this process probably operated historically in combination with frequent surface fire to set the position of the tree line ecotone. As at alpine tree lines, reversed tree lines bordering montane and subalpine grasslands can represent temperature-sensitive boundaries of the tree life form.

Influences of prior wildfires on vegetation response to subsequent fire in a reburned Southwestern landscape

Large and severe wildfires have raised concerns about the future of forested landscapes in the southwestern United States, especially under repeated burning. In 2011, under extreme weather and drought conditions, the Las Conchas fire burned over several previous burns as well as forests not recently exposed to fire. Our purpose was to examine the influences of prior wildfires on plant community composition and structure, subsequent burn severity, and vegetation response. To assess these relationships, we used satellite-derived measures of burn severity and a nonmetric multidimensional scaling of pre- and post- Las Conchas field samples. Earlier burns were associated with shifts from forested sites to open savannas and meadows, oak scrub, and ruderal communities. These non-forested vegetation types exhibited both resistance to subsequent fire, measured by reduced burn severity, and resilience to reburning, measured by vegetation recovery relative to forests not exposed to recent prior fire. Previous shifts toward non-forested states were strongly reinforced by reburning. Ongoing losses of forests and their ecological values confirm the need for restoration interventions. However, given future wildfire and climate projections, there may also be opportunities presented by transformations toward fire-resistant and resilient vegetation types within portions of the landscape.

Gradient analysis of reversed treelines and grasslands of the Valles Caldera, New Mexico

Abstract Objective: Treeless meadows and parks are widespread but poorly understood features of the montane vegetation of the western USA. These communities frequently form reversed treelines where grassy valleys occur below forested slopes above. Our purpose was to assess the environmental correlates of such treelines, as well as patterns in the composition and diversity of grasslands and forest margins in the Valles Caldera National Preserve. Location: Valles Caldera National Preserve (35°50′–36°00′ N, 106°24′–106°37′ W, 2175–3150 m), Jemez Mountains, New Mexico, USA. Methods: We conducted a gradient analysis based on 200 nested quadrats on transects crossing reversed treelines and spanning the compositional heterogeneity of grasslands. We used cluster analysis and non-metric multidimensional scaling to assess relationships between compositional variation and environmental variables. Results: We found strong, highly significant relationships of the vegetation to gradients in slope inclination, soil texture, moisture, nutrient availability, and nighttime minimum temperatures. Reversed treelines are most strongly associated with shifts in the thermal regime, exhibit weaker relationships with soil texture and nutrient content, and show no relationship with gravimetric soil moisture. Gradients in aspect, soil moisture, and annual mean temperature are associated with compositional variation within grasslands and forest margins. Conclusions: Lower nightly minimum temperatures and fewer consecutive frost-free days resulting from cold-air drainage may prevent tree seedling establishment in valley bottoms via photo-inhibition, tissue damage, or frost heaving. Finetextured soils may also impede tree seedling establishment in valley bottoms. These findings lay the groundwork for experimental and physiological tests of these potential causes of these reversed treelines. Nomenclature: Hartman & Nelson (2005); Anon. (2006)

Range-wide conservation of Pinus aristata: a genetic collection with ecological context for proactive management today and resources for tomorrow

Tree species are highly vulnerable to anthropogenic environmental change, and are increasingly being challenged by non-native pests and climate change. Rocky Mountain bristlecone pines are long-lived, exhibit delayed maturation, have low genetic diversity, and inhabit cold, high-elevation environments. They are threatened by the non-native disease white pine blister rust, warming temperatures, changing precipitation patterns, and altered disturbance regimes. The goal of this work was to (1) sample the range-wide genetic diversity of bristlecone pine for ex situ seed and tissue collections and research before the populations are impacted, (2) assess the health and ecological conditions of the species across its geographic range (61 stands) to provide a baseline by which to evaluate future changes, and (3) assess relationships between stand and regeneration characteristics and topographic, geographic, and climatic factors to identify vulnerabilities and proactive management interventions to increase population resilience. Local variation in topoclimate was strongly related to stand structure, composition, health, and regeneration. Bristlecone pine currently showed only minor impacts from insects and pathogens, but relationships between cone production and regeneration with climate variables suggest vulnerabilities to projected climate change. Both cone production and seedling regeneration were also linked to stand structure and ground cover that provide opportunities for proactive in situ management to increase population size to mitigate potential future climate- and pathogen-driven declines. These efforts represent the first proactive coordinated range-wide genetic collection design and forest health assessment for a threatened, but not yet impacted, tree species and offers a model for other species at risk.

BLACK BEARS FORAGE ON ARMY CUTWORM MOTH AGGREGATIONS IN THE JEMEZ MOUNTAINS, NEW MEXICO

Abstract We observed black bears (Ursus americanus) foraging on aggregations of army cutworm moths (Euxoa auxiliaris) (Lepidoptera: Noctuidae) in subalpine felsenmeers (block fields) in the Jemez Mountains of New Mexico. Moth aggregations serve as food for grizzly bears (Ursus arctos horribilis) in the northern Rocky Mountains of Wyoming and Montana. However, black bears have not been reported to use these aggregations, nor have such aggregations been documented to occur this far south in the Rocky Mountains.

The future of subalpine forests in the Southern Rocky Mountains: Trajectories for Pinus aristata genetic lineages

Like many other high elevation alpine tree species, Rocky Mountain bristlecone pine (Pinus aristata Engelm.) may be particularly vulnerable to climate change. To evaluate its potential vulnerability to shifts in climate, we defined the suitable climate space for each of four genetic lineages of bristlecone pine and for other subalpine tree species in close proximity to bristlecone pine forests. Measuring changes in the suitable climate space for lineage groups is an important step beyond models that assume species are genetically homogenous. The suitable climate space for bristlecone pine in the year 2090 is projected to decline by 74% and the proportional distribution of suitable climate space for genetic lineages shifts toward those associated with warmer and wetter conditions. The 2090 climate space for bristlecone pine exhibits a bimodal distribution along an elevation gradient, presumably due to the persistence of the climate space in the Southern Rocky Mountains and exclusion at mid-elevations by conditions that favor the climate space of other species. These shifts have implications for changes in fire regimes, vulnerability to pest and pathogens, and altered carbon dynamics across the southern Rockies, which may reduce the likelihood of bristlecone pine trees achieving exceptional longevity in the future. The persistence and expansion of climate space for southern bristlecone pine genetic lineage groups in 2090 suggests that these sources may be the least vulnerable in the future. While these lineages may be more likely to persist and therefore present opportunities for proactive management (e.g., assisted migration) to maintain subalpine forest ecosystem services in a warmer world, our findings also imply heighted conservation concern for vulnerable northern lineages facing range contractions.