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Venezuela will soon become the first Andean country to lose all of its glaciers, as the "eternal snows" of the Sierra Nevada de Mérida (SNM) disappear. Although documentation of glacier retreat in Venezuela commenced over a century ago, an updated reconstruction at sufficient spatial resolution is warranted. Our objectives were to (a) produce detailed multitemporal maps of glacier cover in the SNM since 1952 and revise historical estimates for 1910 and (b) document changes in cover and rates of glacier retreat of the last Venezuelan glacier at Humboldt Peak. We integrated multiple information sources, including topographic/geomorphic evidence, high-resolution aerial photographs, satellite images, historical maps, panoramic photos, and field observations. We present 1:5,000 maps of glacier cover between 1952 and 2019 and revise the maps of 1910. In Venezuela, glacial area has decreased 98 percent between 1952 and 2019 (from 2.317 km² to 0.046 km²). Glacier retreat rates increased after 1998, with a maximum of -16.9 percent year-1 between 2016 and 2019. These new maps should provide an essential tool for biophysical, ecological, and historical studies and a key reference for promoting awareness on climate change in the tropical Andes, where glaciers are intimately linked with cultural identity. [ABSTRACT FROM AUTHOR]

In many of the alpine-treeline ecotones (ATE) of the Rocky Mountains, Pinus albicaulis (whitebark pine) is the most common conifer initiating tree islands through facilitation. We examined whether microsites leeward of P. albicaulis experience more moderate microclimate, less sky exposure, and more total soil carbon and nitrogen than other common types of leeward microsites. From July to September 2010, 2011, and 2012, in two study areas on the eastern Rocky Mountain Front, we compared microclimate, sky exposure, and total soil carbon and nitrogen leeward of four common microsites, P. albicaulis, Picea engelmannii (Engelmann spruce), rock, and unprotected (exposed). Microsites leeward of P. albicaulis did not consistently experience the most moderate microclimate, but both P. albicaulis and P. engelmannii leeward microsites had lower daily photosynthetically active radiation (PAR), lower average wind speeds, lower soil temperature maxima, and higher soil temperature minima. In general, conifer microsites had significantly lower values for sky exposure; but, the performance of each microsite type varied from micro to study-area scales. Our results highlight the importance of conifers as nurse objects for facilitating treeline community development in the ATE, and especially P. albicaulis because of its high abundance. High losses of P. albicaulis from infection by Cronartium ribicola may alter community dynamics and treeline response to climate warming. [ABSTRACT FROM AUTHOR]

Facilitative interactions among plant species enable plant community development under stressful environmental conditions. Previous studies in two treeline communities within the alpine-treeline ecotone on the Rocky Mountain Front in northwestern Montana, U.S.A., indicated that whitebark pine ( Pinus albicaulis) serves as the majority tree island initiator, thus facilitating the development of tree islands (Resler and Tomback, 2008). However, 33.7% of whitebark pine at these treeline study sites were infected by the introduced pathogen Cronartium ribicola, which causes white pine blister rust. We examined the prevalence of whitebark pine, its ecological role, and the incidence of blister rust within two study sites, Tibbs Butte and Wyoming Creek, to the south on the Beartooth Plateau, and within two study sites, Stanley Glacier and Gibbon Pass, to the north in Kootenay and Banff National Parks, for comparison with information from the Rocky Mountain Front (Resler and Tomback, 2008). We found that whitebark pine was an important component of treeline communities in both the southern and northern study areas, although its abundance and ecological role varied with study site. Nearly half the solitary trees sampled overall in the northern and southern study areas were whitebark pine. Whitebark pine was also the most frequently occurring conifer species among tree islands at three of the four study sites. Across all study sites, whitebark pine served as tree island initiator for 29.4% of the tree islands sampled, but was a more frequent initiator within two study sites. Blister rust incidence for the Wyoming Creek study site in the southern study area was 7.7%, and for the Stanley Glacier study site in the northern study area, 16.2%. Damage and mortality over time from Cronartium ribicola will diminish the current ecological role of whitebark pine as a facilitator of landscape vegetation pattern development and may confound predictions of upward movement of treeline in response to climate warming. [ABSTRACT FROM AUTHOR]

For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research-that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, "What causes treeline?" with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research. [ABSTRACT FROM AUTHOR]

Whitebark pine (Pinus albicaulis) is a foundation and keystone species of upper subalpine and treeline ecosystems throughout the western United States and Canada. During the past several decades, Cronartium ribicola, an introduced fungal pathogen that causes white pine blister rust in five-needled pines, has caused significant declines in whitebark pine throughout its range. Our research objectives were to examine geographic variation in blister rust infection (total canker density) in whitebark pine found at six alpine treelines east of the Continental Divide in Glacier National Park, Montana, and to determine which environmental factors have the greatest influence on blister rust infection at treeline. Within a total of 30 sampling quadrats (five at each treeline study site), we measured the number of cankers on each whitebark pine in order to assess how blister rust infection varied throughout our study area. We created high-resolution digital elevation models to characterize surface microtopography, and used a geographic information system (GIS) to derive environmental variables of interest. A mixed effects, Poisson regression model determined environmental correlates of blister rust from the resulting set of field and GIS-derived variables. We found that rates of infection varied considerably among treelines, and that treeline sites exhibiting high flow accumulation rates, greater distances to wetlands, slopes facing southwest, higher curvature, greater wind speeds, and close proximity to Ribes and perennial streams had the highest rates of blister rust infection. [ABSTRACT FROM AUTHOR]

Whitebark pine (Pinus albicaulis), an important treeline conifer in northern Montana, is considered both a keystone and foundation species in high-elevation ecosystems. The introduced fungal pathogen Cronartium ribicola, which causes white pine blister rust, has resulted in severe declines in whitebark pine in subalpine forest communities throughout the northern Rockies during past decades. However, the prevalence of blister rust in whitebark pine within the alpine treeline community and its impact remain to be determined. We gathered field data on blister rust infection incidence in the treeline ecotone at two locations east of the Continental Divide in the northern Rocky Mountains, Montana, U.S.A. Our objectives were (1) to examine the potential importance of whitebark pine in tree island formation, (2) to determine if blister rust is present in whitebark pine within the alpine treeline community, and (3) to characterize the incidence and intensity of blister rust in krummholz tree islands. We found that whitebark pine is the primary initial colonizer in tree island formation, indicating that the species is important in generating vegetation pattern in these communities. Thirty-five percent of all sampled whitebark pine were infected with blister rust. Although more cankers were found in solitary whitebark pine trees, highest infection incidence per tree occurred in trees that were part of multitree islands. Finally, we found a significant correlation between tree island length and infection incidence. These results have important implications with respect to alpine treeline dynamics on a landscape scale, especially in the context of climate change. [ABSTRACT FROM AUTHOR]