Altitudinal Ecotones Research Articles

Establishing the Position and Drivers of the Eastern Andean Treeline with Automated Transect Sampling

The eastern Andean treeline (EATL) is the world’s longest altitudinal ecotone and plays an important role in biodiversity conservation in the context of land use/cover and climate change. The purpose of this study was to assess to what extent the position of the tropical EATL (9°N–18°S) is in near-equilibrium with the climate, which determines its potential to adapt to climate change. On a continental scale, we have used land cover maps (MODIS MCD12) and elevation data (SRTM) to make the first-order assessment of the EATL position and continuity. For the assessment on a local scale and to address the three-dimensional nature of environmental change in mountainous environments, a novel method of automated delineation and assessment of altitudinal transects was devised and applied to Landsat-based forest maps (GLAD) and fine-resolution climatology (CHELSA). The emergence of a consistent longitudinal gradient of the treeline elevation over half of the EATL extent, which increases towards the equator by ~30 m and ~60 m per geographic degree from the south and north, respectively, serves as a first-order validation of the approach, while the local transects reveal a more nuanced aspect-dependent pattern. We conclude that the applied dual-scale approach with automated mass transect sampling allows for an improved understanding of treeline dynamics.

Holocene vegetation dynamics on the Apakará summit of the neotropical Guayana Highlands and potential environmental drivers

The Guayana Highlands (GH) are natural laboratories to study the influence of environmental drivers on neotropical ecosystems. The GH summits have been characterized by constant vegetation patterns during the Holocene, except for a few sites close to altitudinal ecotones. Here, we report a new pollen record showing two significant vegetation shifts, from Myrica forests to tepuian meadows (5340calyrBP) and then to Chimantaea shrublands (2720calyrBP). These changes are analyzed in terms of potential forcing factors such as regional climate changes, fire and autogenic succession. The first shift occurred shortly after the Holocene Thermal Maximum and the onset of a cooling/drying trend. A charcoal peak was coeval with this change suggesting some potential influence of fire. Autogenic processes related to lake infilling might have also been involved. The second vegetation shift took place during a phase of increased precipitation variability due to an ENSO intensification. The establishment of Chimantaea shrublands started at the end of this phase coinciding with another charcoal peak (2300calyrBP). These results support the idea of relevant ecological changes in the GH during the Holocene, the apparent vegetation constancy recorded in other GH summits could be due to site insensitivity.

Soil nutrient’s spatial variability in forest–tundra ecotones on the Kola Peninsula, Russia

This article addresses spatial variability in soil nutrients in altitudinal and latitudinal forest–tundra ecotones in the Kola Peninsula, Russia. Higher total carbon, nitrogen, and bio-available nutrients were found in the organic horizons of latitudinal ecotone against the background of lower nutrients in soil-forming rock. This is supposedly related to higher contribution of nutrient-rich plants in latitudinal ecotone and intense downward within-profile matter migration in the altitudinal ecotone. Elevated bio-available soil nutrients in spruce forests compared with birch forests and tundra sites, and in birch forests compared with tundra against the background of different trends in the soil-forming rock were attributed to the effects of predominant plants. The effects of predominant plants on soil nutrients were distinctly pronounced at the level of site patches. Soil-forming rock effects on soil nutrients were clear at the level of zones/belts and the whole ecotones. Strong negative correlations between the soil nutrients and altitudes were explained by replacement of vascular plants by low-ash lichens at higher elevations.

Preliminary palynological analysis of a Holocene peat bog from Apakará-tepui (Chimantá Massif, Venezuelan Guayana)

Preliminary palynological analysis of a Holocene peat bog from Apakara-tepui (Chimanta Massif, Venezuelan Guaya- na).- This paper reports the preliminary palynological results, at a millennial scale, of a Holocene peat bog sequence, since around 8.0 cal kyr BP to the present, obtained in the summit of the Apakara-tepui (2170 m elevation), in the Chimanta massif, located in the neotropical Venezuelan Guayana. The early Holocene was characterized by a vege- tation different to the present, in which trees and shrubs dominated and Myrica (Myricaceae) was the main element of the gallery forests around a permanent water body, as indicated by the continuous presence of Isoetes (Isoetaceae) in high percentages. Around the middle Holocene (5.3 cal kyr BP), a shift towards more herbaceous and non-flooded communities occurred, and the present day vegetation established. This has been interpreted as a shift from warmer and wetter climates to cooler and drier conditions. The first phase, from 8.0 to 5.3 cal kyr BP, falls within a warming phase widely documented worldwide, known as the Holocene Thermal Maximum. The millennial trends shown here will be refined with further studies at centennial to decadal time scales. These results support the hypothesis that the best sites to detect paleoenvironmental changes in the summits of the tabular Guayana mountains are close to altitudinal ecotones.

Paleoecology of the Guayana Highlands (northern South America): Holocene pollen record from the Eruoda-tepui, in the Chimantá massif

The previously recorded vegetation constancy during most of the Holocene, atop some summits of the Guayana tabular mountains (or tepuis), led to the hypotheses of either environmental stability or site insensitivity. As high-mountain biomes are considered to be especially well suited for recording past environmental changes, a palynological study on the uppermost summit of the Chimantá massif was designed to test its suitability for these purposes. A peat sequence was obtained spanning the last ~13.0 cal kyr BP, but an acceptable resolution for paleoecological reconstruction is available only for the last ~4000 years. Around 4.3 cal kyr BP, the modern vegetation was established and has remained virtually unchanged until today; minor paleoenvironmental changes recorded in other sequences around 2.5 cal kyr BP were not detected here. The main paleoclimatic trends are in good agreement with other neotropical records, especially from Lake Valencia and the Cariaco Basin. It is concluded that high-altitude tepuian sites are useful to record paleoenvironmental changes of moderate to high intensity but once a dense vegetation cover is established, gentle shifts remain hidden due to the capacity of plant communities to absorb the changes. The best sites for paleoecological research atop the tepuis are those lying on or near altitudinal ecotones, especially between the meadows and the paramoid shrublands (~2200 m elevation). Sites within the meadow domain, as most well-studied so far, are relatively insensitive to Holocene paleoenvironmental changes.

Effects of climate on the radial growth of tree species in the upper and lower distribution limits of an altitudinal ecotone on Mount Norikura, central Japan

Tree‐ring width chronologies were developed for Abies veitchii, Betula ermanii and Betula platyphylla var. japonica in their altitudinal ecotone (approximately 1600 m a.s.l.) on Mount Norikura, central Japan, to determine what climatic conditions affect the growth of tree species in the upper and lower distribution limits of an altitudinal ecotone. This altitude was the lower distribution limit for A. veitchii and B. ermanii in the subalpine zone, and was the upper distribution limit for B. platyphylla var. japonica in the montane zone on Mount Norikura. Tree‐ring widths of the two Betula species and A. veitchii were positively correlated with the August precipitation of the current and previous years, respectively. Precipitation in August (the hottest month) was reduced compared with other months during summer. Tree‐ring width of B. platyphylla var. japonica showed no correlation with temperatures in any month in its upper distribution limit. In contrast, tree‐ring widths of B. ermanii and A. veitchii were negatively correlated with the August temperatures of the current and previous years, respectively, at the lower distribution limit of these species. Therefore, the two Betula species and A. veitchii responded to climatic conditions of the current and previous years, respectively. The present study also suggests that a water deficit in August reduces growth of these three species in this altitudinal ecotone, irrespective of the upper or lower distribution limits, and that a high August temperature is more detrimental to the growth of A. veitchii and B. ermanii in their lower distribution limits. Thus, the three species with different altitudinal distributions examined in the present study responded differently to climatic conditions in this altitudinal ecotone on Mount Norikura.

Effects of Climatic Conditions on Shoot Elongation of Alpine Dwarf Pine (Pinus pumila) at Its Upper and Lower Altitudinal Limits in Central Japan

The effects of climatic conditions on the shoot elongation of alpine dwarf pine (Pinus pumila) were examined at its lower and upper altitudinal limits on Mt. Norikura (2500 and 2840 m a.s.l.) and Mt. Shogigashira (2640 and 2675 m a.s.l.) in central Japan. Altitudinal forest-structural changes were also described. Shoot elongation and stem height of P. pumila increased with decreasing altitude, but its abundance was markedly decreased at the altitudinal ecotone between the upper P. pumila zone and the lower Betula ermanii zone because of the suppression by tall B. ermanii. Thus, the lower altitudinal limit of P. pumila was probably determined by the competition with B. ermanii. The interannual variation in the shoot elongation of P. pumila was related to climatic conditions; long shoot length was associated with high summer temperatures of the previous year at both the upper and lower altitudinal limits on the two mountains. In addition, rates of the increase of shoot elongation in response to the increase of air temperature were not different between the upper and lower altitudinal limits. Thus, the increase of summer temperatures would enhance the growth of P. pumila from its upper to lower altitudinal limits. However, it is harder to predict the altitudinal distribution shift of P. pumila due to environmental change because its lower altitudinal limit is largely affected by competition with B. ermanii. Therefore, this study concluded that long-term monitoring of the population dynamics at the P. pumila–B. ermanii ecotone is necessary to predict the distribution shift of P. pumila.

Pollen production of Alnus incana at its south Norwegian altitudinal ecotone

The pollen production (number of pollen grains per catkin) of Alnus incana is studied along two vertical transects across the altitudinal limit of the species. Annual variation in pollen quantity, and a considerable reduction in the number of pollen are found with increasing altitude. Normal pollen is virtually not produced at the ecotone. It is suggested that parameters such as the temperature of the preceding summer, length of the growing season, and soil factors are limiting factors. A test study shows no germination of seed from the ecotone, and reduced fertility is suggested.

Ancestral semiochemical attraction persists for adjoining populations of siblingIps bark beetles (Coleoptera: Scolytidae)

Three species ofIps pine bark beetles in Hopping's group IX (S.L. Wood'sgrandicollis group),Ips confusus, I. lecontei, andI. paraconfusus, are parapatrically distributed in the American Southwest. They share post-Pleistocene altitudinal ecotones with their host pines. Adjacent to these areas of host overlap, we tested the hypothesis that aggregation pheromones produced by male beetles and/or host volatiles are sufficient to elicit the speciesspecific colonization behaviors typical of these threeIps species in nature. A more distantly related species,I. pini (Hopping's group IV, S.L. Wood'spini group) was used for outgroup comparison. Under the influence of pheromone, males ofI. confusus andI. paraconfusus do not discern among unin-fested log bolts of host and nonhost pine prior to bark contact. Males responding to pheromones emanating from infested bolts are similarly undiscriminating. Females ofI. confusus andI. lecontei olfactorily discern the combination of conspecific males in host pine from other possible beetlepine combinations; females ofI. paraconfusus do not. FemaleI. pini discerned conspecific pheromone from that ofI. lecontei. The bark beetle predator,Enoclerus lecontei, is attracted by, but does not discriminate among, the male-produced volatiles of these Group IXIps species. These results support a hypothesis that divergence in pheromonal responses by these group IXIps species has evolved following their speciation, having been manifested first in the female sex. Evolutionarily, the derived pheromonal messages have preceded their behavioral discrimination by these beetles. Additional speciesspecific cues may operate between the sexes in the field that may preclude heterospecific pairings.