Tropical Treeline Research Articles

Patterns in climate and seedling establishment at a dry tropical treeline

In some internal valleys of the northern Andes, precipitation at treeline elevation is seasonal, with one or two dry periods that could exert a strong control on tree seedling survival. However, little is known about (1) the severity of the dry periods, due to a lack of climate data, or (2) the effect of drought on tree seedling demographic dynamics and how this is modulated by local plant interactions in these tropical-alpine ecosystems. We studied annual precipitation variation and seedling demography and ecophysiology in relation to microclimate and neighboring plants in a dry treeline ecotone at 3100 m a.s.l in the Venezuelan Andes. We selected seedlings of the dominant tree species Cybianthus marginatus and Clusia multiflora at three sites, each with three adjacent 10 × 10-m plots within the forest, forest border, and páramo. For both species, seedling density was zero in the páramo and did not differ between border and forest. Growth rates were low for both species, but higher for Cybianthus in the border than in the forest. Survival rates were high in both environments. Low assimilation and transpiration rates were found in both species regardless of the environment. The spatial association of seedlings with moisture-regulating elements such as mosses, ferns and basal rosettes may indicate a benefit of better hydric conditions in their neighborhood. Our study shows a strong limitation for tree establishment in the páramo, but high rates of survival at the edge of the forest. This suggests a scenario of slow potential forest expansion at this tropical treeline, tree cover advancing in a closed front by the successive formation of forest-edge vegetation allowing tree seedling establishment outside of the forest.

Climate-induced reversal of tree growth patterns at a tropical treeline.

Globally, cold-limited trees and forests are expected to experience growth acceleration as a direct response to warming temperatures. However, thresholds of temperature limitation may vary substantially with local environmental conditions, leading to heterogeneous responses in tree ecophysiology. We used dendroecological and isotopic methods to quantify shifting tree growth and resource use over the past 143 years across topographic aspects in a high-elevation forest of central Mexico. Trees on south-facing slopes (SFS) grew faster than those on north-facing slopes (NFS) until the mid-20th century, when this pattern reversed notably with marked growth rate declines on SFS and increases on NFS. Stable isotopes of carbon, oxygen, and carbon-to-nitrogen ratios suggest that this reversal is linked to interactions between CO2 stimulation of photosynthesis and water or nitrogen limitation. Our findings highlight the importance of incorporating landscape processes and habitat heterogeneity in predictions of tree growth responses to global environmental change.

Fire and grazing controlling a tropical tree line: Effects of long‐term grazing exclusion in Bale Mountains, Ethiopia

AbstractAimsTropical tree lines are often associated with abrupt shifts in vegetation, soils and disturbance regimes, but the underlying mechanisms are poorly understood. We analysed the role of grazing, fuels and fire in maintaining a sharp tree line with flammable heathland above non‐flammable forest.LocationBale Mountains, Ethiopia.MethodsThe study used grazing exclosures, repeated vegetation sampling, soil analyses and burning and sowing experiments along an altitudinal gradient with Hagenia abyssinica forest, Erica trimera forest and Erica heathland; all were heavily grazed, the Erica heathland also burnt on short rotation.ResultsContrary to expectation, livestock exclusion did not increase flammability in the forest, but instead resulted in a dense carpet of non‐flammable herbs. In the heathland, livestock exclusion led to somewhat faster post‐fire fuel recovery, but no major change in vegetation. Seeding of tree species resulted in some seedling establishment, but notably Hagenia grew poorly in the heathland, even when protected from livestock. A bioassay, as well as observations of outpost trees on atypical soil above the tree line, suggests that this poor growth is caused by the acidic soils, rather than harsh climate. Despite frequent fires, heathland soils had lower pH and higher organic matter content than forest soils. Below the tree line, tree seedling establishment was successful only in forest gaps, and if livestock was excluded. In both forest and heathland rapid vegetative regeneration in the ground flora after disturbance restricted major species shifts.ConclusionsThese results suggest that the contrasting fire potential between heathland and forest, and thus the sharp tree line would be maintained, or possibly even accentuated, in the absence of livestock grazing, and that Hagenia colonization upwards into the heathland is restricted not only by fire and grazing, but also the acidic soils, which is a legacy of centuries of dominance by Erica.

Modelling the effect of temperature changes on plant life-form distribution across a treeline ecotone in the tropical Andes

ABSTRACTBackground: Classifying species from the high tropical Andes into plant life forms (PLF) can provide a more functional understanding of tropical treeline dynamics. However, little is known about the potential response of PLFs to climate warming.Aims: The objective of this work was to evaluate the response of PLFs to environmental conditions above the upper montane forest in the Venezuelan Andes and forecast their potential distribution under warming scenarios using habitat suitability models.Methods: We classified species into nine PLFs, analysed their current distribution using CCA and non-linear multiple regressions and forecasted their potential distribution under warming scenarios. We used anatomical leaf traits for shrubs and caulescent rosettes to refine their potential responses to climate.Results: Tree cover sharply decreased with increasing elevation, while shrub and caulescent rosette distribution differed depending on their leaf traits. Projections suggested an upslope shift of all PLFs with warming and an increase in tree cover.Conclusions: The analysis of the distribution of PLFs, by linking plant adaptive strategies to topography/climate, allowed projecting changes in vegetation physiognomy in response to warming. Projections of a moderate increase in tree cover in the grassland páramo did not indicate the formation of closed forests above the forest line.

Seedling transplants reveal species-specific responses of high-elevation tropical treeline trees to climate change.

The elevations at which tropical treelines occur are believed to represent the point where low mean temperatures limit the growth of upright woody trees. Consequently, tropical treelines are predicted to shift to higher elevations with global warming. However, treelines throughout the tropics have remained stationary despite increasing global mean temperatures. The goal of thestudy reported here was to build a more comprehensive understanding of the effects of mean temperature, low-temperature extremes, shading, and their interactions on seedling survival at tropical treelines. We conducted a seedling transplant study using three dominant canopy-forming treeline species in the southern tropical Andes. We found species-specific differences and contrasting responses in seedling survival to changes in mean temperature. The most abundant naturally occurring species at the seedling stage outside the treeline, Weinmannia fagaroides, showed a negative relationship between the survival of transplanted seedlings and mean temperature, the opposite of a priori expectations. Conversely, Clethra cuneata showed increased survival at higher mean temperatures, but survival also increased with higher absolute low temperatures and the presence of shade. Finally, the survival of Gynoxys nitida seedlings was insensitive to temperature but increased under shade. These findings show that multiple factors can determine the upper distributional limit of species forming the current tropical treeline. As such, predictions of future local and regional tropical treeline shifts may need to consider several factors beyond changes in mean temperature. If the treeline remains stationary and cloud forests are unable to expand into higher elevations, there may be severe species loss in this biodiversity hotspot.

Freezing temperatures as a limit to forest recruitment above tropical Andean treelines.

The elevation of altitudinal treelines is generally believed to occur where low mean temperatures during the growing season limit growth and prevent trees from establishing at higher elevations. Accordingly, treelines should move upslope with increasing global temperatures. Contrary to this prediction, tropical treelines have remained stable over the past several decades despite increasing mean temperatures. The observed stability of tropical treelines, coupled with the drastically different temperature profiles between temperate and tropical treelines, suggests that using mean measures of temperature to predict tropical treeline movements during climate change may be overly simplistic. We hypothesize that frost events at tropical treelines may slow climate driven treeline movement by preventing tree recruitment beyond the established forest canopy. To assess this hypothesis, we measured freezing resistance of four canopy-forming treeline species (Weinmannia fagaroides, Polylepis pauta, Clethra cuneata, and Gynoxys nitida) at two life stages (juvenile and adult) and during two seasons (warm-wet and cold-dry). Freezing resistances were then compared to microclimatic data to determine if freezing events in the grassland matrix above treeline are too harsh for these forest species. Freezing resistance varied among species and life stages from -5.7 degrees C for juveniles of P. pauta to -11.1 degrees C for juveniles of W. fagaroides. Over a four-year period, the lowest temperatures recorded at 10 cm above ground level in the grasslands above treeline and at treeline itself were -8.9 degrees C and -6.8 degrees C, respectively. Juveniles maintained freezing resistances similar to adults during the coldest parts of the year and ontogenetic differences in freezing resistance were only present during the warm season when temperatures did not represent a significant threat to active plant tissue. These findings support the hypothesis that rare extreme freezing events at and above tropical treelines can prevent recruitment outside of closed canopy forest for some tree species and may significantly slow treeline advancement despite warming mean temperatures. Predictions of treeline shifts under climate change should be reevaluated to include species-specific' climatic tolerances and measures of climatic variability.

Historic and recent fragmentation coupled with altitude affect the genetic population structure of one of the world's highest tropical tree line species

ABSTRACTAim To assess the effects of altitude and historic and recent forest fragmentation on the genetic diversity and structure of the wind‐pollinated tropical tree line species Polylepis incana.Location One of the highest mountain forest regions of the world, located in the Eastern Cordillera of the Ecuadorian Andes.Methods We compared genetic diversity and structure of adult trees with those of seedlings (n= 118 in both cases) in nine forest stands spanning an altitudinal gradient from 3500 to 4100 m a.s.l. using amplified fragment length polymorphisms (AFLPs). Genetic diversity was calculated as percentage of polymorphic bands (P) and Nei's expected heterozygosity (He); genetic differentiation was assessed using analysis of molecular variance, ΦST statistics and Bayesian cluster analysis.Results Estimates of genetic diversity at the population level were significantly lower in seedlings than in adults. Genetic diversity (He‐value) was, in both cases, negatively correlated to altitude and positively correlated to population size in the seedlings. Genetic differentiation of the seedlings was approximately as high (φST= 0.298) as that of the adults (φST= 0.307), and geographical differentiation was clearly reflected in both AFLP profiles, with mountain ridges acting as barriers to gene flow.Main conclusions Our study provides evidence of a historic upslope migration of P. incana in central Ecuador. In addition, it highlights the detrimental effects of unexpectedly strong genetic isolation, both recent and historical, particularly for our wind‐pollinated species where the distance between forest stands was less than 25 km. We therefore additionally propose that in habitats with pronounced high‐mountain landscape structures, gene flow may be hampered to such an extent that species have a more pronounced sensitivity to habitat fragmentation, even among populations of wind‐pollinated trees.

Using Automated Point Dendrometers to Analyze Tropical Treeline Stem Growth at Nevado de Colima, Mexico

The relationship between wood growth and environmental variability at the tropical treeline of North America was investigated using automated, solar-powered sensors (a meteorological station and two dendrometer clusters) installed on Nevado de Colima, Mexico (19° 35′ N, 103° 37′ W, 3,760 m a.s.l.). Pure stands of Pinus hartwegii Lindl. (Mexican mountain pine) were targeted because of their suitability for tree-ring analysis in low-latitude, high-elevation, North American Monsoon environments. Stem size and hydroclimatic variables recorded at half-hour intervals were summarized on a daily timescale. Power outages, insect outbreaks, and sensor failures limited the analysis to non-consecutive months during 2001–2003 at one dendrometer site, and during 2002–2005 at the other. Combined data from the two sites showed that maximum radial growth rates occur in late spring (May), as soil temperature increases, and incoming short-wave radiation reaches its highest values. Early season (April–May) radial increment correlated directly with temperature, especially of the soil, and with solar radiation. Stem expansion at the start of the summer monsoon (June–July) was mostly influenced by moisture, and revealed a drought signal, while late season relationships were more varied.

Recent warming at the tropical treeline of North America

Tropical treelines are critical zones for observing and understanding regional responses to climatic change(Diaz et al. 2003), especially because the low latitudes play a prominent role in the global climate system(Hoerling and Kumar 2003), andmountain areas regulate downstream availability of water resources (Brad-ley et al. 2004). In North America, tropical treelines are also part of the North American Monsoon System(NAMS); this system’s control over summer precipitation, thunderstorm activity, and lightning patterns in the southwestern US extends to other regions via atmospheric connections (Vera et al. 2006; Dominguez et al.2009). Few weather stations with uninterrupted data series exist above 3000 m in rural areas throughout theentire American Cordillera, from Alaska to Tierra del Fuego (Bradleyet al. 2004), making it difficult to testhypotheses, calibrate models, and detect landscape feedbacks to human activities.

Morphological and ecophysiological traits shaping altitudinal distribution of three Polylepis treeline species in the dry tropical Andes

Numerous species of the genus Polylepis form the highest treeline in the world, with striking dissimilarities in their upper altitudinal limits. The commonly accepted hypothesis is that growth at a treeline is limited by temperature. Here, using in situ records of various morphological and ecophysiological traits, we aimed to identify other factors influencing altitudinal distribution of three congeneric species from the dry tropical Andes: Polylepis rugulosa, Polylepis tarapacana and Polylepis tomentella. While P. tarapacana and P. tomentella reach their altitudinal limit at around 5000 m asl, P. rugulosa does not thrive above 4300 m, but precipitation is markedly lower in its distribution area. The three species responded to altitude by a change of morphological (e.g. decreased tree height and leaf size) and ecophysiological (e.g. decrease of transpiration rate, nutrient concentration or enrichment in the 13C isotope) traits, and this response was generally more pronounced in P. rugulosa. In comparison with P. tarapacana and P. tomentella, P. rugulosa displayed higher transpiration rates. Waxes from the abaxial (stomatous) leaf side of P. rugulosa were most strongly enriched in 13C. Furthermore, leaves of all species studied here had exceptionally low N and P concentrations. Trade-offs linked to changes in leaf area (e.g. bigger leaves, higher photosynthetic capacity but elevated transpiration) seem to drive differentiation and adaptations to altitude among these three congeneric species. We hypothesize that, while the upper distribution limit of P. tarapacana and P. tomentella is largely driven by low temperature, water is an important additional factor controlling the altitudinal distribution of P. rugulosa. Our results suggest that water stress needs to be taken into account among the factors shaping the altitudinal distribution of tropical treeline species.

Stable Isotope Characterization of the Ecohydrological Cycle at a Tropical Treeline Site

ABSTRACTWe investigated the seasonal variation in pools of water available to mature trees growing at high elevation in a tropical environment. The study focused on the dominant tree species (Pinus hartwegii) at about 3800 m a.s.l. on Nevado de Colima, Mexico, where climate is typical of the North American Monsoon System. Stable isotope ratios of hydrogen and oxygen in water extracted from soil, xylem, and leaves were measured through a cycle of two dry and two wet seasons in 2003–2004. Isotopic ratios were also measured in accumulated precipitation, a few single precipitation events, and in spring water over the two-year period. Based on evidence from water, stable isotopes in soil, and xylem samples, trees utilized water from relatively shallow soil depths, which are representative of current conditions, rather than tapping groundwater, which is more representative of long-term trends. While the stable isotope signature in environmental waters showed a slightly different pattern before and during the monsoon, the more pronounced differences in leaf water isotopes between the two seasons, due to drought stress, will lead to a clear seasonal isotopic signal in tree ring cellulose. This study represents a unique snapshot of water cycling in a tropical treeline ecosystem, where our understanding of eco-hydrological pathways is limited. This type of analysis is also useful for proper calibration of stable isotopic signals in tree ring records.

Dominant factors controlling glacial and interglacial variations in the treeline elevation in tropical Africa

The knowledge of tropical palaeoclimates is crucial for understanding global climate change, because it is a test bench for general circulation models that are ultimately used to predict future global warming. A longstanding issue concerning the last glacial maximum in the tropics is the discrepancy between the decrease in sea-surface temperatures reconstructed from marine proxies and the high-elevation decrease in land temperatures estimated from indicators of treeline elevation. In this study, an improved inverse vegetation modeling approach is used to quantitatively reconstruct palaeoclimate and to estimate the effects of different factors (temperature, precipitation, and atmospheric CO(2) concentration) on changes in treeline elevation based on a set of pollen data covering an altitudinal range from 100 to 3,140 m above sea level in Africa. We show that lowering of the African treeline during the last glacial maximum was primarily triggered by regional drying, especially at upper elevations, and was amplified by decreases in atmospheric CO(2) concentration and perhaps temperature. This contrasts with scenarios for the Holocene and future climates, in which the increase in treeline elevation will be dominated by temperature. Our results suggest that previous temperature changes inferred from tropical treeline shifts may have been overestimated for low-CO(2) glacial periods, because the limiting factors that control changes in treeline elevation differ between glacial and interglacial periods.

Daily Weather and Tree Growth at the Tropical Treeline of North America

We present here the 2001–2004 results of observational field studies aimed at quantifying tropical timberline climate and radial increment of Pinus hartwegii Lindl. trees on Nevado de Colima, in the middle of the North American Monsoon region. An automated weather station was installed at 3760 m a.s.l., 19°34.778′N latitude, 103°37.180′W longitude, within a forest where multi-century tree-ring records had been previously developed. At the same time, automated electronic sensors for recording tree growth at 30-min intervals were set up at two sites within a 1-km radius from the weather station. Meteorological observations recorded every 30 min were summarized on a daily basis. Time-series patterns are reported for atmospheric pressure, precipitation, incoming solar radiation, air and soil temperature, relative humidity, soil moisture, and wind speed and direction. Of particular interest is the sudden decrease in air temperature after the onset of the monsoon season, which determines very high relative humidity over the summer and results in air temperature having a spring maximum. Despite sub-freezing air temperatures in most months, soil temperatures never drop below 0°C. Dendrometer data show that the timberline growing season begins in March–April as temperature increases, then radial growth continues throughout the cool-wet summer monsoon, and ends in October–November. As an unexpected result, it was also possible to measure the progressive decline of Pinus hartwegii stem increment in response to an outbreak of roundheaded pine beetle (Dendroctonus adjunctus Blandford), which ultimately killed most trees at one of our two experimental sites.

A 400-year tree-ring chronology from the tropical treeline of North America.

High-elevation sites in the tropics may be particularly sensitive to rapid climate change. By sampling treeline populations, I have developed the first extensive (> 300 years) tree-ring chronology in tropical North America. The site is Nevado de Colima, at the western end of the Mexican Neovolcanic Belt, and the species studied is Mexican mountain pine (Pinus hartwegii). Despite past logging in the area, 300 to 500-year old pines were found at 3600-3700 m elevation, about 300 m below the present treeline. The Nevado de Colima tree-ring chronology is well replicated from 1600 to 1997. Calibration with Colima climatic records points to summer monsoon precipitation as the strongest dendroclimatic signal. Most trees also exhibit extremely low growth in 1913 and 1914, following the January 1913 Plinian eruption of the Volcan de Colima. Because P. hartwegii is found on top of high mountains from Mexico to Guatemala, there is potential for developing a network of tropical treeline chronologies.

A 400-year Tree-ring Chronology from the Tropical Treeline of North America

A 400-year Tree-ring Chronology from the Tropical Treeline of North America

Carbon and water balance in Polylepis sericea, a tropical treeline species

Polylepis sericea trees grow well above the continuous forest line in the Venezuelan Andes. In these environments, extreme daily temperature ranges can occur at any time of the year and trees experience a 4 month dry period. The purpose of this work was to study carbon and water relations of this species in the field during wet and dry seasons in order to understand this species' success at such high altitudes. Leaf gas exchange (portable system in open mode) and leaf water potential (pressure chamber) were measured at 1–2 h intervals during several daily courses at 4000 m elevation in the Páramo de Piedras Blancas. CO2 assimilation versus leaf temperature curves were also obtained for this species in the laboratory. Clear differences in the measured parameters were observed between seasons. For a wet season day, maximum CO2 assimilation rate was 7.4 μmol m-2 s-1 and leaf conductance was relatively constant (approximately 100 mmol m-2 s-1)In the dry season day, maximum CO2 assimilation rate was 5.8 μmolm-2 s-1 and leaf conductance was close to 60 mmolm-2 s-1. Minimum leaf water potentials measured were -1.3 MPa for the wet and -2.2 MPa for the dry season. The CO2 assimilation-leaf temperature relationship showed a 13.4°C leaf temperature optimum for photosynthesis with maximum and minimum compensation points of 29.5 and -2.8°C, respectively. Maximum night-time respiration was relatively high (2.7 (imol) m-2 s-1)Our results show thatP. sericea maintains a highly positive carbon balance through all daily courses, even though there is a slight water stress effect during the dry season; this suggests that its carbon assimilation machinery is well adapted to the low temperatures and seasonal water stress found in the high tropical mountains.

Daily and Seasonal Osmotic Changes in a Tropical Treeline Species

Daily and Seasonal Osmotic Changes in a Tropical Treeline Species