Cloud frequency climatology at the Andes/Amazon transition: 2. Trends and variability

Halladay, Kate; Malhi, Yadvinder; New, Mark

doi:10.1029/2012jd017789

Cited by 18 publications

(28 citation statements)

References 57 publications

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“…There is also a trend of decreasing cloud cover throughout the Andes with increasing north Atlantic sea surface temperatures and frequency of El Niño events (Halladay et al ). Of particular interest is that cloud cover, and hence precipitation, appears to be decreasing over the southwest Amazon during the dry season, which in turn reduces moisture in the cloud forest zone of Peru and Bolivia, intensifying the dry seasons experienced by tropical cloud forests (Halladay et al ). Changes in precipitation and cloud inundation may be increasing water stress and fires near tropical treelines, perhaps preventing forest species from expanding into the drier, more seasonal grasslands, as has been documented in African mountains (Hemp ).…”

Section: Past and Present Tropical Treeline Shifts And Conditionsmentioning

confidence: 99%

The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences

Rehm

Feeley

2015

Ecography

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The upper elevational range edges of most tropical cloud forest tree species and hence the ‘treeline’ are thought to be determined primarily by temperatures. For this reason, the treeline ecotone between cloud forests and the overlying grasslands is generally predicted to shift upslope as species migrate to higher elevations in response to global warming. Here, we propose that other factors are preventing tropical trees from shifting or expanding their ranges to include high elevation areas currently under grassland, resulting in stationary treelines despite rising mean temperatures. The inability of cloud forest species to invade the grasslands, a phenomenon which we refer to as the ‘grass ceiling’ effect, poses a major threat to tropical biodiversity as it will greatly increase risk of extinctions and biotic attrition in diverse tropical cloud forests. In this review, we discuss some of the natural factors, as well as anthropogenic influences, that may prevent cloud forest tree species from expanding their ranges to higher elevations. In the absence of human disturbances, tropical treelines have historically shifted up‐ and down‐slope with changes in temperature. Over time, increased human activity has limited forests to lower elevations (i.e. has depressed treelines), and often broken the equilibrium between species range limits and climate. Yet even in areas where anthropogenic influences are halted, cloud forests have not expanded to higher elevations. Despite the critical importance of understanding the distributional responses of tropical species to climate change, few studies have addressed the factors that influence treeline location and dynamics, severely hindering our ability to predict the fate of these diverse and important ecosystems.

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Section: Past and Present Tropical Treeline Shifts And Conditionsmentioning

confidence: 99%

The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences

Rehm

Feeley

2015

Ecography

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“…It appears that C. cuneata will need to experience both increased mean and low temperatures during future climate change before it may expand its range beyond the treeline boundary. Mean temperature will continue to increase throughout the Andes (Urrutia and Vuille 2009) but concurrent increases in low temperature extremes are less certain and will largely depend on shifts in cloud base, which themselves are difficult to predict (Quintana-Gomez 1999, , Halladay et al 2012. Complicating matters even further is the fact that C. cuneata seedlings had higher survival when provided with shade, which indicates that factors other than temperature can also play a significant role in explaining tropical treeline dynamics.…”

Section: Discussionmentioning

confidence: 99%

“…Although many treeline species maintain adaptations that specifically protect them from excess solar radiation and photoinhibition, the exposed environment of the open grassland treatment may have been unsuitable due to light intensities beyond what treeline species can tolerate. The decreasing trend in cloud cover throughout much of Andes suggests that solar radiation loads and the likelihood of frequent photoinhibition will increase in the future making the already harsh grassland environment even more difficult for trees to invade (Halladay et al 2012). …”

Section: Discussionmentioning

confidence: 99%

“…There is also a trend of decreasing cloud cover throughout the Andes with increasing north Atlantic sea surface temperatures and frequency of El Niño events (Halladay et al 2012b). Of particular interest is that cloud cover, and hence precipitation, appears to be decreasing over the southwest Amazon during the dry season, which in turn reduces moisture in the cloud forest zone of Peru and Bolivia, intensifying the dry seasons experienced by tropical cloud forests (Halladay et al 2012b). This research suggests that changes in precipitation and cloud inundation may be increasing water stress and fires near tropical treelines, perhaps preventing forest species from expanding into the drier, more seasonal grasslands, as has been documented in African mountains (Hemp 2005).…”

Section: Past and Present Tropical Treeline Shifts And Conditionsmentioning

confidence: 99%

“…However, this predicted migration does not account for changes in precipitation and cloud cover. In some tropical regions such as the Andes, precipitation is predicted to increase during the wet season and decrease during the dry season, increasing the seasonality experienced by cloud forest species (Vuille et al 2008, Halladay et al 2012b. Exacerbating this uncertainty varying precipitation patterns is the projection that cloud cover will continue to decrease throughout the tropical Andes into the future, especially during the dry season (Halladay et al 2012b).…”

Section: Tropical Treelines In the Futurementioning

confidence: 99%

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Factors Affecting Current and Future Treeline Locations and Dynamics in the Peruvian Andes

Rehm¹

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The elevational distributions of tropical treelines are thought to be determined by temperature, and are predicted to shift upslope in response to global warming. In contrast to this hypothesis, global-scale studies have shown that only half of all studied treelines are shifting upslope. Understanding how treelines will respond to climate change has important implications for global biodiversity, especially in the tropics, because tropical treelines generally represent the upper-elevation distribution limit of the hyper-diverse cloudforest ecosystem. In Chapter 1, I introduce the idea that grasslands found above tropical treelines may represent a potential grass ceiling which forest species cannot cross or invade. I use an extensive literature review to outline potential mechanisms which may be acting to stabilize treeline and prevent forest expansion into high-elevation grasslands.In Chapters 2-4, I begin to explore these potential mechanisms through the use of observational and experimental methods. In Chapter 2, I show that there are significant numbers of seedlings occurring just outside of the treeline in the open grasslands and that seed rain is unlikely to limit seedling recruitment above treeline. I also show that microclimates outside of the closed-canopy cloudforest are highly variable and that mean vi temperatures are likely a poor explanation of tropical treeline elevations. In Chapter 3, Ishow that juvenile trees maintain freezing resistances similar to adults, but nighttime radiative cooling near the ground in the open grassland results in lower cold temperatures relative to the free atmosphere, exposing seedlings of some species growing above treeline to lethal frost events. In Chapter 4, I use a large-scale seedling transplant experiment to test the effects of mean temperature, absolute low temperature and shade on transplanted seedling survival. I find that increasing mean temperature negatively affects seedling survival of two treeline species while benefiting another. In addition, low temperature extremes and the presence of shade also appear to be important factors affecting seedling survival above tropical treelines. This work demonstrates that mean temperature is a poor predictor of tropical treelines and that temperature extremes, especially low temperatures, and non-climatic variables should be included in predictions of current and future tropical treeline dynamics. vii ABSTRACTThe upper elevational range edges of most tropical cloud forest tree species and hence the "treeline" are thought to be determined primarily by temperatures. For this reason, the treeline ecotone between cloud forests and the overlying grasslands is generally predicted to shift upslope as species migrate to higher elevations in response to global warming.Here, we propose that, despite rising mean temperatures, other factors are preventing tropical trees from shifting or expanding their ranges to include high elevation areas currently under grassland, resulting in stationary treelines. The inability of cloud for...

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Examining variation in the leaf mass per area of dominant species across two contrasting tropical gradients in light of community assembly

Neyret

Bentley

Oliveras

et al. 2016

Ecology and Evolution

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Understanding variation in key functional traits across gradients in high diversity systems and the ecology of community changes along gradients in these systems is crucial in light of conservation and climate change. We examined inter‐ and intraspecific variation in leaf mass per area (LMA) of sun and shade leaves along a 3330‐m elevation gradient in Peru, and in sun leaves across a forest–savanna vegetation gradient in Brazil. We also compared LMA variance ratios (T‐statistics metrics) to null models to explore internal (i.e., abiotic) and environmental filtering on community structure along the gradients. Community‐weighted LMA increased with decreasing forest cover in Brazil, likely due to increased light availability and water stress, and increased with elevation in Peru, consistent with the leaf economic spectrum strategy expected in colder, less productive environments. A very high species turnover was observed along both environmental gradients, and consequently, the first source of variation in LMA was species turnover. Variation in LMA at the genus or family levels was greater in Peru than in Brazil. Using dominant trees to examine possible filters on community assembly, we found that in Brazil, internal filtering was strongest in the forest, while environmental filtering was observed in the dry savanna. In Peru, internal filtering was observed along 80% of the gradient, perhaps due to variation in taxa or interspecific competition. Environmental filtering was observed at cloud zone edges and in lowlands, possibly due to water and nutrient availability, respectively. These results related to variation in LMA indicate that biodiversity in species rich tropical assemblages may be structured by differential niche‐based processes. In the future, specific mechanisms generating these patterns of variation in leaf functional traits across tropical environmental gradients should be explored.

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Cloud frequency climatology at the Andes/Amazon transition: 2. Trends and variability

Cited by 18 publications

References 57 publications

The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences

The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences

Factors Affecting Current and Future Treeline Locations and Dynamics in the Peruvian Andes

Examining variation in the leaf mass per area of dominant species across two contrasting tropical gradients in light of community assembly

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