In cloud forests, fog water can provide a potentially important moisture subsidy to communities that are endemic to these habitats such as the threatened spruce-fir communities of the southern Appalachian Mountains. These mountain-top communities grow only above ~1500 m elevation, harbor the endemic Abies fraseri, and have been proposed to exist because of frequent cloud immersion. We utilized stable isotopes and mathematical mixing models to determine the relative contribution of fog water to saplings at the upper and lower elevational limits both in May (beginning of growing season) and October (end of growing season). Additionally, we simultaneously measured gas exchange and water potentials. Fog water contributed up to 31% of plant water at the upper elevation sites in May. High elevation plants of both species also experienced greater fog frequency and had greater cloud water absorption (14-31%) compared to low elevation plants (4-17%). Greater fog uptake occurred in May compared to October, despite similar rainfall and fog frequencies. High elevation saplings also had improved water status and photosynthetic rates in response to fog. These results demonstrate the important water subsidy that cloud immersion water can provide. With a warming climate leading potentially to increases in the ceiling of the cloud base and, thus, less frequent cloud immersion, persistence of these relic mountain-top forests may depend on the magnitude of these changes and the compensating capabilities of other water sources.
Costs of Chemical Defense and Ant Symbiosis in Two Rain Forest Understory Shrubs in the Genus Piper
The costs of plant defenses against herbivores and pathogens are commonly assumed to be associated with trade-offs in the allocation of limited plant resources to other plant functions, but costs and trade-offs have proven to be challenging to quantify and often are not detected in circumstances where theory predicts they should be substantial. In tropical environments, these costs and trade-offs are even more difficult to discern as there are many layers of complex interactions across trophic levels that complicate study. Most influential of these interactions are the widely varying relationships between insect herbivores and mutualists and their plant hosts. We believe that the variable results of studies that have attempted to quantify the costs and trade-offs associated with defense stem from the methods used to quantify defense and a lack of appropriate control of environmental conditions. In this experiment, we examined the varying response of two closely related species in the genus Piper (Piper cenocladum, which has a well-documented ant mutualism with Pheidole bicornis and Piper imperiale, which is facultatively inhabited by ants) to different light, fertilizer and ant treatments in a factorial design. In order to examine how ant mutualism alters the trade-offs between defense and other plant functions, we measured plant growth, photosynthetic rate, chlorophyll content, and herbivory. We found that the growth of these two species is limited by different factors, with P. cenocladum being limited by nutrient availability and P. imperiale being limited by light availability.
1 Dept of Biological and Environemntal Science, Samford University, Birmingham, AL; 2 Organization for Tropical Studies, Costa Rica
Approximately 500 species of plants are known to hyperaccumulate heavy metals and metalloids. The majority are obligate metallophytes, species that are restricted to metalliferous soils. However, a smaller but increasing list of plants are facultative hyperaccumulators that hyperaccumulate heavy metals when occurring on metalliferous soils, yet also occur commonly on normal, non-metalliferous soils. This presentation will describe the biology of facultative hyperaccumulators and the opportunities they provide for ecological and evolutionary research. The existence of facultative hyperaccumulator populations across a wide edaphic range allows intraspecific comparisons of tolerance and uptake physiology. This approach has been used to study zinc and cadmium hyperaccumulation by Noccaea (Thlaspi) caerulescens and Arabidopsishalleri, and it will be instructive to make similar comparisons on species that are distributed even more abundantly on normal soil. Over 90% of known hyperaccumulators occur on serpentine (ultramafic) soil and accumulate nickel, yet there have paradoxically been few experimental studies of facultative nickel hyperaccumulation. Several hypotheses that have been suggested to explain the evolution of hyperaccumulation seem unlikely when most populations of a species occur on normal soil, where plants cannot hyperaccumulate due to low metal availability. In such species, it may be that hyperaccumulation is an ancestral phylogenetic trait or an anomalous manifestation of physiological mechanisms evolved on normal soils, and may or may not have direct adaptive benefits.
