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Plant-Water Relations in Seasonally Dry Tropical Montane Cloud Forests

  • Author(s): Goldsmith, Gregory Rubin
  • Advisor(s): Dawson, Todd E
  • et al.
Abstract

Seasonal changes in water availability have been demonstrated to play a fundamental role in determining plant survival-mortality dynamics in a number of different ecosystems. Tropical montane cloud forests are often considered to be aseasonal environments that do not experience significant water deficits; however, there is growing recognition that many of these rare ecosystems experience one or more dry seasons annually. Moreover, many tropical montane cloud forests are projected to experience decreases in dry season precipitation and increases in dry season temperature as a function of climate change. While the regular presence of clouds may mitigate plant water stress occurring during the dry season, clouds are also projected to decrease in frequency, intensity and cover. At present, little is known about the plant-climate interactions in tropical montane cloud forests.

In order to improve our understanding of the effects of climate change on tropical montane cloud forests, I studied how seasonal changes in water availability affect plant functioning using observational and experimental approaches at a number of different scales:

In Chapter 1, I present the results from a study on tropical montane cloud forest ecohydrology. Despite longstanding recognition of the unique nature of hydrologic cycling in tropical montane forests, comprehensive and comparative studies remain limited. I studied the intra- and inter-annual variation in the inputs, pools and fluxes of water in a seasonally dry tropical montane cloud forest near Veracruz, Mexico using hydrogen and oxygen stable isotope ratios in water. There was significant seasonal variation in the hydrogen and oxygen stable isotope ratios of precipitation inputs driven by differences in the origin and size of storm events. This variation facilitated the separation of different pools of water, revealing the presence of two separate soil water pools, one highly mobile pool contributing to streams and a second less mobile pool being used by plants. At the peak of the dry season, the predominant deciduous and evergreen tree species were accessing shallow soil water from this second, less mobile pool. The results provide a foundation upon which to better understand the coupling between the hydrology and ecology of tropical montane cloud forests now and given projected scenarios of climate change.

In Chapter 2, I present the results from a study on the effects of a sustained decrease in water availability on the growth and physiology of tropical montane cloud forest plant seedlings. Research on the functional response of tropical plants to seasonal changes in water availability has largely focused on tropical lowland ecosystems. I conducted an experimental dry-down of seedlings from four common tropical montane plant species in the genus Ocotea at a common site near Monteverde, Costa Rica. Despite only a small decrease in soil water availability, plants subjected to the experimental dry down demonstrated species-specific reductions in both physiology and growth. The results indicate that water is likely to

play a strong role in tropical montane cloud forest plant functioning.

In Chapter 3, I present the results from a study on the interactions between clouds and tropical montane cloud forest plants, with a focus on the prevalence and significance of foliar water uptake. Foliar water uptake, the direct uptake of water accumulated on leaf surfaces into leaves, is a common phenomenon in ecosystems that experience frequent fog or cloud immersion, but has not been studied in tropical montane cloud forests. I quantified cloud cover patterns in two neighboring, seasonally dry tropical montane cloud forests near Monteverde, Costa Rica using remote sensing data. I then correlated these patterns with ground-based observations of leaf wetting occurring due to the physical impaction of cloud moisture on leaf surfaces. During the dry season, when rainfall is reduced and leaf wetting due to clouds is the primary source of water in the ecosystem, leaf wetting events resulted in foliar water uptake in all species studied. While all the species demonstrated the capacity to improve their leaf water potential as a result of foliar water uptake, this capacity differed between the two forests and among the species studied within a forest. The results indicate that changes in the frequency, intensity, and duration of cloud cover projected to occur as a function of regional warming will not affect all species or forests equally.

In Chapter 4, I present the results from a study on the effects of leaf wetting events and the resultant foliar water uptake on leaf water pressure-volume relations. Research on foliar water uptake has almost exclusively focused on the implications of additional water for leaf water potential and its impacts on photosynthetic performance. However, the study of pressure-volume relations can provide important insights into whether foliar water uptake alters traits responsible for the movement and conservation of water in leaves. I compared the pressure-volume relations of leaves rehydrated through both xylem and foliar water uptake compared to xylem alone in four tropical montane plant species near Monteverde, Costa Rica. Rehydration through both pathways caused inconsistent differences among species; however, one species demonstrated a decrease in the modulus of elasticity and an increase in capacitance that may be a function of differences in leaf anatomy. The results indicate that changes in pressure-volume traits associated with leaf wetting and foliar water uptake may have consequences for plant functioning and suggest that future research should more explicitly consider the role of leaf wetting in plant-water relations.

Taken together, these studies suggest that the annual dry season experienced by the tropical montane cloud forests studied herein result in plant water deficits that impact plant functioning. However, these plant water deficits can be mitigated by the presence of clouds. The species-specific nature of many of the results indicate that projected changes in dry season water availability may alter plant survival-mortality dynamics and thus affect the species composition of tropical montane cloud forests in the future.

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