This review illuminates key findings in our understanding of grapevine xylem resistance to fungal... more This review illuminates key findings in our understanding of grapevine xylem resistance to fungal vascular wilt diseases. Grapevine (Vitis spp.) vascular diseases such as esca, botryosphaeria dieback, and eutypa dieback, are caused by a set of taxonomically unrelated ascomycete fungi. Fungal colonization of the vascular system leads to a decline of the plant host because of a loss of the xylem function and subsequent decrease in hydraulic conductivity. Fungal vascular pathogens use different colonization strategies to invade and kill their host. Vitis vinifera cultivars display different levels of tolerance toward vascular diseases caused by fungi, but the plant defense mechanisms underlying those observations have not been completely elucidated. In this review, we establish a parallel between two vascular diseases, grapevine esca disease and Dutch elm disease, and argue that the former should be viewed as a vascular wilt disease. Plant genotypes exhibit differences in xylem morphol...
Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability... more Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability and limitation, but the foliar ratios of woody plants have proven more complicated to interpret than ratios from whole biomass of herbaceous species. This may be related to tissues in woody species acting as nutrient reservoirs during active growth, allowing maintenance of optimal N:P ratios in recently produced, fully expanded leaves (i.e., "new" leaves, the most commonly sampled tissue). Here we address the hypothesis that N:P ratios of newly expanded leaves are less sensitive indicators of soil nutrient availability than are other tissue types in woody plants. Seedlings of five naturally established tree species were harvested from plots receiving two years of fertilizer treatments in a lowland tropical forest in the Republic of Panama. Nutrient concentrations were determined in new leaves, old leaves, stems, and roots. For stems and roots, N:P ratios increased after N addi...
ABSTRACT Background/Question/Methods Plant functional traits are quantitative traits that relate ... more ABSTRACT Background/Question/Methods Plant functional traits are quantitative traits that relate to a species’ ecological role within an environment. These traits are often coordinated, as demonstrated by the ‘leaf economic spectrum,’ seed mass to output, and others. In this study, we examined how the functional traits of wood density and leaf mass per area are correlated with each other and with seasonal diurnal water potentials. Water potentials are a measure of plant water status, and vary based on environmental conditions and species physiological processes. We measured woody density, leaf mass per area, and wet and dry season diurnal stem and leaf water potentials for 17 chaparral and coastal sage scrub species at the Santa Margarita Ecological Reserve in California. We hypothesized that wood density and leaf mass per area would be correlated with dry season rather than wet season water potentials if these functional traits serve these species survival during annual seasonal drought. Alternatively, we hypothesized that these functional traits would be better correlated with wet season water potentials if these traits serve to optimize resource capture when conditions are favorable. Results/Conclusions Woody density and leaf mass per area were positively correlated with each other. Leaf mass area was negatively correlated with wet season predawn stem and leaf water potential and midday stem water potential. Leaf mass per area was not correlated with any dry season water potential measurements. Wood density was negatively correlated with wet season predawn and midday stem and leaf water potentials, and dry season midday stem water potential. Woody density was not correlated with dry season predawn stem or leaf water potentials, or midday leaf water potential. Overall, our functional traits were correlated with more wet season measures of water potential than dry season water potentials. This supports our alternative hypothesis that wood density and leaf mass per area are optimized for resource capture during favorable environmental conditions rather than survival during stressful drought conditions, when many of these chaparral and coastal sage scrub species “shut down.”
bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence... more bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the ''leaf economic spectrum,'' characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient 3 growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested.
ABSTRACT This chapter is organized along increasing levels of plant morphological complexity. Fir... more ABSTRACT This chapter is organized along increasing levels of plant morphological complexity. First, it addresses leaves, then moves to the supply-and-demand coordination between stem and leaf water transport, and, finally, the whole plant scale. The chapter addresses the physiological implications of the liana growth form with an emphasis on how being a liana leads to certain physiological outcomes. Comparisons are made with trees to encompass the competitive framework that structures the co-dominance of woody growth forms in tropical forest. The chapter attempts to pull together demonstrated physiological affinities of lianas and use this information to evaluate physiological mechanisms for liana abundance patterns in tropical forest. It focuses on woody species, which are the major lianoid forms of tropical forest. Overall, the available data comparing the physiological propensity of lianas relative to trees provide several illuminating patterns that could help explain curiosities in the abundance and distribution of lianas worldwide.
Coordination of water movement among plant organs is important for understanding plant water use ... more Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, 'expendable' organs such as leaves and longer lived, more 'expensive' organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or 'safety valve'. We tested the HSH in woody species from a Mediterranean-type ecosystem by measuring leaf hydraulic conductance (Kleaf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kleaf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψstem-Ψ50)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole-plant water use strategies. For hydrated shoots, Kleaf was negatively correlated with KS , supporting the HSH. Additionally, Kleaf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio. Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS , or a low safety margin. This critical role of leaves appears to contribute importantly to plant ecological specialization in a drought-prone environment.
Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands ... more Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands growing in East Maui, Hawaii were investigated to assess physiological limitations associated with flooding as a mechanism of reduced canopy leaf area in waterlogged sites. Whole-tree sap flow, stomatal conductance, microclimate, soil oxidation-reduction potential, stand basal area and leaf area index (LAI) were measured on moderately sloped, drained sites with closed canopies (90%) and on level, waterlogged sites with open canopies (50-60%). The LAI was measured with a new technique based on enlarged photographs of individual tree crowns and allometric relationships. Sap flow was scaled to the stand level by multiplying basal area-normalized sap flow by stand basal area. Level sites had lower soil redox potentials, lower mean stand basal area, lower LAI, and a higher degree of soil avoidance by roots than sloped sites. Foliar nutrients and leaf mass per area (LMA) in M. polymorpha were similar between level and sloped sites. Stomatal conductance was similar for M. polymorpha saplings on both sites, but decreased with increasing tree height (r 2 = 0.72; P < 0.001). Stand transpiration estimates ranged from 79 to 89% of potential evapotranspiration (PET) for sloped sites and from 28 to 51% of PET for level sites. Stand transpiration estimates were strongly correlated with LAI (r 2 = 0.96; P < 0.001). Whole-tree transpiration was lower at level sites with waterlogged soils, but was similar or higher for trees on level sites when normalized by leaf area. Trees on level sites had a smaller leaf area per stem diameter than trees on sloped sites, suggesting that soil oxygen deficiency may reduce leaf area. However, transpiration per unit leaf area did not vary substantially, so leaf-level physiological behavior was conserved, regardless of differences in tree leaf area.
Although crown wetting events can increase plant water status, leaf wetting is thought to negativ... more Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.
Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem... more Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem pressure declines. Several techniques for constructing vulnerability curves use centrifugal force to induce negative xylem pressure in stem or root segments. Centrifuge vulnerability curves constructed for long-vesselled species have been hypothesised to overestimate xylem vulnerability to cavitation due to increased vulnerability of vessels cut open at stem ends that extend to the middle or entirely through segments. We tested two key predictions of this hypothesis: (i) centrifugation induces greater embolism than dehydration in long-vesselled species, and (ii) the proportion of open vessels changes centrifuge vulnerability curves. Centrifuge and dehydration vulnerability curves were compared for a long- and short-vesselled species. The effect of open vessels was tested in four species by comparing centrifuge vulnerability curves for stems of two lengths. Centrifuge and dehydration vulnerability curves agreed well for the long- and short-vesselled species. Centrifuge vulnerability curves constructed using two stem lengths were similar. Also, the distribution of embolism along the length of centrifuged stems matched the theoretical pressure profile induced by centrifugation. We conclude that vulnerability to cavitation can be accurately characterised with vulnerability curves constructed using a centrifuge technique, even in long-vesselled species.
The proportional light absorptance by photosynthetic tissue (α) is used with chlorophyll (Chl) fl... more The proportional light absorptance by photosynthetic tissue (α) is used with chlorophyll (Chl) fluorescence methods to calculate electron transport rate (ETR). Although a value of α of 0.84 is often used as a standard for calculating ETR, many succulent plant species and species with crassulacean acid metabolism (CAM) have photosynthetic tissues that vary greatly in color or are highly reflective, and could have values of α that differ from 0.84, thus affecting the calculation of ETR. We measured ETR using Chl fluorescence and α using an integrating sphere in 58 plant species to determine the importance of applying a measured value of α when calculating ETR. Values of α varied from 0.55-0.92 with a mean of 0.82 across species. Differences between ETR values calculated with measured α values ranged from 53% lower to 12% greater than ETR values calculated with a standard α value of 0.84 and were significantly different in 39 out of 58 species. While measurements of ETR using Chl fluorescence represent a rapid and effective assessment of physiological performance, the value of α needs to be considered. Measurements of α, especially on species with light-colored or reflective photosynthetic tissue, will allow more accurate determination of photosynthesis in succulent and CAM species.
We studied C stable isotopic composition (d 13 C) of bulk leaf tissue and extracted sugars of fou... more We studied C stable isotopic composition (d 13 C) of bulk leaf tissue and extracted sugars of four epiphytic Tillandsia species to investigate flexibility in the use of crassulacean acid metabolism (CAM) and C 3 photosynthetic pathways. Plants growing in two seasonally dry tropical forest reserves in Mexico that differ in annual precipitation were measured during wet and dry seasons, and among secondary, mature, and wetland forest types within each site. Dry season sugars were more enriched in 13 C than wet season sugars, but there was no seasonal difference in bulk tissues. Bulk tissue d 13 C differed by species and by forest type, with values from open-canopied wetlands more enriched in 13 C than mature or secondary forest types. The shifts within forest habitat were related to temporal and spatial changes in vapor pressure deficits (VPD). Modeling results estimate a possible 4% increase in the proportional contribution of the C 3 pathway during the wet season, emphasizing that any seasonal or habitat-mediated variation in photosynthetic pathway appears to be quite moderate and within the range of isotopic effects caused by variation in stomatal conductance during assimilation through the C 3 pathway and environmental variation in VPD. C isotopic analysis of sugars together with bulk leaf tissue offers a useful approach for incorporating short-and long-term measurements of C isotope discrimination during photosynthesis.
We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian for... more We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost-benefit theories of leaf life span. Nitrogen content per unit mass (N mass ) and light-and CO 2 -saturated photosynthetic rates per unit mass (P mass ) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P mass , N mass , and SLA at wetter sites. Shorter leaf life spans and more enriched foliar δ 15 N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods.
This study addresses patterns of nutrient dynamics on a precipitation gradient (1800-3500 mm y −1... more This study addresses patterns of nutrient dynamics on a precipitation gradient (1800-3500 mm y −1 ) in lowland tropical forest with heterogeneous soil parent material, high plant species diversity and large changes in species composition. Mean foliar concentrations of phosphorus, potassium, calcium and magnesium decreased with increasing precipitation, whereas foliar carbon:nitrogen increased with increasing precipitation. Mean foliar nitrogen:phosphorus varied from 16.4-23.8 suggesting that plant productivity at these sites is limited by phosphorus. Total soil nitrogen increased as a function of foliar litter lignin:nitrogen, whereas net nitrogen mineralization rates decreased with increasing lignin:N indicating that as litter quality decreases, more soil nitrogen is held in soil organic matter and the mineralization of that nitrogen is slower. Extractable phosphorus in soil was negatively correlated with foliar litter lignin:phosphorus, illustrating effects of litter quality on soil phosphorus availability. Overall, the results suggest that variation in plant community composition along this precipitation gradient is tightly coupled with soil nutrient cycling. Much of our understanding of effects of precipitation on nutrient cycling in tropical forest is based on precipitation gradients across montane forest in Hawaii, where species composition and soil parent material are constant. Our results suggest that variation in parent material or species composition may confound predictions developed in model island systems. Resumen: Este estudio trata sobre los patrones de dinámica de nutrientes en un gradiente de precipitación (1800-3500 mm y −1 ) en un bosque tropical de tierras bajas con material parental heterogéneo, alta diversidad de especies de plantas, y un gran cambio en la composición de especies. Las concentraciones promedio de fósforo, potasio, calcio y magnesio disminuyen con un aumento en la precipitación, mientras que la proporción carbono:nitrógeno aumenta con un aumento en la precipitación. La proporción promedio de nitrógeno foliar:fósforo varía de 16.4 a 23.8, lo cual sugiere que estos sitios tienen limitaciones de fósforo. El nitrógeno total del suelo aumenta como una función de la proporción de lignina foliar de la hojarasca:nitrógeno, mientras que la tasa de mineralización del nitrógeno neto disminuye con un aumento de la proporción de lignina:nitrógeno, lo cual indica que mientras la calidad de la hojarasca disminuye, más nitrógeno del suelo es retenido en la materia orgánica y la mineralización de ese nitrógeno es más lenta. El fósforo extraíble está correlacionado negativamente con la proporción de lignina foliar de la hojarasca:fósforo, lo cual ilustra el efecto de la calidad de hojarasca en la disponibilidad de fósforo del suelo. En resumen, estos resultados sugieren que la variación en la composición de plantas de una comunidad a lo largo de este gradiente de precipitación tiene un efecto considerable en la retroalimentación de los ciclos nutricionales del suelo. Gran parte de nuestros conocimientos sobre el efecto de los gradientes de lluvia en ciclos nutricionales del suelo han sido basados en estudios de gradientes de precipitación de bosques de montaña en Hawaii, en donde tanto la composición de especies y el material parental del suelo son constantes. Nuestros resultados sugieren que la variación del material parental del suelo y la composición de especies pueden confundir las predicciones desarrolladas en sistemas modelos de islas.
Structure and demographics in many tropical forests is changing, but the causes of these changes ... more Structure and demographics in many tropical forests is changing, but the causes of these changes remain unclear. We studied 5 y (2005-2010) of species turnover, recruitment, mortality and population change data from a 20-ha subtropical forest plot in Dinghushan, China, to identify trends in forest change, and to test whether tree mortality is associated with intraspecific or interspecific competition. We found the Dinghushan forest to be more dynamic than one temperate and two tropical forests in a comparison of large, long-term forest dynamics plots. Within Dinghushan, size-class distributions were bell-shaped only for the three most dominant species and reverse J-shaped for other species. Bell-shaped population distributions can indicate a population in decline, but our data suggest that these large and long-lived species are not in decline because the pattern is driven by increasing probabilities of transition to larger size class with increasing size and fast growth in saplings. Spatially aggregated tree species distributions were common for surviving and dead individuals. Competitive associations were more frequently intraspecific than interspecific. The competition that induced tree mortality was more associated with intraspecific than interspecific interactions. Intraspecific competitive exclusion and density-dependence appear to play important roles in tree mortality in this subtropical forest.
Aroma is an important quality factor in foods. The aroma of bee honey depends on volatile fractio... more Aroma is an important quality factor in foods. The aroma of bee honey depends on volatile fraction composition, which is influenced by nectar composition and floral origin. Honey of unifloral origin usually commands higher commercial value, thus the floral determi- nation and certification of unifloral honey plays an important role in quality control. This review concerns investigations made on the volatile fraction of bee honey by gas chromatography/mass spectrometry. Recent advances in extraction methods, results achieved, and comparisons of alternative dependable methods for determining floral origin of bee honey are discussed. We emphasize solid phase micro-extraction gas chromatography (SPME/GC) methodology and present some of the results obtained to date, plus the advantages and drawbacks of SPME/GS in comparison with other methods. 2006 Elsevier Ltd. All rights reserved.
This review illuminates key findings in our understanding of grapevine xylem resistance to fungal... more This review illuminates key findings in our understanding of grapevine xylem resistance to fungal vascular wilt diseases. Grapevine (Vitis spp.) vascular diseases such as esca, botryosphaeria dieback, and eutypa dieback, are caused by a set of taxonomically unrelated ascomycete fungi. Fungal colonization of the vascular system leads to a decline of the plant host because of a loss of the xylem function and subsequent decrease in hydraulic conductivity. Fungal vascular pathogens use different colonization strategies to invade and kill their host. Vitis vinifera cultivars display different levels of tolerance toward vascular diseases caused by fungi, but the plant defense mechanisms underlying those observations have not been completely elucidated. In this review, we establish a parallel between two vascular diseases, grapevine esca disease and Dutch elm disease, and argue that the former should be viewed as a vascular wilt disease. Plant genotypes exhibit differences in xylem morphol...
Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability... more Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability and limitation, but the foliar ratios of woody plants have proven more complicated to interpret than ratios from whole biomass of herbaceous species. This may be related to tissues in woody species acting as nutrient reservoirs during active growth, allowing maintenance of optimal N:P ratios in recently produced, fully expanded leaves (i.e., "new" leaves, the most commonly sampled tissue). Here we address the hypothesis that N:P ratios of newly expanded leaves are less sensitive indicators of soil nutrient availability than are other tissue types in woody plants. Seedlings of five naturally established tree species were harvested from plots receiving two years of fertilizer treatments in a lowland tropical forest in the Republic of Panama. Nutrient concentrations were determined in new leaves, old leaves, stems, and roots. For stems and roots, N:P ratios increased after N addi...
ABSTRACT Background/Question/Methods Plant functional traits are quantitative traits that relate ... more ABSTRACT Background/Question/Methods Plant functional traits are quantitative traits that relate to a species’ ecological role within an environment. These traits are often coordinated, as demonstrated by the ‘leaf economic spectrum,’ seed mass to output, and others. In this study, we examined how the functional traits of wood density and leaf mass per area are correlated with each other and with seasonal diurnal water potentials. Water potentials are a measure of plant water status, and vary based on environmental conditions and species physiological processes. We measured woody density, leaf mass per area, and wet and dry season diurnal stem and leaf water potentials for 17 chaparral and coastal sage scrub species at the Santa Margarita Ecological Reserve in California. We hypothesized that wood density and leaf mass per area would be correlated with dry season rather than wet season water potentials if these functional traits serve these species survival during annual seasonal drought. Alternatively, we hypothesized that these functional traits would be better correlated with wet season water potentials if these traits serve to optimize resource capture when conditions are favorable. Results/Conclusions Woody density and leaf mass per area were positively correlated with each other. Leaf mass area was negatively correlated with wet season predawn stem and leaf water potential and midday stem water potential. Leaf mass per area was not correlated with any dry season water potential measurements. Wood density was negatively correlated with wet season predawn and midday stem and leaf water potentials, and dry season midday stem water potential. Woody density was not correlated with dry season predawn stem or leaf water potentials, or midday leaf water potential. Overall, our functional traits were correlated with more wet season measures of water potential than dry season water potentials. This supports our alternative hypothesis that wood density and leaf mass per area are optimized for resource capture during favorable environmental conditions rather than survival during stressful drought conditions, when many of these chaparral and coastal sage scrub species “shut down.”
bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence... more bstract. Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the ''leaf economic spectrum,'' characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient 3 growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested.
ABSTRACT This chapter is organized along increasing levels of plant morphological complexity. Fir... more ABSTRACT This chapter is organized along increasing levels of plant morphological complexity. First, it addresses leaves, then moves to the supply-and-demand coordination between stem and leaf water transport, and, finally, the whole plant scale. The chapter addresses the physiological implications of the liana growth form with an emphasis on how being a liana leads to certain physiological outcomes. Comparisons are made with trees to encompass the competitive framework that structures the co-dominance of woody growth forms in tropical forest. The chapter attempts to pull together demonstrated physiological affinities of lianas and use this information to evaluate physiological mechanisms for liana abundance patterns in tropical forest. It focuses on woody species, which are the major lianoid forms of tropical forest. Overall, the available data comparing the physiological propensity of lianas relative to trees provide several illuminating patterns that could help explain curiosities in the abundance and distribution of lianas worldwide.
Coordination of water movement among plant organs is important for understanding plant water use ... more Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;expendable&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; organs such as leaves and longer lived, more &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;expensive&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;safety valve&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;. We tested the HSH in woody species from a Mediterranean-type ecosystem by measuring leaf hydraulic conductance (Kleaf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kleaf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψstem-Ψ50)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole-plant water use strategies. For hydrated shoots, Kleaf was negatively correlated with KS , supporting the HSH. Additionally, Kleaf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio. Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS , or a low safety margin. This critical role of leaves appears to contribute importantly to plant ecological specialization in a drought-prone environment.
Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands ... more Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands growing in East Maui, Hawaii were investigated to assess physiological limitations associated with flooding as a mechanism of reduced canopy leaf area in waterlogged sites. Whole-tree sap flow, stomatal conductance, microclimate, soil oxidation-reduction potential, stand basal area and leaf area index (LAI) were measured on moderately sloped, drained sites with closed canopies (90%) and on level, waterlogged sites with open canopies (50-60%). The LAI was measured with a new technique based on enlarged photographs of individual tree crowns and allometric relationships. Sap flow was scaled to the stand level by multiplying basal area-normalized sap flow by stand basal area. Level sites had lower soil redox potentials, lower mean stand basal area, lower LAI, and a higher degree of soil avoidance by roots than sloped sites. Foliar nutrients and leaf mass per area (LMA) in M. polymorpha were similar between level and sloped sites. Stomatal conductance was similar for M. polymorpha saplings on both sites, but decreased with increasing tree height (r 2 = 0.72; P < 0.001). Stand transpiration estimates ranged from 79 to 89% of potential evapotranspiration (PET) for sloped sites and from 28 to 51% of PET for level sites. Stand transpiration estimates were strongly correlated with LAI (r 2 = 0.96; P < 0.001). Whole-tree transpiration was lower at level sites with waterlogged soils, but was similar or higher for trees on level sites when normalized by leaf area. Trees on level sites had a smaller leaf area per stem diameter than trees on sloped sites, suggesting that soil oxygen deficiency may reduce leaf area. However, transpiration per unit leaf area did not vary substantially, so leaf-level physiological behavior was conserved, regardless of differences in tree leaf area.
Although crown wetting events can increase plant water status, leaf wetting is thought to negativ... more Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.
Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem... more Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem pressure declines. Several techniques for constructing vulnerability curves use centrifugal force to induce negative xylem pressure in stem or root segments. Centrifuge vulnerability curves constructed for long-vesselled species have been hypothesised to overestimate xylem vulnerability to cavitation due to increased vulnerability of vessels cut open at stem ends that extend to the middle or entirely through segments. We tested two key predictions of this hypothesis: (i) centrifugation induces greater embolism than dehydration in long-vesselled species, and (ii) the proportion of open vessels changes centrifuge vulnerability curves. Centrifuge and dehydration vulnerability curves were compared for a long- and short-vesselled species. The effect of open vessels was tested in four species by comparing centrifuge vulnerability curves for stems of two lengths. Centrifuge and dehydration vulnerability curves agreed well for the long- and short-vesselled species. Centrifuge vulnerability curves constructed using two stem lengths were similar. Also, the distribution of embolism along the length of centrifuged stems matched the theoretical pressure profile induced by centrifugation. We conclude that vulnerability to cavitation can be accurately characterised with vulnerability curves constructed using a centrifuge technique, even in long-vesselled species.
The proportional light absorptance by photosynthetic tissue (α) is used with chlorophyll (Chl) fl... more The proportional light absorptance by photosynthetic tissue (α) is used with chlorophyll (Chl) fluorescence methods to calculate electron transport rate (ETR). Although a value of α of 0.84 is often used as a standard for calculating ETR, many succulent plant species and species with crassulacean acid metabolism (CAM) have photosynthetic tissues that vary greatly in color or are highly reflective, and could have values of α that differ from 0.84, thus affecting the calculation of ETR. We measured ETR using Chl fluorescence and α using an integrating sphere in 58 plant species to determine the importance of applying a measured value of α when calculating ETR. Values of α varied from 0.55-0.92 with a mean of 0.82 across species. Differences between ETR values calculated with measured α values ranged from 53% lower to 12% greater than ETR values calculated with a standard α value of 0.84 and were significantly different in 39 out of 58 species. While measurements of ETR using Chl fluorescence represent a rapid and effective assessment of physiological performance, the value of α needs to be considered. Measurements of α, especially on species with light-colored or reflective photosynthetic tissue, will allow more accurate determination of photosynthesis in succulent and CAM species.
We studied C stable isotopic composition (d 13 C) of bulk leaf tissue and extracted sugars of fou... more We studied C stable isotopic composition (d 13 C) of bulk leaf tissue and extracted sugars of four epiphytic Tillandsia species to investigate flexibility in the use of crassulacean acid metabolism (CAM) and C 3 photosynthetic pathways. Plants growing in two seasonally dry tropical forest reserves in Mexico that differ in annual precipitation were measured during wet and dry seasons, and among secondary, mature, and wetland forest types within each site. Dry season sugars were more enriched in 13 C than wet season sugars, but there was no seasonal difference in bulk tissues. Bulk tissue d 13 C differed by species and by forest type, with values from open-canopied wetlands more enriched in 13 C than mature or secondary forest types. The shifts within forest habitat were related to temporal and spatial changes in vapor pressure deficits (VPD). Modeling results estimate a possible 4% increase in the proportional contribution of the C 3 pathway during the wet season, emphasizing that any seasonal or habitat-mediated variation in photosynthetic pathway appears to be quite moderate and within the range of isotopic effects caused by variation in stomatal conductance during assimilation through the C 3 pathway and environmental variation in VPD. C isotopic analysis of sugars together with bulk leaf tissue offers a useful approach for incorporating short-and long-term measurements of C isotope discrimination during photosynthesis.
We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian for... more We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost-benefit theories of leaf life span. Nitrogen content per unit mass (N mass ) and light-and CO 2 -saturated photosynthetic rates per unit mass (P mass ) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P mass , N mass , and SLA at wetter sites. Shorter leaf life spans and more enriched foliar δ 15 N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods.
This study addresses patterns of nutrient dynamics on a precipitation gradient (1800-3500 mm y −1... more This study addresses patterns of nutrient dynamics on a precipitation gradient (1800-3500 mm y −1 ) in lowland tropical forest with heterogeneous soil parent material, high plant species diversity and large changes in species composition. Mean foliar concentrations of phosphorus, potassium, calcium and magnesium decreased with increasing precipitation, whereas foliar carbon:nitrogen increased with increasing precipitation. Mean foliar nitrogen:phosphorus varied from 16.4-23.8 suggesting that plant productivity at these sites is limited by phosphorus. Total soil nitrogen increased as a function of foliar litter lignin:nitrogen, whereas net nitrogen mineralization rates decreased with increasing lignin:N indicating that as litter quality decreases, more soil nitrogen is held in soil organic matter and the mineralization of that nitrogen is slower. Extractable phosphorus in soil was negatively correlated with foliar litter lignin:phosphorus, illustrating effects of litter quality on soil phosphorus availability. Overall, the results suggest that variation in plant community composition along this precipitation gradient is tightly coupled with soil nutrient cycling. Much of our understanding of effects of precipitation on nutrient cycling in tropical forest is based on precipitation gradients across montane forest in Hawaii, where species composition and soil parent material are constant. Our results suggest that variation in parent material or species composition may confound predictions developed in model island systems. Resumen: Este estudio trata sobre los patrones de dinámica de nutrientes en un gradiente de precipitación (1800-3500 mm y −1 ) en un bosque tropical de tierras bajas con material parental heterogéneo, alta diversidad de especies de plantas, y un gran cambio en la composición de especies. Las concentraciones promedio de fósforo, potasio, calcio y magnesio disminuyen con un aumento en la precipitación, mientras que la proporción carbono:nitrógeno aumenta con un aumento en la precipitación. La proporción promedio de nitrógeno foliar:fósforo varía de 16.4 a 23.8, lo cual sugiere que estos sitios tienen limitaciones de fósforo. El nitrógeno total del suelo aumenta como una función de la proporción de lignina foliar de la hojarasca:nitrógeno, mientras que la tasa de mineralización del nitrógeno neto disminuye con un aumento de la proporción de lignina:nitrógeno, lo cual indica que mientras la calidad de la hojarasca disminuye, más nitrógeno del suelo es retenido en la materia orgánica y la mineralización de ese nitrógeno es más lenta. El fósforo extraíble está correlacionado negativamente con la proporción de lignina foliar de la hojarasca:fósforo, lo cual ilustra el efecto de la calidad de hojarasca en la disponibilidad de fósforo del suelo. En resumen, estos resultados sugieren que la variación en la composición de plantas de una comunidad a lo largo de este gradiente de precipitación tiene un efecto considerable en la retroalimentación de los ciclos nutricionales del suelo. Gran parte de nuestros conocimientos sobre el efecto de los gradientes de lluvia en ciclos nutricionales del suelo han sido basados en estudios de gradientes de precipitación de bosques de montaña en Hawaii, en donde tanto la composición de especies y el material parental del suelo son constantes. Nuestros resultados sugieren que la variación del material parental del suelo y la composición de especies pueden confundir las predicciones desarrolladas en sistemas modelos de islas.
Structure and demographics in many tropical forests is changing, but the causes of these changes ... more Structure and demographics in many tropical forests is changing, but the causes of these changes remain unclear. We studied 5 y (2005-2010) of species turnover, recruitment, mortality and population change data from a 20-ha subtropical forest plot in Dinghushan, China, to identify trends in forest change, and to test whether tree mortality is associated with intraspecific or interspecific competition. We found the Dinghushan forest to be more dynamic than one temperate and two tropical forests in a comparison of large, long-term forest dynamics plots. Within Dinghushan, size-class distributions were bell-shaped only for the three most dominant species and reverse J-shaped for other species. Bell-shaped population distributions can indicate a population in decline, but our data suggest that these large and long-lived species are not in decline because the pattern is driven by increasing probabilities of transition to larger size class with increasing size and fast growth in saplings. Spatially aggregated tree species distributions were common for surviving and dead individuals. Competitive associations were more frequently intraspecific than interspecific. The competition that induced tree mortality was more associated with intraspecific than interspecific interactions. Intraspecific competitive exclusion and density-dependence appear to play important roles in tree mortality in this subtropical forest.
Aroma is an important quality factor in foods. The aroma of bee honey depends on volatile fractio... more Aroma is an important quality factor in foods. The aroma of bee honey depends on volatile fraction composition, which is influenced by nectar composition and floral origin. Honey of unifloral origin usually commands higher commercial value, thus the floral determi- nation and certification of unifloral honey plays an important role in quality control. This review concerns investigations made on the volatile fraction of bee honey by gas chromatography/mass spectrometry. Recent advances in extraction methods, results achieved, and comparisons of alternative dependable methods for determining floral origin of bee honey are discussed. We emphasize solid phase micro-extraction gas chromatography (SPME/GC) methodology and present some of the results obtained to date, plus the advantages and drawbacks of SPME/GS in comparison with other methods. 2006 Elsevier Ltd. All rights reserved.
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Papers by Louis Santiago