The Pneumatron: An automated pneumatic apparatus for estimating xylem vulnerability to embolism at high temporal resolution.

Xylem vulnerability to embolism represents an important trait to determine species distribution patterns and drought resistance. However, estimating embolism resistance frequently requires time-consuming and ambiguous hydraulic lab measurements. Based on a recently developed pneumatic method, we present and test the "Pneumatron", a device that generates high time-resolution and fully automated vulnerability curves. Embolism resistance is estimated by applying a partial vacuum to extract air from an excised xylem sample, while monitoring the pressure change over time. Although the amount of gas extracted is strongly correlated with the percentage loss of xylem conductivity, validation of the Pneumatron was performed by comparison with the optical method for Eucalyptus camaldulensis leaves. The Pneumatron improved the precision of the pneumatic method considerably, facilitating the detection of small differences in the (percentage of air discharged [PAD] < 0.47%). Hence, the Pneumatron can directly measure the 50% PAD without any fitting of vulnerability curves. PAD and embolism frequency based on the optical method were strongly correlated (r2 = 0.93) for E. camaldulensis. By providing an open source platform, the Pneumatron represents an easy, low-cost, and powerful tool for field measurements, which can significantly improve our understanding of plant-water relations and the mechanisms behind embolism.

Drought sensitivity shapes species distribution patterns in tropical forests.

Although patterns of tree species distributions along environmental gradients have been amply documented in tropical forests, mechanisms causing these patterns are seldom known. Efforts to evaluate proposed mechanisms have been hampered by a lack of comparative data on species' reactions to relevant axes of environmental variation. Here we show that differential drought sensitivity shapes plant distributions in tropical forests at both regional and local scales. Our analyses are based on experimental field assessments of drought sensitivity of 48 species of trees and shrubs, and on their local and regional distributions within a network of 122 inventory sites spanning a rainfall gradient across the Isthmus of Panama. Our results suggest that niche differentiation with respect to soil water availability is a direct determinant of both local- and regional-scale distributions of tropical trees. Changes in soil moisture availability caused by global climate change and forest fragmentation are therefore likely to alter tropical species distributions, community composition and diversity.

Short dry spells in the wet season increase mortality of tropical pioneer seedlings.

Variation in plant species performance in response to water availability offers a potential axis for temporal and spatial habitat partitioning and may therefore affect community composition in tropical forests. We hypothesized that short dry spells during the wet season are a significant source of mortality for the newly emerging seedlings of pioneer species that recruit in treefall gaps in tropical forests. An analysis of a 49-year rainfall record for three forests across a rainfall gradient in central Panama confirmed that dry spells of > or = 10 days during the wet season occur on average once a year in a deciduous forest, and once every other year in a semi-deciduous moist and an evergreen wet forest. The effect of wet season dry spells on the recruitment of pioneers was investigated by comparing seedling survival in rain-protected dry plots and irrigated control plots in four large artificially created treefall gaps in a semi-deciduous tropical forest. In rain-protected plots surface soil layers dried rapidly, leading to a strong gradient in water potential within the upper 10 cm of soil. Seedling survival for six pioneer species was significantly lower in rain-protected than in irrigated control plots after only 4 days. The strength of the irrigation effect differed among species, and first became apparent 3-10 days after treatments started. Root allocation patterns were significantly, or marginally significantly, different between species and between two groups of larger and smaller seeded species. However, they were not correlated with seedling drought sensitivity, suggesting allocation is not a key trait for drought sensitivity in pioneer seedlings. Our data provide strong evidence that short dry spells in the wet season differentially affect seedling survivorship of pioneer species, and may therefore have important implications to seedling demography and community dynamics.

Desiccation tolerance of five tropical seedlings in panama. Relationship to a field assessment of drought performance.

Studies of the desiccation tolerance of the seedlings of five tropical trees were made on potted plants growing in a greenhouse. Pots were watered to field capacity and then dehydrated for 3 to 9 weeks to reach various visual wilting stages, from slightly wilted to dead. Saturated root hydraulic conductance was measured with a high-pressure flowmeter, and whole-stem hydraulic conductance was measured by a vacuum chamber method. Leaf punches (5.6-mm diameter) were harvested for measurement of leaf water potential by a thermocouple psychrometer method and for measurement of fresh and dry weight. In a parallel study, the same five species were studied in a field experiment in the understory of a tropical forest, where these species frequently germinate. Control seedlings were maintained in irrigated plots during a dry season, and experimental plants were grown in similar plots with rain exclusion shelters. Every 2 to 4 weeks, the seedlings were scored for wilt state and survivorship. After a 22-week drought, the dry plots were irrigated for several weeks to verify visual symptoms of death. The field trials were used to rank drought performance of species, and the greenhouse desiccation studies were used to determine the conditions of moribund plants. Our conclusion is that the desiccation tolerance of moribund plants correlated with field assessment of drought-performance for the five species (r(2) > 0.94).

Drought until death do us part: a case study of the desiccation-tolerance of a tropical moist forest seedling-tree, Licania platypus (Hemsl.) Fritsch.

Studies of the desiccation tolerance of 15-month-old Licania platypus (Hemsl.) Fritsch seedlings were performed on potted plants. Pots were watered to field capacity and then dehydrated for 23-46 d to reach various visible wilting stages from slightly-wilted to dead. Root hydraulic conductance, k(r), was measured with a high-pressure flow meter and whole-stem hydraulic conductance, k(ws), was measured by a vacuum chamber method. Leaf punches were harvested for measurement of leaf water potential by a thermocouple psychrometer and for measurement of fresh- and dry-weight. L. platypus was surprisingly desiccation-tolerant, suggesting that most species of central Panama may be well adapted to the seasonality of rainfall in the region. The slightly-wilted stage corresponded to leaf water potentials and relative water contents of -2.7 MPa and 0.85, respectively, but plants did not die until these values fell to -7.5 MPa and 0.14, respectively. As desiccation proceeded k(r) and k(ws) declined relative to irrigated controls, but k(ws) was more sensitive to desiccation than k(r). Values of k(ws) declined by 70-85% in slightly-wilted to dead plants, respectively. By comparison, k(r) showed no significant change in slightly-wilted plants and fell by about 50% in plants having severely-wilted to dead shoots.

Comparison of hydraulic architecture of woody plants of differing phylogeny and growth form with special reference to freestanding and hemi-epiphytic Ficus species from Panama.

Hydraulic parameters were measured in seven species of Fiats (three free-standing and four hemi-epiphytic) on stem segments of 3-35 mm in wood diameter. Parameters measured included Kh (water flux per unit pressure gradient), Kh (leaf specific conductivity =Kh per unit leaf area), Ks (specific conductivity =Hh per unit wood cross section), and Hv (Huber value = wood cross section per unit leaf area). The hemi-epiphytes tended to have less conductive stems per unit leaf area (lower Ki and invested less wood cross section per unit leaf area (lower Hv ) than free-standing species. Hydraulic parameters of Ficus are compared to 21 other temperate and tropical species to see if there is any discernible pattern in hydraulic architecture that correlates with phylogeny, growth form or habitat occupied by diverse species. Figs, as a group, had relatively high hV . and relatively low hv compared to other tropical trees. A possible correlation between KL and transpiration and growth form is discussed.

Physiological determinants of Ficus fruit temperature and implications for survival of pollinator wasp species: comparative physiology through an energy budget approach.

Figs are completely dependent for pollen dispersal on species-specific fig-pollinating wasps that develop within developing fig fruits. These wasps are very sensitive to heat and die at temperatures only a few degrees above ambient. Such temperatures are expected and observed in objects exposed to full sunlight, as fig fruits frequently are. In detailed field and experimental studies of 11 species of Panamanian figs with fruit ranging in size from 5 mm to 50 mm in diameter, we found that both the relative and absolute contribution of transpiration to maintaining non-lethal fruit temperatures increased with fruit size. Small and large fruits reached temperatures of 3 and 8°C, respectively, above air temperature in full sunlight when transpiration was prevented by grease. The temperature reached by large, nontranspiring fruits was sufficient to kill their pollinators. Control fruits which transpired reached temperatures of 2-3°C above air temperature in sunlight, regardless of size. An analysis of the solar energy budget of fruit revealed that large fruits must transpire to maintain tolerable temperatures for the wasps because heat diffusion from fruit to air was too low to balance net radiation in sunlight. By contrast, small fruits do not need to transpire to maintain tolerable temperatures for the pollinators.

Hydraulic architecture, water relations and vulnerability to cavitation of Clusia uvitana Pittier: a C3 -CAM tropical hemiepiphyte.

Clusia uvitana Pittíer (Clusiacea) is a tropical hemiepiphyte that has been shown to display a high plasticity in the expression of CAM in response to the environment. When water is available CO2 is taken up mostly during the- day. This study of the water relations and hydraulic architecture has revealed that leaf water potentials, £ ranged from 0-7 to -0.9 MPa and changed very little with time or water availability. The absolute hydraulic conductivity of stem segments (K,) and the specific conductivity (K1 ) were comparable to many other temperate and tropical species, but the leaf specificity conductivity (K1 ) was 1/3 to 1/30 that of many other species. So stems supported high leaf areas per unit of hydraulic conductivity. C uvitana was very vulnerable to cavitation, reaching 50 % loss of hydraulic conductivity at stem £=1.3 MPa. The species survives in spite of low K1 and high xylem vulnerability, because the CAM physiology insures low transpiration rates and high ability to evade dehydration.