Biomedical subjects

Bettina M J Engelbrecht

Publications and source records attributed to Bettina M J Engelbrecht.

6 recordsLinked to original sources

PubMed2006

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.

Bombacaceae↗

PubMed2006

Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle.

Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity (Ks), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stem hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced mid-day loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle.

Phosphorus↗

PubMed2006

Differences in plant function in phosphorus- and nitrogen-limited mangrove ecosystems.

Mangrove ecosystems can be either nitrogen (N) or phosphorus (P) limited and are therefore vulnerable to nutrient pollution. Nutrient enrichment with either N or P may have differing effects on ecosystems because of underlying differences in plant physiological responses to these nutrients in either N- or P-limited settings. Using a common mangrove species, Avicennia germinans, in sites where growth was either N or P limited, we investigated differing physiological responses to N and P limitation and fertilization. We tested the hypothesis that water uptake and transport, and hydraulic architecture, were the main processes limiting productivity at the P-limited site, but that this was not the case at the N-limited site. We found that plants at the P-deficient site had lower leaf water potential, stomatal conductance and photosynthetic carbon-assimilation rates, and less conductive xylem, than those at the N-limited site. These differences were greatly reduced with P fertilization at the P-limited site. By contrast, fertilization with N at the N-limited site had little effect on either photosynthetic or hydraulic traits. We conclude that growth in N- and P-limited sites differentially affect the hydraulic pathways of mangroves. Plants experiencing P limitation appear to be water deficient and undergo more pronounced changes in structure and function with relief of nutrient deficiency than those in N-limited ecosystems.

Belize↗

PubMed2003

Comparative drought-resistance of seedlings of 28 species of co-occurring tropical woody plants.

Quantifying plant drought resistance is important for understanding plant species' association to microhabitats with different soil moisture availability and their distribution along rainfall gradients, as well as for understanding the role of underlying morphological and physiological mechanisms. The effect of dry season drought on survival and leaf-area change of first year seedlings of 28 species of co-occurring woody tropical plants was experimentally quantified in the understory of a tropical moist forest. The seedlings were subjected to a drought or an irrigation treatment in the forest for 22 weeks during the dry season. Drought decreased survival and growth (assessed as leaf-area change) in almost all of the species. Both survival and leaf-area change in the dry treatment ranged fairly evenly from 0% to about 100% of that in the irrigated treatment. In 43% of the species the difference between treatments in survival was not significant even after 22 weeks. In contrast, only three species showed no significant effect of drought on leaf-area change. The effects of drought on species' survival and growth were not correlated with each other, reflecting different strategies in response to drought. Seedling size at the onset of the dry season had no significant effect on species' drought response. Our study is the first to comparatively assess seedling drought resistance in the habitat for a large number of tropical species, and underlines the importance of drought for plant population dynamics in tropical forests.

Adaptation, Physiological↗

PubMed2003

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).

Desiccation↗

PubMed2002

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.

Adaptation, Physiological↗