Managing semi-arid oak forests (Quercus brantii Lindl.): Mature oak trees of different dimensions create contrasted microhabitats influencing seedling quality.

We investigated the influence of mature of oak trees of various dimensions on soil properties, acorn and oak seedling characteristics in semi-arid forests in western Iran. A total of 24 oak trees were selected in comparable site conditions according to three size categories: small trees (DBH< 20 cm), medium trees (DBH: 20-50 cm) and large trees (DBH> 50 cm). Soil properties, light availability below canopy, acorn dimensions and weight, various below- and above-ground seedling morphological traits were measured. Besides, a seedling quality index (SQI) was also produced as an integrative measure of the seedling response. We found an increasing light availability from small trees to large trees (1512-103 µmol m-2 s-1) and soil fertility was largely improved from small trees to large trees: soil organic carbon (1.33-2.2%), available phosphorus (12.9-18.1 ppm) and potassium (301.2-470.4 ppm). However, soil properties did not significantly differ between medium and large trees. In contrast, acorn weight and dimensions as well as many seedling traits, including the aerial and belowground biomass and the SQI, were the highest in the medium tree category. To fully explore the relationships among our large set of variables, we produced a partial least square path model which explained 72% of the variation of SQI across the three tree classes. To conclude, we identified a clear effect by mature trees which provided favourable conditions for seedling establishment, but that effect was mediated by tree size and optimal conditions were found below the canopy of medium trees.

Resources from another place and time: responses to pulses in a spatially subsidized system.

As the theoretical bases for the dynamics of spatially subsidized communities emerge, ecologists question whether spatially subsidized communities exhibit similar structure or dynamics to communities that receive strongly pulsed resources. In both cases, communities may be structured by responses to resources that are potentially absent at any given point in time (pulsed communities) or space (subsidized communities), even if pulsed resources are part of the in situ productivity of the system or the subsidies arrive as a relatively constant input from a nearby system. The potential for significant spatial or temporal resource limitation, therefore, may be a key factor influencing in similar ways the persistence of populations, the structure and dynamics of communities, and the evolution of specific life history traits. In most complex systems, however, multiple resources may arrive for various trophic entities at various points in time and from various points in space, and thus it may be difficult to separate or compare the dynamics of spatially subsidized and pulsed systems. In this paper, we explore the effects of interactions between pulses and subsidies in plant and animal populations and communities on highly pulsed and variably subsidized islands in the Gulf of California. While many of the plant and animal communities on the unsubsidized islands in this system respond to pulses of rain in classic ways, responses to these rain pulses on islands subsidized by seabird guano or other marine resources are quite different and variable, and depend on a combination of life history characteristics, physiology, competitive interactions, and trophic relationships. These variable responses to rain pulses then translate into large differences in dynamics and community structure of subsidized vs. unsubsidized islands. Indeed, most systems experience both temporal pulses and spatial subsidies. When considered in tandem, complementary or synergistic effects of the multiple, temporally and spatially variable resources may emerge that help explain complex food web structure and dynamics.

A stage-based study of drought response in Cryptantha flava (Boraginaceae): gas exchange, water use efficiency, and whole plant performance.

Models of global climate change predict an increase in the frequency of major droughts, yet we know little about the consequences of drought for the demography of natural populations. This study examined a population of the semi-desert perennial Cryptantha flava (Boraginaceae) to determine how plants of different developmental stages respond to drought through changes in leaf gas exchange, leaf water potential, water use efficiency, growth, and reproduction. In two of the four years, drought was applied using rainout shelters, and a severe natural drought occurred in another. Small, presumably younger, plants sometimes had lower rates of maximum photosynthesis, lower leaf water potentials, and lower instantaneous or integrated water-use efficiency than large plants. Small plants also had higher relative growth rates and lower reproductive effort. Large plants with evidence of shrinkage from a previously larger size often produced less growth and reproduction than large healthy plants, suggesting a decline in plant vigor with age. Drought depressed gas exchange and leaf water potentials equally in all plant stages. Thus, leaf-level physiological attributes provide no clues for why drought reduces growth more strongly in large plants. The results point to several additional avenues of research relevant to understanding stage-dependent or age-dependent plant performance under drought conditions.

The fraction of expanding to expanded leaves determines the biomass response of Populus to elevated CO2.

We examined whether the effects of elevated CO2 on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for 100 days at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO2 in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected by elevated CO2. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly greater in expanding leaves of saplings grown at elevated compared to ambient CO2. The Rubisco content in expanding leaves was not affected by CO2 concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated compared to ambient CO2, indicating CO2-induced down-regulation in fully expanded leaves. Elevated CO2 likely had no overall effect on biomass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings grown at elevated compared to ambient CO2. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source balance and acclimation. Our data suggest that variation in growth responses to elevated CO2 can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated CO2.