Kimberlyn Williams

Growth Depression in Mycorrhizal Citrus at High-Phosphorus Supply (Analysis of Carbon Costs).

Mycorrhizal-induced growth depression of plants in high-P soil has been reported in many species. The carbon costs of factors contributing to this growth depression were analyzed in Volkamer lemon (Citrus volkameriana Tan. & Pasq.) colonized by the mycorrhizal (M) fungus Glomus intraradices Schenck and Smith. M and nonmycorrhizal (NM) plants were each grown at two P-supply rates. Carbon budgets of M and NM plants were determined by measuring whole-plant carbon assimilation and respiration rates using gas-exchange techniques. Biomass, M colonization, tissue-P concentration, and total fatty acid concentration in the fibrous roots were determined. Construction costs of the fibrous roots were estimated from heat of combustion, N, and ash content. Root-growth respiration was derived from daily root growth and root-construction cost. M and NM plants grown in high-P soil were similar in P concentration, daily shoot carbon assimilation, and daily shoot dark respiration. At 52 d after transplanting (DAT), however, combined daily root plus soil respiration was 37% higher for M than for NM plants, resulting in a 20% higher daily specific carbon gain (mmol CO2 [mmol carbon]-1 d-1) in NM than M plants. Estimates of specific carbon gain from specific growth rates indicated about a 10% difference between M and NM plants. Absolute values of specific carbon gain estimated by whole-plant gas exchange and by growth analysis were in general agreement. At 52 DAT, M and NM plants at high P had nearly identical whole-plant growth rates, but M plants had 19% higher root dry weight with 10% higher daily rates of root growth. These allocation differences at high P accounted for about 51% of the differences in root/soil respiration between M and NM plants. Significantly higher fatty acid concentrations in M than NM fibrous roots were correlated with differences in construction costs of the fibrous roots. Of the 37% difference in daily total root/soil respiration observed between high-P M and NM plants at 52 DAT, estimated daily growth respiration accounted for only about 16%, two-thirds of which was associated with construction of lipid-rich roots, and the remaining one-third with greater M root growth rates. Thus, of the 37% more root/soil respiration associated with M colonization of high-P plants, 10% was directly attributable to building lipid-rich roots, 51% to greater M root biomass allocation, and the remaining 39% could have been used for maintenance of the fungal tissue in the root and growth and maintenance of the extramatrical hyphae.

Relationships among leaf construction cost, leaf longevity, and light environment in rain-forest plants of the genus Piper

Among seven Piper species characteristic of multiple stages of rain-forest secondary succession, leaf construction cost increased and leaf longevity decreased as available light increased. Our results extend the observations of several recent studies reporting a striking constancy of leaf construction costs among co-occurring plants that differ in growth form or phenology. When Piper species characteristic of different habitats co-occurred, their leaf construction costs were similar. However, mean leaf construction costs for sun and shade species, based on plants sampled from the entire range of light environments, were quite different. We reject the hypothesis that leaf construction cost and leaf longevity are generally positively related. The arguments previously used to support the hypothesis lack generality because they fail to consider carbon income as well as cost. The data presented here are consistent with the results of several other recent studies in failing to support the hypothesis. We conclude that the more general hypothesis, predicting a positive relationship between longevity and the ratio of cost to carbon gain, is conceptually more robust and consistent with the data we present. This more general hypothesis derives logically from the arguments previously used to predict a positive relationship between longevity and cost.

SEA‐LEVEL RISE AND COASTAL FOREST RETREAT ON THE WEST COAST OF FLORIDA, USA

We investigated patterns, rates, and mechanisms of forest replacement by salt marsh in relation to sea-level rise on the west coast of Florida, USA. The geomorphology of this region typifies that of low-lying, limestone coastlines considered highly susceptible to sea-level rise (e.g., much of the eastern Gulf of Mexico, the Yucatan Peninsula, and low-lying limestone islands throughout the world). This coast is microtidal, shallowly sloping, and has a rate of relative sea-level rise similar to that of eustatic rise. To determine patterns of forest change in relation to sea-level rise, we examined patterns of tree species zonation, tree recruitment, and tree mortality in relation to site elevation and tidal-flooding frequency. To reconstruct histories of forest change in relation to sea-level rise, we estimated age distributions of Sabal palmetto, the most widely distributed tree species at our site, relating age structures of stands to reconstructed histories of tidal flooding in the stands. Finally, to assess the relative roles of flooding stress (hypoxia), salt exposure, and competition from encroaching salt-marsh vegetation in the decline of forest stands, we examined patterns of soil redox potential, groundwater salinity, and density of halophytic vegetation among stands in different stages of decline. Zonation among tree species was related to tidal-flooding frequency. For most trees, seedlings were absent from the most frequently flooded stands in which the species occurred. Reconstructed flooding histories of stands and age estimates for S. palmetto suggest that many decades elapse between cessation of regeneration and local elimination of a tree species. Even during the relatively short duration of the study (4 yr), however, composition of some stands changed in the direction predicted from species zonation and sea-level rise. Forest understory replacement by halophytic vegetation appeared to follow, rather than cause, failure of tree regeneration. Tidal flooding rarely produced severe reducing conditions in soil, but groundwater salinity was correlated with tidal-flooding frequency. Forest retreat in this system, therefore, involves the development of relict (non-regenerating) stands of different tree species at different flooding frequencies. Exposure to salt appears to be the major cause of tree regeneration failure, with flooding stress and interference from marsh vegetation playing minor or negligible roles. These interactions differ somewhat from those on deltaic coasts or coasts with high freshwater outflows, where flooding stress may play a larger role in regeneration failure, and from sandy coasts, where erosion may play a larger role in forest retreat. Regardless of the cause of tree regeneration failure, the development of relict stands may be a general forest response to sea-level rise.

Growth, carbon allocation and cost of plant tissues

The capacity to change in size, mass, form and/or number is an essential feature of life, and the term ‘growth’ can refer to any or all of these types of change. In this chapter, we focus on methods to analyze one type of growth — the increase in dry mass of plants or plant parts through time. We consider components of growth that occur over time periods ranging from minutes to years, and at structural levels ranging from tissues to the whole plant. Our central theme is that a variety of processes at different temporal and structural scales contribute to plant growth and success. In some studies, the control of photosynthate partitioning may be of critical interest in understanding growth, while in others, it may be the relative costs of twigs versus leaves.

Coevolution of the checkerspot butterfly Euphydryas chalcedona and its larval food plant Diplacus aurantiacus: larval response to protein and leaf resin

SummaryPrediapause larvae of the checkerspot butterfly Euphydryas chalcedona were raised from hatch until entrance into diapause on artificial diets. The proportions of protein and host plant leaf resin differed among the diets. Larval size growth rates and mortality were monitored and overall rates and efficiencies of food use were computed.Larval survivorship, growth rate and size of larvae at idapause were significantly enhanced by increasing dietary protein content, particularly over the range found in leaves of the host plant. In contrast, an increasing dietary content of Diplacus aurantiacus leaf resin significantly depressed larval surviviorship, growth rates and size of larvae at diapause. A simple dosedependent interaction was observed between the effects of dietary leaf resin and protein on larval success. Dietary content of leaf resin and protein significantly influenced some measures of food utilization efficiency (ECI and ECD), but not others (AD and NUE).The negative interaction between the effects of dietary leaf resin and protein content suggests the leaf resin phenolic compounds reduce the availability of protein to the larvae. The results for efficiency indices of larval food use are potentially in conflict with this interpretation.The influence of host plant leaf resin and protein on larval success, coupled with the relation between photosynthesis and leaf nitrogen content, are consistent with the hypothesis that productivity can be enhanced by herbivore deterrence resulting from leaf resin production.

The role of carbohydrate reserves in the growth, resilience, and persistence of cabbage palm seedlings (Sabal palmetto)

Sabal palmetto (Walt.) Lodd. ex Schultes (cabbage palm) is an arborescent palm common in many plant communities throughout Florida, U.S.A., and the Caribbean. Although its seedlings grow very slowly in forest understories, they survive damage and defoliation well, and the species may increase in dominance following disturbances such as fire, logging, and hurricanes. We investigated the potential importance of total nonstructural carbohydrate (TNC) pools in the ability of cabbage palm seedlings to recover from the loss of aboveground tissue such as that caused by fire, grazing, or shallow burial by storm debris. TNC concentrations in belowground organs of seedlings from a forest understory were high, and TNC pools were sufficient to theoretically replace >50% of a seedlings canopy. The largest fraction of the belowground TNC pool was in stem tissue, where TNC in unclipped plants accounted for 26–54% of stem dry mass. Experimental reduction of TNC pools by repeated defoliation slowed seedling regrowth, and seedlings with inherently smaller pools (smaller seedlings) suffered higher mortality after repeated defoliation than did larger seedlings. Although regrowth and recovery after the loss of aboveground tissue was related to the size of the TNC pool in belowground organs, even the smallest seedlings with the smallest pools had sufficient stores to withstand at least two defoliations at frequent (7-week) intervals. Large belowground TNC pools in S. palmetto seedlings appear to enable them to survive all but the most frequent defoliations (e.g., frequent grazing or mowing). Allocation of resources to these stores, however, may contribute to the slow growth rates of S. palmetto seedlings in natural communities.

Control of shrub establishment by springtime soil water availability in an annual grassland

SummaryPrevious studies have shown that the shrub, Baccharis pilularis spp. consanguinea, invades annual grasslands in the San Francisco Bay region in a sporadic manner. Invasion was shown to be positively correlated with the amount of rainfall received in the spring. Here we show that, although Baccharis seeds are dispersed near the beginning of the winter rainy season, seedling root growth is extremely slow until spring. At this time, cessation of the winter rains and transpiration by the grassland annuals results in drying of the upper soil profile. We conclude that establishment of Baccharis seedlings at our study site usually fails because seedling roots cannot reach depths of permanently moist soil, below the depth of the grass roots, before this soil drought occurs. The continuation of rains into the warmer spring months provides a window of time when favorable temperatures and adequate soil moisture allow shrubs to establish.

Invasion of an annual grassland in Northern California by Baccharis pilularis ssp. consanguinea

SummaryWe studied the invasion of a California annual grassland by the shrub, Baccharis pilularis ssp. consanguinea (DC) C.B. Wolf. A series of aerial photographs indicated that the shrub had been present at the study site for 35–45 years but spread very little until the early 1970s. A period of slow expansion was followed by rapid outward spread of the shrub population and large increases in shrub density. Detailed demographic studies showed a period of rapid population increase from 1978 to 1983, with the highest stablishment rates in 1982–83. Establishment was strongly correlated with annual and spring rainfall amounts. An iniiall period of slow outward spread is thought to have profided foci for rapid spread during years of high rainfall.

The influence of shade and clouds on soil water potential: The buffered behavior of hydraulic lift

In the sagebrush/bunchgrass steppe of the North American Great Basin soil water potential has been shown to exhibit diel fluctuations with water potential increasing during the night as a result of water loss from roots in relatively dry soil layers. We hypothesized that environmental conditions promoting low transpiration rates (shading, cloudiness) would cause a net increase in soil water potential as a result of reduced soil water depletion during the day and continuing water efflux from roots during the night. We examined the response of soil water potential to artificial shading in sagebrush/bunchgrass plantings and used a simple model to predict how soil water potential should respond to reduced transpiration. Field measurements of soil water potential indicated that shading reduced daytime soil water depletion, but that the magnitude of the soil water potential increase during the night was related to the magnitude of the soil water potential decline during the preceding day. As a result, shading had little net effect on soil water potential. This behavior was consistent with model results and appears to result from the fact that soil water depletion during the day is largely responsible for creating the water potential gradients that drive nocturnal recharge of the shallow soil layers. The overall effect of such behavior is to buffer the seasonal course of soil water depletion in the rooting zone against day-to-day fluctuations in evapotranspiration. Despite the buffered behavior of soil water potential change, reduced evapotranspiration during light summer rains, and resulting soil water redistribution in the rooting zone, may enhance plant water status to a greater extent that would be expected on the basis of the rainfall received.

The coevolution of Euphydryas chalcedona butterflies and their larval host plants

SummaryThe interactions between the checkerspot butterfly, Euphydryas chalcedona, and two of its principal host plants, Diplacus aurantiacus and Scrophularia californica, were studied to test the hypothesis that feeding behavior in nature reflects food quality in terms of leaf nitrogen and defensive chemical contents. Larvae preferentially fed on Diplacus leaves containing the highest nitrogen: resin ratio in the field and laboratory. Larvae did not feed selectively among Scrophularia leaves, which show little variation in quality. Seasonal timing of feeding activity and larval development rates were closely related to the availability of any Scrophularia leaves and high-quality Diplacus leaves.