Leaf physiology of shade-grownCycas micronesica leaves following removal of shade

The photosynthetic characteristics ofCycas micronesica K.D. Hill were studied from August 1998 until February 1999 using chlorophyll fluorescence and gas-exchange techniques to determine the responses to long-term shade of 35% ambient light transmission, followed by the transfer of shade-grown leaves into full-sun conditions. The shade-grown leaves exhibited increased photosynthetic light use efficiency and effective quantum efficiency of photosystem II (PS II) and decreased photosynthetic light saturation point and dark respiration when compared with leaves grown in full sun. Shade was removed from shade-grownC. micronesica leaves during midday on December 14, 1998, when effective quantum efficiency of shaded leaves was 45% greater than that of sun leaves. Following one hour in full sun, effective quantum efficiency of the shade-grown leaves declined to below that of the sun-grown leaves. After receiving full sunlight for the rest of the photoperiod, maximum quantum efficiency of PS II photochemistry for shade-grown leaves was below that of sun-grown leaves throughout the night. The damage caused by excessive light to shade-grown leaves progressed for the first three days after shade removal. On day 3, effective quantum efficiency during midday was 30%, net photosynthesis was 47%, apparent quantum yield was 65%, and light compensation point was 136% of that for sun-grown leaves. After day 3, the relationship between full-sun leaves and the previously shaded leaves for these response variables was relatively stable. Two months following removal of shade, the previously shaded leaves continued to exhibit damage from high light. These results have application to transplanting cycad plants from a shaded nursery to a field site or, after tropical cyclones, where protective forest canopy cover has been destroyed and cycad plants in the forest subcanopy are abruptly exposed to full-sun conditions.     

Effects of scale insect herbivory and shading on net gas exchange and growth of a subtropical tree species (Guaiacum sanctum L.)

Summary The scale insect, Toumeyella sp., feeds exclusively on the subtropical hammock tree lignum vitae (Guaiacum sanctum L.). The combined effects of scale herbivory and shading on leaf gas exchange characteristics and growth of lignum vitae trees were studied using a factorial design. Trees grown in full sun or in 75% shade were manually infested with scale or left noninfested. Beginning 4 weeks after infestation, net CO₂ assimilation, stomatal conductance, transpiration, internal partial pressure of CO₂, and water-use efficiency were determined on single-leaves at 4-week intervals for trees in each treatment. At the end of the experiment, net CO₂ assimilation was determined for whole plants. Total leaf area, leaf, stem, and root dry weights, and leaf chlorophyll and nitrogen concentrations were also determined. Scale infested trees generally had lower net CO₂ assimilation, stomatal conductance, and transpiration rates as well as less leaf area, and root, stem, and leaf dry weights than noninfested trees. Twenty four weeks after the shade treatment was imposed, sun-grown trees had approximately twice the leaf area of shade-grown trees. Shade-grown trees compensated for the reduced leaf area by increasing their photosynthetic efficiency. This resulted in no difference in light saturated net CO₂ assimilation on a whole plant basis between sun-grown and shade-grown trees. Chlorophyll and nitrogen concentrations per unit leaf area were greater in leaves of shade-grown trees than in leaves of sun-grown trees. Shading and herbivory by Toumeyella sp. each resulted in decreased growth of Guaiacum sanctum. Scale insect herbivory did not result in greater detrimental effects on leaf gas exchange characteristics for shade-grown than for sun-grown trees. Herbivory by Toumeyella resulted in a greater decrease in tree growth for sun-grown than for shade-grown trees.     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

Effects of moderate high-temperature stress on photosynthesis in three saplings of the constructive tree species of subtropical forest

During the exposition to moderate high-temperature stress, photosynthetic rates and fluorescence of chlorophyll a were measured with a photosynthetic measurement system (Li-Cor 6400) and leaf chamber fluorometer (Li-Cor6400 LCF), respectively, in leaves of saplings, sun-adapted species (Schima superba), shade-adapted species (Cryptocarya concinna), and in mesophytic plant (Castanopsis hystrix) (42°C). The results showed that moderate high-temperature stress led to a decrease in F_v/F_>m, namely the primary photochemical quantum efficiency, indicating that moderate high-temperature stress causes a partial inhibition of PSII. It also showed that such an effect was more severe in the shade-adapted plant C. concinna than in the sun-adapted species S. superba. However, except for the sun-grown leaves of C. concinna, the moderate high-temperature stress increased the photosynthetic rate of leaves at high light intensity. Simultaneously, less photoinhibition was found to occur under high-light intensity, suggesting that the capacity of resistant-photoinhibition was stimulated by moderate high-temperature stress. The quantum yield of PSII (?_PSII) decreased in the sun-grown leaves of S. superba and C. hystrix but did not show any significant change in leaves of the shade plant C. concinna and shade-grown leaves of sun plant S. superba or the mesophytic plant C. hystrix because they already had a very low ?_PSII under this condition. Moderate high-temperature stress led to a decrease in ?_PSII/?_CO2 ratios, an estimate of the quantum requirement for CO2 assimilation, in the sun plant S. superba and the mesophytic plant C. hystrix because they were associated with the dissipation of a lower fraction of excitation energy. However, no significant changes were found in shade plant C. concinna and in shade-grown leaves of the other examined plants. The effect of moderate high-temperature (42°C) on photosynthesis depends on species and leaf type (sun and shade leaves) in the saplings of subtropical broad-leaved trees.     

Phototaxis, Host Cues, and Host-Plant Finding in a Monophagous Weevil, Rhinoncomimus latipes

Rhinoncomimus latipes is a monophagous weevil used as a biological control agent for Persicaria perfoliata in the eastern United States. Density of adult R. latipes and resulting feeding damage has been shown to be higher in the sun than in the shade. This study aimed to determine whether phototaxis, sensitivity to enhanced host cues from healthier sun-grown plants, or a combination is driving this behavior by the weevil. A series of greenhouse choice tests between various combinations of plant and light conditions showed that R. latipes is positively phototactic, responsive to host cues, and preferentially attracted to sun-grown plants over shade-grown plants. From our experiments, we hypothesize two phases of dispersal and host finding in R. latipes. The initial stage is controlled primarily by phototaxis, whereas the later stage is controlled jointly by host cues and light conditions.     

Soil Effects on Forest Structure and Diversity in a Moist and a Dry Tropical Forest

Soil characteristics are important drivers of variation in wet tropical forest structure and diversity, but few studies have evaluated these relationships in drier forest types. Using tree and soil data from 48 and 32 1 ha plots, respectively, in a Bolivian moist and dry forest, we asked how soil conditions affect forest structure and diversity within each of the two forest types. After correcting for spatial effects, soil‐vegetation relationships differed between the dry and the moist forest, being strongest in the dry forest. Furthermore, we hypothesized that soil nutrients would play a more important role in the moist forest than in the dry forest because vegetation in the moist forest is less constrained by water availability and thus can show its full potential response to soil fertility. However, contrary to our expectations, we found that soil fertility explained a larger number of forest variables in the dry forest (50 percent) than in the moist forest (17 percent). Shannon diversity declined with soil fertility at both sites, probably because the most dominant, shade‐tolerant species strongly increased in abundance as soil fertility increased.     

Recovery of Tawa-dominated forest fragments in the Rotorua Basin, New Zealand, after cessation of livestock grazing

Summary   Tawa ( Beilschmiedia tawa )-dominated forest fragments on farms within the Rotorua Basin were surveyed to quantify the likely recovery processes following exclusion of domestic livestock grazing, using a space-for-time substitution approach. Vegetation structure, plant diversity and soil fertility were measured at 24 sites within 15 forest fragments on six farms, covering a range in time since exclusion from grazing of 1–53 years. The forest fragments were compared with a large area of ungrazed forest in the nearby Lake Okataina Scenic Reserve. As time since exclusion from grazing increased, indigenous plant species diversity increased (up to 30–35 years); ground fern and epiphyte abundance increased (up to 30–35 years); tree seedling and sapling numbers, and litter cover also increased (up to 10–15 years); and overall tree numbers increased, while average tree diameter at breast height and overall tree basal area did not differ significantly. The soil fertility status was highly variable, obscuring clear patterns, although Olsen P status decreased with time since grazing exclusion. Once grazing of forest fragments ceases, significant changes in their diversity, structure and soil characteristics can be expected, which indicate recovery of these plant communities towards the conditions observed in ungrazed forest.     

Carbon stock assessment for a forest-to-coffee conversion landscape in Sumber-Jaya (Lampung, Indonesia): from allometric equations to land use change analysis

The change in stored carbon (C) stocks was assessed for a 700 km~2 area where forestcover decreased from 60% to 10% in the last 30 years. At the same time, the area under coffee increased from 7% to 70% with a gradual evolution from open "sun coffee" systems to multi-strata "shade coffee" systems that provide a partial compensation for C loss. The use of a generic tropi-cal forest rather than tree-specific allometric equation can lead to substantial (up to 100%) overes-timates of aboveground biomass depending on wood density and tree shape. The shoot: root ratio (biomass) of coffee shifted with age, from the 4∶1 value often assumed for tropical trees to 2∶1.Annual aboveground C stock accumulation rates during the establishment stage after slash-and-burn land clearing were 1, close to 2 or 3.5 Mg C ha~(-1)a~(-1) for sun coffee, shade coffee and fallowregrowth, respectively. Forest remnants, shade coffee and sun coffee had soil C stocks in the up-per 30 cm of the soil that were 79%, 60% or 45%, respectively, of the values expected for primary forest in Sumatra. Total C stock (time averaged, above-0.3 m in the soil) for forest, shade and sun coffee was 262, 82 and 52 Mg C ha~(-1), respectively. In the 1970-1984 period, while forest cover was reduced from 59.5% to 19.7%, the landscape lost on average 6.8 Mg C ha~(-1) a~(-1). In the1984-2000 period forest cover was further reduced to 12.6%, but the landscape lost only 0.39 MgC ha~(-1) a~(-1), as forest loss was partially compensated by an increase in shade coffee systems. Conversion of all current sun coffee to shade coffee systems while protecting the remaining forest,could increase average landscape level C stocks by 10 Mg ha~(-1) over a time frame of say 20 years,or 0.5 Mg C ha~(-1) a~(-1).     

Carbon stock assessment for a forest-to-coffee conversion landscape in Sumber-Jaya (Lampung, Indonesia): from allometric equations to land use change analysis

The change in stored carbon (C) stocks was assessed for a 700 km2 areawhere forest cover decreased from 60% to 10% in the last 30 years. At the same time, the area under coffee increased from 7% to 70% with a gradual evolution from open "sun coffee" systems to multi-strata "shade coffee" systems that providea partial compensation for C loss. The use of a generic tropical forest rather than tree-specific allometric equation can lead to substantial (up to 100%) overestimates of aboveground biomass depending on wood density and tree shape. The shoot:root ratio (biomass) of coffee shifted with age, from the 4:1 value often assumed for tropical trees to 2:1. Annual aboveground C stock accumulation rates during the establishment stage after slash-and- burn land clearing were 1, closeto 2 or 3.5 Mg C ha-1a-1 for sun coffee, shade coffee and fallow regrowth, respectively. Forest remnants, shade coffee and sun coffee had soil C stocks in the upper 30 cm of the soil that were 79%, 60% or 45%, respectively, of the values expected for primary forest in Sumatra. Total C stock (time averaged, above - 0.3m in the soil) for forest, shade and sun coffee was 262, 82 and 52 Mg C ha-1, respectively. In the 1970-1984 period, while forest cover was reduced from 59.5%to 19.7%, the landscape lost on average 6.8 Mg C ha-1 a-1. In the 1984-2000 period forest cover was further reduced to 12.6%, but the landscape lost only 0.39Mg C ha-1 a-1, as forest loss was partially compensated by an increase in shadecoffee systems. Conversion of all current sun coffee to shade coffee systems while protecting the remaining forest, could increase average landscape level C stocks by 10 Mg ha-1 over a time frame of say 20 years, or 0.5 Mg C ha-1 a-1.