Influence of woody and herbaceous vegetation control on leaf gas exchange,water status,and nutrient relations of eastern white pine (Pinus strobus L.) seedlings planted in a central Ontario clearcut

The influence of herbaceous and woody vegetation control, either singly or in combination, on leaf gas exchange, water status, and nutrient relations of planted eastern white pine (Pinus strobus L.) seedlings was examined in a central Ontario clearcut over four consecutive growing seasons (GSs). Net carbon assimilation (A_n), leaf conductance to water vapour (G_wv), water use efficiency (WUE), and midday leaf water potential (ψ_m) were measured periodically during the second to fourth GSs of vegetation control treatments, while leaf nutrient relations were examined in GS five. Leaf A_n and G_wv were reduced (p ≤ 0.05) in the presence of herbaceous vegetation in GS two, by both herbaceous and woody vegetation in GS three, and only by woody vegetation (largely trembling aspen (Populus tremuloides Michx.)) in GS four. Leaf WUE was increased (p ≤ 0.05) in all three GSs in which herbaceous vegetation control was applied and where woody vegetation provided partial shading of planted white pine. Leaf water status was comparatively less responsive to vegetation control treatments, but leaf ψ_m was increased (p ≤ 0.05) in the presence of woody vegetation in GSs two and four, likely due to shading and reduced atmospheric evaporative demand of the white pine seedling environment. Within a given GS, the effects of vegetation control on A_n, G_wv, and ψ_m were strongly linked to treatment-induced changes in total vegetative cover, and light and soil moisture availability. Seedling height, diameter, and volume growth rates were positively correlated with A_n and WUE in GSs two and three, but less so in GS four. Vector analysis suggested that herbaceous competition induced foliar N, P, and K deficiencies in five-year-old white pine seedlings while competition from aspen resulted in foliar Ca deficiency.     

Competition for water in a pecan (Carya illinoensis K. Koch) – cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Understanding the belowground interactions between trees and crops is critical to successful management of agroforestry systems. In a study of competition for water in an alley cropping system consisting of pecan (Carya illinoensis) and cotton (Gossypium hirsutum) in a sandy loam soil (Rhodic Paleudult) in Jay, Florida, root systems of the two species were separated by trenching to 120 cm depth. A polyethylene barrier was installed in half of the plots. Spatial and temporal variations in soil water content, root distribution and water uptake by both species, and leaf area development and height of cotton were measured. Interspecific competition for water was greater in the non-barrier treatment near tree rows than at the alley center. Competition became evident 3 to 4 weeks after emergence of cotton and increased during the following 7 to 8 weeks. Compared with the non-barrier treatment, the barrier treatment had higher soil water content and better growth of cotton (height, leaf area, and fine root biomass). Cotton lint yield in the barrier treatment (677 kg ha^–1) was similar to that in a sole-crop stand, but higher than in the non-barrier (502 kg ha^–1) treatment. Lint production efficiency of plants was higher in the interior rows in the non-barrier treatment (0.197 kg lint per square meter of leaf area, compared to 0.117 kg in the barrier treatment). The results suggest that trenching or even deep disking parallel to the tree row may reduce competition for water, but the impact on tree growth cannot be established from this study. This revised version was published online in June 2006 with corrections to the Cover Date.     

Short-term impacts of nutrient manipulations on leaf gas exchange and biomass partitioning in contrasting 2-year-old Pinus taeda clones during seedling establishment

We conducted a 1-year greenhouse experiment to assess the impact of nutrient manipulations on seedling growth, biomass partitioning, and leaf gas exchange between two fast growing Pinus taeda clones that differed in growth efficiency. After 1 year we observed significant treatment and treatment by clone effects on growth, biomass partitioning, and gas exchange parameters. Fertilization increased total seedling biomass 18% primarily through an increase in foliage and coarse-roots. Clones did not differ in total seedling biomass, however, clone 85 produced more stem than clone 93 leading to 37% greater stem:leaf, while clone 93 maintained more branch biomass. The logging residue treatment increased stem:leaf by 30%, but had no effect on total biomass or partitioning. Differences in leaf morphology resulted in significantly greater canopy leaf area in clone 93 than clone 85. Increased foliar N concentration from fertilization had only minor effects on specific photosynthesis under saturating light (A_Sat), but lowered stomatal conductance (g_s), transpiration (E), and internal to external CO₂ concentration ratio (C_i/C_a) as well as improved water use efficiency (WUE) independently of genotype. When gas exchange data was scaled to the canopy level both genotypes achieved similar canopy level CO₂ assimilation rates, but our data suggests they did this by different means. Although we did see a small effect of nutrient limitations in total canopy photosynthesis under saturating light (A_Canopy), A_Sat, and total leaf area (TLA), our foliar N concentration ([N]) indicated that our level of logging residue incorporation did not cause [N] to decrease below sufficiency limits. From a practical standpoint, a better understanding of strategies for capturing and partition C may lead to better selection of clonal material, thereby, optimizing productivity.     

Response of Quercus pyrenaica (melojo oak) to soil water deficit: a case study in Spain

In the present study, carried out from 2004 to 2006, leaf and stem water potential and stem water content were measured in Quercus pyrenaica Willd. individuals in an experimental forested catchment located in Central Western Spain under Mediterranean subhumid conditions. These indicators of tree water status were compared with soil moisture contents measured in the same area from 0 to 1 m depth and from 0 to 2.5 m depth during the last year of the study. The objectives were to clarify the seasonal and year-to-year variations in tree water status, to examine applicability of stem water content as useful water stress indicator and to discuss how deep soil water and root uptake contribute to survival during the long dry summer. Seasonal variations in the tree variables measured revealed a typical pattern, with maximum values at the end of spring followed by a progressive decline during the summer drought in response to the decrease in soil water content (almost exhausted at 0–100 cm depth). The relatively high values and the non-significant variation in predawn leaf water potentials (except for 2005, which was exceptionally dry) indicate that no clear water stress situations occurred. This may be explained in terms of a progressive absorption of water from the deeper layers. The results also suggest that the stem water content is a more sensitive indicator of long-term water limitation than the other variables measured.     

Effects of water state of soil on ecological boundary of leaf Surface ofpopulus davidiana

The effect of water state of soil on eco-boundary over leaf surface was disscussed by takingPopulus davidiana as experimental material. The results show that to change soil water state by some methods was to adjust and control the forming quality of eco-boundary in fact. The plant in good water condition must be accompanied by thinner eco-boundary. This eco-boundary is beneficial to exchange and flow of the substances between plant and environment. Suifenhe Animal and Plant Quarantine Bureau The study was supported by The Chinese Natural Science Foundation, No. 39470136 (Responsible Editor: Chai Ruihai)     

Transpiration along an age series of Eucalyptus globulus plantations in southeastern Australia

Many of the world's Eucalyptus plantations are grown on short rotations of 15 years or less, which often covers the most rapid phase of stand development and peaks in growth rates and leaf areas. Since transpiration is related to stand leaf area these short rotations that make use of rapid early growth rates, may also maximise plantation water use, which has implications for predicting their water requirements and impacts on catchment hydrology. This study examined the transpiration, leaf area and growth rates of Eucalyptus globulus Labill. plantations aged 2–8 years. Transpiration (E), estimated using the heat pulse technique, increased from 0.4 mm day⁻¹ at age 2 years to a peak of about 1.6–1.9 mm day⁻¹ in stands aged 5–7 years. This was associated with similar trends for stand leaf area index (LAI) and periodic annual increments of aboveground biomass, which both peaked at about age 4–6 years resulting in a linear relationship between E and LAI. While stand sapwood areas were continuing to increase at age 8 years, E was already declining due to reductions in sap velocity, from 13.5 cm h⁻¹ at age 2 years to 6.3 cm h⁻¹ at age 8 years and reduced sapwood area growth rates. Trees compensated for this reduction in sap velocity with declines in the leaf area (A_L) to sapwood area (A_S) relationship (A_L:A_S) with age. There was also a reduction in growth efficiency (aboveground biomass increment per LAI) with age. However, reductions in WUE were small after age 4 years, which explained the linear relationship between E and LAI. If E continues to decline successive short rotation lengths may not only make use of rapid early growth rates but could also increase plantation water use compared to longer rotations over the same period of time.     

Sap flow of birch and Norway spruce during the European heat and drought in summer 2003

In this study the hydrological regime of Norway spruce (Picea abies) and birch (Betula pendula) growing on heavy soils in the south east of Austria was analysed. Results from the year 2003 characterised by an extremely hot and dry summer are presented in this paper. Due to the extreme weather conditions the soil water content in August 2003 was very low (0.10–0.25 m³ m⁻³) in the topsoil (0–50 cm) with no explicit difference between both tree species.     

Managing productivity and drought risk in Eucalyptus globulus plantations in south-western Australia

More than 2.5 million ha of Eucalyptus globulus are now planted across the globe including approximately 500 000 ha in southern Australia. In this region average annual rainfall has declined since 1960 and this trend is predicted to continue in the coming decades. E. globulus is a premium species for paper manufacture and grows well under moderate seasonal water stress. The traits that underpin this rapid early growth also make the species vulnerable to prolonged water stress. We established nitrogen rate and nitrogen-by-stocking experiments in five 2-year-old E. globulus plantations along a climatic gradient in south-western Australia. We measured volume growth, predawn leaf water potential and leaf area index over 7 years or until the plantations were 9 years old. These data were used to explore the relationship between growth and water stress, to understand the mechanistic basis for the relationship and to identify best-bet management strategies for E. globulus plantations in southern Australia.     

Changes in leaf water use after removal of leaf lower surface hairs on <Emphasis Type="Italic">Mallotus macrostachyus</Emphasis> (Euphorbiaceae) in a tropical secondary forest in Malaysia

Leaf hairs may assist in maintaining high leaf water use efficiency in tropical secondary forest tree species. We compared leaf temperature, transpiration, photosynthesis and water use efficiency between hairy and depilated leaves in Mallotus macrostachyus (Euphorbiaceae), to determine the role of leaf hair in leaf water use efficiency (WUE) in tropical degraded secondary forest in Malaysia. Measurements were made on five mature individuals growing in sun-exposed conditions and five in shaded conditions. The hair dry weight per unit leaf area was significantly greater in sun leaves than in shade leaves. The transpiration rate (Tr_max) of depilated leaves in sun-exposed conditions was slightly higher than in hairy leaves in both morning and afternoon measurements. In contrast, Tr_max in the shade leaves was almost identical in hairy and depilated leaves. Leaf stomatal conductance (g _s) in the morning showed almost the same value among leaf types and light conditions. In the afternoon, g _s slightly decreased from the morning values in both sun and shade conditions. In the morning, the leaf water use efficiency (A _max/Tr_max) in both conditions did not differ significantly between hairy and depilated leaves. However, in the afternoon, WUE in the depilated leaves was significantly lower than in hairy leaves in sun-exposed conditions. These observations suggest that leaf hairs in M. macrostachyus contribute to the high leaf water use efficiency in drought conditions, such as high vapor pressure deficit experienced at midday in degraded tropical secondary forests.     

Coast live oak, Quercus agrifolia, susceptibility and response to goldspotted oak borer, Agrilus auroguttatus, injury in southern California

Oak mortality is often associated with a complex of decline factors. We describe the morphological and physiological responses of coast live oak, Quercus agrifolia Née, in California to an invasive insect, the goldspotted oak borer (GSOB), Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), and evaluate drought as a potential inciting factor. Morphological traits of 356 trees were assessed and physiological traits of 70 of these were monitored intensively over one growing season. Morphological characteristics of tree health included crown thinning and dieback; bole staining resulting from larval feeding; density of GSOB adult exit holes; and holes caused by woodpecker feeding. These characteristics were used to rank GSOB infestation/injury into four classes, and taken together, they explained 87% of the variation in a principal component analysis. Drought stress on various size/age and infestation classes of Q. agrifolia was measured by assessing branchlet pre-dawn and solar noon xylem water potential, leaf cell turgor potential, and water use efficiency over one growing season. Both morphological and physiological traits were highly variable in mature and old growth trees. Early summer plant water status (branchlet xylem water potential and water use efficiency) was similar between uninfested and newly colonized trees, suggesting that GSOB are not pre-selecting drought-stressed Q. agrifolia for oviposition. By late summer, leaf water and cell turgor potentials were lower in infested than in uninfested mature trees, suggesting that GSOB infestation causes drought stress in these trees. Among the tree size/age classes, infested old growth trees exhibited the greatest change in water use efficiency over the growing season, and showed greater morphological injury symptoms of decline than infested mature trees. Morphological attributes of decline in Q. agrifolia associated with GSOB were correlated weakly with increasing physiological drought stress among infestation classes of trees. We propose that the collection of morphological responses of Q. agrifolia to GSOB described here can be used to monitor the future expansion of the GSOB distribution as well as the GSOB-induced decline of Q. agrifolia in California.     

Field measurements of gas exchange in Avicennia marina and Bruguiera gymnorrhiza

Diurnal gas exchange characteristics were measured simultaneously in two mangrove species, Avicennia marina and Bruguiera gymnorrhiza, over 7 d in summer (February–March), to compare their productivity. The study was undertaken in the Beachwood Mangroves Nature Reserve, Durban, South Africa, using fully expanded leaves of young and mature trees at the top of the canopy. Gas exchange was strongly influenced by photosynthetic photon flux density (PPFD), leaf temperature and the accompanying leaf to air vapour pressure deficit ( w). Carbon dioxide exchange was saturated at a PPFD of about 600 mol m^-2s^-1 in B. gymnorrhiza compared to 800 mol m^-2s^-1 in A. marina. Maximal CO₂ exchange occurred between 12h00 and 14h00 and was consistently greater in A. marina (8.8 mol m^-2s^-1) than in B. gymnorrhiza (5.3 mu;mol m^-2s^-1). Mean internal CO₂ concentrations ( c_i) were 260 l l^-1 in A. marina and 252 l l^-1 in B. gymnorrhiza. Photorespiratory activity was 32% in A. marina and 30% in B. gymnorrhiza. Mean water use efficiency (WUE) was 8.0 mol mmol^-1 in A. marina and 10.6 mol mmol^-1 in B. gymnorrhiza. Diurnal leaf water potentials ranged from –0.8 to –3.5 MPa and were generally lower in A. marina.     

Effects of irrigation on water use and water use efficiency in two fast growing Eucalyptus plantations

Eucalyptus plantations occupy almost 20 million ha worldwide and exceed 3.7 million ha in Brazil alone. Improved genetics and silviculture have led to as much as a three-fold increase in productivity in Eucalyptus plantations in Brazil and the large land area occupied by these highly productive ecosystems raises concern over their effect on local water supplies. As part of the Brazil Potential Productivity Project, we measured water use of Eucalyptus grandis × urophylla clones in rainfed and irrigated stands in two plantations differing in productivity. The Aracruz (lower productivity) site is located in the state of Espirito Santo and the Veracel (higher productivity) site in Bahia state. At each plantation, we measured stand water use using homemade sap flow sensors and a calibration curve using the clones and probes we utilized in the study. We also quantified changes in growth, leaf area and water use efficiency (the amount of wood produced per unit of water transpired). Measurements were conducted for 1 year during 2005 at Aracruz and from August through December 2005 at Veracel. Transpiration at both sites was high compared to other studies but annual estimates at Aracruz for the rainfed treatment compared well with a process model calibrated for the Aracruz site (within 10%). Annual water use at Aracruz was 1394 mm in rainfed treatments versus 1779 mm in irrigated treatments and accounted for approximately 67% and 58% of annual precipitation and irrigation inputs respectively. Increased water use in the irrigated stands at Aracruz was associated with higher sapwood area, leaf area index and transpiration per unit leaf area but there was no difference in the response of canopy conductance with air saturation deficit between treatments. Water use efficiency at the Aracruz site was also not influenced by irrigation and was similar to the rainfed treatment. During the period of overlapping measurements, the response to irrigation treatments at the more productive Veracel site was similar to Aracruz. Stand water use at the Veracel site totaled 975 mm and 1102 mm in rainfed and irrigated treatments during the 5-month measurement period respectively. Irrigated stands at Veracel also had higher leaf area with no difference in the response of canopy conductance with air saturation deficit between treatments. Water use efficiency was also unaffected by irrigation at Veracel. Results from this and other studies suggest that improved resource availability does not negatively impact water use efficiency but increased productivity of these plantations is associated with higher water use and should be given consideration during plantation management decision making processes aimed at increasing productivity.     

Environmental and plant-based controls of water use in a Mediterranean oak stand

In this work, new information is reported on water relations of the Mediterranean oak species Quercus pyrenaica based on environmental and physiological measurements carried out during the growing seasons of 2006 and 2007. The interest in this species has increased due to its use in reforestation programs and its impacts on the water resources due to the extensive spontaneous afforestation after the abandonment of forest (firewood, charcoal, livestock, etc.) and agricultural activities, in general in some areas in the Mediterranean region and in particular, in the studied area (Sistema Central range, Spain). The objectives were to evaluate the long-term water use of this stand and its limit and to analyse the specific traits to cope with summer drought, especially the use of stem water storage and deep soil water reserves. Tree water stress associated with depletion of soil water reserves was not observed since the oak trees appeared to avoid a marked water stress using water reserves from deeper soil layers as summer drought progresses. The contribution of mean daily stem water storage to transpiration was low (4%), although it could be greater under dryer conditions. Only at the end of summer of 2006, the transpiration and canopy conductance were reduced due to soil drought. Despite the absence of marked water stress an upper limit was found in transpiration (slightly higher than 3 mm day⁻¹). The heavy use of soil water resources by this species (75% of available soil water in this study) should be considered when evaluating the impact of spontaneous afforestation and reforestation programs on water resources.     

Relationship of water status to vegetative growth and leaf gas exchange of peach (Prunus persica) trees on different rootstocks

We investigated relationships between tree water status, vegetative growth and leaf gas exchange of peach trees growing on different rootstocks under field conditions. Tree water status was manipulated by partially covering (0, approximately 30 and approximately 60%) the tree canopies on individual days and then evaluating the effects of tree water status on vegetative growth and leaf gas exchange. Early morning stem water potentials were approximately -0.4 MPa for trees in all treatments, but mean midday values ranged from -1.1 to -1.7 MPa depending on rootstock and canopy coverage treatment. Relative shoot extension growth rate, leaf conductance, transpiration rate and net CO2 exchange rate differed significantly among trees in the different rootstocks and canopy coverage treatments. Shoot extension growth rate, leaf conductance, leaf transpiration rate and leaf net CO2 exchange rate were linearly correlated with midday stem water potential. These relationships were independent of the rootstock and canopy coverage treatments, indicating that tree water relations are probably directly involved in the mechanism that imparts vegetative growth control by selected peach rootstocks.     

Foliar morphological variation in the white oak Quercus rugosa Née (Fagaceae) along a latitudinal gradient in Mexico: Potential implications for management and conservation

Quercus rugosa Nee (section Quercus) has a distribution from the southern United States to Honduras. This study characterized leaf variation across the whole distribution of the species in Mexico. Ten foliar morphometric characters were scored in each of 241 individuals from 25 localities. A principal component analysis resolved four principal components (PCs) that explained 76.4% of the total variation. A nested analysis of variance revealed significant differences among populations (29.2% average contribution to total variance for the four PCs) and among-trees within populations (39.2%), while 31.6% was due to intra-individual variation. For the first PC (related to leaf size), 52.1% resulted from among population differences. This variation was negatively correlated with latitude (r = −0.86; P < 0.0001), indicating a steep clinal reduction in leaf size from south to north. Mean annual precipitation and an annual aridity index also significantly decreased and increased with latitude, respectively. It is suggested that the morphological cline is the result of plastic and/or adaptive responses to environmental conditions, and indicative of further ecophysiological latitudinal differences among Q. rugosa populations. Additionally, we estimated the magnitude of the least significant difference among populations for the first PC and translated it into a delineation of six latitudinal zones (each with a width of 2°30′), to be considered as preliminary zones for the movement of Q. rugosa seeds with management and conservation purposes, including management in response to global warming.     

Establishment and growth of oak (Quercus alba, Quercus prinus) seedlings in burned and fire-excluded upland forests on the Cumberland Plateau

Recurrent problems with regeneration of oaks (Quercus spp.) have been documented across a wide range of ecosystems. In oak-dominated forests of the central and Appalachian hardwood regions of the United States, a lack of competitive oak regeneration has been tied, in part, to fire suppression in these landscapes, and managers throughout the region are using prescribed fire to address this concern. To examine fire effects on oak regeneration, researchers have generally relied on inventories or population studies of existing seedlings. These studies are valuable but do not permit examination of the role of fire in enhancing the establishment and growth of new oak seedlings stemming from oak mast events. In this study, white (Quercus alba) and chestnut oak (Quercus prinus) acorn mast crops serendipitously occurred in year three (fall 2005) of a landscape-scale prescribed fire experiment. We examined establishment, survival, height and diameter of new seedlings on sites on the Cumberland Plateau in eastern Kentucky. Treatments were fire exclusion, a single prescribed fire (1x-burn; 2003), and repeated prescribed fire (3x-burn; 2003, 2004, and after acorn drop in 2006), all conducted in late spring. Initial densities of newly established chestnut and white oak seedlings were statistically similar across treatments (P = 0.42), despite fires on the 3x-burn site having occurred after acorns were on the ground. Oak seedling density was significantly predicted by oak basal area on all sites (R² = 0.12–0.46), except for chestnut oak on fire-excluded sites (R² = 0.04). Litter depth was less on 3x-burn sites compared to 1x-burn and fire-excluded sites, whereas canopy openness was greater on both burn treatments compared to fire-excluded sites. Seedling mortality was generally higher on fire-excluded sites compared to burn sites, especially for white oak. Oak seedling mortality in the first two growing seasons was significantly predicted by initial litter depth and open sky, with greater litter depth and lower percent open sky leading to higher mortality. In the third growing season none of the measured variables predicted chestnut oak seedling survival; for white oak, percent open sky remained a significant predictor of mortality. Initially, seedlings on the fire-excluded sites had similar height but smaller diameter; after three growing seasons there were few differences in seedling height or diameter among treatments. Our findings suggest a potential role for prescribed fire in establishing forest floor and light conditions that may enhance the success of new oak germinants, although different responses among species may suggest the need to target management for individual oak species.     

Tree-rings reflect the impact of climate change on Quercus ilex L. along a temperature gradient in Spain over the last 100 years

We analyzed tree rings over the past 100 years to understand the response of Quercus ilex L. to climate change at four different sites along a temperature gradient in a highly anthropogenically transformed ecosystem. To test the hypothesis of a climate change related decrease in productivity at warmer sites, we discuss the effect of historical management on the growth of forest stands and the spatio-temporal variability of growth in response to climate, analyzing departures from linearity in that relationship. We reconstructed stand history and investigated past growth trends using tree-rings. Then we used a dendroecological approach to study the regional, local and age-dependent response to climate, analyzing the relationship between precipitation and tree growth using non-linear mixed models. Tree rings reflected the origin of the studied landscape, mainly a simplification of an original closed forest and progressive canopy opening for agrosilvopastoral purposes after the mid 1800s. As expected, trees were principally responding to water availability, and regional growth (as expressed by the first principal component from the matrix of chronologies) was highly responsive to hydrological year precipitation (r = 0.7). In this water limited ecosystem, the response of growth to precipitation was asymptotic and independent of age, but variable in time. Maximum growth was variable at the different sites and the non-linear function of growth saturated (i.e. reached an asymptote) at temperature dependent site specific precipitation levels within the range considered in the region to lead a shift towards deciduous species dominated woodlands (around 600 mm, variable with mean temperature). Only trees at warmer sites showed symptoms of growth decline, most likely explained by water stress increase in the last decades affecting the highly transformed open (i.e. low competition) tree structure. Stands at colder locations did not show any negative growth trend and may benefit from the current increase in winter temperatures. Coinciding with the decrease in productivity, trees at warmer sites responded more to moisture availability, exhibited a slower response to precipitation and reached maximum growth at higher precipitation levels than trees at colder sites. This suggests that warmer stands are threatened by climate change. The non-linear response of growth to precipitation described is meaningful for different ecological applications and provides new insights in the way trees respond to climate.     

辽西地区水肥处理对核桃叶水势的影响

采用312-D最优饱和设计,在辽西地区开展水肥试验,研究不同水肥处理对2年生核桃幼苗叶水势的效应.结果表明：叶片水势的最适条件为土壤含水率17.52％,施磷570.56 kg/hm2附近;氮具有降低水势效应,氮磷耦合对叶片水势具有相互替代效应;水磷耦合对叶片水势具有相互促进的效应.在生产实践中,合理控制水肥才能提高核桃生产效应.     

Gas exchange responses of <Emphasis Type="Italic">Eucalyptus</Emphasis>, <Emphasis Type="Italic">C. africana</Emphasis> and <Emphasis Type="Italic">G. robusta</Emphasis> to varying soil moisture content in semi-arid (Thika) Kenya

A dramatic decline in forest cover in eastern Africa along with a growing population means that timber and poles for building and fuelwood are in short supply. To overcome this shortage, the region is increasingly turning to eucalyptus. But eucalyptus raises environmental concerns of its own. Fears that it will deplete water supply, affect wildlife and reduce associated crop yields have caused many countries in the region to discourage farmers from planting this exotic. This paper is part of a series of investigations on the growth and water use efficiency of faster growing eucalyptus hybrids, which was introduced from South Africa to Kenya. The hypothesis is that the new hybrids are more efficient in using water and more suitable for the semi-arid tropics than existing eucalyptus and two popular agroforestry species. Gas exchange characteristics of juvenile Eucalyptus grandis (W. Hill ex Maiden), two eucalyptus hybrids (E. grandis × Eucalyptus camaldulensis Dehnh.), Grevillea robusta (A. Cunn) and Cordia africana (Lam) was studied under field and pot conditions using an infrared gas analyzer was used to measure photosynthetic active radiation (PAR), net photosynthetic rate (A), stomatal conductance (g _s) and transpiration rate (E) at CO₂ concentrations of 360 μmol mol⁻¹ and ambient humidity and temperature. A, E and g _s varied between species, being highest in eucalyptus hybrid GC 15 (24.6 μmol m⁻² s⁻¹) compared to eucalyptus hybrid GC 584 (21.0 μmol m⁻² s⁻¹), E. grandis (19.2 μmol m⁻² s⁻¹), C. africana (17.7 μmol m⁻² s⁻¹) and G. robusta (11.1 μmol m⁻² s⁻¹). C. africana exhibited high E values (7.0 mmol m⁻² s⁻¹) at optimal soil moisture contents than G. robusta (3.9 mmol m⁻² s⁻¹) and eucalyptus (5.3 mmol m⁻² s⁻¹) in field experiment and G. robusta (3.2 mmol m⁻² s⁻¹) and eucalyptus (4.2 mmol m⁻² s⁻¹) in pot-grown trees. At very low soil moisture content, extremely small g _s values were recorded in GC 15 and E. grandis (8 mmol m⁻² s⁻¹) and G. robusta (14 mmol m⁻² s⁻¹) compared to GC 584 (46.9 mmol m⁻² s⁻¹) and C. africana (90.0 mmol m⁻² s⁻¹) indicating strong stomatal control by the species. Instantaneous water use efficiency ranged between 3 and 5 μmol mmol⁻¹ and generally decreased with decline in soil moisture in pot-grown trees but increased with declining soil moisture in field-grown trees.     

Effect of cold storage treatments and transplanting stress on gas exchange, chlorophyll fluorescence and survival under water limiting conditions of Pinus radiata stock-types

The capacity of radiata pine seedlings to overcome planting shock in wet and dry conditions and their dependence on previous history was analysed by studying post-planting resumption of gas exchange and photochemical reactions, and survival 2 months later. Even under well-irrigated soil conditions, seedlings showed the effects of stress: gas exchange was reduced, but a clear difference between soil-plugged (PR) seedlings and bare-root (BR) seedlings was observed. Drought enhanced the severity of photosynthesis deprivation. Photochemical reactions, analysed by chlorophyll a fluorescence parameters, were not affected by planting shock in conditions of available soil water, but altered dramatically when drought stress was raised, suggesting structural damage of photosynthetic machinery. Despite the dramatic sensitivity of radiata pine to water availability, rewatering produced remarkable recovery, indicating good photosynthetic components repair capacity, which depended, however, on stock quality at the moment of planting. The ability of radiata pine to cope with drought in terms of post-planting performance depended on both storage conditions and water availability at the planting site. These findings provide information for tree physiologists and foresters as to how the management of radiata pine seedlings before planting can affect post-planting performance potential under wet or dry environmental conditions.