Defoliation Intensity Research Articles

Radial-growth and wood-anatomical changes in overaged Quercus pyrenaica coppice stands: functional responses in a new Mediterranean landscape

Recent land-use changes in intensively managed forests such as Mediterranean coppice stands might profoundly alter their structure and function. We assessed how the abandonment of traditional management practices in coppice stands, which consisted of short cutting-cycles (10–15 years), has caused overaging (stems are usually much older than when they were coppiced) and altered their wood anatomy and hydraulic architecture. We studied the recent changes of wood anatomy, radial growth, and hydraulic architecture in two stands of Quercus pyrenaica, a transitional Mediterranean oak with ring-porous wood forming coppice stands in W–NW Spain. We selected a xeric and a mesic site because of their contrasting climates and disturbance histories. The xeric site experienced an intense defoliation after the severe 1993–1994 summer drought. The mesic site was thinned in late 1994. We studied the temporal variability in width, vessel number and diameter, and predicted the hydraulic conductivities (Kh) of earlywood and latewood. In the mesic site, we estimated the vulnerability to xylem cavitation of earlywood vessels. Overaging caused a steep decline in latewood production at a cambial age of 14 years., which was close to the customary cutting cycle of Q. pyrenaica. The diameter distribution of vessels was bimodal, and latewood vessels only accounted for 4% of the Kh. Overaging, acting as a predisposing factor in the decline episode, was observed at the xeric site, where most trees did not produce latewood in 1993–1995. At the mesic site, thinned trees formed wider tree-rings, more latewood and multiseriate tree-rings than overaged trees. The growth enhancement remained 8 years after thinning. Most of the hydraulic conductivity in earlywood was lost in a narrow range of potentials, between −2.5 and −3.5 MPa. We have shown how hydraulic conductivity and radial growth are closely related in Q. pyrenaica and how aging modulates this relationship.

Distribuição dos fotoassimilados em plantas de Panicum maximum cv. Mombaça

Os fotoassimilados produzidos pelas folhas são translocados por toda a planta, podendo variar conforme a fase de desenvolvimento, inserção da folha, desfolha, entre outros fatores. A distribuição dos fotoassimilados de folhas individuais para os diversos componentes da planta de Panicum maximum cv. Mombaça foi avaliada em função do nível de inserção da folha exposta ao 14CO2 (folha recém-expandida x folha no segundo nível de inserção abaixo), de diferentes perfilhos (perfilho principal, primeiro e terceiro perfilhos primários), de planta intacta ou submetida à remoção das lâminas foliares no nível da lígula, com exceção da folha exposta ao 14CO2. Apesar de os componentes estudados apresentarem quantidades distintas de 14C, significância para o efeito dos tratamentos foi constatada apenas para a porcentagem de 14C presente na parte aérea dos perfilhos, com valores mais altos correspondendo ao perfilho portador da folha exposta, independentemente da hierarquia do perfilho. A parte aérea reteve quantidade de 14C maior que o sistema radicular. A translocação de assimilados da folha da base tendeu a priorizar o conjunto de outros perfilhos, enquanto a folha do topo priorizou a translocação de assimilados para o próprio perfilho. De todos os componentes estudados, a folha do topo teve como principal dreno o meristema terminal, independentemente do tipo de perfilho portador da folha exposta e da condição de desfolhação.

Fundamentos de manejo de praderas para mejorar la productividad de la ganadería del trópico colombiano

Las especies forrajeras constituyen la principal fuente de alimentación de los bovinos en Colombia; en la mayoría de las explotaciones son el único alimento debido a las condiciones climáticas favorables para la producción de forraje a través del año y a lo poco práctico que resulta alimentar rumiantes con granos o subproductos agroindustriales, debido al alto costo y la baja oferta de estos recursos en el mercado nacional. El manejo del pastoreo es el aspecto menos entendido en las explotaciones y ocurren con frecuencia eventos de sobrepastoreo y subpastoreo a lo largo de un mismo año, con reducciones drásticas en la producción animal y en la persistencia del pasto, lo que conduce a una degradación rápida de las praderas. En este artículo se revisan las principales respuestas fisiológicas de las praderas en pastoreo y se incluye un análisis de los efectos del animal sobre las plantas y el suelo. Así mismo, se compendian los mecanismos de respuesta a la defoliación, el papel de las reservas orgánicas, de los meristemos y del área foliar residual, al igual que el manejo del pastoreo, con sus variables de frecuencia e intensidad de la defoliación y su efecto sobre la composición botánica el valor nutritivo y la producción de forraje de la pradera. Finalmente, se analiza brevemente la productividad de las praderas, la producción animal y las variables de respuesta bajo pastoreo.

Spatiotemporal Description of Epidemics Caused by Phoma ligulicola in Tasmanian Pyrethrum Fields

ABSTRACT Spatial and temporal patterns of foliar disease caused by Phoma ligulicola were quantified in naturally occurring epidemics in Tasmanian pyrethrum fields. Disease assessments (defoliation incidence, defoliation severity, incidence of stems with ray blight, and incidence of flowers with ray blight) were performed four times each year in 2002 and 2003. Spatial analyses based on distribution fitting, runs analysis, and spatial analysis by distance indices (SADIE) demonstrated aggregation in fields approaching their first harvest for all assessment times between September and December. In second-year harvest fields, however, the incidence of stems with ray blight was random for the first and last samplings, but aggregated between these times. Spatiotemporal analyses were conducted between the same disease intensity measures at subsequent assessment times with the association function of SADIE. In first-year harvest fields, the presence of steep spatial gradients was suggested, most likely from dispersal of conidia from foci within the field. The importance of exogenous inoculum sources, such as wind-dispersed ascospores, was suggested by the absence of significant association between defoliation intensity (incidence and severity) and incidence of stems with ray blight in second-year harvest fields. The logistic model provided the best temporal fit to the increase in defoliation severity in each of six first-year harvest fields in 2003. The logistic model also provided the best fit for the incidence of stems with ray blight and the incidence of flowers with ray blight in four of six and three of six fields, respectively, whereas the Gompertz model provided the best fit in the remaining fields. Fungicides applied prior to mid-October (early spring) significantly reduced the area under disease progress curve (P < 0.001) for defoliation severity, the incidence of stems with ray blight, and the incidence of flowers with ray blight for epidemics at all field locations. This study provides information concerning the epidemiology of foliar disease and ray blight epidemics in pyrethrum and offers insight on how to best manage these diseases.

Competition between Artemisia frigida and Cleistogenes squarrosa under different clipping intensities in replacement series mixtures at different nitrogen levels

AbstractThe relative competitive ability of Artemisia frigida Willd. (C3, semi‐shrub) and Cleistogenes squarrosa (Trin.) Keng (C4, perennial bunchgrass), grown in a greenhouse, was studied under three nitrogen fertilizer levels, three defoliation intensities and five competitive replacement series in a factorial arrangement. Relative yield total and the respective crowding coefficients for each species were calculated with reference to aboveground, below‐ground and total biomass. The index of aggressivity was also determined relative to total biomass. Cleistogenes squarrosa was a consistently weak competitor at all nitrogen fertilizer and clipping levels compared with A. frigida. Artemesia frigida had a higher relative yield when grown in mixtures with C. squarrosa than in monocultures, whereas the yield of C. squarrosa was lower in mixtures with A. frigida than in monocultures. The relative competitive ability of A. frigida over C. squarrosa declined significantly at different clipping intensities with increasing nitrogen fertilizer. The interaction of nitrogen fertilizer with clipping was significant, although the clipping effect on aggressivity was not significant.

Automated detection and mapping of crown discolouration caused by jack pine budworm with 2.5 m resolution multispectral imagery

Jack pine budworm (Choristoneura pinus pinus (Free.)) is a native insect defoliator of mainly jack pine (Pinus banksiana Lamb.) in North America east of the Rocky Mountains. Periodic outbreaks of this insect, which generally last two to three years, can cause growth loss and mortality and have an important impact ecologically and economically in terms of timber production and harvest. The jack pine budworm prefers to feed on current year needles. Their characteristic feeding habits cause discolouration or reddening of the canopy. This red colouration is used to map the distribution and intensity of defoliation that has taken place that year (current defoliation). An accurate and consistent map of the distribution and intensity of budworm defoliation (as represented by the red discolouration) at the stand and within stand level is desirable.Automated classification of multispectral imagery, such as is available from airborne and new high resolution satellite systems, was explored as a viable tool for objectively classifying current discolouration. Airborne multispectral imagery was acquired at a 2.5m resolution with the Multispectral Electro-optical Imaging Sensor (MEIS). It recorded imagery in six nadir looking spectral bands specifically designed to detect discolouration caused by budworm and a near-infrared band viewing forward at 35° was also used. A 2200nm middle infrared image was acquired with a Daedalus scanner. Training and test areas of different levels of discolouration were created based on field observations and a maximum likelihood supervized classification was used to estimate four classes of discolouration (nil-trace, light, moderate and severe). Good discrimination was achieved with an overall accuracy of 84% for the four discolouration levels. The moderate discolouration class was the poorest at 73%, because of confusion with both the severe and light classes. Accuracy on a stand basis was also good, and regional and within stand discolouration patterns were portrayed well. Only three or four well-placed spectral bands were needed for a good classification. A narrow red band, a near-infrared and short wave infrared band were most useful. A forward looking band did not improve discolouration estimation, but further testing is needed to confirm this result.This method of detecting and classifying discolouration appears to provide a mapping capability useful for conducting jack pine budworm discolouration surveys and integrating this information into decision support systems, forest inventory, growth and yield predictions and the forest management decision-making process.

Infrequent and intense defoliation benefits dry‐matter accumulation and persistence of clipped Arrhenatherum elatius

AbstractThe objective was to determine how the frequency and intensity of defoliation of tall oat grass [Arrhenatherum elatius (L.) Presl.], affected (i) annual dry‐matter (DM) accumulation, (ii) seasonal distribution of the accumulated forage, and (iii) morphological traits related to persistence and vigour. Data were collected twice a year throughout three growing years, beginning the second year after establishment. Two frequencies (A: each time modal height of plants reached 20 cm; B: each time modal height of plants reached 40 cm) and two defoliation intensities (I: 5 cm; II: 10 cm stubble) were arranged in a completely randomized block design with a fully factorial combination and four replicates. Measurements were made of annual DM accumulation, seasonal distribution of accumulated DM and morphological traits related to persistence and vigour, i.e. number of tillers m−2, number of tussocks m−2, crown diameter, and crown area of tussocks. The infrequent defoliation (B) increased annual DM accumulation besides maintaining levels of persistence and vigour. The most productive treatment (BI) had a significantly higher annual DM accumulation than treatments AI and AII. Frequent and severe defoliations (treatment AI) led to plant depletion, which reduced crown diameter, number of tillers m−2 and crown area at the end of the third year compared with infrequent but severe defoliation (treatment BI). Annual DM accumulation was intermediate for less intense defoliations (AII), which produced forage during winter, but resulted in less vigorous plants by the end of the experiment. Crown area was a good indicator of tiller number, particularly under frequent defoliation.

Rapid chemical responses of Acacia karroo to early dormant season defoliation in a semi-arid subtropical savanna

Research was conducted to investigate rapid chemical responses of Acacia karroo to severe defoliation in a semi-arid, subtropical savanna. A flock of 250 goats reduced available foliage by 87% in a 10.5-ha area of Acacia woodland over 3 weeks during May (early dormant season). During the first 10 days, concentrations of nitrogen (N), neutral detergent fibre (NDF) and condensed tannin (CT) did not change. The concentration of potentially hepatotoxic alkaloids (PHAs) decreased. There were no immediately detectable effects of defoliation intensity or recovery time on any of the measured chemical variables. There were no relationships between N and NDF, CT or PHA. Given that the treatments were applied at the start of the dormant season, the metabolism of mature or senescing leaves may have precluded rapid increases in defence. Because a previous study under similar browsing conditions showed increased defence in A. karroo at the same time of year, it was concluded that intra-annual variations in rainfall, and the resulting variations in plant growth activity, lead to intra-annual variations in plant responses to defoliation in semi-arid subtropical savannas. Such variations need to be accounted for in future experiments aimed at investigating induced responses to browsing in savannas.

The productivity of irrigated legumes in northern Victoria. 3. Frequency and intensity of defoliation of subterranean clover

An experiment was undertaken to consider the effects of a range of defoliation treatments on aspects of the productivity of a subterranean clover (Trifolium subterraneum L.) sward (predominantly cv. Trikkala), with particular emphasis on flowering and seed production. There were 2 heights of defoliation, 2.8 and 7.0 cm (rising plate meter) above ground level, and at each height of defoliation, there were 4 intervals of harvest, 4, 6, 9 and 12 weeks. Treatments were arranged randomly within blocks, and there were 4 replicates. Harvests were conducted between autumn (6 April) and spring (13 November). The total production of herbage varied from 5.7 to 8.7 t DM/ha of harvested material, with variations in response due to both height and frequency of defoliation. Harvested yield increased from 6.2 to 7.4 t DM/ha (P&lt;0.05) as interval between harvests was extended, and plots harvested to 2.8 cm above ground level yielded more (7.7 t DM/ha; P&lt;0.05) harvested herbage than those harvested to 7.0 cm (6.0 t DM/ha). While there were differences in production due to defoliation management, nutritive characteristics (DM digestibility, crude protein, neutral detergent fibre) of harvested herbage were similar in all treatments throughout the year. Production of above and below ground seed increased (P&lt;0.05) as interval between harvest increased, and was greater (P&lt;0.05) at the higher defoliation height. Defoliating at 4- and 6-week intervals interrupted the reproductive development of Trikkala; late September–early October harvests substantially reduced both subsequent petiole extension and flower production. This resulted in Trikkala seeds of lower weight in the above ground seed pool (P&lt;0.05). This may have impacted on regenerating ability in the next autumn because Trikkala seeds that were less than about 4 mg in weight were less likely to germinate than heavier seed. In conclusion, the highest dry matter production occurred with intensive, infrequent defoliation whereas highest seed yields were produced with lax, infrequent defoliation.

Effect of defoliation intensity of barley when grazed as green forage by sheep on grain production at harvest

Grazing of barley as green forage is a common practice in the Mediterranean area due to its active growth along autumn and winter, and its ability to recover after a mild grazing (Bonachela et al., 1995). However, grain production at harvest might be affected depending on the intensity of defoliation suffered by the plant (Yau, 2003). Those studies were carried out mainly by hand clipping or allowing only a few hours of grazing per day. Hence, the objective of this experiment was to establish the relationship between grazing intensity and grain production at harvest when animals are kept in the paddocks for the whole day. Moreover, the ability of barley height and/or residual biomass after grazing (kg dry matter/ha) to predict final grain production was also tested.

Obligatory Summer‐Dormant Cool‐Season Perennial Grasses for Semiarid Environments of the Southern Great Plains

Introduced cool‐season perennial grasses may become an important complementary winter forage to dual‐use wheat (Triticum aestivum L.) in high‐risk semiarid environments of the southern Great Plains. Currently recommended, summer semidormant cultivars are not adapted to prolonged and severe summer drought and not productive in the autumn grazing season. In an experiment planted at Vernon, TX, on a Miles fine sandy loam (fine‐loamy, mixed, thermic Udic Paleustalfs) in October 2000, we evaluated productivity and plant survival of an obligatory summer‐dormant ‘Grasslands Flecha’ and summer semidormant ‘Georgia 5’, ‘Jesup’, and ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.); a highly summer semidormant ‘Grasslands Maru’ hardinggrass (Phalaris aquatica L.); and summer semidormant ‘Grasslands Matua’ and ‘Grasslands Tango’ prairiegrass (Bromus wildenowii Kunth) under two defoliation intensities of 7.5 and 15 cm. Georgia 5, Jesup, and Grasslands Flecha were either infected with the novel Neotyphodium coenophialum Glenn, Bacon, and Hanlin endophyte strain AR542, with their endemic endophytes (except for Grasslands Flecha), or noninfected (E–). Only Grasslands Flecha and Grasslands Maru successfully survived summer droughts during 2001–2004. Prairiegrass behaved as an annual but did not reseed in 2003. Aboveground biomass was greater at 15‐ vs. 7.5‐cm defoliation height, except for 2004 growing season. In Grasslands Flecha, the novel endophyte increased tiller survival during summer drought by 150% (2001) and 121% (2002) when compared with E– plants. Obligatory and highly summer semidormant cultivars of perennial cool‐season grasses may be productive and persistent in the southern Great Plains and have the potential to complement forage from dual‐use wheat pastures.

Septoria urticae . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Septoria urticae . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. DISEASE: Leaf spot, severe foliar necrosis, defoliation. Sometimes spots eventually fall out and leave a round shot-hole (GROVE, 1935). In inoculation experiments the fungus has caused complete wilting of the leaves followed by the intense defoliation. The disease can affect plant growth and can considerably reduce numbers of seeds produced (DAL-BELLO et al. , 1993). HOSTS: Laportea canadensis, Parietaria officinalis, Urtica angustifolia, U. cannabina, U. chamaedryoidis, U. dioica, U. gracilenta, U. gracilis, U. lyallii, U. pilulifera, U. urens ( Urticaceae ). GEOGRAPHICAL DISTRIBUTION: NORTH AMERICA: Canada, USA. SOUTH AMERICA: Argentina. ASIA: Azerbaijan, Republic of Georgia [ www.cybertruffle.org.uk/gruzmaps/index.htm ], India, Iran, Israel, Kazakhstan. AUSTRALASIA: Australia. EUROPE: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, France, Germany, Great Britain, Greece, Hungary, Italy, Latvia, Lithuania, Moldova, Netherlands, Norway, Poland, Romania, Spain, Sweden, Ukraine [ www.cybertruffle.org.uk/ukramaps/index.htm ], former Yugoslavia. TRANSMISSION: Not reported, but almost certainly by airborne or splash-dispersed conidia, also from infected plant debris. Successful application of spore suspensions in water and humid chambers in inoculation experiments with S. urticae confirms the role of moisture in infection transmission.

Regrowth and production of herbaceous riparian vegetation following defoliation

Stubble height regulations are frequently used to manage livestock grazing of herbaceous riparian vegetation. The objective of this study was to determine the impact of stubble height, time of clipping and soil water status on production and regrowth of herbaceous riparian vegetation. We used a randomized block design with 4 study sites on each of 3 small (< 2m width) streams in northern Harney County, Ore. In June and July of 2000–2003, 40 × 50 cm experimental plots were clipped to stubble heights of 5.1 (2 inch), 10.2 (4 inch), or 15.2 cm (6 inch), and paired control plots were left unclipped. Complete treatment sets were located adjacent to the stream and 4 m from the stream at each site. All plots were clipped to 1 cm in October and regrowth was calculated by comparing clipped and control plots. Water table depth was measured weekly using PVC wells. Results indicate that height regrowth was associated positively with stubble height (P < 0.01) and was less with July compared to June clipping (P = 0.02). Weight regrowth was also positively related to stubble height (P < 0.01) and decreased with July compared to June clipping (P = 0.04) whereas annual aboveground production increased with July clipping (P = 0.02). Annual production values for clipped plots were higher than for unclipped plots, indicating compensatory production in response to defoliation. Plots distant from the stream had less water availability, but regrowth and production were not strongly influenced by distance from active stream channel. Timing and intensity of defoliation were reliable predictors of regrowth and production performance. Most clipping height × time combinations produced end of season heights sufficient to meet current federal stubble height requirements (i.e., 10–15 cm). Our results provide insight on the timing and intensity of defoliation that will allow for adequate regrowth to meet different management objectives. However, other factors such as stream channel morphology, animal selectivity, and annual weather variation will need to be considered.

Sawfly (Hym., Diprionidae) outbreaks on Scots pine: effect of stand structure, site quality and relative tree position on defoliation intensity

From 1997 to 2001, Finland has experienced its largest-ever documented outbreak of Diprion pini L. (500 000 ha). We investigated—in four affected areas—the effects of site quality, stand structure and relative tree position on outbreak dynamics during two consecutive years (1999–2000). Most of the variation in outbreak intensity, as measured by defoliation intensity, was accounted for by variables pertaining to tree size, indicating a preference for more mature trees. However, we also found elevated and systematic defoliation of younger stands. Tree species composition, age difference with surrounding stands, stand area, stand openness and soil characteristics are secondary to the tree size variables in explaining the defoliation intensity, and often had ambiguous effects. As for the common assumption linking outbreaks to shallow, well-drained and unproductive soils, our data only suggest such a link on the level of pine forest vulnerability on a landscape level. As for the various tree canopy strata, we found only in young stands significant differences in defoliation intensity between them, with the dominant trees most affected. This has implications for forest management, because these trees, now undergoing the highest growth losses due to defoliation, normally benefit as trees of the future from thinning activities.

Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus

In the present study, ‘natural 13 C tracer method’ was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C3 plant species, Helianthus annuus (sunflower), was grown in ‘C4’ soil in microcosms so that the CO2 evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO2 evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and <74%.

Sawfly (Hym., Diprionidae) outbreaks on Scots pine: effect of stand structure, site quality and relative tree position on defoliation intensity

From 1997 to 2001, Finland has experienced its largest-ever documented outbreak of Diprion pini L. (500 000 ha). We investigated—in four affected areas—the effects of site quality, stand structure and relative tree position on outbreak dynamics during two consecutive years (1999–2000). Most of the variation in outbreak intensity, as measured by defoliation intensity, was accounted for by variables pertaining to tree size, indicating a preference for more mature trees. However, we also found elevated and systematic defoliation of younger stands. Tree species composition, age difference with surrounding stands, stand area, stand openness and soil characteristics are secondary to the tree size variables in explaining the defoliation intensity, and often had ambiguous effects. As for the common assumption linking outbreaks to shallow, well-drained and unproductive soils, our data only suggest such a link on the level of pine forest vulnerability on a landscape level. As for the various tree canopy strata, we found only in young stands significant differences in defoliation intensity between them, with the dominant trees most affected. This has implications for forest management, because these trees, now undergoing the highest growth losses due to defoliation, normally benefit as trees of the future from thinning activities.

Herbivory and climatic warming: a Mediterranean outbreaking caterpillar attacks a relict, boreal pine species

Climate change can harm many species by disrupting existing interactions or by favouring new ones. This study analyses the foreseeable consequences of climatic warming in the distribution and dynamics of a Mediterranean pest that causes severe defoliation, the pine processionary caterpillar Thaumetopoea pityocampa, and the effects upon the relict Andalusian Scots pine Pinus sylvestris nevadensis in the Sierra Nevada mountains (southeastern Spain). We correlated a set of regional data of infestation by T. pityocampa upon Scots pine, from a broad ecological gradient, with climatic data for the period 1991-2001, characterized by alternating warm and cold winters. Defoliation intensity shows a significant association with previous warm winters, implying that climatic warming will intensify the interaction between the pest and the Scots pine. The homogeneous structure of the afforested pine woodlands favours the outbreak capacity of the newcomer, promoting this new interaction between a Mediterranean caterpillar pest and a boreal tree at its southern distribution limit.

Regrowth and production of herbaceous riparian vegetation following defoliation

Stubble height regulations are frequently used to manage livestock grazing of herbaceous riparian vegetation. The objective of this study was to determine the impact of stubble height, time of clipping and soil water status on production and regrowth of herbaceous riparian vegetation. We used a randomized block design with 4 study sites on each of 3 small ( 2m width) streams in northern Harney County, Ore. In June and July of 2000-2003, 40 × 50 cm experimental plots were clipped to stubble heights of 5.1 (2 inch), 10.2 (4 inch), or 15.2 cm (6 inch), and paired control plots were left unclipped. Complete treatment sets were located adjacent to the stream and 4 m from the stream at each site. All plots were clipped to 1 cm in October and regrowth was calculated by comparing clipped and control plots. Water table depth was measured weekly using PVC wells. Results indicate that height regrowth was associated positively with stubble height (P &lt; 0.01) and was less with July compared to June clipping (P = 0.02). Weight regrowth was also positively related to stubble height (P &lt; 0.01) and decreased with July compared to June clipping (P = 0.04) whereas annual aboveground production increased with July clipping (P = 0.02). Annual production values for clipped plots were higher than for unclipped plots, indicating compensatory production in response to defoliation. Plots distant from the stream had less water availability, but regrowth and production were not strongly influenced by distance from active stream channel. Timing and intensity of defoliation were reliable predictors of regrowth and production performance. Most clipping height × time combinations produced end of season heights sufficient to meet current federal stubble height requirements (i.e., 10-15 cm). Our results provide insight on the timing and intensity of defoliation that will allow for adequate regrowth to meet different management objectives. However, other factors such as stream channel morphology, animal selectivity, and annual weather variation will need to be considered.

Direct assessment of competitive ability and defoliation tolerance in perennial grasses

Stipa clarazii is a dominant, late-seral perennial grass species under exclosure or lightly grazed rangelands in semiarid Argentina, whereas S. tenuis and S. ambigua are earlier-seral perennial tussock grasses. Recent studies have demonstrated that late-seral species are more competitive and can have either similar or greater herbivory tolerance than earlier-seral species. We hyphothesized that (1) tolerance to defoliation is greater on defoliated plants of S. clarazii than on those of S. tenuis and S. ambigua, (2) competitive ability is greater in S. clarazii than in S. tenuis and S. ambigua when plants remain undefoliated or are exposed to a comparable defoliation intensity, and (3) competitive ability decreases in S. clarazii when this species is selectively defoliated within a non-defoliated, nearby neighbourhood of S. tenuis or S. ambigua. The study objectives included obtaining a direct measure of competitive ability and defoliation tolerance in the three perennial grasses when they grew either in isolation or in different neighbourhoods and were exposed to various defoliation patterns in the field. In 1998, two parallel studies were conducted within a 2-yr-old field exclosure during one growing season. In one study, responses were evaluated on either defoliated or undefoliated plants of all three species without neighbours. In a second experiment, four types of neighborhoods were selected such that a central (target) plant of one species was surrounded by five neighboring plants of a different species. Three different defoliation patterns were imposed on each neighbourhood type. Results led to acceptance of the first and second and rejection of the third hypothesis. When plants grew without nearby neighbours, greater growth rates for height, dry matter production and end-of-season daughter tiller production per parent tiller in S. clarazii relative to S. tenuis and S. ambigua contributed to greater regrowth in S. clarazii. Amounts of 15N atom excess were always greater in S. clarazii than in S. tenuis and S. ambigua independently of neighbourhood type and pattern of defoliation. However, uptake of 15N was similar in S. clarazii to that in S. tenuis or S. ambigua when S. clarazii was selectively defoliated within a non-defoliated, nearby neighbourhood of any of the other two species. Selective herbivory of S. clarazii, rather than factors associated with its competitive ability and defoliation tolerance, were found to be the major determinant driving its replacement by earlier-seral, less desirable perennial grasses in rangelands of central Argentina. Key words: 15N, Argentina rangelands, biomass, nitrogen uptake, species replacement, Stipa

EFFECTS OF A SEVERE DROUGHT ON GROWTH AND WOOD ANATOMICAL PROPERTIES OF QUERCUS FAGINEA

We studied the growth response to drought of a Quercus faginea Lam. stand in a xeric site in NE Spain, that experienced an intense defoliation in 1993–94. This event coincided with very low precipitation from November to February, the period when total monthly precipitation exceeds evapotranspiration. We evaluated the effects of November–February precipitation (recharge precipitation, RP) on internode length, radial growth, and wood anatomy. Quercus faginea showed reduced longitudinal and radial growth during the years with low RP, and most sampled trees did not produce latewood in 1993–94 but showed wide earlywood vessels. We observed the reverse for years with a high RP. Radial growth was enhanced by increased precipitation during January and May of the growth year. If severe droughts become more frequent, due to a greater climatic variability, extensive dieback of marginal Q. faginea populations may be expected.