Increasing Defoliation Intensity Research Articles

Physiological and growth responses to defoliation of older needles in Abies alba trees grown under two light regimes

In most defoliation experiments with evergreen trees, leaves are removed proportionally across all leaf age classes or only the younger ones are removed. However, it remains unclear how the ecophysiological functions of older foliage potentially differ from those of younger leaves. Hence, we conducted a field experiment with five intensities of artificial defoliation (0%, 11% needle biomass loss = all ≥ 5-year-old needles removed, 27% loss = all ≥ 4-year-old needles removed, 46% loss = all ≥ 3-year-old needles removed, and 71% loss = all ≥ 2-year-old needles removed) of Abies alba trees grown under low-light (LL, 28% of full sunlight) and high-light conditions (HL, 70%). Photosynthesis of the remaining one-year-old needles; concentrations of non-structural carbohydrates (NSCs = soluble sugars + starch), total nitrogen (N), and soluble protein (SP) in stem sapwood and one-year-old needles; and tree height and stem radial growth rates were measured. We found that growth light conditions, rather than old needle defoliation up to a needle biomass loss of 70%, significantly affected growth, photosynthetic capacity and concentrations of tissue N, SP, and NSCs, with levels of all parameters greater in HL trees than in LL tress. Defoliation-induced compensatory growth was found in HL trees only, whereas tree height growth rates tended to decrease with increasing defoliation intensity in LL trees. Our results indicate that light availability, rather than damage to older foliage, plays an important role in determining plant physiology and growth. These results from A. alba trees suggest that the resource uptake by older foliage almost balances their resource consumption, leading to the conclusion that the older needles from evergreen trees physiologically contribute less to the whole-tree carbon and nitrogen balance. Our findings also suggest that the effects of older foliage defoliation on forest survival and productivity may have been overestimated in past forest ecosystem management practices, while the physiological functions of the youngest foliage may have been underestimated.

Are economic thresholds for IPM decisions the same for low LAI soybean cultivars in Brazil?

Economic thresholds (ETs) are well-established for defoliation of soybean, Glycine max, and have been updated for many of the newer cultivars; however, there is increasing grower adoption of cultivars with a reduced leaf area index (LAI). It is of theoretical and practical interest to determine low LAI cultivar tolerance to defoliation. We conducted experiments during two consecutive crop seasons (2017/2018 and 2018/2019) using three soybean cultivars (NS 5959 IPRO, NS 5445 IPRO, and DON MARIO 5.8i) and three defoliation levels (0%, 16.7%, and 33.3%) to evaluate the tolerance of reduced LAI soybean cultivars under different defoliation levels. We observed differences among cultivar's LAI during plant development during both years. Soybean LAI was reduced with increasing defoliation intensity. Tested continuous defoliation levels from plant development stages of V2 to R6 reduced the weight of 1000 seeds and yield but did not impact oil or protein content. Despite our findings that current ET for defoliators in soybean (30% defoliation during vegetative stage and 15% defoliation during reproductive stage) are valid, it is important to consider that continuous defoliation injury impacts the capacity of the plant to respond to injury and must be further evaluated for ET refinement in future research. © 2020 Society of Chemical Industry.

Does defoliation frequency and severity influence plant productivity? The role of grazing management and soil nutrients

Rangeland management approaches, including rotational grazing, rely on assumptions about plant growth responses to the intensity, or severity (sward height) plus frequency, of defoliation. We tested these assumptions at the farm, patch and plant scale using data from a grazing management trial in an Eastern Cape mesic grassland of South Africa along with field plot and glasshouse pot experiments. The grazing trial tested season-long grazing (SLG), four-camp grazing (FCG) and holistic planned grazing (HPG) at equivalent stocking rates over three years. We found that grass growth responses in both potted plants and field plots were reduced under more frequent and severe defoliation but that this was mitigated under elevated soil nutrients, in line with the Compensatory Continuum Hypothesis which predicts that compensatory growth will increase across an increasing fertility gradient. In the farm trial, SLG, which theoretically causes high frequency, low severity defoliation, reduced bare ground cover and increased vegetation greenness with increasing defoliation intensity on nutrient-rich soils. This effect was not present under FCG or HPG and disappeared under very high defoliation intensities and on relatively water- and nutrient-poor soils. Managers are advised to only increase grazing frequency on relatively high nutrient soils, while maximizing recovery on poorer nutrient soils.

Acúmulo de forragem e características morfogênicas e estruturais de Megathyrsus maximus cv. Tamani sob intensidades de desfolhação

Abstract. The effects of defoliation intensity (10, 20, 30 and 40 cm above soil level) on green dry matter (GDM) yield and morphogenetic and structural characteristics of Megathyrsus maximus cv. Tamani were evaluated under field conditions in Roraima´s savannas. The effects of defoliation intensity on the GDM yields, leaf area index and leaf elongation rates was adjusted to the quadratic regression model and maximum values recorded with cutting at 28.2; 33.3 and 21.6 cm above soil level, respectively. The population tiller density, number of leaves tiller-1 and leaf appearance rate was inversely proportional to the defoliation intensity, while the opposite occurred for to average leaf length and leaf senescence rate. Apical meristem removing percentage was higher with increasing defoliation intensity. Regrowth vigor showed close negative correlation with defoliation intensity. Pastures of M. maximus cv. Tamani managed under 25 to 30 cm residue provides higher forage productivity, larger efficiency of forage utilization, greater tissue renewal and canopy structure more favorable to grazing.Keywords: Green dry matter, leaves, tillering, senescence

Experimental Assessment of the Economic Injury Level of the Cereal Leaf Beetle (Coleoptera: Chrysomelidae) in Winter Wheat.

The cereal leaf beetle, Oulema melanopus L., is an economically important pest species, reducing crop yields in many cereal growing parts of the world. Available studies on yield depression in winter wheat in relation to the density of O. melanopus revealed inconsistent results, creating uncertainty about economic thresholds for pest control. We present results from a series of controlled field trials over a 4-yr period (2014-2017) in Switzerland to examine the impact of artificial defoliation (0, 20, 40, and 60% of flag leaf area removed in 1-m2 field plots) on yield and yield components in winter wheat. The applied defoliation treatments correspond to field infestations levels of approximately 2-6 larvae per stem. Analysis of the combined data from 11 location-years revealed a significant linear decrease in yield per ear with increasing defoliation intensity. Yield per ear declined at a rate of 1.14% (SE = 0.28) per 10 percentage-point increase in defoliation. Similar results were found in a parallel series of field trials in which a 40% defoliation treatment was applied to eight common wheat cultivars. Interestingly, however, three cultivars showed complete tolerance in yield to defoliation. Data from two trials with natural defoliation by larvae revealed equally low levels of yield loss by feeding of O. melanopus. Based on the experimentally derived yield loss function, we calculated economic injury levels for a range of crop values. These will serve as benchmarks in the development of IPM decision-support tools for managing cereal leaf beetles in winter wheat.

Impacts of experimental defoliation on native and invasive saplings: are native species more resilient to canopy disturbance?

Many non-native, invasive woody species in mesic forests of North America are both shade tolerant and more productive than their native counterparts, but their ability to tolerate disturbances remains unclear. In particular, complete defoliation associated with herbivory and extreme weather events may have larger impacts on invaders if natives maintain greater resource reserves to support regrowth. On the other hand, invaders may be more resilient to partial defoliation by means of upregulation of photosynthesis or may be better able to take advantage of canopy gaps to support refoliation. Across a light gradient, we measured radial growth, new leaf production, non-structural carbohydrates (NSCs), chlorophyll content and survival in response to varying levels of defoliation in saplings of two native and two invasive species that commonly co-occur in deciduous forests of Eastern North America. Individuals were subjected to one of the four leaf removal treatments: no-defoliation controls, 50% defoliation over three growing seasons, 100% defoliation over one growing season and 100% defoliation over two growing seasons. Contrary to our hypothesis, native and invasive species generally did not differ in defoliation responses, although invasive species experienced more pronounced decreases in leaf chlorophyll following full defoliation and native species' survival was more dependent on light availability. Radial growth progressively decreased with increasing defoliation intensity, and refoliation mass was largely a function of sapling size. Survival rates for half-defoliated saplings did not differ from controls (90% of saplings survived), but survival rates in fully defoliated individuals over one and two growing seasons were reduced to 45 and 15%, respectively. Surviving defoliated saplings generally maintained control NSC concentrations. Under high light, chlorophyll concentrations were higher in half-defoliated saplings compared with controls, which may suggest photosynthetic upregulation. Our results indicate that native and invasive species respond similarly to defoliation, despite the generally faster growth strategy of invaders.

Morphogenetic and structural characteristics of Megathyrsus maximus cv. Centenário under defoliation intensities

The effect of defoliation intensities (20, 30, 40 and 50 cm above soil level) on green dry matter (GDM) yield and morphogenetic and structural characteristics of Megathyrsus maximus cv. Centenário were evaluated under field conditions in Roraima´s savannas. The effects of defoliation intensities on the GDM yields, leaf area index and leaf elongation rates was adjusted to the quadratic regression model and maximum values recorded with cutting at 35.8; 44.9 and 30.2 cm above soil level, respectively. The tiller population density, number of leaves tiller-1 and leaf appearance rate was inversely proportional to the intensity of defoliation, while the opposite occurred for to average leaf length and leaf senescence rate. Apical meristem removal percentage was higher with increasing defoliation intensity. Vigor regrowth showed close negative correlation with defoliation intensity level. Pastures of M. maximus cv. Centenário managed under 30 to 35 cm residue provides higher forage productivity, larger efficiency of forage utilization, greater tissue renewal and canopy structure more favorable to grazing.

Clipping defoliation and nitrogen addition shift competition between a C3 grass (Leymus chinensis) and a C4 grass (Hemarthria altissima).

Human-induced disturbances, including grazing and clipping, that cause defoliation are common in natural grasslands. Plant functional type differences in the ability to compensate for this tissue loss may influence interspecific competition. To explore the effects of different intensities of clipping and nitrogen (N) addition on compensatory growth and interspecific competition, we measured accumulated aboveground biomass (AGB), belowground biomass (BGB), tiller number, non-structural carbohydrates concentrations and leaf gas exchange parameters in two locally co-occurring species (the C3 grass Leymus chinensis and the C4 grass Hemarthria altissima) growing in monoculture and in mixture. For both grasses, the clipping treatment had significant impacts on the accumulated AGB, and the 40% clipping treatment had the largest effect. BGB gradually decreased with increasing defoliation intensity. Severe defoliation caused a significant increase in tiller number. Stored carbohydrates in the belowground biomass were mobilised and transported aboveground for the growth of new leaves to compensate for clipping-induced injury. The net CO2 assimilation rate (A) of the remaining leaves increased with clipping intensity and peaked under clipping intensities of 20% or 40%. Nitrogen addition, at a rate of 10g·N·m-2 ·year-1 , enhanced A of the remaining leaves and non-structural carbohydrate concentrations, which benefited plant compensatory growth, especially for the C3 grass. Under the mixed planting conditions, the clipping and N addition treatments lowered the competitive advantage of the C4 grass. The results suggest that a combination of defoliation and N deposition have the potential to benefit the coexistence of C3 and C4 grasses.

Short-term effects of defoliation intensity on sugar remobilization and N fluxes in ryegrass.

In grassland plant communities, the ability of individual plants to regrow after defoliation is of crucial importance since it allows the restoration of active photosynthesis and plant growth. The aim of this study was to evaluate the effects of increasing defoliation intensity (0, 25, 65, 84, and 100% of removed leaf area) on sugar remobilization and N uptake, remobilization, and allocation in roots, adult leaves, and growing leaves of ryegrass over 2 days, using a 15N tracer technique. Increasing defoliation intensity decreased plant N uptake in a correlative way and increased plant N remobilization, but independently. The relative contribution of N stored before defoliation to leaf growth increased when defoliation intensity was severe. In most conditions, root N reserves also contributed to leaf regrowth, but much less than adult leaves and irrespective of defoliation intensity. A threshold of defoliation intensity (65% leaf area removal) was identified below which C (glucose, fructose, sucrose, fructans), and N (amino acids, soluble proteins) storage compounds were not recruited for regrowth. By contrast, nitrate content increased in elongating leaf bases above this threshold. Wounding associated with defoliation is thus not the predominant signal that triggers storage remobilization and controls the priority of resource allocation to leaf meristems. A framework integrating the sequential events leading to the refoliation of grasses is proposed on the basis of current knowledge and on the findings of the present work.

Características morfogenéticas y estructurales de Megathyrsus maximus cv. Tanzânia-1 bajo intensidades de defoliación

The effect of defoliation intensities (20, 30, 40 and 50 cm above soil level) on green dry matter (GDM) yield and morphogenetic and structural characteristics of Megathyrsus maximus cv. Tanzânia-1 were evaluated under field conditions in Roraima´s savannas. The effects of defoliation intensities on the GDM yields, leaf area index and leaf elongation rates was adjusted to the quadratic regression model and maximum values recorded with cutting at 40.9; 38.7 and 35.5 cm above soil level, respectively. The population tiller density, number of leaves plant-1 and leaf appearance rate was inversely proportional to the intensity of defoliation, while the opposite occurred for to medium blade length and leaf senescence rate. Apical meristem removing percentage was higher with increasing defoliation intensity. Aftermath regrowth showed close negative correlation with defoliation intensity level.

Moth outbreaks alter root-associated fungal communities in subarctic mountain birch forests.

Climate change has important implications on the abundance and range of insect pests in forest ecosystems. We studied responses of root-associated fungal communities to defoliation of mountain birch hosts by a massive geometrid moth outbreak through 454 pyrosequencing of tagged amplicons of the ITS2 rDNA region. We compared fungal diversity and community composition at three levels of moth defoliation (intact control, full defoliation in one season, full defoliation in two or more seasons), replicated in three localities. Defoliation caused dramatic shifts in functional and taxonomic community composition of root-associated fungi. Differentially defoliated mountain birch roots harbored distinct fungal communities, which correlated with increasing soil nutrients and decreasing amount of host trees with green foliar mass. Ectomycorrhizal fungi (EMF) abundance and richness declined by 70-80% with increasing defoliation intensity, while saprotrophic and endophytic fungi seemed to benefit from defoliation. Moth herbivory also reduced dominance of Basidiomycota in the roots due to loss of basidiomycete EMF and increases in functionally unknown Ascomycota. Our results demonstrate the top-down control of belowground fungal communities by aboveground herbivory and suggest a marked reduction in the carbon flow from plants to soil fungi following defoliation. These results are among the first to provide evidence on cascading effects of natural herbivory on tree root-associated fungi at an ecosystem scale.

Effects of defoliation on current-year stem growth and fine root dynamics in Fraxinus mandschurica and Larix gmelinii seedlings

Aims Defoliation by insects leads to significant changes in tree growth and physiological metabolism. However, how fine root dynamics would respond to defoliation is still poorly understood. The objectives of this study were to: 1) compare the responses of height and stem collar diameter to defoliation in manchurian ash (Fraxinus mandschurica) and Dahurian larch (Larix gmelinii) seedlings; 2) quantify the effects of defoliation on the seasonal dynamics of fine root production and mortality in the two species. Methods Manchurian ash and Dahurian larch seedlings, with different biomass allocation and height growth strategy, were used to investigate the effects of different levels of artificial defoliation (0% (control), 40% and 80% leaf area removal) on the growth of aboveground (height and stem collar diameter) and belowground (production and mortality of fine roots (diameter ≤ 2 mm)). Minirhizotron approach was employed to determine the seasonal dynamics of fine roots, and seedling height and stem collar diameter were measured concurrently. Important findings The results showed that: 1) defoliation reduced the growth of seedling height (but statistically not significant) and stem collar diameter in both species, with a stronger effect on the stem collar diameter. Effects on aboveground growth by defoliation were enhanced by increasing defoliation intensity in both species. Compared with the control seedlings, the height and collar diameter of ash were reduced by 3.3% to 12.1% and 5.7% to 23.1%, respectively, while the height and collar diameter of larch were only slightly reduced (< 12%). 2) Defoliation significantly reduced live fine roots in both species (p < 0.001), with the reduction in relative growth rate of fine roots being enhanced with defoliation intensity. 3) Defoliation significantly reduced fine root production in both species (p < 0.05), had not apparent effect on fine root mortality. We conclude that defoliation imposes significant effects on aboveground growth (especially stem collar diameter) in ash and belowground growth 李俊楠等: 去叶对水曲柳和落叶松苗木当年生长及细根动态的影响 1083 doi: 10.3724/SP.J.1258.2014.00102 (particularly root production) in larch. Our findings provide some insights into the inter-specific difference in the response of fine root dynamics to canopy carbon supply.

Simulated impacts of insect defoliation on forest carbon dynamics

Many temperate and boreal forests are subject to insect epidemics. In the eastern US, over 41 million meters squared of tree basal area are thought to be at risk of gypsy moth defoliation. However, the decadal-to-century scale implications of defoliation events for ecosystem carbon dynamics are not well understood. In this study, the effects of defoliation intensity, periodicity and spatial pattern on the carbon cycle are investigated in a set of idealized model simulations. A mechanistic terrestrial biosphere model, ecosystem demography model 2, is driven with observations from a xeric oak–pine forest located in the New Jersey Pine Barrens. Simulations indicate that net ecosystem productivity (equal to photosynthesis minus respiration) decreases linearly with increasing defoliation intensity. However, because of interactions between defoliation and drought effects, aboveground biomass exhibits a nonlinear decrease with increasing defoliation intensity. The ecosystem responds strongly with both reduced productivity and biomass loss when defoliation periodicity varies from 5 to 15 yr, but exhibits a relatively weak response when defoliation periodicity varies from 15 to 60 yr. Simulations of spatially heterogeneous defoliation resulted in markedly smaller carbon stocks than simulations with spatially homogeneous defoliation. These results show that gypsy moth defoliation has a large effect on oak–pine forest biomass dynamics, functioning and its capacity to act as a carbon sink.

Effects of simulated browsing on growth and leaf chemical properties in Colophospermum mopane saplings

AbstractBrowsing intensity influences a plant’s response to herbivory. Plants face a trade‐off between investment in the production of secondary compounds and investment in growth. To elucidate this trade‐off, we simulated four browsing intensities (0%, 50%, 75% and 100%) on mopane saplings, Colophospermum mopane (J.Kirk ex Benth.) J.Léonard, in a greenhouse experiment. This showed that, with increasing defoliation intensity, plants change their investment strategy. At intermediate levels of defoliation (50%), mopane saplings increased the synthesis of condensed tannins, so that tannin concentrations followed a hump‐shaped relation with defoliation intensity, with significantly higher tannin concentration at intermediate defoliation levels. When defoliated heavily (75% and 100%), tannin concentrations dropped, and plants were carbon stressed as indicated by a reduced growth rate of the stem diameter, and leaf production and mean individual leaf mass were reduced. This suggests that, at intermediate defoliation intensity, the strategy of the plants is towards induced chemical defences. With increasing defoliation, the relative costs of the secondary metabolite synthesis become too high, and therefore, the plants change their growing strategy. Hence, browsers should be able to benefit from earlier browsing by either adopting a low or a relatively high browsing pressure.

Effects of fine-scale simulation of deer browsing on soil micro-foodweb structure and N mineralization rate in a temperate forest

The effects of low densities of native browsing mammals on nutrient cycling are not fully understood. Weak browsing may improve nitrogen (N) mineralization in soil and positively affect plant regrowth at the forest floor. To investigate the effects of weak browsing by sika deer ( Cervus nippon) on soil subsystems, we defoliated a dwarf bamboo ( Sasa nipponica)-dominated understory layer in a natural forest at different intensities to realistically simulate deer browsing. Defoliation (0–18% leaf removal) was performed three times at approximately 1-week intervals in summer. We measured water-soluble carbon (C) concentration, phospholipid fatty acid (PLFA) profiles as indicators of microbial community structure, a PLFA of 20:4 as an indicator of protozoan abundance, nematode community structure at the family level, and the N mineralization rate in 28 days of incubation. The effects of defoliation on each soil parameter were determined by comparing before and after defoliation values. The N mineralization rate in the first 10 days of incubation showed a unimodal response to defoliation intensity, with a peak mineralization rate at a defoliation rate (number of removed leaves/total leaves) of 7.6%, correlating with protozoan PLFA and the abundance of Plectidae (the most dominant family of bacterivorous nematodes). In contrast, the N mineralization rate during the following 18 days of incubation decreased monotonically with increasing defoliation intensity, correlating with the water-soluble C concentration in the soil and the C content of new leaves. These results suggest that removing <15% of leaves may have induced a pulse-like release of labile organic matter from roots that lasted for less than 1 week and stimulated N mineralization through microbial loop in soil in the short term (in the first 1–2 weeks after defoliation). N mineralization, however, was reduced with increase of defoliation intensity in the longer term (3–5 weeks after defoliation), possibly because of the reduction in labile organic matter supply from roots 1 week after defoliation. As a result, N mineralization rates over the 28-day incubation period responded to defoliation intensity in a unimodal pattern with a small peak (at a defoliation rate of 4.9%) and were negatively affected by high defoliation rates (>10%). This study suggests that browsing on forest floor plants has positive or negative effects on soil N mineralization potential depending on browsing intensity level.

Defoliation changes mycorrhizal benefit and competitive interactions between seedlings and adult plants

Summary Competition is considered to be one of the main biotic factors shaping plant communities. Experiments on the role of mycorrhizal symbiosis in plant competition have reported conflicting results. We studied the effect of arbuscular mycorrhizal symbiosis on below‐ground interactions between seedlings and established adult plants in a system where plants compete for below‐ground resources. In this glasshouse experiment, seedlings of Gnaphalium norvegicum were grown with and without an arbuscular mycorrhizal fungus, in the presence and absence of a large conspecific adult plant. The ability of adult plants to support the fungus was modified by defoliating 0%, 50% or 75% of its leaf area. We found that mycorrhiza increased below‐ground competitive intensity. The mycorrhizal benefit to the seedlings was low in the vicinity of non‐defoliated adult plants, but increased with increasing defoliation intensity of the adult plant. This was mirrored by reductions in mycorrhizal benefit that adult plants gained at the highest level of defoliation. These results emphasize the importance of below‐ground competition during seedling establishment and show that competition for mycorrhiza‐mediated resources may be an important factor underlying seedling establishment in nutrient‐poor systems. Defoliation of neighbours can increase the beneficial effect of mycorrhizae to seedlings establishing in the vicinity of larger plants, suggesting that grazing or mowing may improve seedling establishment by decreasing below‐ground competition.

Impact of defoliation intensity and frequency on N uptake and mobilization in Lolium perenne

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.

Defoliation Impacts on Festuca campestris (Rydb.) Plants Exposed to Wildfire

Wildfires commonly occur in the Fescue Prairie of Alberta, but little information exists to provide a basis for making grazing recommendations after burning. A wildfire in April 1999 provided an opportunity to study the effect of season and intensity of post-burn defoliation on foothills rough fescue (F. campestris Rydb.) in southwestern Alberta. A 3 (date of defoliation) x 2 (defoliation intensity) factorial experiment with 10 replicates (plants) was established in both a burned and a non-burned grassland and analyzed as a nested design. Plants were defoliated once during active vegetative growth (17 May), inflorescence development (2 July), or dormancy (30 September), at either 5 or 15-cm clipped stubble heights in the first growing season after fire. Burning increased tiller numbers by 54% compared to non-burned plants but reduced plant ANPP by 51% in the second growing season. While a single defoliation of burned plants, particularly early in the year, had little effect on growth, delaying defoliation into July decreased tillers 1 year later. Increasing defoliation intensity had the greatest impact on non-burned plants, reducing plant height (15%) as well as tiller (21%) and plant (32%) ANPP in the second year. May defoliation reduced etiolated growth 1 year later regardless of burn treatment. A single grazing event after wildfire does not necessarily appear to detrimentally affect rough fescue; however, the low herbage available immediately after fire may not justify the increased risk to the plant with subsequent grazing. DOI:10.2458/azu_jrm_v56i4_bogen

Defoliation impacts on Festuca campestris (Rydb.) plants exposed to wildfire

Wildfires commonly occur in the Fescue Prairie of Alberta, but little information exists to provide a basis for making grazing recommendations after burning. A wildfire in April 1999 provided an opportunity to study the effect of season and intensity of post-burn defoliation on foothills rough fescue (F. campestris Rydb.) in southwestern Alberta. A 3 (date of defoliation) x 2 (defoliation intensity) factorial experiment with 10 replicates (plants) was established in both a burned and a non-burned grassland and analyzed as a nested design. Plants were defoliated once during active vegetative growth (17 May), inflorescence development (2 July), or dormancy (30 September), at either 5 or 15-cm clipped stubble heights in the first growing season after fire. Burning increased tiller numbers by 54% compared to non-burned plants but reduced plant ANPP by 51% in the second growing season. While a single defoliation of burned plants, particularly early in the year, had little effect on growth, delaying defoliation into July decreased tillers 1 year later. Increasing defoliation intensity had the greatest impact on non-burned plants, reducing plant height (15%) as well as tiller (21%) and plant (32%) ANPP in the second year. May defoliation reduced etiolated growth 1 year later regardless of burn treatment. A single grazing event after wildfire does not necessarily appear to detrimentally affect rough fescue; however, the low herbage available immediately after fire may not justify the increased risk to the plant with subsequent grazing.

Intermediate Wheatgrass and Russian Wildrye Responses to Defoliation and Moisture

Perennial forage grasses in the Northern Great Plains are often grazed under water-limiting conditions. The effects of defoliation and soil moisture dynamics on herbage yield, tiller recruitment and number of crown positions for 2 perennial forage grasses were evaluated in a greenhouse experiment at Mandan, N.D. Intermediate wheatgrass (Thinopyrum intermedium (Host) Barkw. &D.R. Dewey] and Russian wildrye (Psathyrostachys juncea (Fisch.) Nevski] were grown at 75, 50, or 40% of field capacity and left as undefoliated controls or defoliated at 10-day intervals to an 8- or 4-cm stubble height. Cumulative herbage yield exhibited defoliation level by soil moisture (P = 0.0001) and species by defoliation (P = 0.007) interactions. Yield decreased with increasing defoliation intensity at 75 and 50% of field capacity, but at 40% of field capacity only the most intense defoliation level was significantly affected. Russian wildrye produced more herbage (1.97 g plant(-1)) than intermediate wheatgrass (1.36 g plant(-1)) under severe defoliation level. Increased defoliation intensity (P = 0.0001) but not water availability (P > 0.05) decreased tiller numbers. Pooled across all treatments, Russian wildrye produced 10 tillers per plant and intermediate wheatgrass produced 7 (P = 0.0001). This may partially explain Russian wildrye's greater grazing tolerance. The number of crown positions (potential axillary bud sites in the bottom 20mm of the plant) was similar between species suggesting that increased tiller numbers in Russian wildrye occurred because its axillary buds were more readily activated than intermediate wheatgrass. Moderate and severe defoliation reduced the number of crown positions on parent seedlings to only 62 and 50% of the number of crown positions of control seedlings, respectively. Water stress decreased (P = 0.004) number of crown positions at 40% of field capacity but only when crown positions of both parent and daughter tillers were pooled. In this experiment, defoliation had a greater effect than water stress. Moisture level mainly affected tiller size not numbers. The interaction between defoliation and water stress should be examined in field studies to foster improved management of these 2 grasses. DOI:10.2458/azu_jrm_v55i1_hendrickson