Unclipped Plots Research Articles

Plasticity in over-compensatory growth along an alpine meadow degradation gradient on the Qinghai-Tibetan Plateau

Over-compensatory growth of plants after disturbance is generally preferred by grassland users and managers because of more forage. How the grassland productivity and the plant growth condition before disturbance affect the compensatory growth are important for grazing management and the understanding of grassland degradation, yet they are not well understood. A clipping experiment was conducted on the Qinghai-Tibetan Plateau to understand the compensatory growth and conditions for the occurrence of over-compensatory at alpine meadows with different degradation status. Results showed the competition for light constrains the plant growth post-clipping at non-degraded and slightly degraded alpine meadows, while the reduction of soil nitrogen limits it at heavily degraded alpine meadow. The biomass accumulated post-clipping was positively correlated with the growing season biomass in unclipped plots and the biomass at clipping in clipped plots. When the aboveground biomass at clipping was less than 40.10 g m−2 and the growing season biomass was between 38 and 97 g m−2, the over-compensatory growth of alpine meadow could occur. Higher clipping rate is required for the alpine meadow with high productivity but the maximum clipping rate should be less than 0.71 to induce the over-compensatory growth. Equal-compensatory occurred at non-degraded and slightly degraded, while over-compensatory growth was observed at moderately degraded and a marginally significant over-compensatory growth at heavily degraded alpine meadow. The over-compensatory growth occurred at moderately degraded alpine meadow is mainly due to the performance of forbs. Our results suggest that grazing at moderately degraded alpine meadow may induce the over-compensatory growth at the community level, but the over-compensatory growth of forbs at moderately degraded alpine meadow may aggravate the alpine meadow degradation.

Response of Soil Moisture to Infrequent Heavy Defoliation of Chemically Thinned Juniper Woodland

In semiarid regions of the western United States, there is heightened interest in tree removal to increase water availability for other uses such as forage growth and groundwater recharge. This study was conducted in central New Mexico to determine the effects of heavy infrequent defoliation of chemically thinned juniper woodland (Juniperus monosperma) on soil moisture. Each of three cattle-grazing exclosures (CD, FG, and KI) was instrumented: 1) beneath trees with a set of three soil moisture probes (0−25, 25−50, and 50−75 cm depth) and one soil temperature probe under live trees (control) and dead trees (herbicide-treated); and 2) between trees with one soil moisture and one soil temperature probe in control and herbicide-treated intercanopy plots. Each plot had three clipped and three unclipped subplots. Mean daily maximum surface soil temperature was highest (17.19°C) in intercanopy, intermediate (16.13°C) under herbicide-treated, and lowest (14.90°C) under control trees. Topsoil moisture (0−25 cm depth) was different among treatment combinations from late July to early September 2006. Thus, the control unclipped combination had the highest topsoil moisture while the herbicide-treated unclipped combination had the lowest topsoil moisture. Comparing other depths, control unclipped plots had higher soil moisture in the middle layer (25−50 cm) and bottom layer (50−75 cm) than at the top from late August to early November 2006. Results imply that clipping on chemically thinned juniper woodlands does not increase soil moisture at any depth, yet macropore flow and water absorption on deep soil layers, underneath live trees, might help to store soil moisture for longer periods in water-limited environments.

Climate change and defoliation interact to affect root length across northern temperate grasslands

Abstract Grassland plants, especially their root systems, are dynamic and can buffer changes resulting from exposure to multiple stressors; however, the interactive stressor effects on root dynamics and associated above‐ground growth are poorly understood. Here, we examine the effects of changed precipitation and air temperature, and defoliation intensity on root length dynamics and above‐ground biomass using the third year data from a multifactor experiment conducted across three northern temperate grasslands. We found that root length was more sensitive to the changes in environmental and management conditions than root mass, demonstrating the importance of root length as an indicator of rapid root system changes. Across all sites, warming, altered precipitation and defoliation intensity interacted to affect root length while above‐ground biomass was only affected by defoliation intensity, indicating that the root system was more responsive than above‐ground biomass when climatic conditions change. Overall, drought reduced root length, particularly under low defoliation intensity, as well as in combination with warming and heavy defoliation, highlighting the risk of additive effects of such environmental stresses. Across unclipped plots, above‐ground biomass was positively associated with total root length, the latter of which further interacted with precipitation, to affect above‐ground biomass. Compared to defoliated communities, non‐defoliated plant communities exhibited a greater ability to maintain above‐ground biomass under drought conditions via increases in root system efficiency (the amount of above‐ground biomass produced per unit of root length invested). Our results highlight the rapid change of root length in the face of interactive stressors. We postulate that the degree of stability in above‐ground biomass is driven by the altered root system dynamics or species turnover. Future studies are warranted that more directly assess how root length responses under climate change impact other important plant traits in grasslands. A free Plain Language Summary can be found within the Supporting Information of this article.

How precipitation and grazing influence the ecological functions of drought-prone grasslands on the northern slopes of the Tianshan Mountains, China?

Drought-prone grasslands provide a critical resource for the millions of people who are dependent on livestock for food security. However, this ecosystem is potentially vulnerable to climate change (e.g., precipitation) and human activity (e.g., grazing). Despite this, the influences of precipitation and grazing on ecological functions of drought-prone grasslands in the Tianshan Mountains remain relatively unexplored. Therefore, we conducted a systematic field investigation and a clipping experiment (simulating different intensities of grazing) in a drought-prone grassland on the northern slopes of the Tianshan Mountains in China to examine the influences of precipitation and grazing on aboveground biomass (AGB), soil volumetric water content (SVWC), and precipitation use efficiency (PUE) during the period of 2014–2017. We obtained the meteorological and SVWC data using an HL20 Bowen ratio system and a PR2 soil profile hydrometer, respectively. We found that AGB was clearly affected by both the amount and seasonal pattern of precipitation, and that PUE may be relatively low in years with either low or excessive precipitation. The PUE values were generally higher in the rapid growing season (April–July) than in the entire growing season (April–October). Overall, moderate grazing can promote plant growth under water stress conditions. The SVWC value was higher in the clipped plots than in the unclipped plots in the rapid growing season (April–July), but it was lower in the clipped plots than in the unclipped plots in the slow growing season (August–October). Our findings can enhance the understanding of the ecological effects of precipitation and grazing in drought-prone grasslands and provide data that will support the effective local grassland management.

Experimental warming amplified opposite impacts of drought vs. wet extremes on ecosystem carbon cycle in a tallgrass prairie

Climate warming is leading to greater precipitation variability, resulting in increased frequency and intensity of both drought and wet extremes. However, how these extreme events interact with climate warming and hay-harvest in grasslands to impact ecosystem functions has not yet been well explored. In this study, we took advantage of a long-term experiment to examine how climate warming and clipping (i.e., mimicking hay harvest) regulated impacts of naturally occurring drought and wet extremes on ecosystem CO2 fluxes of a tallgrass prairie in the Great Plains, USA. Warming resulted in net ecosystem carbon release (i.e., positive net ecosystem CO2 exchange, NEE) in the extreme drought year of 2011, but significantly enhanced net carbon uptake in the extremely wet year of 2015 in comparison with NEE in normal years. Warming-induced carbon release in the drought year was due to significantly enhanced ecosystem respiration (ER) from mid-summer to early-autumn, whereas warming-enhanced NEE in the wet year was due to an increase in aboveground net primary production (ANPP) compared to those in normal years. Drought diminished warming-induced increases in ANPP to about one sixth of that in the wet year in the unclipped plots. Interestingly, clipping offset the drought-mediated ecosystem carbon loss by increasing GPP and weakened the wet-enhanced ANPP. Overall, our results suggest that a future, warmer climate may exacerbate carbon losses in terrestrial ecosystems during drought extremes but stimulate the ecosystem carbon sink under wet extremes.

Postdefoliation ecosystem carbon and water flux and canopy growth dynamics in sagebrush steppe bunchgrasses

AbstractDeveloping land‐use practices that lead to sustainable net primary productivity in rangelands are important, but understanding their consequences to population and community processes is not often accounted for in basic ecosystem studies. Grazed and ungrazed upland ecosystems generally do not differ in net ecosystem CO2 exchange (NEE), but the underlying mechanisms and the concurrent effects of defoliation to vegetative and reproductive biomass allocation are unclear. To address this, we measured evapotranspiration (ET), NEE, and its constituent fluxes of ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) with live canopy leaf area index (LAIlive; m2 live leaf area/m2 ground area) and aboveground leaf, culm, and reproductive biomass in plots of clipped and unclipped squirreltail (Elymus elymoides) and bluebunch wheatgrass (Pseudoroegneria spicata) growing in intact sagebrush steppe. Clipping reduced LAIlive by 75%, but subsequent re‐growth rates in clipped plots was similar to LAIlive accumulation in unclipped plots. Concurrently, ET and NEE was similar between clipped and unclipped plots, with NEE primarily determined by GEP. GEP was initially lower in clipped plots, but then converged with unclipped GEP even as LAIlive continued to increase in both treatments. GEP convergence was driven by higher whole‐plant photosynthesis (GEPlive = GEP/LAIlive) in clipped plots. Ecosystem water use efficiency (GEP/ET) was reduced by 16% with clipping, due to low GEP/ET 2 weeks following defoliation, but GEP/ET converged before GEP levels did. Proportional reproductive biomass was higher in E. elymoides (21.4% total biomass) than in P. spicata (0.5% total biomass) due to lower allocation to specific leaf and culm mass. Clipping reduced reproductive effort in E. elymoides, in terms of total reproductive biomass (−56%), seed mass per unit leaf area (−64%), and seed mass per flowering head (−77%). We concluded defoliation increased canopy‐level light penetration, facilitating rapid recovery of ecosystem fluxes, but that allocation to vegetative regrowth supporting this led to lower reproductive effort in these range grasses. Insights from studies such as this will be useful in formulating systems‐based land management strategies aimed at maintaining annual productivity and long‐term population and community goals in semiarid rangeland ecosystems.

Seedling germination success and survival of the invasive shrub Scotch broom ( Cytisus scoparius) in response to fire and experimental clipping in the montane grasslands of the Nilgiris, south India

The spread of the exotic shrub Scotch broom in the montane grasslands of the Nilgiris is one of the major threats to biodiversity and ecosystem function there. It is likely that fire suppression over the past few decades is the proximate cause of expansion of broom populations. This study capitalizes on a wildfire event to examine fire effects on mature broom populations and soil seedbanks. Fire resulted in widespread death of mature broom stands but also stimulated broom soil seedbanks. However, this initial difference in seedling densities in burned and unburned plots was lost over time due to continuous recruitment in unburned plots. In a seed addition experiment, plots which were clipped prior to the fire showed higher germination success, possibly because fire temperature was moderated by biomass removal. In another experiment, non-dormant broom seeds were added to burned plots, which then received clipping treatments; there were no differences in broom seedling survival in clipped vs. unclipped plots. Overall, these results suggest that prescribed burning might contribute to the control of Scotch broom invasion by helping eliminate mature stands without significantly increasing regeneration from seed.

Plant clipping may cause overestimation of soil respiration in a Tibetan alpine meadow, southwest China

AbstractThe chamber‐based method is currently the most popular approach used for measuring soil respiration of various ecosystems. When this method is applied, aboveground plant tissues within the chamber need to be clipped some time (usually 24 h) before measuring soil respiration. However, plant clipping may change soil temperature and hence soil respiration because soil respiration is highly temperature‐dependent, particularly in cold regions. To determine to what extent soil respiration may be changed by the clipping, we measured soil temperature and respiration of an alpine meadow of southwest China using a chamber‐based method over an annual cycle. Based on the measurements, an exponential equation was built to describe the relationship between soil respiration and temperature. Concurrently we measured the soil temperature in clipped and unclipped plots on sunny days of the study months in another independent experiment; subsequently soil respiration was estimated for these plots using the exponential equation. Though daily mean soil temperature was insignificantly different between the clipped and unclipped plots, the clipping increased soil temperature at 5 cm depth by up to 4.3°C at daytime but decreased by up to 1.4°C at nighttime during the growing season. The changes were 2.2 and 1.5°C at daytime and nighttime, respectively, in the non‐growing season. It was calculated that the clipping manipulation caused an overestimation of soil respiration by 28.6 and 21.2% for the growing and non‐growing seasons, respectively; nevertheless, this calculated overestimation should differ from the actual one because the data were collected on sunny days only.

Herbivory and plant tolerance: experimental tests of alternative hypotheses involving non‐substitutable resources

A mechanistic understanding of the highly variable effects of herbivores on plant production in different ecosystems remains a major challenge. To explain these patterns, the compensatory continuum hypothesis (CCH) predicts plants to compensate for defoliation when resources are abundant, whereas the growth rate hypothesis (GRH) makes the opposite claim of high herbivory tolerance under resource‐poor conditions. The limiting resource model (LRM) tries to reconcile this dichotomy by incorporating the indirect effects of herbivores on plant resources and predicts that the potential for plant compensation is dependent upon whether, and how, herbivory influences limiting resources. Although extensively evaluated in laboratory monocultures, it remains uncertain whether these models can also explain the response of heterogeneous and multi‐species natural plant communities to defoliation. Here we investigate community‐wide plant response to defoliation and report data from a field experiment in the arid and primarily water‐limited Trans‐Himalayan grazing ecosystem in northern India involving clipping, irrigation and nutrient‐feedback with herbivore dung. Without nutrient‐feedback, plants compensated for defoliation in absence of irrigation but failed to compensate under irrigation. Whereas, in the presence of nutrient‐feedback plants compensated for defoliation when irrigated. This divergent pattern is not consistent with the CCH and GRH, and is only partially explained by the LRM. Instead, these pluralistic results are consistent with the hypothesis that herbivory may alter the relative strengths of water and nutrient limitation since irrigation increased root:shoot ratio in absence of fertilization in unclipped plots, but not in the corresponding clipped plots. So, herbivory appears to increase relative strength of nutrient‐limitation for plants that otherwise seem to be primarily water‐limited. Extending the LRM framework to include herbivore‐mediated transitions between water and nutrient‐limitation may clarify the underlying mechanisms that modulate herbivory‐tolerance under different environmental conditions.

Climate warming increases soil erosion, carbon and nitrogen loss with biofuel feedstock harvest in tallgrass prairie

Anthropogenic soil erosion severely affects land ecosystems by reducing plant productivity and stimulating horizontal carbon and nitrogen movement at the surface. Climate warming may accelerate soil erosion by altering soil temperature, moisture, and vegetation coverage. However, no experiments have been carried out to quantify soil erosion with warming. In a long-term field experiment, we explored how annual clipping for biofuel feedstock production and warming caused soil erosion and accompanying carbon and nitrogen losses in tallgrass prairie in Oklahoma, USA. We measured relative changes in soil surface elevation between clipped and unclipped plots with or without experimental warming. Our results show that average relative erosion depth caused by clipping was 1.65±0.09 and 0.54±0.08 mm yr−1, respectively, in warmed and control plots from November 21, 1999 to April 21, 2009. The soil erosion rate was 2148±121 g m−2 yr−1 in the warmed plots and 693±113 g m−2 yr−1 in the control plots. Soil organic carbon was lost at a rate of 69.6±5.6 g m−2 yr−1 in the warmed plots and 22.5±2.7 g m−2 yr−1 in the control plots. Total nitrogen was lost at a rate of 4.6±0.4 g m−2 yr−1 in the warmed plots and 1.4±0.1 g m−2 yr−2 in the control plots. The amount of carbon and nitrogen loss caused by clipping is equivalent to or even larger than changes caused by global change factors such as warming and rising atmospheric CO2 concentration. In addition, soil erosion rates were significantly correlated with clipping-induced changes in soil moisture. Our results suggest that clipping for biofuel harvest results in significant soil erosion and accompanying losses of soil carbon and nitrogen, which is aggravated by warming.

Nitrogen effects on net ecosystem carbon exchange in a temperate steppe

AbstractIt has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO2 exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m−2 yr−1 with NH4NO3 alone or in combination with phosphorous (P, 5 g P2O5 m−2 yr−1) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing‐season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N‐induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species (Stipa krylovii) and standing litter accumulation induced light limitation on the lower canopy species (Artemisia frigida). Thus, N‐induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.

Labile, recalcitrant, and microbial carbon and nitrogen pools of a tallgrass prairie soil in the US Great Plains subjected to experimental warming and clipping

Carbon (C) and nitrogen (N) fluxes are largely controlled by the small but highly bio-reactive, labile pools of these elements in terrestrial soils, while long-term C and N storage is determined by the long-lived recalcitrant fractions. Changes in the size of these pools and redistribution among them in response to global warming may considerably affect the long-term terrestrial C and N storage. However, such changes have not been carefully examined in field warming experiments. This study used sulfuric acid hydrolysis to quantify changes in labile and recalcitrant C and N fractions of soil in a tallgrass prairie ecosystem that had been continuously warmed with or without clipping for about 2.5 years. Warming significantly increased labile C and N fractions in the unclipped plots, resulting in increments of 373 mg C kg −1 dry soil and 15 mg N kg −1 dry soil, over this period whilst clipping significantly decreased such concentrations in the warmed plots. Warming also significantly increased soil microbial biomass C and N in the unclipped plots, and increased ratios of soil microbial/labile C and N, indicating an increase in microbial C- and N-use efficiency. Recalcitrant and total C and N contents were not significantly affected by warming. For all measured pools, only labile and microbial biomass C fractions showed significant interactions between warming and clipping, indicating the dependence of the warming effects on clipping. Our results suggest that increased soil labile and microbial C and N fractions likely resulted indirectly from warming increases in plant biomass input, which may be larger than warming-enhanced decomposition of labile organic compounds.

Impact of Grazing on Forage Quality and Quantity for Ungulates of the Kainji Lake National Park, Nigeria

The study examined the effect of grazing as simulated by clipping on forage quality and quantity in terms of above ground biomass, live, total production and nutrients content of forages utilized by ungulates of Kainji Lake National Park. Three 2.5 x 2.5 m plots were constructed in the three main vegetation communities in the Park, the plots were in three replicates in each vegetation community. Each plot received separate treatment including medium clipping, heavy clipping and unclipped, average forage biomass production and forage nutrients were measured. The results revealed that higher total annual above ground biomass was recorded in clipped plots which values ranged between 1642 and 2458 g m year when compared to unclipped plots which values ranged between 1.394 and 1.435 g m year. Equally, the values of annual total production of clipped and unclipped plots ranged between 250-471 and 210-254 g m year respectively, the values were significantly different at (p = 0.05). It was observed that forages in clipped plots contained higher values of crude protein and fat. Also they contained nitrogen, phosphorus, calcium and sodium values above minimum values required by herbivores for body maintenance, pregnancy and lactation. Measures to improve forage quality and quantity in the Park are also discussed.

Sideoats Grama Growth Responses to Seasonal Fires and Clipping

There is increased interest in the use of summer-season fires to limit woody plant encroachment on southern prairie grasslands, but collateral effects of these fires on grasses are poorly understood. We quantified effects of repeated winter fires, repeated summer fires, simulated grazing (clipping), and their interaction on yields of the C4 midgrass, sideoats grama (Bouteloua curtipendula) in northern Texas. Monoculture patches of sideoats grama were exposed to 1 of 3 fire treatments: 1) no burn, 2) 2 winter fires in 3 years, or 3) 2 summer fires in 3 years; and to 1 of 2 clip treatments (no clip or clip once each spring). Total yield (live þ standing dead), live yield, percent live tissue, and foliar cover were measured in spring and late-growing season (late-season) over a 7-year period. In unclipped plots, late-season total yield did not fully recover until 2 growing seasons after winter fires and 3 growing seasons after summer fires. By 5 years postfire, total yield was greater in both fire treatments than in the no burn. Live yields recovered more quickly than total yields following summer fires but never exceeded the no burn. Percent live tissue was greater in both fire treatments than in the no burn for up to 2 years postfire. Clipping reduced total and live yields in the no burn and winter-fire treatments but not in the summer-fire treatment. By 5 years postfire, total and live yields were greater in the summer fire þ clip than the no burn þ clip or winter fire þ clip treatments. Results suggest that 1) sideoats grama is tolerant of summer fires but full recovery may require at least 3 years, and 2) in the long-term, summer fire þ clipping may stimulate sideoats grama production more than winter fire þ clipping or clipping alone.

Evapotranspiration of two vegetation communities in a high‐elevation riparian meadow at Hart Prairie, Arizona

Hart Prairie, Arizona, has the largest Bebb willow (Salix bebbiana) community in the United States; however, greater than 95% of willows are older than 80 years and regeneration by seed is not occurring. This study examined the evapotranspiration of two herbaceous communities that dominate the Hart Prairie watershed: (1) a mixture of bracken fern (Pteridium aquilinum) and graminoids and (2) dominantly graminoids. Transpiration during premonsoon and postmonsoon dry periods of 2000, 2001, and 2002 was estimated for each community by the difference in volumetric soil‐water content (0–30 cm soil depth) between replicated plots that were clipped of all vegetation and control, unclipped plots. Transpiration rates estimated under conditions of minimal soil drainage varied between 0.63 and 2.4 mm/d for the fern‐graminoid community and 0.57 and 1.1 mm/d for the graminoid community over the study. The fern‐graminoid community produced more biomass than the graminoid community in all years, but generally had lower transpiration rates. Severe drought in year 2002 reduced growth and transpiration of the fern‐graminoid community more than the graminoid community. Evaporation rates were estimated by temporal changes in soil‐water content in clipped plots during dry periods, and were 54 to 474% of transpiration rates estimated under conditions of minimal soil drainage because of the dry and windy conditions that occur at the study site. Based on this study and a study of transpiration of scattered trees invading the meadow, transpiration by the herbaceous understory was higher than transpiration by trees during similar seasons.

Sideoats Grama Growth Responses to Seasonal Fires and Clipping

There is increased interest in the use of summer-season fires to limit woody plant encroachment on southern prairie grasslands, but collateral effects of these fires on grasses are poorly understood. We quantified effects of repeated winter fires, repeated summer fires, simulated grazing (clipping), and their interaction on yields of the C4 midgrass, sideoats grama (Bouteloua curtipendula) in northern Texas. Monoculture patches of sideoats grama were exposed to 1 of 3 fire treatments: 1) no burn, 2) 2 winter fires in 3 years, or 3) 2 summer fires in 3 years; and to 1 of 2 clip treatments (no clip or clip once each spring). Total yield (live + standing dead), live yield, percent live tissue, and foliar cover were measured in spring and late-growing season (late-season) over a 7-year period. In unclipped plots, late-season total yield did not fully recover until 2 growing seasons after winter fires and 3 growing seasons after summer fires. By 5 years postfire, total yield was greater in both fire treatments than in the no burn. Live yields recovered more quickly than total yields following summer fires but never exceeded the no burn. Percent live tissue was greater in both fire treatments than in the no burn for up to 2 years postfire. Clipping reduced total and live yields in the no burn and winter-fire treatments but not in the summer-fire treatment. By 5 years postfire, total and live yields were greater in the summer fire + clip than the no burn + clip or winter fire + clip treatments. Results suggest that 1) sideoats grama is tolerant of summer fires but full recovery may require at least 3 years, and 2) in the long-term, summer fire + clipping may stimulate sideoats grama production more than winter fire + clipping or clipping alone.

Lateral Clipping of Canopy Influences the Microclimate and Development of Apothecia ofSclerotinia sclerotiorumin Carrots

Four canopy management treatments were evaluated in carrot (Daucus carota) production in Bradford Marsh, Ontario, in 2001 and 2002: (i) unclipped control; (ii) unclipped canopy with manual removal of collapsed senescing leaves at 2-week intervals following the first appearance on the soil; (iii) lateral clipping of the canopy at the initial emergence of apothecia, leaving the debris in the furrow, and (iv) lateral clipping of the canopy with manual removal of the debris from the furrow. Clipping reduced the canopy width by ca. 20% on both sides of the carrot bed by cutting off overlapping leaves above the furrow and senescing foliage on the soil surface. Maximum air and soil temperatures were up to 9.2 and 3.1°C lower, respectively, and relative humidity was up to 30% higher in unclipped canopies than in clipped canopies. The total number of apothecia in clipped plots was reduced by 74 and 76% compared with unclipped plots in 2001 and 2002, respectively. Canopy clipping reduced the quantity of apothecia in the crop by creating an unfavorable microclimate for the development of S. sclerotiorum without affecting the fresh foliar and root weights of carrot at harvest. The presence of clipped foliar debris in the furrow affected the number of apothecia in 2001; however, apothecia under the debris are unlikely to contribute to the overall inoculum in the crop. Lateral clipping also appeared to control Sclerotinia rot of carrot where it occurred (in 2001). This is the first report that documents and quantifies the effects of canopy architecture on the microclimate, development of apothecia of S. sclerotiorum, and Sclerotinia rot in carrot crops.

Simulated green turtle grazing affects structure and productivity of seagrass pastures

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 305:235-247 (2005) - doi:10.3354/meps305235 Simulated green turtle grazing affects structure and productivity of seagrass pastures Kathleen L. Moran, Karen A. Bjorndal* Archie Carr Center for Sea Turtle Research and Department of Zoology, University of Florida, PO Box 118525, Gainesville, Florida 32611, USA *Corresponding author. Email: kab@zoology.ufl.edu ABSTRACT: The seagrass Thalassia testudinum evolved under much greater grazing pressure than exists today because overexploitation by humans caused the severe decline of green turtles Chelonia mydas, the major seagrass herbivore in the Caribbean. Understanding the effects of grazing on the structure and function of seagrass ecosystems is critical to evaluating how ecosystem processes have changed since the major herbivore was essentially removed from modern Caribbean seagrass systems. We evaluated effects of grazing on the structure and productivity of T. testudinum pastures by simulating green turtle grazing in 3 × 3 m plots for 16 mo in the central Bahamas and comparing these clipped plots to adjacent control (unclipped) plots. Simulated grazing affected the physical structure of T. testudinum plots, resulting in a system with reduced structural complexity. Simulated grazing resulted in compensatory growth in T. testudinum. Clipped plots maintained levels of growth comparable to unclipped plots over the 16 mo clipping trial, and specific mass growth was significantly elevated in clipped plots, even without simulating the nutrient return from green turtle feces and urine. The green turtle grazing pattern of re-cropping T. testudinum plots is sustainable for long periods—at least 16 mo. We estimated a range of carrying capacities of T. testudinum for green turtles of 1567 to 3748 kg turtle ha–1. Our study provides a foundation for reconstruction models and restoration plans for Caribbean seagrass ecosystems. KEY WORDS: Seagrass · Thalassia testudinum · Chelonia mydas · Effects of grazing · Herbivory · Productivity · Compensatory growth Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 305. Online publication date: December 23, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

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.

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.