Root Mortality Research Articles

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year−1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m−2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m−2) was found in the driest stand with 520 mm year−1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.

Fine root dynamics in two atlantic white-cedar wetlands with contrasting hydroperiods

Using minirhizotrons, fine root dynamics were compared in two Atlantic white-cedar swamps with contrasting hydroperiods (intermittently and persistently flooded) in order to evaluate the effects of flooding regime on fine root depth, abundance, production, and longevity. Despite the higher mean watertable depth in the persistently flooded, site, no differences in rooting depth were observed. Root length density, root abundance, and root longevity were generally greater in the persistently flooded site throughout the 439-day study period, although root length production was similar between the two sites. These results suggest that prolonged inundation in Atlantic white-cedar swamps decreases root elongation rates, but due to greater observed root densities, stand-level belowground production remains similar to that of intermittently flooded stands. Significant increases in root mortality were observed in the intermittently flooded side during a period of water-table drawdown, suggesting less than optimal hydrologic conditions at this site.

Snow Removal and Ambient Air Temperature Effects on Forest Soil Temperatures in Northern Vermont

We measured deciduous forest soil temperatures under control (unmanipulated) and snow-free (where snow is manually removed) conditions for four winters (at three soil depths) to determine effects of a snow cover reduction such as may occur as a result of climate change on Vermont forest soils. The four winters we studied were characterized as: ‘cold and snowy’, ‘warm with low snow’, ‘cold with low snow’, and ‘cool with low snow’. Snow-free soils were colder than controls at 5- and 15-cm depth for all years, and at all depths in the two cold winters. Soil thermal variability generally decreased with both increased snow cover and soil depth. The effect of snow cover on soil freeze-thaw events was highly dependent on both the depth of snow and the soil temperature. Snow kept the soil warm and reduced soil temperature variability, but often this caused soil to remain near 0°C, resulting in more freeze–thaw events under snow at one or more soil depths. During the ‘cold snowy’ winter, soils under snow had daily averages consistently >0°C, whereas snow-free soil temperatures commonly dropped below −3°C. During the ‘warm’ year, temperatures of soil under snow were often lower than those of snow-free soils. The warmer winter resulted in less snow cover to insulate soil from freezing in the biologically active top 30 cm. The possible consequences of increased soil freezing include more root mortality and nutrient loss, which would potentially alter ecosystem dynamics, decrease productivity of some tree species, and increase sugar maple (Acer saccharum Marshall) mortality in northern hardwood forests.

Uncertainties in interpretation of isotope signals for estimation of fine root longevity: theoretical considerations

AbstractThis paper examines uncertainties in the interpretation of isotope signals when estimating fine root longevity, particularly in forests. The isotope signals are depleted δ13C values from elevated CO2 experiments and enriched Δ14C values from bomb 14C in atmospheric CO2. For the CO2 experiments, I explored the effects of six root mortality patterns (on–off, proportional, constant, normal, left skew, and right skew distributions), five levels of nonstructural carbohydrate (NSC) reserves, and increased root growth on root δ13C values after CO2 fumigation. My analysis indicates that fitting a linear equation to δ13C data provides unbiased estimates of longevity only if root mortality follows an on–off model, without dilution of isotope signals by pretreatment NSC reserves, and under a steady state between growth and death. If root mortality follows the other patterns, the linear extrapolation considerably overestimates root longevity. In contrast, fitting an exponential equation to δ13C data underestimates longevity with all the mortality patterns except the proportional one. With either linear or exponential extrapolation, dilution of isotope signals by pretreatment NSC reserves could result in overestimation of root longevity by several‐fold. Root longevity is underestimated if elevated CO2 stimulates fine root growth. For the bomb 14C approach, I examined the effects of four mortality patterns (on–off, proportional, constant, and normal distribution) on root Δ14C values. For a given Δ14C value, the proportional pattern usually provides a shorter estimate of root longevity than the other patterns. Overall, we have to improve our understanding of root growth and mortality patterns and to measure NSC reserves in order to reduce uncertainties in estimated fine root longevity from isotope data.

Metals in Northern Forest Ecosystems: Role of Vegetation in Sequestration and Cycling, and Implications for Ecological Risk Assessment

Mass estimates of phytoaccumulated trace metal contaminants and transfers to soils are necessary to properly evaluate the impact of historic and continued anthropogenic metal deposition to northern forest ecosystems. An investigation of Cu, Ni, Pb, and Zn mass balances in plant communities subject to metal contamination from smelter emissions in Canada, found that accumulation of metals differed significantly among plant vegetation compartments (foliage, fine roots, bark, trunk, and branches). Analysis of plant community biological accumulation factors (BAFs), calculated using total soil metal and free soil metal ion (Me2+) found that free soil metal BAFs were more similar among locations than total soil metal BAFs, but that disparity still existed. Fine roots were found to dominate annual vegetation transfer of Cu, Ni, and Pb to soils, relative to foliage; fine root mortality played a smaller role than foliage for Zn plant-soil transfer. Plant-associated metal inputs were found to rival or exceed current estimates of atmospheric metal deposition, suggesting that potential benefits of future reductions in emissions to forests need to be evaluated within the context of phytocycling of metals already present.

Environmental control of fine root dynamics in a northern hardwood forest

AbstractUnderstanding how exogenous and endogenous factors control the distribution, production and mortality of fine roots is fundamental to assessing the implications of global change, yet our knowledge of control over fine root dynamics remains rudimentary. To improve understanding of these processes, the present study developed regression relationships between environmental variables and fine root dynamics within a northern hardwood forest in New Hampshire, USA, which was experimentally manipulated with a snow removal treatment. Fine roots (&lt; 1 mm diameter) were observed using minirhizotrons for 2 years in sugar maple and yellow birch stands and analyzed in relation to temperature, water and nutrient availability. Fine root dynamics at this site fluctuated seasonally, with growth and mortality peaking during warmer months. Monthly fine root production was strongly associated with mean monthly air temperature and neither soil moisture nor nutrient availability added additional predictive power to this relationship. This relationship exhibited a seasonal temperature hysteresis, which was altered by snow removal treatment. These results suggest that both exogenous and endogenous cues may be important in controlling fine root growth in this system. Proportional fine root mortality was directly associated with mean monthly soil temperature, and proportional fine root mortality during the over‐winter interval was strongly related to whether the soil froze. The strong relationship between fine root production and air temperature reported herein contrasts with findings from some hardwood forest sites and indicates that controls on fine root dynamics vary geographically. Future research must more clearly distinguish between endogenous and exogenous control over fine root dynamics in various ecosystems.

Growth and Physiological Response of Creeping Bentgrass To Elevated Night Temperature

Growth of cool-season grasses declines with increasing temperatures. The objective of this study was to determine the effects of elevated night temperature on turf quality, root mortality, and carbohydrate metabolism in creeping bentgrass (Agrostis stoloniferous L. var. palustris (Huds.) Farw (syn. A. palustris Huds.). Plants of `Penncross' were exposed to two night temperature regimes: 24 °C (higher night temperature); and 19 °C (lower temperature control) under the same day temperature (24 °C) in growth chambers for 45 days. Prolonged exposure of plants to higher night temperature reduced turf quality, canopy photosynthetic rate, whole-plant and root respiration rates during the day, translocation of newly fixed 14C assimilate to roots, and total nonstructural carbohydrate content in shoots and roots (including dead and live roots). Elevated night temperature increased root mortality and whole-plant and root respiration rates at night. Our results indicated that a decline in turf quality and increase in root dieback with high night temperature was mainly associated with increased night respiration rates of whole plant and roots and reduced carbohydrate availability.

Experimentally induced root mortality increased nitrous oxide emission from tropical forest soils

We conducted an experiment on sand and clay tropical forest soils to test the short‐term effect of root mortality on the soil‐atmosphere flux of nitrous oxide, nitric oxide, methane, and carbon dioxide. We induced root mortality by isolating blocks of land to 1 m using trenching and root exclusion screening. Gas fluxes were measured weekly for ten weeks following the trenching treatment. For nitrous oxide there was a highly significant increase in soil‐atmosphere flux over the ten weeks following treatment for trenched plots compared to control plots. N2O flux averaged 37.5 and 18.5 ng N cm−2 h−1 from clay trenched and control plots and 4.7 and 1.5 ng N cm−2 h−1 from sand trenched and control plots. In contrast, there was no effect for soil‐atmosphere flux of nitric oxide, carbon dioxide, or methane.

Architectural analysis and synthesis of the plum tree root system in an orchard using a quantitative modelling approach

A dynamic 3D representation of the root system architecture of plum is proposed by gathering quantitative and morphological observations of the tree root system in a model. The model includes two information levels: (i) a typology of root axes, based on morphological and developmental characteristics; (ii) a set of basic processes (axial and radial growth, ramification and reiteration, decay). The basic processes are qualitatively identical in space and time. An original approach was used to investigate these processes and to formalize them in the model. Concerning the main roots, a mechanism of reiteration is described that has a substantial influence on the structuring of the root system. Root mortality is assessed using the variation in branching density along the root axes. Radial growth is calculated from the ramification of root axes, using root section conservation properties. This model enables a link between static field observations and a dynamic simulation of the root system architecture. The architectural model allows examination of the global consequences of the basic processes at the level of the root system. The simulations provide useful output, from a simple root depth profile to a simulation of the dynamic 3D root system architecture, to investigate plant functioning and especially water and nutrient uptake.

Estimating root lifespan of two grasses at contrasting elevation in a salt marsh by applying vitality staining on roots from in-growth cores

Contrasting soil conditions caused by different inundation frequenciesrequire different root growth strategies along the elevational gradient ofcoastal salt marshes. The objective of this study was to examine (1) if rootlifespan was shorter in Elymus pycnanthus, a relativelyfast-growing competitive species dominating high marshes, than inSpartina anglica, a relatively slow-growingstress-tolerating species dominating low marshes, and (2) if the species withlonger lifespan had higher tissue density (g cm−3) and lowerspecific root length (m g−1) than the species with shorterlifespan. Root production and mortality rates were established by samplingrootsin in-growth cores, and using triphenyltetrazolium chloride (TTC) staining todistinguish vital from dead roots. Root lifespan was estimated by dividing theliving root biomass (Elymus: 36 gm−2, Spartina: 100 gm−2) by root production (Elymus:0.28 g day−1 m−2,Spartina: 0.25 g day−1m−2) or root mortality rates(Elymus: 0.42–0.53 g day−1m−2). Spartina did not exhibitsubstantial mortality. Despite the present method only yielding rough estimatesof average root lifespan, it is evident that root longevity is much shorter inElymus than in Spartina. Rootlifespanranged between 10–19 weeks for Elymus but was closeto 1 year in Spartina, indicating thatElymus replaces it's roots continuously throughout thegrowing season, whereas Spartina maintains its roots overthe growing season. Fine roots of Elymus had slightlylowertissue density (0.094) than those of Spartina (0.139),whereas coarse roots of Elymus andSpartina had similar tissue density (0.100 gcm−3). Fine roots of Elymus andSpartina had similar specific root length (195 mg−1). However, coarse roots ofElymus (50 m g−1) had higherspecific root length than those of Spartina (20 mg−1) due to having smaller root diameter(Elymus: 548 μm,Spartina: 961 μm). We conclude thatpresentobservations on Elymus and Spartinasupport our first hypothesis that the ‘competitive’ species fromthehigh marsh had short-lived roots compared to the'stress-tolerating'species from the low marsh. However, our result provide only weak support forthe existence of a positive correlation between root longevity and tissuedensity and a negative correlation between root longevity and specific rootlength.

Summer Root Decline: Production and Mortality for Four Cultivars of Creeping Bentgrass

A better understanding of the seasonal dynamics of root activity for creeping bentgrass [Agrostis palustris Huds. = A stolonifera var. palustris (Huds.) Farw.] would be beneficial for developing effective breeding and management programs to maximum summer turf quality. The objectives of this study were to examine dynamics changes in root production and mortality for four creeping bentgrass cultivars that differ in summer shoot performance. The study was conducted on a USGA‐specification putting green in Manhattan, KS, during 1997 and 1998. The cultivars evaluated were ‘L‐93’, ‘Penncross’, ‘Providence’, and ‘Crenshaw’. Grasses were mowed daily at 4 mm and irrigated on alternate days. Root production and mortality were monitored using the minirhizotron technique. In both years, total root length and number were highest in August and then decreased in September for all four cultivars. From July to September, the length and number of newly produced roots decreased while those of dead roots increased. The ratio of dead roots to live roots in length and number increased in late summer for all cultivars. While the differences in total root length and number among cultivars were not consistent between 2 yr, Penncross consistently had more dead roots, fewer new roots, and higher root mortality compared with the other cultivars. The results indicate that summer root decline of creeping bentgrass resulted from both decreased new root production and increased root mortality, which could be associated with high soil temperatures during the summer. Variation in root production and mortality may contribute to the differences in shoot summer performance of the four cultivars.

Summer Root Decline

A better understanding of the seasonal dynamics of root activity for creeping bentgrass [Agrostis palustris Huds. = A stolonifera var. palustris (Huds.) Farw.] would be beneficial for developing effective breeding and management programs to maximum summer turf quality. The objectives of this study were to examine dynamics changes in root production and mortality for four creeping bentgrass cultivars that differ in summer shoot performance. The study was conducted on a USGA-specification putting green in Manhattan, KS, during 1997 and 1998. The cultivars evaluated were ‘L-93’, ‘Penncross’, ‘Providence’, and ‘Crenshaw’. Grasses were mowed daily at 4 mm and irrigated on alternate days. Root production and mortality were monitored using the minirhizotron technique. In both years, total root length and number were highest in August and then decreased in September for all four cultivars. From July to September, the length and number of newly produced roots decreased while those of dead roots increased. The ratio of dead roots to live roots in length and number increased in late summer for all cultivars. While the differences in total root length and number among cultivars were not consistent between 2 yr, Penncross consistently had more dead roots, fewer new roots, and higher root mortality compared with the other cultivars. The results indicate that summer root decline of creeping bentgrass resulted from both decreased new root production and increased root mortality, which could be associated with high soil temperatures during the summer. Variation in root production and mortality may contribute to the differences in shoot summer performance of the four cultivars.

Hydraulic properties of Pinus halepensis, Pinus pinea and Tetraclinis articulata in a dune ecosystem of Eastern Spain

The hydraulic properties of Pinus pinea, Pinus halepensis and Tetraclinis articulata were studied in a coastal dune area from Eastern Spain. The measured variables include vulnerability to xylem embolism (vulnerability curves), hydraulic conductivity and carbon isotopic discrimination in leaves. Leaf water potentials were also monitored in the three studied populations during an extremely dry period. Our results showed that roots had always wider vessels and higher hydraulic conductivity than branches. Roots were also more vulnerable to xylem embolism and operated closer to their hydraulic limit (i.e., with narrower safety margins). Although it was not quantified, extensive root mortality was observed in the two pines during the study period, in agreement with the high values of xylem embolism (> 75%) predicted from vulnerability curves and the water potentials measured in the field. T. articulata was much more resistant to embolism than P. pinea and P. halepensis. Since T. articulata experienced also lower water potentials, safety margins from hydraulic failure were only slightly wider in this species than in the pines. Combining species and tissues, high resistance to xylem embolism was associated with low hydraulic conductivity and with high wood density. Both relationships imply a cost of having a resistant xylem. The study outlined very different water-use strategies for T. articulata and the pines. Whereas T. articulata had a conservative strategy that relied on the low vulnerability of its conducting system to drought-induced xylem embolism, the two pines showed regulatory mechanisms at different levels (i.e., embolism, root demography) that constrained the absorption of water when it became scarce.

Optimizing minirhizotron sample frequency for an evergreen and deciduous tree species

• When using minirhizotrons to study fine dynamics in natural ecosystems, it is important to determine how sample collection frequency influences estimates of fine root production and mortality. Minirhizotron images were collected twice per week from mature Pseudotsuga menziesii and Tilia cordata trees and analyzed to estimate fine root production and mortality. These data were used to create data sets reflecting sample frequencies of 1, 2, 4 or 8wk. • When the sampling interval is long, fine roots can appear and disappear between samplings, leading to underestimates of production and mortality. For example, with an 8-wk sample frequency, 24 and 35% of the fine root production in P. menziesii and T. cordata , respectively, is not measured. Fine root mortality displays the same sensitivity to sample frequency. • Our experimental observations supported the previously published simulation analysis, which provides an estimate of the proportion of fine roots missed at different sample frequencies and is a tool that can be used to select a sample frequency to balance production and mortality accuracy with sampling and analytical effort.

Rapid fine root disappearance in a pine woodland: a substantial carbon flux

Fine root production and mortality are difficult to estimate accurately, because some fine roots die within days of being produced, and many apparently healthy roots disappear rapidly with no obvious period of senescence. Such root dynamics are difficult to analyze without very fine-scaled temporal observations. To capture the behavior of short-lived and rapidly disappearing roots, we sampled minirhizotron tubes weekly for 11 months in a Pinus palustris Mill. woodland. Fine root ([Formula: see text]2 mm diameter) length production and length mortality during this period were 1.57 ± 0.23 mm·cm–2 (mean ± SE) and 1.19 ± 0.17 mm·cm–2, respectively. Depending on the type of estimate used, rapid disappearance accounted for between 21 and 37% of total fine root mortality. Rapidly disappearing roots had relatively short life-spans, a median of just 10.5 days. Monthly sampling of the same data set underestimated length production by 15%, overestimated median root life-span by 60%, and obscured causes of root loss. If short-lived roots are not accounted for, total net primary productivity in temperate forests may be underestimated by as much as 10%. We propose that belowground herbivory is the leading explanation for this rapid disappearance.

Unexpected results of a pilot throughfall exclusion experiment on soil emissions of CO 2 , CH 4 , N 2 O, and NO in eastern Amazonia

The eastern Amazon Basin may become drier as a result of less regional recirculation of water in a largely deforested landscape and because of increased frequency and intensity of El Nino events induced by global warming. Drier conditions may affect several plant and soil microbial processes, including soil emissions of CO2, CH4, NO, and N2O. We report here unanticipated results of a pilot study that was initiated to test the feasibility of a larger-scale throughfall exclusion experiment. In particular, soil drying caused a switch from net consumption of atmospheric CH4 by soils in the control plot to net CH4 emission from soils in the experimentally dried plot. This result is surprising because production of CH4 requires anaerobic microsites, which are uncommon in dry soil. A plausible explanation for increased CH4 emissions in the dried plot is that dry soil conditions favor termite activity and increased coarse root mortality provides them with a substrate. Another surprise was that both NO and N2O fluxes were elevated several years after initiation of the drying experiment. Apparently, a pulse of N availability caused by experimental drying persisted for at least 3 years. As expected, CO2 emissions were lower in the dried plots, which is consistent with lower rates of root growth observed in root in-growth cores placed in the dried plots. More work is needed to test these explanations and to confirm these phenomena, but these results demonstrate that changes in climate could have unanticipated effects on biogeochemical processes in soils that we do not adequately understand.

Root production and demography in a california annual grassland under elevated atmospheric carbon dioxide

AbstractThis study examined root production and turnover in a California grassland during the third year of a long‐term experiment with ambient (LO) and twice‐ambient atmospheric CO2 (HI), using harvests, ingrowth cores, and minirhizotrons. Based on one‐time harvest data, root biomass was 32% greater in the HI treatment, comparable to the stimulation of aboveground production during the study year. However, the 30–70% increase in photosynthesis under elevated CO2 for the dominant species in our system is considerably larger than the combined increase in above and belowground biomass. One possible explanation is, increased root turnover, which could be a sink for the additional fixed carbon. Cumulative root production in ingrowth cores from both treatments harvested at four dates was 2–3 times that in the single harvested cores, suggesting substantial root turnover within the growing season. Minirhizotron data confirmed this result, demonstrating that production and mortality occurred simultaneously through much of the season. As a result, cumulative root production was 54%, 47% and 44% greater than peak standing root length for the no chamber (X), LO, and HI plots, respectively. Elevated CO2, however, had little effect on rates of turnover (i.e. rates of turnover were equal in the LO and HI plots throughout most of the year) and cumulative root production was unaffected by treatment. Elevated CO2 increased monthly production of new root length (59%) only at the end of the season (April–June) when root growth had largely ceased in the LO plots but continued in the HI plots. This end‐of‐season increase in production coincided with an 18% greater soil moisture content in the HI plots previously described. Total standing root length was not affected by CO2 treatment. Root mortality was unaffected by elevated CO2 in all months except April, in which plants grown in the HI plots had higher mortality rates. Together, these results demonstrate that root turnover is considerable in the grassland community and easily missed by destructive soil coring. However, increased fine root turnover under elevated CO2 is apparently not a major sink for extra photosynthate in this system.

Leaf area renewal, root retention and carbohydrate reserves in a clonal tree species following above‐ground disturbance

Summary Removal of the above‐ground portion of clonally regenerating trees results in a massive imbalance in the ratio of root to leaf area. We investigated the resilience of clones following above‐ground disturbance in terms of root carbohydrates, leaf area renewal and root retention. In a 2 × 2 factorial experiment, 40 Populus tremuloides saplings were cut at times of high (late fall) or low (spring after leaf flush) root carbohydrate reserves. Leaf area renewal was manipulated by allowing either only one or all root suckers to re‐grow. Root starch concentrations were 10 times higher at the time of cutting in the fall compared with the spring, whereas sugar concentrations were only 10% higher. When saplings were cut in fall and all suckers were allowed to develop (FA treatment), suckers were taller, and had more biomass and leaf area and higher leaf area ratio than all other treatments. In particular, leaf area and leaf area ratio recovered within a year to near pre‐treatment levels compared with at least a 50% reduction in the other treatments. At the end of the first season after cutting FA saplings also had the greatest root mass and lowest amount of dead root mass, and root starch concentrations of these saplings had returned to pre‐treatment values, compared with a 20% recovery for spring‐cut saplings with a single sucker. Root mass and root starch concentrations were correlated with leaf mass in all treatments. Adequate carbohydrate reserves at the time of disturbance and rapid redevelopment of leaf area by extensive sprouting seem critical for resilience of the clone and allow for the retention of the clonal root system and a rapid rebuilding of carbohydrate reserves. Clones with poor sucker and leaf area development showed extensive root mortality and reduced carbohydrate reserves and their prospects for growth in the following year were comparatively poor.

Mowing Effects on Root Production, Growth, and Mortality of Creeping Bentgrass

Understanding the response of root growth for creeping bentgrass {Agrostis palustris Huds. [ = A stolonifera var. palustris (Huds.) Farw.]} to different mowing regimes will help develop effective management practices to maintain high quality turf. The objective of this study was to examine the effects of mowing height on root elongation, production, and mortality for two creeping bentgrass cultivars throughout the growing season, particularly during summer months. ‘Crenshaw’ and ‘Penncross’ grown in a USGA-specification putting green were mowed daily at two mowing heights: 3 and 4 mm. In comparison with mowing at 4 mm, mowing at 3 mm reduced cumulative new root length (CNRL), total root length (TRL), and maximum rooting depth (RD), but increased dead root length (CDRL), root turnover rate in both length and number, and root mortality at 3- and 9-cm soil depths in both cultivars. The decline in new root production and increase in root mortality occurred earlier in the season and were more pronounced at the 3-cm soil depth than at the 9-cm depth for both cultivars. The effects of low mowing on root growth were more pronounced during summer months than in the spring. Lowering mowing height during summer would be more detrimental for root growth than maintaining a higher mowing height, which could lead to turf quality decline of creeping bentgrass.

ROOT DYNAMICS AND SPATIAL PATTERN IN PRAIRIE AND FOREST

The invasion of grasslands by woody plants is related to increased heterogeneity in soil resources. At the northern edge of the Great Plains, the amount of heterogeneity in soils reciprocally transplanted between prairie and aspen forest can change within a single growing season, suggesting that heterogeneity is strongly influenced by vegetation. Here we examine several mechanisms by which fine roots could influence soil resource heterogeneity. We measured the total length, spatial pattern, and turnover of fine roots in both prairie and forest using minirhizotrons installed horizontally 10 cm below the soil surface. The total length of fine roots in forest (0.7 cm/cm2) was significantly less than that in prairie (1.4 cm/cm2), which may contribute to more spatially variable resource uptake in forest. The coefficient of variation of root length was significantly higher in forest (92%) than in prairie (65%). Semivariograms showed that the scale of patchiness of root length was significantly greater in forest (8–12 cm) than in prairie (3–4 cm). Lastly, the rate of fine root turnover was significantly higher in forest (49%) than in prairie (20%). Turnover may contribute to spatial variability because a system with high rates of root turnover would appear to have high spatial variability at any given time. Root mortality was also significantly higher in forest than prairie. In summary, many aspects of root distributions and dynamics were consistent with relatively high heterogeneity in forest soil resources. The greater total root length in prairie may account for the significantly higher intensity of root competition in grasslands than forests and contribute to the exclusion of woody plants from prairies. Trees may be able to overcome this resistance by both causing and benefiting from higher levels and scales of soil resource heterogeneity.