Forage production and quality of 4 perennial grasses grown under and outside canopies of matureProsopis glandulosa Torr. var.glandulosa (mesquite)

Comparisons were made of dry matter production and forage quality of two coolseason grasses, Canada wildrye (Elymus canadensis L.), Virginia wildrye (Elymus virginicus L.), and two warm-season grasses, green panic (Panicum maximum var.trichoglume Eyles) and plains bristlegrass (Setaria leucopila Scribn. & Merr.), planted under and outside the canopies of mature honey mesquite (Prosopis glandulosa Torr. var.glandulosa). Green panic gave the greatest cumulative dry matter yield in both canopy (5120 kg ha^–1) and open (3370 kg ha^–1) locations, followed by plains bristlegrass under mesquite (1130 kg ha^–1) and in the open (570 kg ha^–1). One-time yields from Canada wildrye and Virginia wildrye, under and outside mesquite canopy, were 247 and 329 kg ha^–1, and 272 and 268 kg ha^–1, respectively. Dry matter production of green panic (p=0.001) and plains bristlegrass (p=0.026) was greater under mesquite only for the first of four harvests, although both warm-season grasses exhibited a trend for greater production under mesquite than in the open. Average nitrogen content of green panic was greater (p=0.0004) under mesquite than in the open, while plains bristlegrass exhibited a trend for greater crude protein content in the open than under mesquite. Overall, moisture content of the warm-season grasses was greater (p=0.0001) under mesquite than in the open, while Virginia wildrye was more (p=0.002) succulent under mesquite than in the open. Averagein-vitro dry matter digestibility of warm-season grasses was almost significantly greater (p=0.0501) in the open than under mesquite. Canopy soils contained significantly more organic C (p=0.0004) and total N (p=0.0001) than open soils, with differences of 8.3 Mg organic C ha^–1 and 1.3 Mg total N ha^–1. Correlations indicated that soil fertility was more limiting to grass production than light intensity.     

Five-year radial growth of red oaks in mixed bottomland hardwood stands

We studied the relationships among 5-year radial (diameter and basal area) growth of red oak (genus Quercus, subgenus Erythrobalanus) crop trees and predictor variables representing individual tree vigor, distance-dependant competition measures, and distance-independent competition measures. The red oaks we examined are representative of the commercially and ecologically important oak species of the bottomland hardwood forests of the southeastern US. The crown class score, a quantitative measure of crown class and tree vigor, performed best in accounting for the variability in tree diameter growth. Plot-level variables failed to account for a significant proportion of the variability in tree radial growth. The basal area of the first-order neighbors that were taller than the crop trees and located within 2.4 times the mean overstory crown radius had the highest negative correlation with crop tree 5-year radial growth. Red oaks were a major part of these competitors and likely exerted the greatest competitive pressure. However, crop tree radial growth was positively associated with the basal area of the red oaks which were indirect (second order) neighbors and which were taller than the crop trees. It is possible that indirect neighbors do not compete with the crop trees, but they likely compete with the direct competitors of the crop trees, thus having an indirect positive influence on crop tree growth. Such reasoning is consistent with previously observed spatial dependence up to four times the mean overstory crown radius. The findings may have implications for thinning hardwoods stands and crop tree management in that foresters need to take into account (1) oak intra-genus competition, (2) the negative competitive effect of direct neighbors, and (3) the potentially positive effect of the indirect neighbors, the competitors’ competitors.     

Impact of artificial pruning on growth and secondary shoot development of wild cherry (Prunus avium L.)

Producing high value veneer wood requires that the tree bole be branch-free. This can be accomplished by natural or artificial pruning. Since wild cherry does not self prune well, pruning artificially is the only practical option. The study analysed the effect of conventional whorl-wise pruning and selective pruning, on height growth, diameter growth and secondary shoot development of wild cherry. Four pruning treatments were applied on cherry trees in summer 2007, one group of cherries was left unpruned to serve as a control: treatment C1 (upper 5 whorls left), C2 (upper 3 whorls left), S1 (removal of branches larger than 3 cm or with an angle to the stem < 40°), S2 (removal of branches larger than 2 cm or with an angle to the stem < 40°), N (unpruned). Data showed that height growth was not affected by pruning. In contrast, diameter growth at breast height of the C2 pruned cherry was reduced by approximately 5% (SE = 2.7%) in the year of pruning (trees were pruned in July). This pruning treatment produced significant (p = 0.028) nine percent less diameter growth than the control in the second year following pruning. The diameter increment of the C1 pruned trees with five whorls left after pruning and the selective pruned cherries were only about 4% (SE = 4.0%) smaller than the control after two years. This loss was statistically not significant. Analyses showed that on selective pruned trees the survival rate of secondary shoots was significantly reduced compared to those on whorl-wise pruned trees. Significant differences in the size of the secondary shoots were only found between the C1 and S1 (p < 0.05) pruned trees. We did not find differences in the total number of secondary shoots per tree among pruning treatments. Solely from a tree growth perspective, the moderate whorl-wise pruning treatment C1 and the selective prunings were equally effective in minimizing the reduction of diameter growth and are recommended in practice. However it was found that the survival of secondary shoots was reduced on selective pruned trees although the amount of pruning work needed in selective pruning was slightly greater than conventional moderate pruning.     

Extent of vegetation-free zone necessary for silvopasture establishment of eastern black walnut seedlings in tall fescue

Silvopasture—the integration of trees, forage, and livestock can be established by planting trees in existing pastures. Successful tree establishment and acceptable tree growth in existing tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] pastures requires a vegetation-free zone near the tree base. This study was conducted to determine how large a vegetation-free zone was necessary for the establishment of black walnut (Juglans nigra L.) in tall fescue pastures. Half-sib black walnut seeds were planted in seven different-sized vegetation-free zones [0.0 (control), 0.3, 0.6, 0.9, 1.2, 1.5, and 1.8 m radii] that were created and maintained by applying glyphosate. The study included two consecutive planting years at two locations, one in central Missouri and another in north-central Missouri. The central Missouri site was underlain with well drained, deep soil while the other site was underlain with a poorly drained soil with a defined argillic horizon. Tree height growth was greatest in 0.9-m or larger vegetation-free zones. Diameter growth was greatest in 1.2-m vegetation-free zones and larger. The results suggest that a minimum of a 1.2-m vegetation-free zone in tall fescue pastures should be used to maximize black walnut height and diameter growth in the critical first years of tree establishment.     

The role of disturbance severity and canopy closure on standing crop of understory plant species in ponderosa pine stands in northern Arizona,USA

Concerns about the long-term sustainability of overstocked dry conifer forests in western North America have provided impetus for treatments designed to enhance their productivity and native biodiversity. Dense forests are increasingly prone to large stand-replacing fires; yet, thinning and burning treatments, especially combined with other disturbances such as drought and grazing, may enhance populations of colonizing species, including a number of non-native species. Our study quantifies plant standing crop of major herbaceous species across contrasting stand structural types representing a range in disturbance severity in northern Arizona. The least disturbed unmanaged ponderosa pine stands had no non-native species, while non-native grasses constituted 7–11% of the understory plant standing crop in thinned and burned stands. Severely disturbed wildfire stands had a higher proportion of colonizing native species as well as non-native species than other structural types, and areas protected from grazing produced greater standing crop of native forbs compared to grazed unmanaged stands. Standing crop of understory plants in low basal area thinned and burned plots was similar to levels on wildfire plots, but was comprised of fewer non-native graminoids and native colonizing plants. Our results also indicate that size of canopy openings had a stronger influence on standing crop in low basal area plots, whereas tree density more strongly constrained understory plant standing crop in dense stands. These results imply that treatments resulting in clumped tree distribution and basal areas <10 m² ha⁻¹ will be more successful in restoring native understory plant biomass in dense stands. Multiple types and severity of disturbances, such as thinning, burning, grazing, and drought over short periods of time can create greater abundance of colonizing species. Spreading thinning and burning treatments over time may reduce the potential for non-native species colonization compared to immediately burning thinned stands.     

Growth and yield of sorghum or cowpea in an agrisilviculture system in semiarid India

This study was conducted near Hyderabad, India during 1991–1994 to quantify the effects of shoot pruning, fertilization, and root barriers around Leucaena leucocephala trees on intercropped sorghum(Sorghum bicolor) or cowpea (Vigna unguiculata) crop production under rainfed conditions. Crop plants grown with pruned trees attained higher dry matter and leaf area index than did those with unpruned trees. Two-year mean grain yields of sorghum with no root barriers were76% and 39% of pure crop yield (1553 kg ha–1)for pruned and unpruned trees, respectively. Corresponding values for cowpea were 49% and 26% of pure crop yield (1075 kgha–1). Sorghum or cowpea intercropped with trees responded to fertilizer application more strongly than did their respective pure crops, suggesting an increased need for fertilizer application in this agrisilviculture system over that currently used for pure crops. Impact of root barriers was small on either crop. Irrespective of root barriers, a high response to tree pruning suggested above ground competition for light dominated tree/crop interactions in this agrisilviculture system. This revised version was published online in June 2006 with corrections to the Cover Date.     

Intensive,site-specific silviculture: Manipulating resource availability at establishment for improved stand productivity. A review of South African research

An important window of opportunity to increase and sustain productivity in short-rotation plantations is the period from felling through re-establishment to canopy closure. This paper explores the effects, interactions and response mechanisms of intensive silvicultural practices on plantation productivity and sustainability, using five South African case studies (a–e). (a) Land preparation trials showed that complete surface cultivation by ploughing had a significant beneficial effect when afforestation is done for the first time in grasslands, improving basal area growth by 11–52% over pitting only. However, similar treatments have not resulted in significant growth responses under re-establishment conditions. (b) Stand growth suppression resulting chiefly from soil compaction during mechanised harvesting operations is strongly related to soil type, soil textural class and residue management options. Volume growth reduction in short-rotation eucalypt crops ranged from 25% on compaction sensitive loamy soils to less than 2% in resistant sandy soils. (c) The response mechanism whereby vegetation management improves stand productivity is a reduction in both inter-specific and intra-genotypic competition for resources, as well as a decrease in stand variability. Operationally, the most important criteria in a vegetation management programme relate to the timing of control operations across diverse site conditions. In local trials, the primary factors controlling the time taken for competition-induced tree growth suppression to occur were related to altitude, slash burning and the interaction between these factors, which facilitated the development of regional vegetation management strategies. (d) Empirical fertilizer trials in short-rotation hardwood stands have shown significant improvements in final productivity (commonly 20–90 m³ ha⁻¹ in eucalypts and 30–50 m³ ha⁻¹ in Acacia), as well as wood density (15–30 kg m⁻³ for eucalypts) following improvements in early nutrition. Improved nutrition was achieved through fertilization at planting or indirectly through residue management. The response mechanism is primarily due to early canopy development and associated increases in light capture, coupled with a more modest increase in canopy quantum efficiency and above-ground carbon allocation on a dry site. On sites with abundant water supply, increased quantum efficiency is likely to be the dominant response mechanism. (e) A series of operational gains trials tested the interactive effect of genetic tree improvement, site–genotype interaction, stand density and vegetation management + fertilization on eucalypt stand growth across five sites. There were no significant interactions between factors, but importantly, the results were additive, emphasizing the need to optimise each practice in the value chain to achieve maximum productivity.     

To prune or not to prune Faidherbia albida: competing needs for water,wheat and tree products in semi-arid Ethiopia

Faidherbia albida is one of the scattered trees commonly intercropped with most cereals in Ethiopia due to its positive impacts. The tree is pruned for various purposes including for fencing and fuelwood. In this study, the impact of pruning on water relations of F. albida and on understorey wheat productivity was investigated. The on-farm study was conducted in Ejerssa Joro, semi-arid Ethiopia. Six mature trees were selected; three were fully pruned and three were left unpruned. Sap flow and leaf water potential were measured on these trees. Crop gas exchange, aboveground biomass and grain yield were measured under and outside tree canopies. The highest and the lowest sap volumes, recorded from unpruned F. albida, during the dry period, were 153 L day^?1 and 20 L day^?1, respectively. The highest and the lowest sap volumes were 13.4 L day^?1 and 0.04 L day^?1 recorded during the wet period. Wheat CO₂ assimilation was highest (7.8 µmolm^?2 s^?1) at 1 m distance and declined away from the tree trunk under unpruned trees. Aboveground biomass and grain yield under unpruned treatments were significantly (P?<?0.05) higher than outside of canopy of same tree and outside canopies of pruned trees. Pruning reduced aboveground biomass and grain yield by 30% and 27%, respectively; despite the higher water uptake by unpruned trees. We recommend that intensive pruning of F. albida be discouraged and propose further studies on optimal pruning for increased food production and provision of tree products to meet farmers’ needs.

     

Afforestation method affects the isotopic composition of planted Pinus halepensis in a semiarid region of Spain

We used an isotopic approach to evaluate the effects of three afforestation methods on the ecophysiology of an Aleppo pine plantation in semiarid Spain. The site preparation methods tested were excavation of planting holes (H), subsoiling (S), and subsoiling with addition of urban solid refuse to soil (S + USR). Five years after plantation establishment, trees in the S + USR treatment were over three times larger than those in the S treatment, and nearly five-fold larger than those planted in holes. Differences in tree biomass per hectare were even greater due to disparities in initial planting density and pine tree mortality among treatments. Pine trees in the S + USR treatment showed higher foliar P concentration, δ¹³C and δ¹⁵N than those in the S or H treatments. Foliar δ¹⁵N data proved that trees in the S + USR treatment utilized USR as a source of nitrogen. Foliar δ¹³C and δ¹⁸O data suggest that improved nutrient status differentially stimulated photosynthesis over stomatal conductance in the pine trees of the S + USR treatment, thus enhancing water use efficiency and growth. In the spring of 2002, trees in the S + USR treatment exhibited the most negative predawn water potentials of all the treatments, indicating that the rapid early growth induced by USR accelerated the onset of intense intra-specific competition for water. The results of this study have implications for the establishment and management of Aleppo pine plantations on semiarid soils. Planting seedlings at low density and/or early thinning of pine stands are strongly recommended if fast tree growth is to be maintained beyond the first few years after USR addition to soil. Foliar C, O and N isotope measurements can provide much insight into how resource acquisition by trees is affected by afforestation techniques in pine plantations under dry climatic conditions.     

Influence of Acacia senegal agroforestry system on growth and yield of sorghum, sesame, roselle and gum in north Kordofan State, Sudan

We examined the effects of intercropping with Acacia senegal (L.) Willd on growth and yield of sorghum (Sorghum bicolor L.), sesame (Sesamum indicum L.) and roselle (Hibiscus sabdariffa). Field experiments were conducted in El-Obeid Research farm (13°10’ N; 30°12’ E), North Kordofan State, Sudan, during 2002 2003 in an 11-year-old A. senegal plantation. The experimental design was randomized complete block design (RCBD) with four replications. Data were recorded for plant height (cm), fresh weight (kg ha -1 ), dry weight (kg ha -1 ), crop yield (kg ha -1 ), and gum yield (kg ha -1 ). We used Land Equivalent Ratios (LER) and simple financial analyses of gross surpluses to evaluate the productivity and profitability of the different treatments. The results indicated that A. senegal trees had a beneficial effect on crop performance and yield as well as gum yield. Significant differences (p<0.05) were obtained for plant height, fresh weight, dry weight and crop yield. Therefore, yield of sorghum, sesame and roselle under intercropping system were 13.7%, 23.8% and 20.9% higher than that obtained in the sole cropping system respectively. The highest yield increase was observed with sesame (23.8%). Gum yield (g/tree/picking) was signifi- cantly (p<0.05) increased for sorghum, sesame and roslle under inter-cropping system. The highest yield of (298g/tree/picking) was obtained when roselle was intercropped with A. senegal, while the least gum yield of (239 g tree-1 ) was recorded in pure A. senegal plot. All the treatments gave land equivalent ratio (LER) of more than one-indicating the superiority of growing the field crops in intercropping over the sole cropping systems. The highest LER of 3.8 was obtained for sesame intercropped with A. senegal (Hashab), followed by 3.7, when sorghum was intercropped with A. senegal and 3.3 when roselle intercropped with A. senegal. All the treatments gave positive net revenues, the highest being for intercropped sorghum (558 SDG ha -1 ) (SDG=Sudanese gienh). The intercropping of roselle gave the second net revenue (518 SDG ha -1 ),while the sole sorghum gave the lowest net revenue (501 SDG ha -1 ).     

Agroforestry on alkali soil: effect of planting methods and amendments on initial growth,biomass accumulation and chemical composition of mesquite (Prosopis juliflora (SW) DC) with inter-space planted with and without Karnal grass (Diplachne fusca Linn. P. Beauv.)

Growth responses of mesquite (Prosopis juliflora (SW) DC.) during establishment stage to planting methods and amendments were studied in a highly alkali soil (Aquic Natrustalf, ESP 94), at the experimental farm of the Central Soil Salinity Research Institute, Karnal in July, 1984. Six combinations of planting methods and amendments with and without Karnal grass (Diplachne fusca Linn. P Beauv) in the inter-row space were compared in a four times replicated field experiment in randomised block design. The mean plant height (MPH), diameter at stump height (DSH), diameter at breast height (DBH), lopped biomass 16 months past planting and total biomass attained in 2 years period were significantly less when inter-row space was planted with Karnal grass. The MPH (cm), DSH and DBH (mm) attained in 2 years period were 319, 43.4 and 15.1 in with grass as compared to 405, 53.4 and 20.3 in without grass treatments. Similarly, the total biomass attained in 2 years was about 3 times more where inter-row space was not planted with grass. Growth was better when mesquite was planted by auger hole and pit methods than in trench plantation, when the original soil was treated with gypsum at 3 kg plant^–1 and then refilled. However, using trenches of dimensions 30 × 30 cm and filled with a mixture of original soil, 3 kg gypsum and 8 kg farm yard manure plant^–1 appeared to be promising method for establishing mesquite plantations on highly deteriorated alkali soils. The effect of amendments on growth decreased in the order gypsum + FYM, gypsum + rice husk, gypsum, control. In two years period, 37 percent mesquite died in the trenches in which the original soil was left unchanged. Effect of treatments on nutrient concentrations and total accumulations in different segments of mesquite is discussed. Karnal grass gave 25.3 t ha^–1 green forage yield in 8 cuts without amendment in a growth period of 26 months proving its great potential as a folder crop for the adverse sites. The inorganic chemical composition of the shoots, including trace elements Fe, Mn, Zn and Cu, is such as to make this species a highly promising plant for the exploitation of alkali soils. Growing Karnal grass with mesquite for a period of about 2 years reduced soil pH and EC significantly and improved organic carbon and available N contents. The grass improved water infiltration rates and moisture storage in the lower layers of the profile.     

Thinning and prescribed fire effects on dwarf mistletoe severity in an eastern Cascade Range dry forest,Washington

Forest thinning and prescribed fire practices are widely used, either separately or in combination, to address tree stocking, species composition, and wildland fire concerns in western US mixed conifer forests. We examined the effects of these fuel treatments alone and combined on dwarf mistletoe infection severity immediately after treatment and for the following 100 years. Thinning, burning, thin + burn, and control treatments were applied to 10 ha units; each treatment was replicated three times. Dwarf mistletoe was found in ponderosa pine and/or Douglas-fir in all units prior to treatment. Stand infection severity was low to moderate, and severely infected trees were the largest in the overstory. Thinning produced the greatest reductions in tree stocking and mistletoe severity. Burning reduced stocking somewhat less because spring burns were relatively cool with spotty fuel consumption and mortality. Burning effects on vegetation were enhanced when combined with thinning; thin + burn treatments also reduced mistletoe severity in all size classes. Stand growth simulations using the Forest Vegetation Simulator (FVS) showed a trend of reduced mistletoe spread and intensification over time for all active treatments. When thinned and unthinned treatments were compared, thinning reduced infected basal area and treatment effects were obvious, beginning in the second decade. The same was true with burned and unburned treatments. Treatment effects on infected tree density were similar to infected basal area; however, treatment effects diminished after 20 years, suggesting a re-treatment interval for dwarf mistletoe.     

Biomass and nitrogen dynamics in an irrigated hybrid poplar plantation

A 3-year study measured the effects of ground cover treatments and N fertilization on biomass and nitrogen dynamics in an irridiated hybrid poplar (Populus deltoides Bartr. X P. trichocarpa Torr. and Gray, clone NC-9922) plantation in northern Wisconsin, U.S.A. Annually fertilized (112 kg N ha⁻¹ year⁻¹) and unfertilized plots were maintained weed free (bare soil), allowed to revegetate with native weeds, or seeded to birdsfoot trefoil (Lotus corniculatus L.). Biomass and N in trees and ground-cover vegetation were sampled before and after each growing season.Trees in bare-soil plots responded to fertilization primarily in the third growing season, but total biomass of 3-year-old trees was not increased by annual fertilization. In plots with a ground cover,fertilization increased tree growth but cover crop treatment had no effect. Ground cover biomass peaked during the second growing season, but declined thereafter, primarily due to reductions in below-ground biomass. Estimated recovery of fertilizer N was low in bare soil plots after 3 years, with 2% in the ‘perennial’ portion of the trees and 13% in the leaf litter. In contrast, recovery in the cover crop plots was 44%–51% in years 2–4. During that period, both biomass and N pool dominance shifted from primarily cover crop to primarily trees. The ground cover appeared to reduce tree growth in years 1–3, but total tree biomass after 4 years was greater in fertilized plots with ground cover (22.7 Mg/ha) than in fertilized bare soil plots (16.7 Mg/ha). Biomass production in fertilized trefoil plots in the fourth year (15.1 Mg ha⁻¹ year⁻¹, excluding leaves) exceeds that of local forests by 50%, and may be comparable to corn productivity in the area.     

Mitigation of effects of extreme drought during stage III of peach fruit development by summer pruning and fruit thinning

A water deficit during stage III of fruit growth was established with the aim of determining if it is possible to achieve an improvement in tree water status by summer pruning and fruit thinning. The experiment was set up as a randomized block split-plot design across trials (irrigation) where pruning was assigned to the main plot and fruit thinning to the sub-plots. The irrigation treatments were (1) standard full irrigation (FI), and (2) suppression of irrigation during stage III of fruit growth until leaves visibly withered (LWI); the pruning treatments were (1) experimental summer pruning (EP), and (2) standard summer pruning (CP); and three fruit thinning intensities were applied to facilitate analysis of the effects of the treatments in relation to fruit load. Changes in amount of light intercepted and in tree stem water potential (Psi stem) were evaluated. The EP treatment reduced the amount of light intercepted by the tree. In the FI treatment, there was a significant reduction in fruit growth measured as both water accumulation and dry mass accumulation. Under FI conditions, reductions in fruit load as a result of EP were not accompanied by a significant improvement in Psi stem. In the LWI treatment, EP produced a significant improvement of 0.17 MPa in Psi stem, but there was no improvement in fruit growth compared with CP trees. A reduction in fruit load from 350 (commercial load) to 150 per tree significantly improved Psi stem by 0.3 MPa at the end of stage III of fruit growth. These results indicate that improvements in water status in response to pruning may be insufficient to promote fruit growth if the pruned trees are unable to provide an adequate supply of assimilates to the developing fruits.     

Effects of fertilization and irrigation on American sycamore and black locust planted on a reclaimed surface mine in Appalachia

Short rotation woody biofuel plantations on reclaimed surface mines in Appalachia can diversify domestic energy supplies and facilitate the reforestation of these disturbed lands. This study examined growth, survival, biomass accumulation and allocation, and nitrogen concentrations following two growing seasons in American sycamore (Platanus occidentalis L.) and black locust (Robinia pseudoacacia L.) seedlings receiving irrigation, granular fertilization, and irrigation + fertilization compared with untreated controls. Fertilization increased basal diameter, height, and stem mass of American sycamore following two growing seasons. Increased stem production was attributed to accelerated development and not shifts in biomass allocation due to treatment. Irrigation and irrigation + fertilization treatments did not enhance growth or biomass accumulation of American sycamore. Similarly, black locust basal diameter, height, and stem mass did not differ among the treatments. Browse surveys showed that more than 76% of black locust seedlings experienced arrested or retrogressed growth due to browse; less than 3% of American sycamores were browsed. This intensive browse by ungulates, likely including reintroduced elk, may have affected growth differences among species and confounded the effects of treatments on black locust. Survival was unaffected by treatment in both species, but mean survival was greatest in American sycamore (80%) compared to black locust (58%). Per tree total nitrogen uptake was highest in fertilized American sycamore (7.9 g) and lowest in irrigated American sycamore (0.9 g). The results of our study suggest that granular fertilizer applications can accelerate seedling growth on reclaimed surface mines in the Appalachian region.     

Dynamics and determinants of Quercus alba seedling success following savanna encroachment and restoration

The scattered tree layer that defines savannas is important for structuring the understory community and determining patterns of overstory recruitment. However, encroachment by woody plants has altered overstory tree densities and regeneration dynamics. We characterized seedling success of the savanna-forming species Quercus alba within Midwestern (USA) oak savannas that had been degraded by encroachment (control; n = 4) or experimentally restored by removal of encroaching woody vegetation (treatment; n = 4). In early 2004, 981 seedlings were transplanted along transects radiating from tree boles of overstory Q. alba trees to inter-canopy gaps and monitored for three growing seasons. Seedlings in restored sites had greater survival (>2×), height growth (by >50%), and basal diameter growth (by >20%). In general, seedling survival and growth parameters increased with distance from overstory trees and were greatest in inter-canopy gaps of restored sites. By the final growing season (2006), the seedling survival-by-distance from tree correlation was stronger in control (r² = 0.25) than treatment sites (r² = 0.18), due to relatively uniform (and greater) survival at all distances from trees in treatment sites. In 2006, growth parameters (seedling height, diameter, Δ height, Δ diameter, and # leaves) were significantly (and more strongly) positively correlated with distance from trees in treatment sites. However, seedling herbivory was also greater after treatment and increased with distance from overstory trees. To understand seedling/microenvironment relationships, we created logistic (survival) and linear regression models (Δ height, Δ basal diameter, # leaves in 2006). Control seedling models had consistently greater predictive power and included more variables, suggesting that savanna restoration may decouple seedlings from their microenvironments, potentially by decreasing competition for limiting resources. Encroachment of the savannas in this study is limiting regeneration of Q. alba, suggesting substantially altered regeneration dynamics from those under which these savannas originally formed. Initial responses from our test of restoration, however, were promising and mechanical encroachment removal may be a means to promote overstory regeneration of this species. Finally, the savannas in this study appear inherently unstable and a scattered canopy tree configuration is unlikely to persist without regular disturbance, even in the restoration sites. Repeated mechanical thinning treatments with selected retention of recruiting Q. alba individuals or reintroduction of understory fire or grazing animals may be potential mechanisms for promoting long-term persistence of savannas at these sites.     

Epicormic sprout development in pruned coast redwood: pruning severity, genotype, and sprouting characteristics

? Young coast redwood (Sequoia sempervirens (D. Don.) Endl.) trees were pruned to various heights to examine the effect of pruning severity on epicormic sprouting. Seven separate stands were used with as many as six treatments per stand in coastal Humboldt County, California, USA.

? Epicormic sprout development was affected by pruning severity but primarily at the most severe pruning treatments that removed all but the branches in the top 15% of tree height. Less severe treatments produced sprouts but the number and size of these sprouts were comparable to unpruned trees.

? Natural clonal patterns were also used to explore patterns of sprouting between genotypes. Linear mixed-effects models were developed to predict sprouting frequency as a function of pruning severity while accounting for the nested data structure (i.e., stem sections sampled nested within genotypes within treatments within sites).

? Comparing variances attributed to each of these random effects indicated that at any level of pruning severity, differences in epicormic sprouting between genotypes and sites expressed soon after pruning had disappeared after six growing seasons. Epicormic branches were more common two years after pruning than six years indicating many branches were dying. Branches were more common in the middle of the pruned bole, possibly because of competition from basal sprouts and the expanding tree crown.

     

Soil water dynamics in cropping systems containing <Emphasis Type="Italic">Gliricidia sepium</Emphasis>, pigeonpea and maize in southern Malawi

The water dynamics of cropping systems containing mixtures of Gliricidia sepium (Jacq.) Walp trees with maize (Zea mays L.) and/or pigeonpea (Cajanus cajan L.) were examined during three consecutive cropping seasons. The trees were pruned before and during each cropping season, but were left unpruned after harvesting the maize; prunings were returned to the cropping area in all agroforestry systems to provide green leaf manure. The hypothesis was that regular severe pruning of the trees would minimise competition with crops for soil moisture and enhance their growth by providing additional nutrients. Neutron probe measurements were used to determine spatial and temporal changes in soil moisture content during the 1997/98, 1998/99 and 1999/00 cropping seasons for various cropping systems. These included gliricidia intercropped with maize, with and without pigeonpea, a maize + pigeonpea intercrop, sole maize, sole pigeonpea and sole gliricidia. Soil water content was measured to a depth of 150 cm in all treatments at 4–6 week intervals during the main cropping season and less frequently at other times. Competition for water was apparently not a critical factor in determining crop performance as rainfall exceeded potential evaporation during the cropping season in all years. The distribution of water in the soil profile was generally comparable in all cropping systems, implying there was no spatial complementarity in water abstraction by tree and crop roots. However, available soil water content at the beginning of the cropping season was generally lower in the tree-based systems, suggesting that the trees continued to deplete available soil water during the dry season. The results show that, under rainfall conditions typical of southern Malawi, the soil profile contains sufficient stored water during the dry season (ca. 75–125 mm) to support the growth of gliricidia and pigeonpea, and that gliricidia trees pruned before and during the cropping season did not deleteriously compete for water with associated crops. Water use efficiency also appeared to be higher in the tree-based systems than in the sole maize and maize + pigeonpea treatments, subject to the proviso that the calculations were based on changes in soil water content rather than absolute measurements of water uptake by the trees and crops.     

Silvicultural treatments enhance growth rates of future crop trees in a tropical dry forest

Silvicultural treatments are often needed in selectively logged tropical forest to enhance the growth rates of many commercial tree species and, consequently, for recovering a larger proportion of the initial volume harvested over the next cutting cycle. The available data in the literature suggest, however, that the effect of silvicultural treatments on tree growth is smaller in dry forests than in humid forest tree species. In this study, we analyze the effect of logging and application of additional silvicultural treatments (liana cutting and girdling of competing trees) on the growth rates of future crop trees (FCTs; i.e., trees of current and potentially commercial timber species with adequate form and apparent growth potential). The study was carried out in a tropical dry forest in Bolivia where a set of 21.25-ha plots were monitored for 4 years post-logging. Plots received one of four treatments that varied in intensity of both logging and silvicultural treatments as follows: normal (reduced-impact) logging; normal logging and low-intensity silviculture; increased logging intensity and high-intensity silviculture; and, unlogged controls. The silvicultural treatments applied to FCTs involved liberation from lianas and overtopping trees. Results showed that rates of FCT stem diameter growth increased with light availability, logging intensity, and intensity of silvicultural treatments, and decrease with liana infestation degree. Growth rate increment was larger in the light and intensive silvicultural treatment (22–27%). Long-lived pioneer species showed the strongest response to intensive silviculture (50% increase) followed by total shade-tolerant species (24%) and partial shade-tolerant species (10%). While reduced-impact logging is often not sufficient to guarantee the sustainability of timber yields, application of silvicultural treatments that substantially enhanced the growth rates of FCTs will help move the management of these forests closer to the goal of sustained yield.     

Establishing American sycamore seedlings on land irrigated with paper mill sludge

A study was established to evaluate several management practices for improving American sycamore (Platanus occidentalis L.) establishment on land subjected to daily paper-mill sludge spray irrigation, and where a dense herbaceous cover dominated by pokeweed (Phytolacca americana L.) was nearly 2 m tall. Treatments studied included: two sludge application treatments (spray vs no spray), five weed control treatments (control, hand-release, oxyfluorfen, glyphosate and atrazine, and sulfometuron methyl), and two tree shelter treatments (control vs tree shelter). Survival and growth of sycamore seedlings were evaluated after one growing season. Sludge application treatment had the greatest effect on tree survival; survival was greatest on plots that were not sprayed during the year of tree growth. Among the weed control treatments, the hand-release treatment resulted in best survival; survival was 100% when hand release was used on unsprayed plots. Tree shelters provided no benefit to tree establishment.