Twig Length Research Articles

Long-term effects of liming on health and growth of a Masson pine stand damaged by soil acidification in Chongqing, China.

In the last decades, the Masson pine (Pinus massoniana) forests in Chongqing, southwest China, have increasingly declined. Soil acidification was believed to be an important cause. Liming is widely used as a measure to alleviate soil acidification and its damage to trees, but little is known about long-term effects of liming on the health and growth of declining Masson pine forests. Soil chemical properties, health condition (defoliation and discoloration), and growth were evaluated following application of limestone powder (0 (unlimed control), 1, 2, 3, and 4 t ha−1) in an acidified and declining Masson pine stand at Tieshanping (TSP) of Chongqing. Eight years after liming, in the 0–20 cm and 20–40 cm mineral soil layers, soil pH values, exchangeable calcium (Ca) contents, and Ca/Al molar ratios increased, but exchangeable aluminum (Al) levels decreased, and as a result, length densities of living fine roots of Masson pine increased, with increasing dose. Mean crown defoliation of Masson pines (dominant, codominant and subdominant pines, according to Kraft classes 1–3) decreased with increasing dose, and it linearly decreased with length densities of living fine roots. However, Masson pines (Kraft classes 1–3) in all treatments showed no symptoms of discoloration. Mean current-year twig length, twig dry weight, needle number per twig, needle length per twig, and needle dry weight per twig increased with increasing dose. Over 8 years, mean height increment of Masson pines (Kraft classes 1–3) increased from 5.5 m in the control to 5.8, 6.9, 8.3, and 9.5 m in the 1, 2, 3, and 4 t ha−1 lime treatments, and their mean DBH (diameter at breast height) increment increased from 3.1 to 3.2, 3.8, 4.9, and 6.2 cm, respectively. The values of all aboveground growth parameters linearly increased with length densities of living fine roots. Our results show that liming improved tree health and growth, and these effects increased with increasing dose.

Tree genotype and genetically based growth traits structure twig endophyte communities

Fungal endophytes asymptomatically inhabit plant tissues where they have mutualistic, parasitic, or commensal relationships with their hosts. Although plant-fungal interactions at the genotype scale have broad ecological and evolutionary implications, the sensitivity of endophytes in woody tissues to differences among plant genotypes is poorly understood. We hypothesize that (1) endophyte communities in Populus angustifolia (Salicaceae) twigs vary among tree genotypes, (2) endophyte variation is linked to quantitative tree traits, and (3) tree genotype influences interspecific fungal interactions. Endophytes were isolated from twigs of replicated P. angustifolia genotypes in a common garden and characterized with PCR-RFLP and DNA sequencing. Twig length and diameter, aboveground tree biomass, and condensed tannins were also quantified. (1) Aspects of fungal community structure, including composition and total isolation frequency (i.e., abundance), varied among genotypes. (2) Aboveground biomass and twig diameter were positively associated with isolation frequency and covaried with composition, whereas twig length and condensed tannin concentration were not significantly correlated to endophytes. (3) Fungal co-occurrence patterns suggested negative species interactions, but the presence of significant co-occurrences was genotype dependent. The species is often assumed to be the most important ecological unit; however, these results indicate that genetically based trait variation within a species can influence an important community of associated organisms. Given the dominance of plants as primary producers and the ubiquity of endophytes, the effect of host genetic variation on endophytes has fundamental implications for our understanding of terrestrial ecosystems.

Evaluation of the Influence of Shaking Frequency and Duration in Mechanical Harvesting of Sweet Cherry

<abstract><title><italic>Abstract. </italic></title> Almost all fresh-market sweet cherry is currently hand-picked. A decreasing labor supply and rising harvesting costs have caused producers to begin moving toward mechanical harvesting. This study focused on studying different combinations of shaking frequency and duration for a hydraulically-powered mechanical shaking end-effector for effectively harvesting fresh-market sweet cherry from “Y” trellis trees. Shaking frequencies of 14 and 18 Hz, combined with shaking durations of 2 and 5 s, were evaluated in an orchard environment. Dislodged fruit was captured on a sheet laid under the tree, collected and weighed after each shaking interval. This procedure was repeated 4 or 10 times based on two different shaking durations used in the study to reach a cumulative shaking time of 20 s. Each combination was replicated on three trees. Results indicated that the first 2-s interval of 14 Hz shaking removed 29% of total fruit on average from the test trees, and the first 5-s interval of 18 Hz shaking removed 38% on average. Multiple shaking intervals with a total of 20 s shaking duration removed 81%, 73%, 62% and 59% of total fruit on average from four 5-s 18 Hz intervals, ten 2-s 14 Hz intervals, four 5-s 14 Hz intervals and ten 2-s 18 Hz intervals respectively, indicating that both frequency and duration have effects on fruit removal efficiency of a mechanical cherry harvesting system. Based on visual observation, over 60% of remaining fruit were growing on long and slender twigs. This result suggested that a proper pruning of cherry trees to minimize the number and/or length of small and long twigs could contribute substantially to the improvement of the overall fruit removal efficiency of the shaking-based mechanical harvesting system.

Effect of Paclobutrazol Application on Nutrient Dynamics, Vigour and Fruit Yield in 'Alphonso' Mango (Mangifera indica L.)

Application of Paclobutrazol to 22 year-old 'Alphonso' mango trees significantly retarded plant height, plant spread and tree volume. Number of flushes and vigour of emerging new flush also decreased significantly besides production of fewer leaves, reduced leaf area, twig length and dry matter content. Fruit yield increased significantly in trees receiving Paclobutrazol treatment, compared to 'control' trees in all four years of study. This increase was distinctly higher during the on-years 2005 and 2007 by 133% and 77%, respectively, over 'control' due to more profuse flowering and fruit-set. Differences in mineral composition of various tree parts were significant except for N and P content. Paclobutrazol application caused significant increase in Ca, Mg and Mn content in the leaf. Substantial reduction observed in dry matter content and reduced leaf area accompanied by greater removal of nutrients by increased fruit production under Paclobutrazol, application may weaken the tree significantly. The trees would then need proper and adequate nutrient management vis-à-vis untreated trees, to achieve sustainable productivity.

Freezing pattern and frost killing temperature of apple (Malus domestica) wood under controlled conditions and in nature

The freezing pattern and frost killing temperatures of apple (Malus domestica Borkh.) xylem were determined by differential thermal analysis and infrared differential thermal analysis (IDTA). Results from detached or attached twigs in controlled freezing experiments and during natural field freezing of trees were compared. Non-lethal freezing of apoplastic water in apple xylem as monitored during natural winter frosts in the field occurred at -1.9 ± 0.4 °C and did not change seasonally. The pattern of whole tree freezing was variable and specific to the environmental conditions. On detached twigs high-temperature freezing exotherms (HTEs) occurred 2.8 K below the temperature observed under natural frosts in the field with a seasonal mean of -4.7 ± 0.5 °C. Microporous apple xylem showed freezing without a specific pattern within a few seconds in IDTA images during HTEs, which is in contrast to macroporous xylem where a 2D freezing pattern mirrors anatomical structures. The pith tissue always remained unfrozen. Increasing twig length increased ice nucleation temperature; for increased twig diameter the effect was not significant. In attached twigs frozen in field portable freezing chambers, HTEs were recorded at a similar mean temperature (-4.6 ± 1.0 °C) to those for detached twigs. Upon lethal intracellular freezing of apple xylem parenchyma cells (XPCs) low-temperature freezing exotherms (LTEs) can be recorded. Low-temperature freezing exotherms determined on detached twigs varied significantly between a winter minimum of -36.9 °C and a summer maximum -12.7 °C. Within the temperature range wherein LTEs were recorded by IDTA in summer (-12.7 ± 0.5 to -20.3 ± 1.1 °C) various tiny clearly separated discontinuous freezing events could be detected similar to that in other species with contrasting XPC anatomy. These freezing events appeared to be initially located in the primary and only later in the secondary xylem. During the LTE no freezing events in the bark and central pith tissue were recorded. Attached twigs were exposed to various freezing temperatures at which LTEs occur. Even if 60% of XPCs were frost-damaged twigs were able to recuperate and showed full re-growth indicating a high regeneration capacity even after severe frost damage to XPCs.

In-situ measurement of twig dieback and regrowth in mature Acer saccharum trees

Crown shyness is thought to influence both the productivity and dynamics of forests, but few studies have examined the underlying causes of this common phenomenon. The few studies that exist suggest that crown shyness is caused by the reciprocal abrasion of neighboring tree crowns, resulting in the death of peripheral buds and/or the breakage of peripheral twigs (referred to here as twig dieback). However, twig dieback has not been directly observed due to the difficulty of accessing the crowns of mature canopy trees. In this study, we used a mobile canopy lift to obtain in-situ measurements of twig dieback in mature sugar maple (Acer saccharum Marsh.) trees. We measured: (1) percent dieback (%year−1) – the proportion of twigs that broke off or died; (2) dieback (cmyear−1) – the length of the broken or dead portion; and (3) regrowth (cmyear−1) – the length of the new twigs that sprouted from axillary buds. One third of the twigs suffered dieback over the course of 1year, resulting in 1.41cm of dieback, on average. Percent dieback was significantly higher in tree crowns located within 3m of another crown, indicating that dieback is the result of the reciprocal abrasion of neighboring crowns. Percent dieback also increased with tree size, suggesting that tree sway increases as trees grow larger, resulting in more frequent and more intense abrasion. These trends were exacerbated by similar trends in regrowth, which was slower both in large trees and in trees located with 3m of another crown. Our results suggest dieback may represent a substantial drain on both tree growth and stand productivity – a drain that increases as trees grow larger and stands mature.

Twig Morphology and Host Effects on Reproductive Success of the Twig GirdlerOncideres cingulata(Say) (Coleoptera: Cerambycidae)

The reproductive success of the twig girdler Oncideres cingulata (Say) was examined relative to twig morphology of the host tree species used for oviposition. There were significant differences in twig morphology between hosts, in twig length, volume, and surface area. Ovipositing beetles appeared to adjust reproductive efforts to these host differences, as reflected by egg density. Twig length was strongly correlated with the number of eggs laid across all host species, although regression analysis found that a model consisting of twig diameter and surface area best accounted for the total number of eggs per twig. Adult emergence was not related consistently to any aspect of twig morphology across host species. However, emergence rates varied between hosts, ranging from 6.1% from eggs laid on birch (Betula nigra L.), 7.7% from American elm (Ulmus americana L.), 10.0% from pecan (Carya illinoensis (Wangenh.) K. Koch), to 28.2% on honey locust (Gleditisia triacanthos L.).

Effect of pruning history on growth and dry mass partitioning of jatropha on a plantation site in Madagascar

While technical aspects of oil processing of seeds of jatropha are under intensive investigation, comparably little is known about the performance of jatropha in the field. We investigated the effects of water availability (rainfed versus irrigated) and pruning-induced differences in plant stature on growth, biomass partitioning, and canopy size at a plantation site in Madagascar in 2010. Plants of different pruning types differed in trunk height (43 versus 29 cm) and primary branches total length (171 versus 310 cm). The two pruning types had effects on dry mass formation and leaf area projection (LAP) during the vegetation period. Trees which had a shorter trunk and longer lateral branches produced more biomass and had a higher LAP. Total dry mass formation varied from 489 to 912 g m � 2 and LAP from 3.26 to 7.37. Total aboveground biomass increased from 2.3 � 0.5 to 4.89 � 1.4 kg tree � 1 and from 4.6 � 1.8 to 8.9 � 1.0 kg tree � 1 for the pruning types with shorter and longer lateral branches, respectively. Growth of twigs and leaves was positively correlated with total length of branches. Relative dry mass allocation to branches, twigs and leaves, length of twigs per cm of branches and specific leaf area (13.57 � 0.72 m 2 kg � 1 ) were not affected by pruning and water supply. Trees with shorter branches had higher LAD. Results indicate that pruning type should be considered as a management tool to optimize biomass production. Detailed studies on effects of canopy size and shape on radiation interception and growth are required to improve the productivity of jatropha. a 2011 Published by Elsevier Ltd.

Top working method and bloom density of pollinizers as productive determinant for spur type apple ( Malus x domestica Borkh.) cultivars

Apple ( Malus x domestica Borkh.) shows gametophytic self incompatibility and requires cross pollination by suitable pollinizers. Gradual decline in natural pollinators and insufficient proportion of pollinizers have been observed in the apple orchards over the years across the North Western Himalayan region. This situation resulted in considerable pollination problem in the region. The present study was undertaken (during the years 2006–2010) to examine the effect of top working methods and time as well as type of pollinizers on different growth parameters for efficient pollination management. Chip budding performed in fall, late winter and summer gave significantly higher success rate (90.0, 89.8 and 80.9%, respectively). Other methods viz., side rind graft (85.6%) and cleft graft (71.2%) during February–March, and T-bud (71.7%) during July–August also gave better success. The highest shoot numbers (4.1) and length (92.8 cm) were recorded when chip budding was performed during September–October and February–March, respectively. In 4th year, chip budded scion branches of all the pollinizers recorded significantly higher number of spurs/m twig length with a range of 22.8 (December–January) to 24.7 (September–October). Significantly the highest bloom density (22.1) was recorded on chip budded (February–March) branches of pollinizers in 4th year. Fruit set on top worked trees of ‘Oregon Spur’ was only in the range of 18.0–20.1% in 1st year and reached to the range of 35.6–41.5% in 4th year. Seed numbers/fruit also increased to the range of 7.3–7.9 in 4th year from 2.9 to 4.6 in 1st year. All these factors resulted in better yield efficiency (2.6–3.8) of ‘Oregon Spur’ in 4th year. ‘Manchurian’ crab produced significantly higher shoot length of 86.5–87.2 cm and 146.1–149.2 cm in 1st and 4th years, respectively. However, ‘Stark Spur’ produced the highest number of spurs/m twig length of 28.9 when grafted/chip budded during December–January or February–March, and 27.1 when chip/T-budded during September–October or July–August. Top working with ‘Manchurian’ crab also resulted in significantly higher fruit set (44.2–45.4%) and yield efficiency (3.6–3.8) on ‘Oregon Spur’ in 4th year. This was followed by ‘Stark Spur’ as pollinizer (37.1–38.2% and 2.9–3.1, respectively). Highly significant positive correlation of shoot numbers, spur density and bloom density of pollinizers with fruit set and yield efficiency of ‘Oregon Spur’ were observed. ‘Manchurian’ crab was found to be very efficient as pollinizer, followed by ‘Stark Spur’ for spur type ‘Oregon Spur’ apple cultivar on the basis of higher bloom density and fruit set parameters.

EVALUATION OF SOME IMPORTED PECAN VARIETIES UNDER GIZA GOVERNORATE ENVIRONMENTAL CONDITIONS

This study was carried out during two successive seasons of 2007 & 2008 to examine the performance of six new pecan varieties imported from Georgia, U.S.A. namely: Burkett, Desirable, Pawnee, Success, Western Schley, and Wichita. The experimental trees were grown at the Horticulture Research Institute, Giza, Egypt. This evaluation includedmorphological, flowering, yield and fruit quality, physical characteristics as well as kernel oil fatty acids composition and the ability to vegetative propagation by grafting. The Effect of environmental conditions on dichogamy phenomenon was also studied.It can be clearly noticed that, there is a positive relation between shoot length & diameter, av. No. of leaves per shoot and internodes length, where Burkett variety was the superior in this concern, while, Pawnee variety exceeded the others in twig length and No. of shoots/twig. Desirable var. recorded the highest No. / of leaflets/leave and leaflet area. Western Schley had smaller tree size. There was an obvious varietals difference in pecan bud developmental stages (dormant, vegetative and starting buds for staminate or pistillate inflorescences). The highest No. of the abovementioned buds as well as total No. of such buds and No. of fruits/shoot was observed in Western Schley variety except for No. of vegetative buds. Fruit set (%) averaged between 66.04 & 66.04 in Wichita to reach 85.60 & 82.57 in Desirable. Number of days in which pollen shed coincides with stigma receptivity varied between the studied varieties, where there was more consistent period of overlap in Pawnee, Success & Western Schley varieties. The previously mentioned varieties can be classified as protandrous (type, 1), whereas, Burkett, Desirable & Wichita varieties as protogynous (2). Moreover, Wichita can be considered a good pollinator for most other studied varieties. Pollen shedding period in Wichita extended with the increase of relative humidity and lower temperature. Burkett followed by Success varieties started and ended to crack shuck and harvesting earlier than other varieties. Western Schley, Desirable & Burkett varieties had an excellent kernel ercentage, respectively, whereas, Pawnee variety was the poorest. Varieties with high nut weight had lower No. of nuts /kg. Desirable followed by Burkett produced the highest yield, while, Success was the lowest. In regard to kernel oil content (%), Desirable followed by Western Schley kernels were the richest. The main compositional characteristic of pecan kernel was the high concentration of oleic acid and very low content of Linolinec acid. It is also interesting to note that, successful grafts (%) varied between varieties from 33.33 % in Western Schley to reach 93.33 & 91.66 % in Desirable & Success, respectively. As a conclusionunder the same conditions of the present study Burkett & Success may be recommended as early cropping varieties; Wichita as a good pollinator; Western Schley as a small size tree which can be helpful in increasing number of trees / Fed. which in turn increases yield/Fed. as well as may facilitate the trees cultural practices. The primary pecan varieties Desirable & Burkett performed superior to other varieties in terms of yield; Western Schley and Desirable as varieties that have higher percentages of kernel and oil content and Desirable & Success are characterized by their easy to vegetative propagation. Moreover, all of these varieties were likely to complete their developmental growth cycle, which indicates low chilling requirements character and can be grown commercially in Egypt.

The effect of twig architecture and seed number on seed size variation in subtropical woody species

This study aims to determine the effect of twig (current year shoot) size on seed size variation and to test whether a seed size vs number tradeoff occurs for the twigs of subtropical broad-leaved species. Fruit-bearing twigs were sampled for 55 woody species (including 33 evergreen and 22 deciduous dicot species) from a southwest Chinese forest. Twig size, fruit size and number, and seed size and number were measured for each species. The relationships among these functional traits were determined both across species and across correlated phyletic divergences. Total fruit mass and total seed mass were positively correlated with twig size. Seed size was positively associated with fruit size, which was, in turn, positively correlated with twig diameter, but negatively correlated with the ratio of twig length to diameter. The effect of twig size on seed size variation was not significant, possibly as a result of the large variation in seed number per fruit. Cross-species and across-phyletic divergence analyses revealed that seed size was negatively and isometrically correlated with seed number per twig mass in both the evergreen and deciduous species groupings, demonstrating the existence of tradeoff between seed size and number. A seed size vs number tradeoff is detectable in the twigs of woody species. Comparatively little of the variance in seed size was attributable to twig size variation.

Clipping Frequency Affects Canopy Volume and Biomass Production in Planeleaf Willow (Salix planifolia var. planifolia Prush)

Willows (Salix) are often a key component of riparian ecosystems and are often browsed by both wildlife and livestock. However, little is understood about how the frequency of browsing affects aboveground and belowground willow production. The objectives of this study were to determine how the frequency of simulated browsing events in a controlled environment affected 1) the aboveground, belowground, and total biomass production and 2) the canopy volume of planeleaf willow (Salix planifolia var. planifolia Prush) plants. The experiment was a completely randomized block design consisting of 2 groups of willow plants with different clipping histories. Within each group, plants were randomly assigned to 1 of 11 subgroups. Clipping events comprised of all combinations of early, middle, and late season periods were imposed on the treatment subgroups, while 1 subgroup served as the control. Canopy volume measurements were made before and after each clipping event. Canopy volume change was related to the harvested twig length (cm) and weight (g). Results suggested that frequency of clipping alone did not explain differences in aboveground and belowground willow production. Instead, willow production was influenced by an accumulation of specific combinations of seasonal clipping events and was dependent on the clipping history of the plants. Early season clipping, alone or in combination with other events, was more detrimental to willows with prior clipping histories than middle or late season clipping treatments. Willow with prior clipping histories treated in either the middle or late seasons, and the late/middle combination produced more than the controls. These results should be verified in willow communities subjected to natural environmental variations and browsing.

Clipping Frequency Affects Canopy Volume and Biomass Production in Planeleaf Willow (Salix planifolia var. planifolia Prush)

Willows (Salix) are often a key component of riparian ecosystems and are often browsed by both wildlife and livestock. However, little is understood about how the frequency of browsing affects aboveground and belowground willow production. The objectives of this study were to determine how the frequency of simulated browsing events in a controlled environment affected 1) the aboveground, belowground, and total biomass production and 2) the canopy volume of planeleaf willow (Salix planifolia var. planifolia Prush) plants. The experiment was a completely randomized block design consisting of 2 groups of willow plants with different clipping histories. Within each group, plants were randomly assigned to 1 of 11 subgroups. Clipping events comprised of all combinations of early, middle, and late season periods were imposed on the treatment subgroups, while 1 subgroup served as the control. Canopy volume measurements were made before and after each clipping event. Canopy volume change was related to the harvested twig length (cm) and weight (g). Results suggested that frequency of clipping alone did not explain differences in aboveground and belowground willow production. Instead, willow production was influenced by an accumulation of specific combinations of seasonal clipping events and was dependent on the clipping history of the plants. Early season clipping, alone or in combination with other events, was more detrimental to willows with prior clipping histories than middle or late season clipping treatments. Willow with prior clipping histories treated in either the middle or late seasons, and the late/middle combination produced more than the controls. These results should be verified in willow communities subjected to natural environmental variations and browsing.

Growth and Reproductive Responses of True Mountain Mahogany to Browsing

True mountain mahogany (Cercocarpus montanus Raf.) compensates for annual growth lost to browsing under conditions of high resource availability. To develop better guidelines for its management for big game winter forage, twig demography was studied under natural herbivory and resource availability inside and outside exclosures at 1 site in the Wasatch Mountains and on 4 sites on the North Slope of the Uinta Mountains, Utah. Annual and previous years' twig lengths, as well as location and numbers of flowers and seeds were diagrammed on branches of browsed and unbrowsed shrubs in the spring or summer and fall between 1996 and 1999. Annual twig growth and flower and seed numbers of both browsed and unbrowsed shrubs were greatest in 1997 or 1998 when precipitation was highest. Utilization of annual growth varied among sites within a year and among years within a site and ranged from 300 % when previous years' growth was browsed. Despite differences in utilization, browsed twigs compensated similarly for length lost to herbivory, so that total twig lengths remained the same over the course of the study. Although twigs on unbrowsed shrubs had less annual growth per unit branch length than those on browsed shrubs, lack of length lost to herbivory resulted in an increase in total twig length over time. Years of high resource availability are important in allowing grazing tolerant shrubs such as true mountain mahogany to compensate for years of heavy utilization. Flower and seed numbers were much higher (P 100 %) even on years of high resource availability. DOI:10.2458/azu_jrm_v56i6_turley

Initial Beaked Hazel Growth Responses following Protection from Ungulate Browsing

Beaked hazel (Corylus cornuta Marsh) dominates the understory of many Boreal Mixedwood forests in central Alberta including those in Elk Island National Park, where this species demonstrates a high tolerance to browsing. This research quantified changes in beaked hazel stem morphology (total twig length and number) and leaf and twig biomass of current annual growth, both inside and outside 4 newly established exclosures during the growing seasons of 1999 and 2000. At 2 sites, leaf and twig current annual growth of beaked hazel shrubs recently protected from herbivory increased significantly (P 0.05) changes in current annual growth at the individual shrub stem level, potentially due to intense intra-specific competition. These results indicate that at several locations in the Park, the recent history of intense browsing appears to be limiting the annual growth of beaked hazel, including browse production. Despite the general increase in growth of individual beaked hazel stems, however, no changes in production were evident at the community level (P > 0.05) with the removal of browsing after 2 years. Protection from browsing did increase average beaked hazel height by 40% over the same period. DOI:10.2458/azu_jrm_v56i5_best

Growth and reproductive responses of true mountain mahogany to browsing

True mountain mahogany (Cercocarpus montanus Raf.) compensates for annual growth lost to browsing under conditions of high resource availability. To develop better guidelines for its management for big game winter forage, twig demography was studied under natural herbivory and resource availability inside and outside exclosures at 1 site in the Wasatch Mountains and on 4 sites on the North Slope of the Uinta Mountains, Utah. Annual and previous years' twig lengths, as well as location and numbers of flowers and seeds were diagrammed on branches of browsed and unbrowsed shrubs in the spring or summer and fall between 1996 and 1999. Annual twig growth and flower and seed numbers of both browsed and unbrowsed shrubs were greatest in 1997 or 1998 when precipitation was highest. Utilization of annual growth varied among sites within a year and among years within a site and ranged from &lt; 21 % to &gt; 300 % when previous years' growth was browsed. Despite differences in utilization, browsed twigs compensated similarly for length lost to herbivory, so that total twig lengths remained the same over the course of the study. Although twigs on unbrowsed shrubs had less annual growth per unit branch length than those on browsed shrubs, lack of length lost to herbivory resulted in an increase in total twig length over time. Years of high resource availability are important in allowing grazing tolerant shrubs such as true mountain mahogany to compensate for years of heavy utilization. Flower and seed numbers were much higher (P &lt; 0.05) on unbrowsed than browsed shrubs. Compensatory growth was enough to maintain, but not increase total twig lengths after high utilization (&gt; 100 %) even on years of high resource availability.

Initial beaked hazel growth responses following protection from ungulate browsing

Beaked hazel (Corylus cornuta Marsh) dominates the understory of many Boreal Mixedwood forests in central Alberta including those in Elk Island National Park, where this species demonstrates a high tolerance to browsing. This research quantified changes in beaked hazel stem morphology (total twig length and number) and leaf and twig biomass of current annual growth, both inside and outside 4 newly established exclosures during the growing seasons of 1999 and 2000. At 2 sites, leaf and twig current annual growth of beaked hazel shrubs recently protected from herbivory increased significantly (P &lt; 0.05) by 85 to 114% relative to that of browsed shrubs. At another site, the removal of browsing changed the morphology of beaked hazel shrub growth (P &lt; 0.05), with protected shrubs producing 26% more twigs that were shorter in aggregate length by 27%. The final site exhibited no significant (P &gt; 0.05) changes in current annual growth at the individual shrub stem level, potentially due to intense intra-specific competition. These results indicate that at several locations in the Park, the recent history of intense browsing appears to be limiting the annual growth of beaked hazel, including browse production. Despite the general increase in growth of individual beaked hazel stems, however, no changes in production were evident at the community level (P &gt; 0.05) with the removal of browsing after 2 years. Protection from browsing did increase average beaked hazel height by 40% over the same period.

Pruning of host plant neighbours as defence against enemy ant invasions: Crematogaster ant partners of Macaranga protected by "wax barriers" prune less than their congeners.

Many ant partners of tropical ant-plants prune the leaves and shoot tips of other plants growing around their hosts. According to the hypothesis proposed by Davidson et al. (Ecology 69:801-808), this specialized behaviour not only protects the host plants against overgrowth, but it also conveys a direct benefit to the ant colony as it removes contact points to the neighbouring vegetation where invasions of enemy ants could occur. Here we test this hypothesis by comparing pruning intensity in five closely related Crematogaster (subgenus Decacrema) plant-ant species (and one species of Technomyrmex) that differ in their exposure to competition by other ants. Pruning intensity was quantified by measuring the area loss of paper tape pieces wrapped around the stems of Macaranga host plants. All Crematogaster (Decacrema) ants tested but not Technomyrmex sp. pruned, but the intensity of the behaviour varied strongly between and within species. Pruning was significantly weaker in the three tested Crematogaster species inhabiting Macaranga host plants with a slippery, waxy stem surface, which functions as a mechanical barrier protecting the specific ant partners against generalist competitors. Pruning was generally stronger on more densely ant-populated trees. Even though the number of ants per twig length was lower in associations of ants with glaucous Macaranga hosts, only part of the variation of pruning activity could be explained by "ant density". When corrected for ant density, "wax-running" Crematogaster (Decacrema) ants still pruned more weakly than their congeners inhabiting non-glaucous Macaranga hosts. Pruning is obviously most important when an ant-plant is potentially accessible to intruders, but less necessary when the ant colony is isolated by a protective wax barrier. Our results support the hypothesis that "selfish" defence against invasions is the major selective pressure that has led to the development and maintenance of pruning behaviour in weakly competitive plant-ants.

Browsing of Antelope Bitterbrush(Purshia tridentata: Rosaceae) in the South Okanagan Valley, British Columbia: Age Preferences and Seasonal Differences

We compared browsing on twigs of small and large antelope bitterbrush (Purshia tridentata) shrubs among ten sites in the south Okanagan valley, British Columbia. We tested whether there were any age preferences by browsers and determined whether these preferences changed between seasons and mode of browsing. Two different types of browsing were observed: leaf stripping which occurred in the summer and twig clipping which occurred predominantly in the winter. We calculated age and size relationships showing that shoot volume and especially stem diameter were good predictors of shrub age. Among the ten sites, clipping removed 0.02 to 15.7% of a shrub's total twig length and stripping removed leaves from 0 to 5.2% of total twig length. Observations suggested that California bighorn sheep (Ovis canadensis california) stripped antelope bitterbrush leaves in late summer, mule deer (Odocoileus hemionus hemionus) clipped twigs in the winter and cattle clipped twigs in the summer. Browsers preferred to clip twigs on smaller and hence younger antelope bitterbrush shrubs. In contrast, larger and older shrubs were preferred for leaf stripping. Since twig clipping was more prevalent than leaf stripping in antelope bitterbrush, overall preference for younger shrubs may lead to difficulties in seedling establishment in regions where it is heavily used as winter forage.

Wind-induced stresses in cherry trees: evidence against the hypothesis of constant stress levels

We calculated the wind-induced bending moments and stresses generated in the stems of five Prunus serotina conspecifics differing in height and canopy shape and size (based on detailed measurements of stem projected area and location with respect to ground level) to test the hypothesis that wind-loads generate uniform and constant stress levels along the lengths of tree twigs, branches, and trunks. These calculations were performed using five different wind speed profiles to evaluate the relative importance of the shape of wind speed profiles versus the ’geometry’ of tree shape on stem stress distributions and magnitudes. Additionally, we evaluated the effect of absolute tree size and stem taper on wind- induced stresses by scaling the size of smaller conspecifics to the absolute height of the largest of the five trees yet retaining the original stem proportions (i.e., diameter relative to stem length) for each plant. Finally, we also determined how the factor of safety for wind-loading (i.e., the quotient of stem yield stress and wind-load stress) changed as a function of tree size (and, presumably, age). Our results indicate that wind-load stress levels (1) vary along stem length even for the same wind speed profile and the same maximum wind speed; (2) would increase to dangerous levels with increasing tree height if it were not for ontogenetic changes in stem taper and canopy shape that reduce stress intensities to manageable levels; (3) tend to be more dependent on stem taper and canopy shape and size than on the shape of the wind speed profile; and (4) the factor of safety against wind-induced mechanical failure decreases as trees get larger, but varies along the length of large trees such that preferential stem failure is likely and functionally adaptive. We thus (1) reject the hypothesis of constant wind-induced stress levels; (2) support the view that size-dependent changes in stem taper are required to maintain wind-load mechanical reliability; and (3) suggest that certain portions of mature trees are ’designed’ to fail under high winds speeds, thereby reducing drag and the bending moments and stresses experienced by trunks.