Cm In Stem Diameter Research Articles

Evaluation of the yield of corn cultivation (Zea mays l.) Under plastic Mulch conditions, Ecuador

Grasses are important in the Ecuadorian agricultural sector, as a staple food for the population and an important source of income. This article explored profitable technologies to increase efficiency in agronomic management. Therefore, the objective of this research was to evaluate the yield of maize (Zea mays L.) under plastic mulch conditions, on morphological variables. A completely randomized block design (CRBD) was implemented, with four treatments: control (T0), black (T1), blue (T2) and green (T3) and three replications. The results revealed that the development variables showed significant differences: in plant height, T2 and T3 increased between 8.5 and 9.5 cm with respect to the control, while T2 increased 0.182 cm in stem diameter, which represents 4.53%, favoring the use of mulch. In addition, T3 was associated with a high index of chlorophyll color intensity and an increase in fresh and dry root biomass. On the other hand, when analyzing the production variables, it was found that T1 was associated with greater ear length and diameter, while T3 produced an increase in ear weight.

Comparison of the effects of two laser photobiomodulation techniques on bio-physical properties of Zea mays L. seeds.

BackgroundLaser applications in agriculture have recently gained much interest due to improved plant characteristics following laser treatment before the sowing of seeds. In this study, maize seeds were exposed to different levels of laser treatment prior to sowing to improve their field performance. The aim of this study is to evaluate the impact of pre-sowing laser photobiomodulation on the field emergence and growth of treated maize seeds.MethodsThe maize seeds were first photobiomodulated with two lasers: 1) a helium-neon (He–Ne) red laser (632.8 nm), and 2) a neodymium-doped yttrium aluminum garnet (Nd:YAG) green laser (532 nm). Following three replications of randomized complete block design (RCBD), four irradiation treatments were applied (45 s, 65 s, 85 s, and 105 s) at two power intensities (2 mW/cm2 and 4 mW/cm2).ResultsBased on the results, maize seeds pretreated with a green laser and 2 mW/cm2 power intensity for 105 s exhibited the highest rate of seed emergence (96%) compared to the untreated control seeds with a lower seed emergence rate (62.5%). Furthermore, maize seeds treated with a red laser for 45 s showed an increased vigor index compared to the other treatment options and the control (P < 0.01). The treatment groups also showed statistically significant differences in seedling growth characteristics compared to the control group p < 0.01. The green laser produced a significant enhancement of about 24.20 cm in seedling length, 8.2 leaves/plant, and 3.4 cm in stem diameter compared to the untreated seeds. Moreover, the green laser treatment showed 57.4 days to anthesis, which was earlier than the untreated seeds (61.4 days). The results showed that the protein, oil, and starch contents of the seeds irradiated with the green laser were 17.54%, 6.18%, and 73.32%, respectively, compared to the seeds irradiated by the red laser with 16.51%, 6.33%, and 71.05%, respectively.ConclusionsThe photo biomodulation of maize seeds using a green laser light can improve the field emergence, seedling growth, and seed quality of the treated seed compared to the red laser treatment.

Rapid and transient changes during 20 years of restoration management in savanna-woodland-prairie habitats threatened by woody plant encroachment

Open-structured habitats, such as heathlands, grasslands, and savannas, support unique plant diversity but are threatened by woody plant encroachment in diverse locations globally. During 20 years of vegetation management aimed at restoring and sustaining oak savannas, woodlands, and wet prairies in a Midwestern USA oak savanna region, we examined change in 17 plant community metrics ranging from cover of rare species to pollinator floral resources. Metrics indicative of open-habitat quality increased rapidly but also declined rapidly with time since disturbance spanning a first decade of intensive management (tree cutting and 4–5 fires/decade) and a second decade of less-intensive management (> 4 years between fires). After initial restoration treatments reduced overstory tree density, changes in cover of open-habitat specialist species correlated with fluctuations of a brushy layer of small trees 1–10 cm in stem diameter. Between 2002 and 2018, 92% of sites where small trees increased by over 100/ha had declines in open-habitat species, whereas 72% of sites not experiencing that level of woody encroachment had sustained or increasing open-habitat species. Conserving open habitats in contemporary environments likely requires perpetually frequent low-severity disturbance (at least every 3–4 years), periodic severe disturbance (e.g., growing-season fires), or multiple treatment types (e.g., managed herbivory and fire) to synergistically limit woody encroachment. Fluctuations between positive and negative trends in the 20-year dataset also highlight that perhaps restoration success should not be evaluated on the basis of a net change from beginning to end, but rather on a time-weighted accrual of restoration benefits.

Frost record in tree rings linked to atmospheric circulation in northern Patagonia

Cold air incursions over subtropical South America are the precursor conditions for frost, a recognized extreme thermal weather event affecting plant growth damaging agricultural production over the whole Argentinian territory. Given that sub-freezing temperatures occurring during the active growing season may harm the cambium tissues and their daughter cells, frost injury can be recorded in annual tree rings in the form of anatomical anomalies. For this study, six forest sites of Araucaria araucana, a multi-centennial tree species, from NW Patagonia were considered. From 321 trees, 1374 frost injuries in the growth rings, named frost rings, were identified and their calendar dating allowed the development of a regional chronology of frost rings covering the period AD 1256 to AD 1993 (738 yrs.). This represents the longest record at present of extreme low thermal events for Patagonia. Frost injuries were mostly restricted to the middle section of the growth rings, suggesting the incidence of late spring frosts. Moreover, frost rings were observed mainly in the juvenile portions (<50 yrs. of age and <6 cm in stem diameter) of the tree stem, indicating that young trees display a greater sensitivity to frost events. Large-scale geographical freezing events were recorded in the years AD 1889, 1916, 1941 and 1948. The atmospheric genesis of these events was sustained by a trough at 500 hPa approaching from the west toward the continent, along with an approach of the Pacific anticyclone at 1000 hPa which invaded the southern part of South America on the day of the frost event. At the hemispheric scale, it was found that atmospheric circulation patterns related to La Niña were associated with the regional frost ring record in A. araucana. The regional frost ring record from A. araucana is proposed as a regional proxy of sub-freezing temperatures in paleoclimate reconstruction and as a background for models about the future behavior of the climate in scenarios of change.

Geographic variations of wood properties of &lt;i&gt;Larix sibirica&lt;/i&gt; naturally grown in Mongolia

Geographic variations in growth, stress-wave velocity of stem, dynamic Young’s modulus of stems and logs, annual ring width, latewood percentage and basic density were investigated for (Münchh.) Ledeb. naturally grown in Mongolia. A total of 250 trees with 20 to 30 cm in stem diameter at a height of 1.3 m above ground level were selected from each natural stand in five different provenances in Mongolia. In addition, five trees in each stand were cut for measuring dynamic Young’s modulus of stems and logs, annual ring width, latewood percentage and basic density. Mean values of stress-wave velocity of stems in each stand ranged from 2.92 to 3.41 km s, and the mean value of five stands was 3.23 km s. Mean values of dynamic Young’s modulus of logs in each stand ranged from 5.17 to 9.72 GPa. A significant correlation ( = 0.798,  &lt; 0.01) was found between stress-wave velocity of stems and dynamic Young’s modulus of logs. Among the five stands, the highest and the lowest values of average annual ring number were 193 and 44, respectively. Mean values of basic density in five trees within each stand were examined and ranged from 0.52 to 0.56 g cm. Significant differences among five stands were found in tree height, stress-wave velocity of stem, dynamic Young’s modulus of stems and logs, annual ring width and latewood percentage, suggesting that trees naturally grown in Mongolia have geographic variations in mechanical properties of wood.Larix sibirica–1–1rp–3L. sibirica

Recruitment Success for Mast Year Cohorts of Sugar Maple (Acer saccharum) Over Three Decades of Heavy Deer Browsing

We examined whether particular years of mast seed production in sugar maple (Acer saccharum) are associated with increased likelihood of cohort recruitment success into the sapling stage over three decades of heavy browsing pressure from white-tailed deer in a mature woodland population in southeastern Ontario, Canada. The population was sampled in 2014 for seedling and sapling stages (≤6 cm in stem diameter) to obtain an age frequency distribution spanning about 80 y and including survivors of seed cohorts produced in two known mast years at the study site (2013 and 1984) and in other mast years known to have occurred within the broader region (but not confirmed for the study site). The age frequency distribution is roughly bimodal with zero to very few individuals recorded for ages 9 through 29 y, corresponding with the known time period (early 1980's to late 2000's) of regional overabundance for white-tailed deer in eastern Ontario and at the study site in particular. The 1 y old seedlings (from the 2013 mast year) and the survivors of putative mast year cohorts from 2006 and 2000, however, are especially conspicuous, with less striking recruitment success indicated for the older confirmed mast year cohort from 1984 (which had more years of accumulated impact from mortality risks). Our results suggest seed masting in sugar maple can bolster cohort recruitment success that otherwise would virtually (or completely) fail when severe impact from deer browsing is combined with other typical early life-stage mortality risks, e.g., from drought, neighborhood competition, and persistent overhead canopy shade.

Conserving Large Oaks and Recruitment Potential while Restoring Midwestern Savanna and Woodland

Restoring Midwestern oak savannas and woodlands over the long term requires balancing mortality of large oaks with recruitment of oaks into large size classes. At restoration sites in northwestern Ohio, we tracked survival of large oak trees (≥20 cm in stem diameter) and recruitment between 2002 and 2015 after initial tree thinning and prescribed burning treatments. The 24 study sites spanned a gradient of canopy cover from unrestored forests to restored woodlands and savannas. Of 141 large black oak (Quercus velutina) and white oak (Quercus alba) trees alive in 2002, 79% were alive 14 y later. Large oak survival varied among vegetation types, with restored savannas not losing a single tree, compared with 18% mortality in unrestored forests and 28% mortality in restored woodlands. At least some mortality in the woodlands was associated with a tornado that damaged two sites. Diameter distributions changed over the 14 y in all three vegetation types. Unrestored forests shifted toward proportionally greater large and fewer small diameter oaks. Meanwhile, restored woodlands, despite having the highest mortality of large oaks, still exceeded, via recruitment of new large oaks, the reconstructed pre-Euro-American-settlement tree density. Restored woodlands and savannas exhibited four-fold increases in oak recruitment potential (saplings 1 to 10 cm in diameter) between 2002 and 2015. Ecological restoration processes have been compatible with conserving large oaks and sustaining oak recruitment potential.

Allometric relationships for the estimation of dry mass of aboveground organs in young highland Norway spruce stand

This study falls into site-specific studies (here in the Drahanská vrchovina Highland) focusing on the determination of allometric relationships between the stem dendrometric and biomass parameters in young Norway spruce stands. The total aboveground biomass (TB) of a sampled tree with 14.9 m in height and 15.5 cm in stem diameter at 1.3 m (DBH) averaged to 110.3 kg. The stem biomass participated on average by 54 %, branch biomass by 24 % and needle biomass by 22 % on the TB of the sampled spruce tree. TB of the tree and the biomass of individual aboveground tree organs were predicted with the highest accuracy (over 91 %) from DBH or a stem diameter at the one tenth of the tree height using allometric-power functions. The stem diameters up to 70 % of the relative tree height predicted TB accurately (over 95 %) as well. The biomass expansion factors based on the stem volume expansion to TB of the tree, as well as the biomass of each of the aboveground tree organs did not show functional dependency on DBH.

Geographical Range and Local Abundance of Tree Species in China

Most studies on the geographical distribution of species have utilized a few well-known taxa in Europe and North America, with little research in China and its wide range of climate and forest types. We assembled large datasets to quantify the geographic ranges of tree species in China and to test several biogeographic hypotheses: 1) whether locally abundant species tend to be geographically widespread; 2) whether species are more abundant towards their range-centers; and 3) how abundances are correlated between sites. Local abundances of 651 species were derived from four tree plots of 20–25 ha where all individuals ≥1 cm in stem diameter were mapped and identified taxonomically. Range sizes of these species across China were then estimated from over 460,000 geo-referenced records; a Bayesian approach was used, allowing careful measures of error of each range estimate. The log-transformed range sizes had a bell-shaped distribution with a median of 703,000 km2, and >90% of 651 species had ranges >105 km2. There was no relationship between local abundance and range size, and no evidence for species being more abundant towards their range-centers. Finally, species’ abundances were positively correlated between sites. The widespread nature of most tree species in China suggests few are vulnerable to global extinction, and there is no indication of the double-peril that would result if rare species also had narrow ranges.

Size-dependent growth responses to competition and environment in Nothofagus menziesii

The increased availability of permanent-plot and environmental data allows examination of the relative influence of environment and competition as controls of individual tree growth at large spatial scales. To understand and predict stem diameter growth of a dominant tree, Nothofagus menziesii (Hook. F.) Oerst., within natural forest in the South Island of New Zealand we tested the relative importance of stem diameter, competition by neighbors, soil water balance and elevation as explanatory variables. The analysis was based on 482 permanent sample plots and GIS data. Because only relatively short trees are susceptible to shading, models were fitted independently for trees smaller and larger than 25cm in stem diameter at breast height, using multilevel log-linear models. N. menziesii exhibited slow diameter growth rates in natural forest, with a median 0.39mmyear−1 and 1.39mmyear−1 for small and large trees, respectively. Growth rates were broadly uniform across 11 sampled regions, but an important fraction of the explained variation was attributed to plot-level growth estimates. Stem diameter, elevation and cross-sectional area of neighbors at the tree top (a refinement of basal area of larger neighbors, a commonly used variable) were important explanatory variables for growth of small stems. The effect of elevation was important for large stems, but size and competition had much weaker effects; there was a clear negative effect of elevation on growth but only moderate statistical support for a humped effect of tree size and for a negative effect of stand density. Thus, our results indicate that whilst asymmetric competition for light controls diameter growth in small trees, the physical environment is relatively more important in large canopy trees.

Characterising wide spatial variation in population size structure of a keystone African savanna tree

Elephant impact on trees is of conservation concern in African savannas, but it’s effect on population size structure of heavily utilised species has received little attention. Although studies have shown that the population structure and density of Sclerocarya birrea subsp. caffra (marula), a heavily utilised savanna tree species, varies widely across its range, the causes of this variation are poorly understood. This study aimed to describe and compare these divergent structures and develop an understanding of the relationship between population structure and site characteristics, specifically disturbance. Four distinct groups of population structure were identified: (1) adult dominated populations consisting of only adults and no juveniles or seedlings. The size class distributions (SCDs) of this group were bell-shaped, indicating an unstable population which may have come about through circumstances that no longer occur in these areas, for example human settlement or a massive episodic recruitment event. High impala utilisation of seedlings or rodent seed predation are proposed to be the main cause of the lack of recruitment; (2) rotated sigmoid (or negative J shaped) curves, representing stable population structures, when protected from elephants, as well as at high rainfall sites with elephants, indicating that without elephants and in suitable habitats with elephants, marulas are able to attain a relatively stable size structure and instability is not inherent; (3) juvenile dominated populations with almost no adults. Adults are relegated to a few refugia, where elephants are unable to reach them; (4) juvenile dominated populations consisting of mostly juveniles and few adults. These populations, all exposed to fire and elephants, have “missing size classes” varying between 5 and 40 cm in stem diameter and 2–8 m in height. This unstable structure was most likely caused by fire suppression of juveniles combined with elephant utilisation of individuals in the “missing size classes”. Consistent application of this pressure is postulated to result in the progression from stable to “missing size class” structure, to juvenile or adult dominated populations, and finally to local extirpation, with increasing elephant densities and concomitant utilisation, interacting with fire, rainfall and soil type.

What Makes a Leaf Tough? Patterns of Correlated Evolution between Leaf Toughness Traits and Demographic Rates among 197 Shade-Tolerant Woody Species in a Neotropical Forest

Slow-growing juveniles of shade-tolerant plant species are predicted to have tough leaves because of the high cost of leaf replacement in shade relative to potential carbon gain. We assessed the degree of correlated evolution among eight traits associated with leaf toughness and the relationships of those traits with the growth and mortality rates of 197 tree and shrub species from the understory of the 50-ha forest dynamics plot on Barro Colorado Island, Panama. Path analysis with phylogenetically independent contrasts revealed that leaves attained material toughness (resistance to fracture per unit fracture area) through increases in tissue density, percent cellulose per unit dry mass, and vein fracture toughness. Lamina density and cellulose content evolved independently and thus represent different paths to material toughness. Structural toughness (resistance to fracture per unit fracture length) depended on material toughness and lamina thickness. Mortality rates of individuals 1-10 cm in stem diameter were negatively correlated with material toughness and lamina density but were independent of structural toughness and cell wall fiber contents. Leaf toughness traits were uncorrelated with relative growth rates. Results imply that material toughness enhances resistance to natural enemies, which increases survival and offsets the biomass allocation cost of producing tough leaves in the shaded understory.

Photosynthetic capacity peaks at intermediate size in temperate deciduous trees

Studies of age-related changes in leaf functional biology have generally been based on dichotomous comparisons of young and mature individuals (e.g., saplings and mature canopy trees), with little data available to describe changes through the entire ontogeny of trees, particularly of broadleaf angiosperms. Leaf-level gas-exchange and morphological parameters were quantified in situ in the upper canopy of trees acclimated to high light conditions, spanning a wide range of ontogenetic stages from saplings (approximately 1 cm in stem diameter) to trees >60 cm d.b.h. and nearing their maximum lifespan, in three temperate deciduous tree species in central Ontario, Canada. Traits associated with growth performance, including leaf photosynthetic capacity (expressed on either an area, mass or leaf N basis), stomatal conductance, leaf size and leaf N content, generally showed a unimodal ('hump-shaped') pattern, with peak values at an intermediate ontogenetic stage. In contrast, leaf mass per area (LMA) and related morphological parameters (leaf thickness, leaf tissue density, leaf C content) increased monotonically with tree size, as did water-use efficiency; these monotonic relationships were well described by simple allometric functions of the form Y = aX(b). For traits showing unimodal patterns, tree size corresponding to the trait maximum differed markedly among traits: all three species showed a similar pattern in which the peak for leaf size occurred in trees approximately 2-6 cm d.b.h., followed by leaf chemical traits and photosynthetic capacity on a mass or leaf N basis and finally by photosynthetic capacity on a leaf area basis, which peaked approximately at the size of reproductive onset. It is argued that ontogenetic increases in photosynthetic capacity and related traits early in tree ontogeny are general among relatively shade-tolerant tree species that have a low capacity for leaf-level acclimation, as are declines in this set of traits late in tree ontogeny.

Comparative resprouting response of Savanna woody plant species following harvesting: the value of persistence

Understanding the relative ability of favoured fuelwood species to resprout following harvesting is integral to designing strategies for sustainable woodland resource utilisation. Harvester preference for particular woody plant species and plant size was examined in a formerly protected area, severely harvested over a short period of time. The resprouting response of harvested plants was measured five months after harvesting, towards the end of the wet season. Although a large proportion of stems of the dominant species ( Terminalia sericea and Dichrostachys cinerea) had been harvested, certain less common species were particularly targeted (e.g., Pterocarpus rotundifolius and Peltophorum africanum). Harvested plants were mostly larger than 4 cm in stem diameter. Almost all harvested plants of all species resprouted. Species differed in whether resprout growth was channelled into many small resprout shoots or fewer large shoots. The species most highly favoured by harvesters produced relatively large resprout shoots. Plant stem diameter was the most important determinant of resprout output, with both shoot number and their composite diameter increasing with increasing plant size. Cut height, although having no significant effect on resprout output, influenced the location of resprouting, with taller stumps having a higher proportion of resprout shoots originating from the stem than from the root crown. For all species, approximately a third of the resprout shoots of harvested plants died during the five months following harvesting. Shoots originating from the stem had a higher dieback than those growing from the root crown. Resprout biomass production over the five month period was considerable, amounting to 989 kg ha −1 (wet mass). An abundance of seedlings of the most common species may be a consequence of woodland thinning through harvesting.

Sex ratio and gender-dependent neighboring effects in Podocarpus nagi, a dioecious tree

We analyzed sex ratio, growth rates, and spacing among individuals of Podocarpus nagi, a dioecious tree, on Mt. Mikasa, Nara City, Japan. The sex ratio of reproductive trees ≥ 5 cm in stem diameter at breast height (dbh, 130 cm above ground level) was significantly male-biased. The sex ratio was male-biased in the < 20 cm and ≥ 50 cm size classes, while it did not depart from 1:1 in the 20 ≤ dbh < 50 cm class. Growth rate varied with tree size in males but not in females. The precocity and vigor of males suggests that differences in reproductive costs between sexes induce the biased sex ratio. Random labeling tests on the positions of reproductive trees showed that in the < 30 cm class, males and females were distributed randomly and independently from each other. In the ≥ 30 cm class, males were significantly clumped, whereas females were randomly distributed. Males and females showed significant repulsion, i.e., a spatial segregation of sexes. Both intra- and intersexual effects on the growth rate of crowding by neighbors were significant for females, but not for males. Maximum competitive interference was observed at a distance of 5 m, which corresponded approximately to the radius of clumps of large males and to the significant repulsive distance between large males and females. These results suggest that sexual differences in sensitivity to local crowding are related to the formation of gender-dependent spatial patterns. Formation of female-repulsive male clumps and a male-biased sex ratio may intensify the decreased probability of regeneration near males, as suggested by the limited seed-dispersal range of this species, thereby promoting coexistence with other species.

Three buried forests of the Last Glacial Stage and middle Holocene at Ooyazawa on northern Honshu Island of Japan

Three extensive buried forests of the Late Quaternary were unearthed at Ooyazawa, Aomori, in the northern part of Honshu Island of Japan during construction of a flood water reservoir. The lower and middle buried forests were formed by activities of the Towada Volcano and dated from the early period of the Last Glacial Maximum and the Late Glacial Period, respectively. The upper buried forest was formed as a result of sea level changes and dated from the Early Jomon Period of middle Holocene. Picea and Larix dominated in the lower and middle buried forests of the Last Glacial Maximum and the Late Glacial Period, accompanied by occasional Abies. In the middle buried forest Picea and Larix grew evenly intermixed, and Larix trees were larger (up to 84 cm in stem diameter) than Picea trees (up to 33 cm in stem diameter). The taxonomic composition of the lower and middle forests was similar to that of the buried forests on the eastern slope of the Towada Volcano of the Late Glacial Period. The buried forests of northern Honshu Island, as well as those of south to central Honshu Island, indicate that Picea–Larix forests were the dominant vegetation through the Last Glacial Maximum to the Late Glacial Period on central to northern Honshu Island. In the upper buried forest of the middle Holocene, Fraxinus and Alnus dominated, accompanied by Juglans, Castanea, Quercus, Fagus, Magnolia, Hydrangea, and Acer. This upper buried forest contemporaneously formed woody peat below the forest floor. According to a pollen analytical study at Ooyazawa, similar buried forests continually established themselves after frequent destructions by flooding for at least 1000 years of middle Holocene. Lowland forests of Fraxinus and Alnus that simultaneously formed woody peat grew extensively in the Kanto Plain of central Honshu Island during the middle Holocene. The upper buried forest at Ooyazawa was the first record of this type of lowland forest outside the Kanto Plain and showed that the establishment of marsh forests with contemporaneous woody peat was a general phenomenon in a large area of Honshu Island during the middle Holocene.

Incidence of browsing by guanaco (Lama guanicoe Müller) on early regeneration of lenga [Nothofagus pumilio (Poepp et Endl) Krasser] forests in Tierra de Fuego, Argentina

In this work we analyze the early regeneration of Nothofagus pumilio forests, as well as the impact of browsing by guanacos (Lama guanicoe), in Tierra del Fuego, Argentina. First, we describe the structure of three N. pumilio stands with differing management schedules: a native virgin forest stand (BV), a stand that was logged in 1991-93 leaving a certain homogeneous density of parent trees («shelterwood cut», CP), and a third stand that was selectively and intensively logged (FL) in 1995. In these three stands we quantify the use by guanacos (estimated from the density of fecal remains) and the frequency of browsed seedlings (&lt; 0.5 cm) and saplings (&gt; 0.5 cm in stem diameter). Results show a high incidence of browsing, which was higher for the native forests stand (100 % and 95.7 %), than for CP (69.9 % and 72.1 %) and FL (67.3 % and 61.5 %, for basal and apical damage of browsed plants, respectively). These differences are mainly due to the fact that patches where big trunks accumulate in managed stands, provide shelter from damage by guanacos. Comparison of observed and expected (according to calculated growth models) sapling heigth show that recurrent damage by guanacos prevent successful growth, the corresponding height loss being 83.1 % for BV, 45.7 % for CP, and 50.3 % for FL. We conclude that guanacos effectively limits early regeneration in both native and managed stands, though a wider time scale is needed for a conclusive assessment of the incidence of browsing in forest dynamics and production.

Impacts of dry and hazy weather in 1997 on a tropical rainforest ecosystem in West Sumatra, Indonesia

An equatorial rainforest in the coastal region of West Sumatra suffered a long period of dry and hazy weather in 1997. Canopy trees with higher growth rates before the event had reduced growth rates during the 8 months from June 1997 to an average of 23% of their 1996 growth rate. The size‐dependency of growth depression in forest trees greater than 5 cm in stem diameter could be observed during this period. The weather induced defoliation, and high litterfall rates of leaves continued throughout the period. The impacts of the weather on growth rates were analyzed by using Saeki’s mathematical model for community production. This model suggested that the great reduction in growth‐rates of active canopy trees would be caused mainly by the interacting effects of potential photosynthetic rate and leaf biomass because of their sensitive responses to drought. In the severe weather conditions, these variables were estimated to have decreased to more than 50% of their 1996 values.

CRECIMIENTO Y PRODUCCIÓN DE CUATRO CULTIVARES DE AGUACATE (Persea Americana MilI.), EN EL SUR DE SONORA, MÉXICO

The southern Sonora valleys in Mexico are grown to 377,125 ha under irrigated conditions, high percent of the surface is devoted to small grain crops. The NAFTA (North American Free Trade Agreement) signing offers a series of advantages and challenges to Mexican agriculture in order to share vegetable and fruit intemational markets. In this scenery Mexican avocado has played an important roll during the past couple of years because of the United States commercial opening. This research was conducted in Southem Sonora, Mexico (26. 41' and 290 NL, and 1090 12' and 109041: WL), from 1989 to 1997. Four cultivars ('Fuerte', 'Hass', 'San Miguel' and 'Ensenada') were planted on a sandy loam soil during march, 1989 in a array 10 x 10 m with five trees each cultivar. A completely random design was used with five replications a DMS P$0.05 test to separate treatrnents under study was used. Measured parameters were round-year stem diameter growth and height, number of fruits per tree, average weight of fruits and yield (t. ha"'). Analysis showed that stem diameter growth and height was !¡Iow during the first year in the field, but after that time, values increase gradually, with a maximum of 15 cm, and 150 cm in stem diameter and height respectively. The before-mentioned was as the result of an absence of strong sink and source relationship. A strong phenomenon of altemate bearing was observed in better performers such as 'Fuerte' and 'San Miguel', even when in a accumulative yield analysis, were high yielders. 'Hass' always yielded in an uprising manner throw the period of study average weight of fruits ranked between 107 to 242 9 being distributed from bigger to smaller size as follows: 'Hass', 'San Miguel', 'Fuerte' and 'Ensenada'.

Pinyon-Juniper Chaining and Seeding for Big Game in Central Utah

Vegetation and soils were evaluated on 5 different-aged, mechanically treated and seeded pinyon-juniper sites and compared to adjacent untreated areas. Plant cover was significantly changed after treatment: trees were reduced from 26 to 6% total ground cover; shrubs were increased from 2 to 8% ground cover; and herbaceous plants increased from 2 to 13% ground cover. Annuals and perennial forbs were 75% of the total plant cover on the 2-year-old site, perennial grasses and shrubs dominated the plant cover (52 to 83%) on three, 14- to 20-year-old sites, while shrubs and trees combined for 94% of the plant cover on the 24-year-old site. Two Agropyron grass species showed good establishment and persistence after seeding. Seeded forbs contributed about 5% of the total plant cover on the 2-year-old treated site and they declined on older treated sites. Seeding of shrubs was only successful on sites where the shrub species was already present in the understory naturally. Seeding of nonnative shrub seed did not produce stands. Even though tree cover was reduced after treatment, total tree density was not. Shrub density increased from an average of 800 plants/ha on untreated areas to 2,750 plants/ha on treated areas. Juniper mortality during mechanical treatment varied from 60 to 91% and was related to the percentage of trees estimated to be 60+ years old (r = 0.97) and with the number of trees greater than 5 cm in stem diameter (r = 0.71) on the adjacent untreated sites. Big game pellet group counts were not different between untreated and treated sites, suggesting that big game make use of these treated areas because of increased forage and browse and in spite of reduced security cover.