摘要:

Phenology was monitored in natural stands of subalpine larch (Larix lyallii Parl.) in parts of four decades. Bud-burst date varied by about six weeks from year to year, and leaf-fall date by a month, suggesting that the importance of photoperiod in these processes has been over emphasized, even in the case of leaf-fall. Most of the variation in bud-burst date is accounted for by spring temperature, while date of leaf-fall depends on both spring and summer temperatures. This rather curious observation might be explained in terms of heat-mediated leaf aging. Early bud-burst, due to warm temperatures in spring, results in early leaf-fall, and warm summers also tend to terminate the life of the leaves sooner, from which it might be inferred that longevity is governed by a heat-sum above a certain threshold temperature. In controlled environments it was shown that the threshold temperature for bud-burst is very low, less than 1.5OC, and the heat-sum requirement is less than 75 degree-days above this threshold. This is the cause of the extremely early flushing date of subalpine larch relative to that of it's common associates.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_01

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Four clones of Picea sitchensis, each growing on two sites with contrasting growth rates, were examined for the rate at which wood density decreased as growth rate increased during successive years of juvenile wood formation. The clones with the greatest and least rates of change were consistent between contrasting sites. It was determined that the most effective measure of change in wood density was the relative rates of increase in wood volume and wood weight increment in successive annual increments. Wood volume increased more rapidly than wood weight for all clones but there were differences between them. The rankings for these relative rates of change were not (he same as those for mean density nor density of the last annual wood increment. Within a single annually produced sheath of wood, wood density was found to decrease with increasing distance down the main stem. In the outermost sheath, volume increment was at a maximum at least two and more usually three interwhorl positions. Dierences between clones in the patterns of weight and volume increment along the main stem are related to differences in crown structure. It is suggested that small height increments along the main stem lead to inadequate surface area for cambial development relative to the available resources for growth.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_02

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Variation in xylem anatomy was found both between and within clones of Picea sifchensis over successive years, and between sites supporting fast and slow growth rates. Increases in tracheid transverse area contributed most to increases in annual wood cross-sectional area. However, changes in wood density, associated with the development of juvenile wood, were not simply related to measured changes in anatomy, including mean cell numbers, tracheid area, tracheid shape, cell wall projection area, estimates of cell wall density, and ray sizes and areas. Interactions between anatomical development processes in the histogenesis of xylem tissue are discussed, and it n suggested that different compensatory processes may occur in histogenesis as the tree grows. Anatomical measures are less likely to be effective in selecting for high juvenile wood density than are differences in the relative rates of wood volume and wood weight growth along the main steam.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_03

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

There are wide gaps in the demand and production of forest based requirements. Available estimates are variable and also suffer from serious methodological drawbacks. Elements of unsustainability are thus qualitatively well understood but not precisely defined in quantitative terms. There is lack of methodology which could project the multiple utility values of forests in a perceptible way. A competition amongst the alternative values of forests, particularly the subsistence needs and economic returns, exists. Shortsightedness in analyzing the possible impacts of recently introduced management systems is likely to accompany more problems than heir anticipated benefits. The need of undertaking long term analysis is logical to ensure sustainability. The data available suggest that productive superiority of exotics like Eucalyptus spp. over the indigenous ones like Shorea robusra and Tectona grandis is apparent only in a short term perspective. In case of fast growing species, the cost of production in terms of the fundamental resources like minerals, needs to be looked into. It is concluded that sustainable forestry though felt to be desirable, may not be achievable exclusively by ways of emphasizing on higher yields and current method of conservation. Reduction in requirements of forest based resources and direct participation of people in the management must be ensured to achieve sustainability.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_04

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

No abstract available for this article.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_05

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

It is hypothesized that root hairs serve as specialized respiratory cells concomitantly with their other functions. Occurrence of "pores" (unthickened cell walls), presence of little or no cuticle (offering minimum resistance for oxygen diffusion) and dense cytoplasm studded with mitochondria at Ule tip of the root hairs, are some of the special ultrastructural adaptations of root hairs which offer an indication that at least one of their main functions involves respiration. This hypothesis is based on ultrastructural evidence and comparative anatomy such as: (1) development of more dense root hairs in soil voids, (2) occurrence of dense root hairs on the juvenile primary root of viviparously germinating seeds inside the pod (e.g., Pisum sarivum and Phaseolus vulgaris seedlings grown in paper towel and glass culture); (3) occurrence of permanently open stomata in the root hair zone of some species which is believed to be a double advantage as it may enable additional diffusion of oxygen to meet greater energy requirement of growing seedling; (4) occurrence of well developed root hairs in most of the terrestrial plants and their absence or poorly developed nature in most hydrophytes; and (5) development of maximum number and length of root hairs in relatively dry and porous soils rather than wet soils. Further, the increase in surface area that root hairs provide serves as a sort of "plant gill" to maximize uptake of oxygen as well as water and nutrients.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_06

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Plants of Pinus caribaea var. hondurensis raised from seeds obtained from a general collection from one seed source (Peten, Guatemala) were categorized into fast and slow growing individuals to study the relationship of growth rate with needle soluble sugars, starch content and velocity of phloem transport. Soluble sugar levels in the slow growing plants were significantly higher than the levels seen in the fast growing individuals. There was no difference between the levels of starch in plants of the two growth types during the day, but starch concentration remained signiftcantly higher in needles of the slow growing plants during the dark period. Results of phloem transport studies, using a fluorescence tracing technique on forest trees, reveal a 3-fold faster velocity of phloem translocation in plants of the fast growth catego~y. Rapid removal of soluble carbohydrates and rapid conversion of starch to soluble carbohydrates in pine needles were highly correlated with rapid growth of the seedlings while starch accumulation and inefficient removal of photosynthate from the needles were characteristic of the slow growing plants. These results are useful in evaluating seedlings, trees and tissue culture plantlets in tropical forest reforestation and afforestation programs.

ISSN:1054-9811    

DOI:10.1300/J091v01n02_07

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor