摘要:

Field exclosure studies have shown that mammalian browsers such as white-tailed deer (Odocoileusvirginianus) can have pervasive effects on forest dynamics in eastern North America. Direct experimental tests of the effects of browsing on growth and survival of a wide range of tree species, however, have yielded conflicting results. This study was designed to assess the effects of variation in the frequency, seasonal timing, and intensity of browsing (simulated by mechanical clipping) on the growth and mortality of three of the major tree species of the Hudson Valley, New York. The clipping treatments were applied to seedlings grown under two different light regimes (full sun and 8% of full sun) to examine seedling responses under different levels of shade-induced carbon stress. Our results demonstrate that even 2 successive years of heavy winter clipping (75% of new shoot growth removed) has little immediate effect on growth or survival of any of the three species. It is possible that winter browsing only has significant negative effects when seedlings are browsed repeatedly over long periods of time. However, comparable levels of summer browsing for only 2 years significantly reduced both growth and survival of all three species. While most natural browsing occurs in the dormant season, our results suggest that it is the less frequent browsing during late spring and early summer that has the greatest immediate effect on tree seedlings. Shading reduced growth and increased mortality in all three species; however, there was only a limited interaction between light level and the simulated browsing treatments. The effects of browsing on survival were similar in all three species; however, the effects of browsing on cumulative height and annual growth varied enough among the species to suggest that browsing could cause significant variation among these species in their rate of invasion in old fields and rights of way, and their rate of regeneration following logging or disturbance of forests.

ISSN:0045-5067    

DOI:10.1139/x94-107

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The influence of air-fall tephra (ash) from the May 18 1980 eruption of Mount St. Helens on decomposition of Pacific silver fir (Abiesamabilis(Dougl.) Forbes) needles was studied at two sites along the ash-fall plume northeast of the mountain, Elk Pass and Chambers Lake (20 and 60 km from the crater, respectively). Ash depths beneath the canopy of the old-growth stands at Elk Pass and Chambers Lake were 18 and 5 cm, respectively. Objectives of the study were to determine: (i) the decomposition rates of needles on the ash surface, buried under ash, and in control plots with ash removed; (ii) the effect of site on decomposition rates; and (iii) the influence of ash on forest floor temperature, moisture, and oxygen levels. Ash had considerable influence on litter decomposition. After 3 years, needles buried under ash had faster decomposition rates at both sites (k = 0.34 and 0.29/year at Elk Pass and Chambers Lake, respectively) than needles on the ash surface or in cleared control plots. There was a trend for needles on the ash surface to have slower decomposition (k = 0.18–0.23/year) than needles on control plots (k = 0.22–0.28/year). Site had little influence on buried needle decomposition; rates at Elk Pass and Chambers Lake were not significantly different, despite differences in ash texture and depth. Ash apparently did not reduce oxygen levels enough to reduce decomposition, but instead increased decomposition by influencing substrate moisture and temperature. Fastest decomposition occurred under the ash where conditions were moist and cool; slowest decomposition occurred on the ash surface where conditions were drier and warmer.

ISSN:0045-5067    

DOI:10.1139/x94-108

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The influence of air-fall tephra (ash) from the 1980 Mount St. Helens eruptions on decomposition of Douglas-fir (Pseudotsugamenziesii(Mirb.) Franco) needles was studied in a field experiment at the Cedar River watershed near Seattle, Wash. Ash that fell on Yakima, Wash., was transported to the site and placed on 5 × 5 m plots in a 50-year-old Douglas-fir stand at depths of 5 and 20 cm. Objectives of the study were to determine: (i) decomposition rates of Douglas-fir needles on the ash surface, buried under ash, and in an untreated control; (ii) the effect of ash depth on decomposition rates; and (iii) the influence of ash on forest floor temperature and moisture. Yakima ash had considerable influence on forest floor temperature and moisture and Douglas-fir needle decomposition rates. Needles buried under 5 cm of ash had increased rates of decomposition after 2 years (k = 0.595/year) relative to control needles (k = 0.347/year), while those buried under 20 cm of ash had a similar rate of decomposition to control needles (k = 0.349/year), perhaps owing to ash compaction. Those on the ash surface had decreased rates of decomposition relative to buried needles. These results are similar to results found in the ash fall zone near Mount St. Helens, where needles located on top of ash decomposed slower than buried needles. Increased decomposition of tephra-affected forest floors, relative to unaffected areas, may have facilitated plant regrowth by increasing the availability of limiting nutrients. Ash created a more favorable temperature environment for decomposition beneath the ash with the forest floor under ash being cooler in summer and warmer in the cooler months. Forest floor moisture was reduced under the ash but did not appear to be limiting to decomposition.

ISSN:0045-5067    

DOI:10.1139/x94-109

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The biogechemical cycle in a highly productive and healthy Norway spruce (Piceaabies(L.) Karst.) stand was studied in the west part of the Vosges Mountains, France. The plot, an 85-year-old plantation established on an acidic brown soil, was considered the healthy control of a series of stands in varying stages of decline in this area. The nutrient reserves were evaluated in the soil and in the living biomass. A classical design was set up and monitored for 3 years to measure the total atmospheric input and the transfer of solution through the ecosystem using tensionless lysimeters. The current soil functioning was dominated by the nitrogen cycle (nitrification) and the occurrence of large amounts of Al in the solution. Nitrification, essentially endogenous, was the main acidifying factor. Nitrates contribute to the leaching of large amounts of Ca and Mg. The mean annual losses in the seepage water at a depth of 60 cm were 17.2 kg•ha−1of N, 10.2 kg•ha−1of Ca, and 6 kg•ha−1of Mg. The mean annual input-output budget of these elements was always negative (−6.3 kg•ha−1for N; −4.7 kg•ha−1for Ca; −2 kg•ha−1for Mg). The seasonal budgets show that the atmospheric input is only helpful to the stand if it occurs during the growing period. The question of the perenniality of the current ecosystem productivity is of paramount interest, even if at present the available soil reserves and the nutrient content of solutions (absolute values and ratios between elements like Ca/Al and Mg/Al) indicate that tree nutrition is not a limiting factor.

ISSN:0045-5067    

DOI:10.1139/x94-110

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Patterns of tree mortality and seedling responses to canopy disturbance were investigated in northern Maine, where an outbreak of the spruce budworm (Choristoneurafumiferana(Clem.)) affected the forests of balsam fir (Abiesbalsamea(L.) Mill.) and spruce (Piceaspp.) continuously between 1972 and 1984. The outbreak created a gradient of canopy tree mortality that ranged between 8.5 and 100% of the cumulative basal area in 1984. This was a result of the difference in vulnerability among the host species (balsam fir > spruce) and of their spatial distribution patterns along the site drainage gradient. Two groups of plant species responded differently to the gradient of canopy disturbance: balsam fir, spruce, and white birch (BetulapapyriferaMarsh.) regenerated mostly at the intermediate levels of mortality (≈20%by basal area) of the canopy balsam fir; raspberry (RubusidaeusL.) and pin cherry (PrunuspensylvanicaL.) were most abundant at ≈100% fir mortality. Overall, the observed responses in space and time of the seedlings to budworm-caused canopy disturbance could be mostly explained by the concept of patch dynamics. Long-term changes in species composition of the spruce–fir forests cannot be predicted with precision with the present knowledge. However, I hypothesize, based on the species-specific vulnerability to budworm damage and patterns of regeneration, that the proportion of spruce to fir trees would not differ very much in the long run regardless of extensive tree mortality by the spruce budw

ISSN:0045-5067    

DOI:10.1139/x94-111

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Whole-tree harvest of Pacific yew (TaxusbrevifoliaNutt.) to provide bark for production of the new anticancer drug taxol may adversely affect stand structure where yews provide thermal cover, browse, or riparian benefits. Harvesting only a portion of the bark on standing trees would maintain existing stand structure if the affected trees continued to grow, but partial bark removal seldom has been applied because its long-term effects are unknown. We measured 121 yews that had been scarred by windthrow or logging damage 3–92 years ago and found that their growth did not differ significantly from the growth of nearby unscarred yews. Radial growth of the scarred trees was strongly correlated with growth before scarring; it was slightly associated with overstory canopy density, elevation, and the amount of bark removed. Partial bark removal from one side of the tree probably will not seriously affect the growth of Pacific yew if less than 50% of the bark is removed.

ISSN:0045-5067    

DOI:10.1139/x94-112

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Fourteen Scots pines (PinussylvestrisL.) were felled in Thetford and Alice Holt forests, southern England, for needle retention studies. In the field, the main stem of each tree was divided into annual sections, with each section being reduced to 30 cm long bolts or less, omitting branch whorls and using the internodal part of each annual shoot. The bolts were then planed one annual ring at a time to reveal the location of needle traces. In the summers of 1950–1990, the average needle retention was 2.6. Average needle retention varied annually from 1.5 to 3.3. There was a slight reduction in needle retention following high values at the end of the 1960s, and this accelerated during the 1980s. Needle retention history was similar at both Thetford and Alice Holt. The amount of annual needle loss varied between 0.3 and 1.6 needle sets, and the 40-year average of 1.02 sets indicated a long-term reduction in the number of needle sets during the lifetime of the trees.

ISSN:0045-5067    

DOI:10.1139/x94-113

出版商:NRC Research Press    

年代:1994

数据来源: NRC