ISSN:0012-9658    

DOI:10.2307/1941140

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941141

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941142

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941143

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941144

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1941145

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

We monitored the dynamics of natural gaps in vegetation in permanent plots in three old—field communities for 3 yr. The vegetation in all three fields was dominated by long—lived clonal perennials. Total area in gaps with exposed soil was very low, with<0.2% of the area of each field in gaps ≥10 cm in diameter (our minimum size). Most gaps were small; only 5 to 21% of all gaps found were °20 cm in diameter. Most (95%) of the new gaps were created by animal activity, with moles being the most common disturbance agent. There was considerable variation in disturbance and closure rates over space and time that resulted in differences in the availability of gaps within fields, among fields, among seasons, and among years. Within each field, disturbance rates were higher but closure rates tended to be lower in more xeric areas of the field, resulting in higher availability of gaps in the more xeric areas at each census. Within each year, both disturbance and closure rates were higher during the growing season than during the winter. These two trends seemed to balance each other, resulting in little change in the availability of gaps over the year. Disturbance rates did not differ among years, but closure rates were higher in a year with higher precipitation, resulting in a lower availability of gaps in the wetter year. Overall these gaps appear to be too rare and too small to play an important role in the persistence of the dominant perennials in the old—field communities we studied, many of which require large gaps to establish from seed. However, they may be important for the persistence of annuals with seed banks in these communities. Such gaps may also influence the rate of succession in these fields by influencing the pattern of colonization by woody plants.

ISSN:0012-9658    

DOI:10.2307/1941146

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Amounts and structural characteristics of coarse woody debris (CWD) were examined in relation to stand age and site moisture condition in 196 Pseudotsuga menziesii stands in western Oregon and Washington. Stands ranged from 40 to 900 yr old, and most if not all, originated after fire. In a chronosequence from the Cascade range, the amount of CWD followed a U—shaped pattern for stands<500 yr old, with moderate levels (92 Mg/ha) in stands<80 yr old, lowest levels (1000 yr, and that the nature and timing of disturbance play a key role in the dynamics of CWD in the dynamics of CWD in the region.

ISSN:0012-9658    

DOI:10.2307/1941147

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Vegetation changes were studied for 21 yr in two clearcut logged and slash—burned Pseudotsuga forests in the western Cascade Range of Oregon. Detrended correspondence analysis (DCA) was used to examine the successional relationships among six understory communities exposed to a gradient of disturbance intensity. Euclidean disturbances between pre— and postdisturbance samples in ordination space were used to compare community resistance to disturbance and long—term recovery, or resilience. Ordination through time for plant communities revealed a common pattern of rapid floristic change away from predisturbance composition, followed by gradual, unidirectional return. Early, but transient, convergence of successional pathways was common among mesic— and dry—site communities, reflecting the broad distribution of colonizers and the floristic similarity of predisturbance understories. Distinct sequences were observed on moist sites, reflecting more unique residual and colonizing floras. Ordinations also revealed increasing compositional change with disturbance intensity. Successional sequences were dominated by residual species on relatively undisturbed sites and by alternate suites of invading species on moderately disturbed and burned sites. Variation in the response gradient between watersheds reflected the modifying influence of local environment, stand history, and change in succession. Resistance and resilience varied little among plant communities but were generally lowest for the depauperate Coptis community and greatest for the compositionally and structurally diverse Polystichum and Rhododendrom—Gaultheria types. Both measures were strongly influenced by disturbance intensity. The stability of Pseudotsuga understories derives from the moderate tolerance of initial understory dominants to burning and in their ability to subsequently perennate from subterranean structures. Variation in the long—term response of communities reflects complex interactions between species life history, disturbance intensity, and chance, suggesting that both deterministic and stochastic factors must be considered in evaluating community stability and response to disturbance.

ISSN:0012-9658    

DOI:10.2307/1941148

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

To determine the effect of environmentally induced stress on reproductive characters of perfect—flowered hermaphrodites, I grew 14 clonal pairs of Lobelia cardinalis plants under common—garden conditions of 27% available light, sufficient water, and equal amounts of soil from initiation of growth until flower bud formation. At that time, one member of each clonal pair was stressed by decreasing its available light to 10% of ambient. I then assessed nectar, pollen grain, and seed production from flowers on stressed and nonstressed ramet. Stress significantly decreased seed and nectar production but did not affect pollen grain number and mean seed mass. In addition, ganets produced significantly different amounts of nectar, pollen grains, and seed per flower. These data on the effects of stress complement the generalization that stressful environmental conditions induce greater male emphasis in monoecious plants and plants that change sex.

ISSN:0012-9658    

DOI:10.2307/1941149

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY