Statistics and Data Analysis in Geology

The forests of eastern North America have been subjected to repeated introductions of exotic insect pests and pathogens over the last century, and several new pests are currently invading, or threatening to invade, the region. These pests and pathogens can have major short- and long-term impacts on forest ecosystem processes such as productivity, nutrient cycling, and support of consumer food webs. We identify six key features of the biology of exotic animal pests and the ecology of their hosts that are critical to predicting the general nature and severity of those impacts. Using three examples of introduced pests and pathogens in eastern forest ecosystems, we provide a conceptual framework for assessing potential ecosystem-scale effects.

https://www.jstor.org/stable/10.1641/0006-3568(2006)056%5B0395:FERTEP%5D2.0.CO;2

Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America

Hemlock woolly adelgid (HWA; Adelges tsugae Annand) infestations have resulted in the continuing decline of eastern hemlock (Tsuga canadensis (L.) Carriere) throughout much of the eastern United States. In 1994 and 2003, we quantified the vegetation composition and structure of two hemlock ravines in the Delaware Water Gap National Recreation Area. This is the first study to use pre-adelgid disturbance data, annual monitoring of infestation severity, and annual records of hemlock health to assess forest response to HWA infestation. In 2003, 25% of monitored hem- lock trees were either dead or in severe decline. Measures of hemlock decline (crown vigor, transparency, density, and dieback) were correlated with HWA infestation severity and changes in light availability over the study period. Average percent total transmitted radiation more than doubled at these sites from 5.0% in 1994 to 11.7% in 2003. The total per- cent cover of vascular plants increased from 3.1% in 1994 to 11.3% in 2003. Species richness increased significantly, and more species were gained (53) than lost (19) from both ravine floras over the 9-year study period. Though exotic invasive plants were absent from these ravines in 1994, our 2003 resurvey found invasive plants in 35% of the perma- nent vegetation plots.

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Vegetation dynamics in declining eastern hemlock stands: 9 years of forest response to hemlock woolly adelgid infestation

Summary 1 Although it is well established that nitrogen and light play major roles in structuring plant communities across the landscape, it is not as clear how they structure communities within forest stands. Virtually nothing is known about within-stand structure of understorey communities of herbs and small shrubs in near-boreal forests. 2 We tested the hypothesis that fine-scale (5‐20 m) variability in N and light structure forest-floor plant communities in two old-growth mixed Pinus resinosa and Pinus strobus forests in north-eastern Minnesota, USA. 3 In each forest, all trees > 1.4 m tall were mapped on a 0.75‐1.0 ha area. A grid of subplots 5‐10 m apart was established (total n = 147), and N mineralization ( µ g g − 1 soil day − 1 ), soil depth (cm), light (% canopy openness), and percentage cover of all herbs and small shrubs were measured on each subplot. 4 Cluster analysis showed that the dominant understorey species fall into three groups. Group 1 is unrelated to N and light, and is negatively associated with a midstorey of the small tree Acer rubrum and the most abundant tall shrub Corylus cornuta . Group 2 reaches maximum abundance in places (mostly gaps) with relatively high light, but is unrelated to within-stand variation in N availability. Group 3 consists of a single species, Aster macrophyllus , and reaches maximum abundance in areas with low N availability and low abundance of Corylus , but higher than average abundance of P. strobus and Betula papyrifera overstorey trees. 5 N and light have a moderate influence on understorey plant community structure. The plant species do arrange themselves along N and light gradients, but the gradients are likely to be too narrow to allow the degree of differentiation seen at the landscape level. Spatial patterning of the species groups is probably influenced by other factors, including disturbance history, chance and neighbourhood effects such as clonal reproduction.

Fine‐scale environmental variation and structure of understorey plant communities in two old‐growth pine forests

2 circular sub-plots in each sample plot, for a total of 140 1-m 2 sub-plots. Sampling was carried out in mid-July of 1991 and repeated at approximately the same time in 1992. In 1994, these same plots were sampled each month from May to October. Seasonal patterns of herb layer dynamics were assessed for the complete 1994 data set, whereas inter-annual variability was based on plot data from 1991, 1992, and the July sample of 1994. There were no significant differences between watersheds for any sample year for any of the other herb layer characteristics measured, including herb layer cover, species richness, evenness, and diversity. Cover on WS4 decreased significantly from 1991 to 1992, followed by no change to 1994. By contrast, herb layer cover did not vary significantly across years on WS3. Cover of the herbaceous layer of both watersheds increased from early in the growing season to the middle of the growing season, decreasing thereafter, with no significant differences between WS3 and WS4 for any of the monthly cover means in 1994. Similar seasonal patterns found for herb layer cover—and lack of significant differences between watersheds—were also evident for species diversity and richness. By contrast, there was little seasonal change in herb layer species evenness, which was nearly identical between watersheds for all months except October. Seasonal patterns for individual species/species groups were closely similar between watersheds, especially for Viola rotundifolia and Viola spp. Species richness and species diversity were linearly related to herb layer cover for both WS3 and WS4, suggesting that spatial and temporal increases in cover were more related to recruitment of herb layer species than to growth of existing species. Results of this study indicate that there have been negligible responses of the herb layer to 6 yr of N additions to WS3.

/pdf/effects-of-atmospheric-nitrogen-deposition-on-the-herbaceous-2in88ef21z.pdf

Effects of atmospheric nitrogen deposition on the herbaceous layer of a central Appalachian hardwood forest

We examined understory succession in current and former canopy gaps in mature Tsuga canadensis (L.) Carriere forests in southeastern Ohio. First, we reconstructed understory succession in current g...

Understory succession and the gap regeneration cycle in a Tsuga canadensis forest

We examined the correlation between canopy gap formation and the initial growth of forest trees by reconstructing the gap history of a Tsuga canadensis (L.) Carr. forest community in southeastern Ohio. We cored each tree (>10 cm dbh, n = 156) in a 40 × 90 m plot and examined the cores for release events, characterized by dramatic increases in radial growth. We identified 80 former gaps in the 79 yr sample period by clustering release events in time and space. Thirteen of the 80 former gaps coincided with the initial growth of trees. These 13 gaps were usually large gaps containing few established trees or gaps undergoing repeat disturbance. Of the 36 trees >10 cm dbh that began growth during the sample, 21 (58%) began growing inside a gap within 6 yr of gap formation-three times the rate predicted by chance (p = 0.001). We also measured the distance in time and space between the first year of growth and the closest canopy gap for each tree. We called the inverse of this measure the gap affinity i...

W. T. Rankin

Papers

Understory succession and the gap regeneration cycle in a Tsuga canadensis forest

The gap dynamics of canopy trees of a tsuga canadensis forest community