Sixteen microclimatic stations with differences in slope, exposure, vegetation cover, and seasonal change were established in a heavily dissected 180—acre Indiana tract. Correlations, based on cumulative air temperature duration—summations, were made between microclimatic differences and variation in phenological events. Nine widespread species of spring wildflowers had a collective mean range in dates of first flowering of 7.2 days for all stations. The maximum range for a single species was 11 days. Flowering dates of nine species of a large gorge were retarded an average of 6.0 days in the north—facing slope with respect to the opposing south—facing slope. This 6.0—day difference between gorge slopes 150 ft apart is equal to the expected to occur in about 110 miles of latitude, assuming standard exposures. Six species of the north—facing slope in a small gorge were retarded an average of 2.8 days with respect to the opposing south—facing slope. Flowering on north—facing gorge slopes was retarded more than in gorge bottoms, and upland stations had earlier than normal flowering dates. Mean flowering dates for the entire area were retarded more at cooler locations than warmer slopes were advanced. Air—temperature sums and flowering dates correlated well in a given microclimate. The results suggest that phenological research could be expedited by making observations in diverse microclimates during a few seasons rather than acquiring long—term phenological records.

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Question: Are canopy gap dynamics responsible for driving the structural and compositional changes that have occurred over a 26-year period in a mature Quercus forest remnant? Location: Dobbs Natural Area, an unlogged 3.6-ha forest preserve in west-central Indiana, USA. Methods: We analyzed mapped permanent plot data for a site that illustrates a trend common in Quercusdominated forests in eastern North America, where recruitment of new stems is dominated by mesophytic, shade-tolerant species such as Acer saccharum, rather than Quercus. We developed a GIS database from stand census measurements taken in 1974 and 2000, employing it to conduct tree-by-tree comparisons that allow direct determination of ingrowth, mortality and survivorship, and to relate the spatial patterns of subcanopy dynamics to canopy gap occurrence. Results: The re-census shows modest changes in canopy composition, but much greater turnover in the subcanopy. Nearly half of all individuals originally present died; much of this mortality resulted from a major decline in subcanopy Ulmus americana. While overall density remained fairly constant, the subcanopy experienced substantial ingrowth of shade-tolerant Acer saccharum, Fagus grandifolia, and Tilia americana. Canopy gaps, although forming at rates in the upper range of regional averages, did not significantly benefit subcanopy populations of Quercus spp. or most other taxa with limited shade tolerance.Conclusions: Canopy gaps play a minor role in driving the recent demographic trends of this stand. The spatial and temporal scales of light availability in gaps do not support regeneration of most shadeintolerant species. Compositional change parallels a historical shift in light regimes.

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Wizard Island, a ½—square—mile volcanic cinder cone located in Crater Lake, Oregon, was sampled by continuous belt transects. Separation of the one herbaceous and four forested community types, which were mapped as major vegetation units, was remarkably distinct, as 60% of the herbaceous species were found in only one or two communities. Physical factors such as the distribution and stability of substrate materials, temperature extremes, and soil—moisture availability exert the greatest control over plant community structure and development. Biotic factors have reduced effects on the generally sparsely vegetated island. The unstable cinder slope community was dominated by Polygonum newberryi Small., Eriogonum pyrolaefolium Hook. var. coryphaeum T. & G., and Arenaria pumicola Cov. & Leib. Pinus albicaulis Engelm. constituted 55% of the importance value of the crater rim community, but appears to be declining in importance. Pinus contorta is invading the crater area and may slowly replace P. albicaulis. Other characteristic crater rim species were Holodiscus microphyllus Rydb. var. glabrescens (Greenm.) Ley. and Castilleja applegatei Fern. The lower cone and north slope forests were dominated by Abies magnifica Murr. var. shastensis Lemm. and Tsuga mertensian (Bong.) Carr., with Pinus monticola Dougl. the only other important tree species. An importance value of 51% for T. mertensiana in the north slope community as compared with 25% in the lower cone reflects the cooler, moist microclimate and greater community maturity on the north slope. The more tolerant T. mertensiana increases in importance as the stands mature. Characteristic shrub and herb species were Vaccinium membranaceum Dougl. and Luzula glabrata (Hoppe) Desv. in the north slope, and Arctostaphylos nevadensis Gray and Pyrola secunda L. in the lower cone communities, respectively. The lava flow community was largely restricted to recesses between lava ridges where moisture and soil conditions are improved. Tsuga mertensiana was the dominant tree species on the lava flow at 51% importance value, with Sambucus microbotrys Rydb. and Cheilanthes gracillima D. C. Eaton as indicators of the shrub community development was and herb strata. Diversity, total plant density, and herbaceous and shrub community development was greatest in the favorable forest microclimates of the north slope. The north slope, crater rim, lower cone, cinder slope, and lava flow communities had 28, 23, 21, 12, and 10 nonarborescent species, respectively. The reported vascular flora of the island comprises 105 species and varieties. The best forest development occurred in an encircling belt at about 6,400—6,500 ft (1,950—1,980 m) elevation, but tree species characteristic of an elevational range of 2,500 ft (820 m) are telescoped onto the 760—ft—(250—m) high island. Substrate and local climatic diversity and climatic moderation due to lake effects and shielding by the caldera wall permit survival of trees above their usual elevation range. Tree invasion on the upper cone (particularly o...