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Despite striking differences in climate, soils, and evolutionary history among diverse biomes ranging from tropical and temperate forests to alpine tundra and desert, we found similar interspecific relationships among leaf structure and function and plant growth in all biomes. Our results thus demonstrate convergent evolution and global generality in plant functioning, despite the enormous diversity of plant species and biomes. For 280 plant species from two global data sets, we found that potential carbon gain (photosynthesis) and carbon loss (respiration) increase in similar proportion with decreasing leaf life-span, increasing leaf nitrogen concentration, and increasing leaf surface area-to-mass ratio. Productivity of individual plants and of leaves in vegetation canopies also changes in constant proportion to leaf life-span and surface area-to-mass ratio. These global plant functional relationships have significant implications for global scale modeling of vegetation-atmosphere CO2 exchange

The relative importance of competition and facilitation has been hypothesized to change with variation in abiotic conditions. I examined the relative importance of competition and facilitation along elevation gradients in the northern Rocky Mountains where Pinus albicaulis and Abies lasiocarpa dominate the overstory. At lower elevations and in more sheltered sites, A. lasiocarpa seedlings, saplings, and trees were not spatially associated with mature P. albicaulis, whereas at high-elevation sites along exposed ridges near timberline A. lasiocarpa were highly aggregated around mature P. albicaulis. I also compared growth rates of A. lasiocarpa trees before and after the death of adjacent P. albicaulis to growth rates of A. lasiocarpa in the same years but adjacent to living trees. In the Bitterroot Mts. A. lasiocarpa responded positively to the death of adjacent P. albicaulis at low-elevation sites (7% increase), but negatively at high-elevation sites (24% decrease). This suggests that facilitation was more important at timberline sites characterized by abiotic extremes and competition was more important in more moderate abiotic conditions. At high-elevation sites in both mountain ranges, large A. lasiocarpa were 2-4 times more aggregated with P. albicaulis than A. lasiocarpa seedlings. At the high-elevation site in the Bitterroots, growth rates of large A. lasiocarpa were significantly lower in open microsites than when trees were adjacent to either living or dead P. albicaulis. In contrast, growth rates of small saplings did not differ among these microsites. Stronger facilitative effects on mature trees than on seedlings or saplings may develop because the winter snowpack protects small A. lasiocarpa from blowing ice and snow. After trees grow above the snowpack shelter from large P. albicaulis may be crucial. These results emphasize the importance of studying interspecific interactions over a range of conditions; in these forests both positive and negative interactions occur between A. lasiocarpa and P. albicaulis, but their relative importance depends on abiotic conditions and plant life history stage.

Ecosystem carbon cycle feedbacks to climate change comprise one of the largest remaining sources of uncertainty in global model predictions of future climate. Both direct climate effects on carbon cycling and indirect effects via climate-induced shifts in species composition may alter ecosystem carbon balance over the long term. In the short term, climate effects on carbon cycling may be mediated by ecosystem species composition. We used an elevational climate and tree species composition gradient in Rocky Mountain subalpine forest to quantify the sensitivity of all major ecosystem carbon stocks and fluxes to these factors. The climate sensitivities of carbon fluxes were species-specific in the cases of relative aboveground productivity and litter decomposition, whereas the climate sensitivity of dead wood decay did not differ between species, and total annual soil CO2 flux showed no strong climate trend. Lodgepole pine relative productivity increased with warmer temperatures and earlier snowmelt, while Engelmann spruce relative productivity was insensitive to climate variables. Engelmann spruce needle decomposition decreased linearly with increasing temperature (decreasing litter moisture), while lodgepole pine and subalpine fir needle decay showed a hump-shaped temperature response. We also found that total ecosystem carbon declined by ~50% with a $2.8\\textdegree C$ increase in mean annual temperature and a concurrent 63% decrease in growing season soil moisture, primarily due to large declines in mineral soil and dead wood carbon. We detected no independent effect of species composition on ecosystem C stocks. Overall, our carbon flux results suggest that, in the short term, any change in subalpine forest net carbon balance will depend on the specific climate scenario and spatial distribution of tree species. Over the long term, our carbon stock results suggest that with regional warming and drying, Rocky Mountain subalpine forest will be a net source of carbon to the atmosphere.

We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii, 9 of Abies lasiocarpa, 4 of Pinus contorta var. latifolia, and 1 of Pinus flexilis) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta, sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differences among subalpine habitats.

The fine-scale genetic structure of a subalpine conifer, whitebark pine (Pinus albicaulis Engelm.), was studied at nested geographic levels from watershed to adjacent stems in the eastern Sierra Nevada Range of California. A combination of several characteristics contributed to unpredicted genetic structure in this species. This includes being one of only 20 pine species worldwide with wingless, bird-dispersed seeds; having the reputed capacity to reproduce vegetatively; and forming distinct growth morphologies at different elevations in this part of its natural range. Genetic differentiation, as measured with 21 allozyme loci, among the three studied watersheds is virtually negligible (FST= 0.004). This is a surprising result because the upper-elevation sites vary somewhat in slope aspect; thus, aspect was confounded with watershed effect. Differentiation between the upper-elevation prostrate krummholz thickets and lower-elevation upright tree clump growth forms is modest (FST= 0.051). Much stronger differentiation was measured among the individual thickets and clumps within their sample sites (FST= 0.334). Within krummholz thickets, multiple individuals are present and genetic relationships often resemble half- to full-sibling family structure (mean r = 0.320). Canonical trend surface analysis in two intensively sampled thickets indicates greatest genotypic variation in the direction of the prevailing wind. At lower elevations, most (72%) of the tree clumps contained more than one genotype; the remaining clumps are probably multistemmed trees. Within tree clumps, family relationships are closer than those for krummholz thickets-commonly full-sibling to selfed structure (mean r = 0.597). Genetic structure is apparently profoundly influenced by the seed-caching behavior of Clark's nutcracker (Nucifraga columbiana Wilson). Western pine species typically show little among-population differentiation and high levels of within-population genetic variation. In whitebark pine in the eastern Sierra Nevada of California, genetic variation is highly structured, especially within the natural groupings-krummholz thickets and upright tree clumps.

The composition and structure of the subalpine coniferous forest communities on Changbai mountain, China, were studied. Two mixed-conifer forest types, indicated by different combinations of Picea jezoensis var. komarovii, Picea koraiensis, Abies nephrolepis, Pinus koraiensis, and one type dominated by Larix olgensis and Abies nephrolepis were sampled. The number of woody species become less towards the higher elevation, while the herb layer was richer in the upper part due to the lower crown density. The Larix-dominated stand was established in a large gap presented as a building phase, and its composition and structure were simpler than the other two types. The dynamics of the forest zone, which had a mosaic structure consisting of Picea- and Larix-dominated patches, is characterized by patches transforming from one to another. This mosaic structure, with patch interchanging, is defined as the climax of the forest zone. Larix played an important role in sustaining the coniferous forest communities as a gap revegetator and a conditioner for the regeneration of Picea and Abies. Windfall was considered to be a frequent disturbance factor which caused the vegetation to become more heterogeneous and offered opportunities for Larix to establish.

Montane temperate forests of the Cordillera de Piuchué Ecosystem Study, Isla Chiloé, Chile, are unaffected by air pollution, timber exploitation and agricultural clearing, and the current floristic assemblage has been relatively stable for the past 7500 years. The apparent absence of major perturbation at this location makes it an appropriate baseline site for ecosystem analysis. We measured soil bulk density, pH, soil organic C (SOC), total N, and NH₄Cl-exchangeable cations (Ca⁺², Mg⁺², K⁺, Na⁺, Al⁺³) in 0-10 and 10-40 cm depth samples from 72 soil profiles representing three vegetation zones: Fitzroya cupressoides Forest, Pilgerodendron uvifera-Tepualia stipularis Forest, and Magellanic Moorland. Fitzroya and Pilgerodendron-Tepualia Forests were indistinguishable for all measured soil characteristics (P > 0.05, Dunn's multiple comparison test on ranked data); these included very high median SOC concentrations (0-10 cm = 49.6%) and correspondingly low bulk density values (0-10 cm = 0.07). Moorland soil median values (0-10 cm) were significantly higher for bulk density (0.12) and lower for SOC (28.5%), but not for total N (Forests = 0.99%, Moorland = 0.95%), resulting in lower median C: N ratios for the moorland (Forests = 44.7; Moorland = 30.3). Across both depths and all three vegetation zones regression analysis indicated that SOC was an excellent predictor (R²= 0.93, P < 0.001) of ∑ (exchangeable Ca⁺² + Mg⁺² + K⁺ + Na⁺).Comparison with other old growth montane environments indicates that the Fitzroya and Pilgerodendron-Tepualia soil profiles are characterized by C:N ratios typical of other relatively unpolluted conifer forest soils (33.0-49.3). Soil profiles of representative polluted montane conifer forests have lower C:N ratios (16.2-23.5).Organic horizons from representative polluted montane conifer forests also retain fewer exchangeable base cations per unit SOC than are retained by organic horizons from the Cordillera de Piuchué forests.

Spatial patterning in the disturbance regime of a forest affects the vegetation dynamics. Therefore, the distribution of canopy gaps was examined in detail for a subalpine Abies-Picea forest in the northeastern United States. Gaps were not randomly distributed. The fraction of forest area in gaps and the abundance of gaps varied significantly with topographic position, elevation, and slope percent. On average, 15 % of the forest was influenced by gaps, but the gap fraction was greater near ridges (23 %) and near streams (27 %) than on the backslope (13 %). Also, gaps were larger and more abundant near streams and ridges. Gap fraction varied with elevation as well: more of the forest was disturbed at lower and higher elevations than at mid-elevations. Significantly more of the forest on steep slopes (≥ 30 %) was under gaps. As a result of this patterning, some parts of the Abies-Picea forest were predictably more disturbed than others. A remaining question is whether this patterning is sufficient to influence the regeneration environment and thus forest composition.

Tree age and tree structure are important determinants for epiphytic lichen communities, affecting substrate stability, light availability, and propagule dispersal. However, little is known about the relative importance of these 2 factors on many species. This work aims to evaluate the importance of tree age and tree structure in explaining the within-stand frequency of the macrolichen Letharia vulpina. The study was carried out in 2 larch–stone pine forests in the Eastern Italian Alps. The frequency of Letharia was evaluated using a standard sampling method. To explain the within-stand frequency of Letharia, tree age and several variables related to tree structure were considered. Multiple ordinary least square regression was applied to clarify the influence of the set of variables. Several partial regressions were then computed to evaluate the relative importance of each significant predictor. The frequency of Letharia increases with increasing tree age, tree diameter, and first branch height. Tree age is the most important variable, accounting for one third of the total explained variation, although tree structure also has a significant effect on lichen frequency. Due to its dispersal limitations and old-tree dependence Letharia is proposed as a suitable indicator of forest continuity. The results may be of interest for conservation purposes, since the frequency of Letharia may be enhanced by protecting old trees. Nomenclature: Nimis & Martellos, 2003.