Variation in ecophysiological properties among conifers at an ecotonal boundary: comparison of establishing seedlings and established adults at timberline

Question: Environmental limitations on carbon acquisition and use can impact successful establishment and restrict a species range, such as for trees at timberline. How do ecophysiological properties associated with carbon uptake and allocation change along an elevation gradient for adult compared to seedling conifers in a timberline ecotone? Location: Teton Range in the Rocky Mountains, Wyoming, USA Methods: Photochemical efficiency (F_v/F_m), specific leaf area (SLA) and foliar nonstructural carbohydrates (NSC) were compared along an elevation gradient (2200‐3050 m) among two age classes (seedling and adult) and species (Abies lasiocarpa and Pseudotsuga menziesii) at timberline during mid‐summer. Results: F _v/F_m values were relatively high in both seedlings and adults across the elevation gradient, with the exception of a low F_v/F_m for seedlings in the site having the lowest soil temperatures. SLA was surprisingly constant within each age class and species across the timberline ecotone. Foliar NSCs did not increase or decrease consistently with elevation in either age class. Nonetheless, NSCs were highly variable among sites, but only in seedlings and not in adults. Conclusions: Elevation effects on these indicators of the efficiency of interception and use of sunlight in the timberline ecotone were minimal during the optimum period of the growing season. However, establishing seedlings had a tendency to exhibit greater responses to the timberline environment, particularly in their allocation of photosynthate to NSC, which may be a constraint to tree establishment at high elevations.     

Sky exposure, crown architecture, and low-temperature photoinhibition in conifer seedlings at alpine treeline

In the alpine-treeline ecotone of the Snowy Range in Wyoming, USA, microsite sky exposure of Englemann spruce ( Picea englemannii ) and subalpine fir ( Abies lasiocarpa ) seedlings (< 5 years) was associated with the avoidance of low-nocturnal temperatures and high insolation, factors which appeared to result in low-temperature photoinhibition. In a field experiment, light-saturated photosynthesis ( A _sat) in current-year seedlings (newly germinated) of fir increased significantly (approximately seven-fold) in response to increased long-wave irradiance at night (warming), solar shading (approximately five-fold), and the combination of the two treatments (approximately eight-fold). A _sat in current-year spruce remained unchanged in response to all treatments, but was over four-times higher than fir in control plots. These results indicated substantial low-temperature photoinhibition, and were supported by similar A _sat trends in natural seedlings. Increased needle inclination and clustering in more exposed microsites for both species implicates the possible role of structural adaptations for decreased sky exposure and warmer leaf temperatures at night.     

Ultraviolet-B and visible light penetration into needles of two species of subalpine conifers during foliar development

The depth of penetration of Ultraviolet-B (UV-B, 300 and 320 nm) and visible (680 nm) light was measured in foliage of Abies lasiocarpa and Picea engelmannii using a fibre-optic microprobe. Measurements were made on foliage at four times during development: needles were sampled from within expanding buds (in bud); within 72 h of emergence from the bud scales (emergent); from elongating branches (elongating); and from foliage that emerged the previous summer (mature). Light attenuation in pre-emergent needles of both species was steep and showed strong wavelength dependence. Short wavelength 300-nm light was attenuated strongly in the developing epidermal layer, but a significant proportion of this potentially damaging UV-B radiation penetrated into the mesophyll. For A. lasiocarpa and P. engelmannii, 99% attenuation of 300-nm light occurred at 51 and 96 μm, respectively, well within the mesophyll. At this stage, however, the bud scales were opaque to light below 400nm. As the epidermal cell walls and cuticle continued to develop and chlorophyll accumulated following emergence from the bud scales, light attenuation, particularly of UV-B radiation, increased. Although no UV-B is transmitted through the epidermis-hypodermis of mature needles, small but measurable quantities of 300- and 320-nm light were measured in the photosynthetic mesophyll of post-emergent and elongating needles. Thus, shortly after emergence from the bud scales in mid-June to mid-July, when incident UV doses are highest, absorption of UV-B radiation by potentially sensitive chromophores in the mesophyll may disrupt physiological and developmental processes in these species. Soluble UV-absorbing pigments accumulated during needle maturation for P. engelmannii but not A. lasiocarpa, suggesting that, for A. lasiocarpa at least, the development of effective UV screening properties in the epidermis may not be related to the induction of soluble flavonoids.     

Germination as a determinant of seedling distributions among natural substrates in Picea engelmannii (Pinaceae) and Abies lasiocarpa (Pinaceae)

In Rocky Mountain (USA) subalpine forests, seedlings of Picea engelmannii (Engelmann spruce) colonize logs more frequently than seedlings of its codominant associate Abies lasiocarpa (subalpine fir). We hypothesized that spruce germinates more readily on logs than fir, perhaps because small spruce seeds are more likely to lodge in log crevices than larger fir seeds. Our objectives were to test this hypothesis and compare both species' germination among several substrates to assess germination influences on natural seedling distributions. Spruce and fir seeds were sown on field-collected logs, litter, and soil in the greenhouse and monitored for 36 d. To test the crevice hypothesis, seeds were either scattered on logs or wedged into crevices, assuming that if both species were placed in crevices, interspecific germination differences on logs would decrease. Spruce mean germination percentages were significantly greater than fir's in all substrate treatments except when seeds were wedged in log crevices. The difference in means between the two log treatments was greater for fir (68%) than spruce (21%). Spruce germinated more rapidly than fir on all substrates. We suggest that large seed size reduces fir's success in colonizing logs, and that germination and establishment factors interact to determine natural seedling distributions for these subalpine conifers.     

Subarctic and alpine tree line dynamics during the last 400 years in north‐western and central Canada

Aim The objectives of this study were to: (1) identify episodes of establishment and mortality of young and mature trees at several sites at the alpine tree line in the western Northwest Territories and the latitudinal tree line in northern Manitoba; (2) infer changes in the structure and location of the tree line from patterns of establishment; (3) evaluate any relationship between these changes and climate; and (4) investigate sources of variability between sampling sites and study areas. Location Taiga Cordillera of the western Mackenzie Mountains in the Northwest Territories, and the western Hudson Bay Lowlands in northern Manitoba, Canada. Methods Recent tree line dynamics were examined at six climatically similar sites: three in the western Mackenzie Mountains and three around Churchill, Manitoba. Dendroecological techniques were employed to construct static age distributions of species present at each site. Static age structures, residuals from modelled age distributions, and reconstructions of dynamic stand density were used to identify patterns of establishment and mortality and to compare these to changes in climate. Results Tree line locations advanced and stand density increased during the early‐to‐mid‐20th century around Churchill, although responses were not uniform across sites or species. Results were less conclusive in the Mackenzie Mountains, although the tree line probably advanced during the late 18th century, and stand infilling occurred during the mid‐20th century. Correlation analyses with temperature suggest that conditions during establishment and particularly during recruitment are crucial for controlling tree line dynamics. Main conclusions Tree line advance and stand infilling have continued to the present at Churchill, while the tree line has stagnated in the western Mackenzie Mountains. The results of this study indicate that site‐ and species‐specific responses play a large role in determining the tree line response at multiple scales, illustrating the complexity of tree line dynamics in the context of a changing climate.     

Fire history and composition of the subalpine forest of western Colorado during the Holocene

Pollen and plant macrofossils from the Keystone Ironbog are used to document changes in species composition and the dynamics of the subalpine forest in western Colorado over the past 8000 years. Modern pollen spectra (particularly pollen influx), plant macrofossils, observations on modern species composition, and quantified densities and mean basal areas of forest trees are used to interpret the paleoecology of the forest. From 8000 to 2600 years ago the fen was surrounded by a subalpine forest. However, unlike the modern subalpine forest where Abies lasiocarpa (Hooker) Nuttall is slighlty more abundant than Picea engelmannii (Parry) Engelmann, these Holocene forests had a greater dominance of P. engelmannii , perhaps reflecting a summer wet climate like that of the modern southern Rocky Mountains and Colorado Plateau. Mesic conditions promoted a dense understory of Sphagnum moss, forbs, grasses, and shrubs which periodically burned with long (centennial) return-interval and stand-replacing fires. Populus tremuloides Michaux was the dominant successional forest tree 8000–6400 and 4400–2600 years ago, with Picea engelmannii and Abies lasiocarpa becoming reestablished within a couple hundred years. A subalpine meadow or grassland covered the fen for about 2000 years between 6400 and 4400 years ago. Over the past 2600 years a stable, non-successional Pinus contorta (Douglas) spp latifolia (Engelmann) Critchfield forest grew around the fen. This forest stand had a relatively sparse understory. The persistence of Pinus contorta at this elevation (2920 m) probably reflects a shift to drier climatic conditions, perhaps coupled with a change in fire regime to relatively frequent (decadal) surface fires. Following fire Pinus contorta became reestablished at least within 200 years, but the subalpine Picea engelmannii-Abies lasiocarpa forest never regenerated at this elevation     

Penetration of blue nad UV radiation measured by fiber optics in spruce and fir needles

The penetration of blue light (460 nm) and UV-A (360 nm) radiation into needles of spruce ( Picea engelmannii Parry ex. Engelm.) and fir [ Abies lasiocarpa (Hook.) Nutt. var. Iasiocrpa ] was examined. The distribution of collimated and scattered light in intact needles was measured by quartz fiber optic microprobes. Both blue light and UV-A were attenuated more rapidly with depth in fir than in spruce, although the light profiles for the two light regimes were different; blue light penetrated to a greater depth than UV-A, Removal of the epicuticular wax from the needle surface increased the amount of internal blue light slightly, but had little effect on the amount of UV-A. The differences in light penetration in light penetration in spruce and fir may be caused bhy different pigmentation and leaf anatomy.     

Propagule bank buildup of Chara aspera and its significance for colonization of a shallow lake

Several aspects of the propagule bank dynamics including germination, burial and storage characteristics, and the relationship between the distribution of the propagule bank and cover of an expanding Chara vegetation (dominated by C. asperaDeth. ex Willd.) was studied in the shallow lake Veluwemeer. The density of oospores in the sediment was positively correlated with the number of years that Chara was present at that particular site. After six years of Chara presence, at least 1.7 × 10⁶ oospores m^–2 had accumulated in the sediment. Oospores in sediment were evenly distributed in the 15 cm top layer, which was very similar to the foraging depth of Bewick's Swans (Cygnus columbianus bewickii Yarr). Burial in the sediment may be an important mechanism by which oospores are stored. On the other hand, at shallow areas about 50% of the biomass was consumed enhancing the potential dispersal of oospores by water birds. In a laboratory experiment, 100% of the tested bulbils of C. aspera emerged and thus may be important for short time survival of established vegetation. In contrast, oospore germination varied between 1 and 15% depending on light level and burial depth. The low germination and the high accumulation of oospores suggest that oospores are adapted to long time survival in a dormant state. Charophytes colonized Veluwemeer step by step in the course of about nine years. Not all the sites with suitable light conditions were colonized at the same speed. C. aspera established a dense vegetation only at sites with high oospores densities (> c. 1 × 10⁴ m^–2). The results indicate that the high density of oospores needed for attaining complete vegetation cover may be an important limiting factor during colonization.