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

Abstract

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 fourtimes 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.

Ecology and the ratchet of events: Climate variability, niche dimensions, and species distributions

“…Thus, a proximal temperature predictor of a species range limit might consist of the probability that temperature falls below a threshold within x days after bud break, or probability of winter temperatures falling below a freezing threshold for y hours, or frequency of years (relative to mean generation time) in which summer temperatures persist below a threshold for z days. Some may be more subtle: Seedling survival of subalpine conifers may be limited by the frequency of growing-season days in which early-morning temperatures are below a low-temperature photoinhibition threshold (48).…”

Treelines Will be Understood Once the Functional Difference Between a Tree and a Shrub Is

“…Quite often, the number of seedlings is greater above the treeline, than within the uppermost forest. These seedlings initially profit from shelter among alpine vegetation and microtopography, but fully sky exposed ones, without any shelter, may experience stress during their earliest life stage, like other low stature plants would (Germino and Smith 1999). There are many examples of successful seedling establishment above treeline that ended in crippled shrub rather than upright tree stature.…”

The Role of Wood Microsites at Timberline-Alpine Meadow Borders for Conifer Regeneration

“…seedlings that may have initially established on wood and soil at lowshadow/low-canopy overstory locations may have been prone to drought and photoinhibition, a common cause of frost-related death for seedlings growing at high elevation with minimal overstory protection , Germino and Smith 1999, Germino et al 2002. Lack of substrate moisture, typically limiting seedling growth (Cui andSmith 1991, Smith andJohnson 2007), is ameliorated by wood substrates.…”

“…In drier areas of the world, concavity microsites supply seedlings with needed moisture during periods of drought (Mellmann-Brown 2005, Anschlag et al 2008. Where night frost is common, rocks and trees may alter wind, sun, and long-wave radiation patterns thereby protecting seedlings (Ball et al 1991, Germino andSmith 1999). In the Pacific Northwest (PNW)…”

“…Although seedlings growing in areas with less overstory canopy benefit from having warmer substrates, increased likelihood of exposure to intense solar radiation may make seedlings prone to photoinhibition (Cui and Smith 1991, Germino and Smith 1999, Germino et al 2002. Photoinhibition occurs when excess excitation caused by high levels of light radiation arriving at the photosystem II reaction center in the chloroplast leads to its inactivation and damage (Long et al 1994) thereby inhibiting photosynthesis (Taiz and Zeiger 2006).…”