In addition to increasing atmospheric CO2 and warming global temperatures, changes in cloud patterns will accompany predicted alterations in precipitation regimes. Yet, no studies to our knowledge have systematically evaluated the impact of natural or experimentally-altered cloud regimes on daily and seasonal carbon gain or transpirational water loss for any species. Increased cloudiness acts to transform parallel-beam (collimated) direct sunlight of higher intensity into non-collimated sunlight (NCS) with a more diffuse directional quality and lower intensity. At the high-altitude treeline of the Rocky Mountains (RM), USA, frequent cloudiness in the afternoon appears to ameliorat daily drought stress and lead to increased carbon gain and growth. In direct contrast to RM, cloud patterns (particularly cloud-immersion during morning hours) appear to influence the altitudinal distribution of another congeneric pair of spruce and fir species in the southern Appalachian Mountains (SA), USA. In these mountain-top areas, spruce-fir forests exist as refugial, relic populations that have persisted since the last glacial maximum. Also, cloud-immersion has been implicated in the altitude zonation and perseverance of these boreal remnants.
The specific impacts of different cloud forms on incident sunlight was measured and compared for the afternoon clouds of the RM and the low cloud and cloud-immersion of SA, along with accompanying responses in photosynthesis and water relations for these spruce-fir congeneric pairs. Sunlight levels (photosynthetically active radiation, PAR) measured with horizontal sensors varied from a low of <140 μmol m-2 s-1 during cloud immersion at SA to maximum values near 2800 μmol m-2 s-1 at RM when direct sunlight was reflected from the edges altocumulus clouds at RM. At SA, photosynthesis became light-saturated between ~400-500 μmol m-2 s-1 PAR and corresponded closely to the sunlight environment measured during immersion. At RM, Understory PAR was ~45% less and much less variable on immersed than clear days. At both RM and SA, estimated daily carbon gain was increased >20% during normal cloud patterns compared to clear afternoons and mornings, respectively. The efficiency of Photosystem II (Fv/Fm) under exposure to clear-sky conditions at both RM (afternoon) and SA (morning) was significantly reduced (up to 36%). Photoinhibition of photosynthesis also corresponded to the lower PS II values (<0.55). Leaf temperatures and corresponding transpiration values were higher (with lower water use efficiency) on completely clear days and severe water stress developed at both RM and SA. Implications for changing cloud patterns with continued global warming are discussed.