Influence of krummholz mat microclimate on needle physiology and survival

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Abstract
Microclimate and photosynthesis of krummholz mat growth forms of Picea engelmanii (Parry) and Abies lasiocarpa [Hook.] Nutt. were investigated to determine structural features which may aid survival in alpine environments. The structure of krummholz mats was described in terms of the vertical distribution of leaf area index and leaf area density, which exceeded 50 m-1 (based on total leaf surface area) near the canopy surface and approached zero below 30 cm from the surface in both species. Photosynthetic photon flux density (PPFD, 0.4–0.7 μm wavelengths) and wind decreased by an average of 6 and 50-fold, respectively, between 1 m above and 10 cm below mat surfaces in both species. Needle temperatures on a P. engelmannii krummholz mat during July averaged about 2°C above air temperature during the day, with a maximum overtemperature of greater than 20°C above T air during one sunlit period. At night, needle temperatures averaged 3–4°C below T air. Net photosynthesis in year-old P. engelmannii shoots reached a maximum at 15–20°C during July and August. Surface shoots were light saturated at near 1200 μmoles m-2s-1 PPFD, and had higher photosynthetic rates than subsurface, predominantly shaded shoots above 800 μmoles m-2s-1. Shade shoots had higher photosynthetic rates when PPFD was below 600 μmoles m-2s-1, and at 250 μmoles m-2s-1 shade shoots maintained about 50% of the net photosynthetic rate of sun shoots at light saturation. Shade shoots appeared capable of benefitting photosynthetically from elevated temperatures within krummholz mats despite relatively low light levels. Especially rapid photosynthesis may occur when canopy needles are illuminated by sunflecks and needle temperatures rise by 10° C or more. Snow cover appears crucial for the survival of needles during winter. Snow accumulated within krummholz needle canopies before the sub-canopy zone of unfoliated branches became filled. The concentrated needle growth in the krummholz canopy captured snow in early autumn without support from ground-level snowpack. Early snow cover in both species prevented cuticle abrasion and resulted in high winter needle water contents and viabilities for subsurface compared to surface needles which became abraded, severely dehydrated, and had high mortality between December and February, especially on windward sides of shoots. Extremely high concentrations of needles within krummholz mat canopies created an aerodynamic structure which elevated needle temperatures to more optimal photosynthetic levels in summer and resulted in more efficient snow accumulation in winter. These factors appear crucial for winter needle survival. Thus, krummholz mats appear to be an important adaptation in growth form which provides survival benefits in both summer and winter.