Birch Seedling Responses to Daily Time Courses of Light in Experimental Forest Gaps and Shadehouses

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Abstract
This study examined the consequences of differing daily time courses of light availability, while controlling for total photon flux density (PFD, measured in moles per square metre), on the physiology, architecture, and growth of seedlings of two birch species (Betula populifolia and B. alleghaniensis) that differ in shade tolerance. In an experimental garden, seedlings were grown in two sets of contrasting diurnal light regimes, "gaps" and "shadehouses," and at four levels of total integrated PFD (°12, 27, 50, and 70% of full sun.) In gaps, seedlings received relatively more heterogeneous diurnal light regimes, with midday full sun peaks lasting between 40 and 280 min on sunny days, depending on gap size. In shadehouses, seedlings received daily total and average PFDs similar to that in gaps, but received no midday direct sunlight, and an overall more uniform distribution of light. The daily time course of light availability, independent of total PFD, significantly affected growth of seedlings. For both species, seedlings in shadehouses grew significantly larger than gap seedlings. Differences between diurnal treatments were greatest at lower integrated light levels in B. populifolia, but not for the more shade—tolerant B. alleghaniensis. Diurnal light regimes also significantly influenced the degree of plasticity in a seedling's sun—shade responses. However, the magnitude of effect due to diurnal light regime depended on the particular trait investigated, the integrated light level, and the species. At similar total PFDs most characters (e.g., specific leaf mass, leaf mass ratio) exhibited more of a sun—type response in shadehouses that in gaps, however, maximum net photosynthesis and chlorophyll a/b ratios reflected more of a sun—type response in gap light regimes. For most growth, physiological, and morphological characters, seedling responses to increasing total PFD were generally more sensitive in gap than in shadehouse regimes. Our results also show that differences between species were greater when compared along the total PFD gradient in gaps vs. shadehouses. Both these results suggest that experimental studies employing uniform light regimes, which incorporate little of the temporal variability experienced by seedlings in natural gaps, may underestimate plasticity within regenerating tree species, and the potential for species niche differences and coexistence.

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