Photosynthetic adaptation and acclimation to exploit seasonal periods of direct irradiance in three temperate, deciduous‐forest herbs

Summary We evaluated the potential for three species of deciduous‐forest herbs to exploit seasonal periods of direct irradiance. In particular, we investigated the importance of photosynthetic acclimation as a mechanism for shade‐tolerant herbs to utilize direct light reaching the forest floor before canopy expansion in the spring and after canopy leaf drop in the autumn. We measured the photosynthetic and growth characteristics of three co‐occurring herbs of a northern hardwood forest: the spring ephemeral Allium tricoccum Ait., the summer‐green Viola pubescens Ait., and the semi‐evergreen Tiarella cordifolia L. Leaf CO 2 exchange, leaf mass per area, and leaf biochemistry differed among species and seasonally within species to match the changing light environment below the forest canopy. From spring to summer, as irradiance dropped with the expansion of the overstorey canopy, Viola leaves exhibited reduction of both photosynthetic capacity and light compensation point. Weaker acclimation of less magnitude occurred in Tiarella leaves over the spring–summer light transition; this was followed by further acclimation to the stronger autumn irradiance. Viola ’s greater range of photosynthetic acclimation was associated with shifts in allocation between Rubisco and chlorophyll, as well as changes in total leaf nitrogen (N) concentration and leaf mass per area (LMA). In contrast, Tiarella ’s narrow range of acclimation was associated solely with changes in allocation to Rubisco versus chlorophyll, with no changes in total leaf N or LMA. Seasonal changes in leaf chemistry and structure in Viola suggest a stepwise ontogeny whereby individual leaves are able to function as ‘sun leaves’ for 3–5 weeks in the spring, and then as ‘shade leaves’ for up to 3 months in the summer. Whole‐plant biomass accumulation showed that all three species accumulated most of their annual biomass increment during periods of direct irradiance. These results demonstrate the importance of brief seasonal periods of strong irradiance to the growth of deciduous forest herbs, even shade‐tolerant, summer and evergreen species.