Spatial variation of photosynthetic capacity of early-, mid-, or late-successional broad-leaved tree species in a temperate mixed forest

Abstract

Forests are important not only as a habitat for animal and plant species, but also for economy, protection of soil, local recreation areas and they provide air-purification. Forests which were earlier almost solely monocultures are now more and more converted into mixed species stands. For this conversion it is important to get a deeper knowledge about the involved species. The photosynthetic capacity of a species is additionally needed for the up-scaling to canopy- or stand-level photosynthesis. We investigated the photosynthetic capacity of the five broad-leaved tree species <i>Fraxinus excelsior</i>, <i>Acer pseudoplatanus</i>, <i>Carpinus betulus</i>, <i>Fagus sylvatica</i> and <i>Tilia cordata</i> in the Hainich National Park, Thuringia. Additionally, we focused on the species ability to adapt to different light regimes and compared our findings with a literature compilation of European temperate broad-leaved tree species. We measured light saturated net photosynthesis rate under ambient CO₂ concentrations (A_max), maximum carboxylation efficiency (V_cmax), maximum electron transport rate (J_max) and leaf day respiration rate (R_d) along a light gradient inside the canopies. All investigated tree individuals grew in the immediate vicinity of each other. We included measurements about relative irradiance, N content per leaf area and leaf mass and height of the trees. The sun leaves of the five investigated species had similar photosynthetic capacities and did not differ significantly, which can be explained by similar light conditions in the uppermost canopy layer. On the contrary, the ability to produce shade leaves differed between the species. Shade leaves were at least 20 % larger and had a specific leaf area (SLA) over 220 cm² g^-1. The A_max rate on leaf area basis was as high or slightly lower as in sun leaves and the A_max rate on mass basis tended to be higher in sun leaves. The photosynthetic nitrogen use efficiency was higher in shade leaves, whereas R_d was lower. <i>F. sylvatica</i> was fulfilled all the above mentioned criteria and the other species showed only partly a significant sun-shade differentiation. The species showed different flexibilities to produce shade leaves and their adaptation consisted morphologically mainly on the alteration of SLA. V_cmax and J_max correlated in almost every species with RI, nitrogen per leaf area (N_a) and SLA whereas both V_cmax and J_max were better explained by N_a than by RI or SLA. Additionally, we found that J_max was optimized with respect to light availability in light-demanding species whereas V_cmax was optimized in shade-tolerant species. Area-related A_max correlated with RI, SLA and N_a only in <i>T. cordata</i> and <i>C. betulus</i> whereas mass-based A_max correlated in <i>F. excelsior</i>, <i>A. pseudoplatanus</i> and <i>F. sylvatica</i>. We conclude that the five temperate broad-leaved tree species differ in their strategy to adapt leaf morphology and function to light availability. In a comparison to high-light grown juveniles of the same species we demonstrated that the juvenile leaves had similar photosynthetic capacities like the mature sun leaves which is in debt to the high growing irradiance of the juveniles. Finally, we compared V_cmax, J_max and A_max over many European broad-leaved tree species. We focussed on a clear separation of juvenile (age over one year) and mature (age over 20 years) trees as well as on the separation of sun and shade leaves and the categorisation of successional status. We found differences between mature sun and shade leaves and mature shade and juvenile leaves. A significant differences between the successional groups could not be detected which is contrary to the several studies and the common text book knowledge. These contrasting results may be explained by the fact that most other studies mix data of juveniles and mature trees or sun and shade leaf values. Additionally, we found an increase in V_cmax and A_max over the last 40 years in <i> F. sylvatica</i>, which can be explained by increasing foliar nitrogen contents. In conclusion, we would strongly recommend not to mix between juvenile and mature photosynthetic capacity data for photosynthesis up-scaling to global terrestrial biosphere models. Here, a detailed knowledge of the species photosynthetic capacity is of advantage as well as for the forest conversion.