Purple moor grass induces a rapid decrease of photosynthesis in young oak after forest clear-cutting

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Purple moor grass induces a rapid decrease of

photosynthesis in young oak after forest clear-cutting

Antoine Vernay, Philippe Malagoli, Ludivine Guinard, Thierry Ameglio,

Philippe Balandier

To cite this version:

Antoine Vernay, Philippe Malagoli, Ludivine Guinard, Thierry Ameglio, Philippe Balandier. Purple

moor grass induces a rapid decrease of photosynthesis in young oak after forest clear-cutting. 3rd

International Symposium on Plant Signaling and Behavior 2015, Jun 2015, Paris, France. PSB2015,

134 p., 2015, International Symposium on Plant Signaling and Behavior 2015. �hal-01269279�

1 _{Clermont Université, Université Blaise Pascal, UMR547 PIAF, BP 10448, F-63000 Clermont-Ferrand, France}

2 _{INRA, UMR547 PIAF, F-63100 Clermont-Ferrand, France}

3 _{Irstea, Research Unit on Forest Ecosystems (EFNO), Domaine des Barres, F-45290 Nogent-sur-Vernisson, France}

Introduction

Results

The purple moor grass (Molinia caerulea (L.) Moench) is a well-known resource competitor to the detriment of tree regeneration in many boreal or temperate forests of the Northern hemisphere. This study aimed at investigating to what extent soil nitrogen capture in interaction with light availability drives the early establishment of competition between oak (Quercus petraea (Matt.) Liebl.) and Molinia seedlings.

Purple moor grass induces a rapid decrease of

photosynthesis in young oak after forest clear-cutting

Vernay

_{A., Malagoli}

_{P., Guinard}

_{L., Améglio}

_{T . and P. Balandier}

Experimental design

Fig. 1 Experimental set-up. O : oak ; M : Molinia ;15_{N : pot labeled with K}15_NO 3. SP : sole-grown; MP : mixed plants receiving either 11% or 55% iPAR

Two-year-old oaks were grown in 20 L pots, alone or in combination with Molinia, for two levels of light availability (11 and 55% of incident

photosynthetically active radiations) in a greenhouse. Leaf

photosynthesis measurements and soil 15_{N-labelling were used to}

monitor changes in carbon assimilation and soil nitrogen uptake between and within species under well-watered conditions during early time of growth (Fig. 1).

O 15_N O O O O 15_N 15_N 5 X 5 X _{5 X 5 XBare soil} M M M O 5 X M M M O 5 X M M M 15_N Unlabelled 55% of iPAR 11 % of iPAR 15_N-labelling 15_N-labelling SP MP

Conclusion

Photosynthetically active radiations (µmoles m-2s-1)

G ro ss C O2 a ssi m ila ti o n (µ m o le s m -2 s -1) G ro ss C O2 a ssi m ila ti o n (µ m o le s m -2 s -1) G ro ss C O2 a ssi m ila ti o n (µ m o le s m -2 s -1) G ro ss C O2 a ssi m ila ti o n (µ m o le s m -2 s -1) Oak Oak Molinia Molinia
SP  MP
MP
MP A) B) C) D)

Photosynthetically active radiations (µmoles m-2s-1)

Photosynthetically active radiations (µmoles m-2s-1)

Photosynthetically active radiations (µmoles m-2s-1)

55% iPAR 11% iPAR
SP  MP -4 -2 0 2 4 6 8 10 12 14 16 18 0 200 400600800100012001400160018002000

Fig. 2 Photosynthetic light response curves (µmoles CO2m-2s-1) in oak (Quercus

petraea (Matt.) Liebl.) seedlings (A and C) and Molinia (Molinia caerulea (L.) Moench) (B and D) leaves when oak was sole-grown (SP) or grown with Molinia (MP) under 11% or 55% iPAR. Points and lines correspond to experimental measurements and fitted equations, respectively. Doted arrows indicate the levels of PAR reaching the canopy under 11% or 55% iPAR growing conditions

Fig. 3 Relationships between maximum gross assimilation (Amax), apparent quantum yield (QYa) and atomic excess15N (%) in leaves in oak (Quercus

petraea (Matt.) Liebl.) seedlings and Molinia (Molinia caerulea (L.) Moench) when oak was sole-grown (SP) or grown with Molinia (MP) under 11% (A and C) or 55% iPAR (B and D). Regression equations and coefficients for each species are listed into figures.

Am ax (µ m o l C O2 . s -1. m -2) Q Ya (µ m o l C O2 .m o le -1 H2 O ) 11% iPAR A) B) C) D) Oak: y = 15.726x + 6.0314 R2= 0.57 Molinia: y = 3.8853x + 3.4903 R2= 0.93 Oak: y = 19.743x + 4.3863 R2= 0.91 Molinia: y = 19.35x - 2.34 R2= 0.46 Oak: y = -0.0281x + 0.028 R² = 0.05 Molinia: y = 0.0115x + 0.0207 R2= 0.88 Oak: y = 0.0916x + 0.0132 R² = 0.77 Molinia: y = 0.0267x + 0.0129 R2= 0.25 55% iPAR

Atomic excess 15N (%) Atomic excess 15N (%)

Presence of Molinia had no significant effect on short-term oak seedling growth regardless of the light availability. However, increase in incident light resulted in strongly decreased photosynthesis capacity in oak. Meanwhile, photosynthesis capacity strongly increased in Molinia (Fig. 2).

• In both species, all parameters characterizing photosynthesis were positively correlated to 15_{N atomic excess in} leaf (Fig. 3 A, B, C, D),

except for QYa in oak

under 11% iPAR (Fig. 3C).

A response model to explain such a behavior of oak grown with Molinia under unbalanced light resource may fit supply/demand theory (Davis et al. 1998). In our experimental design, there is a switch from a competition for light to a competition for below-ground resources. As Molinia can easily grow under a wide range of irradiance, growth in Molinia was strongly improved when

light availability increased due to a larger photosynthetic capacity (especially Amax). Improved photosynthesis increased shoot

and root growth that, in turn, increased both N demand and subsequent N capture by Molinia. Relationships between

parameters characterizing photosynthesis (maximal gross CO2assimilation (Amax), apparent quantic yield (QYa)) and15N atomic

excess (i.e. newly N taken up) in leaf in both species suggests that soil N availability as well as capacity to efficiently capture soil inorganic nitrogen might be one of the key-players among soil resources in driving the short-term establishment of dominance of

species in an oak-Molinia mixture in the early phase of the regeneration, ultimately driving success or not of oak trees.

Sole Oak  Mixed Oak
Mixed Molinia