Early effects of tree species identity and richness on the herb layer

The forest herb layer provides a multitude of ecosystem services as a
result of its species-rich character. Herb layer diversity and biomass are both
influenced by tree layer composition and species richness through species-specific
influences on environmental conditions. The results of observational studies
on richness–biomass relationships between tree and herb layer have not
been unequivocal. We examined tree species identity and richness effects on
herb layer species richness, composition, biomass and nutrient concentrations at the BIOTREE tree diversity experiment in Kaltenborn (Germany).

Sixteen plots were planted in 2004, using a pool of four tree species
(beech, oak, Douglas-fir, Norway spruce) and four richness levels, comprised of
all possible species combinations. In this way, complete dilution was avoided,
allowing separation of tree species identity and richness effects. Mixed plots consisted
of amatrix ofmonospecific patches. One permanent vegetation quadrat of
1 m2 was established in the centre of four patches per plot. The herb layer was
monitored in 2004 and 2010; in 2010 light measurements were performed in
each quadrat, and in 2011 above-ground biomass was sampled on 0.25 m2
within the quadrat.

Community composition shifted markedly between 2004 and 2010.
Tree species identity did not yet influence temporal compositional turnover or
herb layer species richness in 2004 and 2010. Ellenberg N indicated a temporal
shift towards lower soil fertility under all tree species, whereas Ellenberg R indicated
decreasing soil acidity under beech and Douglas-fir. Ellenberg L and F
showed no shift from 2004 to 2010. Apart from the significantly lower Ellenberg
N for beech, none of the Ellenberg indicators indicated interspecific differences.
Douglas-fir, and especially Norway spruce, negatively influenced total aboveground
herb layer biomass. Douglas-fir also induced lower relative light availability,
higher potassium, magnesium and nitrogen concentrations and lower
carbon:nitrogen ratios in the total biomass. Higher tree species richness positively
affected graminoid and total biomass and also slightly increased plot-level
herb layer species richness.

Thus, despite the young age of the experiment, tree species identity
and richness effects on the herb layer could already be discerned.We expect these relations
to become stronger with time.

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