Humidity Manipulation) is conducted in Estonia with silver birch and hybrid aspen,
representing widespread deciduous species in northern Europe. The experiment
revealed that high humidity is an important climatic factor, causing reduction in
growth rate of the aboveground parts, leaf biomass and area, bud size, sap flux and
earlier bud break and delayed leaf fall. In the belowground parts, root biomass (fineroot biomass in particular) was increased although the root compartment was
exposed to anaerobic conditions, increased soil pH and soil water potential, with
lowered soil respiration and altered microbial and fungal communities. In the leaves,
high humidity shifted the metabolism towards nonstructural carbohydrates, antioxidants and phenolic compounds, while nutrient (N and P) content, photosynthesis,
dark respiration, hydraulic conductance and the density of glandular trichomes were
reduced. The change in the chemical composition of the surface wax layer resulted in
lower hydrophobicity, exposing the leaves for fungal pathogen attacks. In the stem,
N and P content in wood and number of living parenchyma cells were increased,
while the stem wood density was reduced and wood chemistry altered. Reduction in
the growth rates under high humidity was more pronounced in hybrid aspen than in
silver birch. Birch showed more efficient adjustment of leaf and root morphology,
hydraulic architecture, primary carbon and nitrogen metabolism and leaf surface
properties mitigating the negative impacts of high humidity. At the ecosystem level,
high humidity altered soil processes (moisture content, microbiota, nitrification),
with potential to affect competition and species composition. A complementary
laboratory study with birch demonstrated that additional N supply can counteract
the effects of low VPD on cellular metabolism and leaf surface wax quality.
Keywords Aspen, Birch, Forest ecosystem, Growth, Leaf epicuticular waxes,
Metabolism, Nitrogen, Nutrients, Relative humidity (RH), Sap flux, Soil processes,
Trichomes, Vapour pressure deficit (VPD)
1 Introduction
Several climate change scenarios predict increasing precipitation at the northern
latitudes leading to increase in atmospheric water vapour content and wetter conditions (IPCC 2013). The predicted rise in annual precipitation in northern Europe
varies from 4 to 34% for the time period of 2071–2100, depending on the scenario
(Jacob et al. 2014). In addition to soil water availability, plants are affected by the
relative humidity of the surrounding air (often measured as relative humidity, RH, or
as water vapour pressure deficit, VPD). Dai (2006) showed a slightly positive trend
in relative humidity (RH) in the northern hemisphere over the recent decades
(1976–2004). Besides the northern European area, an increase in RH has been
reported over oceans and regionally in Africa, the Middle East, India and Asia
(China), whereas decreasing trends were reported in the low and midlatitude areas
318
E. Oksanen et al.
representing widespread deciduous species in northern Europe. The experiment
revealed that high humidity is an important climatic factor, causing reduction in
growth rate of the aboveground parts, leaf biomass and area, bud size, sap flux and
earlier bud break and delayed leaf fall. In the belowground parts, root biomass (fineroot biomass in particular) was increased although the root compartment was
exposed to anaerobic conditions, increased soil pH and soil water potential, with
lowered soil respiration and altered microbial and fungal communities. In the leaves,
high humidity shifted the metabolism towards nonstructural carbohydrates, antioxidants and phenolic compounds, while nutrient (N and P) content, photosynthesis,
dark respiration, hydraulic conductance and the density of glandular trichomes were
reduced. The change in the chemical composition of the surface wax layer resulted in
lower hydrophobicity, exposing the leaves for fungal pathogen attacks. In the stem,
N and P content in wood and number of living parenchyma cells were increased,
while the stem wood density was reduced and wood chemistry altered. Reduction in
the growth rates under high humidity was more pronounced in hybrid aspen than in
silver birch. Birch showed more efficient adjustment of leaf and root morphology,
hydraulic architecture, primary carbon and nitrogen metabolism and leaf surface
properties mitigating the negative impacts of high humidity. At the ecosystem level,
high humidity altered soil processes (moisture content, microbiota, nitrification),
with potential to affect competition and species composition. A complementary
laboratory study with birch demonstrated that additional N supply can counteract
the effects of low VPD on cellular metabolism and leaf surface wax quality.
Keywords Aspen, Birch, Forest ecosystem, Growth, Leaf epicuticular waxes,
Metabolism, Nitrogen, Nutrients, Relative humidity (RH), Sap flux, Soil processes,
Trichomes, Vapour pressure deficit (VPD)
1 Introduction
Several climate change scenarios predict increasing precipitation at the northern
latitudes leading to increase in atmospheric water vapour content and wetter conditions (IPCC 2013). The predicted rise in annual precipitation in northern Europe
varies from 4 to 34% for the time period of 2071–2100, depending on the scenario
(Jacob et al. 2014). In addition to soil water availability, plants are affected by the
relative humidity of the surrounding air (often measured as relative humidity, RH, or
as water vapour pressure deficit, VPD). Dai (2006) showed a slightly positive trend
in relative humidity (RH) in the northern hemisphere over the recent decades
(1976–2004). Besides the northern European area, an increase in RH has been
reported over oceans and regionally in Africa, the Middle East, India and Asia
(China), whereas decreasing trends were reported in the low and midlatitude areas
318
E. Oksanen et al.