Increasing air humidity - a climate change factor in Northern latitudes - alters wood chemistry of silver birch and hybrid aspen

Transcription

1 Increasing air humidity - a climate change factor in Northern latitudes - alters wood chemistry of silver birch and hybrid aspen Co-authors: Priit Kupper 1, Arne Sellin 1, Reimo Lutter 2, Linnar Pärn 2, Hardi Tullus 2, Krista Lõhmus 1, Anu Sõber 1 1 University of Tartu 2 Estonian University of Life Sciences Arvo Tullus postdoctoral researcher Chair of Ecophysiology Department of Botany University of Tartu Workshop in Uppsala 11th 12th of November 2013

2 Introduction and aims Wood chemistry varies among and within tree species, it reflects the impact of environmental conditions on wood formation during xylogenesis and it is essential for forestry as it determines the industrial usability of the given species, nutrient removal with harvest and consequent need for fertilization. Knowledge about climate change effects on wood chemistry is scanty and mainly restricted to elevated temperature, CO 2 and O 3. Alterations in these factors have been shown to cause significant changes in wood chemistry of Betula and Populus spp (e.g. Kaakinen et al. 2004; Kostiainen et al. 2008; Richet et al. 2011; ). The aim of the current study was to analyse the stemwood chemistry in young saplings of silver birch (Betula pendula Roth) and hybrid aspen (Populus tremula L. P. tremuloides Michx.) grown for four years under artificially elevated air humidity. The study was conducted in the Free Air Humidity Manipulation (FAHM) research facility in Estonia. Why humidity? What is FAHM research facility?

3 Why to study increasing air humidity? Due to global warming, wetter conditions (increased precipitation and air humidity) are predicted in Northern Europe (IPCC 2007; IPCC 2013; EEA 2012). Figure 1. Projected changes in annual (left) and summer (right) precipitation (%) between and (European Environment Agency Report 2012) Warmer air can hold more water vapour, more frequent rain events mean that time when relative air humidity is high, will also increase. Water vapour acts as one of the most important greenhouse gases, which is believed to cause up to two thirds of the greehouse effect.

4 Free Air Humidity Manipulation (FAHM) experiment The plant responses to altered air humidity have been studied mainly in growth-chamber experiments. Ecosystem responses to such changes have not been studied under field conditions. From a unique open-air research facility was established in Estonia. Free Air Humidity Manipulation (FAHM) is the first free-air experiment, where the long-term effect of altered atmospheric humidity on growth and development of trees and functioning of forest ecosystems is studied. Photo of the FAHM site in summer 2009

5 H4 C4 FAHM N E C3 H2 D1 C1 H3 Aerial photo: June C2 H1 The FAHM experimental facility is located at Järvselja Experimental Forest District in North-East Estonia. It is a 2.7 ha fenced area where nine experimental circle plots (d = 14m) are situated. Three experimental plots act as control plots. In three plots the relative air humidity (RH) is elevated 7% over ambient level. Three experimental plots were open-top plots from (one is a control plot, one is a humidification plot and in one plot air humidity is decreased below the ambient level). Half of each plot was planted with silver birch (Betula pendula) and another half with hybrid aspen (Populus tremula x P. tremuloides) in The experimental plots are surrounded by a buffer zone, which is a hybrid aspen plantation.

6 Spring 2006 Summer 2012 Humidity manipulation experiment started in 2008 and has been running during all growing seasons (May-Oct) since then. The first experimental period with silver birches ended in 2011, after that a new birch generation was established with planted seedlings. Hybrid aspens were harvested in 2012 and a new generation emerged vegetatively from root and stump sprouts.

7 Humidification treatment Relative humidity inside the plots is elevated by ca 7 units (%) using misting technique (a droplet size ca 10 μm) and FACE-like technology to mix humidified air inside the plots. Humidification is applied when RH < 75%, air temperature is > 10 C and wind speed < 4 m/s.

8 Methods Altogether 43 silver birch and 48 hyrid aspen model trees were selected for wood analysis at the end of the first FAHM study period (birches: autumn 2011, aspens: autumn 2012), when all trees were harvested in the plots. From each model tree wood sample from the stem section which had formed and grown during four years under treatment (birches: , aspens: ) was separated. The following chemical properties of stemwood were analysed: concentrations of structural carbohydrates (cellulose, acid insoluble lignin, hemicellulose); concentrations of macronutrients (N, P, K); calorific value; ash content; Wood density was also analysed.

9 Results: above-ground growth of trees Height and diameter increment of all hybrid aspens (n = 342) in FAHM experimental plots. Arrows show the direction of statistically significant difference between control (C) and humidified (H) plots. Growth characteristics of the harvested model trees (n = 91). Asterisks show significance of treatment effect. Letters indicate significant differences between tree species.

10 Results: above-ground growth of trees Height and diameter increment of all silver birches (n = 330) in FAHM experimental plots. Arrows show the direction of statistically significant difference between control (C) and humidified (H) plots. Growth characteristics of the harvested model trees (n = 91). Asterisks show significance of treatment effect. Letters indicate significant differences between tree species.

11 Physiological, functional and morphological responses Higher air humidity caused ca 20% reduction in the water vapour pressure difference (VPD) between the plant leaf interior and the surrounding atmosphere, a primary driving force of transpiration. Transpiration (sap flow, sap flux density) and photosynthetic capacity (A max, V cmax, J max ) of both birches and aspens decreased under elevated humidity (Kupper et al., 2011: Env. Exp. Bot.; Tullus et al., 2012: PLoS ONE; Sellin et al., 2013: Ecol. Res.). Tree leaves were smaller in H plots, and contained less N and P. This could be partly due to the decreased transpirational flux from roots to leaves and transpiration-driven mass flow of nutrients in soil. Generally birches showed better adaptability with elevated humidity. Leaf scenescence and leaf fall of birches was postponed in H plots resulting in longer growing season, similar reaction has been observed in CO 2 enrichment staudies with birches (Godbold et al., in prep.). Birches formed longer and thinner short roots in H plots, which can be interpreted as a morphological adaptation leading to an increase in the absorptive area, however such response declined with age (Parts et al., For. Ecol. Manage., in press).

12 Elevated humidity effect on macronutrients in wood Macronutrient concentrations in stemwood of birches (Bir) and aspens (Asp). Asterisks show significance of treatment effect. Letters indicate significant differences between tree species. Macronutrients in stemwood can be found in different forms in living parenchyma cells as enzymes, structural proteins and storage proteins formed during leaf scenescence as a result of nutrient retranslocation. As macronutrient content in tree leaves was generally smaller in H plots (Tullus et al. 2012; Sellin et al. 2013), higher quantity of living cells looks more plausible explanation, currently dead/live cell ratio is analysed from microscope images. Lower K content and especially lower K/N ratio in H plots could indicate lower need for osmolytes to maintain turgor in living cells in H plots, where transpirational flux was generally lower (Kupper et al. 2011; Tullus et al. 2012; Sellin et al. 2013). Higher K content in C trees could reflect upregulation of xylem hydraulic conductivity to support higher water fluxes through the stem.

13 Elevated humidity effect on structural carbohydrates in wood Structural carbohydrate concentrations in stemwood of birches (Bir) and aspens (Asp). Asterisks show significance of treatment effect. Letters indicate significant differences between tree species. Significant differences between tree species. In H plots birch wood had significantly lower and aspen wood higher cellulose concentration compared to the control, however the differences were very small. Lignin concentration showed similar although statistically insignificant trend in both species, being higher in H plots, which could be a result of more stressful conditions. Hemi-cellulose concentration in aspen wood was positively correlated with growth rate, which could explain its significantly lower average content in H plots.

14 Elevated humidity effect on density, calorific value and ash content of wood Density, calorific value and ash content of stemwood of birches (Bir) and aspens (Asp). Asterisks show significance of treatment effect. Letters indicate significant differences between tree species. Significant differences between tree species. These were 6-yr-old model trees, where analysed wood had formed during the last four years under treatment. Previous results with birches grown for two years under treatment indicated significantly lower wood density in H plots, which was explained by possible differences in xylem anatomy (Sellin et al. 2013).

15 Conclusions Increasing air humidity will probably cause significant changes in functioning of Northern forest ecosystems. It would be beneficial to include/update humidity effect in long-term forest growth prediction models although more empirical studies are needed including also ohter species (conifers) and their mixtures. The major effect of elevated humidity on wood chemistry of birch and aspen was altered macronutrient content in dormant-season-harvested stems, with minor changes in structural carbohydrates. In short-rotation forestry with fast-growing deciduous trees, can we expect changes in nutrient removal per unit of harvested biomass? The observed responses during the first 5-yr study period of the FAHM experiment suggest that the expected climate-change-induced increase in the growth rate of trees at northern latitudes (boreal areas) due to the earlier start of the growing season in spring or higher carbon assimilation rate could be smaller than expected if temperature rise is accompanied by a rise in atmospheric humidity.

16 FAHM website: Thank you! Environmental Conservation and Environmental Technology R&D (KESTA) project ( ) Biosphere and atmosphere related R&D in the Estonian Environmental Observatory (BioAtmos)