Consistent response of crown transparency, shoot growth and leaf traits on Norway spruce (Picea abies (L.) H. Karst.) trees along an elevation gradient in northern Italy

doi:10.1016/j.ecolind.2015.09.006

Ecological Indicators

Volume 60, January 2016, Pages 1041-1044

https://doi.org/10.1016/j.ecolind.2015.09.006 Get rights and content

Highlights

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An elevation gradient was used to test tree health indicators in Trentino, Italy.
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Crown transparency, productivity and leaf traits were compared on Picea abies trees.
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A significant effect of elevation was consistently detected by all the indicators.
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Shoot length, needle weight, F_V/F_M, δ¹⁸O decreased when crown transparency increased.
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This pattern is partly independent from the effect of elevation.

Abstract

The significance of crown transparency as forest health indicator has been frequently challenged. Here we examined crown transparency together with damage, crown productivity (shoot length) and leaf traits (needle weight, O and C stable isotopes, chlorophyll a fluorescence) on 9–27 Picea abies (L.) H. Karst. trees along an elevation gradient in Trentino, northern Italy. The gradient was assumed to represent several environmental stressors. A fully randomized sampling design was adopted and data were analyzed by means of one-way ANOVA and partial correlation. Results show that (i) crown transparency and visible damage on trees increase with elevation; (ii) such a pattern is consistent with a reduction of shoot length, needle weight, photosynthetic potential, and δ¹⁸O. Even when the driving effect of elevation is controlled, significant relationships remain between crown transparency and needle weight and δ¹⁸O. These results provide evidence that crown transparency reflects changes in tree condition and health, and are supportive of the use of crown transparency as a rapid method to monitor tree and forest health.

Introduction

Forest health is important for resource managers, policy makers and the society at large, and to evaluate the sustainability of forest management (SFM) (Forest Europe, UNECE and FAO, 2011). Since decades crown transparency and defoliation are key indicators adopted in forest health monitoring programmes in Europe and elsewhere (Innes, 1993, Ferretti and Fischer, 2013). Although based on different definitions (see Eichhorn et al., 2010), crown transparency and defoliation have some similarities: (i) they both provide estimates of the foliage density on a tree crown in comparison with a reference standard, either a real tree (Eichhorn and Roskams, 2013), a photograph (e.g. Mueller and Stierlin, 1990), or a transparency card (Belanger and Anderson, 1992); (ii) they are both assessed according to the same measurement scale, ranging from 0 (no transparency/defoliation) to 100 (dead tree) (Eichhorn et al., 2010; (iii) they have been both criticized because of their subjectivity and their weak connection with robust, objective, quantitative measurements of tree health and vigour (e.g. Innes, 1993). The former is a typical problem with observer-based surveys (e.g. Scott and Hallam, 2002), and can be controlled by Quality Assurance procedures (Ferretti et al., 2013). The latter is more controversial, although various investigations pointed out a distinct loss of periodical basal area increment at increasing levels of crown transparency/defoliation (e.g. Solberg and Tveite, 2000).

Here we investigate the value of crown transparency as a proxy indicator of the actual condition of Norway spruce trees (Picea abies (L.) H. Karst.) in comparison with measurements obtained by objective methods, such as leaf traits (needle biomass, chlorophyll a fluorescence, stable oxygen and carbon isotopes) and annual shoot length. We concentrated on crown transparency (i.e., the additional amount of skylight visible through the target crown compared to the amount of skylight visible through a fully foliated one; Eichhorn et al., 2010) for practical and operational reasons: crown transparency can be promptly assessed in relation to standard photographs (e.g. Mueller and Stierlin, 1990), allows for more robust spatial and temporal comparison (e.g. Redfern and Boswell, 2004), and is consistent with the Italian national assessment method (Bussotti et al., 2008). Leaf traits have a strong predictive power in assessing plant responses to biotic and abiotic stress factors (e.g. Bussotti and Pollastrini, 2015). Shoot and needle morphology are important in relation to solar radiation interception, photosynthetic and carbon sequestration capacity (e.g. Niinemets and Kull, 1995a, Niinemets and Kull, 1995b, Slaney et al., 2007, Pokorný et al., 2010). Measurement of direct chlorophyll a fluorescence is used to assess plant vitality (Kalaji et al., 2014) and provides information on structure and function of photosynthetic apparatus (Strasser et al., 2004). Stable isotopes of carbon (C) and oxygen (O) are valuable tools for understanding tree physiological processes (Gessler et al., 2014) and climate (Hartl-Meier et al., 2014). An increase in carbon isotope δ¹³C in trees can be the result of reduced stomatal conductance and/or increased photosynthetic capacity (Farquhar et al., 1982). Oxygen isotope ratio (δ¹⁸O) is useful to interpret δ¹³C data (Farquhar et al., 1989) and permits to evaluate plant responses to environmental variables (e.g. atmospheric humidity, temperature) that influence stomatal opening.

Norway spruce is the most economically important forest tree species in Europe, widely distributed across the Alps. Understanding whether a relationship exists between crown transparency and objective, quantitative measurements of crown productivity and leaf traits is therefore important, and may open the way towards a functional interpretation of data obtained by forest health monitoring networks. We investigated whether consistency exists between crown transparency and shoot length and the above referred leaf traits in response to an elevation gradient in Trentino, northern Italy. Elevation can be considered as a proxy gradient for several environmental stressors (e.g. climate-related variables, tropospheric ozone) that may affect tree condition and health (De Vries et al., 2014). Actually, a distinct pattern with elevation is well known for oxidative stress like tropospheric ozone (Chevalier et al., 2007) and irradiance (Blumthaler et al., 1997).

Section snippets

Study area and design

The study was carried out in Trentino, northern Italy, over a 920 m x 3200 m area, along an elevation gradient from 800 to 1600 m a.s.l., i.e. within the optimal distribution range of Norway spruce (Aeschimann et al., 2004) (Fig. 1). Nine sampling points were randomly selected, three for each elevation range considered (800–900, 1100–1200, 1500–1600 m a.s.l.) within the gradient. The three mature Norway spruce trees closest to each sampling point were considered. Mean temperature recorded at the

Results and discussion

Mean values for tree circumference, crown transparency, visible damage on tree crown, shoot length, needle weight, F_V/F_M and stable isotopes for the trees at each elevation range are reported in Table 1. Tree circumference (a variable adopted to describe tree size) was not significantly different among elevation ranges. A significant increase of crown transparency and damage (consisting of dead shoots/branches from previous years, and of one single case of resin flow on a branch, all of them

Conclusion

There was a consistent response of morphological, physiological and biometric indicators and crown transparency on Norway spruce along the elevation gradient explored by our study. Crown transparency was negatively and significantly related to crown productivity and leaf traits as measured by quantitative methods. This is promising and supportive of the use of crown transparency/defoliation as a rapid method for large-scale assessment and monitoring of tree and forest condition.

Acknowledgements

This project was supported by the Autonomous Province of Trento, Department for Territory, Agriculture, Environment and Forestry. We are grateful to Parco Naturale Paneveggio – Pale di San Martino for the help in field surveys; to Maria Cristina Viola for biometric analysis and sample preparation for isotopic analysis.

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NoteConsistent response of crown transparency, shoot growth and leaf traits on Norway spruce (Picea abies (L.) H. Karst.) trees along an elevation gradient in northern Italy

Highlights

Abstract

Introduction

Section snippets

Study area and design

Results and discussion

Conclusion

Acknowledgements

J. Photochem. Photobiol. B: Biol.

Ecol. Indic.

For. Ecol. Manag.

For. Ecol. Manag.

A Guide for Visually Assessing Crown Densities of Loblolly and Shortleaf Pines. Res. Note SE-352

Status and changes of tree condition at the CONECOFOR plots, 1996–2005

Ann. CRA

Influence of altitude on ozone levels and variability in the lower troposphere: a ground-based study for western Europe over the period 2001–2004

Atmos. Chem. Phys.