Thinning Response and Potential Basal Area in a Mixed Sub-humid Low-elevation Oak-Hornbeam Forest

doi:10.21203/rs.3.rs-698917/v1

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Thinning Response and Potential Basal Area in a Mixed Sub-humid Low-elevation Oak-Hornbeam Forest

https://doi.org/10.21203/rs.3.rs-698917/v1

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Competition for resources (light, water, nutrients, etc.) limits the size and abundance of alive trees a site can support. This carrying capacity determines the potential carbon sequestration in alive trees as well as the maximum growing stock. Lower stocking through thinning can change growth and mortality. We were interested in the relations between stand structure, increment and mortality using a long-unmanaged oak-hornbeam forest near Vienna, Austria, as case study. We expected lower increment for heavy thinned compared to unmanaged stands. We tested the thinning response using three permanent growth plots, whereas two were thinned (50% and 70% basal area removed) and one remained unmanaged. We calculated stand structure (basal area, stem density, diameter distribution) and increment and mortality of single trees. The heavy thinned stand had over ten years similar increment as the moderate thinned and unthinned stands. Basal area of the unthinned stand remained constant and stem density decreased due to competition-related mortality. The studied oak-hornbeam stands responded well even to late and heavy thinning suggesting a broad “plateau” of stocking and increment for these forest types. Lower stem density for thinned stands lead to much larger tree increment of single trees, compared to the unthinned reference. The findings of this study need verification for other soil and climatic conditions.

Forestry

Quercus petraea

Carpinus betulus

abandoned forest management

conservation

biodiversity

tree growth

Oaks (Quercus spp.) are widespread worldwide and Quercus petraea (Matt.) Liebl. (sessile oak) grows in Europe over an area of about 300 Million hectares (Caudullo et al. 2017). In Austria, Quercus petraea together with Quercus robur L. (pedunculate oak) are after Fagus sylvatica L. (European beech) the second most important broadleaf species, in terms of growing stock, increment and harvesting (BFW 2017). Q. petraea and Q. robur forests cover an area of about 69,000 hectares in Austria, which is about 2% of the total forest area in Austria. Not much in absolute terms, oak-dominated forests in Austria are common at low elevations near cities and probably have been managed since early times of settlement (Haneca et al. 2005).

Large parts of the original oak-dominated forests have been lost to settlement and agriculture and the remaining forests are often under some form of conservation, such as the Vienna Woods near the city of Vienna in Austria. With about 75% the largest part of the Vienna woods is now a UNESCO biosphere reserve of about 105,000 ha (Pötzelsberger et al. 2015). Oak species are common at the lower elevations of the Vienna woods and on pseudogley and shallow soils, where European beech is less competitive and productivity is lower. These forests are unmanaged or only extensively managed, should approach their carrying capacity and eventually reach old-growth conditions with diverse stand structure, featuring significant number of large old trees (Oettel et al. 2020). Stand structure is not only important for habitat conditions (Mölder et al. 2019), regeneration (Ortmann-Ajkai et al. 2016) and carbon sequestration (Bruckman et al. 2016), but also broadly determine tree growth (Zeller & Pretzsch 2019, Stimm et al. 2021).

Thinning reduces the numbers of trees in a stand and thus concentrates the growth on fewer trees, which grow faster than under unthinned conditions (Assmann 1970). Stand increment can be increased by thinning, compared to unthinned conditions, when forests are water-limited and interception losses in dense stands occurr (Gavinet et al. 2020). This is also supported by Assmann (1970) working on temperate forests and suggesting that increment of thinned stands can be higher than an unthinned stand. Assmann (1970) then introduced the concept of an optimal basal area, where stand increment is highest. The research of the influential forester Emil Assmann was summarized by Pretzsch et al. (2015) and has still relevance as increment determines the rate of carbon uptake and basal area determines the carbon stocks in trees (Hochbichler et al. 2006, Suchomel et al. 2012). On the other hand, thinning cannot increase volume stocks and the volume is in general highest in unthinned stands, composed of a large number of trees allthough with smaller dimensions (Río et al. 2008). Average thinning intensity is preferred by visitors, compared to very dense stands (Jensen & Skovsgaard 2009) and thus improve the recreation value of urban forests.

Q. petraea stands in Czech Republic responded well to thinning with a clear effect on diameter increment, less on height increment (Fedorová et al. 2016). Thinning has been shown to reduce adverse effects of droughts and increases drought resistance of Q. petraea in France (Breda et al. 1995, Schmitt et al. 2020). Survival of sessile oak growing in Hungary was found to be moderately increased after thinning, with even stronger positive effects for coexisting Carpinus betulus L., the European hornbeam (Tinya et al. 2020). Improving survival is particularly important for oak forests, where summer conditions are usually critical for growth (Führer et al. 2011). We are not aware of studies for Austrian mixed oak-hornbeam stands on thinning response and relations between basal area and stand increment.

There are indications that the forest carbon stocks are saturating (Canadell et al. 2007), that is, the potential for further net carbon uptake is decreasing, in Europe (Nabuurs et al. 2013) and in Amazonia (Hubau et al. 2020). This saturation is often coupled with unmanaged forests and lack of recent disturbances (Seidl et al. 2014), both can considerably reduce carbon stocks, but allow for continued net carbon uptake. In forests, that are in equilibrium (steady-state of carbon pools), gains and losses balance each other (Paul et al. 2021). We hypothesize that net gain (increment minus losses) approaches zero, when potential stocking is reached (i.e., growing stock or basal area is constant and not increasing further). We test this hypothesis for low-elevation sub-humid oak-hornbeam forests near the city of Vienna. This study has the following objectives: (1) to determine the stand structure of oak-hornbeam stands in urban forests near Vienna, and (2) to explore relations between stocking (basal area), increment and mortality for three management alternatives, unthinned, moderate and heavy thinning.

Data

We used data from a thinning trial established in 2009 in a mixed oak-hornbeam forest in the ‘Vienna Woods’, near Vienna, Austria (Fig. 1). The site locations are approximatly 48.22°N and 16.24°E. The forests in the study region are owned and managed by the Vienna Forest Administration (public entity of the City of Vienna). After World War II, large parts of the forests near Vienna were harvested to meet the demand for wood and the current stands are a result of that. Thus the stands in this area have similar age and stand structure and have been unthinned prior 2009. Parts of the Vienna Woods are now UNESCO biosphere reserve. Average annual precipitation is 630 mm, average annual potential evapotranspiration 768 mm and average daily temperature 9.4°C (Fick & Hijmans 2017, Abatzoglou et al. 2018). The aridity index using the UNEP definition is 0.82 mm/mm (annual precipitation sum / potential evapotranspiration) and Führer’s aridity index is 6.64°C/mm (Führer et al. 2011). From 1960 to present, in less than ten years precipitation exceeded potential evapotranspiration (Fig. A1), confirming that these forests are usually water-limited (sensu Boisvenue and Running 2006; Creed et al. 2014). More details on the study region can be found elsewhere (Pötzelsberger et al. 2015).

There are three plots with a size between 0.2 to 0.25 hectares. Larger plot size was favoured over replications, as the terrain is heterogeneous and replicating the plots would have compromised the comparability of the site conditions. Within the same area a grid-based forest inventory system (not used here) was established, using angle count sampling (‘Bitterlich sampling’) (Hasenauer & Eastaugh 2012). The elevation is about 300 m above sea level. The three plots are located within 20 m distance (about one tree length) along a footpath / skid trail used harvesting (Fig. 1b)

Two plots were thinned in 2009 (chainsaw and skidding on the ground) and one plot was kept unthinned as control. The treatments were (1) moderate thinning (removal of ~ 50% basal area) and (2) heavy thinning (removal of ~ 70% basal area). The moderate thinning intended to induce more regeneration from seeds by more homogenous seed fall, in particular of Quercus spp. The thinning targeted foremost small-middle sized trees until about 30 cm DBH to promote growth of larger-sized retained trees, to achieve valuable dimensions.

The plots were measured three times, 2009, 2014 and 2019. For every tree reaching breast height (1.3 m above ground) the diameter at breast height (DBH), tree height, tree status (alive, standing dead, harvested) and species was determined.

Analysis

We computed the following stand structure metrics, basal area, stem density, average tree height, stand volume (using functions of Pollanschütz 1974) and yield class, defined as average periodic increment at stand age of 100 years using the yield table ‘Eiche Ungarn’ (Marschall 1992). We calculated species mixture (%) based on contribution to basal area of sessile oak, Quercus petraea (Matt.) Liebl. (pooled with occasional Turkey oak, Quercus cerris L.), European hornbeam, Carpinus betulus L. and European beech, Fagus sylvatica L.. We calculated these stand structure metrics for 2009, 2014 and 2019 and for the two thinned plots, for both before and after thinning in 2009.

We calculated stem density by 10-cm diameter classes to explore potential changes in stand structure and contributions of species to overall stem density, shown in Table 1. We calculated stand-level increment and losses (BA2 is zero) for the two periods with remeasurements (2009–2014 and 2014–2019, each spanning 5 years), after Eq. 1.

Table 1

Stand structure of three stands in year 2009. For thinned stands we show condition before thinning (“2009 –“) and after thinning (“2009 +“). BA is basal area at breast height (1.3 m above ground) and NHA is stem density. Mixture represent share of sessile Oak, *Quercus petraea* (Matt.) Liebl. (pooled with Turkey oak, *Quercus cerris* L.), European hornbeam, *Carpinus betulus* L., and European beech, *Fagus sylvatica* L., in percent of BA (separated by commas). Volume is stem volume using the Austrian form factor functions (Pollanschütz 1974). Yield class is the cummulated increment at age 100 years using Austrian yield tables (Marschall 1992).
	Unthinned	Moderate thinning		Heavy thinning
Measurement date	2009	2009 -	2009 +	2009 -	2009 +
BA (m² ha^− 1)	37.3	37.0	16.9	36.4	11.1
NHA (ha^− 1)	1736	2170	410	2135	180
Mixture (%)	88, 12, <1	69, 30, 1	87, 12, 1	59, 41, <1	91, 8, 1
Mean tree height (m)	12.2	12.6	17.1	12.9	17.9
Volume (m³ ha^− 1)	327	308	157	300	106
Yield class (m³ ha^− 1 yr^− 1)	4.5	4.5		4.5
Plot size (ha)	0.25	0.2		0.2

BA_inc = (BA2-BA1)/5 (1)

We explored the increment of single trees by calculation annual diameter increment per tree and analysed the role of tree size on increment using DBH. As few trees had DBH greater 40 cm, we separated 0–20 cm DBH and > 20 cm DBH trees and used Welch’s t-test to check for significant differences in diameter increment with tree size. We finally compared tree survival over a 10-year period between 2009 (after thinning) and 2019 in understanding the combined effects of tree size and thinning intensity on tree survival.

Thinning and stand structure

The basal area of unthinned oak-hornbeam forests near the city of Vienna does not increase after reaching 38-40 m² ha^-1 (Fig. 2a). While basal area remains largely constant, stem density keeps decreasing for the unthinned stand and thus the average tree size increases (Fig. 2b). For thinned forests on the other hand, basal area is increasing and approaching the basal area of the unthinned stand. The net increase of basal area is caused by increment exceeding mortality independent (see next section). We also noted a constant stem density for thinned forests (independent of thinning intensity) and no mortality and no recruitment were observed within 10 years after thinning (Fig. 2b). The thinnings removed proportionally more stem density, than basal area, and in consequence, the harvesting targeted smaller diameter stems (see also diameter distribution in Fig. A2).

The stem density distributions of the three stands before thinning in 2009 were similar (Fig. A2). We noted initially more trees with 10-20 cm DBH in the heavily thinned stand, while the moderately thinned stand had fewer trees with 0-10 cm and 20-30 cm DBH than the two other stands (Fig. A2a). The thinning removed trees with diameters smaller 30 cm and the moderate thinning retained trees with 0-10 cm with about 60 ha^-1 partly (Fig. A2b). For the unthinned stand, stem density was decreasing over, particularly for trees below 20 cm DBH. From these results, we cannot determine the processes behind the observed changes in stand structure over time. Since each tree was observed separately, we were able to analyse increment and losses in terms of basal area.

Increment and losses

We measured the plots three times after thinning in 2009 and calculated increment and losses (i.e., mortality and harvesting) for two periods, 2009-2014 and 2014-2019. The balance of increment and losses determines the net change, whether a stand accumulates or loses basal area. The forests in the study region were mixed and composed predominantly of oak and hornbeam (Table 1, Fig. A3). In Fig. 3, we show increment and losses separeted for oak, hornbeam and beech – a minor admixed species. hornbeam and beech had smaller diameters (Fig. A3).

Between 2009-2014, immediately after thinning in 2009, the increment ranged between 0.54 and 0.69 m² ha^-1 year^-1 and oak contribute by far most (>85%) to increment. The stand with highest basal area (unthinned, 37 m² ha^-1) had the second largest increment (0.58 m² ha^-1 year^-1) and the stand with lowest basal area (coppice with standards, 11 m² ha^-1) was fastest growing (0.69 m² ha^-1 year^-1). Smallest increment was obsered between 2009 and 2014 for the moderatly thinned stand (high forest conversion, 16 m² ha^-1) with 0.54 m² ha^-1 year^-1. Increment was considerable lower than losses for the two thinned stands, as harvesting removed 30-50% of basal area and the ratio of between losses and increment was about 7.4, thus seven times more basal area was lost then gained by increment (Fig. 3). The net change for unthinned stand, on the other hand, was positive (0.58 - 0.29 = +0.29 m² ha^-1 year^-1).

Five to ten years after thinning in 2014-2019, these patterns considerably changed. The unthinned stand lost three times more basal area (solely due to mortality) than grew through increment. The climate conditions did not vary considerable between the two periods (Fig. A1). The stand growing less in the previous period (high forest conversion), was now the only stand with a positive net change in basal area (Fig. 3). We note here that despite the reduction of plot basal area by 30-50%, there was still considerable mortality in the two thinned stands, allthough three times smaller than under unthinned conditions. The largest loss due to mortality was found for the stand with highest basal area (unthinned in 2014-2019, Figs 2, 3). Thinning targeted foremost small diameter trees (Fig. 3) and also natural mortality under unthinned conditions was more frequent for trees < 20 cm DBH (Fig. A4). We were next interested, whether thinning (naturally through mortality or artificially through harvesting) has lead to an increase in diameters of larger trees.

Larger trees grew faster across the three studied stands based on 10-year diameter increment between 2000 and 2019 and grouped using 20 cm DBH to separate large and small trees (Fig. 4ac). Small trees were more abundant than large trees (Fig. 4bd). A more detail grouping of trees according to their DBH show that only few trees hat DBH larger 40 cm (Figs A2-A4). For the dense, unthinned stand, we noted a significant effect of tree size on diameter increment, with large trees growing significantly faster (p < 0.001). For the two thinned stands, the faster increment of large trees was not significantly different from small trees (p 0.123 heavy thinning, p 0.149 moderate thinning). There was a clear link between faster diameter increment and thinning intensity (see also Fig. A5). Under unthinned conditions, diameter increment was smallest (Fig. 4), consistent with larger stem density but comparable increment on plot level (Table 2, Fig. 2). Thinning increased the diameter increment 2-2.5 fold for large trees > 20 cm DBH and even 7-9 times for trees with DBH smaller 20 cm. Diameter increment was not significantly by thinning intensity, for both large and small trees, despite larger average increment for the strongly thinned stand (Fig. 4). Comparing both thinned stands with unthinned conditions showed significantly larger increment associated with thinning, irrespective whether 30 or 50% of basal area are removed.

Our study indicates (1) that the studied oak-hornbeam forests respond well to thinning, both on stand and tree-level, (2) that maximum basal area is reached at 35–40 m² ha^− 1, where increment stagnates and mortality becomes more frequent and (3) both harvesting and natural mortality result in reduction of the density of small diameter trees, while harvesting concentrates the growth on the (fewer) remaining trees more efficiently. We discuss below these topics separatly and their relevance for carbon sequestration and habitat value.

Thinning response

The thinning applied in the study region removed a considerable share (50–70%) of basal area, mostly composed of trees with DBH smaller 30 cm. This reduced the basal area and in-situ carbon storage, assuming that basal area is correlated with tree carbon (Burrows et al. 2002). Basal area measurements require less time (e.g. using sampling-by-diameter, Bitterlich 1948; Hasenauer and Eastaugh 2012) than measuring both diameters and tree height – needed to accuratly estimate tree volume using allometric function (Pollanschütz 1974, Repola 2008, Ruiz-Peinado et al. 2012). In this study, the reduction in tree carbon by removing small diameter trees (‘thinning from below’) is most likely smaller, as mostly hornbeam trees with smaller diameter and smaller tree height were removed (Fig. A6). A large number of small trees have collectivly a smaller volume than few large trees (Neumann et al. 2016).

Diameter increment increased more than two-fold for trees larger 20 cm DBH, thus the carbon allocation into larger sized wood is more than doubled (Fig. 4). For trees smaller 20 cm DBH the increase in increment by thinning is even larger, in particular after removing 50% of basal area. Large diameter coarse woody debris has slower decay and thus longer carbon residence time (Harmon et al. 2020). Additional benefits of more and larger-sized trees include more flexibility in potential wood products (Pukkala 2017) and more diverse habitat, for instance for hollow-dwelling animals and other organisms specialized on large-dimension wood (Thom & Seidl 2016). For carbon accounting purposes, internal decay of large trees need consideration to avoid overestimating biomass and in consequence carbon stocks by applying allometric functions (Roxburgh et al. 2006, Dean et al. 2018).

Maximum basal area

Our results suggest that the studied oak-hornbeam stands have maximum basal area of 35–40 m² ha^− 1 (Fig. 2). In the unthinned stand, increment range between 0.25 and 0.6 m² ha^− 1 year^− 1 and mortality due to to competation is in the same range. The basal area remained largely unchanged over the 10-year measurement period. There is scant published inforamtion on the maximum basal area of unmanaged oak-hornbeam forests. Grotti et al. (2019) reported for oak-hornbeam stands on floodplains in Northern Italy stagnating basal area at about 25–30 m² ha^− 1. With an average temperature of 13.2°C and annual precipitation sum 658 mm, the stands studied by Grotti et al. (2019) had similar precipitation but allmost 4°C higher temperature. These stands were reportedly unmanaged since the 1980s and earlier a coppice with standards system was in place (similar to the management applied on the intensively thinned stand of this study).

In Romania, the Runcu-Grosi Natural Reserve has stem density of 713 ha^− 1, growing stock of 577 m³ ha^− 1 and basal area of 39.3 m² ha^− 1 (Petritan et al. 2012). Annual temperature ranged between 7.6 and 9.4°C and precipitation sum from 750 to 925 mm and thus the Romanian stands had similar temperature and higher precipitation than the stands used in this study (9.4°C, 630 mm) based on gridded climate data (Fick & Hijmans 2017). In a nearby study (about 2 km northwest of our study site) an ‘overmature’ coppice (dominated by oak and hornbeam) had stem density (> 5 cm DBH) of 1881 ha^− 1, growing stock of 412 m³ ha^− 1 and basal area of 45.6 m² ha^− 1 (Bruckman et al. 2016). Bruckman et al. (2016) did not measure or report increment or mortality, thus we can only assume that these stands were close to their maximum basal area. This suggests that oak-hornbeam stands in this region can have up to 46 m² ha^− 1 and thus about 6 m² ha^− 1 more than observed in this study. We have to assume that Bruckman et al. (2016) included dead trees in their basal area (i.e., carbon study reporting aboveground biomass, but not standing deadwood). We note, that in this study dead trees were not tallied.

The available literature suggest that oak-hornbeam forests have maximum basal area of about 40 m² ha^− 1, where carbon pool in alive trees is highest (using basal area as proxy for carbon). This study provides evidence that increment stagnates at this point and mortality exceeds increment (cf. Figure 4) resulting in net decrease of basal area/carbon in alive trees. Plotting increment versus basal area (sensu Assmann 1970) indicates that oak-hornbeam stands might have a rather wide ‘plateau’ where basal area is largely unaffected by decreasing basal area (Fig. 5). This was an unexpected result as initially we expected a stronger decrease in increment for the heavy thinned stand. This means (1) that oak and hornbeam are plastic tree species and utilize the additional available resources (light, water, nutrients) made available through thinning and (2) that oak-hornbeam stands can be thinned by up to 70% basal area without decreasing increment. It is important to maintain crown cover across the area with even distribution of retained trees and targetting foremost small-mid diameter trees, to reach the same outcome as found by this study. Further research to test the effects of thinning targetting large diameters such as selective final harvest or shelterwood cut.

Harvesting versus natural mortality

Tree death by harvesting or mortality results in decreasing stem density. Over ten years natural mortality reduced stem density by about 250 ha^− 1, while thinning removed 1760–1955 ha^− 1. Assuming net basal area increment of 0.25 m² ha^− 1 year^− 1, it will take another 60–70 years to reach pre-thinning basal area. A thinned stand will be composed of a smaller number of large trees (Fig. A4), than can be expected under unthinned conditions. The trees in the studied stands grow in height until about 30 cm DBH reaching their maximum height at about 25 m (Fig. A5). Since thinning targetted mostly small trees, the average tree height in the two thinned stands increase by 4.5 to 5 m (Table 1). As tree volume can be estimated as the product of basal area, average tree height and a form factor, the increase in average tree height through thinning partly compensates for the lower basal area after thinning. In consequence, considering two stands with the same basal area but different tree diameters (and in consequence tree height) the stand with large trees can be expected to have larger stand volume.

In conclusion, harvesting speeds up the natural process of tree mortality. Thinned stands have for several decades lower basal area than unthinned stands. Evaluating the effects of thinning on forest carbon stocks need measurements of standing and lying deadwood and coarse woody debris (Woldendorp et al. 2004, Harmon et al. 2020), reliable biomass functions and accurate tree height measurements (Neumann et al. 2016, Stereńczak et al. 2019). This case study provides some evidence that oak-hornbeam forests may have a broad “plateau-like” relationship between increment and stocking (Assmann 1970), when stands approach their maximum basal area. Our results encourage verifying whether similar patterns can be found on contrasting soil and climate condition and other widespread Central-European tree species, such as Fagus sylvativa or Picea abies.

We demonstrate that the Oak-hornbeam forests studied here have a maximum basal area of 35–40 m²/ha, where further net accumulation of basal area slows considerably. This decrease in net accumulation is caused by increasing mortality and stagnating increment as stands approach their carrying capacity. We cannot generalize this behaviour for other forests and tree species. There is, however, increasing evidence globally for diminishing net gain as forest ecosystems approach their steady state, e.g. in New Zealand (Paul et al. 2021) and Africa and Amazonia (Hubau et al. 2020). This study suggests, that for continued uptake (i.e., carbon, biomass or volume), the stocking of forest ecosystems has to be below the potential steady state; this reduction is possible through forest management or natural disturbances.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Acknowledgements

We thank the Viennese Forest Administration for permission to establish and maintain the research sites. We are grateful for the logistic support and field measurements by Raphael Klumpp and Franz Reinthaler.

Authors’ contributions

MN conceived the idea of the study, analysed the data and wrote the manuscript. HH designed the experimental design, coordinated the fieldwork and helped with interpretation and statistical analysis. All authors contributed to the drafts and gave approval to publication.

Funding

The study was supported by the University of Natural Resources and Life Sciences, Vienna.

Availability of data and materials

The data used in this analysis is available over https://figshare.com/s/b745b22d333ffbda883c (this preliminary link will be replaced with final DOI upon acceptance) and is also available from the corresponding author upon request.

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Thinning Response and Potential Basal Area in a Mixed Sub-humid Low-elevation Oak-Hornbeam Forest

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Thinning response

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