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Gap expansion is the dominant driver of canopy openings in a temperate mountain forest landscape.

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Title: Gap expansion is the dominant driver of canopy openings in a temperate mountain forest landscape.
Authors: Krüger, Kirsten, Senf, Cornelius, Jucker, Tommaso, Pflugmacher, Dirk, Seidl, Rupert
Source: Journal of Ecology; Jul2024, Vol. 112 Issue 7, p1501-1515, 15p
Subject Terms: MOUNTAIN forests, TEMPERATE forests, FOREST canopy gaps, FOREST regeneration, FOREST dynamics, TREE mortality, LANDSCAPE assessment, NATURAL ventilation
Geographic Terms: ALPS
Abstract (English): Natural disturbances are important drivers of forest dynamics, and canopy gaps are their fingerprints in forest ecosystems. Gaps form and persist because of the interplay of tree mortality and regeneration. They can have long‐lasting impacts on ecosystems, yet the temporal dynamics of gap formation and closure remains poorly quantified.We analysed 11,331 canopy gaps and their changes through time across 3999 ha of unmanaged temperate mountain forests at Berchtesgaden National Park (Germany). We assessed gap formation and closure using three repeat lidar acquisitions between 2009 and 2021, analysing canopy height changes at 1 m horizontal resolution. Our objective was to determine the dominant mode of gap formation, distinguishing the creation of new gaps from the expansion of existing ones. Additionally, we studied the rate of gap closure, considering closure from tree regeneration and lateral crown expansion.Gap formation was primarily driven by gap expansion rather than the initiation of new gaps. Gap expansion accounted for 81.3% of gap formation, although new gaps were on average twice as large as gap expansions. Only 1.4% of gaps did not expand over the 12‐year study period, and Norway spruce forests had the highest rate of gap expansion.Overall, gap closure rate (0.74 ha 100 ha−1 year−1) was higher than gap formation (0.58 ha 100 ha−1 year−1) in our study system. Ingrowth of the regenerating tree cohort was the primary mode of gap closure, with lateral crown expansion accounting for 20% of all gap area closed. Mixed‐species stands had the highest rate of gap closure, and gaps <0.1 ha closed faster than larger gaps.Synthesis. While canopy openings are generally small in the European Alps, we show that they keep growing over multiple years, underlining that gap expansion is an important driver of temperate forest dynamics. Canopy gaps closed faster than they were created, highlighting the resilience of European mountain forests to natural disturbances. However, as disturbances are projected to increase under climate change, this resilience might be challenged in the future, requiring a continuous monitoring of gap dynamics as an important early warning indicator of forest change. [ABSTRACT FROM AUTHOR]
Abstract (German): Zusammenfassung: Natürliche Störungen sind wichtige Treiber der Walddynamik, und Lücken im Kronendach sind ihre „Fingerabdrücke" in Waldökosystemen. Lücken entstehen aufgrund des Zusammenspiels von Baumsterben und Baumverjüngung. Sie können langanhaltende Auswirkungen auf Ökosysteme haben, doch die zeitliche Dynamik der Lückenbildung und des Lückenschlusses ist nach wie vor kaum quantifiziert.Wir analysierten 11.331 Lücken im Kronendach und ihre zeitlichen Veränderungen auf 3.999 ha unbewirtschafteter Bergwälder der gemäßigten Zone im Nationalpark Berchtesgaden (Deutschland). Wir quantifizierten die Bildung und Schließung von Lücken anhand von drei wiederholten Lidar‐Aufnahmen zwischen 2009 und 2021 und analysierten die Veränderungen der Baumhöhe mit einer horizontalen Auflösung von 1 m. Unser Ziel war es, den vorherrschenden Modus der Lückenbildung zu bestimmen und dabei die Entstehung neuer Lücken von der Ausdehnung bestehender Lücken zu unterscheiden. Außerdem untersuchten wir die Rate des Lückenschlusses, wobei wir Lückenschluss durch Baumverjüngung und seitliche Kronenausdehnung berücksichtigten.Die Lückenbildung wurde in erster Linie durch Lückenerweiterung und nicht durch die Entstehung neuer Lücken bestimmt. Lückenerweiterungen machten 81,3% der Lückenbildung aus, obwohl die neuen Lücken im Durchschnitt doppelt so groß waren wie die Erweiterungen bestehender Lücken. Nur 1,4% der Lücken vergrößerten sich während des 12‐jährigen Untersuchungszeitraums nicht, wobei Fichtenwälder den höchsten Anteil an Lückenvergrößerungen aufwiesen.Insgesamt war die Lückenschlussrate (0,74 ha 100 ha−1 Jahr−1) in unserem Studiensystem höher als die Rate der Kronenöffnung (0,58 ha 100 ha−1 Jahr−1). Das Einwachsen der sich verjüngenden Baumkohorte war der wichtigste Mechanismus des Lückenschlusses, wobei seitliche Kronenausdehnung 20% der gesamten geschlossenen Lückenfläche ausmachte. Mischbestände wiesen die höchste Lückenschlussrate auf, und Lücken <0,1 ha schlossen sich schneller als größere Lücken.Synthese. Obwohl die Öffnungen im Kronendach in den europäischen Alpen im Allgemeinen klein sind, zeigen wir, dass sie über mehrere Jahre hinweg wachsen. Das dokumentiert, dass die Ausdehnung von Lücken ein wichtiger Mechanismus in der Dynamik gemäßigter Wälder ist. Lücken im Kronendach schließen sich schneller als sie entstehen, was die Resilienz europäischer Bergwälder gegenüber natürlichen Störungen unterstreicht. Da jedoch prognostiziert wird, dass Störungen im Zuge des Klimawandels deutlich zunehmen werden, könnte diese Resilienz in Zukunft in Frage gestellt werden, was eine kontinuierliche Überwachung der Lückendynamik als wichtigen Frühwarnindikator für Waldveränderungen erfordert. [ABSTRACT FROM AUTHOR]
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Database: Complementary Index
Description
Abstract:Natural disturbances are important drivers of forest dynamics, and canopy gaps are their fingerprints in forest ecosystems. Gaps form and persist because of the interplay of tree mortality and regeneration. They can have long‐lasting impacts on ecosystems, yet the temporal dynamics of gap formation and closure remains poorly quantified.We analysed 11,331 canopy gaps and their changes through time across 3999 ha of unmanaged temperate mountain forests at Berchtesgaden National Park (Germany). We assessed gap formation and closure using three repeat lidar acquisitions between 2009 and 2021, analysing canopy height changes at 1 m horizontal resolution. Our objective was to determine the dominant mode of gap formation, distinguishing the creation of new gaps from the expansion of existing ones. Additionally, we studied the rate of gap closure, considering closure from tree regeneration and lateral crown expansion.Gap formation was primarily driven by gap expansion rather than the initiation of new gaps. Gap expansion accounted for 81.3% of gap formation, although new gaps were on average twice as large as gap expansions. Only 1.4% of gaps did not expand over the 12‐year study period, and Norway spruce forests had the highest rate of gap expansion.Overall, gap closure rate (0.74 ha 100 ha−1 year−1) was higher than gap formation (0.58 ha 100 ha−1 year−1) in our study system. Ingrowth of the regenerating tree cohort was the primary mode of gap closure, with lateral crown expansion accounting for 20% of all gap area closed. Mixed‐species stands had the highest rate of gap closure, and gaps <0.1 ha closed faster than larger gaps.Synthesis. While canopy openings are generally small in the European Alps, we show that they keep growing over multiple years, underlining that gap expansion is an important driver of temperate forest dynamics. Canopy gaps closed faster than they were created, highlighting the resilience of European mountain forests to natural disturbances. However, as disturbances are projected to increase under climate change, this resilience might be challenged in the future, requiring a continuous monitoring of gap dynamics as an important early warning indicator of forest change. [ABSTRACT FROM AUTHOR]
ISSN:00220477
DOI:10.1111/1365-2745.14320