An individual-based growth and competition model for coastal redwood forest restoration

Abstract

Thinning treatments to accelerate coastal redwood forest stand development are in wide application, but managers have yet to identify prescriptions that might best promote Sequoia sempervirens (Lamb. ex D. Don) Endl. (redwood) growth. The creation of successful thinning prescriptions would be aided by identifying the underlying mechanisms governing how individual tree growth responds to competitive environments in coastal redwood forests. We created a spatially explicit individualbased model of tree competition and growth parameterized using surveys of upland redwood forests at Redwood National Park, California. We modeled competition for overstory trees (stems ≥ 20 cm stem diameter at breast height, 1.37 m (dbh)) as growth reductions arising from sizes, distances, and species identity of competitor trees. Our model explained up to half of the variation in individual tree growth, suggesting that neighborhood crowding is an important determinant of growth in this forest type. We used our model to simulate the effects of novel thinning prescriptions (e.g., 40% stand basal area removal) for redwood forest restoration, concluding that these treatments could lead to substantial growth releases, particularly for S. sempervirens. The results of this study, along with continued improvements to our model, will help to determine spacing and species composition that best encourage growth.Résumé : Les traitements d'éclaircie visant à accélérer le développement des peuplements de séquoia côtier (Sequoia sempervirens (Lamb. ex D. Don) Endl.) sont largement appliqués, mais les aménagistes n'ont pas encore identifié les modalités qui optimiseraient la croissance du séquoia. La mise au point de prescriptions adéquates seraient facilitée par l'identification des mécanismes qui déterminent la réaction de croissance des arbres à leur environnement compétitif dans les forêts de séquoia côtier. Nous avons paramétré un modèle de croissance et de compétition spatialement explicite à l'échelle de l'arbre à partir d'inventaires de forêts bien drainées de séquoia du parc national de Séquoia en Californie. Nous avons modélisé la compétition des arbres dominants (arbres d'au moins 20 cm de DHP) en termes de réduction de croissance causée par des arbres concurrents selon leur taille, leur distance et leur espèce. Notre modèle explique jusqu'à 50 % de la variation de la croissance des arbres individuels, ce qui indique que la concurrence des arbres voisins est un facteur important de la croissance dans ce type forestier. Nous avons utilisé notre modèle pour simuler les effets de nouvelles modalités d'éclaircie (p. ex. : prélèvement de 40 % de la surface terrière du peuplement) pour restaurer les forêts de séquoia, ce qui nous a permis de conclure que ces traitements peuvent engendrer des réactions de croissance substantielles, particulièrement dans le cas de S. sempervirens. Les résultats de cette étude, de même que les améliorations continues apportées à notre modèle, devraient aider à déterminer l'espacement et la composition en espèces qui ...

Improving estimates of tree mortality probability using potential growth rate

“…Although the best estimates of a given tree's maximum potential growth rate would require a prohibitive degree of knowledge about a particular tree's genetics and various site factors, a number of recent studies have taken advantage of robust, spatially explicit datasets to estimate the potential diameter growth rate (PDGR) of trees in the absence of competition (Canham et al , 2006Uriarte et al 2004;Coates et al 2009;Gomez-Aparicio et al 2011;Das 2012). PDGR is an estimate of the expected mean potential growth rate without competition for a given species at a given set of sites for a given size tree and should, therefore, be a reasonable measure with which to calculate GI.…”

Climate Impacts on Tree Growth in the Sierra Nevada

“…This is not surprising, as diameter growth is expected to increase with tree size until leveling off or even sometimes decreasing for the largest trees [40], and most of the trees observed in this study have small DBH (mean 20 cm). The parameter linking growth to the competition index (β 2 ) is negative ( Table A5), confirming that growth declines with increasing crowding [40,41]. The average temperature of previous year has a negative effect on growth of PICO, QUCH, PILA, CADE and PIPO, but a positive effect on growth of ABMA and QUKE.…”

“…To model competition, we used the Heygi index, which is the neighbor's diameter divided by the distance to the targeted tree summed over all neighbors: [40,41,[48][49][50][51]. This competition index was correlated with tree growth in other studies in the same geographic area [40,52,53].…”

“…Forest inventory data can also help us in identifying growth drivers [40,41]. Inventory data includes all the trees in a stand, while dendrochronological studies have traditionally targeted trees that are expected to be sensitive to climate, which can introduce bias [42].…”

Regeneration Dynamics of Coast Redwood, a Sprouting Conifer Species: A Review with Implications for Management and Restoration

“…Thinning operations accelerate the development of larger diameter redwood trees [73,86,87], promoting old forest structural features beneficial for creation of wildlife habitat [8]. Modeling early redwood stand growth confirms results from empirical thinning studies: thinning can substantially increase growth of remaining trees and accelerate process of stand development [76]. Berrill et al [77] modeled young plantations with and without thinning and found thinning put stands on a more rapid trajectory towards the desired old forest reference condition.…”