Western juniper and ponderosa pine ecotonal climate–growth relationships across landscape gradients in southern Oregon

Abstract

Forecasts of climate change for the Pacific northwestern United States predict warmer temperatures, increased winter precipitation, and drier summers. Prediction of forest growth responses to these climate fluctuations requires identification of climatic variables limiting tree growth, particularly at limits of tree species distributions. We addressed this problem at the pine-woodland ecotone using tree-ring data for western juniper (Juniperus occidentalis var. occidentalis Hook.) and ponderosa pine (Pinus ponderosa Dougl. ex Loud.) from southern Oregon. Annual growth chronologies for 1950-2000 were developed for each species at 17 locations. Correlation and linear regression of climate-growth relationships revealed that radial growth in both species is highly dependent on October-June precipitation events that recharge growing season soil water. Mean annual radial growth for the nine driest years suggests that annual growth in both species is more sensitive to drought at lower elevations and sites with steeper slopes and sandy or rocky soils. Future increases in winter precipitation could increase productivity in both species at the pine-woodland ecotone. Growth responses, however, will also likely vary across landscape features, and our findings suggest that heightened sensitivity to future drought periods and increased temperatures in the two species will predominantly occur at lower elevation sites with poor water-holding capacities.Résumé : Les changements climatiques envisagés pour la région nord-ouest du Pacifique, aux É tats-Unis, prédisent des températures plus chaudes, une augmentation des précipitations en hiver et des étés plus secs. La prédiction de la réaction en croissance des forêts à ces fluctuations climatiques exige qu'on identifie les variables climatiques qui limitent la croissance des arbres, particulièrement à la limite de l'aire de répartition des espèces d'arbres. Nous abordons ce problème à la limite de la forêt claire de pin à l'aide de données dendrochronologiques pour le genévrier occidental (Juniperus occidentalis var. occidentalis Hook.) et le pin ponderosa (Pinus ponderosa Dougl. ex Loud.) dans le sud de l'Oregon. Des chronologies de la croissance annuelle de 1950 à 2000 ont été construites pour chaque espèce à 17 endroits. Les analyses de corrélation et de régression linéaire des relations entre le climat et la croissance ont montré que la croissance radiale chez les deux espèces est fortement dépendante des épisodes de précipitations d'octobre à juin parce qu'elles rechargent le sol en eau pour la saison de croissance. La croissance radiale annuelle moyenne pour les neuf années les plus sèches indique que la croissance annuelle des deux espèces est plus sensible à la sécheresse à basse altitude ainsi que dans les stations avec une pente plus abrupte et un sol sableux ou rocheux. L'augmentation future des précipitations hivernales pourrait accroître la productivité des deux espèces à la limite de la forêt claire de pin. Cependant, la réaction en croissance variera probablement en ...

Phenological Response of an Arizona Dryland Forest to Short-Term Climatic Extremes

“…This result appears inconsistent with the greater sensitivity of ponderosa pines to climate at lower elevations [28,29]. One explanation may be that the NDVI signal from pixels with relatively equal proportions of herbaceous and woody vegetation shifted to ponderosa pine-dominated timing at warmer, lower elevations due to a suppression of herbaceous response that did not occur at higher elevations.…”

“…The higher variability of peak green-up from year to year is not consistent with a concurrent assumption of ponderosa pine-dominated pixels. We do not have an explanation for the dynamics of this particular scenario; the ponderosa pine may be responding to a critical deterministic factor that was not taken into account in this study, such as soil texture [29], species interactions, or resource availability.…”

“…Fire suppression policies over the past century have generally led to denser, more evenly distributed stands of ponderosa pine, but the ecosystems can still display a wide array of vegetative compositions and structures arranged in a variety of mosaics [27]. The drought sensitivity of ponderosa pine is linked to landscape characteristics, such as elevation, slope, and soil texture [28,29], while increased tree density and lower elevation have been related to increased likelihood of mortality by bark beetles [26]. The removal of ponderosa pine overstory due to fire [30], insect infestations [26], and thinning [31] has been documented to stimulate the growth of understory forbs and grasses, which can produce different annual growth patterns than woody vegetation; herbaceous growth typically reacts quickly to precipitation and climatic events, while woody vegetation exhibits a measured and less dynamic response [32].…”

Climate change effects on vegetation in the Pacific Northwest: a review and synthesis of the scientific literature and simulation model projections

“…Drought stress is prevalent, but most species are adapted to dry summers. Interannual variation in tree growth is positively correlated with precipitation or negatively correlated with summer temperatures for ponderosa pine (Carnwath et al 2012, Knutson and Pyke 2008, Kusnierczyk and Ettl 2002, Douglas-fi r (Carnwath et al 2012, Case and Peterson 2005, Chen et al 2010, Griesbauer and Green 2010, Littell et al 2008, and lodgepole pine (Case and Peterson 2007). There is little evidence that summer drought is the direct cause of signifi cant tree mortality; rather, drought may indirectly cause mortality through its effects on wildfi res and forest health.…”

“…Tree ring analyses suggest that western juniper growth is driven primarily by soil moisture availability and drought (Knapp et al 2004, Knutson andPyke 2008).…”