Climate and signature years in west European oaks

From Dendrochronology to Allometry

The contribution of tree-ring analysis to other fields of scientific inquiry with overlapping interests, such as forestry and plant population biology, is often hampered by the different parameters and methods that are used for measuring growth. Here I present relatively simple graphical, numerical, and mathematical considerations aimed at bridging these fields, highlighting the value of crossdating. Lack of temporal control prevents accurate identification of factors that drive wood formation, thus crossdating becomes crucial for any type of tree growth study at inter-annual and longer time scales. In particular, exactly dated tree rings, and their measurements, are crucial contributors to the testing and betterment of allometric relationships.

Old World megadroughts and pluvials during the Common Era

Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other "Old World" climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the "Old World Drought Atlas" (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

Complex climate controls on 20th century oak growth in Central-West Germany

We analyze interannual to multi-decadal growth variations of 555 oak trees from Central-West Germany. A network of 13 pedunculate oak (Quercus robur L.) and 33 sessile oak (Quercus petraea (Matt.) Liebl.) site chronologies is compared with gridded temperature, precipitation, cloud-cover, vapor pressure and drought (i.e., Palmer Drought Severity Index, PDSI) fluctuations. A hierarchic cluster analysis identifies three groups for each oak species differentiated by ecologic settings. When high precipitation is primarily a characteristic for one Q. robur and one Q. petraea cluster, the other clusters are more differentiated by prevailing temperature conditions. Correlation analysis with precipitation and vapor pressure reveals statistically significant (P < or = 0.05) correlations for June (r = 0.51) and annual (r = 0.43) means. Growth of both species at dry sites correlates strongly with PDSI (r = 0.39, P < or = 0.05), and weakly with temperature and cloud-cover. In natural stands, Q. robur responds more strongly to water depletion than Q. petraea. Twenty-one-year moving correlations show positive significant growth response to both PDSI and precipitation throughout the 20th century, except for the 1940s - an anomalously warm decade during which all oak sites are characterized by an increased growth and an enhanced association with vapor pressure and temperature. We suggest that the wider oak rings that are exhibited during this period may be indicative of a nonlinear or threshold-induced growth response to drought and vapor pressure, and run counter to the general response of oak to drought and precipitation that normally would result in suppressed growth in a warmer and drier environment. As the wide rings are formed during the severe drought period of the 20th century, a complex model seems to be required to fully explain the widespread oak growth. Our results indicate uncertainty in estimates of future growth trends of Central European oak forests in a warming and drying world.

Predicting plant responses to global environmental change

Predicting the future responses of plants and ecosystems to further changes in the CO₂ concentration of the atmosphere and to the possibility of global warming are important current concerns. Predictions have been most frequently attempted using short-term, single-factor experiments in controlled environments. However, these experiments have failed to indicate the outcome of field experiments at larger spatial and temporal scales. Some of this failure is due to ignorance of environmental conditions and interactions while some is due to the use of inappropriate short-cuts, such as the addition of fertilizers for simulating enhanced mineralization, and some is due to ignorance of the processes involved in scaling-up from individual plants to populations. Long-term observations on plants in ecosystems nevertheless indicate that community-scale experiments may provide a useful but imperfect capacity to predict ecosystem responses. Although difficult to implement in practice, it is concluded that catchment-scale experiments offer the best opportunity to predict plant, community and ecosystem responses to environmental change. CONTENTS Summary 239 I. Introduction 239 II. CO₂ -enrichment experiments 240 III. Experiments with applied nitrogen fertilizer 243 IV. Population ecophysiology 245 V. Ecosystem predictions 248 Acknowledgements 249 References 250.