Oak seedling survival and growth along resource gradients in Mediterranean forests: implications for regeneration in current and future environmental scenarios

Abstract

Understanding seedling performance across resource gradients is crucial for defining the regeneration niche of plant species under current environmental conditions and for predicting potential changes under a global change scenario. A 2‐year field experiment was conducted to determine how seedling survival and growth of two evergreen and two deciduous Quercus species vary along gradients of light and soil properties in two Mediterranean forests with contrasting soils and climatic conditions. Half the seedlings were subjected to an irrigation treatment during the first year to quantify the effects on performance of an alteration in the summer drought intensity. Linear and non‐linear models were parameterized and compared to identify major resources controlling seedling performance. We found both site‐specific and general patterns of regeneration. Strong site‐specificity was found in the identity of the best predictors of seedling survival: survival decreased linearly with increasing light (i.e. increasing desiccation risk) in the drier site, whereas it decreased logistically with increasing spring soil water content (i.e. increasing waterlogging risk) in the wetter site. We found strong empirical support for multiple resource limitation at the drier site, the response to light being modulated by the availability of soil resources (water and P). Evidence for regeneration niche partitioning among Quercus species was only found at the wetter site. However, at both sites Quercus species shared the same response to summer drought alleviation through water addition: increased first‐year survival but not final survival (i.e. after two years). This suggests that extremely dry summers (i.e. the second summer in the experiment) can cancel out the positive effects of previous wetter summers. Therefore, an increase in the intensity and frequency of summer drought with climate change might cause a double negative impact on Quercus regeneration, due to a general reduction in survival probability and the annulment of the positive effects of (infrequent) ‘wet’ years. Overall, results presented in this study are a major step towards the development of a mechanistic model of Mediterranean forest dynamics that incorporates the idiosyncrasies and generalities of tree regeneration in these systems, and that allow simulation and prediction of the ecological consequences of resource level alterations due to global change.

Influences of Forest Structure, Climate and Species Composition on Tree Mortality across the Eastern US

“…1D). A similar pattern of increasing mortality in higher light conditions has been found for oak seedlings in the Mediterranean [56] and linked to higher desiccation risk. Although several species followed this pattern, many others showed the opposite trend of decreasing mortality with increasing radiation, in agreement with many other studies linking light to survival (e.g.…”

Forest dynamics at Minnesota's prairie-forest border driven by invasive buckthorn (Rhamnus cathartica) and native bur oak (Quercus macrocarpa)1

“…These include studies in which the light levels observed were all well below full sun exposure. Asymptotic models provide a satisfying parallel to the idealized photosynthetic response curve found in plant physiology textbooks, but our results and results from at least one other study (Gó mez-Aparicio et al 2008) suggest that a more complicated relationship may exist for saplings growing in the field. Additionally, although we know of no other study of R. cathartica sapling growth, our observations of growth response to light are consistent with two previous studies of R. cathartica seedling growth (Gourley 1985, Grubb et al 1996.…”

Tree-Rings Mirror Management Legacy: Dramatic Response of Standard Oaks to Past Coppicing in Central Europe

“…Oaks have gradually lost their ability to successfully regenerate in many European woodlands. This phenomenon is often referred to as ‘oak decline’ [29]–[31]. Available studies attribute it to various factors [32], [33], e.g.…”