COS 279-1 - Patterns and limitations in ontogenetic niche shifts across Swiss tree species

Abstract: The regeneration niche, or the environmental conditions required for the generational replacement of individuals within natural populations, is a crucial aspect of species' responses to environmental change that remains understudied. Conventionally, conservation and species modeling efforts have primarily focused on the presence or performance of adult individuals, while the vulnerability of seedlings is generally assumed to be similar (or greater) than that of adults. However, if juveniles and adults systematically differ in niches and climate tolerances, as observational and experimental data suggest, inferences based solely on adult populations may not accurately capture changes in species distributions and habitat suitability. However, systematic multispecies assessments of ontogenetic niche shifts and comparisons with experimentally measured tolerances are still not available.

Here, we used a multidisciplinary approach, using: 1) long-term data from the Swiss National Forest Inventory to compare the niches of juveniles and adults across 43 forest species in Switzerland and quantify ontogenetic niche shifts. 2) physiological tolerance to frost, drought, and heat, for seedlings of 22 widespread tree species, which were compared to growth rates, life history traits, and potential niche limits derived from species distribution models (SDMs).

Our results indicated that habitat models using inventory data agreed with theoretical predictions of niche conservatisms, with large overlap between juvenile and adult niches, and large temporal stability for both groups (though consistently lower for juveniles than adults). The mismatches between niche stages was caused by juveniles having larger niche than adults, suggesting potential for rapid adaptation and pre-established migration facilitating responses to environmental changes. However, the comparison between experimental tolerances and SDM-derived potential niche limits highlighted clear limitations in current modeling approaches to accurately capture tolerance to extremes and seedling dynamics, as well as strong interactive effects between adaptation to multiple types of extremes across species that are seldom considering in SDM approaches.

All in all, our results highlight that while responses to average conditions and stable niche are likely to be well captured by current approaches, the relevance and consequences of extremes for forest indicate the need for further improvements in multiage modelling approaches and the consideration of hybrid experimental-modelling approaches to improve the accuracy and realism of forest responses to climate change in the face of changing environments.