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Climatic seasonality drives ecosystem processes (e.g. productivity) and influences plant species distribution. However, it is poorly understood how different aspects of seasonality (especially regarding temperature and precipitation) affect growth continuity of trees in climates with low seasonality because seasonality is often only crudely measured. On islands, exceptionally wide elevational species distribution ranges allow the use of tree rings to identify how growth continuity and climate–growth relationships change with elevation. Here, we present a novel dendroecological method to measure stem growth continuity based on annual density fluctuations (ADFs) in tree rings of Pinus canariensis to indicate low climatic seasonality. The species ranges from 300 to >2000 m a.s.l. on the trade wind-influenced island of La Palma (Canary Islands), where we measured three decades of tree-ring data of 100 individuals distributed over 10 sites along the entire elevational range. The successfully implemented ADF approach revealed a major shift of stem growth continuity across the elevational gradient. In a remarkably clear pattern, stem growth continuity (percentage of ADFs) showed a hump-shaped relationship with elevation reaching a maximum at around 1000 m a.s.l. Low- to mid-elevation tree growth was positively correlated with the Palmer Drought Severity Index (PDSI; indicating aridity) and sea surface temperature (indicating trade wind-influenced moderation of water supply), while high-elevation tree growth was positively correlated with winter temperature (indicating a cold-induced dormancy period). We conclude that ADFs are a useful method to measure stem growth continuity in low-seasonality climates. Growth of P. canariensis on the Canary Islands is more frequently interrupted by winter cold at high elevations and by summer drought at low elevations than in the trade wind-influenced mid elevations, where growth sometimes continues throughout the year. Climate change-associated alterations in trade wind cloud formation might cause non-analogue growth limitations for many unique island species.
In this paper we describe Macrobiotus canaricus sp. nov., a new tardigrade species of the Macrobiotus hufelandi group from the Canary Islands. Moreover, with the use of DNA sequencing, we confirm that Macrobiotus recens Cuénot, 1932 represents the hufelandi group, even though eggs laid by this species do not exhibit the typical hufelandi group morphology. Our study is based on both classical taxonomic methods that include morphological and morphometric analyses conducted with the use of light and scanning electron microscopy, and on the analysis of nucleotide sequences of four molecular markers (three nuclear: 18S rRNA, 28S rRNA, ITS-2, and one mitochondrial: COI). Our analyses revealed that M. canaricus sp. nov. is most similar to Macrobiotus almadai Fontoura et al., 2008 from the Archipelago of the Azores, from which it differs by the absence of granulation patches on the external and internal surfaces of legs I–III as well as by the absence of a cuticular pore in the centre of the external patch on legs I–III. Molecular sequences allowed us to pinpoint the phylogenetic positions of M. canaricus sp. nov. and M. recens within the M. hufelandi group.