Abiotic, biotic or historical limitation of Nothofagus treeline expansion

Background/Question/Methods

Factors that may limit distributions and plant species ability to respond to climate warming - abiotic and biotic limitations and life history traits - have not been fully explored. Here, we evaluate whether inertia by the local treeline forming species in New Zealand, Nothofagus solandri var. cliffortioides, to climate warming reflects insufficient warming, competitive limitation or taxon-specific traits reflecting sensitivity and ability to respond to climatic changes. We used experimental plantings along four vertical transects above treeline of two treeline forming species with contrasting recruitment patterns and life history traits (N. solandri and Pinus contorta) plus microsite manipulations (above ground vegetation removal, passive warming) at one site and recruitment observations of naturally seeded N. solandri across seven sites.

Results/Conclusions

Transplanted seedlings of both species were able to survive up to 150 m above treeline but, in contrast to expectations, survival and growth (basal area increment) did not decline with lower temperatures (distance from treeline) or increase with warmer temperatures (passive warming). Growth and survival did not significantly differ between species (Repeated Measures ANOVA, p > 0.05). Within experimental plantings, N. Solandri seedling survival was lower when neighbors where removed, whether or not passive warming was added; neighbors more likely facilitate than inhibit seedling survival. Consistent with microsite manipulation experiments, observed N. solandri seedlings were found significantly overrepresented in sheltered microsites (based on microsite occupancy:availability ratio; χ² test, p<0.05). However, in contrast to experiments, seedlings occurred less than expected (based on microsite availability) in vegetated microsites and occurred more than expected (based on availability) in sheltered, non-vegetated microsites (eroded banks, beneath established tree). Results do not support abiotic (low temperature) limitation or, at least for established seedlings, taxon specific life history traits. N. solandri treeline inertia most likely reflects the effect of biotic interactions (potentially facilitation and competition) on recruitment patterns.