Leaf Cuticle Reveals Effects of Climate Change on New Zealand Forests
It is well known that changing climate affects the distribution and range of species, where they often ‘track’ conditions most favorable to them. For example, as temperatures warm today, species are moving up slopes and poleward to what had been cooler climates. Glacial-interglacial cycles provide a unique experiment where one can observe a community in the same location, through both cold and warm conditions, comparing how they have changed over time. In the case of vegetation, the warmer interglacial periods provide a window of recovery, however interglacial communities can vary greatly in composition and even soil conditions, providing an interesting history to study in one location over time such as the northern island of New Zealand.
Palynology is the study of pollen and spores, which can help indicate the presence and range of plants both past, through the rock record, and present since related groups produce similarly shaped grains. Pollen is produced in copious amounts and light enough to be dispersed via wind, water, or even animal transport making presence and range estimates for vegetation more generalized. Another way to detect plant presence is by observing leaf cuticle, the waxy outer layer of plant leaves which preserves unique characteristics of each plant species.
Locality map, showing North Island of New Zealand and the sample sites.
Cuticle can be readily found in peat or lignite deposits, a preserved mixture of muddy material and plant debris. Although many studies have focused on palynology to infer vegetation changes in New Zealand, observations are now focusing on leaf cuticle. The primary goal of this research was to see if leaf cuticle identifications could correlate to the established palynological record.
“The general understanding is that cuticle allows more taxonomic resolution than pollen. For example, while pollen may allow identification to family level, and perhaps to genus, cuticle may get you to genus and perhaps species. The difficulty behind either technique really goes back to having an adequate reference collection” explains Mike Pole.
Such a reference collection allows one to compare fossilized cuticle to living representatives which helps with plant identification. “The really hard work is then building the reference collection of microscope slides – thousands of leaf samples, prepared for cuticle, mounted on slides, cleaned, sealed, labelled, databased, and now digitized” says Pole.
Cuticle of Astelia (Asteliaceae), Cyperaceae and Arecaceae; 1. Astelia sp., TLM view (SL5436, Hamiltons Gap, scale bar equals 50 µm); 2. Astelia sp., TLM view (SL5436, Hamiltons Gap, scale bar equals 20 µm); 3. Astelia sp., SEM view of inner surface (S-1661, Hamiltons Gap, scale bar equals 50 µm); 4. Astelia sp., SEM view of outer surface (S-1661, Hamiltons Gap, scale bar equals 50 µm); 5. Cyperaceae gen. et sp. indet., TLM view (SL2376, Maxwell-07, scale bar equals 50 µm); 6. Cyperaceae gen. et sp. Indet., TLM view, detail of stomatal complexes (SL2376, Maxwell-07, scale bar equals 20 µm); 7. Arecaceae gen. et sp. indet., TLM view (SL4671, Rapanui, scale bar equals 100 µm); 8. Extant Rhopalostylus sapida, TLM view (OPH2638, scale bar equals 50 µm).
All this effort eventually starts to paint a picture as to how vegetative communities have varied over time through New Zealand’s changing climates. During the Miocene, New Zealand was covered in a lowland swampy environment home to a huge diversity of plants. Later during the Late Pliocene to Early Pleistocene, New Zealand’s main mountain range rose, dividing the island’s weather into extremely wet conditions on the west side and drier conditions on the east side. Dr. Pole elaborates “Early in the Pliocene, New Zealand was probably warmer than today. Later in the Pliocene, New Zealand started to experience the oscillations of the glacial-interglacial cycles. Climate during the interglacials was roughly similar to today with forests remarkably similar as well. However, at the peak of the glacials, it was much colder and drier.” As climate changed, so did the plants.
“These results confirm the pollen studies in that New Zealand’s forest composition oscillated throughout the glacial interglacial cycles. All else being equal, this could be expected to continue” says Pole. Under natural climate change, New Zealand should eventually descend into another glacial period and then its vegetation ought to resemble what was around during the last glacial period. Yet, humans have shifted the natural course and climate change no longer follows its usual progression.
Looking to the future, Pole explains “However, all else is not equal, and the climatic forcings of greenhouse gas emissions are over-riding those behind the glacial-interglacial cycle. When humans arrived in New Zealand, most of our drier forests were quickly burnt off. European settlers, managed to burn off even more. I think that New Zealanders tend to regard what’s left as ‘unburnable’. We aren’t used to our forests burning. I suspect that we will get a nasty surprise, when global warming results in a very dry summer – and (like in Australia now) some of our remaining forests burn. They will be replaced with something else – and hence it will be a kind of systems-change.”
