Tag Archives: tree mortality

Let the litter fall! Leaf loss to safe water. Kauri and drought part 2.

During the 2013 drought, the most noticable impact on the kauri forest was the increase in litterfall. It’s well established that many tree species lose their leaves in response to drought, leading to decreased plant water loss. Deciduous trees may lose their leaves early while semi deciduous trees may lose more leaves in a dry year. By reducing leaf area, trees reduce the aarea of water-losing surfaces, thereby reducing overall water loss.

In a recently accepted paper in Plant Ecology, we report that litterfall increased 72% in 2013 compared to 2012 because of dry soil conditions (see figure). Most of the extra litter was kauri leaf and twig material.

Kauri are known to self-prune branches but the reason for branch abscission has been unclear.  It may be that this costly process occurs to protect the highly vulnerable hydraulic system of these forest giants. During dry periods, trees through off twigs to prevent failure of the water conducting system of the plant.

We are doing ongoing monitoring to determine the lag impact of this massive biomass loss to the forest floor.

Kauri and drought – part 1. Is drought an issue for these forest giants?

Kauri is one of the most iconic tree species on the planet and is particularly symbolic for New Zealand. Trees are big (with trunk diameters up to 5 m) and long-lived (possibly up to 2000 years or more). Amongst conifers, kauri is the third largest species. It’s culturally significant for Maori and Europeans for it’s place in the forest and the resources it can provide. A single tree could yield enough wood for four houses. While we don’t log native forest any more, kauri face other threats from the mould-like PTA causing kauri dieback and also drought.

My research is looking at drought vulnerability of kauri and in this series of posts, I will explain what we have discovered when the drought of early 2013 presented itself as a natural experiment. In this post, how did I get into this work and why kauri? I’m fascinated by how trees work. How does transpiration work? How do trees work out how to use carbon? And kauri are especially interesting. How do they get so big and how do they live so long?

We know kauri are responsive to climate because their tree rings record climatic conditions, leading to one of the longest proxy records of past climate in the southern hemisphere. The growth response is correlated with the El Nino Southern Oscillation (ENSO) cycle. Larger rings occur during drier spring-times and smaller rings occur during wetter periods. This counter intuitive pattern got me thinking about what the underlying physiological mechanism might be. The first step was to explore the literature and I was surprised to find that there were only snippets about this species and we know comparatively little about kauri responses to changes in climate from season to season and from year to year. As long-lived organisms, kauri must have some potential to deal with variation in environmental conditions. Yet, there are three pieces of evidence that kauri may be vulnerable to drought.

The first piece of evidence is that kauri are highly vulnerable to xylem embolism. Xylem embolism is the formation of air bubbles in the water conducting system of a plant. This can happen when water is scarce or during the freeze-thaw cycle. If too many air bubbles form, the water system can fail completely and the plant dies due to lack of water, or hydraulic failure. Embolism occurs in kauri with only minimal reductions in water availability.

Second, the literature suggests that kauri have shallow roots. Trees with deeper roots have access to deeper water stores which can sustain them during drier periods. During dry periods, shallow soil layers are the first to dry out so plants with shallow roots will run out of soil water first. Reports of fallen trees indicate that deep peg roots are for anchoring only as they do not have any fine roots attached (but more on this on a later post). Furthermore, remaining kauri are often found on ridge-tops which can be the first areas to dry out during drought as water flows downhill.

Finally, there have been reports of groups of dead kauri trees in early timber appraisal reports that have been linked to the extensive 1917 drought. New Zealand’s climate is generally considered to be moist and mild. However, climate projections indicate droughts will become more severe and more frequent. My research is exploring whether kauri can survive dry conditions year after year.

Stay tuned for results in coming weeks.

Ecohydrology in New Zealand. Water supply issues and the threat of drought for forests

Under a changing climate, parts of New Zealand will become warmer and drier. As yet, we know very little about the impact this will have on vegetation. In a recent post on the Waiology blog, I explain the importance of understanding responses of evapotranspiration (ET) to climatic conditions because ET is a major component of water budgets. It is particularly vital to quantify how much water is returning to the atmosphere as through this pathway in water supply catchments. In future climates, overseas studies indicate that water yield will decline, reducing water available for human consumption.

A further reason to explore the impact of future climates on forests is the potential for increased tree mortality. Several instances of drought-induced tree mortality in New Zealand were included in a global review in 2010. Such events are likely to become more common in all biomes because trees across the world operate close to their safety margins and even slight changes in climate can be catastrophic for trees. The combination of increased temperatures and lower rainfall leads to particularly challenging conditions for tress because both the soil and the atmosphere are drier, putting the plant’s water system under stress. This lethal formula has been implicated in California recently and was discussed as a global issue at the Tree Mortality Workshop in Jena, Germany last week. In addition to changing climate, attacks from insects, fungi and other pathogens will further weaken trees and impact on forest ecosystem services. With so many unique plants and ecosystems in New Zealand, there is an urgent need to improve our understanding of drought in the local context.