Here’s a big question to think about. Picture your favorite ecosystem, perhaps it’s a grassland, a heathland or a wet forest. Which ecological process has the biggest impact on how your ecosystem changes over time?
Presumably, the process you selected is the one that you think is most critical to retain (or avoid) in order to conserve a diverse, functioning ecosystem. If the process is altered or removed, ecosystem health will decline.
Now, think about this question: what if you’re wrong?
What if some other process is actually far more important, and the process you selected is actually rather trivial in the big picture?
I’ve been thinking about this question while wandering around box-ironbark forests in central Victoria, an ecosystem I’m pretty new to. The question I’ve asked myself is: what are the dominant processes that control how box ironbark forests function? I don’t have the answer, but the question has triggered lots of interesting discussions. But before we discuss box ironbark forests, let’s widen the canvas and think about the processes that affect most Australian ecosystems.
Fire and Rain
I imagine that the two most popular answers to the question above would be fire and water. Grazing is important in agricultural landscapes, but fire and water are arguably the most important drivers of ecosystem dynamics in most Australian ecosystems.
Fire controls ecosystem processes in lots of Australian ecosystems, especially forests, heathlands and savannahs (see recent blog). Fire regimes affect plant regeneration and mortality, and can alter ecosystem structure and habitat suitability for fauna.
In semi-arid and arid regions, the key factor that affects ecosystem processes isn’t fire, it’s water; how much water arrives in the first place (in floods and droughts), and how water is retained in, and moves through, the landscape.
Intact ecosystems are very good at retaining water. Falling rain hits ground plants and litter, and these slow the flow of water through the ecosystem. The more water that is held in the landscape, and the longer it is available, the greater the biological productivity.
By contrast, degraded dryland systems are very leaky. Degrading processes such as heavy grazing remove the plants and litter that intercept surface water. Water runs off faster and soils dry out faster, and this in turn, leads to further reductions in biological productivity. This creates a vicious cycle as ‘leaky ecosystems’ become more and more degraded.
To restore degraded dryland systems, arid zone ecologists focus on measures that slow the flow of water across the landscape. Understory and ground plants, fallen timber and leaf litter all create small barriers that prevent water from leaking, or running out of the system. Drifts of leaf litter on the ground improve water infiltration and soil condition (including organic carbon levels), so soils can store more water for longer.
In degraded dryland systems, a primary management goal is to improve the system’s ability to retain soil moisture.
It’s easy to view these two world views as applying to completely different ecosystems – fire is important in forests and water in deserts. But sharp boundaries are rare in nature. Some forests grow on poor, degraded, dry soils, and don’t burn that much. Retaining or restoring scarce resources is likely to be very important in these systems. Which brings us back (finally!) to box-ironbark forests.
Water for Elephants Ironbarks?
To return to our original question: which ecological processes have the biggest impact on how box-ironbark ecosystems change over time? How important are fire and water? I emphasise that I don’t know (and I don’t know anyone else who does), so this blog sketches out some early thoughts, and some research questions for future projects.
Box-ironbark forests are really interesting. They occur on soils that are naturally infertile, but which were made less productive by widespread gold mining in the mid-1800s. Mining degraded soils over vast areas, and many soils now have no A horizon at all. The loss of the original topsoil must have reduced soil water holding capacity and, by inference, the biological productivity of the forests as a whole.
Box-ironbark forests in Victoria don’t burn very often. They don’t produce a lot of ground fuels, so they don’t carry frequent fires. Many of the dominant understory shrubs seem to regenerate well without fires (although fire undoubtedly promotes regeneration of many species). Unlike most forests in south-east Australia, there is little evidence that fire is an important driver of ecosystem dynamics. (Who knows what role fire may have played before European settlement, but that’s a matter for conjecture).
By contrast, box ironbark forests are undoubtedly subject to seasonal water limitation. Annual rainfall is moderate (450-600 mm on average) but summers are hot and dry. Annual rates of biomass accumulation are low compared to ecosystems on more fertile, well watered soils. Prolonged droughts (as seen during the past decade) killed many understory shrubs which, in turn, is thought to have contributed to declining numbers of birds during the drought.
Box ironbark forests also seem to be pretty ‘leaky’. Soils are often bare and crusted, and rates of water infiltration are (presumably) very slow. The paucity of humus and organic matter means that their water holding capacity is also low. Because the ground surface is relatively open, there often aren’t a lot of barriers to impede the flow of water during heavy rains. Following heavy summer rains, water rushes down shallow drainage lines, washing leaf litter off the slopes and down to the gullies. This process must further reduce soil productivity. Indeed, this is the vicious cycle that arid zone ecologists try to avoid.
Piecing together these casual observations, one can make a good case that, to maintain biological productivity in box-ironbark forests, it’s useful to view these forests through the lens of arid zone ecology. This approach becomes even more compelling given the perils of climate change. As temperatures increase, box ironbark forests will come under increasing water stress.
As in arid ecosystems, a valuable management strategy to minimize the effects of climate change will be to increase the system’s ability to retain soil moisture.
I’m starting to wonder – is water availability a far more important issue than fire in box ironbark forests? Instead of this simple either/or question, a better question might be: how do fires (where they do occur) affect soil moisture retention in box ironbark forests? Do they make the system more or less leaky?
Litter for water, water for ecosystems
How do we retain soil moisture in water-limited ecosystems? David Tongway, John Ludwig and other arid zone ecologists have studied this for decades now.
The goal isn’t to create ‘dams’ of deep water (this is a dryland system after all). Instead, the goal is to increase the capacity of topsoil and litter layers to hold and slow the flow of surface water. Anything that promotes ‘surface roughness’, slows the flow of water. Branches, leaves and twigs on the soil surface trap fine litter and create small barriers that hold water for longer periods. Soil accumulates, soil micro-organisms become more abundant, and system productivity increases.
In the longer term, as trees grow older and drop more larger limbs, fallen coarse woody debris (‘brown gold’) helps to capture even more litter and water. By contrast, processes that reduce leaf litter accumulation are likely to reduce soil water holding capacity, biological productivity and resilience to climate change.
A key goal for conservation management is to increase ecosystem resilience to changing climate. The most likely impact of climate change in south-eastern Australia is a decline in soil water due to higher evaporation as temperatures increase. So any process that retains soil moisture and reduces system ‘leakage’ seems like a great process to promote (or at least to investigate), yes?
Water for ironbarks may not be the goal, but water for soil development, soil productivity, and biological diversity seems like an important ecosystem process to promote. Ultimately, retaining leaf litter (fallen leaves, twigs and branches) and old trees (that produce more branches) may be the simplest way to hold soil water. Unfortunately, litter for water and resilient ecosystems doesn’t sound that sexy does it? Nevertheless, as climate change intensifies, it may be the issue that we have to focus on more and more.
Environment Conservation Council (1997). Box – Ironbark Forests and Woodlands Investigation Resources and Issues Report. (ECC: Melbourne)
Ludwig J, Tongway D, Freudenberger D, Noble J, Hodgkinson K (1997). Landscape Ecology, Function and Management: Principles from Australia’s Rangelands. (CSIRO: Melbourne).
Mac Nally R, Bennett AF, Thomson JR, Radford JQ, Unmack G, Horrocks G, Vesk PA (2009). Collapse of an avifauna: climate change appears to exacerbate habitat loss and degradation. Diversity and Distributions 15(4), 720-730.
Taylor SG (2008) Leaf litter invertebrate assemblages in box-ironbark forest: composition, size and seasonal variation in biomass. Victorian Naturalist 125(1), 19-27.
Watson DM (2011) A productivity-based explanation for woodland bird declines: poorer soils yield less food. Emu 111(1), 10-18.
- The dirt ring photo blog
- Interact, said the tortoise to the hare
- Growing old in a shrubland: gravity always wins
- Fire and rain: what makes a woodland?