Guest post by Katariina Vuorinen

Cool as a moose: How can browsing counteract climate warming effects across boreal forest ecosystems? (2020) Vuorinen et al., Ecology, https://doi.org/10.1002/ecy.3159

The Crux

When temperatures increase, trees grow more. When a moose struts in and eats the twigs, trees grow less. So, if we just have enough moose around, climate warming won’t be able to increase the growth rate of trees. This is what we call the “cooling” effect. Rather simple – and cool – story, right?

However, every ecologist knows that the biological theatre is more complex than this. What if snow protects saplings against browsing? What if changes in temperature affect moose in such a way that they will not feed on trees in the same way as they used to? What if trees’ response to moose is actually different depending on whether it is warm or cold? In complex ecological systems, tree growth is determined in an intricate network of interactions, where the story line is so mind-bogglingly complicated that it seems almost impossible to say what is actually going on.

Luckily, it’s not quite impossible. In this paper, we set out to model those intricate networks, taking into account everything from the climate, the tree species, the effect of time, to the presence of herbivores and their browsing intensity, in an attempt to disentangle that complex biological theatre.

What We Did

To get a baseline impression of what happens to trees when moose aren’t around, we set up fences to keep the voracious ungulates away. Originally, this fencing was started by NTNU University Museum in Trøndelag, later expanding to 62 sites scattered across Norway and eastern Canada. As moose can only browse on relatively small trees, fences were placed in clear-cut areas where we could monitor the growth of the trees from the sapling phase.

After keeping track of the height of hundreds of trees inside and outside of the fences for more than a decade, we had assembled over 16 000 growth measurements. These were accompanied by annual estimations of the proportion of browsed twigs. Based on existing knowledge about which plant species the moose preferred, we also estimated the amount of food available for moose at each site. The tree growth and browsing data were complemented with data on three climatic variables, namely, growth period temperature, precipitation and winter snow-water equivalent, as well as data on regional moose density.

We analyzed the data with structural equation models (SEMs) that combine multiple predictors and response variables into one big model network. A SEM allows you to treat an environmental variable both as an explanatory variable and a response variable simultaneously. For instance, the amount of competing trees could be explained by moose presence, but it itself could explain tree growth.

Did You Know

Herbivore cooling effects are better documented in arctic and alpine systems, where smaller woody plants namely shrubs, play the role of the trees. Empirical studies have shown that for example reindeer can slow down climate-change driven shrubification that would otherwise result in loss of open tundra. However, also in the arctic, herbivore effects take multiple forms: sheep effect seem to be modified by climate, potentially via plant-plant competition.

What We Found

Three of the studied tree species played along with the simple cooling story: Canadian rowan and birch and Norwegian birch. They benefited from higher temperatures and suffered from moose. However, most of the tree species wrote their own, more nuanced narratives. Canadian fir responded more weakly to temperature when moose were missing. Norwegian rowan flipped its temperature response curve around if moose were present. Norwegian pine responded negatively to temperature, but did not seem to be bothered by moose. This is understandable, as heat turns into an enemy when it gets too hot. The soft, palatable species took more damage from moose than spiky spruce and pine.

From a tree’s point of view, the role of a moose can change from a foe to a friend if the moose browses on a neighbouring competitor tree. Canadian fir and rowan and Norwegian birch and pine benefited from the fact that moose lowered the growth of competing trees. Snow complicated the story even further. Norwegian rowan benefited from increasing winter precipitation, but only outside of the fences, suggesting that individual trees were indeed protected from browsing by a snow layer. This is potentially a result of snow lowering the proportion of browsed twigs. Interestingly, also temperature affected browsing intensity, but the effect size and direction varied between different tree species.

The bottom line given by these results is clear: the moose cooling effect exists, but how important it is really depends on ecological context.

Problems

We always need to careful when assessing results obtained by using datasets of different accuracies. Where locally estimated browsing data was highly precise, regional moose density and climate data were less so. Thus, the effect strengths may partly reflect differences in data quality rather than true differences between explanatory variables. Overcoming these weaknesses might reveal side-plots yet to be unravelled.

So What?

So, if the pathways of climate effects are this complex, what is actually going to happen in the boreal forests when temperatures rise? Some tree species may benefit from increase of temperature just to end up on the moose dinner menu. Less tasty ones may thrive, or suffer from excess heat and increased competition. If global warming brings us snowless springs, cooling potential of browsing may increase. In contrast, if we get more snow with increasing precipitation, moose may turn into a trivial side-character.

In the complex interplay of biotic and abiotic actors, only one thing is certain: that we do not know what will happen outside the observed variable boundaries. Interactions and non-linearities make any future predictions highly uncertain. If we are to place hope on herbivory as a cooler of climate change impacts, constraints imposed by species differences, snow, competition, as well as climate effects on browsing must be acknowledged – not so neat of a story, and perhaps less cool, but nearer the ecological reality.

Katariina E. M. Vuorinen is a PhD candidate at the Norwegian University of Science and Technology. She studies the effects of climate and large herbivores on plants by using data from across boreal and arctic biomes. You can read more about her work at this link.

Title Image Credit: James D. M. Speed, NTNU University Museum, CC BY 2.0, Image Cropped