**4. Discussion**

204 Remote Sensing of Biomass – Principles and Applications

The low diversity regions under historical site conditions have a successional pattern of increasing *Larix* spp. biomass to year 200, followed by the slow establishment of evergreen conifers with *Larix* spp. maintaining a significant presence to the end of simulation (Figure 7a). The temperature treatment accelerates the establishment of evergreen conifers and at some sites causes a complete collapse of larch biomass in many of the low diversity sites around year 200 when the temperature has increased by 4C (Figure 7b). Successional dynamics in northwestern Siberia represent an exception to this general pattern. The colder regional temperatures in northwestern Siberia do not promote transition from *Larix* spp. to evergreen conifer, rather there is persistent *Larix* spp. dominance (Figure 7d). Northwestern Siberia does not experience the collapse of larch that is seen at sites further south (Figure 7b), but does transition to forests dominated by evergreen conifers in late successional stages in response to warming (Figure 7e). This late successional transition is similar to the natural succession dynamics of central and southern Siberia under base climate conditions (Figure 7a). The effect of the *L. decidua* treatment on *Larix* spp. biomass is immediate and continues to the end of simulation in all low diversity regions (Figure 6), indicating that *L. decidua* easily establishes and contributes to overall biomass in these regions. The inclusion of *L. decidua* in the low diversity regions under base climate conditions delays and suppresses the transition to evergreen conifers. *L. decidua* acts to prevent the collapse of larch in response to warming that is observed in low diversity areas in central Siberia

Fig. 7. Simulated mixed species biomass dynamics (tC ha-1) for low diversity sites in Siberia. Species composition by the dominant genera over 500 simulated years starting from bare ground for the base historical climate **(a,d),** temperature increase **(b,e),** and temperature

(Figure 7b,c).

increase with *Larix decidua* **(c,f)**.

#### **4.1 Model simulation across Siberia and RFE**

Biomass patterns from simulation of mature forest under historical climate conditions reflect the idea that areas with increased plant diversity have increased productivity (Tilman and Downing 1994; Chapin et al., 1997, Bengtsson et al., 2000), with areas of higher biomass located in the areas of increased diversity in the Amur region of the RFE. The 93 high diversity sites are all located in the Amur region of the RFE and have an average of 38 individual tree species. The remaining 279 sites have an average of 9 individual tree species. The Amur region of the RFE also has higher average temperatures and precipitation values than across Siberia and the remainder of the RFE which allows a more diverse group of species to actively compete and achieve optimal biomass without climate limitations. Similar biomass results from past simulations which allow 44 individual tree species to grow at all sites without range limitation across Siberia and the RFE suggest it is the severe climate, and not a decreased species diversity, which limits the amount of total biomass across the interior of Russia (Shuman and Shugart 2009).

Successional dynamics across the study area under base climate reflect fundamental competition dynamics among species. Larch (*Larix* spp.) is highly tolerant of cold temperatures, but is one of the most shade-intolerant genera in the region (Nikolov and Helmisaari 1992). As the forest matures, competition for light becomes a key factor in determining which species becomes dominant. In northwestern Siberia, the cold temperatures prevent many species from competing with the cold-tolerant larch. Central and southern Siberia do not experience the severely cold temperatures of northwestern Siberia, and evergreen conifers actively compete with larch. Due to the shade-intolerance of larch, these forests transition to evergreen dominance as seen in the base climate simulation (Figure 2a,b,c). The transition from larch to evergreen conifer is also a product of the lack of insect or wildfire disturbance in these simulations. At each site the results are a landscapelevel approximation of succession, which includes the natural disturbance caused by the death of individual trees. Warming climate is expected to cause increases in total area burned, fire-season length, and the severity of fire (Overpeck et al., 1990; Kasischke et al., 1995; Stocks et al., 1998; Soja et al., 2004; Soja et al., 2007). Similarly, the incidence of insect disturbance is also expected to become more prevalent with warming conditions (Holling 1992, Volney and Fleming 2000; Logan et al., 2003). Understanding the intrinsic successional dynamics isolates the direct response of the system to changing climate. Establishing the response of the system without the added changes of disturbance provides a strong basis for deconstructing the complexities of the system response to climate change.

#### **4.2 Climate sensitivity analysis**

#### **4.2.1 Continental scale discussion**

Larch is shade-intolerant and, in all but the coldest regions, evergreen conifers naturally replace larch over time, especially when no disturbance occurs that can rejuvenate the larch by providing open gaps of sunlight (Nikolov and Helmisaari 1992). The shift from deciduous larch to evergreen conifer forest is accelerated across Siberia under warming conditions (Figure 2), and implies a significant change in albedo. Following 200 years of forest development, larch-dominated forests are replaced with evergreen conifer-dominated forests in areas across Siberia. In southern Siberia, where forests are vulnerable to early replacement of larch by evergreen conifer, there would be a local significant albedo shift of

Resilience and Stability Associated with Conversion of Boreal Forest 207

The response to the temperature and *L .decidua* treatments in the high diversity regions highlights the importance of analysis of local climate and successional dynamics. The climate and *L. decidua* treatments do not have the strong effect on biomass in these high diversity regions of the RFE that is seen at the continental scale and in low diversity regions. Sites in the northern RFE (N RFE) region fall within larch's optimal growth ranges for both temperature and precipitation. Larch establishes strongly and competitively in N RFE in early succession followed by a transition to *Picea* spp. dominance under base climate conditions. The high diversity in the area indicates there is strong competition, and *L. decidua* does not have unique characteristics which allow it to

Under warmer conditions in the N RFE in late succession there is a transition, not to *Picea*  spp., but to *Pinus* spp. dominance. This highlights the importance of genus-level analysis. These genera are typically combined into a single evergreen conifer group, both for the purposes of global climate models and for the regional scale analysis presented here. Such a grouping prevents the detection of this shift between the two evergreen conifer species. This shift has the potential for further exploration in association with altered albedo values. Measured in the boreal forest of Canada, the summer albedo difference between *Pinus* spp. (0.086) and *Picea* spp. (0.081) is negligible, but the winter albedo for *Pinus* spp. (0.150) and *Picea* spp. (0.108) is more significant (Betts and Ball 1997). The albedo values in the RFE are likely similar to these Canadian species, and there is potential, with the winter albedo difference, to alter total annual absorption of surface radiation in response to the shift from *Picea* spp. to *Pinus* spp. with warming. Unlike the low diversity regions, the successional dynamics resulting from including *L. decidua* as a species in simulation are similar to those observed in response to warming, with late successional dominance by *Pinus* spp. This result suggests that introduction of *L. decidua* would not be a useful strategy for mitigation

The response to the climate and *L. decidua* treatments in southwestern (SW) RFE is also a product of local climate conditions. The SW RFE region has a climate with temperatures which place it in the upper limit of tolerance for larch, creating a climate in which the native larch species cannot compete and establish. Under base climate conditions, it is difficult for larch to compete, so it is not surprising that larch continues to be absent under warmer conditions. Even with increased tolerance for warmer conditions, *L. decidua* cannot effectively compete with other species in the SW RFE, and does not have a significant impact on biomass. Under base climate conditions, the SW RFE has mixed evergreen and deciduous species in the mature forest late in succession. With warming temperatures, there is an increase in biomass of mixed deciduous trees, and a decrease in evergreen conifer biomass. The decrease of evergreen conifers is balanced by the increase of mixed deciduous trees, so the effect on total biomass is not consistent and lasts only 60

The high diversity areas in the RFE have high ecological resilience and stability, which allow them to maintain basic ecosystem function following climate change and avoid irreversible shifts to another vegetation state. Stability is the ability of a system to return to equilibrium following temporary disturbance, and resilience is the persistence of a system and its ability to absorb change and disturbance without changing state (Holling 1973). In other terms, stability is a persistence of the system state and a consequence of interactions within the

**4.2.2 High diversity regional and local scale discussion** 

establish in this region.

of vegetation shifts in this region.

years.

approximately 5.1 W m-2 if the larch stands are replaced by evergreen conifers (Shuman et al., 2011). This represents a local increase in average annual absorbed surface radiation of between 2 and 7%. Albedo difference was the driver of the results of the modeling experiments completed by Bonan et al., (1992), Betts (2000), and Snyder et al., (2004) all of whom predicted the effects of such an albedo shift would extend beyond the boundaries of the boreal region. Chapin et al., (2000) documented similar albedo differences between forest and shrub tundra in Alaska with an increase in absorbed radiation leading to a warming trend that extends beyond the tundra. Similar to the modeling results for the boreal region, and the finding from the Alaskan tundra, the albedo shift implied by the successional dynamics shown in our forest simulations have the potential to increase temperatures across the region and create a positive feedback of regional warming. In particular, our results establish that there will be local shifts from larch to evergreen conifer. The resultant increase in the amount of absorbed incoming radiation has the potential to impact surrounding regions and set off a cascade of species shifts towards evergreen conifers, which in turn promote more warming. This positive feedback is of great concern across Siberia.

The response at the continental scale is most similar to the results for the low diversity regions for all treatments. This is not surprising given that 75% of the sites included in the continental-scale analysis are classified as low diversity, with an average of 9 tree species. Historically, this system maintains existing vegetation through cycles of predictable disturbance and succession, which support the regeneration of larch following fire (Chapin et al., 2004). The repetition of this successional sequence across the broad range of climatic conditions found in the boreal region creates a resilient vegetation composition with stable cycles of vegetation states (Chapin et al., 2004). The climate change scenarios considered here modify the vegetation composition in a new way, which in turn alters the successional history and reduces the resilience of vegetation, thereby forcing a new vegetation state to emerge. The larch-dominated forests appear sensitive to an increase in temperature very early in succession when the overall stand age is also young. The evergreen conifer dominated forests seem to be sensitive to changes in precipitation at mid-succession when there is a mix of stand ages from young to mature trees. Mid-succession is also the natural transition point, under base conditions, between larch and evergreen conifer, so the response to precipitation is likely connected with the emergence of evergreen conifers as the dominant species. The connection between precipitation and evergreen conifers is explored in more detail with the regional scale analysis.

At the continental scale, total forest and *Larix* spp. biomass are highly responsive to the *L. decidua* treatment and show a pattern similar to that of the forests in low diversity regions. These results suggest that low species diversity makes the system vulnerable to establishment by a new species, but highlight the potential of the introduction of *L. decidua* to be used in the mitigation or management of the albedo shift caused by transition to evergreen conifer dominance. *L. decidua* has the same characteristics as existing Siberian larch species, and thus forests dominated by this species have higher albedo, and decreased absorbed incoming radiation, when compared to stands of evergreen conifers in the same region. Maintaining larch-dominated stands across the region would delay the positive feedback triggered by the albedo shift that is associated with conversion from deciduous larch to evergreen forest.

approximately 5.1 W m-2 if the larch stands are replaced by evergreen conifers (Shuman et al., 2011). This represents a local increase in average annual absorbed surface radiation of between 2 and 7%. Albedo difference was the driver of the results of the modeling experiments completed by Bonan et al., (1992), Betts (2000), and Snyder et al., (2004) all of whom predicted the effects of such an albedo shift would extend beyond the boundaries of the boreal region. Chapin et al., (2000) documented similar albedo differences between forest and shrub tundra in Alaska with an increase in absorbed radiation leading to a warming trend that extends beyond the tundra. Similar to the modeling results for the boreal region, and the finding from the Alaskan tundra, the albedo shift implied by the successional dynamics shown in our forest simulations have the potential to increase temperatures across the region and create a positive feedback of regional warming. In particular, our results establish that there will be local shifts from larch to evergreen conifer. The resultant increase in the amount of absorbed incoming radiation has the potential to impact surrounding regions and set off a cascade of species shifts towards evergreen conifers, which in turn promote more warming. This positive feedback is of great concern

The response at the continental scale is most similar to the results for the low diversity regions for all treatments. This is not surprising given that 75% of the sites included in the continental-scale analysis are classified as low diversity, with an average of 9 tree species. Historically, this system maintains existing vegetation through cycles of predictable disturbance and succession, which support the regeneration of larch following fire (Chapin et al., 2004). The repetition of this successional sequence across the broad range of climatic conditions found in the boreal region creates a resilient vegetation composition with stable cycles of vegetation states (Chapin et al., 2004). The climate change scenarios considered here modify the vegetation composition in a new way, which in turn alters the successional history and reduces the resilience of vegetation, thereby forcing a new vegetation state to emerge. The larch-dominated forests appear sensitive to an increase in temperature very early in succession when the overall stand age is also young. The evergreen conifer dominated forests seem to be sensitive to changes in precipitation at mid-succession when there is a mix of stand ages from young to mature trees. Mid-succession is also the natural transition point, under base conditions, between larch and evergreen conifer, so the response to precipitation is likely connected with the emergence of evergreen conifers as the dominant species. The connection between precipitation and evergreen conifers is explored in more detail with the regional scale

At the continental scale, total forest and *Larix* spp. biomass are highly responsive to the *L. decidua* treatment and show a pattern similar to that of the forests in low diversity regions. These results suggest that low species diversity makes the system vulnerable to establishment by a new species, but highlight the potential of the introduction of *L. decidua* to be used in the mitigation or management of the albedo shift caused by transition to evergreen conifer dominance. *L. decidua* has the same characteristics as existing Siberian larch species, and thus forests dominated by this species have higher albedo, and decreased absorbed incoming radiation, when compared to stands of evergreen conifers in the same region. Maintaining larch-dominated stands across the region would delay the positive feedback triggered by the albedo shift that is associated with conversion from deciduous

across Siberia.

analysis.

larch to evergreen forest.
